Heterocyclic enols

As expected, heterocyclic enols and potential enols (i.e, compounds existing mainly in the CH form) behave toward diazomethane similarly to the open chain and isocyclic enols, i.e. they form enol methyl ethers by reactions of the SnI type (cf. footnote 29). Examples of this behavior are barbituric acid, picrolonic acid, dchydroacetic acid (64), 3-methyl-l-phenylpyrazolin-5-one, 1-phenylpyrazoli-dine-3,5-dione, 1,2-diphenylpyrazolidine-3,5-dionc, 3-hydroxy-... 

Hydroxypyridine (86, R = H) and its derivatives also belong to the class of heterocyclic enols. In benzene and dioxane, 3-hydroxy-pyridine occurs as the neutral molecule (and not as a betaine).Its reaction with diazomethane, in heterogeneous media, gives a mixture of 3-methoxypyridine (86, R = Me) (10%) and l-methyl-3-hydroxy-pyridinium betaine (87) (30%If tert-butanol is used as a... 

Phosphorothioate Esters of Heterocyclic Enols, Chloropyrifos [2971-88-2], 0,0-diethyl 0-(3,5,6-trichloro-2-pyridinyl)... 

Carbamate Insecticides. These are structurally optimized derivatives of the unique plant alkaloid physostigmine [57-47-6], a cholinergic drug isolated in 1864 from Physostigma venenosum (see Alkaloids) (17,24,35—39). The carbamates maybe considered synthetic derivatives of the synaptic neurotransmitter acetylcholine, with very low turnover numbers. The N,N-dimethylcarbamates of heterocyclic enols (36) and the N-methylcarbamates of a variety of substituted phenols (35) with a wide range of insecticidal activity were described in 1954 (35). The latter are the most widely used carbamate insecticides, and the N-methylcarbamates of oximes have subsequently been found to be effective systemic insecticides. 

Caibamate insecticides in water are subject to photo decomposition under the effects of ultraviolet radiation, the rate of which is related with pH, tending to increase with increasing pH. The first effect of UV irradiation is the cleavage of the ester bond, resulting in the phenol or the heterocyclic enol of the carbamate ester [9]. The studies concerning the half-life for photo decomposition process suggest that the light source characteristics influence the extent of the process. 

Table VI. Concentrations of Phenols or Heterocyclic Enols Formed during the Irradiation of Sevin, Baygon, Pyrolan, and Dimetilan at Different pH Values ...

Concentration of phenol or heterocyclic enol produced (mg/liter). 

Ultraviolet light induced marked changes in the structure of the foui carbamate insecticides. The primary effect of ultraviolet irradiation was to cleave the esteric bond to form the phenol or heterocyclic enols of the carbamate esters which were further decomposed after prolonged exposure. The photodecomposition rate and the nature of the... 

The concept has also been extended to a six-membered homolog, the di-oxanone 61, vhich furnishes the enolate 62 by deprotonation. Here again, subsequent aldol addition proceeds vith high stereoselectivity and the dia-stereomer 63a results predominantly or almost exclusively (Eq. (28)) [109, 110]. With all heterocyclic enolates 58, 60, and 62 stereocontrol is relatively easily accomplished, because of the rigid structure of the heterocycle. 

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