No abstract

To further understand stereoselection in the alkylation of sulfenate anions, a protected cysteinesulfenate was generated in THF solution at low temperature. Introduction of a reactive alkylating agent brings about a cysteinyl sulfoxide in 51-75% yield, with diastereomeric ratios at the sulfinyl group ranging from 83:17 to 95:5. An internally complexed lithium counterion is proposed to account for the stereoselectivity.

An increasing number of reactions of sulfenic acid anions are being demonstrated in the literature. As such, mild, general and reliable means for the generation of sulfenates are due. In the current paper, an addition/elimination of 2-sulfinyl acrylates using various nucleophiles is demonstrated and evaluated as a protocol for alkane- and arenesulfenate generation. Cyclohexanethiolate, methoxide and n-butyllithum each exhibit some merit for the reaction, and the thiolate is established as a mild, selective and effective reagent to release sulfenates from 2-sulfinyl acrylates. The stereospecificity of the addition/elimination of each nucleophile is recognized, and an explanation for the specificity is offered for thiolate and methoxide.

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As a means to circumvent the standard and sometimes troubling conjugate addition reaction of thiols to alkyl propiolates, a new reagent, cesium (Z)‐2‐carbomethoxyethenethiolate, is introduced. The cesium carbonate mediated methanolysis of (Z)‐2‐carbomethoxyethenyl thiolacetate at low temperature creates cesium (Z)‐2‐carbomethoxyethenethiolate in solution. For base‐insensitive and solvolytically stable electrophiles, efficient substitution can be achieved by direct introduction to the electrophile. The conditions were adapted to accommodate base‐sensitive cysteine derivatives, and the use of DMF as a cosolvent permits the isolation of (Z)‐2‐carbomethoxyethenyl cysteinyl thioethers in high enantiomeric purity (>99:1 er) and with good Z/E ratios. There was no evidence of carbon substitution reactions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)