Concise Synthesis of Guanidine-Containing Heterocycles Using the Biginelli Reaction

Abstract

Two general methods for the synthesis of 2-imino-5-carboxy-3,4-dihydropyrimidines were developed using the 3-component Biginelli reaction. The first method utilizes pyrazole carboxamidine, a β-ketoester, and an aldehyde in an initial Biginelli reaction. After Boc protection, these products undergo aminolysis and acidic deprotection to generate 2-imino-5-carboxy-3,4-dihydropyrimidines in a 4-step sequence. The second method utilizes a triazone-protected guanidine, a β-ketoester, and an aldehyde in a Biginelli reaction. Acidic cleavage of the triazone yields 2-imino-5-carboxy-3,4-dihydropyrimidines in a 2-step sequence. We also describe the further elaboration of several of these products using a tethered Biginelli reaction to give triazaacenaphthalene structures similar to those found in crambescidin and batzelladine alkaloids.

Introduction

Guanidine functional groups are found in numerous biologically active natural products and several drugs and drug candidates.^1,2 These guanidine-containing therapeutics include cardiovascular, antihistamine, anti-inflammatory, antidiabetic, antibacterial, antiviral, and antineoplastic drugs.¹ The diversity in activity of these compounds likely derives, in part, from the ability of guanidinium cations to recognize receptors by a variety of noncovalent interactions, including hydrogen-bonding, electrostatic, and π-stacking associations.³

Various natural products contain the guanidine functional group embedded in a 6-membered ring, such as the Na⁺ channel blocker saxitoxin (1), the puffer fish poison tetrodotoxin (2), and the nucleoside base guanine. There are several structurally novel marine alkaloids that contain a guanidine unit within complex polycyclic architectures. These include the crambescidin⁴ (examples 3-5) and batzelladine^4i,5 (6) families, which are related by their triazaacenaphthalene core structures. These guanidine alkaloids, and some of their synthetic derivatives, display promising anticancer4a-h^,4k,6 and antiviral activities.^{4b-d,4h,4j,6c} They have also been shown to inhibit important protein-protein interactions,^4g,5a-b^,7 voltage sensitive Ca²⁺ channels,^4j and Na⁺, K⁺, and Ca²⁺-ATPases.⁸

The complex architecture of the crambescidins and the batzelladines and their compelling biological activities have sparked intense effort toward their total chemical synthesis.⁹ Notably, the Snider¹⁰ and Murphy¹¹ groups developed biomimetic approaches, the Nagasawa¹² group used a 1,3-dipolar cycloaddition approach, and the Overman¹³ group pioneered the use of tethered Biginelli^9b,14 reactions for assembly of crambescidin alkaloids. The batzelladines and their analogs have also attracted considerable synthetic attention from the Overman,¹⁵ Snider,¹⁶ Murphy,¹⁷ and Nagasawa¹⁸ groups, among others.¹⁹

The existing chemistry for the synthesis of complex guanidine natural products of the crambescidin and batzelladine families and their analogs, although highly refined, requires many steps. We have, therefore, directed some effort to the development of new chemistry that will allow access to analogs of these guanidine-containing compounds in just a few steps. The Biginelli reaction²⁰ utilizes urea, an aldehyde, and a β-ketoester in a 3-component condensation giving rise to dihydropyrimidinone products.¹⁴ The related condensation of guanidine, an aldehyde, and a β-ketoester to form 6-membered guanidine-containing heterocycles, 2-imino-5-carboxy-3,4-dihydropyrimidines 10, (eq 1) is much less well developed.^14,21 Cho^21a and Atwal^21b both demonstrated that reactions between the Knoevenagel product of ethyl acetoacetate and 3-nitrobenzaldehyde with guanidine or methylguanidine give Biginelli adducts, albeit in low yields (14-25%). Milcent showed that yields are improved when the R¹ substituent of the β-ketoester is phenyl rather than methyl (eq 2).^21c In these latter examples, double Biginelli adducts 12 are also observed as byproducts when guanidine is used. Kappe also showed that guanidine can be successfully used in a traditional 3-component Biginelli reaction when the β-ketoester possesses a phenyl substituent (as in the case of ethyl benzoylacetate, R¹ = Ph) to give Biginelli adducts in satisfactory yields while avoiding double Biginelli byproducts.^21d Finally, multistep Biginelli-aminolysis strategies in which isoureas or isothioureas are used as guanidine precursors to access 2-imino-3,4-dihydropyrimidines is precedented in the literature.²² For example, Atwal demonstrated the aminolysis of Biginelli adducts containing methyl isourea^22a fragments, whereas Kappe prepared 2-imino-3,4-dihydropyrimidine salts by aminolysis of resin-bound isothiourea Biginelli adducts.^22b

In the course of our efforts to prepare focused libraries of polycyclic guanidines, we discovered that direct 3-component Biginelli reactions with guanidine are useful only with benzoylacetates and aryl aldehydes. Attempted reactions using acetoacetates (R¹ = Me) failed to give useful yields of the desired Biginelli adducts. We thus undertook the development of more general Biginelli-based methods for preparing 2-imino-5-carboxy-3,4-dihydropyrimidines.

Results

Synthesis of 2-Imino-5-carboxy-3,4-dihydropyrimidines

Aminolysis Strategy

Our initial efforts focused on Biginelli reactions with guanidine surrogates followed by aminolysis of the resulting products (Scheme 1). Atwal-type Biginelli reactions with O-methylisourea 13 and Knoevenagel precursors 11 to give dihydropyrimidines 14 are well established.²³ However, aminolysis of these methoxy-1,4-dihydropyrimidines proved to be problematic in our hands. Specifically, it was found that direct aminolysis of methoxydihydropyrimidines 14 was prohibitively sluggish. We hoped that installation of an acyloxy group at N3 to give substrates of type 15 would improve aminolysis rates, similar to effects reported by Atwal.^22a However, <10% conversion of Boc-protected methoxydihydropyrimidines 15 to aminolysis products 16 was observed, even after 45 h at 70 °C.

SCHEME 1.

In an attempt to identify Biginelli products in which subsequent aminolysis to generate a guanidine functional group would be more facile, pyrazole carboxamidine hydrochloride (17) was chosen as a coupling partner for Biginelli condensations (Scheme 2). It was reasoned that Biginelli adducts 18 would be efficient substrates for aminolysis based on the common use of 17 as a guanylating agent.²⁴ The initial Biginelli condensation in this sequence when carried out in DMF at 70 °C proceeded in moderate to good yields (58-73%, Table 1). Reaction times of 48 h were required, as the final dehydration step to form the Δ^5,6 alkene was slow relative to the comparable step in Biginelli reactions employing urea. Both aromatic and aliphatic aldehydes could be effectively used as reaction substrates. However, three of the representative aldehydes we examined did not give rise to the desired products: 2-furaldehyde and pivaldehyde failed to give isolable Biginelli adducts, whereas p-nitrobenzaldehyde provided marginal yields of Biginelli products as part of intractable mixtures. Biginelli products 18 are depicted in Scheme 2 as the Δ^2,3 tautomers, however NMR analysis shows that these heterocycles are mixtures of the Δ^2,3 and the Δ^1,2 tautomers in variable ratios.

SCHEME 2.

TABLE 1.

Synthesis of Biginelli adducts 18 using pyrazole carboxamidine hydrochloride 17

entry	R’	R	product	yield (%)
1	Et	Ph	18a	73
2	Et	o-vinyl-C6H4	18b	58
3	Bn	i-Pr	18c	74
4	Bn	n-Pr	18d	65
5	Bn	cyclohexyl	18e	63
6	Et	m-NO2-C6H4	18f	60

We initially examined direct aminolysis of Biginelli adducts 18 using the conditions described by Atwal^22a for related methyl isourea adducts. These Biginelli products were dissolved in THF, cooled to 0 °C and saturated with ammonia by bubbling ammonia gas through the solution for 15 min. The resulting solution was then heated at 70 °C in a sealed tube. Direct aminolysis under these conditions was too slow to be practical. However, introduction of a Boc group significantly improved the rate of aminolysis. Substrates 18a-f were Boc-protected using standard conditions (DMAP, Boc₂O) to give compounds 19a-f in good yields (Table 2). These products exist as mixtures of tautomers, with the Boc substituent assumed to be at N3 based on precedent for related functionalizations of 2-methoxy- and 2-[[(4-methoxyphenyl)methyl]thio]-1,4-dihydropyrimidines.^22a Substrates 19a-f typically underwent aminolysis at 70 °C within 24 h to give good yields of Boc-protected guanidine products 16a-f (Table 2).²⁵

TABLE 2.

Conversion of pyrazole Biginelli adducts 18 to guanidines 10 by the sequence depicted in Scheme 2

entry	R’	R	product	yield (%)	product	yield (%)	product	yield (%)
1	Et	Ph	19a	67	16a	77	10a	90
2	Et	o-vinyl-C6H4	19b	86	16b	60	10b	65
3	Bn	i-Pr	19c	68	16c	63	10c	86
4	Bn	n-Pr	19d	99	16d	70	10d	68
5	Bn	cyclohexyl	19e	89	16e	86	10e	95
6	Et	m-NO2-C6H4	19f	53	16f	75	10f	94

Removal of the Boc group from aminolysis products 16 was facile. Exposing compounds 16a-f to 50% TFA in dichloromethane at room temperature for 1 h gave 2-imino-5-carboxy-3,4-dihydropyrimidines 10a-f as their trifluoroacetate salts (Table 2). Recrystallization of these salts provided pure products in good to excellent yields.

The pyrazole Biginelli reaction - aminolysis sequence outlined in Scheme 2 provides a reasonably general method for preparing 2-imino-5-carboxy-3,4-dihydropyrimidines in 4 steps from a β-ketoester, an aldehyde and pyrazole carboxamidine hydrochloride (17). For the substrates examined, overall yields ranged from 19-46%. Of most significance, this strategy can be used with aliphatic as well as aromatic aldehydes and is not limited to the use of benzoylacetates as the β-ketoester component.

Protected-Guanidine Strategy

Although Biginelli reactions with guanidines containing a single alkyl substituent proceed in low to moderate yields,^21a-c we postulated that N,N-dialkylguanidines might give improved yields in Biginelli condensations. Initial support for this supposition came from the Biginelli reaction of N,N-diallylguanidine,²⁶ benzaldehyde, and benzyl acetoacetate in sodium bicarbonate-buffered DMF, which gave the Biginelli adduct in 80% yield (70 °C, 11 h). However, preliminary attempts to remove the allyl protecting groups from the resulting product were not encouraging.

Based on this result, other substituted guanidines were surveyed in order to identify a protected guanidine that would both participate in Biginelli condensations and allow subsequent deprotection to be accomplished efficiently. Neither Boc-guanidine²⁷ nor p-tosylguanidine²⁸ were reactive in Biginelli condensations using numerous reaction conditions (acidic, basic, Lewis acidic).¹⁴ The reduced basicity of these guanidines compared to N,N-diallylguanidine likely accounts for this lack of reactivity.²⁹ 1,3,5-Triaz-4-ones have been employed as masked primary amines.³⁰ We reasoned that the related triazone derivative 20 (Scheme 3) would more closely approximate the basicity and nucleophilicity of N,N-dialkylguanidines during Biginelli condensations to give products that potentially could be converted to simple guanidines under acidic conditions (Scheme 3).

SCHEME 3.

The preparation of 3,5-dimethyl-4-oxo-[1,3,5]triazinane-1-carboxamidine (20) is summarized in Scheme 4. Benzylamine was first converted to triazone derivative 25 by standard condensation with N,N’-dimethylurea and aqueous formaldehyde. The benzyl group was subsequently removed by high pressure hydrogenation to give 1,3-dimethyl-[1,3,5]triazinan-2-one (26) in 96% yield. Upon heating triazone 26 and N,N’-bis(tert-butoxycarbonyl)-1H-pyrazole-1-carboxamidine (27) at 70 °C in THF, the di-Boc-protected guanidine precursor 28 was formed in 53% yield (80% based on consumed starting material); extended reaction times or elevated temperatures gave rise to unwanted byproducts. The Boc-protecting groups of product 28 were easily removed by reaction with 50% TFA in dichloromethane at room temperature to give the triazone-protected guanidine 20 in quantitative yield.

SCHEME 4.

Triazone-protected guanidine 20 performed well in 3-component Biginelli condensations. Heating an aldehyde (aromatic or aliphatic), β-ketoester, and guanidine 20 at 70 °C for 12 h in sodium bicarbonate-buffered DMF gave good yields (62-86%) of Biginelli adducts 21a-j (Table 3). These Biginelli condensations reached completion more quickly than the corresponding reactions of pyrazole carboxamidine 17. In addition, 2-furaldehyde, pivaldehyde, and p-nitrobenzaldehyde were used successfully in Biginelli reactions with 20.

TABLE 3.

Biginelli reactions with triazone-protected guanidine 20

entry	R’	R	product	yield (%)
1	Et	Ph	21a	73
2	Et	o-vinyl-C6H4	21b	73
3	Bn	i-Pr	21c	86
4	Bn	n-Pr	21d	76
5	Bn	cyclohexyl	21e	91
6	Et	m-NO2-C6H4	21f	71
7	Et	2-furan	21g	67
8	Bn	t-Bu	21h	43
9	Et	p-NO2-C6H4	21i	86
10	Et	2,4-di-MeO-C6H4	21j	62

We turned to examine conditions for removal of the triazone group from compounds 21a-j in order to reveal the unprotected guanidine functional group. Unfortunately, the triazone ring of these guanidines was not unraveled under the mildly acidic conditions previously used for cleaving triazone derivatives of alkyl amines.^30b For example, exposure of these products to 2-6 N HCl in EtOH at room temperature resulted in no deprotection after 16 h; various other acids gave similar results. Heating adducts 21a-j in 6 N HCl in EtOH to 60 °C gave incomplete deprotection in the same time period. Finally, it was discovered that heating these substrates in 6 N aqueous HCl at 60 °C for 24 h in a sealed tube (to prevent loss of HCl) gave complete deprotection in most cases (Table 4). Not surprisingly, these relatively harsh acidic conditions were problematic with some substrates; compounds 21g, 21h, and 21j underwent extensive decomposition under these conditions.

TABLE 4.

Deprotection of triazone-protected Biginelli adducts

entry	R’	R	product	yield (%)
1	Et	Ph	10a	84
2	Et	o-vinyl-C6H4	10b	69
3	Bn	i-Pr	10c	84
4	Bn	n-Pr	10d	76
5	Bn	cyclohexyl	10e	83
6	Et	m-NO2-C6H4	10f	84
7	Et	2-Furan	10g	-
8	Bn	t-Bu	10h	-
9	Et	p-NO2-C6H4	10i	79
10	Et	2,4-di-MeO-C6H4	10j	-

This triazone-protected guanidine Biginelli strategy provides 2-imino-5-carboxy-3,4-dihydropyrimidines in only 2 steps with good overall yields (53-76%). However, this sequence is limited to substrates that survive the strongly acidic conditions required to remove the triazone-protecting group from the Biginelli product.

Transformation of 2-Imino-5-carboxy-3,4-dihydropyrimidinones Derived From Masked Dialdehydes to Hexahydrotriazaacenaphthalenes

With two methods for the preparation of 2-imino-5-carboxy-3,4-dihydropyrimidines in hand, we applied these procedures in the synthesis of hexahydrotriazaacenaphthalene structures, envisioning the use of a masked dialdehyde 29 (Scheme 5) in an initial Biginelli reaction to give 2-iminodihydropyrimidine products 30. The protected-aldehyde functional group would then be unmasked to generate pyrrolopyrimidinium hemiaminals of type 31. These cyclic hemiaminals would finally be used in tethered Biginelli reactions^9b with a second β-ketoester to give hexahydrotriazaacenaphthalenes

SCHEME 5.

A variety of Boc-protected 2-imino-5-carboxy-3,4-dihydropyrimidines 30a-e were synthesized from masked dialdehydes 29a,b³¹ using the pyrazole variant of our Biginelli-aminolysis strategy (Scheme 6). The initial Biginelli adducts 33a-e were synthesized in yields ranging from 61-73%. These Biginelli condensations were successful with the mono dimethyl acetal-protected dialdehydes 29a,b and β-ketoesters having various groups at R¹ (ethyl and allyl esters) and R² (methyl, phenyl, p-nitrophenyl). Boc-protection and subsequent aminolysis of these Biginelli products proceeded in good yields to give products 30a-e.

SCHEME 6.

The Boc-protected 2-imino-5-carboxy-3,4-dihydropyrimidines products were converted to hexahydrotriazaacenaphthalenes as summarized in Scheme 7. Treatment of compounds 30a-e with trifluoroacetic acid at room temperature resulted in cleavage of the Boc and dimethylacetal groups and ring closure to give tetrahydropyrrolopyrimidinium salts of type 31. The solvent and excess TFA were removed from these products under reduced pressure; these crude intermediates were then redissolved in trifluoroethanol containing an excess of a second β-ketoester. These solutions were then heated in the presence of morpholinium acetate as a promoter and sodium sulfate as a desiccant to give hexahydrotriazaacenaphthalene tethered-Biginelli products 32.

SCHEME 7.

This 5-step sequence was employed to prepare six hexahydrotriazaacenaphthalenes (32a-f, Table 5). The overall yields for these syntheses ranged from 11-42%. For the hexahydrotriazaacenaphthalene products (n = 1), the yields for the tethered Biginelli reactions ranged from 51-90%. However, little diastereoselectivity was observed, a result that stands in contrast to the tethered Biginelli reactions reported previously from our laboratories.^9b,13,15 The syn relative configuration was slightly favored (3:1, verified by ¹H NMR NOE correlations) for several of these products (32a,c, and e). Other hexahydrotriazaacenaphthalene adducts were isolated as 1:1 mixtures of epimers. In all cases, the syn and anti epimers were cleanly separated using standard flash chromatography techniques.

TABLE 5.

Triazaacenaphthalene and related structures

entry	R1	R2	R3	R4	n	product	yield (%)	dr (syn:anti)
1	Et	Me	Me	Me	1	32a	70	3:1
2	Et	Ph	allyl	Me	1	32b	68	1:1.3
3	Et	Ph	Et	C6F5	1	32c	86	3.2:1
4	Et	p-NO2-C6H4	allyl	Me	1	32d	90	1:1.3
5	Et	p-NO2-C6H4	Bn	Me	1	32e	54	3.2:1
6	Et	p-NO2-C6H4	allyl	(CH2)4OBn	1	32f	51	1:1
7	allyl	Me	Et	p-NO2-Ph	2	32g	27	1:0

In addition to the preparation of triazaacenaphthalene structures, one derivative that incorporates a fused piperidine ring, hexahydro-1H-1,9,9b-diazaphenalene 32g, was also synthesized. The inclusion of the 6-membered piperidine ring resulted in a lower yield for the tethered Biginelli reaction (27%). However, this reaction proceeded with high diastereoselection to produce exclusively the syn product.

Discussion

Biginelli strategies utilizing pyrazole carboxamidine 17 or triazone-protected guanidine 20 enable the concise and general synthesis of 2-imino-5-carboxy-3,4-dihydropyrimidines. Only a few members of this class of heterocycles had previously been prepared using Biginelli reactions, which suffered from low yields or lack of generality in one or more of the reactants.^21,22 There are advantages and limitations to each method developed during the present study. The Biginelli reaction with pyrazole carboxamidine 17, followed by aminolysis utilizes relatively mild conditions throughout. However, the initial Biginelli step involves a slow dehydration that requires prolonged heating to drive to completion. In addition, efficient aminolysis is dependent upon the introduction of a Boc substituent. This protection step and the subsequent deprotection reaction add several steps to the route. Despite these limitations, this method is preferable for substrates having sensitive functionality, as most of the chemistry is relatively mild. In contrast, the synthesis of 2-imino-5-carboxy-3,4-dihydropyrimidines from triazone guanidine 20 requires only 2 steps: the Biginelli reaction, which is complete within 12 h, and the final deprotection step. This Biginelli reaction is more general in scope than that employing pyrazole carboxamidine 17 as every aldehyde surveyed provided the desired adduct. The major limitation of this second strategy is the harsh deprotection step (strong acid, heat), which is incompatible with some functionality. Taken together, these two strategies provide ready access to a wide range of 2-imino-5-carboxy-3,4-dihydropyrimidines. Both strategies provide these products as racemates.³²

We have applied the pyrazole carboxamidine 17 Biginelli-aminolysis sequence to masked dialdehydes to ultimately generate tetrahydropyrrolopyrimidinium intermediates 31, which were transformed by tethered Biginelli reactions to a variety of hexahydrotriazaacenaphthalenes 32. This chemistry provides access to the core structures of the crambescidin and the batzelladine alkaloids in a succinct 5-step sequence from readily available starting materials.

The standard Knoevenagel conditions used in this study for the tethered Biginelli condensation (1 equiv of morpholinium acetate, trifluoroethanol) were previously shown to favor syn stereoselection with similar substrates.^15a Thus, tethered Biginelli reactions of hexahydropyrrolopyrimidinium substrates 33 (Figure 2) proceeded with up to 9:1 stereoselectivity under essentially identical conditions.¹⁵ These studies showed that the stereochemical outcome of this reaction can be rather solvent and temperature dependent, with lower temperatures and more polar solvents improving stereoselectivity.^15a We were, therefore, somewhat surprised by the low selectivities observed for tethered Biginelli reactions of tetrahydropyrrolopyridinium substrates 31. The incorporation of a second double bond in substrates of type 31 compared to those of type 33 (Figure 2) leads to some flattening of the bicyclic ring system. This change may account for the erosion in stereoselectivity.

FIGURE 2.

The chemistry reported herein can likely be used to synthesize several heterocyclic architectures in addition to hexahydrotriazaacenaphthalenes. One example was demonstrated, the short construction of hexahydro-1H-1,9,9b-triazaphenalene 32g. This example demonstrates the utility of this chemistry for preparing decidedly non-natural analogs of the crambescidin and the batzelladine alkaloids. In addition to alternative sizes for the central fused ring and the substituents R¹-R⁴, a variety of other analogs undoubtedly can be synthesized from core structures of type 32.

Conclusion

Natural products that contain guanidine units within 6-membered heterocyclic rings possess diverse biological activities. The short synthesis of 2-imino-5-carboxy-1,4-dihydropyrimidines and hexahydrotriazaacenaphthalenes reported herein will likely find use for the synthesis of focused libraries of guanidine alkaloid-like structures. We are applying and extending the strategies described herein to the preparation of such libraries in order to more completely probe the biology of these fascinating guanidine-containing heterocycles.

Experimental Section³³

6-Methyl-4-phenyl-2-pyrazol-1-yl-1,4-dihydropyrimidine-5-carboxylic acid ethyl ester (18a). Representative procedure for synthesis of Biginelli products 18

Benzaldehyde (1.14 mL, 11.3 mmol), ethyl acetoacetate (1.45 mL, 11.3 mmol), pyrazole carboxamidine hydrochloride 17 (1.86 g, 12.5 mmol), NaHCO₃ (3.8 g, 45 mmol) were heated in DMF (16.5 mL) at 70 °C for 48 h. After cooling to room temperature, the solution was diluted with water (50 mL) and washed with Et₂O (3 × 50 mL). The combined organic layers were dried over anhydrous MgSO₄, filtered, and concentrated. The residue was purified by flash chromatography (silica gel, 20% Et₂O in hexanes → 40% Et₂O in hexanes) to give compound 18a as a pale yellow oil (2.65 g, 73% yield, a 1.6:1 mixture of tautomers): ¹H NMR (500 MHz, DMSO-d₆) major tautomer δ 9.56 (d, J = 3.5 Hz, 1 H), 8.46 (d, J = 3 Hz, 1 H), 7.86 (s, 1 H), 7.37-7.31 (m, 4 H), 7.28-7.23 (m, 1 H), 6.58-6.57 (m, 1 H), 5.59 (d, J = 3.5 Hz, 1 H), 4.08-4.03 (m, 2 H), 2.42 (s, 3 H), 1.14 (t, J = 7 Hz, 3 H) ppm; minor tautomer δ 9.88 (s, 1 H), 8.34 (d, J = 2.5 Hz, 1 H), 7.83 (s, 1H), 7.37-7.31 (m, 4 H), 7.28-7.23 (m, 1 H), 6.54-6.53 (m, 1 H), 5.65 (s, 1 H), 4.08-4.03 (m, 2 H), 2.47 (s, 3 H), 1.13 (t, J = 7 Hz, 3 H) ppm; ¹³C NMR (125 MHz, DMSO-d₆) all peaks from both tautomers δ 165.9, 165.8, 156.6, 146.9, 146.2, 145.5, 144.5, 143.1, 141.5, 141.3, 128.8, 128.4, 128.3, 128.0, 127.6, 127.0, 126.8, 126.5, 109.1, 108.6, 103.8, 99.8, 59.3, 59.3, 58.0, 52.7, 23.0, 17.6, 14.1, 14.0 ppm; IR (thin film) 3381, 3335, 2983, 2931, 1743, 1716, 1702, 1628, 1532, 1485, 1395, 1370, 1263, 1240, 1200, 1171, 1151, 1091, 1307 cm^-1; HRMS (ESI) m/z 311.1513 (311.1508 calcd for C₁₇H₁₉N₄O₂⁺ [MH]⁺).

4-Methyl-6-phenyl-2-pyrazol-1-yl-6H-pyrimidine-1,5-dicarboxylic acid 1-tert-butyl ester 5-ethyl ester (19a). Representative procedure for synthesis of Boc-protected Biginelli adducts 19

Compound 18a (262 mg, 0.84 mmol) and di-tert-butyl dicarbonate (Boc₂O, 285 mg, 1.01 mmol) were dissolved in MeCN (3.0 mL) under an N₂ atomosphere. DMAP (10 mg, 78 μmol) was added and the solution was maintained at room temperature for 12 h. The solvent was removed en vacuo and the residue was recrystallized from EtOH to give 19a as a colorless solid (230 mg, 67%, mp 144-145 °C): ¹H NMR (500 MHz, CDCl₃) δ 8.10 (s, 1 H), 7.71 (s, 1 H), 7.47 (d, J = 7.5 Hz, 2 H), 7.32-7.29 (m, 2 H), 7.27 (m, 1 H), 6.45 (s, 1 H), 6.39 (s, 1 H), 4.31-4.22 (m, 2 H), 2.61 (s, 3 H), 1.35 (s, 9 H), 1.29 (t, J = 7 Hz, 3 H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 165. 7, 153.5, 151.6, 143.3, 143.1, 138. 5, 129.5, 128.5, 128.1, 126.9, 113.6, 108.6, 83.7, 60.7, 55.0, 27.6, 21.2, 14.3 ppm; IR (thin film) 2983, 2935, 1732, 1707, 1702, 1623, 1564, 1461, 1397, 1370, 1300, 1267, 1231, 1143, 1090, 1038, 963, 908 cm^-1; HRMS (ESI) m/z 433.1867 (433.1852 calcd for C₂₂H₂₆N₄O₄Na⁺ [MNa]⁺). Anal. Calcd for C₂₂H₂₆N₄O₄: C, 64.37; H, 6.38; N, 13.65; Found: C, 64.24; H, 6.45; N, 13.65.

2-Amino-4-methyl-6-phenyl-6H-pyrimidine-1,5-dicarboxylic acid 1-tert-butyl ester 5-ethyl ester (16a). Representative procedure for synthesis of aminolysis products 16

A solution of compound 19a (219 mg, 0.53 mmol), ammonium chloride (14 mg, 0.27 mmol) and THF (2 mL) was cooled to 0 °C and ammonia gas was bubbled through for 30 min. The resulting solution was heated in a sealed tube at 70 °C for 12 h. After cooling to room temperature, the solvent was removed en vacuo and the residue was purified by recrystallization (EtOH) to give 16a as a colorless solid (147 mg, 77% yield, mp 169-170 °C): ¹H NMR (500 MHz, CDCl₃) δ 7.71 (bs, 1 H), 7.40-7.38 (m, 2 H), 7.35-7.29 (m, 3 H), 6.25 (s, 1 H), 4.17 (q, J = 6.5 Hz, 2 H), 2.37 (s, 3 H), 1.55 (s, 9 H), 1.29 (t, J = 7 Hz, 3 H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 166.5, 156.0, 153.3, 151.8, 142.3, 128.6, 128.0, 127.0, 104.4, 84.7, 59.8, 55.7, 28.1, 22.2, 14.5 ppm; IR (thin film) 3370, 3011, 2986, 1717, 1697, 1651, 1607, 1519, 1370, 1342, 1337, 1297, 1236, 1148, 1115, 1063 cm^-1; HRMS (ESI) m/z 360.1929 (360.1923 calcd for C₁₉H₂₆N₃O ⁺₄ [MH]⁺). Anal. Calcd for C₁₉H₂₅N₃O₄: C, 63.49; H, 7.01; N, 11.69; Found: C, 63.08; H, 7.12; N, 11.85.

5-Ethoxycarbonyl-6-methyl-4-phenyl-3,4-dihydro-1H-pyrimidin-2-ylidene-ammonium trifluoroacetate (10a). Representative procedure for synthesis of heterocyclic guanidines 10

Method A

Compound 16a (674 mg, 1.88 mmol) was dissolved in CH₂Cl₂ (2.5 mL) under nitrogen. TFA (2.5 mL) was added and the resulting solution was stirred for 1 h at room temperature. The solvent and excess TFA were removed under reduced pressure and the resulting residue was purified by trituration with Et₂O to give trifluoroacetate salt 10a as a colorless solid (629 mg, 90%): ¹H NMR (500 MHz, CDCl₃:CD₃OD (1:1)) δ 7.34-7.31 (m, 2 H), 7.28-7.26 (m, 3 H), 5.43 (s, 1 H), 4.10-4.04 (m, 2 H), 2.42 (s, 3 H), 1.14 (t, J = 7 Hz, 3 H) ppm; ¹³C NMR (125 MHz, CDCl₃:CD₃OD (1:1)) δ 164.9, 151.0, 143.4, 141.6, 128.8, 128.4, 126.4, 103.6, 60.5, 53.3, 17.2, 13.6 ppm; IR (thin film) 3247, 3065, 2984, 2875, 1690, 1637, 1556, 1496, 1457, 1386, 1370, 1329, 1273, 1241, 1090, 910 cm^-1; HRMS (ESI) m/z 260.1400 (260.1399 calcd for C₁₄H₁₉N₃O₂⁺ [MH]). Anal. Calcd for C₁₆H₁₈N₃O₆F₃: C, 51.48; H, 4.86; N, 11.26; Found: C, 51.59; H, 4.87; N, 11.19.

Method B

Compound 21a (189 mg, 0.51 mmol) was dissolved in 6 N HCl (10 mL) and heated to 60 °C in a sealed tube for 24 h. After cooling to room temperature, the reaction solution was then extracted with CH₂Cl₂ (8 × 10 mL). The organic layers were dried over MgSO₄, filtered, and concentrated. The residue was purified by flash chromatography (silica gel, 10% MeOH in CH₂Cl₂) to give the HCl salt of 10a as a colorless solid (126 mg, 84%).

5-Benzyl-1,3-dimethyl-[1,3,5]triazinan-2-one (25)

Benzylamine (5 mL, 45.8 mmol), formaldehyde (37% w/w solution, 7.5 mL, 91.6 mmol), and N,N’-dimethylurea (4.03 g, 45.8 mmol) were charged to a reaction flask equipped with a reflux condenser and heated to 100 °C under an argon atmosphere for 16 h. After cooling to room temperature, the reaction was quenched by the addition of H₂O (25 mL) and CH₂Cl₂ (50 mL). The organic layer was separated and washed with brine (1 × 25 mL), dried over anhydrous MgSO₄, filtered, and concentrated. The residue was purified by flash chromatography (silica gel, Et₂O → 10% MeOH in Et₂O) to give 25 as a colorless solid (7.46 g, 74% yield): ¹H NMR (500 MHz, CDCl₃) δ 7.25-7.16 (m, 5 H), 4.00 (s, 4 H), 3.80 (s, 2 H), 2.74 (s, 6 H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 155.9, 137.5, 129.0, 128.5, 127.6, 67.7, 55.3, 32.4 ppm; IR (thin film) 3452, 2924, 2869, 1629, 1514, 1453, 1417, 1404, 1297, 1151, 1024 cm^-1; HRMS (ESI) m/z 242.1276 (242.1269 calcd for C₁₂H₁₇N₃ONa⁺[MNa]⁺).

1,3-Dimethyl-[1,3,5]triazinan-2-one (26)

5-Benzyl-1,3-dimethyl-[1,3,5]triazinan-2-one 25 (1.0 g, 4.56 mmol) was dissolved in EtOH (25 mL). To this solution was added Pd/C (10%) (140 mg) and the resulting mixture was heated to 65 °C under an H₂ atmosphere (750 psi) for 18 h. After cooling to room temperature, the suspension was then filtered through Celite and the eluent was concentrated by rotary evaporation to give 26 as a colorless solid (567 mg, 96% yield): ¹H NMR (500 MHz, CDCl₃) δ 3.99 (s, 4 H), 2.68 (s, 6 H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 155.8, 63.1, 31.5 ppm; IR (thin film) 3420, 3283, 2939, 2871, 1624, 1523, 1418, 1405, 1301, 1029, 906 cm^-1; HRMS (ESI) m/z 152.0801 (152.0800 calcd for C₅H₁₁N₃ONa⁺[MNa]⁺).

Tert-Butyloxycarbonylimino-[(3,5-dimethyl-4-oxo-[1,3,5]triazinan-1-yl)-methyl]-carbamic acid tert-butyl ester (28)

N,N’-Bis(t-butoxycarbonyl)-1H-pyrazole-1-carboxamidine 27 (7.46 g, 24.1 mmol) and 1,3-dimethyl-[1,3,5]triazinan-2-one 26 (3.73 g, 28.9 mmol) were dissolved in dry THF (10 mL) under argon in a flame-dried flask. This solution was heated at 70 °C for 24 h. After cooling to room temperature, the solvent was removed en vacuo and the residue was purified by flash chromatography (silica gel, 4% MeOH in CH₂Cl₂) to give 28 as a colorless solid (4.63 g, 53% yield): ¹H NMR (500 MHz, CDCl₃) δ 10.00 (bs, 1 H), 4.67 (s, 4 H), 2.89 (s, 6 H), 1.46 (s, 18 H) ppm; ¹³C NMR (125 MHz, CD₃OD) δ 157.1, 153.7, 81.6, 62.1, 33.0, 28.1 ppm; IR (thin film) 3179, 2981, 2934, 1750, 1636, 1607, 1517, 1392, 1288, 1254, 1226, 1131, 1124 cm^-1; HRMS (ESI) m/z 394.2064 (394.2066 calcd for C₁₆H₂₉N₅O₅Na⁺ [MNa]⁺).

3,5-Dimethyl-4-oxo-[1,3,5]triazinane-1-carboxamidine trifluoroacetate salt (20)

Compound 28 (4.93 g, 13.3 mmol) was dissolved in CH₂Cl₂ (18.6 mL) under argon and TFA (18.6 mL) was added. The resulting solution was maintained at room temperature for 1 h, the solvent was removed under reduced pressure, and the resulting residue was crystallized from Et₂O to give 20 as a colorless solid (3.8 g, 100% yield, mp 179-181 °C): ¹H NMR (500 MHz, CD₃OD) δ 4.73 (s, 4 H), 2.88 (s, 6 H) ppm; ¹³C NMR (125 MHz, CD₃OD) δ 161.3, 158.4, 63.3, 33.1 ppm; IR (thin film) 3336, 2931, 2882, 2476, 1602, 1527, 1423, 1406, 1313, 1120, 1035, 975 cm^-1; HRMS (ESI) m/z 172.1204 (172.1198 calcd for C₆H₁₄N₅O⁺ [MH]⁺). Anal. Calcd for C₈H₁₄F₃N₅O₃: C, 33.69; H, 5.00; N, 24.55; Found: C, 33.83; H, 5.00; N, 24.43.

2-(3,5-Dimethyl-4-oxo-[1,3,5]triazinan-1-yl)-6-methyl-4-phenyl-1,4-dihydropyrimidine-5-carboxylic acid ethyl ester (21a). Representative procedure for synthesis of Biginelli products 21

Compound 20 (293 mg, 1.71 mmol), ethyl acetoacetate (0.2 mL, 1.56 mmol), benzaldehyde (0.16 mL, 1.56 mmol) and NaHCO₃ (575 mg, 6.85 mmol) were added to DMF (2.5 mL) under a nitrogen atmosphere. This mixture was heated at 70 °C for 22 h. The reaction was cooled to room temperature and poured over crushed ice (25 g). The resulting suspension was extracted with Et₂O (1 × 75 mL) and CH₂Cl₂ (3 × 25 mL) and the organic layers were dried over anhydrous MgSO₄, filtered, and concentrated. The resulting residue was recrystallized from Et₂O to give 21a as a colorless solid (425 mg, 73% yield, mp 198-200 °C): ¹H NMR (500 MHz, CDCl₃) δ 7.30-7.21 (m, 5 H), 5.39 (s, 1 H), 4.79-4.72 (m, 4 H), 4.06-4.02 (m, 2 H), 2.77 (s, 6 H), 2.39 (s, 3 H), 1.19 (t, J = 7 Hz, 3 H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 167.3, 158.7, 156.2, 153.0, 145.2, 128.5, 127.6, 126.5, 102.2, 61.8, 60.0, 54.2, 32.7, 24.0, 14.4 ppm; IR (thin film) 3252, 2981, 2932, 2904, 2886, 2880, 1664, 1629, 1599, 1522, 1503, 1372, 1339, 1303, 1217, 1126, 1061, 1033, 968, 910 cm^-1; HRMS (ESI) m/z 372.2030 (372.2036 calcd for C₁₉H₂₆N₅O₃⁺ [MNa]⁺). Anal. Calcd for C₁₉H₂₅N₅O₃: C, 61.44; H, 6.78; N, 18.85; Found: C, 61.22; H, 6.94; N, 18.67.

4-(3,3-Dimethoxypropyl)-6-methyl-2-pyrazol-1-yl-1,4-dihydropyrimidine-5-carboxylic acid ethyl ester (33a). Representative procedure for synthesis of Biginelli products 33

Compound 17 (1.24 g, 8.32 mmol), ethyl acetoacetate (0.96 mL, 7.57 mmol), and aldehyde 29a (1.0 g, 7.57 mmol) were dissolved in DMF buffered with NaHCO₃ (2.54 g, 30.3 mmol) and heated under an N₂ atmosphere for 48 h. The mixture was cooled to room temperature, diluted with water (20 mL), and the resulting solution was extracted with Et₂O (3 × 15 mL). The combined organic layers were dried over MgSO₄, filtered, and concentrated. The resulting residue was purified by flash chromatography (silica gel, 100% Et₂O) to give 33a as a yellow oil (1.58 g, 62% yield, a 1.3:1 mixture of tautomers): ¹H NMR (500 MHz, CDCl₃) major tautomer δ 8.11 (d, J = 5 Hz, 1 H), 7.87 (s, 1 H), 7.43 (d, J = 1 Hz, 1 H), 6.25-6.24 (m, 1 H), 4.55 (t, J = 5 Hz, 1 H), 4.23 (t, J = 5 Hz, 1 H), 4.09-4.00 (m, 2 H), 3.12 (s, 6 H), 2.21 (s, 3 H), 1.71-1.65 (m, 1 H), 1.59-1.49 (m, 3 H), 1.13 (q, J = 7 Hz, 3 H) ppm; minor tautomer δ 8.20 (d, J = 5 Hz, 1 H), 7.48 (d, J = 1 Hz, 1 H), 7.35 (d, J = 2.5 Hz, 1 H) 6.25-6.24 (m, 1 H), 4.48-4.47 (m, 1 H), 4.18 (t, J = 5 Hz, 1 H), 4.09-4.00 (m, 2 H), 3.14 (s, 3 H), 3.12 (s, 3 H), 2.20 (s, 3 H), 1.71-1.65 (m, 1 H), 1.59-1.49 (m, 3 H), 1.14 (q, J = 7 Hz, 3 H) ppm; ¹³C NMR (125 MHz, CDCl₃) all peaks from both tautomers δ 166.5, 157.2, 147.4, 144.8, 142.6, 141.1, 141.0, 128.4, 127.1, 108.7, 108.5, 104.9, 104.5, 104.4, 101.3, 59.68, 59.7, 54.4, 53.0, 52. 6, 52.5, 52.4, 50.2, 32.0, 31.8, 27.6, 27.2, 23.2, 18.6, 14.3, 14.3 ppm; IR (thin film) 3389, 3320, 2982, 2955, 2937, 1697, 1627, 1531, 1484, 1394, 1377, 1241, 1200, 1126, 1105, 1071, 1038 cm^-1; HRMS (ESI) m/z 359.1689 (359.1695 calcd for C₁₆H₂₄N₄O₄Na⁺ [MNa]⁺).

6-(3,3-Dimethoxypropyl)-4-methyl-2-pyrazol-1-yl-6H-pyrimidine-1,5-dicarboxylic acid 1-tert-butyl ester 5-ethyl ester (34a). Representative procedure for synthesis of Boc-protected Biginelli products 34

Compound 33a (1.45 g, 4.31 mmol) and di-tert-butyl dicarbonate (Boc₂O, 1.13 g, 5.17 mmol) were dissolved in MeCN (17.0 mL) under an N₂ atomosphere. DMAP (53 mg, 0.43 mmol) was added and the solution was stirred at room temperature for 12 h. The solvent was removed en vacuo and the residue was recrystallized from Et₂O/hexanes to give 34a as a colorless solid (1.36 g, 73%, mp 114-115 °C): ¹H NMR (500 MHz, CDCl₃) δ 8.15 (d, J = 2.5 Hz, 1 H), 7.72 (s, 1 H), 6.44 (s, 1 H), 5.19 (dd, J = 10 Hz, 4 Hz, 1 H), 4.39 (t, J = 6 Hz, 1 H), 4.32-4.20 (m, 2 H), 3.30 (s, 3 H), 3.27 (s, 3 H), 2.43 (s, 3 H), 1.94-1.87 (m, 1 H), 1.80-1.73 (m, 1 H), 1.65-1.58 (m, 1 H), 1.54-1.46 (m, 1 H), 1.34 (t, J = 7 Hz, 3 H), 1.25 (s, 9 H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 165.4, 152.5, 151.4, 143.0, 129.5, 114.9, 108.6, 104.18, 83.3, 60.6, 53.2, 52.9, 52.6, 27.8, 27.7, 27.6, 26.8, 21.2, 14.4 ppm; IR (thin film) 3394, 2982, 2935, 2831, 1774, 1732, 1709, 1650, 1623, 1567, 1460, 1398, 1370, 1301, 1288, 1230, 1142, 1070 cm^-1; HRMS (ESI) m/z 459.2212 (459.2220 calcd for C₂₁H₃₂N₄O₆Na⁺ [MNa]⁺).

2-Amino-6-(3,3-dimethoxypropyl)-4-methyl-6H-pyrimidine-1,5-dicarboxylic acid 1-tert-butyl ester 5-ethyl ester (30a). Representative procedure for synthesis of products 30

Compound 34a (1.15 g, 2.63 mmol) and ammonium chloride (71 mg, 1.32 mmol) were dissolved in THF (17 mL), cooled to 0 °C and ammonia gas was bubbled through for 30 min. The resulting solution was sealed in a glass tube and stirred at 70 °C for 12 h. After cooling to room temperature, the solvent was removed en vacuo and the residue was purified by flash chromatography (silica gel, 40% Et₂O in hexanes → 100% Et₂O → 10% MeOH in Et₂O) to give product 30a as a colorless solid (0.90 g, 88% yield, mp 110-111 °C): ¹H NMR (500 MHz, CDCl₃) δ 7.87 (bs, 2 H), 5.21 (s, 1 H), 4.30 (s, 1 H), 4.16 (q, J = 7 Hz, 2 H), 3.26 (s, 6 H), 2.21 (s, 3 H), 1.55-1.54 (m, 4 H), 1.52 (s, 9 H), 1.26 (t, J = 7 Hz, 3 H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 166.3, 156.1, 153.0, 151.6, 104.1, 84.0, 59.5, 52. 7, 52.65, 51.9, 28.9, 28.0, 27.9, 21.5, 14.4 ppm; IR (thin film) 3410, 2982, 2936, 1716, 1645, 1597, 1514, 1371, 1343, 1289, 1265, 1241, 1141, 1064 cm^-1; HRMS (ESI) m/z 408.2104 (408.2111 calcd for C₁₈H₃₁N₃O₆Na⁺ [MNa]⁺). Anal. Calcd for C₁₈H₃₁N₃O₆: C, 56.09; H, 8.11; N, 10.90; Found: C, 56.39; H, 8.05; N, 10.85.

3-Ethoxycarbonyl-8-methoxycarbonyl-4,7-dimethyl-1,2,2a,5,6,8a-hexahydro-5,6,8b-triazaacenaphthylene (32a). Representative procedure for the synthesis of triazaacenaphthalenes 32

Compound 30a (200 mg, 0.52 mmol) was stirred in 50% TFA:CH₂Cl₂ (1.4 mL) under an N₂ atmosphere for 1 h. The solvent and excess TFA were removed under reduced pressure, and the residue was placed under high vacuum for 8 h. The residue was then dissolved in trifluoroethanol (1 mL) and to this solution was added morpholinium acetate (84 mg, 0.57 mmol), Na₂SO₄ (81 mg, 0.57 mmol), and methyl acetoacetate (0.16 mL, 1.56 mmol). This mixture was heated at 70 °C for 72 h and then filtered to remove Na₂SO₄, concentrated, and the residue was purified by chromatography (silica gel, 1% MeOH in CH₂Cl₂ to 5% MeOH in CH₂Cl₂) to give compound 32a as a mixture of diastereomers. These epimers were separated by chromatography (silica gel, EtOAc) to give syn-32a (119 mg, 52% yield) and anti-32a (40 mg, 18% yield) as tan trifluoroacetate salt solids. The relative configuration of the angular hydrogens flanking the pyrrolidine nitrogen was verified by NOESY crosspeaks: syn-32a: ¹H NMR (500 MHz, CDCl₃) δ 4.31 (q, J = 5 Hz, 2 H), 4.27-4.20 (m, 1 H), 4.18-4.12 (m, 1 H), 2.65-2.62 (m, 2 H), 2.29 (s, 6 H), 1.94-1.90 (m, 2 H), 1.30 (t, J = 7 Hz, 3 H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 166.3, 165.9, 153.1, 152.3, 149.6, 103.7, 103.4, 60.1, 55.8, 51.1, 33.1, 33.0, 20.3, 20.1, 14.6 ppm; IR (thin film) 3283, 3145, 2983, 2948, 2872, 2840, 1691, 1685, 1617, 1522, 1443, 1372, 1338, 1317, 1290, 1269, 1248, 1203, 1183, 1130, 1104, 1079 cm^-1; HRMS (ESI) m/z 320.1604 (320.1610 calcd for C₁₆H₂₂N₃O₄⁺ [M]⁺); anti-32a: ¹H NMR (500 MHz, CDCl₃) δ 4.27-4.15 (m, 4 H), 3.75 (s, 3 H), 2.40-2.38 (m, 2 H), 2.37 (s, 6 H), 1.63-1.60 (m, 2 H), 1.30 (t, J = 7 Hz, 3 H) ppm; ¹³C NMR (125 MHz, CDCl₃) δ 166.3, 165.8, 153.1, 152.2, 149.6, 103.7, 103.4, 60.1, 55.8, 51.1, 33.1, 33.0, 20.3, 20.1, 14.6 ppm; IR (thin film) 3190, 3078, 2983, 2908, 2779, 1702, 1697, 1619, 1569, 1541, 1433, 1378, 1320, 1280, 1268, 1238, 1201, 1187, 1166, 1089 cm^-1; HRMS (ESI) m/z 320.1616 (320.1610 calcd for C₁₆H₂₂N₃O₄⁺ [M]⁺).

FIGURE 1.