Carbon dioxide ring closures with

Still another possibility in the base-catalyzed reactions of carbonyl compounds is alkylation or similar reaction at the oxygen atom. This is the predominant reaction of phenoxide ion, of course, but for enolates with less resonance stabilization it is exceptional and requires special conditions. Even phenolates react at carbon when the reagent is carbon dioxide, but this may be due merely to the instability of the alternative carbonic half ester. The association of enolate ions with a proton is evidently not very different from the association with metallic cations. Although the equilibrium mixture is about 92 % ketone, the sodium derivative of acetoacetic ester reacts with acetic acid in cold petroleum ether to give the enol. The Perkin ring closure reaction, which depends on C-alkylation, gives the alternative O-alkylation only when it is applied to the synthesis of a four membered ring ... 

Pyrido[2,3-, pyridazine derivatives 48 have been synthesized by refluxing equimolar amounts of an appropriate 5-benzylidene-2,2-dimethyl-l,3-dioxane-4,6-dione 47 with 5-amino-6-phenylpyridazin-3(2/7)-one 46 in methanol or a methanol acetic acid mixture. The electron-poor carbon atom of the polarized carbon-carbon double bond of 47 is the electrophile attacking C-4 of the 5-aminopyridazinone 46. Imino-enamine tautomerization of the intermediate is followed by ring closure and subsequent loss of acetone and carbon dioxide affording the reaction products 48 as stable crystalline solids in 70-90% yield (Scheme 9) <2000T2473>. 

Heteroallenes (e.g. caibon disulfide, carbon dioxide, carbon oxysulfide, isocyanates, isothiocyanates, ketenes, ketenimines) and vinylidenephosphoranes can form polar intermediates from which two isomeric products may result in a 1,4-cycloaddition (equation 113). The direction of the ring closure depends decisively upon the nucleophilic character of Z compared with that of Y. 

Indoxyls are normally prepared from anthranilic acids via alkylation with a haloacetic acid followed by a cyclising condensation with loss of carbon dioxide. " Indoxyl itself is best prepared by Friedel-Crafts type ring closure of iV-phenylglycine activated with triphenylphosphine oxide/triflic anhydride in the presence of triethylamine at room temperature. " ... 

Alkyl-3-(styrylsulfonyl)thioureas, obtained in the form of their potassium salt by reaction of styrylsulfonamides with alkyl isothiocyanates and potassium carbonate in refluxing acetone, cyclize in weakly alkaline solution, or thermally upon melting (usually at 130 160 °C) via a Michael-induced ring closure to give AT-alkyl-5-aryl-5,6-dihydro-1,4,2-dithiazin-3-amine 1,1-dioxides l.9,16,I9,31,32... 

Substituents such as chloro, bromo, sulfanyl, or phenoxy in position 2 of phenylsulfonamides are sufficiently reactive to allow ring closure reactions by treatment with carbon disulfide/alkali hydroxide in ethanol43-45 or dimethyl sulfoxide.46 The resulting alkali thiolates 4 may then be methylated43 44 by dimethyl sulfate to give the 3-(methylsulfanyl)-1,4,2-benzodithiazine 1,1-dioxides. 

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