Condensation reactions of guanidines with bis-electrophiles: Formation of highly nitrogenous heterocycles

Abstract

2-Amino-1,4-dihydropyrimidines were reacted with bis-electrophiles to produce novel fused bi-pyrimidine, pyrimido-aminotriazine, and pyrimido-sulfonamide scaffolds. In addition, a quinazoline library was constructed using a guanidine Atwal-Biginelli reaction with 1-(quinazolin-2-yl)guanidines. The product heterocycles have novel constitutions with high nitrogen atom counts and represent valuable additions to screening libraries for the discovery of new modulators of biological targets.

1. Introduction

Diversity Oriented Synthesis (DOS) inspires chemists to design or apply chemical methodologies that result in the synthesis of structurally diverse, functionally rich and often architecturally complex small molecule scaffolds.^1,2 In many cases, new biological targets can be explored more effectively with DOS-derived probes and natural products than traditional aromatic “drug-like” heterocycles.³ Novel molecular scaffolds are therefore valuable additions to high throughput screening libraries and improve the odds of identifying lead compounds in drug discovery initiatives.^4,5

As a continuation of our efforts in the preparation of rare heterocyclic scaffolds,⁶ we became interested in developing a convergent synthetic route to 4H-pyrimido[1,2-a]pyrimidines. While there is evidence in the literature demonstrating melanin-concentrating hormone receptor (MCH1R) antagonistic effects,⁷ protein kinase inhibition,⁸ treatment of atherosclerosis and restenosis,⁹ antiviral activities,¹⁰ inhibition of platelet aggregation,¹¹ and anti-MRSA dihydrofolate reductase activity¹² of 4H-pyrimido[1,2-a]pyrimidines 1, their synthesis and application is strikingly rare compared to the structurally and pharmacologically related ring-contracted imidazo[1,2-a]pyridines 2 (Figure 1). The latter heterocycle is an abundant pharmacophore in medicinal chemistry and has been used in antiviral, antibacterial, anti-inflammatory, analgesic, antipyretic, and anxioselective indications. Derivatives of 2 are β-amyloid formation inhibitors and constitute a novel class of orally active nonpeptide bradykinin B2 receptor antagonists.¹³ Several imidazo[1,2-a]pyridines are already clinically used, including zolimidine (an anti-ulcer drug), zolpidem (a hypnotic drug), and alpidem (a nonsedative anxiolytic). Imidazo[1,2-a]pyrimidine moieties are also important as benzodiazepine receptor agonists, antiviral agents, antibacterials, antifungal agents, and calcium channel blockers.¹³

Fig. 1.

The 4H-pyrimido[1,2-a]pyrimidine and imidazo[1,2-a]pyridine scaffolds 1 and 2, and selected biologically active derivatives.

Using a variant of the Biginelli 3-component reaction for the preparation of cyclic guanidines,^14,15 we were able to develop efficient strategies for a tandem condensation with bis-electrophiles to give bipyrimidines 3, pyrimido-aminotriazines 4, cyclic pyrimido-sulfonamides 5 and pyrimido-pyrimidines 6 (Scheme 1). In addition, a small sub-library of quinazoline-containing heterocycles 7 was assembled using Atwal-Biginelli conditions.^16,17

Scheme 1.

Overview of heterocycles obtained by condensation of guanidines with bis-electrophiles.

New strategies for accessing structurally diverse guanidine-based Biginelli products have previously been reported by the Overman group.¹⁸ Specifically, triazone-protected guanidines were found to endure the harsh acidic conditions frequently used for the Biginelli cyclization.¹⁸ In order to access C-4 alkyl-substituted cyclic guanidines 10, we first envisioned using thioureas 8 as starting materials followed by an aminolysis of the corresponding methylthioureas 9, in a manner similar to that reported by Kappe (Scheme 2).¹⁹

Scheme 2.

Thiourea methylation followed by aminolysis of the thioimidate provides access to 2-aminodihydropyrimidines.

2. Results and Discussion

Thioureas 8 with R = methyl, phenethyl, and iso-propyl were prepared through Biginelli multicomponent condensations from β-ketoesters, aldehydes, and thiourea under standard acid-catalyzed conditions.²⁰ The reaction parameters for the one-pot methylation/aminolysis were optimized on substrate 8a (R = Me) (Table 1, Entries 1-5). Initial investigations using methyl iodide resulted in dealkylation of the newly formed methylthiourea 9a, most likely due to the presence of nucleophilic iodide ions in the vigorously heated aminolysis reaction mixture.²¹ Accordingly, methyl triflate was used as an alternative electrophile for the S-alkylation and proved effective for the formation of the desired guanidine 10a in good isolated yields. The use of dimethyl sulfate also provided 10a in 57% yield but required prolonged reaction times (Entry 5). High yields of product 10a were achieved when THF was used as the reaction solvent. However, the methylation step was problematic due to the propensity of methyl triflate to polymerize THF.²² The optimal solvent for this two-step reaction sequence was found to be chlorobenzene (PhCl), which was both inert to the reaction conditions and could safely be heated to 95-100 °C under sealed tube conditions (Table 1, Entry 3). The aminolysis step was found to proceed smoothly on multigram scale (2.5 g) with ammonium acetate or ammonium chloride as additives in the presence of ammonia. Following this one-pot reaction, the hydrochloride salts of 10a-c were prepared and subsequently used as substrates for further cyclization reactions (vide infra).

Table 1.

Conversion of thioureas (8a, R = CH₃; 8b, R = (CH₂)₂Ph; 8c, R = CH(CH₃)₂) to thioimidates 9a-c and 2-aminopyrimidines 10a-c according to Scheme 2.

Entry	8/Conditions	9/Additive/Time	10/Yield %a
1	8a/MeOTf, THF, 1.5 h	9a/NH4OAc/20 h	10a/77
2	8a/MeOTf, ClCH2CH2Cl, 0.75 h	9a/NH4OAc/15 h	10a/30
3	8a/MeOTf, PhCl, 3.5 h	9a/NH4OAc/17 h	10a/61
4	8a/MeOTf, PhCl, 15 h	9a/NH4Cl/21 h	10a/51
5	8a/(MeO)2SO2, THF, 60 h	9a/NH4OAc/17 h	10a/57
6	8b/MeOTf, PhCl, 3.5 h	9b/NH4OAc/17 h	10b/39
7	8c/MeOTf, PhCl, 3.5 h	9c/NH4OAc/20 h	10c/33

^aIsolated yield of purified product.

The 4-arylated 2-aminopyrimidine 13•TFA was obtained in a 2-step sequence starting from PMB-guanidine 11 (Scheme 3).²³ Treatment of a solution of 11 in hexafluoroisopropanol with enone 12 and sodium bicarbonate at 90 °C provided the PMB-protected 13 in 42% yield. Deprotection of the PMB group was accomplished in 70% yield in TFA buffered with water, triethylsilane, and thioanisole.²⁴ A 3-component Biginelli reaction with guanidine hydrochloride 14•HCl, ethyl 3-oxo-3-phenylpropanoate 15 and aldehydes 16 and 17 was performed to access heterocycles 18, 19 and the hydrochloride salt 18•HCl (Scheme 4).

Scheme 3.

2-Step preparation of 2-aminopyrimidine 13•HCl from PMB-protected guanidine 11.

Scheme 4.

Preparation of 2-aminopyrimidines from guanidine hydrochloride.

Cyclization reactions of guanidines with the bis-electrophile 1,1,3,3-tetramethoxypropane (20) were explored under microwave conditions (Scheme 5 and Table 2).^25,26 We found that these reactions proceeded smoothly in 42-76% yield when the hydrochloride salts of 10a-c or 18 were combined with 20 in a microwave vial and heated to 160 °C in trifluoroethanol (21) (Entries 1-4). The use of the hydrochloride salts was essential since the corresponding reactions with free amines did not afford the cyclocondensation products. We also briefly explored the reaction of 18 with 4-methoxybut-3-enone 26 to give 8-methyl 4H-pyrimido[1,2-a]pyrimidine 27 in 64% yield (Table 2, Entry 5).

Scheme 5.

Synthesis of 4H-pyrimido[1,2-a]pyrimidine-3-carboxylates

Table 2.

Cyclocondensation reactions of guanidines with bis-electrophiles 20 and 26 according to Scheme 5.

Entry	Reactants	Solvent/Temperature/Time	Product/Yield %a
1	10a•HCl and 20	21/160 °C/1 h	22/76
2	10b•HCl and 20	21/160 °C/2 h	23/71
3	10c•HCl and 20	21/160 °C/2 h	24/71
4	18•HCl and 20	21/160 °C/1.5 h	25/42
5	18 and 26	THF/150 °C/0.25 h	27/64

^aIsolated yield of purified product.

Additional substituents on pyrimido[1,2-a]pyrimidines could be introduced by cyclocondensations of 18•HCl with heterocyclic TMS-alkynones (Scheme 6).²⁷ In the presence of sodium carbonate and catalytic amounts of DMAP, microwave heating at 170 °C for 30 min provided the 8-heteroaryl substituted pyrimidopyridines 30 and 31 in modest yields. In order to further improve the synthesis of these interesting derivatives, we developed a one-pot Sonogashira coupling/cyclocondensation approach (Scheme 7). Room temperature conversion of acid chlorides 32-34 and trimethylsilyl acetylene in the presence of Pd(0)-catalysts led to ynones 35-37, which were condensed with pyrimidines 18 and 19 under microwave irradiation to give pyrimidopyridines 38a-d in improved overall yields of 28-60% (Table 3). A modest preference for NCy₂Me over Et₃N in the Sonogashira coupling was noted for acid chloride 32 (Entries 1, 2, 5, and 6), whereas the yield of the electron-rich acyl chloride 33 was almost unchanged (Entries 3 and 7). The efficiency of the conversion of the electron-deficient acyl chloride 34 decreased in the presence of NCy₂Me (Entries 4 and 8). Copper- and solvent-free reaction conditions²⁸ also lowered yields (Entries 9 and 10).

Scheme 6.

Synthesis of 8-heteroaryl-4H-pyrimido[1,2-a]pyrimidine-3-carboxylates.

Scheme 7.

One-pot Sonogashira coupling - cyclocondensation for the synthesis of 8-aryl-4H-pyrimido[1,2-a]pyrimidine-3-carboxylates.

Table 3.

Sonogashira couplings - cyclocondensations according to Scheme 7.

Entry	Reactants	Sonogashira Coupling Conditions	Product/R/R1/Yield %a
1	32 and 18	Pd(PPh3)2Cl2 (2 mol %); CuI (4 mol %), Net3, THF, rt	38a/H/H/54
2	32 and 19	Pd(PPh3)2Cl2 (2 mol %); CuI (4 mol %), Net3, THF, rt	38b/H/OMe/39
3	33 and 18	Pd(PPh3)2Cl2 (2 mol %); CuI (4 mol %), NEt3, THF, rt	38c/OMe/H/32
4	34 and 18	Pd(PPh3)2Cl2 (2 mol %); CuI (4 mol %), Net3, THF, rt	38d/CF3/H/55
5	32 and 18	Pd(PPh3)2Cl2 (2 mol %); CuI (4 mol %), NCy2Me, THF, rt	38a/H/H/60
6	32 and 19	Pd(PPh3)2Cl2 (2 mol %); CuI (4 mol %), NCy2Me, THF, rt	38b/H/OMe/51
7	33 and 18	Pd(PPh3)2Cl2 (2 mol %); CuI (4 mol %), NCy2Me, THF, rt	38c/OMe/H/28
8	34 and 18	Pd(PPh3)2Cl2 (2 mol %); CuI (4 mol %), NCy2Me, THF, rt	38d/CF3/H/29
9	32 and 18	Pd(OAc)2 (1 mol %); NCy2Me, 40 °C	38a/H/H/30
10	34 and 18	Pd(OAc)2 (1 mol %), NCy2Me, 40 °C	38d/CF3/H/44

^aIsolated yield of purified product.

Pyrimidopyridines 27, 30, 31 and 38a-d were isolated as single regioisomers, and their assignment was supported by an x-ray analysis of 38d (Figure 2). The regiochemical preference for substitution at C-8 can be explained by a destabilizing steric hindrance exerted between the pseudoaxial 4-aryl group and a peri-substituent at the 6-position.

Fig. 2.

X-ray structure of 38d (CCDC 932463).

Starting with 2-aminopyrimidines 10a-c and 18, we were able to access two additional novel heterocyclic scaffolds, the 2-morpholino-6H-pyrimido[1,2-a][1,3,5]triazine-7-carboxylates 40-43, and the 4-oxo-3,4,6,9-tetrahydropyrimido[2,1-c][1,2,4,6]thiatriazine-7-carboxylate 2,2-dioxides 44 and 45 (Scheme 8). Condensation of pyrimidines 10a-c and 18 with in situ generated cyanoimine 39 furnished the pyrimidotriazines in 25-53% yield under microwave irradiation conditions. A similar transformation has recently been reported for the synthesis of aminotriazine-based anti-HIV agents.²⁹ Alternatively, treatment of 10a and 18 with chlorosulfonylisocyanate afforded the cyclic sulfamides 44 and 45 in 15% and 40% yield, respectively. We were able to obtain an x-ray structure of 44 that confirmed the assignment (Figure 3).

Scheme 8.

Synthesis of 2-morpholino-6H-pyrimido[1,2-a][1,3,5]triazine-7-carboxylates 40-43 and 4-oxo-3,4,6,9-tetrahydropyrimido[2,1-c][1,2,4,6]thiatriazine-7-carboxylate 2,2-dioxides 44-45.

Fig. 3.

X-ray structure of 44 (CCDC 935121). Due to static disorder in the ethyl ester group, the five ethyl hydrogen positions are uncertain and are therefore not shown.

Heterocycles 40-43 and 44-45 are representative examples for target scaffolds 4 and 5 shown in Scheme 1. We were also interested in further extending our conversions of 2-aminopyrimidines and guanidinoquinazolines to the structural variants 6 and 7, which are only featured in very few reported examples in the literature.^30,31 In particular, we wanted to determine if aminopyrimidine 13•TFA could be used as a substrate for a second Atwal-Biginelli condensation reaction. Although prolonged reaction times were required, we were pleased to find that the expected fused bicycle 46 was indeed formed as a 4:1 mixture of anti-syn-isomers in 57% yield (Scheme 9). The major anti-isomer was assigned based on literature precedence³⁰ as well as ab initio HF-6-311G* calculations (Figure 4).³² The latter provided a 5.8 kcal/mol gas-phase energy difference in favor of anti-46, suggesting that the reaction was not under complete thermodynamic control. Indeed, upon heating a sample of the product mixture for 26 h in the presence of deuterated acetic acid at 90 °C in DMSO-d₆, the signals characteristic for syn-46 at 7.00-6.92 and 5.35 ppm disappeared in the ¹H NMR.

Scheme 9.

A second Atwal-Biginelli cyclocondensation reaction converts 2-aminopyrimidine 13•TFA into pyrimidopyrimidine 46.

Fig. 4.

HF-6-311G* structures of anti- (top) and syn- (bottom) 46.

In an analogous synthetic sequence, guanidinoquinazolines 47a-d were converted in moderate to high yields to the amine-linked pyrimidyl-quinazolines 49a-i upon heating with 12 and 48a-d in hexafluoroisopropanol (Scheme 10 and Table 4). The cyclocondensation worked particularly well for the morpholine-substituted enone 48d (Entry 8), whereas the yield for the dimethylamino-substituted enone 48a was consistently low (Entries 4 and 9). Quinazolines 47a-d were prepared in a three-step sequence from the corresponding anilines.³³

Scheme 10.

Atwal-Biginelli cyclocondensation reactions with guanidine-quinazolines 47a-d.

Table 4.

Atwal-Biginelli reactions according to Scheme 10.

Entry	Reactants	Substituents R/R1/R2/R3	Product/Yield %a
1	47a and 12	Cl/H/Cl/H	49a/56
2	47b and 12	Me/H/Cl/H	49b/62
3	47c and 12	F/H/Cl/H	49c/52
4	47c and 48a	F/H/NMe2/H	49d/40
5	47b and 48b	Me/H/H/NO2	49e/67
6	47d and 12	-OCH2O-/Cl/H	49f/41
7	47d and 48c	-OCH2O-/Br/H	49g/57
8	47d and 48d	-OCH2O-/N((CH2CH2)2O)/H	49h/81
9	47d and 48a	-OCH2O-/NMe2/H	49i/45

^aIsolated yield of purified product.

3. Conclusion

Cyclocondensations of heterocyclic guanidines with bis-electrophiles provide a rapid access to previously unexplored or sparsely documented heterocyclic scaffolds. Specifically, we were able to devise concise routes to new bipyrimidines, pyrimidoaminotriazines, pyrimidosulfonamides, pyrimidopyrimidines, and amine-linked pyrimidoquinazolines. Several versatile methodologies were explored in this work: a) a one-pot methylation/aminolysis sequence for the preparation of 2-aminodihydropyrimidine intermediates (Scheme 2); b) an alternative Atwal-Biginelli cyclization of a PMB-protected guanidine for the construction of 2-aminodihydropyrimidines (Scheme 3); c) a one-pot tandem Sonogashira coupling–alkynone cyclization to produce fused bipyrimidines (Scheme 7); d) diverse cyclocondensation strategies, including consecutive Atwal-Biginelli reactions, to afford pyrimidopyrimidines, pyrimidotriazines and pyrimidothiatriazines (Schemes 5, 8 and 9). Finally, quinazoline-dihydropyrimidine amines were prepared by extending Atwal-Biginelli conditions to quinazoline guanidines (Scheme 10). We are currently exploring the activity profile of these new heterocycles in a variety of biological screens.

4. Experimental Section

4.1. General Experimental Procedures

All moisture-sensitive reactions were performed under an atmosphere of dry nitrogen and all glassware was either dried in an oven at 140 °C or flame dried under high vacuum prior to use. THF and Et₂O were dried by distillation over Na/benzophenone, and CH₂Cl₂ and toluene were purified using an alumina filtration system. Reactions were monitored by either ¹H NMR at 300 MHz in DMSO-d₆, high resolution LC/MS (Thermo Scientific Exactive spectrometer), or TLC analysis (EM Science pre-coated silica gel 60 F254 plates). Visualization of TLCs was accomplished with a 254 nm UV light and by staining with a p-anisaldehyde solution (2.5 mL of p-anisaldehyde, 2 mL of AcOH, and 3.5 mL of conc. H₂SO₄ in 100 mL of 95% EtOH) or a KMnO₄ solution (1.5 g of KMnO₄ and 1.5 g of K₂CO₃ in 100 mL of a 0.1% NaOH solution). Flash chromatography was performed on 40-63 μm silica gel (Silicycle) or on a Teledyne ISCO CombiFlash R_f. Melting points were determined using a Laboratory Devices Mel-Temp II and are not corrected. Infrared spectra were obtained on either a Nicolet Avatar 360, a Smiths Detection IdentifyIR FT-IR spectrometer, or Perkin Elmer ATR IR. Mass spectra were obtained on either a Micromass Autospec double focusing instrument or High Res LC/MS instrument (Thermo Scientific Exactive spectrometer). Purities were determined using an Agilent Technologies 385-ELSD. Microwave reactions were performed in a Biotage Initiator 2.0 Microwave reactor. ¹H and ¹³C NMR spectra were obtained on a Bruker Avance 300, 400, 500, or 700 MHz instrument at room temperature unless otherwise noted. Chemical shifts (δ) are reported in parts per million (ppm) with the residual solvent peak used as an internal standard (CHCl₃ δ 7.26 ppm for ¹H and 77.00 ppm for ¹³C, DMSO δ 2.50 ppm for ¹H and 40.45 ppm for ¹³C). ¹H NMR spectra are tabulated as follows: chemical shift, multiplicity (s=singlet, d=doublet, t=triplet, q=quartet, m=multiplet, br=broad), integration, and coupling constant(s) (J) in Hertz (Hz). ¹³C NMR spectra were recorded using a proton-decoupled pulse sequence with a d₁ of 5 sec, and are tabulated by the chemical shifts of the observed peaks. X-ray crystallographic data for 38d (deposition number CCDC 932463) and 44 (CCDC 935121) were deposited with the Cambridge Crystallographic Data Center.

4.2. Ethyl 4,6-dimethyl-2-thioxo-1,3,6-trihydropyrimidine-5-carboxylate (8a).²⁰ General Procedure A

A suspension of ethyl 3-oxobutanoate (2.50 mL, 19.7 mmol), thiourea (2.20 g, 28.9 mmol), ethanal (1.10 mL, 19.7 mmol) and benzyltriethylammonium chloride (0.386 g, 1.97 mmol) was heated in a sealed tube at 75° C for 15 min. The reaction mixture turned light yellow in color and was heated at 105-100 °C for 20 h, allowed to cool to room temperature, diluted with water (20 mL), and stirred for 1 h. The resulting solid residue was isolated by vacuum filtration, washed with water (70 mL), recrystallized from boiling EtOH:H₂O (3:1, 50 mL), washed with EtOH:H₂O (1:1), followed by water, and dried in vacuo to afford 8a (1.81 g, 43%) as pale yellow crystals: ¹H NMR (300 MHz, DMSO-d₆) δ 10.12 (s, 1H), 9.19 (s, 1H), 4.15-4.03 (m, 3H), 2.19 (s, 3H), 1.19 (t, 3H, J=6.9 Hz), 1.09 (d, 3H, J=6.3 Hz). The spectral data are consistent with the literature values.²⁰

4.3. Ethyl 4-methyl-6-(2-phenylethyl)-2-thioxo-1,3,6-trihydropyrimidine-5-carboxylate (8b).³⁴

According to general procedure A, a suspension of thiourea (1.72 g, 22.4 mmol), benzyltriethylammonium chloride (0.29 g, 1.50 mmol), hydrocinnamaldehyde (2.08 mL, 15.0 mmol) and ethyl 3-oxobutanoate (2.50 mL, 15.0 mmol) was heated at 120 °C for 3 h. The viscous orange reaction mixture was allowed to cool to room temperature, diluted with EtOAc (150 mL), and washed with water (3 × 100 mL). The organic layer was dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The crude light orange residue was recrystallized from hexanes:EtOAc (2:1), isolated by vacuum filtration, washed with hexanes and dried in vacuo to afford 8b as a pale yellow solid (2.24 g, 49%, ~90% pure): ¹H NMR (300 MHz, DMSO-d₆) 10.19 (s, 1H), 9.45 (s, 1H), 7.30-7.18 (m, 5H), 4.18-4.16 (m, 1H), 4.07-4.02 (m, 2H), 2.63-2.51 (m, 2H), 2.22 (s, 3H), 1.78-1.66 (m, 2H), 1.13 (t, 3H, J=7.2 Hz). The spectral data are consistent with the literature values.³⁴

4.4. Ethyl 4-methyl-6-(methylethyl)-2-thioxo-1,3,6-trihydropyrimidine-5-carboxylate (8c).²⁰

According to general procedure A, a suspension of thiourea (2.75 g, 35.9 mmol), citric acid (2.30 g, 12.0 mmol), isobutyraldehyde (2.22 mL, 23.9 mmol) and ethyl 3-oxobutanoate (4.0 mL, 23.2 mmol) was heated at 120 °C for 15 h. The viscous yellow/orange reaction mixture was allowed to cool to room temperature, diluted with EtOAc (300 mL), and washed with water (3 × 200 mL). The organic layer was dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The resulting light orange residue was slowly triturated with EtOAc (30 mL) and hexanes (60 mL) at room temperature. The suspension was cooled to -20 °C for 24 h and the resulting off-white precipitate was isolated by vacuum filtration, washed with hexanes:EtOAc (3:1, 100 mL), followed by hexanes (30 mL), and dried in vacuo to afford 8c as an off-white solid (2.13 g, 33%, ~90% pure): ¹H NMR (300 MHz, DMSO-d₆) 10.09 (s, 1H), 9.23 (s, 1H), 4.15-4.02 (m, 2H), 4.00-3.97 (m, 1H), 2.23 (s, 3H), 1.71-1.65 (m, 1H), 1.20 (t, 3H, J=7.2 Hz), 0.82 (d, 3H, J=6.0 Hz), 0.76 (d, 3H, J=6.0 Hz). The spectral data are consistent with the literature values.²⁰

4.5. Ethyl 2-amino-4,6-dimethyl-1,4-dihydropyrimidine-5-carboxylate (10a). General Procedure B

To a light yellow suspension of 8a (1.50 g, 7.00 mmol) in chlorobenzene (40 mL) at 0 °C was added methyl trifluoromethanesulfonate (0.91 mL, 7.7 mmol) in one portion. The reaction mixture was allowed to warm to room temperature and stirred for 3.5 h. Ammonium acetate (5.4 g, 70 mmol) was added and ammonia gas was bubbled through the cold (0 °C) reaction mixture for 10 min. The reaction tube was sealed and heated at 95-100 °C for 17 h. The reaction mixture was cooled to room temperature, diluted with saturated Na₂CO₃ (250 mL) and water (250 mL), extracted with EtOAc (3 × 400 mL), dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The crude residue was recrystallized from hexanes:EtOAc (1:1, 150 mL), isolated by vacuum filtration, washed with hexanes, and dried in vacuo to afford 10a as a colorless solid (0.84 g, 61%, ELSD purity 97.3%): Mp 196-198 °C; IR (ATR) 3390, 3303, 3088, 2975, 1661, 1584, 1493, 1370, 1329, 1303, 1226, 1154, 1092, 1066 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 6.81 (br s, 1H), 6.12 (br s, 2H), 4.18 (q, J=6.3 Hz, 1H), 4.05-3.95 (m, 2H), 2.10 (s, 3H), 1.16 (t, 3H, J=7.2 Hz), 0.97 (d, 3H, J=6.3 Hz); ¹³C NMR (75 MHz, DMSO-d₆,) δ 166.5, 160.8, 156.0, 98.6, 58.4, 45.5, 23.9, 15.0; HRMS (TOF ES⁺) m/z calcd for C₉H₁₆N₃O₂ 198.1243, found 198.1242.

4.6. Ethyl 2-amino-6-methyl-4-(2-phenethyl)-1,4-dihydropyrimidine-5-carboxylate (10b)

According to general procedure B, a mixture of 8b (2.50 g, 8.21 mmol) and methyl trifluoromethanesulfonate (1.1 mL, 9.0 mmol) in chlorobenzene (80 mL) was treated with ammonium acetate (6.3 g, 82 mmol) and ammonia gas to give the crude product which was dissolved in CH₂Cl₂ (20 mL) and triturated with hexanes to yield a tan oil. This oil was isolated by decantation and again dissolved in CH₂Cl₂ (20 mL), triturated with hexanes (30 mL) and cooled to -20 °C to afford 10b as a pale yellow solid (0.92 g, 39%, ELSD purity 97.3%): Mp 129-134 °C; IR (ATR) 3390, 3348, 2975, 1650, 1584, 1465, 1374, 1238, 1172, 1206, 1146, 1077, 1029 cm^-1; ¹H-NMR (300 MHz; DMSO-d₆) δ 7.29-7.15 (m, 6H), 6.87 (br s, 2H), 4.14 (m, 1H), 3.98-3.88 (m, 2H), 2.71-2.59 (m, 1H), 2.54-2.44 (m, 1H), 2.13 (s, 3H), 1.66-1.56 (m, 2H), 1.10 (t, 3H, J=6.9 Hz); ¹³C NMR (75 MHz, DMSO-d₆) δ 165.8, 158.9, 155.1, 141.7, 128.3 (2 C), 128.2 (2 C), 125.6, 97.8, 58.2, 48.5, 38.2, 29.7, 22.6, 14.4; HRMS (TOF ES⁺) m/z calcd for C₁₆H₂₂N₃O₂ 288.1712, found 288.1720.

4.7. Ethyl 2-amino-4-(methylethyl)-6-methyl-1,4-dihydropyrimidine-5-carboxylate (10c)

According to general procedure B, a mixture of 8c (1.80 g, 7.42 mmol) and methyl trifluoromethanesulfonate (0.96 mL, 8.2 mmol) in chlorobenzene (70 mL) was treated with ammonium acetate (6.0 g, 78 mmol) and ammonia gas to give the crude product which was purified by chromatography through a plug of SiO₂ (EtOAc to EtOH). The product slowly eluted with EtOH and was recrystallized from boiling CH₂Cl₂:hexanes (1:1, 40 mL) to afford 10c as a white crystalline solid (0.581 g, 33%, ELSD purity 100.0%): Mp 171-173 °C; IR (ATR) 3497, 3353, 2951, 2869, 1653, 1596, 1489, 1377, 1225, 1154, 1068, 995, 816 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 6.87 (br s, 1H), 6.07 (br s, 2H), 4.04-3.92 (m, 3H), 2.12 (s, 3H), 1.61-1.54 (m, 1H), 1.15 (t, 3H, J=7.2 Hz), 0.78 (d, 3H, J=6.9 Hz), 0.71 (d, 3H, J=6.9 Hz); ¹³C NMR (75 MHz; DMSO-d₆) δ 166.5, 161.1, 156.2, 95.7, 57.9, 54.4, 34.6, 23.5, 18.1, 15.9, 14.4; HRMS (TOF ES⁺) m/z calcd for C₁₁H₂₀N₃O₂ 226.1556, found 226.1569.

4.8. Ethyl 2-imino-4,6-dimethyl-1,2,3,4-tetrahydropyrimidine-5-carboxylate hydrochloride (10a•HCl). General Procedure C

To a suspension of 10a (0.500 g, 2.54 mmol) in MeOH (5.0 mL) was added a solution of HCl in dioxane (1.5 mL, 3.0 mmol, 2.0 M) dropwise at room temperature. After 10 min, the solution was clear and the solvents were removed under reduced pressure. The resulting oil was triturated with ether (2 × 10 mL), isolated by vacuum filtration, and dried in vacuo to afford 10a•HCl as a colorless foam (0.54 g, 91%): IR (ATR) 3361, 3236, 3055, 2975, 2869, 1681, 1551, 1241, 1096, 1070 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 10.90 (s, 1H), 9.06 (s, 1H), 7.89 (br s, 2H), 4.38-4.36 (m, 1H), 4.19-4.08 (m, 2H), 2.26 (s, 3H), 1.22 (t, 3H, J=7.2 Hz), 1.16 (d, 3H, J=6.3 Hz); ¹³C NMR (75 MHz, DMSO-d₆) δ 164.9, 151.8, 144.1, 104.5, 60.5, 45.4, 22.9, 17.9, 14.6. For additional characterization data, see 10a.

4.9. Ethyl 2-imino-6-methyl-4-(2-phenethyl)-1,2,3,4-tetrahydropyrimidine-5-carboxylate hydrochloride (10b•HCl)

According to general procedure C, 10b (0.150 g, 0.522 mmol) was combined with a solution of HCl in ether (0.31 mL, 0.63 mmol, 2.0 M) and MeOH (1.0 mL) at room temperature to afford 10b•HCl as a pale yellow solid (0.143 g, 85%): ¹H NMR (300 MHz, DMSO-d₆) δ 10.59 (br s, 1H), 9.18 (br s, 1H), 7.76 (br s, 2H), 7.31-7.16 (m, 5H), 4.33-4.30 (m, 1H), 4.14-4.02 (m, 2H), 2.73-2.49 (m, 2H), 2.26 (s, 3H), 1.78-1.09 (m, 2H), 1.15 (t, 3H, J=6.9 Hz). For additional characterization data, see 10b.

4.10. Ethyl 2-imino-4-(methylethyl)-6-methyl-1,2,3,4-tetrahydropyrimidine-5-carboxylate hydrochloride (10c•HCl)

According to general procedure C, 10c (0.150 g, 0.666 mmol) was combined with a solution of HCl in ether (0.40 mL, 0.80 mmol, 2.0 M) in MeOH (1.5 mL) at room temperature to afford 10c•HCl as a pale yellow solid (0.121 g, 69%): ¹H NMR (300 MHz, DMSO-d₆) δ 10.77 (br s, 1H), 9.14 (br s, 1H), 7.84 (br s, 2H), 4.19-4.07 (m, 3H), 2.28 (s, 3H), 1.78-1.72 (m, 1H), 1.21 (t, 3H, J=7.2 Hz), 0.87 (d, 3H, J=6.9 Hz), 0.77 (d, 3H, J=6.9 Hz). For additional characterization data, see 10c.

4.11. 1-(4-(4-Chlorophenyl)-2-((4-methoxybenzylamino)-6-methyl-1,4-dihydropyrimidin-5yl)butan-1-one (13). General Procedure D

A microwave vial was charged with 11²³ (0.63 g, 2.8 mmol) and (CF₃)₂CHOH (5.0 mL). To this homogeneous solution was added 12^36,37 (1.4 g, 5.5 mmol) and NaHCO₃ (0.26 g, 3.0 mmol). The vial was sealed and immersed into a pre-heated oil bath at 90 °C and heated for 16 h. The solution was allowed to cool to room temperature, filtered through a small plug of Celite, and rinsed with CH₂Cl₂. The filtrate was concentrated under reduced pressure and purified by chromatography on SiO₂ (ISCO-R_f, 0-10%MeOH/CH₂Cl₂) to afford 13 as an orange crystalline solid (0.485 g, 42%, ELSD purity 99.7%): Mp 60-64 °C; IR (neat) 3311, 2929, 2830, 1661, 1574, 1512, 1212, 1088, 818 cm^-1; ¹H NMR (300 MHz, CDCl₃) δ 7.22-7.11 (m, 6H), 6.78 (d, 2H, J=8.4 Hz), 5.29 (s, 1H), 4.33 (d, 1H, J=15.3 Hz), 4.25 (d, 1H, J= 15.0 Hz), 4.03 (q, 2H, J=7.2 Hz), 3.78 (s, 3H), 2.38 (s, 3H), 1.14 (t, 3H, J=7.2 Hz); ¹³C NMR (100 MHz, CDCl₃) δ 166.6, 159.2, 153.0, 143.2, 133.2 (2 C), 128.9, 128.6 (2 C), 128.3 (2 C), 127.9 (2 C), 114.2 (2 C), 100.0, 59.5, 55.2, 53.7, 44.6, 22.4, 14.2; HRMS (ESI⁺) m/z calcd for C₂₂H₂₅O₃N₃Cl (M+H) 414.1579, found 414.1562.

4.12. Ethyl 2-amino-4-(4-chlorophenyl)-6-methyl-1,4-dihydroprimidine-5-carboxylate 2,2,2-trifluoroacetate (13•TFA)

A solution of 13 (0.420 g, 1.01 mmol) in a mixture of TFA/Et₃SiH/thioanisole/H₂O (92.5:2.5:2.5:2.5, 23 mL) was stirred at 50 °C for 22 h, allowed to cool to room temperature and concentrated under reduced pressure. The crude orange residue was purified by trituration with tert-butyl methyl ether, isolated by vacuum filtration, and dried in vacuo to give trifluoroacetate salt 13•TFA (0.29 g, 70%, ELSD purity 99.9%) as a white solid: Mp 207-218 °C (dec.); IR (neat) 3241, 3205, 3084, 2963, 2901, 1709, 1666, 1239, 1200, 1186, 1139, 1096, 848, 828, 800, 771, 726 cm^-1; ¹H NMR (300 MHz, CDCl₃:MeOD, 1:1) δ 7.03 (d, 2H, J=8.4 Hz), 6.94 (d, 2H, J=8.4 Hz), 5.14 (s, 1H), 3.80 (q, 2H, J=7.2 Hz), 2.13 (s, 3H), 0.87 (t, 3H, J=7.2 Hz); ¹³C NMR (100 MHz, CDCl₃:MeOD, 1:1) δ 164.0, 150.3, 143.1, 139.7, 133.7, 128.3(2 C), 127.5 (2 C), 103.1, 60.1, 52.1, 16.4, 12.8; HRMS (ESI⁺) m/z calcd for C₁₄H₁₇O₂N₃Cl (M+H) 294.1004, found 294.0999.

4.13. Ethyl 2-amino-4,6-diphenyl-1,4-dihydropyrimidine-5-carboxylate (18).³⁰ General Procedure E

To a solution of benzaldehyde (16, 2.0 mL, 20 mmol), urea 14•HCl (3.32 g, 35.0 mmol) and NaHCO₃ (6.72 g, 80.0 mmol) in DMF (40 mL) was added ethyl 3-oxo-3-phenylpropanoate (15, 3.80 mL, 22.5 mmol) at room temperature. The reaction mixture was heated to 70 °C for 17 h, cooled to room temperature, and slowly diluted with water (100 mL). The aqueous mixture was stirred at room temperature for 1 h and the resulting solid was separated by decanting and vacuum filtration. The crude product was purified by heating in hexanes:EtOAc (1:1, 100 mL) for 30 min, separated by vacuum filtration, and dried in vacuo to afford 18 as a white solid (3.35 g, 52%): ¹H NMR (300 MHz, DMSO-d₆,) δ 7.48 (s, 1H), 7.39-7.31 (m, 4H), 7.27-7.21 (m, 6H), 6.29 (s, 2H), 5.30 (s, 1H), 3.67 (q, 2H, J=7.2 Hz), 0.73 (t, 3H, J=7.2 Hz); MS (EI⁺) m/z 321 (M⁺, 43), 292 (38), 274 (32), 248 (52), 244 (100), 104 (37), 76 (42); HRMS (EI⁺) m/z calcd for C₁₉H₁₉N₃O₄ 321.1477, found 321.1476. The spectral data are consistent with the literature values.¹⁸

4.14. Ethyl 2-imino-4,6-diphenyl-1,2,3,4-tetrahydropyrimidine-5-carboxylate hydrochloride (18•HCl)

According to general procedure C, 18 (1.28 g, 3.97 mmol) and a solution of HCl in dioxane (1.25 mL, 5.00 mmol, 4.0 M) and methanol (5.0 mL) afforded 18•HCl as a white crystalline solid (1.42 g, 99%): ¹H NMR (300 MHz, DMSO-d₆) δ 10.86 (s, 1H), 9.75 (s, 1H), 8.09 (br s, 2H), 7.51-7.33 (m, 10H), 5.55 (d, 1H, J=3.6 Hz), 3.78 (q, 2H, J=6.9 Hz), 0.74 (t, 3H, J=6.9 Hz); ¹³C NMR (75 MHz, DMSO-d₆) δ 164.6, 151.7, 144.6, 142.2, 133.7, 130.3, 129.4 (2 C), 129.1 (2 C), 128.8, 128.6 (2 C), 127.1 (2 C), 104.2, 60.4, 52.8, 13.7. For additional characterization data, see 18.

4.15. Ethyl 2-amino-4-(3,4-dimethoxyphenyl)-6-phenyl-1,4-dihydropyrimidine-5-carboxylate (19)

According to general procedure E, ethyl 3-oxo-3-phenylpropanoate (15, 3.80 mL, 22.5 mmol), 3,4-dimethoxybenzaldehyde (17, 3.32 g, 20.0 mmol), urea 14•HCl (2.31 g, 24.0 mmol) and NaHCO₃ (6.33 g, 75.4 mmol) in DMF (40 mL) provided the crude product which was recrystallized from EtOAc:hexanes (2:1, 150 mL), isolated by vacuum filtration, washed with hexanes (50 mL) and dried in vacuo to afford 19 as a white crystalline solid (3.11 g, 41%, ELSD purity 100.0%): Mp 261-263 °C (dec.); IR (ATR) 3403, 3340, 2974, 1655, 1627, 1565, 1372, 1232, 1135, 1100, 1027 cm^-1; ¹H NMR (300 MHz, DMSO-d₆,) δ 7.44 (s, 1H), 7.25-7.21 (m, 5H), 6.98 (s, 1H), 6.93-6.86 (m, 2H), 6.28 (br s, 2H), 5.23 (s, 1H), 3.72 (s, 6H), 3.67, (q, 2H, J=7.2 Hz), 0.75 (t, 3H, J=7.2 Hz); ¹³C NMR (75 MHz, DMSO-d₆,) δ 166.7, 161.6, 155.9, 149.0, 148.4, 143.2, 139.3, 128.5 (2 C), 127.4, 127.3 (2 C), 118.7, 112.2, 110.8, 98.0, 58.5, 56.0, 55.8, 52.9, 14.2; MS (EI⁺) m/z 381 (M⁺, 36), 352 (31), 308 (86), 244 (100), 198 (35), 77 (56); HRMS (EI⁺) m/z calcd for C₂₁H₂₃N₃O₄ 381.1688, found 381.1687.

4.16. Ethyl 2,4-dimethyl-4H-pyrimido[1,2-a]pyrimidine-3-carboxylate (22). General Procedure F

A suspension of 10a•HCl (0.100 g, 0.428 mmol) and 1,1,3,3-tetramethoxypropane (20, 0.09 mL, 0.5 mmol) in trifluoroethanol (21, 1.5 mL) was heated in a microwave reactor at 160 °C for 1 h. The resulting mixture was dissolved in EtOAc (50 mL), washed with saturated sodium carbonate (50 mL), and extracted with EtOAc (2 × 50 mL). The organic layers were combined, dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The resulting red/orange oil was purified via chromatography on neutral Al₂O₃ (0-2% MeOH/CH₂Cl₂) to afford 22 as a red/orange solid (0.076 g, 76%): Mp 151-152 °C; IR (ATR) 3076, 2970, 1657, 1622, 1585, 1491, 1437, 1396, 1273, 1245, 1206, 1109, 1070, 990 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 8.51 (dd, 1H, J=2.4, 1.8 Hz), 8.24 (dd, 1H, J=2.1, 6.3 Hz), 6.70 (dd, 1H, J=3.9, 6.3 Hz), 5.26 (q, 1H, J=6.6 Hz), 4.18-4.03 (m, 2H), 2.25 (s, 3H), 1.23 (t, 3H, J=7.2 Hz), 1.16 (d, 3H, J=6.6 Hz); ¹³C NMR (75 MHz, DMSO-d₆) δ 165.1, 163.4, 157.6, 151.5, 147.2, 110.3, 100.3, 59.5, 56.8, 23.4, 22.2, 14.8; HRMS (ESI⁺) m/z calcd for C₁₂H₁₆N₃O₂ 234.1243, found 234.1243.

4.17. Ethyl 2-methyl-4-phenethyl-4H-pyrimido[1,2-a]pyrimidine-3-carboxylate (23)

According to general procedure F, a mixture of 10b•HCl (0.138 g, 0.426 mmol) and 1,1,3,3-tetramethoxypropane (20, 0.09 mL, 0.5 mmol) in trifluoroethanol (21, 2.0 mL) was heated in a microwave reactor at 160 °C for 2 h. The crude red/orange oil was purified by chromatography on neutral Al₂O₃ (EtOAc to 5% MeOH/CH₂Cl₂) to afford 23 as a yellow/brown oil (0.098 g, 71%, ELSD purity 99.5%): IR (ATR) 2981, 2925, 1665, 1618, 1579, 1491, 1430, 1393, 1275, 1238, 1202, 1096, 1063, 774, 697 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 8.52 (dd, 1H, J=3.9, 2.1 Hz), 8.18 (dd, 1H, J=6.6, 2.1 Hz), 7.26-7.08 (m, 5H), 6.70 (dd, 1H, J=3.9, 6.3 Hz), 5.21 (t, 1H, J=5.1 Hz), 4.20-4.07 (m, 2H), 2.46-2.37 (m, 2H), 2.28 (s, 3H), 1.86-1.76 (m, 2H), 1.19 (t, 3H, J=7.2 Hz); ¹³C NMR (75 MHz, DMSO-d₆) δ 164.7, 162.6, 158.0, 151.3, 147.0, 140.5, 128.4 (2 C), 128.0 (2 C), 125.9, 109.2, 99.0, 59.8, 59.1, 36.3, 30.3, 23.1, 14.4; HRMS (TOF ESI⁺) m/z calcd for C₁₉H₂₂N₃O₂ 324.1712, found 324.1707.

4.18. Ethyl 4-isopropyl-2-methyl-4H-pyrimido[1,2-a]pyrimidine-3-carboxylate (24)

According to general procedure F, a mixture of 10c•HCl (0.113 g, 0.432 mmol) and 1,1,3,3-tetramethoxypropane (20, 0.076 mL, 0.45 mmol) in trifluoroethanol (21, 1.5 mL) was heated in a microwave reactor at 160 °C for 2 h. The crude red/orange oil was purified by chromatography on neutral Al₂O₃ (0-2% MeOH/CH₂Cl₂) to afford 24 as a dark red oil (0.080 g, 71%, ELSD purity 98.4%): IR (ATR) 3064, 2964, 1670, 1620, 1579, 1491, 1432, 1378, 1294, 1234, 1204, 1094, 1063, 999, 774 cm^-1; ¹H NMR (400 MHz, DMSO-d₆) δ 8.52 (dd, 1H, J=4.0, 2.4 Hz), 8.21 (dd, 1H, J=6.4, 2.0 Hz), 6.70 (dd, 1H, J=6.4, 4.0 Hz), 4.94 (d, 1H, J=6.4 Hz), 4.18-4.04 (m, 2H), 2.28 (s, 3H), 1.79-1.74 (m, 1H), 1.22 (t, 3H, J=7.2 Hz), 0.77 (d, 3H, J=6.8 Hz), 0.63 (d, 3H, J=6.8 Hz); ¹³C NMR (100 MHz, DMSO-d₆) δ 165.4, 162.3, 157.6, 151.5, 147.4, 108.7, 98.3, 64.6, 59.0, 34.1, 23.1, 18.0, 17.4, 14.4; HRMS (TOF ESI⁺) m/z calcd for C₁₄H₂₀N₃O₂ 262.1556, found 262.1558.

4.19. Ethyl 2,4-diphenyl-4H-pyrimido[1,2-a]pyrimidine-3-carboxylate (25)

According to general procedure F, a solution of 18•HCl (0.100 g, 0.311 mmol) and 1,1,3,3-tetramethoxypropane (20, 0.055 mL, 0.33 mmol) in trifluoroethanol (21, 1.5 mL) was heated in a microwave reactor at 150 °C for 1.5 h. The crude red/orange oil was purified by chromatography on SiO₂ (EtOAc) to afford 25 as a red/orange solid containing ~5-6% EtOAc (0.047 g, 42%, ELSD purity 100.0%): Mp 185-186.5 °C; IR (ATR) 3070, 2982, 2869, 1694, 1612, 1689, 1558, 1493, 1476, 1441, 1277, 1232, 1189, 1074 cm^-1; ¹H NMR (400 MHz, DMSO-d₆) δ 8.55-8.54 (m, 2H), 7.42-7.30 (m, 10H), 6.78 (dd, 1H, J=4.8. 5.6 Hz), 6.45 (s, 1H), 3.80 (q, 2H, J=7.2 Hz), 0.80 (t, 3H, J=7.2 Hz); ¹³C NMR (100 MHz, DMSO-d₆) δ 165.6, 163.5, 158.5, 151.7, 147.5, 141.5, 141.2, 129.5 (2 C), 129.1 (2 C), 128.5, 127.7 (2 C), 126.7 (2 C), 110.7, 98.7, 63.1, 59.6, 14.0; HRMS (TOF ESI⁺) m/z calcd for C₂₂H₂₀N₃O₂ 358.1556, found 358.1556.

4.20. Ethyl 8-methyl-2,4-diphenyl-4H-pyrimido[1,2-a]pyrimidine-3-carboxylate (27)

A suspension of 18 (0.161 g, 0.500 mmol) and 26 (0.051 mL, 0.500 mmol) in distilled THF (1.0 mL) was heated in a microwave reactor for 15 min at 150 °C. The crude reaction mixture was recrystallized from hexanes:EtOAc (4:1, 20 mL), isolated by vacuum filtration, washed with hexanes, and dried in vacuo to afford 27 as a yellow-orange solid (0.12 g, 64%, ELSD purity 99.5%): Mp 185-187 °C; IR (ATR) 3068, 2969, 1705, 1625, 1511, 1448, 1241, 1196, 1085 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 8.38 (d, 1H, J=6.9 Hz), 7.42-7.32 (m, 10H), 6.72 (d, 1H, J=6.6 Hz), 6.38 (s, 1H), 3.79 (q, 2H, J=7.2 Hz), 2.32 (s, 3H), 0.79 (t, 3H, J=7.2 Hz); ¹³C NMR (75 MHz, DMSO-d₆) δ 173.3, 165.7, 159.1, 151.4, 146.2, 141.8, 141.4, 129.4 (2 C), 129.1, 129.0 (2 C), 128.5, 127.6 (2 C), 126.7 (2 C), 111.2, 98.1, 62.8, 59.5, 25.1, 14.0; MS (EI⁺) m/z 371 (M⁺, 43), 342 (51), 298 (100), 278 (63), 93 (69), 66 (52); HRMS (EI⁺) m/z calcd for C₂₃H₂₁N₃O₂ 371.1634, found 371.1618.

4.21. Ethyl 8-(1H-indol-2-yl)-2,4-diphenyl-4H-pyrimido[1,2-a]pyrimidine-3-carboxylate (30). General Procedure G

A suspension of 18•HCl (0.150 g, 0.419 mmol), 28 (0.111 g, 0.461 mmol), sodium carbonate (0.067 g, 0.63 mmol), and DMAP (0.013 g, 0.10 mmol) in MeOH (0.34 mL, 8.4 mmol) and acetonitrile (2.5 mL) was heated in a microwave reactor at 170 °C for 30 min. The dark red/orange reaction mixture was concentrated under reduced pressure and purified by chromatography on SiO₂ (1:1, EtOAc:hexanes). The resulting product was recrystallized from boiling hexanes:EtOAc (6:1), isolated by vacuum filtration, washed with hexanes, and dried under high vacuum to afford 30 as a red solid (0.050 g, 25%, ELSD purity 99.6%): Mp 170-172 °C; IR (ATR) 3202, 3057, 2975, 1666, 1610, 1472, 1435, 1277, 1251, 1221, 1195, 1092, 753 cm^-1; ¹H-NMR (400 MHz, DMSO-d₆) δ 12.01 (s, 1H), 8.49 (d, 1H, J=6.8 Hz), 7.63 (d, 1H, J=8.0 Hz), 7.50-7.32 (m, 13H), 7.22 (t, 1H, J=7.2 Hz), 7.05 (t, 1H, J=7.2 Hz), 6.42 (s, 1H), 3.83 (q, 2H, J=7.2 Hz), 0.83 (t, 3H, J=7.2 Hz); ¹³C NMR (100 MHz, DMSO-d₆) δ 165.3, 160.2, 158.7, 151.3, 146.2, 141.5, 140.8, 138.6, 133.7, 129.0 (2 C), 128.8 (2 C), 128.5, 128.1, 127.7, 127.1 (2 C), 126.2 (2 C), 124.9, 121.7, 120.2, 112.7, 108.1, 106.6, 97.8, 62.2, 59.0, 13.6; HRMS (TOF ES⁺) m/z calcd for C₃₀H₂₅N₄O₂ 473.1978, found 473.1993.

4.22. Ethyl 8-(benzofuran-2-yl)-2,4-diphenyl-4H-pyrimido[1,2-a]pyrimidine-3-carboxylate (31)

According to general procedure G, a suspension of 18•HCl (0.150 g, 0.419 mmol), 29 (0.112 g, 0.461 mmol), sodium carbonate (0.067 g, 0.63 mmol), and DMAP (0.013 g, 0.10 mmol) in MeOH (0.34 mL, 8.4 mmol) and acetonitrile (2.5 mL) provided a crude product which was purified by chromatography on SiO₂ (1:1, EtOAc:hexanes). The resulting product was recrystallized (1:15, EtOAc:hexanes, 16 mL), isolated by vacuum filtration, washed with hexanes (5 mL) and dried in vacuo to afford 31 as a red/orange solid containing ~5% EtOAc (0.044 g, 22%, ELSD purity 99.1%): Mp 195-196 °C; IR (ATR) 3064, 2975, 1614, 1586, 1504, 1474, 1446, 1430, 1374, 1305, 1281, 1247, 1224, 1206, 1165, 1146, 1092, 761 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 8.68 (d, 1H, J=6.6 Hz), 7.98 (s, 1H), 7.82 (d, 1H, J=7.8 Hz), 7.71 (d, 1H, J=8.4 Hz), 7.53-7.33 (m, 13H), 6.51 (s, 1H), 3.83 (q, 2H, J=6.9 Hz), 0.82 (t, 3H, J=6.9 Hz); ¹³C NMR (125 MHz, DMSO-d₆) δ 165.5, 159.0, 157.6, 156.1, 151.7, 151.2, 148.3, 141.3, 140.3, 129.6 (2 C), 129.2 (3 C), 128.8, 128.4, 128.1, 127.8 (2 C), 126.9 (2 C), 124.6, 123.6, 113.0, 112.4, 107.2, 99.3, 63.0, 59.8, 14.0; HRMS (TOF ES⁺) m/z calcd for C₃₀H₂₄N₃O₃ 474.1818, found 474.1802.

4.23. 1-Phenyl-3-(trimethylsilyl)prop-2-yn-1-one (35).^27,35

To a suspension of bis(triphenylphosphine)palladium(II) chloride (0.035 g, 0.050 mmol) and copper iodide (0.019 g, 0.10 mmol) in freshly distilled THF (10 mL) were added sequentially Et₃N (0.35 mL, 2.5 mmol), benzoyl chloride (0.29 mL, 2.5 mmol) and TMS-acetylene (0.35 mL, 2.5 mmol). The reaction mixture turned from dark orange to pale yellow forming the Et₃N•HCl precipitate over a 1 h period. TLC analysis (10:1, hexanes:EtOAc, product R_f = 0.59) showed full conversion after 1 h. The Et₃N•HCl salt was filtered off and the filtrate was concentrated under reduced pressure. The crude product was purified by chromatography on SiO₂ (10:1:0.1, hexanes:EtOAc:Et₃N) to afford 35 as a pale orange oil (0.48 g, 95%): R_f 0.59 (10:1, hexanes:EtOAc); ¹H-NMR (300 MHz, CDCl₃) δ 8.18-8.14 (m, 2H), 7.63-7.60 (m, 1H), 7.53-7.48 (m, 2H), 0.33 (s, 9H); MS (EI⁺) m/z 202 (M⁺, 43), 187 (100), 159 (38), 105 (24), 76 (31); HRMS (EI) m/z calcd for C₁₂H₁₄OSi 202.0814, found 202.0814. The spectral data are consistent with the literature values.^27,35

4.24. Ethyl 2,4,8-triphenyl-4H-pyrimido[1,2-a]pyrimidine-3-carboxylate (38a). General Procedure H

To a suspension of bis(triphenylphosphine)palladium(II) chloride (0.007 g, 0.01 mmol) and copper iodide (0.004 g, 0.02 mmol) in freshly distilled THF (0.75 mL) was added sequentially N,N-dicyclohexylmethyl amine (0.11 mL, 0.50 mmol), benzoyl chloride (0.058 mL, 0.50 mmol) and TMS-acetylene (0.071 mL, 0.50 mmol). The reaction mixture turned from dark orange to pale yellow forming the Cy₂MeN•HCl precipitate over a 1.5 h period. TLC analysis (10:1 hexanes:EtOAc; R_f = 0.62) showed a quantitative formation of 35 after 1.5 h. The mixture was diluted to 0.15 M with THF (2.3 mL), and 18 (0.13 g, 0.40 mmol), DMAP (0.012 g, 0.10 mmol), and MeOH (0.40 mL) were added. The solution was heated in a microwave reactor at 170 °C for 30 min. The dark red-black reaction mixture was diluted with CH₂Cl₂ (30 mL) and washed with saturated NaHCO₃ (1 × 30 mL). The organic layer was dried (Na₂SO₄) and concentrated under reduced pressure. The resulting dark red oil was purified by chromatography on SiO₂ (1:1, hexanes:EtOAc) to afford 38a as a red solid (0.11 g, 60%, ELSD purity 100%): Mp 210-211 °C; IR (ATR) 3068, 2995, 1705, 1616, 1502, 1446, 1210, 1075 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 8.63 (d, 1H, J=6.9 Hz), 8.16 (d, 2H, J=6.9 Hz), 7.60-7.32 (m, 14H), 6.49 (s, 1H), 3.81 (q, 2H, J=6.9 Hz), 0.80 (t, 3H, J=6.9 Hz); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.9, 165.2, 158.5, 151.1, 147.4, 141.3, 140.9, 134.9, 132.4, 129.0 (4 C), 128.6 (3 C), 128.0, 127.8 (2 C), 127.2 (2 C), 126.3 (2 C), 106.7, 98.2, 62.3, 59.1, 13.6; HRMS (TOF MS ES) m/z calcd for C₂₈H₂₄N₃O₂ (M+H) 434.1869, found 434.1882.

4.25. Synthesis of 38a under copper- and solvent-free conditions

A mixture of palladium(II) acetate (0.001 g, 0.005 mmol), benzoyl chloride (0.058 mL, 0.50 mmol), TMS-acetylene (0.078 mL, 0.55 mmol) and N,N-dicyclohexylmethylamine (0.11 mL, 0.50 mmol) was heated in a sealed tube at 40 °C for 2 h. The reaction mixture turned heterogeneous and dark brown. After 2 h, TLC analysis (10:1, hexanes:EtOAc; R_f = 0.63) showed a nearly quantitative formation of 35. The reaction mixture was diluted to a concentration of 0.15 M with THF (3.0 mL) and 18 (0.129 g, 0.400 mmol), DMAP (0.012 g, 0.10 mmol), and MeOH (0.40 mL) were added. The solution was heated in a microwave reactor at 170 °C for 30 min. The resulting dark red reaction mixture was diluted with CH₂Cl₂ (30 mL) and washed with saturated NaHCO₃ solution (1 × 30 mL). The organic layer was dried (Na₂SO₄), concentrated under reduced pressure, and the crude residue was purified by chromatography on SiO₂ (2:1 to 1:1, hexanes:EtOAc). The resulting red solid was dissolved in EtOAc (1.0 mL) and crystallized by slowly adding hexanes (20 mL) at 0 °C. The solid was isolated by vacuum filtration, washed with hexanes, and dried in vacuo to afford 38a (0.053 g, 30%). The spectral data are consistent with the values obtained in 4.24.

4.26. Ethyl 4-(3,4-dimethoxyphenyl)-2,8-diphenyl-4H-pyrimido[1,2-a]pyrimidine-3-carboxylate (38b)

According to general procedure H, a solution of bis(triphenylphosphine)palladium(II) chloride (0.007 g, 0.01 mmol), copper iodide (0.004 g, 0.02 mmol), benzoyl chloride (0.058 mL, 0.50 mmol), TMS-acetylene (0.071 mL, 0.50 mmol) and N,N-dicyclohexylmethylamine (0.11 mL, 0.50 mmol) in freshly distilled THF (0.75 mL) provided 35. After addition of THF (2.3 mL), treatment with 19 (0.15 g, 0.40 mmol), DMAP (0.012 g, 0.10 mmol) and MeOH (0.40 mL) provided the crude product which was purified by chromatography on SiO₂ (1:1 to 2:1, EtOAc:hexanes). The resulting solid was dissolved in EtOAc (1.0 mL) and crystallized by slowly adding hexanes (20 mL) at 0 °C. The precipitated solid was isolated by vacuum filtration, washed with hexanes, and dried in vacuo to afford 38b as an orange solid (0.10 g, 51%, ELSD purity 99.9%): R_f 0.21 (2:1, EtOAc:hexanes); Mp 95-96 °C; IR (ATR) 3086, 2987, 2932, 1662, 1618, 1517, 1435, 1215, 1075, 1027 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 8.66 (d, 1H, J=6.9 Hz), 8.14 (d, 2H, J=7.2 Hz), 7.60-7.51 (m, 3H), 7.46-7.36 (m, 6H), 7.14 (s, 1H), 7.00-6.94 (m, 2H), 6.39 (s, 1H), 3.79 (q, 2H, J=7.2 Hz), 3.73 (s, 3H), 3.71 (s, 3H), 0.81 (t, 3H, J=7.2 Hz); ¹³C NMR (75 MHz, DMSO-d₆) δ 166.8, 165.2, 158.3, 150.9, 149.0, 148.8, 147.3, 141.1, 134.9, 133.7, 132.3, 129.0 (2 C), 128.5 (2 C), 127.9, 127.8 (2 C), 127.2 (2 C), 118.9, 112.0, 110.2, 106.6, 98.3, 62.3, 59.0, 55.5 (2 C), 13.6; MS (EI⁺) m/z 493 (M⁺, 29), 420 (100), 338 (34), 155 (21); HRMS (EI⁺) m/z calcd for C₃₀H₂₇N₃O₄ 493.2001, found 493.1977.

4.27. Ethyl 8-(4-methoxyphenyl)-2,4-diphenyl-4H-pyrimido[1,2-a]pyrimidine-3-carboxylate (38c)

According to general procedure H, a solution of bis(triphenylphosphine)palladium(II) chloride (0.007 g, 0.009 mmol), copper iodide (0.004 g, 0.02 mmol), p-anisoyl chloride (0.099 g, 0.58 mmol), TMS-acetylene (0.083 mL, 0.58 mmol) and N,N-dicyclohexylmethyl amine (0.12 mL, 0.58 mmol) in freshly distilled THF (0.75 mL) led to 36 (R_f = 0.17; 20:1, hexanes:EtOAc) which was dissolved in THF (2.3 mL) and treated with 18 (0.150 g, 0.467 mmol), DMAP (0.012 g, 0.093 mmol) and MeOH (0.4 mL) to provide a crude product which was purified by chromatography on SiO₂ (1:1, hexanes:EtOAc). The resulting red/orange solid was dissolved in CH₂Cl₂ (1.0 mL) and crystallized by slowly adding hexanes (5.0 mL) and cooling to -20 °C. The crystalline solid was isolated by vacuum filtration, washed with hexanes, and dried in vacuo to afford 38c as a red/orange solid (0.060 g, 28%, ELSD purity 99.8%): R_f 0.31(1:1, hexanes:EtOAc); Mp 210-212 °C; IR (ATR) 2969, 1700, 1594, 1493, 1439, 1297, 1241, 1212, 1169, 1075, 1023, 697 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 8.54 (d, 1H, J=6.9 Hz), 8.15 (d, 2H, J=9.0 Hz), 7.48-7.29 (m, 11H), 7.08 (d, 2H, J=8.7 Hz), 6.44 (s, 1H), 3.84 (s, 3H), 3.77 (q, 2H, J=7.2 Hz), 0.80 (t, 3H, J=7.2 Hz); ¹³C NMR (125 MHz, DMSO-d₆) δ 166.2, 165.2, 162.8, 158.8, 151.2, 146.8, 141.4, 141.0, 129.8 (2 C), 129.0 (2 C), 128.6 (3 C), 127.9, 127.1 (3 C), 126.3 (2 C), 114.4 (2 C), 106.2, 97.9, 62.2, 59.0, 55.5, 13.5; HRMS (TOF ES⁺) m/z calcd for C₂₉H₂₆N₃O₃ 464.1974, found 464.1975.

4.28. Ethyl 2,4-diphenyl-8-(4-(trifluoromethyl)phenyl)-4H-pyrimido[1,2-a]pyrimidine-3-carboxylate (38d)

According to general procedure H, a solution of bis(triphenylphosphine)palladium(II) chloride (0.007 g, 0.01 mmol), copper iodide (0.004 g, 0.02 mmol), 4-(trifluoromethyl)benzoyl chloride (0.074 mL, 0.50 mmol), TMS-acetylene (0.071 mL, 0.50 mmol) and Et₃N (0.071 mL, 0.50 mmol) in freshly distilled THF (3.0 mL) led to 37 (R_f = 0.64; 10:1, hexanes:EtOAc) which was treated with 18 (0.129 g, 0.400 mmol), DMAP (0.012 g, 0.10 mmol) and MeOH (0.40 mL) to give a crude product which was purified by chromatography on SiO₂ (2:1 to 1:1, EtOAc:hexanes) to afford 38d as a red crystalline solid (0.11 g, 55%, ELSD purity 100.0%): R_f 0.31 (1:1, EtOAc:hexanes); Mp 206-208 °C; IR (ATR) 3068, 2974, 1705, 1618, 1506, 1456, 1323, 1114, 1066 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 8.70 (d, 1H, J=6.9 Hz), 8.35 (d, 2H, J=8.1 Hz), 7.91 (d, 2H, J=8.4 Hz), 7.54-7.30 (m, 11H), 6.52 (s, 1H), 3.81 (q, 2H, J=6.9 Hz), 0.80 (t, 3H, J=7.2 Hz); ¹³C NMR (150 MHz, DMSO-d₆) δ 165.6, 165.2, 158.2, 150.9, 148.0, 141.1, 140.7, 138.7, 131.7 (q, J_CF = 33.0 Hz), 129.1 (2 C), 128.7, 128.6 (4 C), 128.1, 127.2 (2 C), 126.4 (2 C), 125.8 (q, 2 C, J_CF = 4.5 Hz), 123.9 (q, J_CF = 271.5), 107.0, 98.5, 62.3, 59.1, 13.5; MS (EI) m/z 501 (M⁺, 11), 428 (100), 278 (14), 223 (17); HRMS (EI) m/z calcd for C₂₉H₂₂N₃O₂F₃ 501.1664, found 501.1658. X-ray crystallographic data for 38d were deposited with the Cambridge Crystallographic Data Center (deposition number CCDC 932463).

4.29. Ethyl 4-amino-6,8-dimethyl-2-morpholino-6H-pyrimido[1,2-a][1,3,5]triazine-7-carboxylate (40). General Procedure I

To a solution of 3,3-diphenoxy-2-azaprop-2-enenitrile (0.133 g, 0.558 mmol) in THF (1.0 mL) was added morpholine (0.049 mL, 0.56 mmol) in one portion at room temperature. The homogeneous colorless reaction mixture was stirred at room temperature for 2 h to form 39 (R_f = 0.33; 1:1, hexanes:EtOAc). To the reaction mixture was added 10a (0.100 g, 0.507 mmol) and the resulting suspension was heated in a microwave reactor at 110 °C for 45 min. The mixture was purified by chromatography on SiO₂ (EtOH) to afford 40 as a white foam (0.090 g, 53%, ELSD purity 99.8%): R_f 0.17 (EtOH); IR (ATR) 3327, 2969, 2906, 2855, 1651, 1584, 1551, 1482, 1420, 1264, 1232, 1107, 1059, 1023, 885, 775 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 7.89 (br s, 2H), 5.15 (q, 1H, J=6.0 Hz), 4.14-4.01 (m, 2H), 3.65-3.58 (m, 8H), 2.16 (s, 3H), 1.22 (t, 3H, J=7.2 Hz), 1.08 (d, 3H, J=6.0 Hz); ¹³C NMR (75 MHz, DMSO-d₆) δ 165.2, 160.8, 158.4, 156.2, 152.1, 100.3, 65.9 (2 C), 58.8, 47.7, 43.5 (2 C), 23.2, 19.4, 14.4; HRMS (TOF ESI⁺) m/z calcd for C₁₅H₂₃N₆O₃ 335.1832, found 335.1825.

4.30. Ethyl 4-amino-8-methyl-2-morpholino-6-phenethyl-6H-pyrimido[1,2-a][1,3,5]triazine-7-carboxylate (41)

According to general procedure I, a solution of 3,3-diphenoxy-2-azaprop-2-enenitrile (0.091 g, 0.38 mmol) and morpholine (0.034 mL, 0.38 mmol) in THF (1.0 mL) led to 39 which was treated with 10b (0.100 g, 0.348 mmol). The reaction mixture was purified by chromatography on SiO₂ (EtOH) to afford 41 as a pale yellow foam (0.037 g, 25%): R_f 0.82 (EtOH); IR (ATR) 3308, 2913, 2850, 1638, 1581, 1551, 1482, 1424, 1266, 1232, 1096, 1059, 1023, 889, 781 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 7.90 (br s, 2H), 7.25-7.06 (m, 5H), 5.31 (t, 1H, J=5.4 Hz), 4.17-4.02 (m, 2H), 3.65-3.59 (m, 8H), 2.58-2.40 (m, 2H), 2.20 (s, 3H), 1.86-1.73 (m, 2H), 1.23 (t, 3H, J=7.2 Hz); ¹³C NMR (125 MHz, DMSO-d₆) δ 165.4, 160.6, 159.4, 156.3, 152.6, 141.3, 128.3 (2 C), 127.9 (2 C), 125.7, 98.6, 65.9 (2 C), 58.8, 51.3, 43.6 (2 C), 35.6, 29.6, 23.1, 14.4; HRMS (TOF ESI⁺) m/z calcd for C₂₂H₂₉N₆O₃ 425.2301, found 425.2301.

4.31. Ethyl 4-amino-6-isopropyl-8-methyl-2-morpholino-6H-pyrimido[1,2-a][1,3,5]triazine-7-carboxylate (42)

According to general procedure I, a solution of 3,3-diphenoxy-2-azaprop-2-enenitrile (0.116 g, 0.488 mmol) and morpholine (0.043 mL, 0.49 mmol) in THF (1.0 mL) led to 39 which was treated with 10c (0.100 g, 0.444 mmol). The reaction mixture was purified by chromatography on SiO₂ (EtOH) to afford 42 as a pale yellow foam (0.070 g, 43%, ELSD purity 99.7%): R_f 0.80 (EtOH); IR (ATR) 3341, 3152, 2964, 1655, 1577, 1547, 1480, 1424, 1275, 1227, 1111, 1079, 1022, 893, 787 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 7.86 (br s, 2H), 5.09 (d, 1H, J=5.4 Hz), 4.16-4.00 (m, 2H), 3.65-3.50 (m, 8H), 2.19 (s, 3H), 1.86-1.77 (m, 1H), 1.23 (t, 3H, J=6.9 Hz), 0.75 (d, 3H, J=6.9 Hz), 0.72 (d, 3H, J=6.9 Hz); ¹³C NMR (75 MHz, DMSO-d₆) δ 166.0, 160.5, 158.6, 156.7, 152.7, 98.0, 65.9 (2 C), 58.7, 55.4, 43.6 (2 C), 34.1, 23.0, 17.8, 17.6, 14.4; HRMS (TOF ESI⁺) m/z calcd for C₁₇H₂₇N₆O₃ 363.2145, found 363.2136.

4.32. Ethyl 4-amino-2-morpholino-6,8-diphenyl-6H-pyrimido[1,2-a][1,3,5]triazine-7-carboxylate (43)

According to general procedure I, a solution of 3,3-diphenoxy-2-azaprop-2-enenitrile (0.163 g, 0.685 mmol) and morpholine (0.060 mL, 0.68 mmol) in THF (1.0 mL) led to 39 which was treated with 18 (0.200 g, 0.622 mmol). The reaction mixture was cooled to room temperature, diluted with ether (1.5 mL) and cooled to -20 °C for 1 h. The resulting precipitate was isolated by vacuum filtration and washed with ether (15 mL). The crude precipitate was recrystallized from boiling hexanes:EtOAc (4:1, 35 mL), isolated by vacuum filtration, washed with hexanes, and dried in vacuo to afford 43 as a yellow/green foam (0.130 g, 46%, ELSD purity 99.9%): IR (ATR) 3334, 3165, 2975, 2855, 1646, 1549, 1476, 1418, 1271, 1230, 1105, 1087, 1021, 995, 764, 695 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 8.04 (s, 2H), 7.47-7.44 (m, 2H), 7.37-7.27 (m, 8H), 6.30 (s, 1H), 3.79-3.69 (m, 4H), 3.58 (s, 6H), 0.77 (t, 3H, J=7.2 Hz); ¹³C NMR (75 MHz, DMSO-d₆) δ 165.5, 160.6, 159.0, 156.3, 152.5, 141.7, 141.5, 128.6 (2 C), 128.2 (2 C), 128.1, 127.5, 127.0 (2 C), 126.5 (2 C), 100.4, 65.9 (2 C), 58.7, 54.6, 43.6, 43.4, 13.5; HRMS (TOF ESI⁺) m/z calcd for C₂₅H₂₇N₆O₃ 459.2145, found 459.2135.

4.33. Ethyl 6,8-dimethyl-4-oxo-3,4,6,9-tetrahydropyrimido[2,1-c][1,2,4,6]thiatriazine-7-carboxylate 2,2-dioxide (44). General Procedure J

To a suspension of 10a (0.100 g, 0.507 mmol) and DIPEA (0.13 mL, 0.76 mmol) in THF (2.5 mL) at 0 °C was added chlorosulfonyl isocyanate (0.068 mL, 0.76 mmol) in one portion. The reaction mixture became homogeneous within 5 min after the addition of the isocyanate. The solution was stirred for 3 h at 0 °C and warmed to room temperature for 14 h. ¹H NMR analysis indicated a 47% conversion, and therefore the mixture was treated at 0 °C with additional DIPEA (0.084 mL, 0.51 mmol) and chlorosulfonyl isocyanate (0.045 mL, 0.51 mmol). The reaction mixture was stirred at 0 °C for 2 h, allowed to warm to room temperature, and stirred for an additional 4 h. The resulting clear pale yellow solution was diluted with EtOAc (30 mL) and concentrated under reduced pressure after addition of SiO₂ (1.0 g). The resulting mixture was added directly to the top of a pre-flushed column and purified by chromatography on SiO₂ (1:1, hexanes:EtOAc to EtOAc). The resulting product was recrystallized from boiling CH₂Cl₂:hexanes (1:1.5, 2.5 mL), isolated by vacuum filtration, washed with hexanes and dried in vacuo to afford 44 as a white solid (0.024 g, 15%): Mp 219-220 °C; IR (ATR) 3308, 3239, 3184, 2988, 1707, 1609, 1471, 1329, 1294, 1253, 1189, 1161, 1083, 902, 753, 738, 662 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 10.89 (s, 1H), 5.45 (q, 1H, J=6.3 Hz), 4.25-4.08 (m, 2H), 2.27 (s, 3H), 1.24 (t, 3H, J=6.9 Hz), 1.17 (d, 3H, J=6.3 Hz); ¹³C NMR (75 MHz, DMSO-d₆) δ 168.0, 151.9, 151.1, 149.2, 108.5, 64.1, 49.9, 23.8, 21.0, 18.1; HRMS (TOF ESI⁺) m/z calcd for C₁₀H₁₅N₄O₅S 303.0763, found 303.0755. X-ray crystallographic data for 44 were deposited with the Cambridge Crystallographic Data Center (deposition number CCDC 935121).

4.34. Ethyl 4-oxo-6,8-diphenyl-3,4,6,9-tetrahydropyrimido[2,1-c][1,2,4,6]thiatriazine-7-carboxylate 2,2-dioxide (45)

According to general procedure J, a solution of 18 (0.250 g, 0.778 mmol), DIPEA (0.19 mL, 1.2 mmol), and chlorosulfonyl isocyanate (0.10 mL, 1.2 mmol) in THF (2.5 mL) led to crude product which was diluted with EtOAc (30 mL), concentrated under reduced pressure with SiO₂ (1.0 g), and purified by chromatography on SiO₂ (1:1, hexanes:EtOAc to EtOAc) to afford 45 as a white foam (0.132 g, 40%, ELSD purity 99.7%): IR (ATR) 3202, 3057, 2982, 1700, 1594, 1444, 1331, 1249, 1163, 1100, 979, 749, 693 cm^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 11.18 (s, 1H), 7.47-7.37 (m, 11H), 6.66 (s, 1H), 3.84 (q, 2H, J=6.9 Hz), 0.78 (t, 3H, J=6.9 Hz); ¹³C NMR (75 MHz, DMSO-d₆) δ 164.1, 148.5, 147.5, 146.4, 139.1, 132.9, 129.8, 129.1 (2 C), 129.0 (2 C), 128.7, 128.0 (2 C), 126.2 (2 C), 104.5, 60.2, 51.8, 13.4; HRMS (TOF ESI⁺) m/z calcd for C₂₀H₁₉N₄O₅S 427.1076, found 427.1077.

4.35. Diethyl 4,6-bis(4-chlorophenyl)-2,8-dimethyl-4,6-dihydro-1H-pyrimido[1,2-a]pyrimidine-3,7-dicarboxylate (46)

According to general procedure D, a solution of 13•TFA (0.030 g, 0.074 mmol), 12 (0.074 g, 0.29 mmol) and NaHCO₃ (0.014 g, 0.016 mmol) in (CF₃)₂CHOH (0.25 mL) was heated for 24 h. LC/MS analysis demonstrated an approximately 50% conversion of 13, and 2 more equivalents of 12 (0.037 g, 0.15 mmol) were added. The reaction mixture was stirred at 90 °C for an additional 18 h after which complete consumption of the starting material was observed via LC/MS. Product 46 (0.022 g, 57%) was obtained as a yellow crystalline 4:1 mixture of anti-syn-isomers that could not be further separated: R_f 0.42 (MeOH:CH₂Cl₂, 0.2:10); Mp 70-75 °C; IR (CH₂Cl₂) 2979, 2922, 1677, 1622, 1544, 1490, 1446, 1369, 1274, 1228, 1199, 1088, 1014, 830, 736 cm^-1; ¹H NMR (400 MHz, CDCl₃) δ 12.05 (s, 1H), 11.95 (s, 0.2H), 7.43-7.41 (d, 2H, J=8.4 Hz), 7.31-7.29 (d, 2 × 2H, J=8.4 Hz), 6.97-6.94 (d, 2 × 0.5H, J=8.8 Hz), 5.38 (s, 2 × 0.25H), 5.18 (s, 2 × 1H), 4.10-4.01 (m, 2 × 4H), 2.49 (s, 2 × 3H), 1.17-1.13 (t, 2 × 3H, J=7.2 Hz); ¹³C NMR (100 MHz, CDCl₃) δ 164.3, 164.0 (2C), 148.2, 145.6, 144.7, 138.6, 137.7, 135.3, 134.4, 129.5 (2C), 129.1, 128.9 (2C), 128.3, 105.1, 103.4, 64.8, 62.3, 60.8, 56.6, 19.3, 14.0; HRMS (ESI⁺) m/z calcd for C₂₇H₂₈O₄N₃Cl₂ (M+H) 528.1451, found 528.1441.

4.36. Ethyl 2-(4-morpholinobenzylidene)-3-oxobutanoate (48d).³⁶

A solution of L-proline (1.21 g, 10.5 mmol) in DMSO (10 mL) was treated with 4-morpholinebenzaldehyde (1.12 g, 5.86 mmol) at room temperature. After 10 min, ethyl acetoacetate (0.80 mL, 6.3 mmol) was added and the mixture was stirred for 17 h. The yellow solution was treated with EtOAc, followed by water, and the organic layer was washed with brine, dried (Na₂SO₄), filtered, and concentrated. The resulting oil was purified by chromatography on SiO₂ (ISCO-R_f, 20-100% EtOAc-hexanes, 15 min gradient) to give 48d as a sticky yellow 4.4:1 mixture of inseparable Z/E isomers (0.713 g, 40%): Mp 94-96 °C; IR (neat) 2974, 1713, 1588, 1514, 1260, 1163, 982 cm^-1; ¹H NMR (400 MHz, CDCl₃) δ 7.53 (s, 0.2 H), 7.43 (s, 1H), 7.35 (d, 2H, J=7.8Hz), 7.28 (d, 0.4H, J=8.8 Hz), 6.82-6.78 (overlap d, 2.4H), 4.33 (q, 2H, J=7.2 Hz), 4.23 (q, 0.4H, J=7.2 Hz), 3.80-3.78 (m, 5H), 3.24-3.21 (m, 5H), 2.37 (s, 0.6H), 2.34 (s, 3H), 1.31-1.28 (2 overlapping t, 3.7H); ¹³C NMR (100 MHz, CDCl₃) δ 204.0, 194.5, 168.4, 164.6, 152.4, 152.1, 141.1, 140.3, 131.5, 131.4, 130.6, 129.7, 122.9, 122.6, 114.0, 113.9, 66.2, 61.2, 60.9, 47.3, 47.2, 31.0, 26.0, 14.0, 13.8; HRMS (ESI⁺) m/z calcd for C₁₇H₂₂NO₄ (M+H) 304.1543, found 304.1535.

4.37. Ethyl 2-(6-chloro-4-methylquinazolin-2-ylamino)-4-(4-chlorophenyl)-6-methyl-1,4-dihydropyrimidine-5-carboxylate (49a)

According to general procedure D, a solution of 47a (0.029 g, 0.12 mmol), 12^36,37 (0.23 g, 0.90 mmol) and NaHCO₃ (0.026 g, 0.31 mmol) in (CF₃)₂CHOH (0.40 mL) afforded 49a as a light yellow solid (0.053 g, 56%, ELSD purity 95.4%): Mp >200 °C; IR (neat) 3059, 2975, 1650, 1575, 1461, 1420, 1305, 1215, 1094, 838 cm ^-1; ¹H NMR (400 MHz, DMSO-d₆) δ 10.87 (br s, 1H), 10.43 (br s, 1H), 8.11 (d, 1H, J=1.6 Hz), 7.85-7.79 (m, 2H), 7.39 (d, 2H, J=8.8 Hz), 7.36 (d, 2H, J=8.8 Hz), 5.70 (s, 1H), 4.07 (q, 2H, J=7.2 Hz), 2.77 (s, 3H), 2.39 (s, 3H), 1.17 (t, 3H, J=7.2 Hz); ¹³C NMR (100 MHz, DMSO-d₆) δ 169.3, 165.6, 161.8, 151.8, 149.0, 148.8, 144.0, 134.3, 132.5, 128.9, 128.7, 128.5, 124.9, 120.8, 100.4, 59.8, 52.7, 21.9, 18.9, 14.6; HRMS (ESI+) m/z calcd for C₂₃H₂₂N₅O₂Cl₂ (M+H) 470.1145, found 470.1146.

4.38. Ethyl 4-(4-chlorophenyl)-2-(4,6-dimethylquinazolin-2-ylamino)-6-methyl-1,4-dihydropyrimidine-5-carboxylate (49b)

According to general procedure D, a solution of 47b (0.055 g, 0.26 mmol), 12^36,37 (0.28 g, 1.09 mmol) and NaHCO₃ (0.049 g, 0.58 mmol) in (CF₃)₂CHOH (0.40 mL) afforded 49b as a light yellow solid (0.073 g, 62%, ELSD purity 98.4%): Mp >200 °C; IR (neat) 3058, 2975, 1655, 1535, 1450, 1420, 1316, 1215, 1077, 839 cm ^-1; ¹H NMR (400 MHz, DMSO-d₆) δ 10.72 (br s, 2H), 7.83 (s, 1H), 7.70 (d, 1H, J=8.4 Hz), 7.63 (d, 1H, J=8.4 Hz), 7.38 (app s, 4H), 5.71 (s, 1H), 4.06 (q, 2H, J=6.4 Hz), 2.75 (s, 3H), 2.45 (s, 3H), 2.39 (s, 3H), 1.16 (t, 3H, J=6.8 Hz); ¹³C NMR (100 MHz, DMSO-d₆, 100 °C) δ 168.1, 164.8, 159.3, 150.4, 149.1, 147.4, 143.3, 135.0, 133.2, 131.4, 127.8, 127.5, 125.4, 123.6, 119.3, 99.3, 58.6, 51.9, 20.7, 20.3, 18.4, 13.5; HRMS (ESI+) m/z calcd for C₂₄H₂₅N₅O₂Cl (M+H) 450.1691, found 450.1680.

4.39. Ethyl 4-(4-chlorophenyl)-2-(6-fluoro-4-methylquinazolin-2-ylamino)-6-methyl-1,4-dihydropyrimidine-5-carboxylate (49c)

According to general procedure D, a solution of 47c (0.048 g, 0.22 mmol), 12 (0.22 g, 0.86 mmol) and NaHCO₃ (0.043 g, 0.51 mmol) in (CF₃)₂CHOH (0.40 mL) afforded 49c as a yellow solid (0.051 g, 52%, ELSD purity 90%): Mp >200 °C; IR (neat) 3059, 2975, 1663, 1536, 1450, 1420, 1310, 1215, 1077, 826 cm ^-1; ¹H NMR (400 MHz, DMSO-d₆) δ 10.71 (br s, 2H), 7.93-7.83 (m, 2H), 7.72 (ddd, 1H, J=2.4, 8.8, 8.8 Hz), 7.39-7.35 (m, 4H), 5.70 (s, 1H), 4.07 (q, 2H, J=7.2 Hz), 2.75 (s, 3H), 2.39 (s, 3H), 1.16 (t, 3H, J=7.2 Hz); ¹³C NMR (100 MHz, DMSO-d₆, 100 °C) δ 168.5, 164.7, 159.9, 157.6 (d, J_CF = 243.5 Hz), 150.5, 148.2, 146.3, 143.0, 131.5, 128.2 (d, J_CF = 8.1 Hz), 127.9, 127.7, 127.5, 122.7 (d, J_CF = 25.2 Hz), 119.3 (d, J_CF = 8.1 Hz), 108.6 (d, J_CF = 22.1 Hz), 99.5, 58.7, 51.9, 20.9, 18.1, 13.5; HRMS (ESI+) m/z calcd for C₂₃H₂₂N₅O₂ClF (M+H) 454.1441, found 454.1435.

4.40. Ethyl 4-(4-dimethylaminophenyl)-2-(6-fluoro-4-methylquinazolin-2-ylamino)-6-methyl-1,4-dihydropyrimidine-5-carboxylate (49d)

According to general procedure D, a solution of 47c (0.032 g, 0.15 mmol), 48a^36,37 (0.15 g, 0.59 mmol) and NaHCO₃ (0.027 g, 0.32 mmol) in (CF₃)₂CHOH (0.60 mL) gave 49d as an orange solid (0.027 g, 40%, ELSD purity 99.4%): Mp >220 °C; IR (neat) 3059, 2974, 1655, 1543, 1420, 1307, 1215, 1077, 838 cm^-1; ¹H NMR (400 MHz, DMSO-d₆) δ 10.57 (br s, 2H), 7.84-7.81 (m, 2H), 7.71-7.67 (m, 1H), 7.15 (d, 2H, J=8.4 Hz), 6.63 (d, 2H, J=8.4 Hz), 5.57 (s, 1H), 4.06 (q, 2H, J=6.8 Hz), 2.82 (s, 6H), 2.74 (s, 3H), 2.38 (s, 3H); 1.19 (t, 3H, J= 6.8 Hz); ¹³C NMR (100 MHz, DMSO-d₆) δ 168.9 (d, J_CF = 4.0 Hz), 165.3, 161.3, 157.8 (d, J_CF = 242.5 Hz), 151.6, 149.7, 147.4, 146.5, 132.2, 128.6 (d, J_CF = 8.1 Hz), 126.7, 123.1 (d, J_CF = 25.2 Hz), 119.5 (d, J_CF = 9.1 Hz), 112.3, 109.3 (d, J_CF = 22.1 Hz), 100.2, 59.2, 51.8, 40.1, 21.8, 18.2, 14.2; HRMS (ESI+) m/z calcd for C₂₅H₂₈N₆O₂F (M+H) 463.2252, found 463.2248.

4.41. Ethyl 2-(4.6-dimethylquinazolin-2-ylamino)-6-methyl-4-(3-nitrophenyl)-1,4-dihydropyrimidine-5-carboxylate (49e)

According to general procedure D, a solution of 47b (0.043 g, 0.20 mmol), 48b^36,38 (0.16 g, 0.60 mmol) and NaHCO₃ (0.057 g, 0.68 mmol) in (CF₃)₂CHOH (0.60 mL) gave 49e as a light yellow solid (0.062 g, 67%, ELSD purity 90.4%): Mp >200 °C; IR (neat) 3059, 2974, 1668, 1525, 1450, 1420, 1340, 1215, 1077, 734, 692 cm^-1; ¹H NMR (400 MHz, DMSO-d₆) δ 10.94 (br s, 1H), 10.53 (br s, 1H), 8.23 (app t, 1H, J=1.6 Hz), 8.12-8.09 (m, 1H), 7.85 (s, 1H), 7.80 (d, 1H, J=7.6 Hz), 7.73 (d, 1H, J=8.4 Hz), 7.67-7.63 (m, 2H), 5.85 (s, 1H), 4.08 (q, 2H, J=7.2 Hz), 2.77 (s, 3H), 2.47 (s, 3H), 2.40 (s, 3H), 1.17 (t, 3H, J=7.2 Hz); ¹³C NMR (75 MHz, DMSO-d₆) δ 169.0, 165.1, 159.8, 151.1, 147.8, 147.5, 147.0, 135.6, 133.8, 132.8, 130.3, 126.0, 124.3, 122.3, 121.0, 119.6, 98.7, 59.4, 51.9, 21.6, 21.0, 19.0, 14.1; HRMS (ESI+) m/z calcd for C₂₄H₂₅N₆O₄ (M+H) 461.1932, found 461.1913.

4.42. Ethyl 4-(4-chlorophenyl)-6-methyl-2-(8-methyl-[1,3]dioxolo[4,5-g]quinazolin-6-ylamino)-1,4-dihydropyrimidine-5-carboxylate (49f)

According to general procedure D, a solution of 47d (0.029 g, 0.12 mmol), 12^36,37 (0.035 g, 0.14 mmol) and NaHCO₃ (0.026 g, 0.31 mmol) in (CF₃)₂CHOH (0.40 mL) afforded 49f as a light yellow solid (0.024 g, 41%, ELSD purity 98.2%): Mp >200 °C; IR (neat) 3059, 2974, 1649, 1450, 1420, 1215, 1094, 1077, 768 cm ^-1; ¹H NMR (300 MHz, DMSO-d₆) δ 10.76 (br s, 2H), 7.43 (s, 1H), 7.38 (d, 2H, J=8.8 Hz), 7.34 (d, 2H, J=8.7 Hz), 7.26 (s, 1H), 6.19 (br s, 2H), 5.65 (s, 1H), 4.06 (q, 2H, J=7.2 Hz), 2.65 (s, 3H), 2.37 (s, 3H), 1.16 (t, 3H, J=7.2 Hz); ¹³C NMR (125 MHz, DMSO-d₆) δ 166.7, 165.2, 159.6, 153.3, 150.9, 148.5, 145.8, 143.7, 131.8, 128.5, 127.9, 115.6, 102.9, 102.1, 101.0, 99.0, 59.2, 51.6, 21.8, 18.9, 14.2; HRMS (ESI+) m/z calcd for C₂₄H₂₃N₅O₄Cl (M+H) 480.1433, found 480.1431.

4.43. Ethyl 4-(4-bromophenyl)-6-methyl-2-(8-methyl-[1,3]dioxolo[4,5-g]quinazolin-6-ylamino)-1,4-dihydropyrimidine-5-carboxylate (49g)

According to general procedure D, a solution of 47d (0.041 g, 0.17 mmol), 48c^36,37 and NaHCO₃ (0.031 g, 0.37 mmol) in (CF₃)₂CHOH (0.40 mL) afforded 49g as a yellow solid (0.039 g, 57%, ELSD purity 98.2%): Mp >200 °C; IR (neat) 3059, 2974, 1700, 1629, 1536, 1420, 1192, 1077, 1028, 839 cm^-1; ¹H NMR (400 MHz, DMSO-d₆) δ 10.61 (br s, 2H), 7.51 (d, 2H, J=8.4 Hz), 7.41 (s, 1H), 7.29 (d, 2H, J=8.4 Hz), 7.25 (s, 1H), 6.20 (br s, 2H), 5.66 (s, 1H), 4.06 (q, 2H, J=7.2 Hz), 2.66 (s, 3H), 2.38 (s, 3H), 1,16 (t, 3H, J=7.2 Hz); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.6, 165.2, 159.7, 153.2, 151.0, 148.5, 145.8, 144.1, 131.4, 128.3, 120.3, 115.5, 102.9, 102.1, 101.0, 98.9, 59.2, 51.7, 21.8, 19.0, 14.2; HRMS (ESI+) m/z calcd for C₂₄H₂₃N₅O₄Br (M+H) 524.0928, found 524.0915.

4.44. Ethyl 6-methyl-2-(8-methyl-[1,3]dioxolo[4,5-g]quinazolin-6-ylamino)-4-(4 morpholinophenyl)-1,4-dihydropyrimidine-5-carboxylate (49h)

According to general procedure D, a solution of 47d (0.046 g, 0.17 mmol), 48d (0.21 g, 0.70 mmol) and NaHCO₃ (0.032 g, 0.38 mmol) in CF₃CHOH (0.40 mL) gave 49h as a yellow solid (0.073 g, 81%, ELSD purity 99.7%): Mp >220 °C; IR (neat) 3058, 2816, 1655, 1543, 1450, 1420, 1316, 1215, 1077, 1028, 956 cm^-1; ¹H NMR (400 MHz, DMSO-d₆) δ 10.56 (br s, 2H), 7.41 (s, 1H), 7.24 (s, 1H), 7.19 (d, 2H, J=8.0 Hz), 6.86 (d, 2H, J=8.0 Hz), 6.19 (br s, 2H), 5.58 (s, 1H), 4.06 (app t, 2H, J=6.0 Hz), 3.68 (br s, 4H), 3.03 (br s, 4H), 2.65 (s, 3H), 2.37 (s, 3H), 1.17 (t, 3H, J=6.8 Hz); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.5, 165.4, 160.1, 153.2, 151.2, 150.2, 148.6, 145.7, 135.4, 126.7, 115.4, 115.0, 102.9, 102.1, 101.0, 99.8, 66.0, 59.1, 51.6, 48.3, 21.9, 18.7, 14.2; HRMS (ESI+) m/z calcd for C₂₈H₃₁N₆O₂ (M+H) 531.2350, found 531.2338.

4.45. Ethyl 4-(4-(dimethylamino)phenyl)-6-methyl-2-(8-methyl-[1,3]dioxolo[4,5-g]quinazolin-6-ylamino)-1,4-dihydropyrimidine-5-carboxylate (49i)

According to general procedure D, a solution of 47d (0.043 g, 0.17 mmol), 48a^36,37 (0.18 g, 0.69 mmol) and NaHCO₃ (0.031 g, 0.37 mmol) in (CF₃)₂CHOH (0.60 mL) gave 49i as an off-white solid (0.038 g, 45%, ELSD purity 100%): Mp >220 °C; IR (neat) 3059, 2934, 1663, 1549, 1450, 1420, 1215, 1077, 1029, 839 cm^-1; ¹H NMR (400 MHz, DMSO-d₆) δ 10.61 (br s, 2H), 7.42 (s, 1H), 7.24 (s, 1H), 7.14 (d, 2H, J=8.4 Hz), 6.64 (d, 2H, J= 8.8 Hz), 6.19 (br s, 2H), 5.54 (s, 1H), 4.06 (q, 2H, J=6.8 Hz), 2.82 (s, 6H), 2.66 (s, 3H), 2.37 (s, 3H), 1.18 (t, 3H, J=6.8 Hz); ¹³C NMR (100 MHz, DMSO-d₆) δ 166.4, 165.4, 160.1, 153.1, 151.1, 149.7, 148.6, 145.6, 132.4, 126.7, 115.4, 112.3, 102.9, 102.1, 101.0, 100.0, 59.1, 51.6, 40.1, 21.8, 18.7, 14.2; HRMS (ESI+) m/z calcd for C₂₆H₂₉N₆O₄ (M+H) 489.2245, found 489.2237.