Diels-Alder Reaction of 2-Ethenyl-1,3,3-trimethylcyclohexene with 4H-Chromen-4-ones: A Convergent Approach to ABCD Tetracyclic Core of Marine Diterpenoids Related to Puupehenone and Kampanols

A rapid assembly of the tetracyclic core of marine diterpenoids related to puupehenone and kampanols by DielsAlder reaction of 2-ethenyl-1,3,3-trimethylcyclohexene with 4H-chromen-4-one (¼ 4H-1-benzopyran-4-one) dienophiles is described. Introduction. -T heDielsAlder reactions of 2-ethenyl-1,3,3-trimethylcyclohexene with dienophiles like dimethyl acetylenedicarboxylate (1), unsymmetrical 1,4-benzo- quinones (2), 1,4-benzoquinone (3), substituted 1,4-benzoquinones (4), 2-(methoxy- carbonyl)-4,4-dimethylcyclohex-2-enone (5), 3-((E)-(methoxycarbonyl)prop-2-eno- yl)-1,3-oxazolidin-2-one (6), acetylenedicarbaldehyde (7), (S)-5-(tert-Butyl)-3-hy- droxy-2-isopropyl-1,4-benzoquinone (8), and conjugated ketones (9) have been reported. There are very few reports on (4 þ 2) cycloaddition reactions using 4H- chromen-4-ones (¼ 4H-1-benzopyran-4-ones) as dienophiles, and in all these cases an activating functionality likeCHO, � COR, � COOR, � CN, � Ar, etc. at C(3) have been utilized (10). Only one DielsAlder reaction of 2-ethenyl-1,3,3-trimethylcyclohexene with 6-bromo-3-cyano-4H-chromen-4-one has been reported (10a).