An enantioselective version of the AB+B→ABCD-type steroid total synthesis

Abstract Diene 1 and dienophile 4a in a Diels/Alder reaction mediated by a chiral ligand-modified Lewis acid enantioselectively furnish adduct 5a (chem. yield: 64%; e.e.: 73%), which after partial deoxygenation and final enantioselection by recrystalization affords 5b . The latter compound can easily be converted via Torgov's pentaenone 6a into estrogens or progestogens.