3,1-Benzothiazin-4-ones and 3,1-Benzoxazin-4-ones: Highly Different Activities in Chymotrypsin Inactivation

3,1-Benzothiazin-4-ones are suIfur analogs of the potent serine protease inactivators of the 3, l-benzoxazin-4-one type, which acylate the serine residue within the active site of the enzymes. A series of 2-amino-3,1-benzothiazinones was synthesized, but these compounds showed only very little inhibitory activity toward chymotrypsin, a model serine protease. Detailed investigations revealed that benzothiazinones and benzoxazinones react with identical mechanisms, but benzothiazinones acylate chymotrypsin with much lower rate constants. Investigations of nonenzymatic hydrolysis showed the benzothiazinones to be intrinsically more stable than benzoxazinones. It was concluded from spectroscopic results, that benzoxazinones are highly activated due to the absence of ester-like resonance. 2-Benzoylamino-4H-3,1-benzoxazin-4-one was found to be a new, highly active chymotrypsin inactivator. In contrast, benzothiazinones were found to be resonance stabilized. The contribution of a resonance structure with an exocyclic oxanion to the overall structure of the benzothiazinones and its nonproductive binding to the active site explained their low reactivity toward chymotrypsin.