Variable Stereocontrol in Cycloadditions of 1,7,9-Decatrien-3-ones by Different Lewis Acidic Promoters – Application to a Short Synthesis of α-Eudesmol

Abstract

Trienones 1–3 were subjected to Lewis acid-promoted intramolecular Diels–Alder reactions. It was shown that with monocoordinating Lewis acids the endo selectivity of the cycloaddition was generally high. The preference for either of the two possible endo products cis-a and cis-b, however, was shown to be highly dependent on the nature of the Lewis acid, and on the substitution pattern of the trienone substrates. Lewis acids with two coordination sites furnished predominantly the exo product trans-b via seven-membered ring chelate intermediates. The chelate-controlled intramolecular Diels–Alder reaction was then utilised as the key step in a stereocontrolled synthesis of the sesquiterpene α-eudesmol. This reaction mode, performed on a model trienone, also paved the way for the synthesis of the pharmaceutically important natural product dihydromevinolin.