Alcohols, their Derivatives, and Halides

Nucleo diiiic Substitution.—Further studies with unhindered secondary hydroxy-steroids confirm earlier reports of almost total inversion of configuration in reactions with PCls. However, thionyl chloride is less efficient than is generally believed in giving chloro-compounds with retention of configuration (SnI reaction). Thus 5a-cholestan-3a- and -3/5-ol with thionyl chloride each gave mixtures of 

3-chloro-compounds with a slight preference for inversion, and the 19-nor-ip-ol mainly the inverted 2a-chloro-compound. (Solvents can influence the course of reaction with thionyl chloride it would seem worthwhile to study the reactions of the 3-ols with thionyl chloride in, for example, ether.) Where the substitution with inversion would encounter serious steric hindrance (e.g. 5a-cholestan-4a-ol), halogeno ompounds are formed with retention, whichever halide is used.  

3/ -Chloro-3a-methyl-5a-cholestane (20), the unstable isomer, is accessible by reaction of 3a-methyl-5a-cholestan-3 -ol (21) with phosphorus pentachloride (hydrogen chloride affords the stable 3a-chloro-isomer). This reaction clarifies the mechanisms of substitution by phosphorus pentachloride. Inversion generally 

For best results, thionyl chloride is conveniently purified by distillation from dipentene. l,l,2-Trifluoro-2-chloro-triethylamine (Et2NCF2CHClF) converts many alcohols into fluoro-compounds, e.g. (22) — (23) (24) — (25), although eliminations and rearrangements involving carbonium ions are not infrequent. A novel variant uses the reagent in the presence of lithium chloride or 

Mousseron-Canet and J.-L. Borgna, Bull. Soc. chim. France, 1969, 613 see also J.-L. Borgna and M. Mousseron-Canet, ibid., 1970, 2218. 

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