Scheme 2.

Preparation of A-ring keto-aldehyde 4. Reagents and conditions: a) NaBH₄, CeCl₃, MeOH, −78 °C to 0 °C; b) m-CPBA, CH₂Cl₂, 0 °C; c) TBSCl, imid, CH₂Cl₂, 0 °C to RT, 65 % over 3 steps; d) BF₃⋅OEt₂, CH₂Cl₂, −20 °C, 78 %, 19:1 d.r.; e) NaH, RT, 72 h, 73 %, >19:1 (E):(Z); f) LiAlH₄, Et₂O, 0 °C; g) Ac₂O, py, DMAP, CH₂Cl₂, RT, 77 % over 2 steps; h) LDA, TBSCl, THF, −78 °C to RT; PhH, reflux, 16 h, 10:1 d.r.; i) LiAlH₄, Et₂O, 0 °C, 63 % over 2 steps; j) 3 n HCl, MeOH, (1:3), RT; k) DMSO, (COCl)₂, Et₃N, CH₂Cl₂, −78 °C to RT, 74 % over 2 steps. m-CPBA=3-chloroperoxybenzoic acid, DMAP=N,N-dimethyl-4-aminopyridine, DMSO=dimethylsulfoxide, imid=imidazole, LDA=lithium diisopropylamide, py=pyridine, TBS=tert-butyldimethylsilyl, THF= tetrahydrofuran.