Further Studies on Quinone Diels-Alder Reactions with 1,3,3-Trimethyl-2-vinylcyclohexenes: Regioselective Synthesis of 12-Methyl-podocarpane Diterpenes and Isolation of a Hetero Diels-Alder Product from 1,4-Benzoquinone

Abstract
Diels-Alder reactions of 2-[(E)-β-alkoxyvinyl]-1,3,3-trimethyl-cyclohexenes with 2-methoxy-3-methyl-1,4-benzoquinone regioselectively produce adducts possessing the 12-methyl-podocarpane ring skeleton. In addition, in the reaction with 1,4-benzoquinone an unusual hetero Diels-Alder product involving the C[dbnd]O group of the quinone is found in low yield.