Metal- and base-free synthesis of imidazo[1,2-a]pyridines through elemental sulfur-initiated oxidative annulation of 2-aminopyridines and aldehydes.

The elemental sulfur-promoted oxidative cyclization reaction for the efficient synthesis of substituted imidazo[1,2-a]pyridines has been developed. Easily available 2-aminopyridines and aldehydes were directly assembled in a highly atom-economical fashion through oxidative annulation under metal- and base-free conditions. Besides arylacetaldehydes, aliphatic aldehydes were also compatible with this system to deliver the alkyl-substituted imidazo[1,2-a]pyridines in excellent yields with the capability of gram-scale synthesis.