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A Microwave-Assisted Cyclotrimerization Route to Phenanthridines
L. Sripada, J. A. Teske, A. Deiters*
North Carolina State University, Raleigh, USA
19 March 2008 (online)
Reported here is a concise synthetic route to phenanthridines through a microwave-assisted intermolecular [2+2+2] cyclotrimerization of a diyne with an alkyne. The diyne fragment was readily prepared from aniline via Sonogashira coupling followed by an acylation-alkylation sequence. Microwave irradiation enhanced the yields for the cyclotrimerization reaction. The reaction proceeds in good yields for terminal monoalkynes but with no regioselectivity. Doubly substituted alkynes underwent smooth reaction using open-vessel microwave conditions with slow addition of the diyne (N. E. Leadbeater et al. Org. Proc. Res. Dev. 2006, 10, 833). The regioselectivity was improved, albeit with decrease in yields, by using TMS acetylene precursors. Aniline substitution patterns were not studied, although a variety of alkynes were used.