A cyclisation reaction of methyl (4R)-3-(2-diazo-3-oxobutanoyl)thiazolidine-4-carboxylate which proceeds with retention of configuration, probably via a planar ester enolate intermediate possessing axial chirality

Under basic conditions, the title compound 1b is converted into methyl (6S)-3-acetyl-2-oxo-8-thia-1,4,5-triazabicyclo[4.3.0]non-3-ene-6-carboxylate 4a, the absolute configuration of which is established by X-ray crystallography.