Ring expansion cyclobutane

Treatment of the alkenylpotassium (210 X = K) with dimethoxyfluoroborane and peroxide gives (+)-myrtenoI (210 X = OH) contaminated with 2% of trans-pinocarveol (213 R = H).331 Alkylation of the ir-allyl complex (230) occurs directly on carbon from the more hindered side of the molecule to yield (231).332 Heating a-or /3-pinene with pentacarbonyliron yields the chiral ring skeletal enantiomers (232) and (233) by 7r-allyl cyclobutane ring-expansion.333... 

Cyclopropanes bearing an electron-donating substituent X as well as an electron-deficient center Y at C1 undergo a facile cyclopropane to cyclobutane ring expansion reaction to give cyclobutanones or related derivatives. 

The formation of 1-oxocyclopropyllithium compounds can also be accomplished starting from 1-alkoxyphenylsuIfanylcyclopropanes using lithium 1-dimethylaminonaphthalenide (LDMAN). The effectiveness of this procedure is illustrated starting from 1-methoxy-l-phenylsulfanyl-cyclopropane (53) giving cyclobutanones 56. An efficient method to initiate cyclopropylmethyl to cyclobutane ring expansion of these cyclopropanes under neutral conditions uses triflic anhydride in dichloromethane in the presence of 2,6-di-rer -butyl-4-methylpyridine. ... 

Vinylcyclopropanes represent particularly useful functionality. They do permit a ring expansion to cyclobutanes via the cyclopropylcarbinyl cation manifold (Eq. 9). Equally important, such systems suffer smooth thermal rearrangement to cyclopen-... 

The ring expansion of cyclobutane derivatives to other carbocycles remains to be one of the most powerful tool in synthetic organic chemistry. Cyclobutanones are exceptionally facile starting materials for the preparations of y-lactones as well as cyclopentanones. 

Cyelobutanone has been prepared by (1) reaction of diazomethane with ketene,4 (2) treatment of methylenecyclobutane with performic acid, followed by cleavage of the resulting glycol with lead tetraacetate,s (3) ozonolysis of methylenecyclobutane, (4) epoxidation of methylene-cyclopropane followed by acid-catalyzed ring expansion,7 and (5) oxidative cleavage of cyclobutane trimethylene thioketal, which in turn is prepared from 2-(co-chloropropyl)-l,3-dithiane.8... 

A new method for the synthesis of 2-substituted, as well as 2,4- and 2,5-disubstituted, cyclopentanones in 53-93% yield has been reported.81 For example, the Lewis acid catalyzed transformation of l-propanoyl-l-(4-tolylsulfanyl)cyclobutane gave 2-ethyl-2-(4-tolylsulf-anyl)cyclopentanone (1) in 93 % yield. The formation of the cyclopentanone is best explained by a mechanism which involves initial coordination of aluminum trichloride to the carbonyl oxygen, followed by ring expansion to form the sulfur-stabilized carbocation. Finally, migration of the ethyl group to the carbocation center regenerates concomitantly the carbonyl function.81... 

The substituents at the 1-position of the cyclopropyl ring can have an important influence on the reaction pathway. Thus, fluorination and simultaneous ring expansion to cyclobutane derivatives is observed on treatment of substituted [l-alkyl(or aryl)cyclopropyl]phenyl-... 

In addition to the ring expansion of cyclobutane derivatives, 1-cyclopentylcyclo-pentanol derivatives and spiro compounds (Scheme 9.41) could be utilized in the iron-initiated transformation utilizing anhydrous iron(III) chloride on silica, with good yields [99]. 

The adducts of 1-arylthiocyclopropyllithium 116 61) to aldehydes and ketones, upon treatment with p-toluenesulfonic acid in refluxing benzene under anhydrous conditions or with the Burgess reagent67), underwent ring expansion to 1 -phenylthiocyclobutenes, which may be hydrolyzed to cyclobutanones, desulfurized to cyclobutanes or thermo-lyzed to dienes. Thus, the cyclopropylcarbinol 233, adduct of 116a to /-butyl methyl ketone, was rearranged to the cyclobutanone enol thioether 234, Eq. (72) 139). 

From three-membered rings In pro tic media, l-acyl-2-cyclopropene derivatives undergo a ring expansion reaction to cyclobutenols [7]. - Ring expansion of cyclopropylmethanols to fluorinated cyclobutans [8]. 

Photochemical cyclobutane annealings are much more promising and find their way into synthesis to a greater extent. Photochemical cycloaddition reactions to enolized 1,3-diketones and consecutive ring openings via retroaldol reactions have been applied in ring expansion reactions numerous times. 

The well-known ring expansion of cyclobutanes to tetrahydrofuranyl carbenes has been exploited in an approach to the synthesis of disaccharides. " ... 

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