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

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 0oC for 24 hrs.


Introduction
Tricarballylic acid (TCA) 5 is found as a fragment in several mycotoxin natural products such as Fumonisin B 1 (1a), Fumonisin B 2 (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] Me  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 B 1 (1a), B 2 (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)-
The 1 H 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 o 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 1 H 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 o 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 1 H 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 CH 2 CO 2 Me-succinimide of generic anti-(3S) diastereomer, the methoxyl resonance will be observed at higher field than in the syn -(3R) diastereomer.
Similarly, the 1 H NMR spectrum of compounds 11s/11a shows the (CO 2 Me) 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 o 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 CH 2 Cl 2 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. .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-5.6m 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