Intramolecular Diels-Alder reactions, 3 - Variable stereocontrol in cycloadditions of 1,7,9-decatrien-3-ones by different Lewis acidic promoters - Application to a short synthesis of alpha-eudesmol

Trienones 1-3 were subjected to Lewis acid-promoted intramolecular Diels-Al der reactions. It was shown that with monocoordinating Lewis acids the endo selectivity of the cycloaddition was generally high. The preference for ei ther 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 substitu tion pattern of the trienone substrates. Lewis acids with two coordination sites furnished predominantly the exo product trans-b via seven-membered ri ng chelate intermediates. The chelate-controlled intramolecular Dials-Alder reaction was then utilised as the key step in a stereocontrolled synthesis of the sesquiterpene alpha-eudesmol. This reaction mode, performed on a mo del trienone, also paved the way for the synthesis of the pharmaceutically important natural product dihydromevinolin.