Isomerism in bicyclic diacetals. Part II. Bicyclic methylene diacetals in the galacto, arabino, and ribo series

Studies of the acid-catalysed methylenation of tetritols with the galacto, arabino, and ribo configurations have led to the following observations: (i) dimethyl galactarate affords dimethyl 2,3:4,5- and 2,5:3,4-di-O-methylene- galactarate, (ii) methyl-D-arabinonate affords methyl 2,3:4,5-, 2,4:3,5-, and 2,5:3,4-di-O-methylene-D-arabinonate, and (iii) methyl D-ribonate affords methyl 2,4:3,5- and 2,5:3,4-di-O-methylene-D-ribonate. Vicinal coupling constant data obtained by ¹H n.m.r. spectroscopy show that dimethyl 2,5:3,4-di-O-methylenegalactarate and methyl 2,5:3,4-di-O-arabinonate exist predominantly in gauche, gauche conformations in solution whereas the gauche, anti conformation is highly populated in solutions of methyl 2,5:3,4-di-O-methylene-D-ribonate. The relative stabilities of the constitutional isomers are discussed in terms of electronic effects associated with gauche oxygen-oxygen interactions in O-C_|^|-C _|^|-O fragments as well as in terms of steric effects. There is no strong evidence to support the view that such gauche oxygen - oxygen interactions are an important stabilising feature in cis-fused 3,5,8,10-tetraoxabicyclo[5.3.0]decanes.