New strategy for convergent steroid synthesis.

We published recently our results on a new and convergent synthesis of natural steroids. The strategy was based on a cycloaddition reaction of Nazarov reagents 2 and 5 with cyclohexenones 1 and 4. In this paper we report results that deal with the synthesis of two new bicyclic Nazarov reagents (13 and 19) and their cycloaddition with two cyclohexenones (1 and 4). These new results constitute an important improvement concerning the versatility of the strategy since tetracycles having the stereochemistry found in natural steroids are now available.