STEREOSELECTIVE ACCESS TO THE BASIC SKELETON OF TETRACYCLIC DITERPENES VIA A SEQUENCE OF CONSECUTIVE [3+2],[2+2+2], AND [4+2] CYCLOADDITION REACTIONS

Benzocyclobutenes 9a, 9b, 23, and 24, prepared from 1,5-hexadiyne in good yields in a sequence involving [3 + 2] annelation and [2 + 2 + 2] cycloaddition reactions, underwent an intramolecular [4 + 2] reaction to afford the basic skeleton of tetracyclic diterpenes phyllocladane and kaurane. The factors governing the stereochemistry of the Diels-Alder reaction have been elucidated. A carbonyl group at C-12 favored the kaurane stereochemistry (58:42) whereas acetal or silyl ether functions at that same position led to a highly (97:3) or totally stereoselective formation of the phyllocladane ring system.