Sigmatropic Ring Expansion : Quantitative Evaluation of the Transition States and its Synthetic Application to Natural Products

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We recently reported that the highly stereoselective synthesis of (Z)-or (E)-double bonds in 10-membered thiolcarbonates was achieved by controlling chairlike-boat-like transition states in the [3,3] sigmatropic rearrangement of 8-membered thionocarbonates. Interestingly, the respective olefin geometry in the staring materials was converted to the opposite geometry with complete isomeric purity.In order to examine the highly stereoselectivity in these systems, a combination of MM and MO-SCAN compyutations was devised to evaluate the proposed transition states (T_c and T_B).The synthetic utility of the present methodology was demonstrated by synthesis of the yellow scale pheromone. Further, mediummembered heterocyclic allenes were synthesized.