Intermolecularly Hydrogen-Bonded Dimeric Helices. Tripyrrindiones

Abstract

A rare and unusual class of tripyrrolic compounds, violet-colored tripyrrin-1,14-diones, can be prepared easily and in moderately high yields from base (piperidine)-catalyzed condensation of 3-pyrrolin-2-ones with 2,5-diformylpyrroles. Dipyrrinones adopt the all syn-Z conformation leading to helical, lock-washer like structures, which form dimers that are held together by intermolecular hydrogen bonds in nonpolar solvents and in the crystal. Strong bathochromic spectral shifts of the tripyrrindione ∼480 nm long wavelength UV-visible absorption band are seen with added base: DBU, 615 nm; TFA, 573 nm; and Zn (OAc)₂, 586 nm.

1. Introduction

Tripyrrolic tripyrrindiones (Fig. 1A) were first reported in the synthetic work of Hans Fischer and Erich Adler in 1931¹ directed toward the syntheses of the tetrapyrrole, mesobilirubin-XIIIα, and its synthetic diyrrinone precursor, neoxanthobilirubinic acid. In the course of that study, and somewhat incidental to it, a brief study of tripyrranes was illustrated by report of two reactions: 4-carboethoxy-5-chloromethyl-3-methyl-3-pyrrolin-2-one with opsopyrrole and with opsopyrrole carboxylic acid to give unreported yields of brown-violet high melting tripyrrins (Fig. 1B). Although written as hydroxypyrrole tautomers of undesignated stereochemistry throughout, in Fischer’s convention, these two tripyrroles apparently constituted the first documented examples of tripyrrindiones. Nearly ten years later, Fischer and Reinecke,² in connection with synthetic and analytical studies of glaucobilin, prepared the methyl ester of the carboxylic acid of Fig. 1B and concluded that the violet pigment of the Gmelin reaction of verdins was not due to a dihydroxytripyrrin (tripyrrindione). No further work on tripyrrindiones came from the Fischer lab, and they escaped further investigation until 1966,^3a when a Fischer student, Henning von Dobeneck, prepared the hexamethyltripyrrindione of Fig. 1C³ in two steps by condensation of 5-bromo-3,4-dimethyl-2-formyl-1H-pyrrole with 2,3,7,8-tetramethyldipyrrinone to give a tripyrrole, followed by treatment with potassium acetate in acetic acid. The E-stereochemistry was designated.

Figure 1.

(A) Tripyrrindione carbon framework and numbering system. (B) Fischer and Adler’s brown-violet-colored synthetic tripyrrindiones (ref. ¹) in the Fischer representations as hydroxypyrroles with no C=C stereochemistry denoted. (C) Hexamethyl tripyrrindione of von Dobeneck (ref. ²) as formulated in 1966 (left) and revised in 1971 (right) with the E stereochemistry as drawn by von Dobeneck. (D) The reddish urinary pigment, uroerythrin as represented in ref. 3. (E) Nakajima’s tripyrrolic pigments from urine, redrawn as represented (ref. ⁴). (F) The synthetic targets of the current work.

The first naturally-occurring tripyrrindione, as it was subsequently characterized,⁴ was isolated repeatedly, over many years, from urine and urate sediments, and named uroerythrin.⁵ The pigment was characterized as its dimethyl ester-lactam monomethyl ether, and its structure was proposed as shown in Fig. 1D.⁴ Apparently only the endo-vinyl isomer was isolated, and the E-stereochemistry was indicated throughout. Whether the predictably less stable exo-vinyl isomer was present is unclear. Subsequently, Nakajima et al.⁶ isolated two urinary pigments to which they assigned tripyrrindione structures (Fig. 1E), one of which is identical to that of Fig. 1D, the other is the “missing” exo-vinyl isomer, and for both the stereochemistry was not indicated. Apparently no further tripyrrindiones have been synthesized or isolated since 1994, leaving a total of 5-6 representatives in this class of compounds.

We were attracted to further investigate simple tripyrrindiones synthesized by a short synthetically logical route. We focused on the synthesis of hexaethyl of 1 (1, Figure 1F) by a more direct method than that employed in the synthesis of hexamethyl analog of Fig. 1C. We also synthesized tetramethyl analog 2 (Fig. 1E) for comparison and showed that 1 would prove to have better solubility in organic solvents, a lower melting point than the mp 315°C of the hexamethyl analog (Fig. 1C), and the potential for growing a crystal suitable for X-ray crystallographic analysis. The conformation of 1 in solution was examined by NMR methods and molecular mechanics calculations, and its molecularity was determined from vapor pressure osmometry (VPO) measurements. Throughout are described improved procedures for the monopyrrole starting materials.

2. Results and Discussion

2.1 Synthesis

Our approach to the syntheses of 1 and 2 was direct: 3,4-diethyl-2,5-diformyl-1H-pyrrole (3)⁷ would be condensed with either 3,4-diethyl-3-pyrrolin-2-one (4)⁸ or its dimethyl analog (5)⁹, as outlined in Scheme 1. Pyrrole dialdehyde 3^7,10 was prepared from the known ethyl 3,4-diethyl-1H-pyrrole-2-carboxylate (6)¹¹ in 71% yield by reaction of carboxylic acid 7 with triethyl orthoformate in the presence of trifluoroacetic acid.⁷ Accordingly, we report herein improved procedures for the syntheses of 3-7: preparing 6 and 7 and converting 7 to 3, 8^8,12 to 4, and 3,4-dimethyl-1H-pyrrole to 5.⁹ Pyrrole dialdehyde 3 was condensed¹³ with excess pyrrolinone, catalyzed by piperidine in ethanol solvent (sealed tube) at 100°C. The reaction proceeds through the monocondensation product, 8 or 9, which reacts further to afford either 1 or 2. The progress of the reaction is monitored by following the disappearance of 8 or 9, which typically takes about 30 hours. Alternatively, and more conveniently on a larger scale, the use of the sealed tube can be dispensed with by carrying out the reaction in refluxing p-dioxane: with 3 + 4 a 57% yield of 1 + 10% of 10 was obtained after 6 days. When a 2:1 ratio of pyrrolinone 4:dialdehyde 3 was used in the sealed tube reaction, with a short reaction time, an 87% yield of dipyrrinone aldehyde 10¹⁴ was obtained.

Scheme 1.

Because of its better solubility and the ease in which 1 gave crystals suitable for X-ray studies, most of what follows in sections 2.3-2.6 was conducted with 1.

2.2 Structures and NMR Spectroscopy

The constitutional structures of 1 and 2 follow from the method of synthesis¹³ and the structures of the starting materials.^7-12 The symmetry of 1 and 2 are recognizable by the (reduced) number of carbon signals (Table 1), and good correlation between the observed chemical shifts and those of the dipyrrinone analogs.^8,15 Similar good correlations may be found in the ¹H-NMR spectra of Table 1, which also reveals shifted pyrrole and lactam signals in CDCl₃ and (CD₃)₂SO solvents. The deshielded signals relative to those found in monomeric dipyrrinones (lactam NH: 7.8-7.0 ppm; pyrrole NH: 8.1-7.8 ppm)¹⁶ point to hydrogen bonding interactions, with the solvent in (CD₃)₂SO and from intermolecular hydrogen bonding in CDCl₃. The data from (CD₃)₂SO differ somewhat from those of typical diyrrinones in that the lactam NH chemical shift of 1 is ∼0.3-0.4 ppm more deshielded, while the pyrrole NH chemical shift is quite comparable to values found in typical dipyrrinones.¹⁵ The NH chemical shifts of 1 in (CD₃)₂SO are consistent with hydrogen bonding to solvent.^16,17 The chemical shifts of 1 and 2 in CDCl₃ are more like those found in bilirubin and semirubins (with the pyrrole and lactam NH signals at ∼10.7 and 9-9.8 ppm, respectively)^12b,14c,17 than they are like dipyrrinone dimers (pyrrole and lactam NHs at ∼10.5 and ∼11 ppm, respectively) or rubin ester dimers (pyrrole and lactam NHs at ∼10.3 and ∼10.6 ppm respectively).^17,18 The data for 1 in CDCl₃ thus clearly point to intermolecular hydrogen bonding, but one where the lactam NH···O=C hydrogen bond is slightly weaker than in an intermolecularly hydrogen bonded dipyrrinone dimer, and/or a dimer in which the lactam NH is located in the shielding core of an aromatic ring or π-bond.

Table 1.

¹H- and ¹³C-NMR chemical shifts^a of tripyrrindiones 1 and 2.

		13C-NMR			1H-NMR
Carbon		1		2	1		2
		CDCl3	(CD3)2SO	CDCl3	CDCl3	(CD3)2SO	CDCl3
1/14	C=O	173.8	172.6	174.4	?	?	?
2/13	=C-	132.6	132.2	132.6	?	?	?
21	CH2CH3	17.7	17.7	8.3	2.18 (q)b	2.27 (q)b	1.72 (s)
22	CH3	13.8	14.6	?	0.95 (t)b	1.12 (q)b	?
3/12	=C-	146.8	146.9	141.4	?	?	?
31	CH2/CH3	17.7	17.6	9.9	2.46 (q)b	2.48 (q)b	2.07 (s)
32	CH3	15.6	16.4	?	1.17 (t)b	1.02 (t)b	?
4/11	=C-	131.1	131.0	130.3	?	?	?
5/10	=CH-	98.8	97.6	98.8	6.04 (s)	5.93 (s)	6.03 (s)
6/9	=C-	130.1	129.7	126.1	?	?	?
7/8	=C-	128.7	128.2	128.4	?	?	?
71	CH2	16.8	17.2	17.6	2.56 (q)b	2.50 (q)b	2.53 (q)b
72	CH3	16.5	17.1	16.5	1.19 (t)b	1.04 (t)b	1.17 (t)b
15/17	NH	?	?	?	9.73 (brs)	10.10 (brs)	9.5 (brs)
16	NH	?	?	?	10.46 (brs)	10.73 (brs)	10.6 (brs)

^aChemical shifts in δ, ppm for 10^-3 M solutions at 22°C.

^bJ = 7.5 Hz.

Nuclear Overhauser Effect (NOE) studies of 1 in CDCl₃ and (CD₃)₂SO revealed a close proximity of the C(5) hydrogen to the flanking lactam ethyl (CH₂ and CH₃) hydrogens at C(3¹) and C(3²) [by symmetry: the C(10) hydrogen to the C(12¹) and C(12²) ethyl hydrogens, thereby indicating the Z-configuration at the C(4) and C(10) exocyclic carbon-carbon double bonds]. Strong NOEs were also found between the lactam and pyrrole NHs, and between the C(5)/C(10) hydrogens and the flanking pyrrole ethyl groups, confirming a syn-Z conformation, as shown in Fig. 2.

Figure 2.

Nuclear Overhauser Effects (NOEs), indicated by curved arrows, seen in 1 in both CDCl₃ and (CD₃)₂SO solvents. Similar NOEs are found in typical dipyrrinones, such as methyl xanthobilirubinate.

2.3 Molecularity in Solution

The suspected dimerization of 1, as revealed by ¹H-NMR NH chemical shifts was further explored by vapor pressure osmometric measurement of 1 in CHCl₃ solution. Calibrated vs a measured molecular weight of 214 ±11 g/mol of benzil (FW = 210.2 g/mol) in CHCl₃ a MW for 1 (FW = 421.6 g/mol) was determined to be 831 ±30 g/mol in ∼10^-3 M solution ? or very nearly twice the molecular weight. These data, together with the ¹H-NMR data, are consistent with complete dimer formation held together by intermolecular hydrogen bonding.

In order to explore aspects of the dimerization of 1, we examined the NH chemical shifts at increasing dilution. Of course, no variation is found in (CD₃)₂SO over the concentration range 1.0 × 10^-3 to 4.5 × 10^-5. However, in CDCl₃, over the range 8.2 × 10^-4 to 4.1 × 10^-5, only a small (∼0.1 ppm) upfield shift is observed (Table 2). The latter contrasts markedly with data for the dipyrrinone methyl xanthobilirubinate (Fig. 2) which also is dimeric in CDCl₃ but shows an order of magnitude larger (∼1 ppm) upfield shift.¹⁶ Given that the association constant K_A for dimerization of methyl xanthobilirubinate at room temperature was determined to be 25-30,000 M^-1, it is clear that tripyrrindiones must have a much larger K_A.

Table 2.

Concentration dependence of pyrrole and lactam NH chemical shifts of 1 in (CD₃)₂SO and in CDCl₃ at 22°C compared with similar data for methyl xanthobilirubinate (XBR-Me).

Pyrrole (P) and Lactam (L) Chemical Shifts (δ, ppm)
Conc. of 1 (mM)	(CD3)2SO		Conc of 1 (mM)	CDCl3		Conc. of XBR-Me (mM)	CDCl3
	PNH	LMH		PNH	LNH		PNH	LNH
0.8907	10.084	10.215	0.8172	9.712	10.302	0.9766a	10.136	10.933a
0.4453	10.084	10.215	0.4086	9.686	10.283	0.4883	10.048	10.765
0.2227	10.084	10.215	0.2643	9.662	10.257	0.1808	9.700	10.371
0.0445	10.084	10.215	0.0409	9.584	10.186	0.09039	9.535	10.200

^aThe data plateau at higher concentrations, 0.125 M; PNH: 10.401, LNH: 11.334.

In further support of dimeric association through hydrogen bonding in nonpolar solvents, we observed small deviations from Beer’s Law¹⁹ in CHCl₃ and n-hexane but no deviations in (CH₃)₂SO.

2.4 X-ray Crystal Structure

Further support for the constitutional structure of 1 comes from X-ray crystallography, which confirms the syn-Z conformation as well as the hydrogen-bonded dimer (Fig. 3). In the dimer, each tripyrrindione adopts a helical conformation by twisitng about the C(5)-C(6) and C(9)-C(10) single bonds by ∼20°, and the pitch of the helix is ∼ 2.35 Å. In order to fit together in the dimer, the two tripyrrindiones adopt the same helicity: M binds to M, P to P; thus, each dimer is chiral. The unit cell includes four dimer pairs (8 tripyrrindiones; 4 M and 4 P). The bond angles and bond distances (Fig. 4) are very similar to those found in the X-ray crystal structure of 7,9-dimethyl-2,3,8-trimethyl-10H-dipyrrinone,²⁰ which is found as a hydrogen bonded dimer in the crystal, with planar monomers.

Figure 3.

Structural drawing of intermolecularly hydrogen-bonded dimeric 1 in the crystal showing the atom numbering system; hydrogens are deleted for clarity of representation.

Figure 4.

Bond distances (Å) and angles (°) found in the X-ray crystallographic structure of tripyrrindione 1 hydrogen-bonded dimer, with numbering system used. Hydrogen bonding distances are indicated by dashed lines, and the following related main atom distances were found: O1-pyrrole N2 (3.19 Å), O1-lactam N3 (2.81 Å), 02-lactam N1 (2.84 Å). In a typical dipyrrinone dimer X-ray crystal structure, 7,9-dimethyl-2,3,8-triethyl-10H-dipyrrinone (ref. ²⁰), exhibits lactam O to pyrrole N and lactam O to lactam N distances: 2.89 Å and 2.86 Å, corresponding to pyrrole NH···O=C lactam distances of 2.09Å and a lactam NH···O=C lactam distance of 2.09 Å and a lactam NH···O=C lactam distance of 1.97 Å. The bond lengths and bond angles are nearly identical.

2.5 Conformation from Molecular Dynamics Calculations

In support of the helical tripyrrindione structure found in the crystal, and consistent with the NMR spectroscopic analysis of pigment conformation, molecular dynamics calculations²¹ of 1 show a helix with comparable torsion angles about the C5-C6 and C9-C10 single bonds: C4-C5-C6-N2 ≅ N2-C9-C10-C11 ∼26°. The same sign of the angles indicates twisting in opposite directions so as to produce the helical structure. There is very little twisting about the exocyclic C=C bonds: N1-C4=C5-C6 ≅ C9-C10=C11-N3 ∼2°. However, an isoenergetic symmetric structure with C4-C5-C6-N2 and N2-C9-C10-C11 torsion angles ∼27° and only ∼-27° deviation from planarity in the N1-C4=C5-C6 and C9-C10=C11-N3 is also found. The calculations indicate conformational mobility in the lactam rings flipping up and/or down relative to the pyrrole ring. The helical structure is required for formation of the hydrogen-bonded dimer.

2.6 Optical spectroscopy

The UV-visible spectral data for 1 in solvents with a wide range of polarity are given in Table 3. Characteristic of orange-red colored solutions, an intense and broad long wavelength absorption band is found near 500 nm, with an equally intense but narrower band near 325 nm. The long wavelength band is characteristic of two overlapping electronic transitions, which becomes more apparent when one views the actual spectra (Fig. 5). The shape of the long wavelength band is reminiscent of exciton splitting seen in bilirubinoids and dipyrrinone dimers. And while one might view a dimeric tripyrrindione to be a molecular exciton, this is more likely to be seen in nonpolar solvents than in a solvent like (CH₃)₂SO, in which dimerization is not evident but which is where the band separation is most recognizable ? suggesting a non-excitonic origin. The data suggest overlapping transitions of an as yet unclear origin.

Table 3.

Solvent dependence of the UV-visible spectral data for tripyrrindione 1.^a

λmax / nm (ε) /L?mol-1?cm-1
Hexane	Benzene	CHCl3	CH3CN	CH3OH	(CH3)2SO
498 (32,800)	496 (23,600)	488 (25,500)	471 (27,300)	479 (31,000)	484 (31,700)
331 (30,300)	326 (32,800)	325 (31,100)	319 (32,300)	321 (29,200)	323 (30,900)

^aSolutions are ∼1 × 10^-5 M in 1 and contain 2% CHCl₃.

Figure 5.

Solvent dependence of the UV-visible spectra of 1 at ∼1 × 10^-5 M at 23°C. The curves are numbered corresponding to the solvents used: 1) methanol; 2) chloroform; 3) acetonitrile; 4) trifluoroacetic acid; 5) hexane; 6) dimethylsulfoxide; 7) benzene.

We observed pronounced UV-visible spectral changes in 1, especially with the ∼490 nm long wavelength band. Upon addition of TFA, the solution becomes more red as the long wavelength band bathochromically shifted and becomes less broad so that in pure TFA, λ_max appears near 530 nm and ε rises to ∼53,000 (Fig. 6). The shorter wavelength band near 325 nm is barely changed. An even more striking effect is found upon addition of base. The organic base DBU was particularly effective (Fig. 6) in shifting the long wavelength band, which is well-separated into two bands in neat DBU. Solutions become blue and a pink fluorescence is observed. The data are summarized in Table 6 and might be interpreted in terms of a protonated lactam in TFA and a lactim enolate ion in DBU.

Figure 6.

Influence of acid (curve 3: trifluoroacetic acid, neat) and base (curve 2: DBU, 1,8-diazabicylclo[5.4.0]undec-7-ene, neat) on the UV-visible spectra of 1, compared with curve 1: CHCl₃ for ∼ 1 × 10^-5 M solutions at 23°C.

Fischer and coworkers reported a color shift in their tripyrrindiones (Fig. 1B) upon addition of Zn(OAc)₂ to alcohol solutions of the pigment.^1,2 Thus, upon addition of alcoholic Zn(OAc)₂ to an alcoholic solution of their tripyrrindiones the color shifted from violet to blue, but showed no fluorescence. Upon addition of ammonium hydroxide it became yellow and intensely fluorescent. Our pigment 1 behaved in a similar way (Fig. 7). The bathochromic shift, blue coloration and absence of fluorescence upon addition of Zn(OAc)₂ are qualitatively similar to those found from 1 in DBU. The shift to yellow and strong fluorescence upon addition of NH₄OH to the solution is again similar. However, attempts to isolate the Zn salt or its product following addition of NH₄OH have been unsuccessful. We assume that the zinc complexes with a lactim tautomer of 1, as it does with dipyrrylmethenes.

Figure 7.

Influence of added zinc acetate (curve 2) followed by added conc. NH₄OH (curve 3) on the UV-visible spectrum of 1 in CH₃OH (curve 1) at ∼1 × 10^-5 M concentration and 23°C.

3. Concluding Comments

Tripyrrindione 1, synthesized smoothly from monopyrrole components, adopts an all-syn-Z lock-washer-like conformation in solution and in the crystal. In chloroform solution, 1 is dimeric, with strong evidence for intermolecular hydrogen bonding. In the crystal, 1 forms intermolecularly hydrogen-bonded pairs of M to M and P to P helical dimers.

4. Experimental Section

4.1. General Procedures

Nuclear magnetic resonance (NMR) spectra were obtained on a Varian Unity Plus 500 MHz spectrometer in CDCl₃ solvent (unless otherwise specified) at 25°C. Chemical shifts were reported in δ ppm referenced to the residual CHCl₃ ¹H signal at 7.26 ppm and ¹³C signal at 77.0 ppm. A combination of heteronuclear multiple bond correlation (HMBC) spectra and ¹H{¹H} NOE data were used to assign ¹H and ¹³C NMR spectra. UV-visible spectra were recorded on a Perkin-Elmer Lambda-12 spectrophotometer. Melting points were taken on a Mel Temp capillary apparatus and are corrected. Combustion analyses were carried out by Desert Analytics, Tucson, Az. Analytical thin layer chromatography was carried out on J. T. Baker silica gel IB-F plantes (125 μ layers). Radial chromatography was carried out on Merck silica gel PF₂₅₄ with gypsum preparative layer grade, using a Chromatotron (Harrison Research, Palo Alto, CA). Spectral data were obtained in spectral grade solvents (Aldrich or Fisher). Deuterated chloroform and dimethylsulfoxide were from Cambridge Isotope Laboratories. 3,4-Diethyl-1H-pyrrole (6)¹¹ and 3,4-dimethyl-1H-pyrrole (7)^22,23 were prepared as reported previously from ethyl 3,4-diethyl-1H-pyrrole-2-carboxylate¹¹ and ethyl 3,4-dimethyl-1H-pyrrole-2-carboxylate,^22,23 respectively.

4.1.1. Ethyl 3,4-Diethyl-1H-pyrrole-2-carboxylate (6)

The following is an improved procedure, which avoids the use of DBU base.^11,12b 3-Acetoxy-4-nitrohexane^11,12b (2.4 g) and ethyl isocyanoacetate^11,12b,24 (1.4 g) were placed in a 100-mL round bottom flask together with a 1:1 mixture of THF and ethanol (30 mL). Anhydrous potassium carbonate (3.5 g) was added in portions while the mixture was stirred vigorously. The reaction mixture was then stirred at room temperature for 3 days. After ascertaining that the reaction was complete (TLC), the mixture was poured onto water (100 mL), acidified to pH 5 with 5% HCl, and extracted with diethyl ether (4 × 25 mL). The solvent was removed at reduced pressure (rotovap) after drying over Na₂SO₄. The resulting oil was passed through a short column of silica gel using 1% CH₃OH in CH₂Cl₂ as eluent. The eluate was collected, and the solvent was evaporated to yield a brown oil (one spot on TLC, eluent CH₂Cl₂): 2 g, 81% [lit.¹¹ oil]. It had ¹H-NMR (CDCl₃) δ: 1.16 (t, J = 6.9 Hz, 3H), 1.2 (t, J = 7.7 Hz, 3H), 1.36 (t, J = 7.7 Hz, 3H), 2.5 (q, J = 7.7 Hz, 2H), 2.8 (q, J = 7.7 Hz, 2H), 4.3 (q, J = 6.9 Hz, 2H), 6.7 (d, J = 2.9 Hz, 1H), 9.1 (bs, 1H) ppm; ¹³C-NMR (CDCl₃) δ: 14.3 (q), 14.8 (q), 15.3 (q), 17.9 (t), 18 (t), 59.6 (t), 118.5 (s), 119 (d), 126 (s), 132 p (s) ppm.

4.1.2. 3,4-Diethyl-3-pyrrolin-2-one (4)

The procedure described below provides 4 in twice the yield of converting 8 → 4 as previously reported.⁸ Pyrrole 6 (2.0 g, 10.3 mmol) was dissolved in ethanol (20 mL), then a solution of NaOH (1.2 g) in water (8 mL) was added, and the mixture was heated at reflux overnight. The volume was reduced under vacuum (rotovap), and the resulting solid was dissolved in a minimum amount of water and acidified with 5% HCl to pH 5 at 0°C. The precipitated carboxylic acid (7) was quickly filtered, washed with water, and air-dried overnight to yield 1.5 g (88%) of almost white, slightly pink solid with ¹H-NMR (CDCl₃) δ: 1.03 (t, J = 7.7 Hz, 3H), 1.1 (t, J = 7.7 Hz, 3H), 2.34 (q, J = 7.7 Hz, 2H), 2.63 (q, J = 7.7 Hz, 2H), 6.64 (d, J = 2.9 Hz, 1H), 11.06 (s, 1H), 11.96 (s, 1H) ppm; ¹³C-NMR (CDCl₃) δ: 15.37 (q), 16.1 (q), 17.8 (t), 118.7 (s), 119.6 (d), 125.1 (s), 131 (s), 162.7 (s) ppm. The acid was taken directly to the next step.

Pyrrole acid 7 (4.0 g, 24 mmol) was added to water (100 mL) and then steam distilled. The distillate was extracted with ether (5 × 20 mL) and dried over Na₂SO₄. The ether solution was filtered and then evaporated at reduced pressure to yield 2.0 g (68%) of 8¹² as a yellow oil that is very sensitive to air and light; ¹H-NMR (CDCl₃) δ: 1.23 (t, J = 7.7 Hz, 6H), 2.5 (q, J = 7.7 Hz, 4H), 6.56 (d, J = 2.5 Hz, 2H), 8 (bs, 1H) ppm; ¹³C-NMR (CDCl₃) δ: 14.4 (q), 18.25 (t), 114.2 (d), 124.4 (s) ppm. 3,4-Diethylpyrrole 8 was used directly in the next step.

3,4-Diethyl pyrrole 8 (0.5 g, 4.1 mmol) was heated at reflux with H₂O₂ (1 mL), dry pyridine (1 mL) and methanol (about 0.3 mL) for about 25 min; then more H₂O₂ (0.5 mL) was added, and the mixture was heated at reflux for a short period (until no more starting material was detected by TLC: eluent 1% MeOH:99% CH₂Cl₂). After the reaction was complete, the solvent volume was reduced under vacuum (rotovap), and the residue was dissolved in diethyl ether, washed with water and dilute (3%) HCl, followed by sat. NaCl. The ether layer was evaporated (rotovap) to yield 0.44 g (80%) of the pyrrolinone product 4 as yellow oil.⁸ It had ¹H-NMR (CDCl₃) δ: 1.1 (m, 6H), 2.3 (q, J = 7.7 Hz, 2H), 2.42 (m, 2H), 3.84 (s, 2H), 6.8 (bs, 1H) ppm. GCMS, m/z: 139, 124, 110, 96, 92, 82, 67, 55 amu.

4.1.3. 3,4-Diethyl-2,5-diformyl-1H-pyrrole (3)

The procedure reported below gives a higher yield of 3 than that reported earlier.⁷ Pyrrole acid 7 (0.5 g, 3.0 mmol) was dissolved in TFA (20 mL) at room temperature in the dark; then triethyl orthoformate (20 mL) was added dropwise at a rate such that the temperature remained close to room temperature. The mixture was then stirred for about 1 h (or until the reaction was complete); then the solvent volume was reduced (rotovap) and the remaining liquid was poured onto water and neutralized with sat. aq. NaHCO₃. After ether extraction (5 × 10 mL), followed by evaporation of the solvent (rotovap), the solid, yellowish product (3) was found to be pure. The yield was 0.38 g (71%), mp 105-107°C [lit.⁷ mp 106°C]. The ¹H- and ¹³C-NMR matched those previously reported.⁷

4.1.4 2,3,7,8,12,13-Hexaethyl-15H,17H-tripyrrin-1,14-dione (1)

Procedure A

3,4-Diethyl-3-pyrrolin-2-one (1.40 g, 10.0 mmol) was added to 3,4-diethyl-2,5-diformyl-1H-pyrrole (0.300 g, 1.68 mmol) dissolved in 20 mL of p-dioxane. Piperidine (30 drops) was then added slowly, and the reaction was heated at reflux under N₂ for 7 days. After cooling to room temperature, the solvent was evaporated in vacuo (rotovap) and the residue was taken up in diethyl ether (100 mL). The aqueous layer contained the side product dipyrrinone (10) which was set aside for further purification. The organic layer was washed with 5% aq. HCl, saturated aq. NaHCO₃, water and brine and dried over Na₂SO₄. The ether was evaporated, and the residue was separated using radial chromatography, 5% MeOH in CH₂Cl₂ as eluent to afford 339 mg (48%) of 1, mp 215-217°C; IR (film) ν: 3228, 2968, 2934, 2874, 1770, 1709, 1676 cm^-1; NMR data in Table 1.

Anal. Calcd for C₂₆H₃₅N₃O₂ (421.6): C, 74.07; H, 8.37; N, 9.97.

Found: C, 73.84; H, 8.46; N. 9.85.

Procedure B

3,4-Diethyl-3-pyrrolin-2-one 4 (0.28 g, 1.93 mmol) was dissolved in 3 mL of a 1:1 mixture of absolute ethanol and CH₂Cl₂ in a glass pressure tube, to which piperidine (0.1 mL, 1 mmol) was added. The pressure tube reaction was immersed in a steam bath, and a solution of 3,4-diethyl-2,5-diformyl-1H-pyrrole 3 (0.088 g, 0.49 mmol) in CH₂Cl₂ (about 1 mL) was added dropwise into the reaction mixture. The progress of the reaction was checked periodically by TLC. When there were no starting materials (colorless) detected, and the yellow-green spot corresponding to dipyrrinone 10 was minimal in size (usually after about 24 h), the reaction was worked up as follows. The solvent was removed (rotovap), and the residue was dissolved in CH₂Cl₂, washed with 5% HCl, followed by 3 washings with sat. NaHCO₃, then sat. NaCl. The resulting CH₂Cl₂ solution was dried over Na₂SO₄, filtered, and evaporated at reduced pressure. The residue was purified by radial chromatography using as eluent a mixture of CH₃OH and CH₂Cl₂ with the concentration of CH₃OH being increased gradually from 1% to 15%. The procedure yielded 115 mg (57%) of a dark red, almost black solid (1) that was pure by TLC and had mp 215-219°C; NMR in Table 1, and FAB-MS: calcd. for C₂₆H₃₅N₃O₂ (421.2729), found: 421.270 [M⁺].

4.1.5. 9-Formyl-2,3,7,8-tetraethyl-10H-dipyrrinone (10)

The following procedures differ from that reported earlier,¹⁴ which added the 9-formyl group to the 9H-dipyrrinone.

Procedure A

The aqueous layer from Procedure A for the synthesis of 1 was allowed to evaporate and the residue was taken up in CH₂Cl₂, washed with 5% aq. HCl, sat. aq. NaHCO₃, water and brine and dried over Na₂SO₄. Dipyrrinone 10 was isolated by radial chromatography (5% CH₃OH in CH₂Cl₂ eluent) to yield 51 mg (10%), with mp 162-163°C. It had IR (film) ν: 3434, 2966, 2945, 1769, 1708, 1673, 1602 cm^-1; ¹H-NMR (CDCl₃) δ: 1.13-1.28 (m, 12H), 2.45 (q, J = 7.6 Hz, 2H), 2.56 (m, 4H), 2.8 (q, J = 7.6 Hz, 2H), 5.97 (s, 1H), 9.7 (s, 1H), 10.4 (s, 1H), 10.75 (s, 1H) ppm; ¹³C-NMR (CDCl₃) δ: 13.8 (q), 15.45 (q), 16.29 (q), 16.9 (t), 17.1 (t), 17.15 (t), 17.36 (q), 17.8 (t), 94.8 (d), 128.8 (s), 130 (s), 130.2 (s), 131.7 (s), 134.2 (s), 135 (s), 146.8 (s), 171.9 (s), 177.4 (d) ppm.

Procedure B

3,4-Diethyl-2,5-diformyl pyrrole 3 (0.086 g, 0.48 mmol) and 3,4-diethyl-3-pyrrolin-2-one 4 (0.135 g, 0.96 mmol) were dissolved in 3 mL of absolute ethanol in a resealable glass pressure tube (Ace Glass); then piperdine (0.1 mL, 1 mmol) was added. The tube was then sealed and put into a steam bath. The reaction progress was monitored from time to time by TLC, 3% CH₃OH:97% CH₂Cl₂ eluent. When there was no trace of the starting material, and the only observed spot was a yellow-green one for 10 (usually within 1 h), the reaction was worked up as follows. Ethanol was removed by vacuum (rotovap), and the residue was dissolved in CH₂Cl₂, washed with 5% HCl, followed by 3 washings with sat. NaHCO₃, then sat. NaCl. The resulting CH₂Cl₂ solution was dried over Na₂SO₄, filtered, and evaporated at reduced pressure (rotovap). The residue was passed through a short plug of silica gel, using CH₂Cl₂ as eluent, to afford 125 mg (85%) of a crystalline yellow-green solid, which was pure by TLC and had mp 161-169°C. FAB-MS: calcd. for C₁₈H₂₄N₂O₂ (300), found: 300 [M⁺].

4.1.6 Ethyl 3,4-Dimethyl-1H-pyrrole-2-carboxylate

The following is a different, higher yield synthesis than that reported earlier.^22,23 2-Acetoxy-3-nitrobutane (19 g, 0.12 mol) and ethyl isocyanoacetate (13.3 g, 0.12 mol) were placed in a 1-L round bottom flask together with a 1:1 mixture of THF and ethanol (300 mL). Potassium carbonate (35 g) was added in portions while the mixture was stirred vigorously. The reaction was stirred at room temperature for 2 days, while the reaction progress was checked by TLC (eluent: 1% CH₃OH:99% CH₂Cl₂). At completion the reaction mixture was poured into water, acidified to pH 5 with 5% HCl, and extracted with diethyl ether (4 × 75 mL). The solvent was removed under reduced pressure after drying over Na₂SO₄, and the solid residue was passed through a short column of silica gel (1% CH₃OH eluent). The eluate was collected, and the solvent was evaporated (rotovap) to give the desired product, which was recrystallized from CH₃OH-H₂O to yield a white crystalline solid, 17.3 g (88%) that showed one spot on TLC (CH₂Cl₂ eluent). It had mp 91-92°C (lit.²³ mp 90-91 °C). The ¹H- and ¹³C-NMR data matched those previously reported.²³

4.1.7 3,4-Dimethyl-1H-pyrrole (9)

Ethyl 3,4-dimethyl-1H-pyrrole-2-carboxylate from above (4.7 g, 28 mmol) was dissolved in ethanol (40 mL), then a solution of NaOH (3.5 g) in water (15 mL) was added, and the mixture was heated at reflux overnight. After 16 h the reaction was complete, the volume was reduced (rotovap), and the resulting solid was dissolved in a minimum amount of water and acidified at 0°C with 5% HCL to pH 5. The precipitated product was filtered quickly, washed with water and air dried overnight to yield 3.9 g (100%) of a white solid. It had ¹H-NMR (CDCl₃) δ: 1.9 (s, 3H), 2.15 (s, 3H), 6.65 (d, 1H), 11.0 (s, 1H), 12.0 (s, 1H) ppm; ¹³C-NMR (CDCl₃) δ: 9.6 (q), 118.4 (s), 118.7 (s), 120.4 (d), 124.7 (s), 162.2 (s) ppm and was used directly in the next step.

The pyrrole acid (3.9 g, 28 mmol) was added to water, heated to boiling and ∼500 mL of water was co-distilled with decarboxylated pyrrole. The distillate was extracted with ether (5 × 75 mL) and dried over Na₂SO₄. The ether solution was filtered and evaporated (rotovap) to yield 1.7 g (65%) of a white solid that is very sensitive to air and light. It had mp 30-31°C (lit.²³ mp 30-31°C)..

4.1.8. 3,4-Dimethyl-3-pyrrolin-2-one (5)

3,4-Dimethyl-1H-pyrrole (1.7 g, 18 mmol) was heated at reflux with H₂O₂ (4 mL), dry pyridine (4 mL) in CH₃OH (10 mL) for about 15 min; then more H₂O₂ (4 mL) was added, and the mixture was heated at reflux for a short while (until no more starting material was detected by TLC, 1% CH₃OH:99% CH₂Cl₂ as eluent). The solvent volume was reduced (rotovap), and the residue was dissolved in diethyl ether, washed with water and dilute (3%) HCl, followed by sat. aq. NaCl. The ether was evaporated (rotovap) to yield 1.7 g (95%) of the desired product (5) as white, low melting crystals, mp 93-94°C (lit.²³ mp 93-95°C). It had ¹H-NMR (CDCl₃) δ: 1.8 (s, 3H), 2.0 (s, 3H), 3.8 (s, 2H), 6.25 (bs, 1H) ppm.

4.1.9. 2,3,12,13-Tetramethyl-7,8-diethyl-15H,17H-tripyrrin-1,14-one (2)

3,4-Dimethyl-3-pyrroline-2-one (5) (0.21 g, 1.9 mmol) was dissolved in 3 mL of a 1:1 mixture of absolute ethanol and CH₂Cl₂ in an unsealed glass pressure tube, to which piperidine (0.1 mL, 1 mmol) was added, and the reaction tube was immersed into a steam bath. A solution of 2,5-diformyl-3,4-diethyl-1H-pyrrole (3) (0.085 g, 0.47 mmol) in CH₂Cl₂ (∼1 mL) was added dropwise into the reaction mixture, and the reaction progress was monitored by TLC (3% CH₃OH:97% CH₂Cl₂ eluent). When no more starting materials (colorless) were no longer detected, and the yellow-green spot due to dipyrrinone was minimal in size (usually after about 30 h), the reaction was worked up as follows. The solvent was removed (rotovap), and the residue was dissolved in CH₂Cl₂, washed with 5% aq. HCl, followed by 3 washings with sat. aq. NaHCO₃, then sat. aq. NaCl. The resulting CH₂Cl₂ solution was dried over Na₂SO₄, filtered and evaporated at reduced pressure. The resulting solid mixture was purified by radial chromatography using gradually increasing concentration of CH₃OH in CH₂Cl₂ as eluent to afford 102 mg (59%) of a dark red, almost black solid, mp 282-284°C, that was pure by TLC. It had ¹H-NMR (CDCl₃) δ: 1.17 (t, J =7.5 Hz, 6H), 1.72 (s, 6H), 2.07 (s, 6H), 2.53 (q, J ∼7.5 Hz, 4H), 6.03 (s, 2H), 9.5 (bs, 1H), 10.6 (bs, 2H) ppm; ¹³C-NMR (CDCl₃) δ: 8.3 (q), 9.87 (q), 16.48 (q), 17.6 (t), 98.8 (d), 126.1 (s), 128.4 (s), 130.3 (s), 132.6 (s), 141.4 (s), 174.35 ppm; FAB-MS: calcd. for C₂₂H₂₇N₃O₂ (365.2103), found: 365.210 [M⁺] amu.

4.2 X-ray Structure and Solution

Crystals of 1 were grown by slow evaporation of CH₂Cl₂. A crystal was placed into the tip of a 0.1 mm diameter glass capillary and mounted on a Bruker SMART Apex system for data collection at 100(2) K. A preliminary set of cell constants was calculated from reflections harvested from 3 sets of 20 frames for 1. These intial sets of frames were oriented such that orthogonal wedges of reciprocal space were surveyed (final orientation matrices determined from global least-squares refinement of 3524 reflections). The data collection was carried out using MoKα radiation (0.71073 Å graphite monochromator) with a frame time of 20 s and a detector distance of 4.94 cm. A randomly oriented region of reciprocal space was surveyed to the extent of 2 hemispheres and to a resolution of 0.66 Å. Four major sections of frames were collected with 0.5° steps in ω at 600 different ϕ settings and a detector position of 27° in 2θ. The intensity data were corrected for absorption and decay (SADABS).²⁵ Final cell constants were calculated from the xyz centroids of strong reflections from the actual data collection after integration (SAINT 6.45, 2003).⁹ Crystal data and refinement information for 1 may be found in Table 5.

Table 5.

Crystal data and structural refinement for 1.

Empirical Formula	C27H37Cl2N3O2
Formula Weight	506.50
Temperature	100 (2) K
Wavelength	0.71073 Å
Crystal System	Monoclinic
Space Group	C2/c
Unit Cell Dimensions	a = 26.878(6) Å   α = 90°
	b = 13.650(3) Å   β = 120.198°
	c = 17.082(5) Å   γ = 90°
Volume	5417(2)
Z	8
Density	1.242 Mg/m3
Absorption Coefficient	0.268 mm-1
F(000)	2160
Crystal Size	0.20 × 0.10 × 0.03 mm3
Theta range for data collection	1.73 to 22.50°
Index ranges	-28<h<28, -14<k<13, -18<l<18
Reflections collected	3524
Independent reflections	3524 [R(int) = 0.1275]
Completeness to theta = 22.50°	99.2%
Absorption correction	SADABS
Max. and min. transmission	0.9920 and 0.9484
Refinement method	Full matrix least-squares on F2
Data \ restraints \ parameters	3524 \ 41 \ 350
Goodness-of-fit on F2	1.061
Final R indices [I>2 signal (I)]	R1 = 0.0658, wR2 = 0.1206
R indices (all data)	R1 = 0.1889, wR2 = 0.1423
Largest diff. peak and hole	0.518 and -0.462 Å-3

The structure was solved and refined using SHELXTL.²⁷ The monoclinic space group C2/c was determined based on systematic absences and intensity statistics. A direct-methods solution was calculated which provided most non-hydrogen atoms from the E-map. Full-matrix least squares/difference Fourier cycles were performed for structure refinement. All non-hydrogen atoms were refined with anisotropic displacement parameters unless stated otherwise. Hydrogen atom positions were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters (a C-H distance fixed at 0.96 Å and a thermal parameter 1.2 times the host carbon atom). Tables of atomic coordinates, bond lengths and angles, anisotropic displacement parameters, hydrogen coordinates and isotropic displacement parameters have been deposited at the Cambridge Crystallographic Data Centre, CCDC No. 651743.

Table 4.

Influence of acid, base and zinc on the UV-visible spectra of 1.^a

λmax / nm (ε) /L?mol-1?cm-1
CF3CO2H	DBUb	CH3OH-Zn(OAc)2c	CH3OH-Zn(OAC)2 + NH4OHd
537 (52,900)	615 (41,300)	586 (20,200)	454 (13,000)
324 (28,100)	355 (24,000)	376 (19,300)	417 (14,000)
		312 (21,500)	276 (25,400)

^aSolutions are 1 × 10^-5 M in 1.

^bContains 385,000 equivs DBU.

^cContains 1 equivalent of Zn(OAc)₂.

^dWith 1 mole equivalent of added conc. NH₄OH.