I tried to conduct the synthesis of a cyclic dipeptide from L-DOPA directly. Following the experiments from some papers, I set the reaction temperature to be 220 celsius degrees and the solvent I chose was glycerol. Every time the temperature is about 180 celsius degrees, the solution quickly got brown and the color would be deeper if continued to be heated. I don't think my desired products are brown as the reaction time is too short, so I guess it is because the L-DOPA was oxidized. but even when the reaction was performed under the nitrogen atmosphere, the situation occurred again (and no paper suggests the synthesis should be isolated from oxygen).

Therefore I would like to learn:

what is the color of cyclic dipeptides generally? should this synthesis be conducted in the absence of oxygen?

Thank you!