Synthesis of optically pure highly functionalized gamma-lactams via 2-azetidinone-tethered iminophosphoranes.

Benito Alcaide, Pedro Almendros, Jose M Alonso

Index: J. Org. Chem. 69(3) , 993-6, (2004)

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Abstract

A synthesis of optically pure densely functionalized gamma-lactams starting from 2-azetidinone-tethered iminophosphoranes has been developed. Full chirality transfer has been accomplished from the enantiomerically pure 2-azetidinones. The addition of lithium acetylides to 4-oxoazetidine-2-carbaldehydes at -78 degrees C smoothly yielded propargylic alcohols with excellent diastereoselectivities. Propargylic alcohols were converted to mesylates, which by exposure to sodium azide afforded the corresponding azides. Treatment of beta-lactams bearing an azido side chain with triphenylphosphine (TPP) gave lambda(5)-phosphazenes (iminophosphoranes, phosphine imines), which were not isolated. The sodium methoxide promoted reaction of the phosphazene beta-lactams smoothly provided gamma-lactams, through a N1-C2 bond breakage process on the four-membered lactam with concomitant ring expansion, followed by hydrolysis.