Absolute configuration of the ·CH(OMe)·CHMe· system in the phthiocerols

Abstract

The absolute configuration of the ·CH(OMe)·CHMe· system in the phthiocerols, RCH(OMe)·CHMe·[CH₂]₄·CH(OH)·CH₂·CH(OH)·[CH₂]_nLMe (R = Et or Me, n= 20 and 22), constituents of tuberculolipids, has been studied. By conversion of methoxyphthiocerane A. EtCH(OMe)·CHMe·[CH₂]_n+7Me, into the methyl-branched hydrocarbon EtCH₂·CHMe·[CH₂]_m+7Me having (–)-rotation it is shown that in the phthiocerols the asymmetric centre bearing the methyl branch has the S-configuration. In view of the threo-configuration of the adjacent asymmetric centres it follows that the centre bearing the methoxy-group has the R-configuration.

(+)- and (–)-erythro-3-Methoxy-2-methylbutyric acid, prepared in the course of the studies on the stereochemistry of the methoxyphthioceranes, are shown to have S- and R-configuration, respectively, at the asymmetric centre bearing the methyl branch.