Alkyl-substituted cyclobutanones

Alkyl-Substituted Cyclobutanones. The molecular predissociation of cyclobutanone may give rise to biradical species, trimethylene and acyltrimethylene, via a ring-opening reaction (23, 31), 

A striking similarity observed between the hydrocarbon product distributions in the photolysis of dimethylcyclobutanones and the photolysis of dimethylpyrazolines has been noted (52,158). 

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In 1967 Turro and Southam reported the first systematic study of alkyl substituted cyclobutanones in methanol solutions (15). As with cyclobutanone [21], all the alkyl ketones studied ([26a-c]) exhibited products characteristic of B-cleavage, decarbonylation, and ring expansion. Interestingly, the selectivity of a-cleavage is reminiscent of similar trends observed for a-alkylated... 

Solvolysis of 3,5-Dinitrobenzenesulfonates. Solvolysis of the crystalline 3,5-dinitrobenzenesulfonate of 3-pentyn-l-ol in CF3C02H-Cp3C02Na gives 2-methylcyclobutanone in 73% yield (eq 1). The yield of 2-methylcyclobutanone was reduced when the p-toluene- and /n-nitrobenzenesulfonates were used. Cyclobutanone and other alkyl-substituted cyclobutanones have also been prepared via this participation of an alk)mic bond during solvolysis of homopropargylic sulfonate esters, but the former requires the use of the more reactive trifluoromethanesulfonate. 

There are several studies of the photodecomposition of alkyl-substituted cyclobutanones for which there is insuffieient data to estimate / -values, but the results may be helpful in understanding the effects of structure on the photodecomposition modes of the cychc ketones. 

An attractive synthesis of cyclobutanone (253) has been recently described using 1-isocyano-l-tosylcyclobutane(252)(equation 151)144. l-Isocyano-l-tosylcyclobutanes252 can be prepared from (tosylmethyl)isocyanide and alkyl-substituted 1,3-dibromobutanes. This method appears to be superior to previously reported methods for the preparation of cyclobutanone because of high purity and high yields. 

Cyclobutanones (11, 560-561). Ketenimium salts are more reactive than ke-tenes in [2 + 2] cycloadditions with alkenes to prepare cyclobutanones. The salts are readily available by in situ reaction of tertiary amides with triflic anhydride and a base, generally 2,4,6-collidine. The cycloaddition proceeds satisfactorily with alkyl-substituted alkenes and alkynes, but not with enol ethers or enamines.1... 

An as yet unexplicable drawback of the described catalytic system (S,S)-5/pivaldehyde/dioxygen is its limitation to 2-aryl substituted cyclohexanones the positional isomer 4-phenylcyclohexanone or 2-alkyl substituted cyclohexanones are not converted. The strained cyclobutanones, however, will almost always react under the catalytic conditions. Thus, the racemic bicyclic cyclobu-... 

Substituents R and have been alkyl, ethoxycarbonylethyl, benzyl, substituted benzyl, and phenyl the latter gave the lowest yield. Cyclobutanone gave a rearranged product, due apparently to ring strain. ... 

There appears to be at least one factor, in addition to angular strain and resonance stabilization, which determines the relative importance of photoinduced decarbonylation, 6-cleavage, and ring expansion of cyclobutanones. It has already been demonstrated that increasing a-alkylation of cyclo-butanone results in an increase in the extent of cyclic acetal formation (i.e., ring expansion). Comparison of [64a] and [64b] reveals a similar trend. The 3-phenyl-cyclobutanones [141] and [144], however, provide the most spectacular effect of a-substitution (25). In both cases, a-cleavage occurs exclusively at the most highly sub-... 

Cycloadditions of ketenes with alkenes and alkynes constitute the most popular method for the synthesis of cyclobutanones and cyclobutenones. Unfoitunately, however, this process is truly general only for highly nucleophilic ketenophiles such as conjugated dienes and enol ethers. In general, unactivated alkenes and alkynes fail to react in good yield with either alkyl- or aryl-substituted ketenes, or with ketene itself To circumvent this limitation, dichloroketene is usually employed as a ketene equivalent, since this electrophilic ketene reacts well with many types of unactivated multiple bonds, and the resultant cycloadducts undergo facile dechlorination under mild conditions. ... 

The pinacol rearrangements of several cyclobutanediols, mostly induced by BFs-OEta, have been examined by Conia. The reactions of symmetrically substituted materials occurred with high specificity and yield. Both isomers cis and trans) gave the same product in instances where this question was examined. As shown in equation (17), for the substituents R = H, alkyl or phenyl, only ring contraction was observed, while equally regiospecific formation of the cyclobutanone product was found for the substituents R = allyl or vinyl. 

Both cyclobutanones and the tertiary cyclobutanols derived from them behave as intramolecular alkylating reagents towards 0-substituted aromatic rings under 4-TsOH catalysis yielding cyclobuta[c]chromans 23. The use of an equimolar amount of 4-TsOH results in subsequent fission of the cyclobutane ring and the formation of chromenes <04T449>. 

Asymmetric synthesis of 3,4-dihydrocoumarin 41 was achieved by rhodium(I) catalyzed intramolecular reaction of phenol-substituted cydobutanone 42 (Scheme 3.21) [33]. When the reaction of 3,3-disubstituted cyclobutanones was performed in the presence of electron-deficient alkenes, the arylrhodium species generated via 1,4-rhodium shift underwent addition to the alkenes, providing 5-alkylated dihydrocoumarins (Scheme 3.22). 

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