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 evaluated in vitro for inhibitory activity toward human leukocyte elastase (HLE). The Gewald thiophene synthesis was utilized to obtain several ethyl 2-aminothiophene-3-carboxylates. These precursors were subjected to a five-step route to obtain thieno[2,3-d][1,3]oxazin-4-ones bearing various substituents at positions 5 and 6. Both thieno[2,3-d] and thieno[3,2-d] fused oxazin-4-ones possess extraordinary chemical stability, which was expressed as rate 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 stability of the acyl-enzyme intermediates.