Diketene 4-pyrones

The synthesis of a large number of y-pyrones and y-pyranols from enamines has been brought about through the use of a wide variety of bifunctional molecules. These molecules include phenolic aldehydes (126,127), phenolic Mannich bases (128), ketal esters (129), and diketene (120-132). All of these molecules have an electrophilic carbonyl group and a nucleophilic oxygen center in relative 1,4 positions. This is illustrated by the reaction between salicylaldehyde (101) and the morpholine enamine of cyclohexanone to give pyranol 102 in a quantitative yield (127). 

On the other hand y-pyrones or 1,3-diketones could be obtained from the reactions of ketone derived enamines with diketene 423-426). The addition of dimethyl ketene dimer to aldehyde or ketone derived enamines produced cyclohexanediones 425,426). 

Pyridones from 1,3,5 triketones, 46,60 Pyrolysis, diketene to ketene, 46, SO a-PYRONE, 46,101 in Diels Alder reactions, 46, 104 4-Pyrones, 2,6 disubstituted, from 1,3,5 pentanetnones by acid cycliza tion, 46, 61... 

The N-silylated amide 1558 reacts with diketene vio the O-silylated intermediate 1559 to give the pyrone 1560, which rearranges when treated with 10% HCl to give pyridin-2-one 1561 [74] (Scheme 9.43). 

The only four- - six-membered ring interconversions of any real synthetic significance are those involving diketene. Base-catalyzed dimerization of diketene is a long-established and efficient method for the preparation of dehydroacetic acid (equation 161), while mild treatment with water in the presence of tertiary amine bases gives 2,6-dimethyl-4-pyrone (equation 162). 1,3-Dioxins are obtained from the acid-catalyzed condensation of diketene with ketones (equation 163). 

Base-catalyzed dimerization of diketene (167) efficiently yields dehydroacetic acid (168) treatment of diketene with aqueous triethylamine gives 2,6-dimethyl-4-pyrone (169). 

Kato et al,94 reported that 2-amino-6-methylpyridine and diketene did not yield 4-oxo-4/f-pyrido[l,2-a]pyrimidines but instead yielded 2-acetyl-acetamido-6-methylpyridine and pyridone or pyrone derivatives. 2-Methyl-4-oxo-4H-pyrido[l,2-a]pyrimidine (47) and its 8-methyl derivative have also been prepared from 2-aminopyridine and 2-amino-4-methylpyridine with N,N-dimethyl-3-aminocrotonamide95 or with acetoacetamide45 in yields of 5 and 39°,. respectively. 

A different approach can be found in the reaction of diketene 44 with Meldrum acid (45). Treatment of the resulting derivative 46 with TsOH in benzene forms 3-carboxy-4-hydroxy-6-methyl-2-pyrone (47) (84SC265). 

Kato et reported that 2-amino-6-methylpyridine and diketene did not yield 4-oxo-4//-pyrido[l,2-a]pyrimidines but instead yielded 2-acetyl-acetamido-6-methylpyridine and pyridone or pyrone derivatives. 2-Methyl-... 

The acylation of the enamine of a cyclic ketone with diketene leads directly to bicycUc 4-pyrones, as indicated below. ° ... 

Glycosyl sulphamides derived from D-glucose and D-ribose have been prepared conventionally further reaction with diketen led to corresponding glycosyl A -sulphamoyl-pyrone carboxamide derivatives. ... 

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