Oxidative ring‐opening of rel ‐(2 R ,3 R ,5 S )‐5‐aryl‐2‐benzoylamino‐6,7‐bis(methoxycarbonyl)‐2,3‐dihydro‐1‐oxo‐3‐phenyl‐1 H ,5 H ‐pyrazolo[1,2‐ a ]‐pyrazoles. Synthesis of rel ‐(2 R ,3 R )‐3‐phenyl‐3‐[5‐aryl‐3,4‐bis(methoxycarbonyl)pyrazolyl‐1]alanine esters

rel ‐(2 R ,3 R )‐ N ‐Benzoylamino‐6,7‐bis(methoxycarbonyl)‐2,3‐dihydro‐1‐oxo‐1 H ,5 H ‐pyrazolot[1,2‐ a ]‐pyrazoles 5 , accesible by cycloaddition of dimethyl acetylenedicarboxylate ( 3 ) to (1Z)‐ rel ‐(4 R ,5 R )‐1‐aryl‐methylidene‐4‐benzoylamino‐5‐phenyl‐3‐pyrazolidinone‐1‐azomethine imines 4 , undergo oxidative ring cleavage with methanolic bromine giving rel ‐(2 R ,3 R )‐ N ‐benzoyl‐3‐phenyl‐3‐[5‐aryl‐3,4‐bis(methoxy‐carbonyl)pyrazolyl‐1]alanine methyl esters 6 as products.