Competing Pathways in the Azomethine Ylide Route to Indoloquinones: An Improved Procedure for the Generation of a Transient 4-Oxazoline from the Oxazolium Salt

Abstract

Azomethine ylide generation from the oxazolium salt 22 and PhSiH3/CsF affords complex products derived from the initial cycloadduct 23 via three competing pathways. Colorless intermediates 24−27 have been detected, and their oxidation products 10, 28, and 29 have been identified. Nucleophilic activation of the oxazolium salt 22 is reported using the organic-soluble benzyltrimethylammonium cyanide. This variation affords the unstable cycloadduct 32, which undergoes aromatization via a single pathway involving 24 and 27, and DDQ oxidation gives the indoloquinone 10 in an improved 63% yield. Lower yields using PhSiH3/CsF or NaCN activation methods are attributed to the heterogeneous conditions and interception of the azomethine ylide by unreacted oxazolium salt.