Unsaturated carboxylic esters rearrangement

MO calculations have been carried out on the isomerization of cyclopropane to propene, and the MNDO method has been used to study the reaction pathway and to optimize the structure of reactant, transition structure, and product of the ring opening reaction of bicyclo[1.1.0]butane. Various methods have been employed to estimate the rate constants for ring opening of the 2-cyclopropyl-2-propyl radical. 1-Acceptor-1-sulfenyl-substituted 2-vinylcyclopropanes of the type (430) have been found to afford 6-sulfenyl-a,jS y, -unsaturated carboxylic esters and nitriles (431) upon treatment with acid, by a process which involves C(l)—C(2) bond fission and a novel 1,5-sulfenyl rearrangement (see Scheme 110). It has been shown that the benzophenone-sensitized photolysis of vinyl norcaradiene derivatives, such as 5-(2-methylprop-l-enyl)-3-oxatricyclo[4.4.0.0 ]deca-7,9-dien-4-ones (432), results in the regioselective cleavage of only one of the cyclopropyl c-bonds to afford isochroman-3-one derivatives (433). It has been reported that the major product obtained from the reaction of structurally diverse a-diazo ketones with an electron-rich alkene in the... 

The addition of alkoxyacetylides to aldehydes and ketones has emerged as a powerful olefination strategy for the production of a,p unsaturated carboxylic esters. This technique was first reported in the synthesis of an antifungal mold metabolite isolated from the fermentation of an acrostalagmus species. Addition of the acetylide anion to 28 and subsequent Meyer-Schuster rearrangement completed the synthesis of the metabolite 29 along with its anomer 30. 

Rearrangements and other side-reactions are rare. The ester pyrolysis is therefore of some synthetic value, and is used instead of the dehydration of the corresponding alcohol. The experimental procedure is simple, and yields are generally high. Numerous alkenes have been prepared by this route for the first time. For the preparation of higher alkenes (> Cio), the pyrolysis of the corresponding alcohol in the presence of acetic anhydride may be the preferable method." The pyrolysis of lactones 9 leads to unsaturated carboxylic acids 10 ... 

Allyl acrylates have been reacted with the combination of ClMe2SiH/ [(cod)RhCl]2/Me-DuPHOS (l,2-bis(2,5-dimethylphospholano)benzene) to bring about reduction of the ,/l-unsaturated ester followed by a Claisen rearrangement to the y,8-unsaturated carboxylic acid (Eq. 293)474 Other silanes did not perform as well in this sequence. 

The benzotriazolyl derivative of acrolein acetal, compound 882, is lithiated, treated with chlorodiphenylphosphine, and the obtained intermediate is oxidized with hydrogen peroxide to phosphine oxide 883 (Scheme 145). The relatively acidic proton in derivative 883 is easily removed by a base, and the obtained anion adds to a carbonyl group of aldehyde or ketone. Subsequent rearrangement and elimination of the phosphorane group generates diene 884. For the derivatives of aldehydes (884, R2 = H), (E)-(E) stereoselectivity of the elimination is observed. Acidic alcoholysis of dienes 884 affords esters of P,y-unsaturated carboxylic acids 885 < 1997JOC4131>. 

Rearrangement of allyl trimethylsilyl ketene acetal, prepared by reaction of allylic ester enolates with trimethylsilyl chloride, to yield Y,5-unsaturated carboxylic a-cids. The Ireland-Claisen rearrangement seems to be advantageous to the other variants of the Claisen rearrangement in terms of E/Z geometry control and mild conditions. 

Phenol esters of a,(3-unsaturated carboxylic acids have an interesting reactivity due to the synthetic utility of the resulting hydroxychalcones (Scheme 19). This aspect will be illustrated in Section IV. However, from the basic point of view, it is worth mentioning that the cis or trans configuration of the olefinic part of the acyl moiety can have a marked influence on the photochemical reactivity of the ester. When para-methoxyphenyl fumarates are irradiated, the normal ortho-rearranged products are obtained. By contrast, irradiation of para-me-thoxyphenyl maleates does not lead to rearrangement. Instead, cyclization products are obtained (Scheme 20). 

There are several variations of the Claisen rearrangement that make it a powerfid tool for the synthesis of y,<5-unsaturated carboxylic acids. The ortho ester modification of the Claisen rearrangement allows carboalkoxymethyl groups to be introduced at the /-position of allylic alcohols.157 A mixed ortho ester is formed as an intermediate and undergoes sequential elimination and sigmatropic rearrangement. 

Esters of allylic alcohols can be rearranged to y,<5-unsaturated carboxylic acids via the O-trimcthylsilyl ether of the ester enolate.161 This rearrangement takes place under much milder conditions than the ortho ester method. The reaction occurs at or slightly above room temperature. Entries 14 and 15 of Scheme 6.12 are examples. The example in entry 16 is a rearrangement of the enolate without intervention of the silyl enol ether. 

Exercise 18-45 Write a mechanism for the base-catalyzed equilibration of a,/3- and /3,-y-unsaturated esters. Which isomer would you expect to predominate Why does this type of isomerization proceed less readily for the carboxylate anions than for the esters Would -y.S-unsaturated esters rearrange readily to the a,/3-unsaturated esters Why, or why not ... 

The product of a Ireland-Claisen rearrangement is a silyl ester. However, silyl esters generally are so sensitive toward hydrolysis that one usually does not attempt to isolate them. Instead, the silyl esters are hydrolyzed completely during work-up. Thus, Ireland-Claisen rearrangements de facto afford carboxylic acids and, more specifically, they afford y,5-unsaturated carboxylic acids. 

Protodesilylation, A new preparation of -unsaturated carboxylic acids involves the amide acetal Claisen rearrangement of a 3-(trimethylsilyl)allyl alcohol such as 1 to allylsilanes (2). Desilylation of 2 by usual methods results in formation of stereoisomers. However, use of liquid HF at low temperatures results only in (E)-3 in 88% yield. The HF-pyridine complex is not so stereoselective. Amides such as 3 can be converted into carboxylic acid esters by Meerwein s trialkyloxonium salts (—70% yield). 

Esters of allyl alcohols can be rearranged to 7,6-unsaturated carboxylic acids via the trimethylsilyl derivative of the ester enolate. This rearrangement takes place... 

In the Ireland-Claisen rearrangement (equation 11.17), an allylic ester is converted to an enolate that reacts with trimethylsilyl chloride to produce an allyl trimethylsilyl ketene acetal, which then imdergoes rearrangement. Depending on the reaction conditions, the product may be a y,S-unsaturated carboxylic acid or ester. ... 

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