A [4+1] cyclopentannulation route to ring A aromatic, C(1)-C(11) methano-bridged steroids

Article abstract of DOI:10.1016/S0040-4039(00)79764-4

In a novel, one-pot [4+1] cyclopentannulation procedure, the steroidal olefin 2 and CH₂O are converted into the corresponding C(1)-C(11) methano-bridged derivative 3 by two sequential Lewis acid-promoted electrophilic substitutions. Birch reduction of the olefin 3, followed by removal of the protecting groups, affords 1,11α-methano-9β-estra-1,3,5(10)-triene-3,17β-diol (6).