The yeast reduction products (2R, 3S)-N-Boc-3-hydroxyproline esters 23 have been converted into the 3-methoxymethoxyprolinal 26 which undergoes an efficient Julia olefination with the L-serine-derived amino sulfone 29. Selective reduction of the resulting alkene 31 using diimide and cyclization leads to N-(benzyloxycarbonyl) slaframine 33c and thence to natural (-)-slaframine 5 and its more stable, crystalline N-acetyl derivative 34.
