Ascorbic Acid. 2. Structural Determination and Synthesis of 2- and 3-Acyl Derivatives of 5,6-O-Isopropylidene-L-ascorbic Acid

Article abstract of DOI:10.1021/jo00259a025

The development of methods for the synthesis and spectroscopic identification of 2-O or 3-O carbon and phosphorus derivatives of 5,6-O-isopropylidene-L-ascorbic acid (IAA) is reported.Noncrystallographic structural identification has been established by pK_a determination based upon pH-dependent UV spectra and 13C NMR spectra of phosphorus esters which show phosphorus couplings to C1, C2, and C3 for the O2 esters and couplings to C2, C3, and C4 for O3 esters.UV spectra of O2 and O3 both demonstrate red shifts of near 26 nm upon ionization.The O2 esters absorb less intensely than their anions, but the reverse is true for O3 esters.Infrared spectra are particularly useful for structural assignments: O3 esters have bands for carbonyl stretching near 1784 cm^-1 and olefinic stretching at 1700-1710 cm^-1; O2 esters have corresponding bands near 1778 cm^-1 and 1680-1690 cm^-1, respectively.In compounds with a 3-O-acyl group, the carbonyl stretch is more intense than the olefinic stretch but the reverse is true in compounds with a 3-OH group.The 1H NMR spectra of H4, H5, H6, and H6' indicate first-order ABCD coupling.A value of near 1 Hz has been found for the long-range J_P-H4 in the phosphorus esters.The monoanion of IAA, which is generated in situ by reacting 1 equiv of IAA with triethylamine, reacts in aprotic solvents to produce predominantly O3 esters.Reactions in aprotic solvents with neutral IAA appear to give 2-esters, which can be prepared nearly quantitatively when 6-10 equiv of methanol are present.