Rearrangement base-promoted rearrangements

The enantioselective base-promoted rearrangement of oxiranes was achieved by White-sell and Fehnan in 1980. Various homochiral lithium amides were used for the isomerization of cyclohexene oxide with an enantiomeric excess (ee) up to 36% with the employment of 50 in refluxing THF (Scheme 24). 

The influence of the oxime geometry on the base-promoted rearrangement is pointed out for compounds 29 (85T5181). Thus, when treated with aqueous potassium hydroxide at room temperature, Z isomers 29Z (R = H, -oxyiminomethyl), arising as cycloadducts of the dimer or trimer of fulminic acid and norbornene, rearrange into 30. On the contrary, the E isomer 29E remains unchanged under similar conditions of aqueous bases (Scheme 6). 

The standard means for preparing oxicams 285, initially developed by Lombardino, is through the base-promoted rearrangement reaction of isothiazole dioxides 286, which in turn are prepared from saccharin derivatives such as 287 (Scheme 40) <1981AHC(28)73>. The oxicam core can be further derivatized by N-alkylation of oxicam 285 and amidation of the C-3 ester functionality of 288 to form the common drug scaffold 289. 

In the case of 3,4-dimethylated derivatives, the base-promoted rearrangement of the corresponding epoxides is a preferable method because it is regiospecific (equation 21 )6y. However, it does not take place in the case of derivatives having an alkenyl group at the heteroatom12,67c. 

Hydroxy cyclobutylidenes 635 undergo base promoted rearrangements to afford 2//-pyran-2-ones via formation and ring closure of the intermediate 1,5-keto ester 636 (Scheme 145) <2005TL8237>. 

Pine, S. H. Base-promoted rearrangements of quaternary ammonium salts. Org. React. 1970, 18, 403-464. 

Another type of ylidic rearrangement, the Sommelet-Hauser rearrangement, was discovered later, in 1937, by Sommelet and studied extensively by Sommelet and by Hauser. This transposition involves the base-promoted rearrangement of non stabilized ammonium and sulfonium ylides. Thus, the ammonium salt (11), when heated with alkali, gave cleanly the substituted diphenylmethane derivative (12 Scheme 5). Wittig found that the dibenzyl ammonium salt (13) reacted with phenyllithium giving two... 

Several limitations of this base-promoted rearrangement are worthy of mentioning (a) when structurally feasible, both Stevens and Sommelet-Hauser reactions will compete (b) certain substituents on the aromatic ring, e.g. Cl, CN, NO2, prevent the formation of the ylide and no rearrangement takes place (c) if a p-hydrogen is present on the ammonium salt, elimination becomes yet another undesirable side reaction, In an attempt to overcome these limitations, Sato has utilized the fluoride-induced desilylation of... 

I.3.2.6. Base-Promoted Rearrangements of Halocyclopropanes with Vicinal C-H Acidic... 

The Base-Promoted Rearrangements of Quaternary Ammonium Salts ... 

This reaction was first reported by Kornblum and DeLaMare in 1951. It is primarily a base-promoted rearrangement of a peroxide into a ketone and an alcohol and is thus known as the Kornblum-DeLaMare reaction or Kornblum-DeLaMare rearrangement. ... 

The base-promoted rearrangement of an a-haloketone to a carboxylic acid, known as the Favorskii rearrangement, is illustrated by the conversion of 2-chlorocyclohexanone to cyclopentanecarboxylic acid. 

A review on the base-promoted rearrangement of oxime 0-derivatives, commonly O-sulfonates, and the similar reaction of AA A-trimethylhydrazonium iodides to give 2H-azirines or, upon subsequent aqueous acidic workup, a-amino ketones has been reported. With excess base, a-amino ketal is formed from the 2//-azirine via an unstable 2-alkoxy aziridine which can be isolated or hydrolysed in situ to a-amino ketone. ... 

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