Structure–reactivity relationships in normal and 19-nor-5,10-seco-steroidal cyclodecenone systems. Part 1. Acid-catalyzed and thermal reactions

Abstract

Under acid-catalyzed conditions both the (Z)- and (E)-19-nor-seco ketones (1a) and (2a) underwent transannular cyclization [forming the C(5)–C(10) bond], accompanied by aromatization of the resulting ring A, to give estra-1,3,5(10)-trien-17β-yl acetate (4)(in 76% and 83% yield, respectively). On the other hand, when the corresponding 19-methyl analogues (1b) and (2b) were treated with acid under similar experimental conditions, the (Z)-diastereoisomer (1b) remained mostly unchanged, while the (E)-19-methyl-seco ketone (2b) afforded the A-nor-B-homo derivative (5)(in 68% yield). The same transannular cyclization of the (E)-seco ketone (2b) was also achieved thermally in the absence of protonation.