Enantioselective cyclopropanations: a study on the catalytic effects of methyl-α-D-mannopyranoside-based lariat ethers

Novel monoaza-15-crown-5 ethers have been synthesized with an α-d-mannopyranoside backbone. These chiral catalysts contain different heteroaromatic side chains, which have never been incorporated in sugar-based crown ethers before. The novel macrocycles together with a few previously synthesized mannose-based crown ethers were examined in four asymmetric phase-transfer cyclopropanations. The reactions proceeded mainly with excellent yields (up to 99%), while enantiomeric excesses ranged from 1 to 44%, proving the mannose-based lariat ethers to be moderate enantioselective catalysts in these reactions. The distance between the crown ring and the functional group of the side chain proved to be important in terms of enantioselectivity, the propylene moiety was more favorable than the ethylene bridge. The type of heteroaromatic ring also affected the catalytic activity of the mannose-based macrocycles, using lariat ethers containing a pyrrole unit, higher enantiomeric excess was measured.