Synthesis of Cyclic Sulfamates by a Cascade Process

Significance

A one-pot synthesis of nitrogen-containing molecules via a rhodium-catalyzed cascade process is described. Treatment of homopropargylic sulfamate esters 1, 3 and 5 with catalytic dirhodium and a stoichiometric oxidant leads to formation of cyclic N-sulfonyl imines. Reductive workup or Grignard addition gives the final products 2, 4 and 6 in moderate to excellent yields. In all cases, the seven-membered ring is formed exclusively. Geometrical constraints prevent the propargylic sulfamate from undergoing cyclization, whilst extending the chain length further also leads to the seven-membered ring via cyclization onto the proximal alkyne carbon. The reaction conditions were optimized with respect to the rhodium catalyst and solvent. The proposed mechanism involves attack of the alkyne on a generated metallonitrene, followed by cyclization to the oxonium ylide and subsequent rearrangement, as evidenced by several control experiments.