A methylene acetal derivative 4 of the necic acid, monocrotalic acid, part of the macropyrrolizidine alkaloid, monocrotaline, was enantioselectively synthesized from 2-furylmethanol derivative 6. Oxidation of 6, followed by silylation of the resulting lactol 9, gave silyl ether 19 as a major product. Introduction of the di-tert-hydroxy groups to 22 was achieved by stereoselective dihydroxylation with osmium tetroxide. After protection of the vicinal diol as a methylenedioxy derivative, acetal 26 was successfully converted into the desired product 4.