Facile Synthesis of (R,R) and of (R,S) Tricarballylic Acid Anhydride and Imide Derivatives

Abstract

The diastereomeric mixture of (R)-2-(methoxycarbonylmethyl)-N-(R)-1-(1-phenylethyl) succinimide 11s and (S)-2-(methoxycarbonylmethyl)-N-(R)-1-(1-phenylethyl) succinimide 11a was synthesized by reaction of 2-(carboxymethyl)succinic anhydride 6 with (R)-(α)-methylbenzylamine in dry THF/room temperature/24 hrs. The diastereomeric mixture of  1-[(R)-(α)-Methylbenzylamideformyl)]propane-2,3-dicarboxylic acid anhydride 9s and  1-[(R)-(α)-methylbenzylamideformyl)]propane-2,3-dicarboxylic acid anhydride 9a was isolated as an intermediate under the reaction conditions. This diastereomeric mixture 9s/9a was also prepared by a different route via reaction of 1-(chloroformyl)propane-2,3-dicarboxylic acid anydride 12 with (R)-(α)-methylbenzylamine in the presence of DMA at 0°C for 24 hrs.

Introduction

Tricarballylic acid (TCA) 5 is found as a fragment in several mycotoxin natural products such as Fumonisin B₁ (1a), Fumonisin B₂ (2b), and AAL Toxin T_A (3) and a macrocyclic polycarboxylic acid which is an inhibitor of Ras Farnesyl-Protein Transferase (FPTase) [1] like Actinoplanic acid (4), (Figure 1). Fumonisins and AAL toxins are sphingosine analog mycotoxins characterized by tricar-ballylic acid side chains.[2] Most notably, fumonisins are known to be natural cancer promoters, and have been linked to human esophageal cancer in parts of China and southern Africa.[3] The occurrence and biological activity of the tricarballylic acid molecule has been studied extensively.[4]

It has been established that TCA 5 is produced by rumen bacteria and that it influences ruminant tissue metabolism.[5,6] Previously [7] we have described the synthesis of the 4-(5-nonyloxycarbonyl)-3-substituted butanoic acids and methyl esters like those found in Fumonisin B₁ (1a), B₂ (2b), AAL Toxin T_A (3) and Actinoplanic acid 4 (Figure 1) as a part of sphingosine analogs.

We now report the preparation of the diastereomeric mixture of (R)-2-(methoxycarbonyl-methyl)-N-(R)-1-(1-phenylethyl)succinimide 11s and (S)-2-(methoxycarbonylmethyl)-N-(R)-1-(1-phenylethyl)-succinimide 11a.

Results and Discussion

Ring-opening of 2-(carboxymethyl) succinic anhydride 6 with (R)-(α)-methylbenzylamine in dry THF (RT, 24 h) gave a white sticky material of diastereomeric and regioisomeric mixture of amides upon concentration (Scheme 1).

The ¹H NMR spectrum allowed us to suggest that the mixture of diastereomeric and regioisomeric amides were the α-amide and β-amide derivatives 7 and 8, respectively. NMR data for these structures 7 and 8 were consistent with the results described by Hoye et al.[8] for the determination of absolute configuration of 3-substituted carboxylic acids (β-chiral acids). The resulting products 7 and 8 were melted at 129-132°C, re-solidified by cooling, transferred into a NMR tube and finally analyzed. The spectrum shows doublet peaks downfield for (RCHc-R') and (RCHd-R') at δ 2.86 (dd, J = 6.86, 16.90 Hz) and δ 3.30 (dd, J = 9.75, 18.54 Hz) of the compound 9s, respectively. The ¹H NMR spectrum of 9a was virtually the same as for 9s with the following differences: δ (RCHc-R') and (RCHd-R') at δ 2.75 (dd, J = 9.76, 17.90 Hz) and δ 3.22 (dd, J = 9.93, 18.42 Hz) ppm. This was considered to be due to the anisotropic shielding effect of the aromatic ring of the phenyl group attached to the succinimide. The major product of the diastereomeric mixture of anhydride was assigned to be 9s (1.3:1). The intermediate compounds 9s/9a were re-melted at 158-160°C, and gave the expected assignment of the diastereomeric mixture of succinimide derivatives 10s/10a in a 1:1 ratio. The product was quite pure after spectroscopic analysis and no attempt was made to purify this diastereomeric mixture further. Esterification of the diastereomeric mixture 10s/10a was achieved with N,N'-dimethylformamide dimethylacetal in dry THF (RT, 24h). Concentration gave a white sticky material of diastereomeric mixture 11s/11a in a 1.2:1 ratio. The diastereomeric mixture was purified by MPLC (1:9 Hexane:EtOAc) to provide compounds (R)-2-(methoxycarbonylmethyl)-N-(R)-1-(1-phenylethyl)-succin-imide 11s and (S)-2-(methoxycarbonylmethyl)-N-(R)-1-(1-phenyl-ethyl) succinimide 11a in 82% yield. We have developed a reliable method for determining the absolute configuration of carboxylic acids containing a stereogenic center at C(3) of the tricarballylic acid derivatives.[8,9]. Figure 2 shows the application of this method to identify a heterotopic group attached to C(3) that contains readily distinguishable ¹H NMR resonances in the diastereomeric pair of 1-arylethylsuccinimides (e.g. the methoxycarbonyl methyl group in 11). The C(3)-configuration has been deduced by comparing the relative shifts of these resonances. For example, in the anti isomer 11 of the CH₂CO₂Me-succinimide of generic anti-(3S) diastereomer, the methoxyl resonance will be observed at higher field than in the syn -(3R) diastereomer.

Similarly, the ¹H NMR spectrum of compounds 11s/11a shows the (CO₂Me) of 11s as a singlet downfield (high chemical shift) at δ (3.66) and the 11a was a singlet upfield (low chemical shift) at δ (3.62), due to the anisotropic shielding effect of the aromatic ring of the phenyl group attached to the succinimide derivatives. We attempted to further separate these diastereomers and confirmed the present of two diastereomers by using gas chromatography at different temperatures (250, 270°C) and also with different retention times, but we obtained a broad peak which included a shoulder of the other diastereomer. HPLC was used for the separation of those diastereomeric mixture of 11s/11a, unfortunately those diastereomeric mixture was enriched to each other.

The intermediate compounds 9s and 9a were also prepared via a different route as shown in (Scheme 2), by treatment of the 1-(chloroformyl) propane-2,3-dicarboxylic acid anhydride 12 with (R)-α-methylbenzylamine in the presence of DMA and CH₂Cl₂ at 0°C. The reaction mixture was vigorously stirred at room temperature overnight to provide a good yield of the crude product which was purified further by flash chromatography (2:1 Hexane:EtOAc) to afford the diastereoisomeric mixture in 60% yield.

Experimental

General

¹H and ¹³C NMR spectra were taken on an IBM NR-200 or IBM NR-300-AF spectrometer. Chemical shifts are given in ppm (δ-scale), coupling constants (J) in hertz. ¹H-chemical shifts (300MHz) were referenced to the residual CHCl₃ signal (7.24). ¹³C-chemical shifts (75 MHz) were referenced to CDCl₃ (77.0). Optical rotations were measured on a Perkin Elmer 1420 ratio recording spectrometer, 1-2% solution in CHCl₃. MPLC refers to medium pressure liquid chromatography (25-60 psi) using hand packed columns of E. Merck silica gel (230-400 mesh) and a Fluid Metering Inc. pump, an ISCO Model 2361 gradient programmer, and a differential refractive index detector (Waters R401). Flash column chromatography was performed as described by Still [10] using E. Merck silica gel (230-400 mesh). The R_f value reported for TLC analysis was determined on Macherey-Nagel 0.25 mm layer fluorescent UV254 plates with the indicated solvent system. Infrared IR spectra listed as recorded ‘neat’ refer to a thin film of material on NaCl disks. Infrared spectra were recorded on a MIDAC Prospect spectrometer. IR spectrum peaks are reported in cm^-1. Elemental analyses were performed by M-H-W Laboratories (Phoenix, AZ) and the Central Lab.of Ain Shams University, Cairo, Egypt. Tandem gas chromatography/ low resolution mass spectrometry (GC/LRMS) using electron impact (EI) ionization was performed on a Hewlett-Packard 5890 series II gas chromatography and 5971A mass selective detector at 70 eV. Gas chromatography retention time is reported along with the capillary column configuration using the following convention: Column C = DB-5.6 m x 0.1 mm ID x 0.1 m film + 1 guard column with a 1 mL/in flow rate.

Propane-1,2,3-tricarboxylic Acid (1,2-Anhydride) 6

Tricarballylic acid 5 (1.76 g, 0.01 mol) was dissolved in acetic anhydride (1.78 mL, 2.04 g, 0.02 mol). The reaction mixture was stirred overnight at room temperature, then excess acetic anhydride was removed under reduced pressure and the product was recrystallized twice from ethyl acetate to give of the title compound as a white micro-crystalline solid (1.37 g, 0.0087 mol, 77.8% yield) that had mp. 132-133°C, lit.[11]: 132-134°C. ¹H NMR: (200 MHz, acetone-d₆): δ 2.89 [dd, J = 2.6 and 18.2 Hz, 1H, RCHa-R'(ax)], 2.91 [dd, J = 6.6 and 18.6 Hz, 1H, RCHc-R'(ax)], 3.002 [dd, J = 5.4 and 18.2 Hz, 1H, RCHd-R'(eq)], 3.27 [dd, J = 10.4 and 18.2 Hz, 1H, RCHb-R'(eq)], 3.62 [dddd, J = 10.4, 6.6, 5.4, and 2.6 Hz, R-CH₂CHxCH₂-R'(eq)], 11.3 (s, R-CO₂H) ppm. ¹³C NMR: (50 MHz, acetone-d₆): δ 176.7, 174.2, 173.4 , 38.2, 35.3 and 35.2 ppm. IR: (KBr pellet): 1727 (vs), 1790 (s), and 1833 (m) cm^-1 Anal. Calcd for C₆H₆O₅: C, 45.58; H, 3.84; O, 50.59% Found: C, 45.55; H, 4.08.

1-(Chloroformyl)propane-2,3-dicarboxylic anhydride 12

Propane-1,2,3-tricarboxylic acid (1,2-anhydride) 6 (0.01 mol) was refluxed with SOCl₂ (1 mol) for 48 hours. The SOCl₂ was evaporated and the residue distilled under reduced pressure. Purification of the fraction collected at b.p. 132-136°C (1.5 mmHg) required the use of a Buchi Kugelrohr apparatus which afforded the product which later crystallized from CH₂Cl₂ upon being cooled to room temperature to give the title product (1.58 g, 0.009 mol, 90% yield), m.p. 96-98°C, lit.[12] 97-98°C; ¹H NMR (200 MHz, acetone-d₆): δ 2.63 (dd, J = 6.2 and 17 Hz, RCHa-R’), 2.80 (dd, J = 6.8 and 17 Hz, RCHc-R’), 3.07 (dd, J = 7.2 and 18.6 Hz, RCHd-R’), 3.33 (dd, J = 10 and 18.8 Hz, RCHb-R’), 3.86 (dddd, J = 6.2, 6.8, 7, and 10 Hz, RCH₂CHxCH₂-R’) ppm. IR (KBr pellet); 1760 (vs) and 1835 (m) cm^-1. Anal. Calcd for C₆H₅ClO₄: C, 40.82; H, 2.85; Cl, 20.08; O, 36.25% Found: C, 45.55; H, 4.08; Cl, 20.93.

3-Carboxypentane-1,5-dioic acid, mono-[1-(R)-(1-phenylethyl)]amide 7, and 3-mono-[(R)-(1-phenylethyl)]amide pentane-1,5-dioic acid 8

Tricarballylic acid anhydride (TCA) 6 (47.4 mg, 0.3 mmol) was placed into an oven dried culture tube (25 mL r.b.), dissolved in dry THF (1.2 mL, 0.25 M), and then treated with (R)-(+)-α-methylbenzyl amine (46.4 μL, 43.6 mg, 0.36 mmol). The resulting solution was stirred overnight at room temperature. The solvent was removed under reduced pressure to give 84.3 mg (92% mass recovery) of a white sticky of diastereoisomer 7 and regioisomeric amides 8 as determined by ¹H NMR analysis and no attempt was made to purify this material further and isolate the diastereoisomers.

Compound 7

¹H NMR (200 MHz, acetone-d₆, from the mixture) : δ 1.43−1.40 (d, J = 7.0 Hz, 3H, Ar CHCH₃), 2.43−2.54 (m, 2H, RCH₂-CONH), 2.61-2.68 (m, 2H, RCH₂-CO₂H), 3.18-3.22 (m, 1H, RCH₂CHCH₂-R'), 5.03 (dq, J = 6.8 Hz, ArCH-), 5.34 (d, J = 7.23 Hz, -NH-), 7. 16-7.52 (m, 5H, Ph-H) ppm.

Compound 8

¹H NMR (200 MHz, acetone-d₆, from the mixture) : δ 1.62−1.59 (d, 3H, J = 6.8 Hz, Ar CHCH₃), 2.43−2.54 (m, 2H, RCHaHb-CO₂H), 2.61−2.68 (m, 2H, RCHa’Hb’-CO₂H), 3.2-3.3 (m, 1H, RCHR'), 5.0-5.08 [dq, J = 6.65 Hz, 1H, ArCHNHMe], 5.63 (d, J = 7.23 Hz, 1H, -NH-), 7.19-7.49 (m, Ph-H) ppm.

The mixture of compounds 7 and 8, were placed into an oven dried melting point tube, fused (129-135°C), solidified, and then transferred into the NMR tube. The diastereoisomers were determined by ¹H NMR analysis. The structural assignments were made by comparison of the spectral data of compounds 9s/9a which were obtained by the reaction of tricarballylic anhydride acid chloride 12 with (R)-(+)-α-methylbenzyl amine. The ratio of the distereoisomers was ~1:1 according to the integration. No attempt was made to resolve the diastereoisomers further, m.p. 129-135°C. The ¹H NMR spectra data was identical consistent with authentic samples 9s/9a in all aspects.

1-[(R)-(α)-methylbenzylamido)formylpropane-(R)-2,3-dicarboxylic-anhydride 9s and 1-[(R)-(α)− methylbenzylamido)formylpropane-(S)-2,3-dicarboxylic anhydride 9a

Into an oven dried flask (50 mL r.b.) equipped with a stirring bar were placed tricarballylic anhydride acid chloride (85.7 mg, 0.5 mmol) in dry CH₂Cl₂ (20 mL). The reaction mixture was stirred until dissolution was complete. (R)-α−methylbenzylamine (60.5 mg, 0.5 mmol) and N,N'-dimethylaniline (DMA) (70 μL, 66.5 mg, 0.55 mmol) in CH₂Cl₂ (5 mL) were added dropwise to the acid chloride solution at 0°C with vigorous stirring. The reaction mixture was warmed slowly to RT and stirred overnight. The solvent was removed under reduced pressure, the residue dissolved in CH₂Cl₂, extracted three times with cooled dilute HCl (2 mol dm-3), and several times with water. The organic layer was dried on Na₂SO₄, filtered, and concentrated under reduced pressure to give the crude product. No attempts were made to purify this material. The compounds 9s and 9a were obtained as mixture of diastereoisomers in 1:1 ratio (NMR).

Compond 9s

LRMS (EI): m/z 148 [M⁺-113, (20)], 106 (20), 94 (20), 77 (25), 44 (100). ¹H NMR (200 MHz, acetone-d₆): δ 1.62 (d, J = 7.2 Hz, ArCHCH₃), 2.65 (dd, J = 6.39 and 16.93 Hz, RCHa-R'), 2.86 (dd, J = 6.86 and 16.90 Hz, RCHc-R'), 3.09 (dd, J = 7.13 and 18.59 Hz, RCHb-R'), 3.30 (dd, J = 9.75 and 18.54 Hz, RCHd-R'), 3.84 (dddd, J = 12.20, 7.31, 5.16, and 2.21 Hz, RCH₂CHxCH₂-R'), 5.37 (q, J = 7.20 and 14.36 Hz, ArCH-), 7.42 (m, Ar-H) ppm.; Cap gc; tR = 7.21 min.; column: DB-5 5 m+ 1m guard ~1 mL / min.; gum temp prog: To = 50°C / 2 min. / 20°C min.-1 / 250°C / 5 min.

The ¹H NMR spectrum of 9a was virtually the same as for 9s with the following differences. δ 2.75 (dd, J = 9.76 and 17.90 Hz, RCHc-R'), 3.22 (dd, J = 9.93 and 18.42 Hz, RCHd-R') ppm. Anal. Calcd for C₁₄H₁₅NO₄: C, 64.36; H, 5.79; N, 5.36; O, 24.49% Found: C, 64.03; H, 4.99; N, 5.01.

The solidified materials 9s/9a were again melted at 158-160°C, and transferred into a NMR tube. The ¹H NMR spectral data was identical with those for compounds 10s and 10a. Tlc: R_f = 0.2 in 2:1:Hex:EtOAc. LRMS (EI): m/z 275 (M⁺, (100), 260 (<3), 244 (20), 232 (55), 215 (20), 200 (15), 172 (20), 146 (40), 120 (65), 105 (100), 104 (100), 94 (12), 77 (60), 59 (75), and 41 (25). The compounds 10s and 10a were obtained as a diastereoisomeric mixture in 1.2:1 ratio (NMR). (10s) ¹H NMR (500 MHz, CDCl₃): δ 8.07 (d, 1H, J = 8.5 Hz, Ar-H), 7.87 (d, 1H, J = 8.0 Hz, Ar-H), 7.80 (d, 1H, J = 1 and 7 Hz, Ar-Hp), 5.42 (dq, J = 7.5 Hz, Ar-CH-Me), 3.66 (s, RR’CHCOOCH₃)₂ 3.51 (dddd, J = 2.3, 3.6, 3.8, and 9.3 Hz, R2CHCO₂Me), 2.99 (dd, J = 3.73 and 15.31 Hz, RCHaHbCONHCHAr), 2.9-2.75 (m, 2H, RCHaHbCOOMe), 2.47-2.44 (dd, J = 6.0 and 7.5 Hz, 2H, RCHcHdCONHAr), 2.43-2.41 (dd, J = 5.5 and 7.5 Hz, RCHcHdCONHAr), 1.827 -1.812 (d, J = 7.5 Hz, 3H, ArCH-Me) ppm. The ¹H NMR spectrum of 10a was virtually the same as that for 10s with the following differences: δ 3.62 (s, 3H, RCH₂COOMe) and 1.821-1.807 (d, J = 7.0 Hz, 3H, ArCH-CH₃) ppm.; Cap gc: tR = 10.44 min.; column: DB-5 5mL = 1m guard ~ 1mL / min.; gum temp prog: To = 50°C / 2 min. / 20°C min.-1 / 250°C / 5 min.

(R)-2-Carboxymethyl-N-[(R)-1-(phenylethyl)succinimide 11s and (S)-2-Carboxymethyl-N-[(R)-1-(phenylethyl)succinimide 11a

Diastereomeric acid mixture 10s/10a (115 mg, 0.44 mmol) was dissolved or suspended in dry THF (1 mL). N,N’-dimethylformamide dimethylacetal (209 mg, 1.76 mmol, 0.5 mL) was added dropwise to the suspended solution within 20 min. The reaction mixture was allowed to stir overnight at room temperature and then the solvent was removed. The residue was washed with water (15 mL) and then the product was extracted with ethyl acetate, washed with saturated NaHCO₃ solution (2x10 mL), and brine (10 mL). The organic layer was dried over MgSO₄ anhydrous. The solvent was removed, to afford 110.5 mg of crude semi-oily product. The residue was purified by MPLC (with pure EtOAc) to provide the diastereoisomeric products 11s and 11a (95 mg, 0.35 mmol., 20 mole% and 82% yield). The material proved to be a 1.2:1 mixture of diastereoisomers according to ¹H NMR analysis. TLC; R_f = 0.2 in 2:1:Hex:EtOAc.; LRMS (EI): m/z 275 (M⁺, (100), 260 (<3), 244 (20), 232 (55), 215 (20), 200 (15), 172 (20), 146 (40), 120 (65), 105 (100), 104 (100), 94 (12), 77 (60), 59 (75), and 41 (25).

Compound 11s

¹H NMR (500 MHz, CDCl₃) : δ 8.07 (d, 1H, J = 8.5 Hz, Ar-H), 7.87 (d, 1H, J = 8.0 Hz, Ar-H), 7.80 (d, 1H, J = 1 and 7 Hz, Ar-Hp), 5.42 (dq, J = 7.5 Hz, Ar-CH-Me), 3.66 (s, RR’CHCOOCH₃)₂ 3.51 (dddd, J = 2.3, 3.6, 3.8, and 9.3,Hz, R2CHCO₂Me), 2.99 (dd, J = 3.73 and 15.31 Hz, RCHaHbCONHCHAr), 2.9-2.75 (m, 2H, RCHaHbCOOMe), 2.47-2.44 (dd, J = 6.0 and 7.5 Hz, 2H, RCHcHdCONHAr), 2.43-2.41 (dd, J = 5.5 and 7.5 Hz, RCHcHdCONHAr), 1.827 -1.812 (d, J = 7.5 Hz, 3H, ArCH-Me) ppm.

The ¹H NMR spectrum of 11a was virtually the same as for 11s with the following differences: δ 3.62 (s, 3H, RCH₂COOMe) and 1.821-1.807 (d, J = 7.0 Hz, 3H, ArCH-CH₃) ppm.; Cap gc: tR = 10.44 min.; column: DB-5 5mL = 1m guard ~ 1mL / min; gum temp prog.: To = 50°C / 2 min. / 20°C min.-1°/ 250°C / 5 min. Anal. Calcd for C₁₅H₁₇NO₄: C, 66.44; H, 6.22; N, 5.09; O, 23.25% Found: C, 66.55; H, 6.09; N, 4.99.