Enol esters Acetic anhydride

The kinetics of the acid-catalyzed esterification reaction of 2,4,6-trimethylbenzoic acid in i-PrOH under microwave irradiation have been investigated [84], A simple and practical technique for MW-assisted synthesis of esters has been reported wherein the reactions are conducted either on solid mineral supports or by using a phase transfer catalyst (PTC) in the absence of organic solvents [85], The esterification of enols with acetic anhydride and iodine has also been recorded [86],... 

Coumarin formation proceeds via an intramolecular attack by enol ester 9 on the ketone to give 10. Dehydration of 10 then affords coumarin 11. It has been observed that coumarins are favored when higher order homologs of acetic anhydride and their corresponding salts such as propionic anhydride/sodium propionate and butyric anhydride/ sodium butyrate are used. 

Due to the nonaromatic character of the oxepin system the oxepinones do not usually form stable enol structures. By O-acylation or O-alkylation, however, the enol forms can be stabilized as enol esters and ethers, respectively. A large number of substituted 1-benzoxepins have been synthesized by this route. Acetylation of l-benzoxepin-3(2//)-ones 1 and l-benzoxepin-5(2/T)-ones 3 was readily achieved with acetic anhydride in the presence of an appropriate base such as pyridine, triethylamine or sodium acetate.t5,t6 t72 176... 

The Grohe-Heitzer sequence (Scheme 4.2) begins with acylation of malonate derivative 37 with benzoyl chloride 36 to give malonate 38 (Mitscher, 2005). Condensation of the malonate with an ortho-ester in the presence of a dehydrating agent such as acetic anhydride affords enol ether 39. The enol ether then undergoes an addition-elimination... 

Condensation products of 4(5//)-oxazolonium salts with aldehydes and orthoesters are the subject of a series of papers by Kosulina and co-workers Reaction of 2-methyl-4(5//)-oxazolonium perchlorates 44 with an ortho ester gives rise to an enol ether, which reacts with furanamides to afford the frani-eneamides 45 (Scheme 6.14). " Using electron deficient anilines in a three component condensation affords either 46 or 47 in 64-80% and 78-84% yields, respectively, depending on whether the reaction is performed in acetic acid or acetic anhydride. Electron-rich anilines are unreactive since they are merely protonated by the perchloric acid present in the reaction medium. ... 

Enol carbonates Di-f-butyl dicarbonate, 94 Potassium hexamethyldisilazide, 257 Enol esters Enol acetates Acetic anhydride, 93 Bis(7i5-cyclooctadienyl)ruthenium(II), 35... 

The position of the equilibrium of Eq. (13) depends markedly on the nature of R. When R = H the product is entirely the ketone, whereas when R = CH3 the product can be shown from IR and NMR spectroscopic data to be completely enolized. Whichever form predominates, Schotten-Baumann benzoylation gives the indolizine / -nitrobenzoate ester (67). Acetic anhydride instead of perchloric acid with 65 gives the 1-acetamidoindolizine which is also available from the 1-acetyl compound via the Schmidt reaction. 

The enol acetate 77 of 3,4-dihydro-7-methoxy-5-methyl-l-(2l/)-naphthalenone was converted to the acid 78 by ozonolysis and hydrolysis and this by a Wittig reaction with a-methoxyethyltriphenyl-phosphonium chloride gave 79. Compound 79 was converted into 80 by a series of reactions, five in number, which in turn was converted into 81 by reaction with potassium in -butanol. The methyl ester of compound 81, one isomer of which was recognized as that having the correct stereo structure, was converted to 82 by heating with acetic anhydride and 10-camphorsulfonic acid. Subsequent steps involved ozonization, reaction with V,iV -carbonyldiimidazole, lactam formation, reaction with pyridinium bromide perbromide, reaction with sodium hydride, and a further series in which (+ )-oxodendrobine (83) was ultimately obtained. Reduction of the latter to ( )-dendrobine... 

For the first example, we chose to acylate olefin alcohol la. This was readily accomplished using acetic anhydride and 4-DMAP in pyridine to provide ester 17. Methylenation, using Takai s (10) protocol, yielded the acyclic enol ether 18 which was subsequently cyclized with 15 mol % of the Schrock catalyst 6 in hot toluene to afford the glycal 19 in good yield. Hydroboration and oxidative work-up led to the methyl-C-glycoside 20 (Scheme 4). With this proof of principle in hand, we then set out to prepare a number of additional examples as shown in Table 1 (11). 

Trost added the sulfone (465) to the ester (464), and reduced the ketone to the alcdiol. In this example the formation of the enol derivative of the keto sulfone failed. The hydroxy group was functionalized with acetic anhydride, and the reduction to the alkene (466) carried out with the standard sodium amalgam conditions in 54% overall yield, as shown in equation (108). 

Reaction of 2-phenylsulfinyl esters such as (133) with acetic anhydride is slow at room temperature, but when a catalytic amount of methanesulfonic acid is present in the reaction medium, conversion to the vinyl sulfide (134) is complete after a few hours at 20-40 C (Scheme 31). Mixtures of ( )/(2) isomers are usually obtained, with the (2)-isomer generally being the more stable. These isomers are in-terconverted on heating, a process which occurs slowly on standing, even in the dark. The discovery that photochemical or free radical induced 1,3-shift of the PhS group in (135) to the y-position can be effected offers an additional opportunity to modulate the a/y selectivity in the extended enolate chemistry of a,3-unsaturated carbonyl systems. 

Dihydrocodeinone [65, 69-70], 1-bromodihydrocodeinone [69], 14-hydroxydihydrocodeinone [38], and the alkyldihydrocodeinones [65] can be converted into esters of the enol form by heating the ketones with acid anhydrides and sodium salts. The enol esters and their salts are stable even in hot solution, but they are hydrolysed to the original ketones by boding with acids [69]. Dihydrocodeinone enol acetate [liv] is marketed as the drug acedicon. 

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