Dicarbonyl compounds Zinc chloride

Chloro-l,3-dithiane (797) has been employed as a formyl cation equivalent . The morpholine enamines of a variety of aldehydes and ketones were shown to react with this dithiane to produce the a-(l,3-dithan-2-yl) aldehydes and ketones (798) in good yield (Scheme 186) (77TL2077). In direct analogy with this work, the reaction of enol silyl ethers with 2-ethoxy-l,3-dithiolane in the presence of zinc chloride has been reported to afford half-protected 1,3-dicarbonyl compounds (81TL3243). 

Monosaccharides react with a variety of 1,3-dicarbonyl compounds in the presence of zinc chloride in ethanolic or aqueous solution to yield substituted furans (Scheme 69) (56MI31200). The reaction of ethyl acetoacetate with D-glucose and D-mannose yielded the trisubstituted furan (252) in 20% yield, while D-fructose under similar conditions yielded (253 7%). These products have been used for the synthesis of dehydromuscarones (63HCA1259). Oxidation of the tetrahydroxybutyl side chains with lead tetraacetate gives the aldehyde, which can be converted to the corresponding acid with alkaline silver oxide. 

Schulte and coworkers (62AP801) have prepared a number of 3-propargyl-4-hydroxy-2-pyrone derivatives (210) by condensation of /3-dicarbonyl compounds (208) with propargyl-malonyl chloride (209). On heating (210) with zinc carbonate, ring closure took place to give the corresponding furo[3,2-c]pyrone derivatives (211 Scheme 40). 

In the case of 2,5-dimethylfuran (51) the dicarbonyl compound is revealed by the disconnection of one carbon-oxygen bond ring construction is of type (44). The forward synthetic reaction is a cyclodehydration reaction effected by the action of acetic anhydride in the presence of zinc chloride (Expt 8.12). 

The Schiff bases (they may be also considered as specific hydrazones) play a great role in the chemistry of N-aminoazoles. In most cases, these compounds are obtained by cyclization of a suitable acyclic compound or by interaction of an N-aminoazole with carbonyl compounds aldehydes, ketones, or their acetals. Usually the reaction is carried out on heating components in acetic acid or in alcohol in the presence of catalytic amounts of a mineral acid. The use of j3-dicarbonyl compounds requires more drastic conditions. Thus, 1-aminobenzimidazoles (83KGS386) or 7-amino-theophiline (87KGS155I) reacts with acetylacetone at I75°C in the presence of anhydrous zinc chloride, yielding hydrazones of type 283. 

On comparing the reactions of 2-amino-2-deoxyaldoses and 1-amino-l-deoxyketoses with 8-dicarbonyl compounds, which yield pyrroles (Scheme A, X = NH, A -alkyl, or A -aryl), with the reactions of the nonnitrogenous aldoses and ketoses with the same compounds, which yield furan derivatives (Scheme A, X = 0), it may be noted that the changes of bonds are the same. It seems reasonable, therefore, to assume that both processes proceed through the same, or at least similar, mechanisms. From the experimental point of view, the main differences between these two reactions concern rates and catalysts. Aldoses and ketoses react very slowly in the absence of such acidic catalysts as zinc chloride and ferric chloride. The reactions of the amino sugars are much faster and are usually performed under neutral or slightly basic conditions. 

However, attempts to apply the sequence to a number of other 1,3-dicarbonyl compounds led only to tar formation. Abramenko15 also reported the above reaction and, in addition, prepared the corresponding dimethylthieno[3,2-6 pyridine from the same dione and 3-aminothiophene double salt, using zinc chloride in ethanol.16 Klemm13 used zinc chloride in dioxane to effect cyclization of 10 and also prepared 4,5.6-trimethylthieno 3,2-Z>]pyridine via the 3-methylpentane-2,4-dione/2-aminothiophene Schiff s base. The acetals and ketals of 1,3-dicarbonyl compounds are also effective in this synthesis. Thus Klemm1 has prepared the parent systems by condensation-cyclization of 2- and 3-aminothiophene double salts with malondialdehyde tetraethyl acetal (MTA) [Eq. (3). ... 

The rate of fluorination can be dramatically accelerated under micro-wave conditions. Adjusting the eonditions with the addition of tetrabuty-lammonium hydroxide can alter the reaetion eourse to give the difluorinated materials as the predominant produets (Seheme 15.35). ° iV-Fluoropyridinium salts have also been widely employed for the fluorination of these substrates. Reactions are commonly run in acetonitrile or dichloromethane at reflux with yields ranging from 50-90%. Fluorination of unsubstituted p-dicarbonyl compounds yield the mono-fluorination products almost exclusively. In some cases, reactions can be facilitated by addition of a catalytic amount of a Lewis acid such as zinc chloride which promotes the enolisation process (Scheme 15.36). °... 

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