Enamino-esters, cycloaddition

When deprotonated dimethyl sulfone is reacted with 2 equiv of benzonitrile, compound 109 is obtained in low yield (Equation 91) <1973JOM(59)53>. Compounds of the general structure 179 can be prepared from two molecules of enamino esters 288 and sulfur dichloride or disulfur dichloride <1984JOG4780> or in low yield using chlorocarbo-nylsulfenyl chloride 289 as the source of sulfur <1985JHC1621> (Equation 92). A series of cycloadditions lead to the formation of 131 from 290 and two molecules of the ynamine 291 (Scheme 69) <1995LA1795>. 

Similarly, (heterocyclic) jS-enamino esters substituted with an electron donor as well as with an electron acceptor group ( push-pull ) show a decreased cycloaddition tendency towards 1,3-dipoles such as azides see (71c3B351o) and Section 4.11.4.7. 

An example of an azide cycloaddition to an open-chain enamino ester (292) has also been reported giving (293) (76BSF2025). 4-Acyl-l,2,3-triazoles (249) have been obtained in fair yield by the reaction of the phenyl azide with a-keto vinyl chlorides (sszoBisee). 

Although cycloadditions to enamines are well known,2 the partially saturated heterocyclic /J-enamino esters, as, e.g., 8, display only a very weak tendency toward cycloaddition reactions, and Diels-Alder-type (4 + 2) cycloadditions have not yet been observed. From the 1,3-dipoles known today,140 170 hr of treatment of the enamino esters 8 (X = O, n = 1) with... 

A cycloaddition reaction of the halogen-substituted nitrone (289) with olefins in the presence of AgBp4 provides reactive iminium salts in a regio- and stereo-specific manner. These react with various nucleophiles to give 1,2-oxazine derivatives as shown. In a study of the reactions of P-enamino-esters with carbonyl compounds, WamholF et have isolated a series of acyclic and... 

Enamino ketones and esters also react with dimethyl acetylenedicarboxylate (67). Again cycloaddition appears to occur and the unstable cyclobutene intermediates rearrange to give insertion of two carbon atoms. 

The a-pyrone analogues (101) have been synthesized by a [24]cycloaddition reaction between cyclic a-keto-enamines (100) and di(thioethyl)ketene/ 5-Enamino- 8-keto-esters, derived from the corresponding acetylenic keto-esters, undergo smooth alkylation to give /35-diketo-esters, which can be cyclized to 4-hydroxy-a-pyrones with hot polyphosphoric acid/ ... 

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