Unusual stereoselectivity in Diels-Alder cycloadditions of 5-bromopyrone

5-Bromopyrone undergoes cycloaddition to poorly activated or unactivated alkenes to afford high yields of cycloadducts. The regiochemistry of the cycloaddition is excellent. The stereoselectivity of cycloaddition depends on both electronic and steric factors in the dienophile but can be controlled to give predominantly endo or predominantly exo cycloadducts. (C) 1997 Elsevier Science Ltd.