Synthesis of (+)-9-Demethyl-7,8-dideoxycalopin as a Model for the Determination of the Absolute Stereochemistry of a New Group of Fungal Metabolites

Abstract

A key step in the synthesis of (+)-9-demethyl-7,8-dideoxycalopin (3) is the highly stereoselective ene reaction between phenylmenthyl glyoxylate 8 and β,β-dimethylstyrene (7). Hydroboration of the resulting homoallyl alcohol 9 afforded only the undesired epimer 10, which was ultimately converted into target 3 by a sequence of oxidation, epimerization, and reduction steps. 1H NMR comparison of the MTPA esters derived from compound 3 with those from natural calopin supported the assignment of the (2S,3R,4S) configuration to this new group of fungal metabolites. Further evidence of the absolute configuration was obtained by comparison of the CD spectra of calopin and of model compound 3. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)