Approaches to pseudopeptidic ergopeptines. Part 2. Consequences of the incorporation of an α-azaproline residue into the oxacyclolic system

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Abstract
As part of a programme to synthesize pseudopeptidic ergopeptines, the introduction of an α-azaproline residue in place of native proline into an ergotamine-like oxacyclolic system has been investigated. Starting material N-[(R)-2- benzyloxypropionyl]cyclo(-Phe-azaPro-)10 was prepared following two alternative synthetic routes and was subjected to reductive O-debenzylation. N,O-Acyl transfer on the resulting N-[(R)-2-hydroxypropionyl]cyclo(Phe-azaPro-)14 leads, through a new type of four- heteroatom tetrahedral adduct, to (5R)-5-methyl-3-{(1S)-2-phenyl-1-[(pyrazolidin-1-yl)carbonyl]ethyl}oxazolidine-2,4-dione 16, as a unique isolable tautomer. Structural and conformational details of compound 14, as revealed by X-ray analysis, are reported and compared with those of previously studied related models.

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