The synthesis of [l-(2-pyridylthio)alkyl]boronic esters was explored as a means of determining the diastereoisomeric purity and the absolute configuration of the carbon 0( to the boron of the corresponding 1-chloroalkyl derivatives. The significant nonequivalences observed in the 1H NMR spectra of the two diastereoisomers were attributed to the presence of a strong intramolecular nitrogen-boron coordination. Rigorous proof of a cyclic configurationally stable structure and determination of the boron configuration were obtained by X-ray crystallographic analysis. Q 1997 Elsevier Science Ltd Pinanediol (1-chloroalkyl)boronates 1 have been shown to play an important role in asymmetric synthesis and very promising results have been reported in the field of pheromones, polyols and 0(amino boronic acids.l An accurate and reliable procedure for the determination of the diastereoisomeric purity of these versatile intermediates is therefore essential. (1-Chloroalkyl)boronic esters can be sometimes directly analysed as a result of the A8 of a pinanyl proton (a doublet near 1.1-1.2 ppm). 2 However, this difference is often slight and, in this zone, overlapping peaks may prevent a precise measurement. Another reported analytical method was based on their conversion to the 1-acetamido derivatives and the measurement of the relative integrations of the broad NH peaks by I H NMR. 3 We here report an alternative simple and more general method based on the conversion of the boronic esters 1 to the 2-pyridylthio derivatives 2 (Scheme 1).