Diastereodifferentiating [2+2] photocycloaddition of chiral cyclohexenone carboxylates with cyclopentene by a microreactor

We performed diastereodifferentiating [2+2] photocycloaddition of chiral cyclohexenone (1) with cyclopentene using a special continuous microflow reactor equipped with UV-LED lamps for photoreaction, and compared the results to batch processing. Even at very high sample concentration, the high conversion was accomplished in this microreactor; however, the reaction under the same conditions did not proceed to completion in the batch. Furthermore, the highest diastereomeric excess (de) of photoproducts was achieved in this microreactor when aromatic compounds were added, which prevented the reaction from proceeding to completion in the batch due to their high absorption coefficients. This diastereoselective enhancement was due to superior light penetration in the microreactor.