Scheme 24. Schreiber's branching pathway exploiting complementary cyclisation reactions. Reagents and conditions: (1) 10 mol% Pd(PPh3)2(OAc)2, benzene, 80 °C, [88a: single diastereomer, 81%; 90d: single diastereomer, 70%] (2) 10 mol% Hoveyda–Grubbs’ 2nd-gen. cat., CH2Cl2, reflux, [88b: trans:cis 85 : 15, 89%; 90a: trans:cis 85 : 15, 87%; 91 : trans:cis 75 : 25, 90%]; (3) 10 mol% CpRu(MeCN)3PF6, acetone, rt, [88c: single diasteromer, 85%; 90c: single diasteromer, 91%]; (4) 10 mol% NaAuCl4, MeOH, rt, single diasteromer, 80%; (5) NaH, toluene, rt, 88%; (6) Co2(CO)8, Et3NO, NH4Cl, benzene, rt, [88f: d.r. >90 : 10, 85%; 90b: single diastereomer, 85%]; (7) m-CPBA, THF, –78 → 0 °C, single diastereomer, 87%; (8) 4-methyl-1,2,4-triazoline-3,5-dione, CH2Cl2, rt [89: single diastereomer from trans diene, 72%; 92: single diastereomer, 65%; 93: 80% (combined yield from mixture of dienes)].
