Scheme 3.
Reagents and conditions: a) 1.0 equiv 6a, 1.1 equiv 12, 0.1 equiv piperidine, THF, 50 °C, 21 h, 91 %; b) 1.5 mmol 13, 0.10 g DOWEX-H+, THF/CH3OH (1:1), 25 °C, 20 h, 75 %.
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