The reaction of N-(tert-butoxycarbonyl)-L-serine methyl ester (I) with hydrazine gives the corresponding hydrazide (II), which is acylated with trifluorothioacetic acid S-ethyl ester (III), yielding the diacylated hydrazine (IV). The cyclization of (IV) by means of DEAD and PPh3 affords the pyrazolidinone (V), which is deacylated with NaOH to furnish 4(S)-(tert-butoxycarbonyl)pyrazolidin-3-one (VI). The condensation of (VI) with vinylphosphonate (VII) gives the adduct (VIII), which is cyclized with the oxalyl monoester chloride (IX) by means of DIEA, yielding the pyrazolopyrazole (X). Finally, this compound is deprotected in acid medium and acylated with thiazole derivative (XI) by means of a Pd(0) catalyst to furnish the target compound.
The cyclization of N-(tert-butoxycarbonyl)-O-(p-toluenesulfonyl)serine (I) with hydrazine gives the pyrazolidinone (II), which by reaction with formaldehyde yields the ylide (III). The cyclization of (III) with the vinyl sulfone (IV) by means of NMM affords the bicyclic pyrazolidinone (V), which is deprotected in acid medium, then condensed with the acid chloride (VI) and treated with a Pd(0) catalyst.