“…The signals at lowest field, in the ranges 170.3e171.0 and 171.2e172.5 ppm, are assigned to C-4 of the pyrone ring and the carbonyl carbon of the acetoxy group, respectively. In the cases of boron heterocycles 6c (R¼p-ClC 6 H 4 ) and 6d (R¼p-O 2 NC 6 H 4 ) only, where both 1 H and 13 C NMR spectra were obtained in DMSO-d 6 , there was evidence for the existence of two stereoisomers, with respect to the tetracoordinated boron atom. Specifically, in the 13 C NMR spectra there were two signals for each of the two types of methyl, two ester carbonyl carbon signals, and two peaks for C-4 of the pyrone ring.…”