2-Benzopyran-3-ones as synthetic building blocks; regioselective Diels–Alder additions with simple olefins leading to aromatic steroids

Abstract

2-Benzopyran-3-one 1a undergoes strongly regioselective Diels–Alder additions to buta-1,3-diene, 2-methylpropene, but-1-ene and the olef in 9. The main adducts 17a and 17b from 1a and 1b respectively and the olefin 9(16) are derived by endo- and exo-addition to the re-face of 9(16). These adducts are converted into the 8α,9α-steroids 22a and 22b by a four-step sequence including Dieckmann cyclisation of 21a and 21b as the key step. The derived 8α,9α-steroids 24a and 24b can be epimerised at C-8 via the enones 26a and 26b and lithium–ammonia reduction. Other aromatic steroids obtained by this general route are the equilenin derivative 28 and the dihydronaphthalene 29.