Diels-Alder cycloadditions of 3-methoxy-8 alpha-estra-1,3,5(10),14,16-pentaen-17-yl acetate: Unexpected trends in regioselectivity

In contrast to Diels-Alder reactions of 3-methoxyestra-1,3,5(10),14,16-pent aen-17-yl acetate, in which 14,17-bridged cycloadducts are formed with high head-to-head beta -face selectivity, product distributions for similar rea ctions of 3-methoxy-8 alpha -estra-1,3,5(10),14,16-pentaen-17-yl acetate di splay sensitivity to the nature and reactivity of the dienophile, with a tr end favouring regioreversal during p-face cycloaddition. Results are interp reted in terms of competing stereoelectronic and steric influences.