Synthesis of aldehydes 16 and 20a
aReagents and conditions: a) MeOH, from 0 °C to 25 °C, 12 h, 100%; b) p-cresol, Cs2CO3, MeCN, 90 °C, 14 h, 74%; c) NBS, AIBN, CCl4, 90 °C, 1 h; d) NBS, DMSO, 100 °C, 2 h, 30% over two steps; e) SOCl2, MeOH, 0 to 25 °C, 12 h, 100%; f) NBS, AIBN, CCl4, 80 °C, 4 h, 60%; g) NMO, MeCN, 25 °C, 12 h, 70%.