This synthesis described by Fortunak in 1996, has an intramolecular Diels-Alder [4+2] cycloaddition as key step between an unactivated alkyne and a imidate generated in situ by action of Me3OBF4 on a N-arylamide.
This synthesis has been recently improved by Yao et al. using a mixture of Ph3PO and Tf2O (2:1) instead of Me3OBF4, by this methodology a wide range of tetracyclic "camptothecin-like" molecules has been synthesized.
Organic Letters, ASAP, April 14, 2007
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