ChemInform Abstract: Acidic, Selective Monoacylation of vic‐Diols.

Abstract

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Using NMR Spectroscopy To Probe the Chemo- and Diastereoselectivity in the NaBH4 Reduction of Benzoin Acetate and Benzoin Benzoate

“…The NMR spectrum of the crude product also displayed two singlets at 1.97 and 2.09 ppm that were attributable to the acetoxy CH 3 groups of 3 and 4 , respectively, further indicating that the ester function had not been reduced by NaBH 4 and that the product was a mixture of diastereomers 3 and 4 . The assignment of the doublets at 5.88 and 4.96 ppm and the singlet at 1.97 ppm to diastereomer 3 came from a perfect match of these peaks with the corresponding peaks from an authentic sample of 3 . NMR integration of the doublets revealed a ratio of 80:20 for diastereomers 3 and 4 , respectively.…”

“…1 H NMR spectra are recorded in CDCl 3 (TMS internal standard). For comparative purposes, 1 H NMR spectra of (±)-benzoin and authentic samples of hydroxyesters 3 and 5 are provided. In a postlaboratory session, NMR spectra for all compounds are analyzed for the chemo- and diastereoselectivity associated with the NaBH 4 reduction of ketoesters 1 and 2 .…”

“…Using 1 H NMR spectroscopy, students ”discover” that NaBH 4 reductions of their substrates are highly chemoselective and that the ketone function is selectively reduced while the ester function remains intact. Also, based on their NMR data, they “discover” that reduction of compounds 1 and 2 affords mixtures of the erythro - and threo -diastereomers 3 / 5 and 4 / 6 , respectively . These findings lead to a postclassroom discussion on the chemo- and diastereoselectivity of NaBH 4 reductions of the chiral ketoesters 1 and 2 as revealed by 1 H NMR data and the general application of the Felkin-Anh model for nucleophilic addition to the carbonyl group.…”