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. Author manuscript; available in PMC: 2011 Aug 12.
Published in final edited form as: J Med Chem. 2010 Aug 12;53(15):5770–5781. doi: 10.1021/jm100582w

Scheme 1.

Scheme 1

a Reagents and conditions: (a) PPh3, DMAD, THF, −78 °C, 20 min, then room temperature, 2.5 h; (b) CF3COOH, p-TsOH, 0 °C, 10–15 min; (c) CH2Cl2/CF3COOH, p-TsOH, 0 °C, 0.5 h.