β-(N,N-Dimethylcarbamoylselenenyl)- and β-(N,N-dimethylcarbamoyltellurenyl)alkenyl ketones were converted into isoselenazoles and isotellurazole Te-oxides, respectively, simply by treating with hydroxylamine-O-sulfonic acid, and deoxygenation of the latter products was successfully carried out by treating with PPh3. Alternative treatment of ynone oxime tosylates with hydrochalcogenide ions or N,N-dimethylchalcogenocarbamate ions also gave the same isochalcogenazole rings. These reactions were assumed to proceed through intramolecular nucleophilic substitution on the nitrogen atom of oxime sulfonates by the attack of in situ generated chalcogen nucleophiles.

Alkyl or alkenyl N,N-dimethylchalcogenocarbamates were easily prepared by a stepwise treatment of bis(N,N-dimethylcarbamoyl) dichalcogenides with NaH or NaBH4, followed by various alkylating agents or acetylenes bearing electron-withdrawing substituents. The one-step conversion of alkyl or alkenyl N,N-dimethylselenocarbamates or N,N-dimethyltellurocarbamates into the corresponding symmetrical dialkyl or dialkenyl dichalcogenides was also achieved efficiently by treating with SnCl4.

Treatment of substituted isotellurazoles or their Te-oxides with acetylenic dienophiles efficiently afforded polysubstituted pyridine derivatives through a pathway involving heteroDiels-Alder reaction of isotellurazoles and the subsequent tellurium extrusion from the intermediary cycloadducts. © 2009 Elsevier Science. All rights reserved. Keywords: Isotellurazole;Hetero Diels-Alder Reaction; Acetylenic Dienophiles; Polysubstituted Pyridine; AzafluorenonePolysubstituted and fused pyridine cores have been found in a wide variety of naturallyoccurring polycyclic alkaloid skeletons having biological, pharmaceutical, and agrochemical activities, and studies on efficient synthesis of pyridine ring systems preserves a considerable importance in current organic synthesis in spite of the long history of pyridine chemistry. Among the modern methodologies for the syntheses of pyridine rings, thermal reactions of chalcogenand nitrogen-containing five-membered heterocycles with dienophiles have been extensively studied. However, synthetic application of such compounds has been limited within the area of oxygen-or nitrogen-bridged five-or six-membered heteroaromatics due to the easiness in preparation and their treatability.1-5 Among the five-membered heterocycles bearing a chalcogen atom, tellurazoles and isotellurazoles have been presumed to behave as heavy chalcogen-bridged reactive azadienes based on the relatively low heteroaromaticity of tellurium-containing fivemembered heteroaromatics 6 and the subsequent feasible extrusion of elemental tellurium from the intermediary cycloadducts B or B' under mild reaction conditions. However, only limited preparative methods for tellurazoles and isotellurazoles had been reported within these decades in spite of their potentiality as reactive heterodienes. 7,8 In the course of our studies on syntheses and reactions of higher-row chalcogen-containing heterocycles, we reported a convenient preparation

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