STUDIES ON THE SYNTHESIS OF THE DITERPENOID MOLD METABOLITE SORDARICIN - EXPLORATION OF A PROSPECTIVE BIOGENETIC INTRAMOLECULAR [4 + 2] CYCLOADDITION

In a model study directed toward the synthesis of the diterpenoid sordaricin 3, the main carbon skeleton was established via intramolecular [4 + 2] cycloadditions of the 5,5-cyclopentadienyl derivatives 24 and 30. These intermediates were prepared by two similar routes, beginning with the alkylation of either the norbornenyl ester 14 or its enantiomer with iodide 16. The oxygenated two-carbon bridge in each of the resulting adducts was then cut away by a sequence of oxidative processes to form the diene component for the cycloadditions. For 24 this began with the Bayer-Villiger oxidation (21 --> 22) and for 30, Vedejs alpha-hydroxylation of 27, followed by periodate cleavage. In the latter case, the benzyloxy group was introduced into the dienophile moiety by selective epoxidation of the isopropenyl group followed by amide-initiated elimination.