Rhizopodin Synthesis Essay

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  • 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 Moriwake’s 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(OOCCH2COOEt)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 β-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 (Ph3P)3RuCl2 catalyst. Ru[(R)-Tol-BINAP]Cl2 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.

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