Oxazole nucleus preparation

Similar in approach to linear methods discussed previously, the cyclodehydration of a-amido ketones is an alternative strategy for preparation of the oxazole nucleus. Treatment of the ketone with PPhs and I2 results in cyclization to provide the fully substituted oxazole nucleus in excellent yield. This strategy was recently used in the synthesis of oxazole-modified glycopeptides, designed to target two classes of arthritis-associated proteins. The oxazole was probed as a conformationally locked dipeptide mimic. 

Thiazole-containing cyclophanes have been prepared by the Hofmann pyrolytic route. Two isomeric [2.2](2,5)thiazolophanes were isolated. From N.M.R. spectral comparisons between normal and deuterated derivatives the isomers were shown to have the anti.anti and anti,syn structures. The aromatic nucleus is conformationally rigid on an N.M.R. time scale whereas their oxygen congeners are conformationally mobile 0. Dipole moments and I.R. spectra for (96 R=H, Ph) were reported. The Cl and 0 are in the trans conformation with a minor amount of Cl, O-ois conformer. This is in contrast to the corresponding oxazole where only the former... 

Copper(II) bromide has been used to a-brominate diketotetraquinanes and to introduce a double bond into a prostanoid nucleus in a one-pot bromination-elimination procedure (eq 3). 3,7-Dibromo-2//,6//-benzodithiophene-2,6-diones (eq 4) and 5-bromo-4-oxo-4,5,6,7-tetrahydroindoles (eq 5) are prepared by the selective a-bromination of their respective ketone starting materials without bromination of the aromatic or heterocyclic rings. 4-Carboxyoxazolines are converted to the corresponding oxazoles using a mixture of CuBra and l,8-diazahicyclo[5.4.0]undec-7-ene (eq 6). ... 

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