oxalic acid T=110 - 120°C; 0.25 h;

With  4-(N,N-dimethlyamino)pyridine in  tetrahydrofuran T=20°C; 2 h;   

Multi-step reaction with 2 steps 1: 1.) Et3N / 1.) THF, -5 deg C to 0 deg C, 2.) hexane, 1 h, 20 deg C2: 4-dimethylaminopyridine / tetrahydrofuran / 2 h / 20 °C