REARRANGEMENT OF ETHERS AND ESTERS

Nucleophilic substitution of the halogen atom of halogenomethylisoxazoles proceeds readily this reaction does not differ essentially from that of benzyl halides. One should note the successful hydrolysis of 4-chloromethyl- and 4-(chlorobenzyl)-isoxazoles by freshly precipitated lead oxide, a reagent seldom used in organic chemistry. Other halides, ethers, and esters of the isoxazole series have been obtained from 3- and 4-halogenomethylisoxazoles, and 3-chloro-methylisoxazole has been reported in the Arbuzov rearrangement. Panizzi has used dichloromethylisoxazole derivatives to synthesize isoxazole-3- and isoxazole-5-aldehydes/ ... 

Other simple alkenois (enols) also rearrange to carbonyl compounds. However, ether and ester derivatives of enols are known and can be prepared by... 

When the reaction is carried out in ether solution, it was found that, whereas addition of acetic acid to a solution of pyrazolenine propionate led to a mixture of propionate and acetate esters in the products, the addition of large amounts of the more strongly nucleophilic methanol gave only a barely detectable fraction of methyl ethers. Such behaviour closely parallels that observed by White in the rearrangement of nitrosoamides and may be accounted for in terms of a similar mechanism, if it is assumed that the initial photolysis of the pyrazolenine gives a diazoalkane. 

Beckmann Rearrangement.59 For a very long time it has been known that the action of acids upon oximes produces amides. More recently it has been found that oxime ethers and esters also undergo rearrangement to form amide derivatives ... 

Since its discovery in 1912, the Claisen rearrangement of allyl vinyl ethers (1 - 2 equation 1) has become one of the most powerful tools for stereoselective carbon-carbon bond formation. Much of its current popularity is due to the subsequent development of a series of new variants of this 3,3-sigma-tropic rearrangement. In 1940, Carroll reported the base-catalyzed rearrangement of p-keto esters and allylic alcohols to alkenic ketones (3 -> 4 5 equation 2). Twenty years later. Saucy and Marbet dem-... 

The different stereochemical course has been explained by unfavorable steric interactions of the methoxy group and the ring proton in the chair transition state B for the ortho ester variant resulting in a preference for the boatlike transition states C and D. On the other hand, chair transition state A predominates in the allyl vinyl ether rearrangement of 7 and 8. 

The Ireland-Claisen rearrangement is not limited to allylic esters, but can also be applied to propargylic glycolates [14]. In the rearrangement of several pro-pargylic esters 14 the diastereoselectivity (ds), the allenic a-hydroxyacids 15 are formed with, was always very high (Scheme 5.2.6, Table 5.2.3). In this reaction, LHMDS in THF is the best base/solvent combination, LDA gave no desired product, and the use of ether and dimethoxyethane resulted in a decrease of both yield and diastereoselectivity. 

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