Reactions of hydriodic acid with aldonolactones and n-alkanolactones. Interconversions between lactones and iodocarboxylic acids

Aldonolactones containing from four to eight carbon atoms, and lactones of the related monohydroxy-n-alkanoic acids, were subjected to reaction with 57% hydriodic acid at 125 °C. As in the classical studies of Kiliani, the reduction of D-glycero-D-ido-heptono-1,4-lactone yielded mainly γ-heptanolactone. Analogously, the corresponding γ-alkanolactones were obtained as major products from the 1,4-lactones of the D-xylono-, D-allono-, and D-erythro-L-talo-octono configuration. Monoiodo-n-alkanoic acids were also formed in admixture with the lactones in all of these reactions. D-Erythrono-1,4-lactone was unique among the aldonolactones in that it led only to an acid, i.e., 3-iodo-n-butanoic acid. The latter was also the product of the non-reductive reaction of hydriodic acid with β-butyrolactone whereas, by contrast, γ-butyrolactone afforded 4-iodobutanoic acid. Among compounds in the five to eight carbon series, it was found that under conditions close to equilibrium the ratio of lactone to iodoacid decreased progressively with the length of the carbon chain; e.g., in the 4 h reactions of γ-valero-, γ-capro-, γ-heptano-, and γ-octanolactone, the ratios were 2.4, 1.2, 0.2, and 0.1, respectively. An accompanying characteristic of these reactions is a progression in the number of isomeric iodoacids formed. Whereas γ-valerolactone was accompanied by 4-iodopentanoic acid, there were two isomers (4- and 5-) of iodohexanoic acid, three monoiodo- (including 6-iodo-) heptanoic acids, and four (including 7-iodo-) octanoic acids. In all instances, the isomer substituted at the penultimate carbon was major. An interplay of several individual reactions, including ring-opening displacements, eliminations–additions, and rearrangements, as well as a probable influence of entropy changes on the lactone-acid equilibria, appear to account largely for these observations.