Abstract
Protected linear hexapeptide 2 undergoes a remarkably facile C(4)-epimerisation when macrolactamisation is attempted with BOP [benzotriazol-1-yloxy-tris(dimethylamino)phosphonium hexafluorophosphate] and DMAP in CH2Cl2 under conditions of high-dilution; compound 6 is isolated in 51% yield from this reaction. In order to confirm its structure, 6 was independently synthesised from 18 by macrolactamisation with HATU [O-(7-azabenzotriazol-1-yl)-N,N,N',N'-tetramethyluronium hexafluorophosphate] and NEM (N-ethylmorpholine); ring-closure now proceeded in 70% yield. After subsequent deprotection by catalytic hydrogenolysis, amine salt 7 was chemoselectively coupled to activated ester 5 to give 4-epi-A83586C (8) after glycal hydration.
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