An organocatalytic reductive coupling and Lewis-acid-catalyzed annulative ring-opening strategy is developed as a two-step protocol for the stereoselective synthesis of dihydropyrans as the major products from the chiral formylcyclopropanes, CH acids, and Hantzsch ester. It is an efficient, catalytic, two-step protocol for the chiral synthesis of dihydropyrans and dihydrofurans. Structurally important and challenging functionally rich cyclopropanes containing cyclic-1,3-diones were synthesized in very good yields with excellent chemo-, enantio-, and diastereoselectivities from the readily available starting materials, chiral formylcyclopropanes, cyclic-1,3-diones, or CH acids and Hantzsch ester through an organocatalytic reductive coupling reaction at ambient conditions, especially without harming the cyclopropane ring. Chiral cyclopropanes containing cyclic-1,3-diones were stereospecifically transformed into dihydropyrans and dihydrofurans found in many bioactive natural products and drugs through an annulative ring-opening reaction by using Lewis-acid (BF 3 ·OEt 2 ) or cesium carbonate (Cs 2 CO 3 ) catalysis. Highly diastereo- and regioselective ring opening of cyclopropanes was explained through a stereospecific intimate ion pair pathway.
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Peraka et al. (2018) studied this question.