Acid catalyzed intramolecular attack of β‐phenylthioureido group on amide function. Parallel formation of thiodihydrouracil and 4‐iminothiodihydrouracil. Different pathways in the Edman degradation reaction in the formation of six‐ versus five‐membered cyclic intermediates

Abstract

Contrary to the cleavage of a-phenylthioureido peptides 1 proceeding through intermediate 2-anilinothiazolinone 2, the b-analog cis-2-(3-phenylthioureido)cyclopentane-carboxamide 5 forms transiently 4-imino-2-thioxopyrimidine 6. Monitoring amide cyclization and hydrolysis of iminopyrimidine 6 in acid by UV showed that an equilibrium between 5 and 6 was reached followed by slower conversion of both compounds into 2-oxo-4-thioxopyrimidine 7. Both processes were characterized by isosbestic points, the first due to parallel conversion of 5 into 6 and 7 (or 6 into 5 and 7) at a constant ratio while the second identical for both reactants -to conversion of equilibrated 5 and 6 into 7. The special isosbestic points allowed the determination of the individual constants of Scheme 2. Further confirmation was obtained from NMR product analysis and following the cyclization of amide 5 in DMSO:DCl. Product 2-oxo-4thioxopyrimidine 7 hydrolyzed reversibly to thioureido acid 8. The cyclization rate of 8 allowed the participation of 6-oxothiazine 10 formed by sulfur attack to be excluded. The absence of sulfur attack in the six-membered case is explained by the longer C-S bond bringing about greater bond angle strain at the tetrahedral ring atoms due to the geometrical characteristics of five-and six-membered rings with planar segments. The cyclizations of amide 5 to iminopyrimidine 6 and to thiodihydrouracil 7 are first order in [H þ ], while the reactions of protonated imine 6H þ are zero order to amide and À1 to thiodihydrouracil. The reaction orders can be reconciled by assuming a rate determining proton transfer from the tetrahedral intermediate in amide cyclization.We now report that in fairly concentrated aqueous acids, a simple b-thioureido amide, namely cis-2-(3-phenylthioureido) cyclopentanecarboxamide 5, cyclizes through a pathway different from the Edman degradation. In the range 0.01-1 M HCl, the following steps are observed: (a) an equilibrium between (www.interscience.wiley.com)