Unexpected Regioselectivity in the Synthesis of Pyranonaphthoquinone via the Diels−Alder Reaction

The unusual regioselectivity in the Diels-Alder reactions of pyranoquinone 1 with (4,4-dimethoxybuta-1,3-dien-2-yloxy)trimethylsilane 2 are explored by both computations and experiments. The regioselectivity is controlled by the electrostatic interaction of the lactone ring-oxygen and the vicinal quinone oxygen on the transition structure, which can be tuned by the terminal methyl group of the butadienes.