Intramolecular 1,4-Dipolar Cycloadditions of Cross-Conjugated Heteroaromatic Betaines. Synthesis of Hexahydrojulolidines and Related Peri- and Ortho-Fused Ring Systems
Intramolecular 1,4-Dipolar Cycloadditions of Cross-Conjugated Heteroaromatic Betaines. Synthesis of Hexahydrojulolidines and Related Peri- and Ortho-Fused Ring Systems
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Potts, K.T.;
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Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας
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Potts, K.T.;
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3-Alkenyl2-lactams react with (chlorocarbony1)phenylketene to give nonisolable anhydro-4-hydroxy-2-oxo-1,3-oxazinium hydroxides which undergo regio- and stereospecific 1,4-dipolar cycloaddition in moderate yields to produce cycloadducts containing a carbon dioxide bridge. Thermolysis of the cycloadduct led to extrusion of carbon dioxide via a nonconcerted 1,5-hydrogen shift to give julolidinetype derivatives. Two of the cycloadducts were characterized by single-crystal X-ray determinations. A significant enhancement in the overall yield of the cycloaddition occurred in related reactions using anhydro-4-hydroxy-2-oxo-1,3-thiazinihuym droxides obtained from 3-alkenyl-3-ethyl(or methyl) 2-thiolactams and a variety of 1,3-bielectrophiles such as (chlorocarbonyllphenylketene, carbon suboxide, substituted malonyl dichlorides, and ethy (chlorocarbonyllacetate. The thiazinium betaines were often isolable, and in one instance, a single-crystal X-ray characterization was possible. Cycloaddition of the thiazinium betaines occurred in a regiospecific manner. The initially formed cycloadducts which retained the carbonyl sulfide bridge could be induced to lose COS on further heating. Julolidine-type derivatives were obtained principally via a nonconcerted process. Variation in lactam ring size, coupled with tether length and substituent in the 1,3-bielectrophile, enabled control of ring size, substituents, and whether an ortho- orperi-fused tricyclic system resulted from the overall cycloaddition. In contrast to the lactam system, the thiolactams require disubstitution in the 3-position to avoid proton loss in the intermediate betaine with formation of a 1,3-thiazoline- 4,6-dione.
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