Chiral silyl acetals as convenient reagents for determining enantiomeric purity of alcohols

Abstract

Dimethyl and diphenylsilyl acetals of structure (3) are convenient reagents for determining the enantiomeric purity of alcohols by n.m.r. spectroscopy.

New Silyl Ether Reagents for the Absolute Stereochemical Determination of Secondary Alcohols

“…It has recently been shown that chiral silanes can be used to resolve the chemical shifts of enantiotopic stereogenic centers and to determine the absolute configuration of natural products. ,, However, the absolute stereochemical determination of a molecule with use of these previously described diphenyl alkyl-substituted silyl ethers was only possible when enantiomerically pure standards were available for comparison. , We suspected that if a derivative could be designed that had a conformationally stabilizing component to its structure, it would be possible to reliably determine absolute stereochemistry by using this type of synthetic tool.…”

New methodologies for enantiomeric excess (ee) determination based on phosphorus NMR

“…Representative examples are given in Table XIV. [106][107][108]0.1 M. It should be noted that alkylphosphonothioic dichlorides are not satisfactory for the analysis of chiral secondary or tertiary amines. However, chiral diamine 109, which is a typical example of a bifunctional compound containing both a primary and a tertiary amine center, readily forms the diastereomeric phosphonothioic diamide 110, which shows weII separated absorptions in the decoupled 31p NMR spectra, allowing accurate ee determination 103 (Scheme 33).…”

“…Fortunately, alkylphosphonothioic dichlorides [RP(=S)CI2] are relatively stable, resistant to hydrolysis and provide in good yields the alkylphosphonothioic diamides upon treatment with amines101.102. Reagent 96 (R = CH 3) reacts in the presence of triethylamine quantitatively in a few minutes at -20°C with two equivalents of a-allylglycine methylester (R = allyl) to afford diastereomeric methylphosphonic diamides 106,107 and 108 (Scheme 32). The decoupled 31 P NMR spectrum shows three nicely separated singlets for the two meso compounds RS1_I07 and RS2_108 and the racemic (RR and SS)-106 adducts, with a meso /D,L ratio (49/51) virtually identical with theory ( Figure 7).…”

Chiral Silylation Reagents for the Determination of Absolute Configuration by NMR Spectroscopy

“…A survey of the literature revealed two investigations of chiral silylating reagents for examining the optical purity of alcohols. Clausen and Bols have shown that trimenthoxychlorosilane can distinguish between enantiomers of a racemic homoallylic alcohol by 13 C NMR spectroscopy, and Chan et al showed that chiral silyl derivatives could be used to determine ee's for simple secondary alcohols by 1 H and 13 C NMR analysis. Surprisingly, no application of this methodology has been reported in elucidating the absolute stereochemistry of natural products.…”