Synthesis and stereochemistry of insect derived spiroacetals with branchedcarbon skeletons

About thirty constitutionally different spiroacetals have been characterise d from insects but only three have branched carbon skeletons. Two are based on the 1,7-dioxaspiro[5.5]undecane system and are certain stereoisomers of the 2,4,8-trimethyl derivative, from the aposematic shield bug, Cantao par entum (White), and a 2,2,8-trimethyl derivative from the rove beetle, Ontho lestes murinus (L). The 1,6-dioxaspiro[4.5]decane system is represented by a stereoisomer of the 2,3,7-trimethyl derivative in the Cantao species. The elucidation of their structures and stereochemistry by spectroscopy, synth esis and enantioselective gas chromatography is described.