Lithium methylthiolate

The commonly used bases in this reaction are commercially available solutions of n-BuLi (in hexane) (140-146), or n-Bu2Mg (in heptane) (144) freshly prepared lithium amide LiNH2 has also been used for the synthesis of lithium methylthiolate (147). Similarly, several bulky arenethiols (e.g., HSC6H2Ph3-2,4,6) have been successfully deprotonated with either a 1 1 mixture of n-Bu,Mg and s< c-Bu2Mg in heptane or Mg N(SiMe3)2 2 in toluene (148). A major shortcoming of this procedure is that thiols that are not commercially available have to be prepared via metal thiolate species—clearly a roundabout approach. 

Sulfenylation of C-7 imino carbanions was also exploited by Spitzer and Goodson (1973) in preparing 7a-methylthioamides 354-357, and by Ratcliffe and Christensen (1973) in their synthesis of cefoxitin. The former workers used lithium diisopropylamide in DMF (-78°C) to generate the Schiff base anion. Methylthiolation was performed with excess methoxy-carbonylmethyl disulfide. Liberation of the modified nucleus was achieved with Girard-T reagent in aqueous DMF. This series of methylthioamides was subsequently transformed with chlorine to the corresponding 7a-methoxyamides, presumably via the intermediacy of acylimines. 

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