The reactions of D-ribono-1,4-lactone, and 2-deoxy-D-ribono-1,4-lactone with benzaldehyde and acetone in acidic media were investigated. The products obtained were isolated and characterized. The 1H NMR spectra of the 1,5-lactone product resulting from the thermodynamically controlled reaction of D-ribono-1,4-lactone with benzaldehyde were examined between 300 °K and 200 °K in a polar solvent. No conformational changes in the 1,5-lactone ring were observed within this temperature range. Detailed NMR studies showed that the acetalization of D-ribono-1,4-lactone proceeded with the initial formation of the endo-2,3-acetal derivative, which in the presence of aqueous acids underwent ring expansion and isomerization to the 3,4-acetal of the 1,5-lactone. The endo preference of benzylidene acetals was explained by the transition state conformation of the reactants and the thermodynamic stability of the products, as calculated with molecular mechanics. © 1993.