Scheme 3: General method for the synthesis of 20a–i. Reagents and conditions: (a) 2, 2-dimethoxypropane, TsOH, anhydrous acetone, reflux, 4h; (b) ethyl bromide,K2CO3, DMF, rt, 12h; (c) LiAlH4, THF, reflux, 4h; (d) 10a-i, EDCI, DMAP, CH2Cl2, rt, 8-12h; (e) 10%HCl, acetone, rt, 2h.