2-(diethylamino)thieno[1,3]oxazin-4-ones as stable inhibitors of human leukocyte elastase

A series of 2-(diethylamino)thieno[1,3]oxazin-4-ones was synthesized and ev aluated in vitro for inhibitory activity toward human leukocyte elastase (H LE). The Gewald thiophene synthesis was utilized to obtain several ethyl 2- aminothiophene-3-carboxylates. These precursors were subjected to a five-st ep route to obtain thieno[2,3-d][1,3]oxazin-4-ones bearing various substitu ents at positions 5 and 6. Both thieno[2,3-d] and thieno[3,2-d] fused oxazi n-4-ones possess extraordinary chemical stability, which was expressed as r ate constants of the alkaline hydrolysis. The kinetic parameters of the HLE inhibition were determined. The most potent compound, 2-(diethylamino)-4H- [1]benzothieno[2,3-d][1,3]oxazin-4-one, exhibited a K-i value of 5.8 nM. 2- (Diethylamino)thieno[1,3]oxazin-4-ones act as acyl-enzyme inhibitors of HLE , similar to the inhibition of serine proteases by 4H-3,1-benzoxazin-4-ones . The isosteric benzene-thiophene replacement accounts for an enhanced stab ility of the acyl-enzyme intermediates.