Synthesis of steroidal derivatives bearing a small ring using a catalytic [2+2] cycloaddition and a ring-contraction rearrangement

Fixing conformation by introducing a ring structure is a common strategy in drug development. We demonstrate a synthesis that installs a small carbocyclic ring as a structurally-rigid unit in drug lead compounds. Both trans- and cis-cyclobutane rings were constructed in excellent selectivities by controlling the reaction temperature of an EtAlCl2-catalyzed [2+2] cycloaddition between a silyl enol ether and an α,β-unsaturated ester. Spirocyclopropane rings were stereospecifically formed by our previously reported ring-contraction rearrangement of fused cyclobutanols. This strategy allowed stereodivergent access to a new class of steroidal derivatives bearing a small ring.