Synthesis of 23a-23d: Reagents and conditions: (a) allylmagnesium chloride, anhyd. diethylether, −78 oC to rt, overnight. (b) ethylvinyl ether, Hg(OCOCF3)2, rt, 4 h. (c) 1st generation Grubb’s catalyst, anhyd. benzene, reflux, 2 h. (d) (i) 9-BBN, anhyd. THF, rt, overnight; (ii) 10% NaOH, 30% H2O2, 50 °C, 1 h. (e) CH3SO2Cl, Et3N, anhyd. DCM, rt, 2 h. (f) NaN3, anhyd. DMF, 80 °C, overnight. (g) H2, Pd/C, MeOH, 50 psi, 2 h. (h) RCHO, NaCNBH3, 1,2-dichloroethane, AcOH rt, overnight. (i) H2, Pd/H2, MeOH, 50 psi, 1 h.