SHORT COMMUNICATIONSCycloaddition reactions of aliphatic diazo compounds are widely used in the synthesis of various nitrogen-containing and other heterocycles [1][2][3]. In these reactions, the most reactive dipoles are believed to be diazoalkanes and 2-diazo ketones, whereas cycloaddition of 2-diazo-1,3-dicarbonyl compounds to common dipolarophiles is not typical [2,3].The goal of the present work was to study cycloaddition of 2-diazo-1,3-dicarbonyl compounds to "superdipolarophiles" [4], i.e., aromatic thioketones which exhibit the highest reactivity in cycloaddition processes, and elucidate the possibility for synthesizing in such a way O-and S-containing heterocycles.As substrates we used acyclic and carbocyclic 2-diazo-1,3-dicarbonyl compounds, diazoacetylacetone (Ia), ethyl diazoacetoacetate (Ib), and diazodimedone (Ic), and thiobenzophenone (II), thiofluorenone, and other aromatic thioketones were selected as dipolarophiles. We have found that diazodicarbonyl compounds Ia-Ic react with aromatic thioketones even at room temperature to give tetrasubstituted 1,3-oxathioles IIIa-IIIc as final products (Scheme 1).Taking into account that diazodicarbonyl compounds Ia-Ic are stable under usual conditions and that they can be stored for a long time without appreciable decomposition, we presumed that the reaction follows a conventional three-step scheme which was proposed for analogous processes with participation of diazo ketones and other diazomonocarbonyl compounds [5,6]. Initially, [3 + 2]-cycloaddition of dipolar diazo group at the C=S bond of thioketone gives unstable thiadiazoles IV which lose nitrogen molecule to generate reactive thiocarbonyl ylides V; the latter undergo intramolecular 1,5-electrocyclization leading to 1,3-oxathioles III.Cyclization of C=S ylide V generated from unsymmetrically substituted diazodicarbonyl compounds could produce two regioisomeric oxathioles. However, in the reaction with diazo ester Ib, electrocyclization of the corresponding ylide occurs chemoselectively at the acetyl group, whereas the ester fragment remains intact, and the product is only one regioisomer, 1,3-oxathiole IIIb. Cyclic diazodiketone Ic reacted with thioketones at a much lower rate, and elevated temperature was necessary to accelerate the process. As a result, the reaction was accompanied by partial decomposition of diazodimedone (Ic) with formation of the Wolff rearrangement product, 2-oxoketene. The I, R = R′ = Me (a); R = MeO, R′ = Me (b); RR′ = CH 2 C(Me) 2 CH 2 (c).