Scheme 4.

New synthesis of C14-C19 building block. Reagents and conditions: (a) 1. catalytic enantioselective Cr-mediated propargylation in the presence of the Cr-catalyst (10 mol %) prepared from CrBr₃ and (R)-sulfonamide E. 2. Ac₂O/pyr/rt. 3. Amano lipase PS-800 (10% wt), phosphate buffer:acetone (1:9), 50 °C; the overall yield of optically pure 8 from 5: 73%. (b) Cl₃CCONH₂, PPh₃, CH₂Cl₂, rt (94%). (c) B-iodo-9-BBN, CH₂Cl₂, 0 °C (85%).¹⁵