Abstract
When tripeptides of type Axxt-Aib-Axx-OH were coupled with amino acid methyl esters by means of commonly used coupling reagents, the formation of 1,3-thiazol-5(4H)-imines and 1,3-oxazol-5(4H)-imines was observed. With the aim of understanding which structure elements are required for this reaction, several model peptides have been prepared according to our recently described methodology, a modification of the 'azirine/oxazolone method', followed by selective isomerization of the peptide thioamides. In addition, attempts to prepare peptides that contain more than one C=S group by the same methodology also led to the formation of 1,3-thiazol-5(4H)-imine-containing derivatives. An additional C=S group can be introduced into the peptide, when the 1,3-thiazol-5(4H)-imines were treated with H2S, although mixtures of epimers were obtained. The structures of an endothiohexapeptide, two 1,3-thiazol-5(4H)-ones, and two peptides containing a 1,3-thiazol- 5(4H)-imine moiety have been established by X-ray crystal-structure analysis.
Breitenmoser, Roland A