Synthesis and Highly Stereoselective Hydrogenation of the Statin Precursor Ethyl (5S)‐5,6‐Isopropylidenedioxy‐3‐oxohexanoate

Abstract

A search for the large-scale preparation of (5S)-5,6-(isopropylidenedioxy)-3-oxohexanoates (2) -a key intermediate in the synthesis of pharmacologially important statins -starting from (S)-malic acid is described. The synthesis of the required initial compound methyl (3S)-3,4-(isopropylidenedioxy)butanoate (1) by Moriwakes reduction of dimethyl (S)-malate (3) has been improved. Direct 2-C chain elongation of ester 1 using the lithium enolate of tert-butyl acetate has been shown to be successful at a 3-to 5-fold excess of the enolate. Unfortunately, the product, tert-butyl (5S)-5,6-(isopropylidenedioxy)-3-oxohexanoate (2a) is unstable during distillation. Ethyl (5S)-5,6-(isopropylidenedioxy)-3-oxohexanoate (2b) was prepared alternatively on a multigram scale from (3S)-3,4-(isopropylidenedioxy)butanoic acid (7) by activation with N,N'-carbonyldiimidazole and subsequent reaction with Mg(OOCCH 2 COOEt) 2 . A convenient pathway for the in situ preparation of the latter is also described. Ethyl ester (2b) can be advantageously purified by distillation. The stereochemistry of the catalytic hydrogenation of b-keto ester (2b) to ethyl (5S)-5,6-(isopropylidenedioxy)-3-hydrohyhexanoate (syn-6 and anti-6) has been studied using a number of homogeneous achiral and chiral Rh(I) and Ru(II) complexes with phosphine ligands. A comparison of Rh(I) and Ru(II) catalysts with (S)-and (R)-BINAP as chiral ligands revealed opposite activity in dependence on the polarity of the solvent. No influence of the chiral backbone of substrate 2b on the enantioselectivity was noted. A ratio of syn-6/anti-6 = 2.3 was observed with an achiral (Ph 3 P) 3 RuCl 2 catalyst. Ru[(R)-Tol-BINAP]Cl 2 neutralized with one equivalent of AcONa afforded the most efficient catalytic system for the production of optically pure syn-(5S)-5,6-isopropylidenedioxy-3-hydroxyhexanoate (syn-6) at a preparative substrate/catalyst ratio of 1000:1.

Glycosyl-Nucleolipids as New Bioinspired Amphiphiles

“…(3). Compound 3 was synthesized according to the literature procedure [34]. The data agreed with the literature values.…”

“…The acetonide-ester 3 is the key starting material for the synthesis of all three GNLs and was therefore required in significant quantities. Although commercially available, acetonide-ester 3 is relatively expensive, so it was synthesized, as was the intermediate dimethyl (S)-malate 3b, from (S)-malic acid 3a according to a literature procedure [34]. Aminolysis of unactivated ester 3 with propargylamine turned out to be less effective.…”

Highly stereoselective hydrogenations—As key‐steps in the total synthesis of statins

“…We recently reported a new approach for the total synthesis of atorvastatin 32 via highly stereoselective hydrogenation of the statin precursor ethyl (5S)-5,6-isopropylidenedioxy-3-oxohexanoate 33 as a key-step. The retrosynthetic pathway to the enantiopure side chain of atorvastatin is depicted on Scheme 1.…”

“…NaBH 4 ). 34,35 The resulting crude methyl (S)-3,4-dihydroxybutanoate was directly transformed without purification into the acetonide (S)-1 33 with a yield of 74% after distillation. Saponification of ester (S)-1 with aqueous NaOH and subsequent neutralization gave the corresponding acid (S)-4 in a chemical yield of 71-78%.…”

“…After work-up keto ester (S)-2 was obtained after distillation in good yield. 33 To define optimal conditions for the stereoselective hydrogenation of b-keto ester (S)-2 into the desired protected polyol (3R,5S)-6 (Scheme 3), we studied the performance of several homogeneous chiral Rh and Ru catalysts.…”