PREPARATION OF CONJUGATED HETEROCYCLES BY [4pi+6pi] CYCLOADDITION OF MESOIONIC COMPOUNDS

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The in situ formation of a mesoionic 4-oxazolone allowed the formation of the [4pi+6pi] cycloadduct with tropone in good yield. The yields of a pyrazolotropone via [4pi+2pi] cycloadducts of a sydnone and the[4pi+6pi] adduct with a 1,3-dithiolone were increased by addition of copper(II) acetate.The reaction of mesoionic 5-thiazolones with tropone gave products by deep-seated rearrangement of the cycloadduct. The interrelationship among the products as well as the structures have been discussed.Theoretical consideration of the regio- and peri-selectivities of these cycloadditions based on PM3-MO calculation has led to the conclusion that the selectivity is governed mainly by steric factors and not by electronic interaction.A series of photoisomerization of a [4pi+6pi] adduct of a mesoionic 4-oxazolone with tropone was discovered. The structures of the products and mechanism leading to the photoproducts have been elucidated.A useful method for a practical preparation of 2-t-butyl-6-(dimethylamino)-fulvene has been established. This fulvene reacted with a sydnone, mesoionic 1,3-dithiolone, 1,3,2-oxathiazolone, and 4-oxazolone in a [4pi+6pi] mode. The cycloadducts underwent extrusion of a group of atoms and elimination of dimethylamine under the reaction conditions to give the desired fully unsaturated cyclopenta-thiapyrane, -pyridazine, -thiazine, and -pyrane derivatives, in one step and generally in good yields. These products have been found to have electronic structures isoelectronic with azulene.