The circular dichroism of the n→π* band of cyclic thionocarbonates. Part II. Studies on the thionocarbonates of some acyclic vicinal diols

Abstract

C.d. curves given by thionocarbonates of configurationally related vicinal diols can differ markedly. Thus the derivatives [(1)–(3)] of 1,4-di-O-methyl-D-threitol, 1,2:5,6-di-O-isopropylidene-D-mannitol and D-butane-2,3-diol show a composite two-band curve, a positively signed curve, and a negatively signed curve, respectively. For the thionocarbonate (4) of the erythro-diol 1,2:5,6-di-O-isopropylidene-D-altritol, the Δε value approaches the magnitudes of those shown by conformationally rigid derivatives. The variation in the c.d. spectrum of compound (1) with solvent and temperature is best rationalized in terms of a two-conformer equilibrium. Analysis of the variable temperature results yields ΔG° for the interconversion, and also the rotational strengths of each conformer.