Isomerism in bicyclic diacetals. Part I. 1,3:2,4- and 1,4:2,3- Di-O-methylene- erythritol

Acid-catalysed methylenation of erythritol affords 1,3 2,4- and 1,4:2,3-di-O-methylene-erythritol and a small amount of 1,4-anhydro-2,3-O-methylene-erythritol. Constitutional assignments have been made to the diacetals on the basis of their ¹H n.m.r. and mass spectra. Deuteriation studies and the Janthanide shift reagent, Eu(fod)₃, have been employed to investigate the conformational behaviour of the 1,4:2,3-diacetal in solution by ¹H n.m.r. spectroscopy. Acid-catalysed equilibration of the 1,3:2,4- and 1,4:2,3-diacetals indicates that there is a free energy difference of 1·37 kcal mol^-1 in favour of the former at room temperature. The significance of these results is discussed in terms of electronic effects associated with the -0-C-C-O-fragments as well as steric effects.