Absolute stereochemistry of the novel dioxaspiro diterpenoids strictanonic and grindelistrictic acids. Stereoselective synthesis of strictanonic acid methyl ester and its C-6 epimer

Abstract

The stereoselective synthesis of (+)-strictanonic acid methyl ester (2b) and its C-6 epimer from a common chiral intermediate, derived from natural grindelic acid (1a) of known absolute configuration, is described. With the aid of one- and two-dimensional multipulse n.m.r. techniques, the complete stereochemistry of strictanonic acid and that of C-9 of the related bisnorditerpene grindelistrictic acid (6a) was established.