Novel polyoxazole-based cyclopeptides from Streptomyces sp. Total synthesis of the cyclopeptide YM-216391 and synthetic studies towards telomestatin

A convergent, complementary, synthetic approach to the contiguously linked tris-oxazole units 10, 11 and 12 in telomestatin (1) and YM-216391 (2) is described. The route involves coupling reactions between oxazole 4-carboxylic acids, viz 16a, 16c, 16d and oxazole 2-substituted methylamines, viz 16b, 16e, 17, leading to the amides 18 and 21, followed by cyclodehydrations to the corresponding bis-oxazole oxazolines, e.g. 19, and oxidations of the latter using well-established protocols. The tris-oxazoles 11 and 12 were next converted stepwise into the hexa-oxazole bis-macrolactams 33. Although the bis-macrolactams 33 (cf. 28) could be converted into the corresponding oxazoline-hexa-oxazoles 34 and to the enamides 35, neither of these intermediates could be elaborated to the hepta-oxazole 30en route to telomestatin 1. Likewise, neither the hexa-oxazole 47 or application of an intramolecular Hantzsch oxazole ring-forming reaction from 44b allowed access to the advanced polyoxazole-macrolactam intermediates 48 and 30a, respectively, towards telomestatin. Combination of the tris-oxazole based methylamine 70 with the dipeptide carboxylic acid 71 derived from D-valine and L-isoleucine, leads to the corresponding amide which, in two straightforward steps, is converted into the -amino acid 78. Macrolactamisation of 78, using HATU, next produces the cyclopeptide 79 which is then elaborated to the thiazole and oxazole based cyclopeptide YM-216391 (2). The synthetic cyclopeptide 2 is shown to be the enantiomer of the natural product isolated from Streptomyces nobilis.