Faworski-rearrangement

Numerous examples of the Favorskii (Faworsky) rearrangement, including some with labeled carbon (see Problem 7.13), are known and the reaction will be encountered again. 

The cyclobutanones mentioned in Reaction schemes 83 and 84 can be halogenated and subjected to the Faworski rearrangement with aqueous alkah to give free cyclopropane carboxylic acid, predominantly the trans-isomer [917] (Reaction scheme 87). 

In search of different conditions to enforce this reaction, an interesting intramolecular rearrangement of the chlorine atoms in 139 across the 4-membered ring catalyzed by tertiary amines as well as by hydrochloric acid in alcohol (Reaction scheme 90) was discovered. The newly formed cis-isomeric diequatorial a-halo-cyclobutanone 141 is now again prone to the Faworski rearrangement under mild alkaline aqueous conditions, to give the cis-permethric acid [231] or analogously, the racemic deltamethric acid as a cis/trans 80 20 mixture (Reaction scheme 90) [224, 228, 230]. 

It would have been surprising if the Faworski rearrangement had not been appUed to this problem, (Reaction scheme 107) [262], utilizing here the 2+2-cycloaddition of an a-chloroenamine 171 (see Reaction scheme 84) and allylacetate ... 

Utilization of knowledge of the Faworski-rearrangement leads to an interesting alternative to Reaction scheme 148 (similar to Reaction scheme 94), which yields the desired trans-Z-isomer as shown in Reaction scheme 151 [435]. The key step is the hydrochloric acid-catalyzed aldolcondensation, by which the newly formed OH-group is immediately transformed into a leaving group for the consecutive ring closure via the cyclobutanone intermediate 228 to the cyclopropane 227. 

The Faworski-rearrangement of a-halocyclobutanones provides a very attractive and in practice, often particularly useful method [488-491], since it usually utilizes readily available acid chlorides and olefins as precursors. Not only the commercially important (fenpropathrin) tetramethyl cyclopropane carboxylic acid 247 as in Reaction scheme 169, but many others, particularly higher alkyl-substituted cyclopropane carboxylic acids are preferentially prepared in this way (Reaction scheme 170). 

Certain relations to the Faworski rearrangement may be involved in the alkaline ring contraction of chlorocyclobutanol acetate 251 (Reaction scheme 176) to a cyclopropane carbaldehyde 252 [500], a precursor for still somewhat active pyrethroids. 

FAWORSKI WALLACH Rearrangement Reanangement of a-haloketones or a,a -dihalokeiones to acids or acrylic adds (via cyclopropanones). 

FAWORSKI - WALLACH Rearrangement 114 FEIST - SENARY Furan synthesis 115 Fedvadian 394 FELKINCycItration 116 Fenton 327... 

A. Faworsky (A. Favorskii), a Russian chemist, who simultaneously held positions and had coworkers at the University of St. Petersburg and the Frauen-hochschule in the same city, published a clear study of the rearrangement reaction in J. Prakt. Chemie, 1913,88,641. 

Scheme 7.44. A representation of a pathway to cyclopentanecarboxylic acid anion from 2-chlorocyclohexanone (the Faworsky [Favorskii] rearrangement). The path is presumed to take place via y- (or 1,3) elimination of hydrogen chloride (FICl) through a substituted cyclo-propanone intermediate.

Ring contraction reactions of the Faworski-type have been extensively investigated to synthesize pyrethroid cyclopropane-carboxylic acids, using the smooth 2 + 2 cycloaddition of haloketenes to olefins [60] (Reaction schemes 30, 31) forming 2-halocyclobutanones 45-49, which in an alkahne medium rearrange to cyclopropane carboxyUc acids. 

Dichloroketene as a component for this 2 + 2-cycloaddition, however, introduces a new problem for the Faworski-reaction. The a,a-dihalocyclobutanones are not suited for the rearrangement of the carbon skeleton. Instead, the tertiary chlorine atom is transferred to the other a-position under basic conditions to give a,a -dihalocyclobutanones 248 [495] (Reaction schemes 172, 173 see also page 43). Even... 

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