Isolation of methyl (RS)‐1‐tert‐butoxycarbonyl‐3‐cyanomethyl‐1,2‐dihydro‐2‐oxo‐5H‐pyrrole‐5‐carboxylate, the key‐intermediate in base‐catalyzed formation of racemic products by 1,3‐dipolar cycloadditions to methyl (S)‐1‐tert‐butoxycarbonyl‐3‐[(E)‐cyanomethylidene]‐2‐pyrrolidinone‐5‐carboxylate