OXAZOLE ACTIVATION FOR AZOMETHINE YLIDE TRAPPING - SINGLY AND DOUBLY TETHERED SUBSTRATES

Bicyclic oxazolium salts 18, 24, 37, and 44 can be generated from tethered haloalkyloxazoles by internal alkylation. Reductive alkylation of the oxazolium salts using CsF/PhSiH3 converts the salts initially into the corresponding 4-oxazoline derivatives. Subsequent electrocyclic ring opening generates stabilized azomethine ylides that can be trapped by suitable dipolarophiles. Intermolecular dipole trapping followed by DDQ oxidation affords the ring-fused pyrroles 22 and 26. When tethered alkynoates are used for internal dipole trapping, the adducts 38 and 45 can be obtained by a similar reductive activation sequence, followed by DDQ workup. Effective procedures for the internal oxazole N-alkylation step are described using an acetonitrile-trifluoroethanol solvent system. Also, an improved method for the generation of the dichlorocerium derivative of ethyl propiolate and intermolecular trapping by an aldehyde is reported.