Quassinoid Synthesis via o-Quinone Diels-Alder Reactions

Article abstract of DOI:10.1021/jo00173a018

The reaction of 3,5-disubstituted o-quinones (2a, 2c) and 4-chloro-3,5-disubstituted o-quinones (2b, 2d) with simple dienes was investigated as a potential route to the quassinoid skeleton.Quinones 2a and 2c reacted in high yield at the 3,4-position with only a small excess of diene.Attempted equilibration of cis-fused cycloadducts to the trans-fused system failed due to the intervention of a stable enol form, as in 20.Compound 2c with ethyl 3,5-hexadienoate gave 15a, which upon reduction and lactonization provided BCD-ring tricyclic quassinoid analogues 18a and 19a.Again isomerization to the BC trans-fused system was not possible.The chloroquinones showed some preference for Diels-Alder reaction at the 5,6-position, but the additions were characterized generelly by low yields, side reactions, and lessened stereoselectivity.