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 lanthanide 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 –O–[graphic omitted]–O– fragments as well as steric effects.