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

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.