Rac-(2R*,3R*)-S-Ethyl-4-Chloro-3-Hydroxy-2-Phenylbuthanethioate and Rac-(2R*, 3R*)-S-Ethyl-2-Phenyl-3-(tosyloxy) buthanethioate: Dichotomy of the Stereoselectivity of the Mukaiyama Reaction

摘要

The title compounds, rac-(2R*, 3R*)-S-ethyl-4-chloro-3-hydroxy-2-phenylbuthanethioate (I) and rac(2R*,3R*)-S-ethyl-2-phenyl-3-(tosyloxy) buthanethioate (III), are both composed of a S-ethyl 2-phenylbutanethioate moiety but have different geometries. Compound I is substituted in the 3 and 4 positions by a hydroxyl group and a chlorine atom, respectively. In compound III the hydroxyl group in the 3 position of rac-(2R*, 3R*)-S-ethyl3- hydroxy-2-phenylbuthanethioate (II), has been tosylated in order to obtain suitable crystals for X-ray analysis. In compound I the phenyl substituent and the hydroxyl group have a syn arrangement, whereas in the tosylate derivative of II, i.e., compound III, they have an anti arrangement. In the crystal structure of I centrosymmetric hydrogen bonded dimers are formed via O-H center dot center dot center dot O hydrogen bonds, involving the hydroxyl group and the carbonyl O-atom. In the crystal structure of III symmetry related molecules are connect via a weak C-H center dot center dot center dot O intermolecular interaction, involving a tosylate O-atom and a phenyl H-atom, so forming zigzag chains propagating in the c direction. The compounds were prepared by the Mukaiyama crossed aldol reaction between the silyl enol ether of S-ethyl 2-phenylethanethioate and simple aldehydes, like 2-chloroacetaldehyde (for I) and acetaldehyde (for II). The syn/anti stereo descriptors clearly indicate that the stereoselectivity of the Mukaiyama aldol reaction has switched from a syn selective process for the reaction using 2-chloroacetaldehyde to an anti selective process for the reaction with acetaldehyde. In both compounds the relative stereochemistry at the newly created chiral centers, positions 2 and 3, is R/R.