Intramolecular Hydrogen Bonding. An Infrared and Nuclear Magnetic Resonance Study of Diastereomeric Episulfides

A detailed ir and nmr study has confirmed the absolute configurational assignments of diastereomeric {3hydroxy (-acetoxy) episulfides. The concentration dependence of the bonded and free hydroxyl absorptions of the alcohols shows that the isomer previollsly assigned the 25,3R, or threo configuration (I, R = H) forms the stronger intramolecular hydrogen bond (~v = 138 cmI ), whereas the isomer assigned the 25,35, or erythro configuration (II, R = H, ~v = 123 cmI ) has a greater tendency to associate intermolecularly (dimers) in carbon tetrachloride. Chloroform may be a better solvent than generally thought for differentiating between closely related isomers that have relatively weak internal hydrogen bonds. Our results indicate that chloroform is a good solvent for estimating the relative strengths of intramolecular hydrogen bonds of the diastereomeric {3-hydroxy episulfides, since chloroform efficiently disrupts interfering weaker intermolecular bonds at episulfide concentrations below 0.03 M while it has little effect on the intramolecular hydrogen bonds. The OH proton chemical shift for either isomer is linearly dependent upon concentration, the threo alcohol exhibiting a smaller limiting slope but larger limiting chemical shift compared with the erythro alcohol. The strong HCOH coupling Uncon = 9.0 ± 0.3 Hz) found for the threo alcohol at all concentrations studied contrasts with the smaller coupling (lucon rv 2-3 Hz) for the erythro isomer observed only at low concentrations «0.007 M). The methylene protons of the episulfide ring (H-4 and H-S) are part of an ABX system with H-3 in both isomers, but in the erythro alcohol this pattern collapses to an AA IX system in dilute chloroform solution (below 0.084 M) apparently as a result of specific interaction with chloroform. Benzene-induced solvent shifts for protons of the acetates suggest an interaction in which the planes of the benzene and episulfide rings are approximately perpendicular. W e previously reported",4 the isolation and characterization of diastereomeric pairs oCB-hydroxy episulfides, l-cyano-2-hydroxy-3,4-epithio butanes, which are formed on enzymic hydrolysis of thioglucosides [potassium (5)and (R)-2-hydroxy-3-butenylglucosinolates] in the seeds of Crambe abyssinica (isomers I and II) and Brassica napus (isomers III and IV, R = R). Whereas the absolute configurations at C-2 of the pairs of diastereomers [2(5) for I and II, 2(R) for III and IV] were established unequivocally by chemical means, 'l-i the absolute configurations at C-3 (Chart I)