Cyclization of a linear substrate to form a medium ring is inherently difficult as a result of enthalpic and entropic factors. The use of a sulfur-containing zwitterionic catalyst in the catalytic bromolactonization of long-chain olefinic acids to form medium-sized lactones is reported, using N-bromosuccinimide (NBS) as the stoichiometric halogen source. The reaction was found to be more efficient at 0 �C than at room temperature, which could be attributed to the temperature dependence of the zwitterionic catalyst. The catalyst was found to exist in equilibrium with its individual components. Low-temperature 1H NMR studies revealed the shifting of this equilibrium to favor the formation of more zwitterionic catalyst species. The catalyst can be easily synthesized via a one-step coupling reaction between N,N-dimethylaminopyridine and 3,5-bis(trifluoromethyl)phenyl isothiocyanate, while NBS is inexpensive and readily available. These enhance the practical utility of the bromolactonization protocol.

Efficient medium ring size bromolactonization using a sulfur-based zwitterionic organocatalyst

245th National Meeting (2013)

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