Isopropenyl enol acetates with

Enol esterification with acetyl chloride-acetic anhydride gives the A -trien-3-acetate, but reaction with isopropenyl acetate or with hot acetic anhydride-pyridine gives A " -trien-3-acetates. " Since A"" -3-ketones react with Girard reagents, these linear dienones can be separated from A ""-3-ketones. ... 

Unsubstituted 20-ketones readily form enol acetates. Reaction with isopropenyl acetate yields the kinetic A -isomer (86, R = CH3CO) which is equilibrated to the A -enol acetate (85, R = CH3CO) on exposure to acetic anhydride-/7-toluenesulfonic acid. Treatment of the 20-ketone according to the latter conditions gives the A -enol acetate directly. 

The dimethyl acetal (94) is readily prepared from the 22-aldehyde (93) by direct reaction with methanol in the presence of hydrogen chloride. Ena-mines (95) are formed without a catalyst even with the poorly reactive piperidine and morpholine.Enol acetates (96) are prepared by refluxing with acetic anhydride-sodium acetate or by exchange with isopropenyl acetate in pyridine.Reaction with acetic anhydride catalyzed by boron trifluoride-etherate or perchloric acid gives the aldehyde diacetate. 

The diacylation of isopropenyl acetate with anhydrides of dicarboxylic acids is applicable for the synthesis of several other cyclic jS-triketones in moderate yield. - It has been used for the synthesis of 2-acetylcyclohexane-l,3-dione (40% yield), 2-acetyl-4-methylcyclopentane-l,3-dione (10% yield), 2-acetyl-4,4-dimethylcyclopentane-l,3-dione (10% yield), 2-acetyl-5,5-dimethylcyclohexane-l,3-dione (10% yield), 2-acetylcyclo-heptane-l,3-dione (12% yield) and 2-acetylindane-l,3-dione (26% yield). Maleic anhydrides under more drastic conditions give acetylcyclopent-4-ene-l,3-diones in yields from 5% to 12%. The corresponding acylation of the enol acetate of 2-butanone with succinic anhydride has been used to prepare 2-methylcyclopentane-l,3-dione, an important intermediate in steroid synthesis. - ... 

Dihydrocarvone [l] was treated with isopropenyl acetate in the presence of p-toluene sulphonic acid and converted into a mixture of enol acetates [2] and [3], separated by GLC Treatment of [2] with boron trifluoride in methylene chloride at room temperature for 10 minutes gave (+) Camphor [U]. This synthesis is particularly interesting in that it is a chemical analogy for the biosynthesis conversion of a monocyclic into a bi-cyclic monoterpenoid. 

Bromination of androstan-i6-ones is complicated by rearrangements and by dibromination, but appears to favour 17a (pseudo-axial) attack initially. Enolisation of the 16-0x0 group is slow and inefficient compared with a 17-ketone, at least when isopropenyl acetate is used [x i], although the same (A ) double bond is produced. The enol acetate bromi-nates selectively at the lya-position [152]. The alternative enol acetate (A ) is formed in minor amount, and gives the 15 -bromoketone. Both 17a- and i5) -attack correspond to pseudo-axial approach of the reagent as required by stereo-electronic factors. 

With higher olefins the product distribution becomes more variable. Not only the expected enol acetates but also allylic acetates are formed. Thus, propene forms isopropenyl acetate along with some n-propenyl acetate and allyl acetate [6, 7]. Higher and cyclic olefins react to form mainly allylic esters [8-18] moreover pre-isomerization of the olefins give rise to an even broader spectrum of products. The results published differ from each other, probably because of different reaction conditions and composition of reaction mixtures. 

House and Trost state that enol acetylation with isopropenyl acetate yields chiefly the less highly substituted isomer whereas the reverse is true of enol acetylation with acetic anhydride, but we fail to find in the paper experimental evidence in support of these generalizations. 

F lykos and Sorm" converted androslerone acetate (8) into the only possible enol acetate (y) by heating 3 g. of (K) with 2.3 ml. of isopropenyl acetate and I ml. of a catalyst solution of 0.02 ml. of coned, sulfuric acid in isopropenyl acetate and collected 10 ml. of distillate in 2 hrs. they then lulded 23 ml. more of reagent and I ml. uf catalyst solution and collected 2,3 ml. In 2 hrs. 

Pyrazole synthesis [before references]. In a new and useful two-step synthesis of pyrazoles developed by Parham and Dooley,12 an enol acetate, such as isopropenyl acetate (1), is condensed with phenyl(trichloromethyI)mercury (1, 851) to give a... 

Enol acetylation. An a-halo-a-arylacetophenone such as (1) cannot be converted into the enol acetate by reaction with isopropenyl acetate, and conversion into the sodium enolate by sodium methoxide followed by treatment with acetyl chloride gives low yields. However, Cooper and Owen28 found that, if the ketone is treated with dry methanol and sodium hydride and the mixture let stand until evolution of hydrogen has ceased and then treated with acetyl chloride, the enol acetate is obtained in yield of50-70%. 

These compounds have been synthesized by several methods. A convenient method is by the interaction of trialkylmethoxytin compounds with enol acetates. For example, reaction of triethylmethoxytin (73) with isopropenyl acetate (74) gives the tin(IV) enolates as a mixture of 0- and C-isomers (75) and (76) (Scheme 2S). The exchange reaction appears to maintain the regiochemistry of the starting enol ester. Thus, reaction of 2-methyl-1-acetoxycyclohexene with tributylmethoxytin gives compound (77). On the other hand, the regioisomer (78) has been generated in situ by treatment of the precursor lithium enolate with tri-n-butylstannyl chloride (Scheme 26). ... 

Enol acetates are obtained in moderate yield by means of acetic anhydride containing /7-toluenesulfonic acid703 or, better, bypassing ketene into the appropriate ketone containing 0.5 % of sulfoacetic acid at 60-80°.704 705 They are elegantly prepared by transesterification with isopropenyl acetate, CH2==CCH3(OCOCH3),706 which is obtained from acetone and ketene at 55°.704... 

The isopropenyl acetate method yields, for example, 96 % of isomer B from ethyl methyl ketone, but 92% of isomer A from isobutyl methyl ketone. With acetic anhydride and p-toluenesulfonic acid 20-oxo steroids give the enol acetates B,707 but with isopropenyl acetate give the enol acetates A.708... 

Enol acetates have been little used as protective groups per se. They are formed from enolizable aldehydes and ketones by an acid-catalyzed exchange reaction with isopropenyl acetate [123, 124, 125, 126], by the action of acetic anhydride in the... 

Keto esters can be converted into either trans or cis enol acetates. The trans isomer is accessible by treatment with AcCl/EtsN (HMPA, rt)i or AcCl/DBU (MeCN, 5°C rt eq 11), while the cis isomer is accessible by treatment with isopropenyl acetate/HOTs. Each isomer couples with dialkylcuprates with retention of configuration to afford stereoisomerically enriched Q ,/3-unsaturated esters. 

The acetylation of amines with isopropenyl acetate appears to be a transition between the highly exothermic reactions of acyl halides and anhydrides with amines on the one hand and the reaction of amines with more conventional esters on the other. While this reagent is of particular value in the preparation of enol acetate, it has been used for the preparation of amides. One interesting aspect of its use is that acetone forms as a coproduct which may distill off as the reaction proceeds. Isopropenyl acetate and other isopropenyl esters may also be used to Y-acylate amides and imides. By the judicious selection of starting amides and isopropenyl esters, tertiary amides with three different acyl groups may be synthesized. This may very well be one of very few reaction systems which permits the synthesis of this rare group of tertiary amides. 

Enol Acetylation. In the presence of catalytic acid, isopropenyl acetate reacts with enolizable ketones to give the enol acetate plus acetone. This reactivity has been exploited to accomplish selective enolization of ketones, and to provide dienol acetates for Diels-Alder reactions. Each of these reactions is described in more detail in the following sections. 

Selective Generation of Kinetic and Thermodynamic Enol Acetates. While the use of isopropenyl acetate with catalytic acid is considered to provide conditions for the generation of kinetic... 

Ketones with labile hydrogen atoms undergo enol acetylation on reaction with ketene. Strong acid catalysis is required. If acetone is used, isoptopenyl acetate [108-22-5] (10) is formed (82—85). Isopropenyl acetate is the starting material for the production of 2,4-pentanedione (acetylacetone) [123-54-6] (11). 

Acyliminium ions are sufficiently electrophilic to react with enolate equivalents such as silyl enol ethers207 and isopropenyl acetate.208... 

With a less reactive olefin such as isopropenyl acetate, diazoketone 86 gives only a low yield of cyclopropane 90 a-acyl enol ether 92, resulting from an intramolecular rearrangement of the ketocarbenoid, becomes the favored reaction product. If 91... 

The oldest known method for producing isopropylidene acetals is treatment of a diol with anhydrous acetone under acid catalysis. However, in order to trap the resulting water it is also necessary to include molecular sieves or copper sulfute 2-Methoxypropene (19) is roughly twice as expensive as acetal 18. but as an enol ether it is also more reactive. In especially problematic cases one can in addition resort to 2-trimethylsilyIoxypropene (IPOTMS = isopropenyl-oxytrimethylsilane) (20), but lor this situation it is inappropriate on the basis of cost. 

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