INVESTIGATIONS OF THE FORMATION OF CYCLIC ACETAL AND KETAL DERIVATIVES OF D-RIBONO-1,4-LACTONE AND 2-DEOXY-D-RIBONO-1,4-LACTONE

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 we'' isolated and characterized. The H-1 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-degrees-K and 200-degrees-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.