Methyl (4R)-3-(2-diazo-3-oxobutanoyl)-1,1-dioxo-1λ⁶,3-thiazolidine-4-carboxylate 14 undergoes a base-induced cyclisation to give methyl (8aS)-3-acetyl-4,7,7-trioxo-1,4,6,7,8,8a-hexahydro-7λ⁶-[1,3]thiazolo[4,3-c][1,2,4]triazine-8a-carboxylate 15 in a state of high enantiomeric purity. Similar stereoselective cyclisations, proceeding with retention of configuration, are observed with methyl (1R,4R)- and (1S,4R)-3-(2-diazo-3-oxobutanoyl)-1-oxo-1λ⁴,3-thiazolidine-4-carboxylates 25 and 27 to give compounds 33 and 34. It is suggested that the cyclisation reactions proceed by way of planar ester enol(ate) intermediates which possess axial chirality, e.g. 35.

The bicyclic sulfone 15 and the bicyclic sulfoxides 33 and 34 are also produced by oxidation of methyl (8aS)-3-acetyl-4-oxo-1,4,8,8a-tetrahydro[1,3]thiazolo[4,3-c][1,2,4]triazine-8a-carboxylate 5 with m-chloroperoxybenzoic acid (in DMF in the case of the sulfone 15 and in CHCl₃ in the case of the sulfoxides 33 and 34). The use of the oxidant in methanol or of hydrogen peroxide in formic acid leads to an oxidative deacetylation to give methyl (8aS)-3,4,7,7-tetraoxoperhydro-7λ⁶-[1,3]thiazolo[4,3-c][1,2,4]triazine-8a-carboxylate 17, the structure of which is established by an X-ray crystallographic analysis. The analysis reveals an interesting packing arrangement of the molecules in the crystal, attributable to an intermolecular H-bonding network. In particular, intermolecular H-bonding between the ester carbonyl oxygen atom and the amino hydrogen atom at position 1 provides a possible explanation for the shift of the ester carbonyl absorption to 1680 cm^–1 in the solid-state IR spectrum of compound 17.