Synthesis of highly functionalized [3]dendralenes and their Diels–Alder reactions displaying unexpected regioselectivity

Abstract

Acyclic tetrasubstituted [3]dendralenes stable towards D–A cyclodimerization were prepared by the double olefination of substituted dienyl phosphonates with dimethylsulfonium methylide, followed by H–W–E olefination of the formed butadienylphosphonoacetates with aldehydes. The X-ray structure of a [3]dendralene with a 4-position benzyl substitution revealed that an electron-rich diene exists in the s-trans conformation, leading to the stability. This structural preference of dendralenes leads to an unusual regioselectivity in the D–A reaction, wherein a deactivated diene participated in the process and displayed endo selectivity. The steric bulk of the groups at the 2 and 4 positions govern the regioselectivity and dominates the electronic effects. For the dendralenes possessing a 4-methyl substituent, the electronically rich diene participated in the D–A reaction. These D–A adducts failed to undergo further cycloaddition owing to steric crowding, which resulted in gauche conformation of the diene, as evidenced by X-ray structures of the adducts.