The chlorinating behaviour of WCl6 towards alpha-aminoacids

Cl/O interchange took place when WCl6 was allowed to interact with a series of alpha-aminoacids. The alpha-ammonium-acylchloride salts [NH2(CH2)(3)CHC(O)Cl][WOCl5], 1a, and [MeNH2CH2C(O)Cl][WOCl5], 1b, were afforded in ca. 55% yields from the reactions of WCl6 with, respectively, L-proline and sarcosine in CH2Cl2. By using other reaction media (hexane or CHCl3), the alpha-amino-acylchloride complexes WOCl4[O=C(Cl) CH(CH2)(3)NH2], 5a, and WOCl4[O=C(Cl)CH(R)NHR'] (R = H, R' = Me, 5b; R = R' = H, 5c; R = Me, R' = H, 5d) were isolated in moderate to good yields from WCl6 and, respectively, L-proline, sarcosine, glycine and L-alanine. The formation of 5a, b is basically the result of HCl release from the parent compounds 1a, b. 5a represents a key intermediate in the course of the reaction leading to (WOCl4)(2)[mu:kappa(2)(O)-dkp], 2, dkp = (S, S)-octahydrodipyrrolo[1,2-a:1',2'-d]pyrazine-5,10-dione. 2 was optimally prepared from WCl6/L-proline under high temperature conditions. Hydrolytic treatment of 2 afforded the L-proline-derived 2,5-diketopiperazine (dkp), which was finally isolated with an overall yield of 70%. 1a, b were characterized by X-ray diffractometry, thus providing very rare examples of crystallographically characterized acylchloride derivatives of alpha-aminoacids. DFT calculations were extensively carried out in order to shed light on structural and mechanistic features.