A general synthetic approach to optically active iridoid aglycones : The total syntheses of β-ethyl descarbomethoxyverbenalol, ethyl catalpol, and (-)-specionin

Abstract

A general approach to the synthesis of iridoid aglycones is reported. A combination Claisen rearrangement/nitrile oxide cycloaddition sequence has permitted the facile annulation of a highly functionalized cyclopentane ring onto a simple glycal derived from D-xylal. The title aglycones have been prepared and the work has culminated with a stereoselective total synthesis of (-)-specionin. Two other anomers of specionin have also been independently prepared. The work confirms Vandewalle's relative stereochemical assignment of specionin and also confirms the absolute stereochemistry of this compound.