Science of Synthesis

Collier, S. J., Science of Synthesis, (2007) 33, 509.

One of the most obvious disconnections of an N-sulfonylenamine gives a primary or secondary sulfonamide and an alkene synthon. As expected, this approach has proved useful in the synthesis of N-sulfonylenamines. In general, the alkene synthons are alkynes bearing an electron-withdrawing group at one terminus. The reactions proceed through conjugate addition of the sulfonamide anion at the opposite end of the electron-withdrawing group (‌Scheme 10‌). This chemistry has been applied to the synthesis of (−)-indolizidine 223AB (‌4‌) (see General Introduction, ‌Scheme 2‌).[‌1‌] In the case of phenylalkynoates, treatment with sulfonamides in the presence of triphenylphosphine results in addition at the same side as the ester unit (‌Scheme 10‌). In this case, the reaction proceeds through an initial conjugate addition of the phosphine at the β-position, followed by attack of the sulfonamide at the position α to the ester. Elimination of the phosphine provides the N-sulfonylenamine.[‌26‌] Secondary 4-toluenesulfonamides undergo a palladium-mediated vinyl-transfer reaction with butyl vinyl ether to give N-tosylenamines; this offers an alternative approach for vinylation.[‌27‌] Palladium-catalyzed oxidative amination of nonactivated terminal alkenes with secondary 4-toluenesulfonamides can also give N-tosylenamines, with amination occurring at the more substituted end of the alkene.[‌28‌] With primary 4-toluenesulfonamides, N-tosylimines are formed, whereas with cyclic alkenes, allylic amines are formed.[‌28‌] The direct alkenation of N-monoalkylsulfonamides by acetylene is possible, but high temperatures and pressures are required.[‌29‌]

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References