DI-O-BROMOETHYLIDENE-D-MANNITOL FORMATION

The stereochemistry and structure of 1,2-O-bromoethylidene-D-mannitol and its role in the reaction pathway leading to 1,2:5,6-di-O-bromoethylidene-D-mannitol were studied. Nmr spectroscopy established that monoacetal formation was from the outset an equilibrium-controlled reaction, giving a 65:35 cis/trans ratio. Previously reported work pointed out the high proportion of cis rings in the diacetal formation, ca. 67:33 cis,cis/cis,trans. When the latest results are coupled with earlier data, the reaction pathway can be explained. Monoacetal formation is equilibrium controlled and the subsequent diacetal formation is kinetically controlled and irreversible because of insolubility. Examination of molecular models suggested that the cis preference in the second ring closure may be accounted for by a long-range directional effect transmitted through hydrogen bonding. The unreported 3,4,5,6-tetra-O-methyl-D-mannitol was prepared as its crystalline 1,2-di-O-p-tolylsulfonyl ester. © 1969, American Chemical Society. All rights reserved.