Stereoselective Syntheses via Esters of Arsinous and Arsinthious Acids

Stereoselective Syntheses via Esters of Arsinous and Arsinthious Acids 

The optically active arsinous acid esters 128-134 and the arsinthious acid ester 135 have been used to prepare optically active tertiary arsines (Table 7). When the arsine sulphide 136 was heated in benzene with n-butyl or n-propyl bromide, ethyl bromide was eliminated and there was a reversal in the sign of the rotation, which was attributed to the formation of the inverted n-butyl and n-propyl esters of (p-carboxyphenyl)phenyl-arsinthious acid, 137a and 137b, respectively (equation 18) . 

Some years later, stereoselective syntheses of a variety of optically active esters of arsinthious acids were reported The esters were prepared by treating secondary chloroarsines with thiols in the presence of (— )-iV,iV-diethyl-a-methylbenzylamine. The -pentyl ester 135, when allowed to react with -propylmagnesium bromide, gave (R)-( —)-65 with low stereoselectivity and inversion at arsenic as indicated in equation 19 (Table 7)  

In thorough work of related interest, the resolution of the arsonthious ester 2-p-carboxyphenyl-5-methyl-l 3-dithia-2-arsindane (141) was described by the fractional crystallization of the (+ )-a-methylbenzylammonium and the (— )-quininium salts the former yielded the (—)-acid, Md —8.7° (CHCI3) the latter the ( + )-acid, [a]o + 8.9° (CHCla). The optical stability of the compound was unaffected by crystallization from boiling ethanol, but in pyridine the rotation of the (—)-acid reached half-value within 6 weeks. The compound racemized more rapidly in 0.1 N sodium hydroxide, which was attributed to fission of the arsenic-sulphur bonds. 

Although ethylmethylphenylphosphine oxide was the first tetrahedral phosphorus compound to be resolved only negative results were obtained when attempts were made to resolve methyl(a-naphthyl)phenylarsine oxide (142) by protonation and separation of (-I-)-a-bromocamphor-7E-sulphonate salts, or (o-carboxyphenyl)methylphenyl-arsine oxide (143) or its anhydride 144 after protonation and use of ( —)-brucine or (H-)-morphine . The reversible combination of a tertiary arsine oxide with water was recognized at the outsetand it is this property that is responsible for the racemization  

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