HIDDEN AXIAL CHIRALITY AS A STEREODIRECTING ELEMENT IN REACTIONS INVOLVING ENOL(ATE) INTERMEDIATES .1. CYCLIZATION REACTIONS OF METHYL -(2-DIAZO-3-OXOBUTANOYL)THIAZOLIDINE-4-CARBOXYLATE AND RELATED-COMPOUNDS

Methyl -(2-diazo-3-oxobutanoyl)thiazolidine-4-carboxylate lb underwent cyclisation, under a variety of basic conditions, to give methyl -1,4 ,5-triazabicyclo[4.3.0]non-3-ene-6-carboxylate 2a in an enantiopure st ate. The absolute configuration of compound 2a was deduced by X-ray cr ystallography. Similar stereoselective cyclisations, proceeding with r etention of configuration, were observed with methyl (methoxycarbonyl) acetyl]thiazolidine-4-carboxylate 1g (to give compound 5a), methyl xob utanoyl)-2,2-dimethylthiazolidine-4-carboxylate 20a (to give compound 21a) and methyl -3-oxobutanoyl)-2-methylthiazolidine-4-carboxylate 22a (to give compound 24). An X-ray crystallographic analysis of compound 22a revealed that the amide and diazo ketone units, although individu ally near planar, were twisted from each other by 35-degrees; it was n otable that the amide linkage adopted the (Z)-geometry required for th e cyclisation reaction whereas the diazo moiety was incorrectly aligne d. It is suggested that the cyclisation reactions proceed by way of pl anar enol(ate) intermediates, e.g. 6a, which possess axial chirality.