Cyclopropanones 2.2- dimethyl

A reactive source of cyclopropane derivatives capable of furnishing cyclopropanone acetals is 1-chloro-l-methoxycyclopropane (33), which reacts with methanol in the presence of silver salts to give cyclopropanone dimethyl acetal (34) without rupture of the three-membered ring. It was subsequently shown that methanolysis, which occurs via an S l mechanism, does not require silver salts and occurs in quantitative yield with or without added base. ... 

Dibrom-3-oxo-2,4-dimethyl-pentan wird kathodisch (an Quecksilber in aproti-schen Solventien) zu 3-Oxo-2,4-dimethyl-penten-(l) reduziert3. Das intermediar gebil-dete Cyclopropanon-Derivat kann mit Nukleophilen (z. B. Alkoholen, Essigsaure) abge-fangen werden (s. S. 669)3,4. 

Dimethyl anthracene and diphenyl isobenzofuran form remarkably stable233 cyclopropanone derivatives (353/354), whilst with other diene components (butadiene, tetracyclone, and fulvene) the primarily formed Diels-Alder adducts either suffer ketalizing attack of the solvent (356 - 357, 359 - 358/360) or undergo irreversible changes such as decarbonylation to 362 or rearrangement to 355. 

Dibromopropanones may also be reduced electrolytically to form cyclopropanone derivatives as illustrated by the formation 12> of 1-methoxytetramethylcyclopropanol, 38, and l-methoxy-2,2-dimethyl-cyclopropanol, 39, in excellent yields. 

Other alkylation reactions are observed in the condensation of cyclo-propanium ions (generated in situ) with ketones 89.92)> enamines6, nitroalkanes 93>, dimethylmalonate 92>, and phenol. 92> Thus, 7-hydroxy-7-pyrrolidinobicyclo[4.1.0]heptane (56) as well as the 7,7-dipyrrolidino derivative (54) react with acetone to give the amino ketone 113. 89> This reaction may be pictured as an addition of the enol form of the ketone to the reactive iminium salt formed from the carbinol amine. In like manner, phenol undergoes ortho substitution with the carbinol amine 114 formed from cyclopropanone and dimethyl amine. 

Approaches that represent a type (ii) synthesis of 277-pyran-2-ones include the self-condensation of 1,3-dicarbonyl compounds, the reaction of cyclopropanones with pyridinium enolbetaines and the reaction of activated methylene groups with acetylenic esters <1984CHEC, 1996CHEC-II>. 4-Perfluoroalkyl-6-aryl-pyran-2-ones are formed by the reaction of the phosphonium salts 631 with 2-perfluoroalkynoates (Equation 254) <1999JFC(95)135, 1998JFC(91)99>. Dimedone reacts with dimethyl acetylenedicarboxylate to afford the pyran-2-one 632 in excellent yield (Equation 255) <2003PS2627>. 

Other, less stable cyclopropanones, such as the 2,2-dimethyl compound, can be made by carbene addition (Chapter 40) to ketenes. This compound did the Favorskii reaction with methoxide in methanol the only product came from the expected loss of the less unstable carbanion. This will, of course, be general-acid-catalysed by methanol as no free carbanion can be released into an alcoholic solvent. 

Cyclopropanon/l,l-Dihydroxy-cyclopropan IV/3, 409 Dibutyl-zinn-dichlorid/Dimethyl-bis-[4-carboxy-phenyl]-silicium XIII/6, 439... 

In other additions, cyclopropanone forms an addition product with HCN which can be hydrolyzed to form the hydroxy acid 35 (equation 13) . The latter is unusually stable to acid. With HCl, cyclopropanone affords the chlorohydrin 36 (equation 14), but the 2,2-dimethyl derivative undergoes ready ring-opening to form a mixture of 37 and 38, as might be expected from the opportunity to form the more stable dimethylhydroxyallyl cation (equation 15). 

Gem-dibromocyclopropanes can be converted into synthetically useful cyclopropanone equivalents by a process consisting of lithium-halogen exchange followed by reaction of lithiocyclopropane (113) with dimethyl disulfide (Scheme 43) . The resulting bromo-methylthio derivative (114) undergoes a variety of substitution reactions. Methanolysis gives S,0-dimethylketal (115) which can be converted into l,l bis(methyl-thio)cyclopropane (116) with methyl mercaptan in trifluoroacetic acid. Reaction of 114 with other nucleophiles provides the derivatives shown in Scheme 44 . The sulfur-... 

Cyclopropanone ketals undergo a unique [2s + 2a] cycloaddition reaction with tetracyanoethylene (TCNE) ". In the case of cis- and trans-dimethyl 0,S ketals 151... 

An oxy radical of related type may be involved in the oxidation by ceric ion of the hemiacetal (29) of cyclopropanone. Although the spectrum of the radical (30) has not been observed, that of its open-chain isomer (31) has been identified using a flow technique. This mechanism may account for the oxidation of (29) with silver or cupric ion to dimethyl adipate, by dimerization of (31) (Schaafsma et al., 1966). 

Irradiation of 2,2-dimethylcyclobutane-l,3-dione, 2,4-dimethyl-2,4-diphenylcyclobutane-l,3-dione and 2,2,4,4-tetraphenylcyclobutane-l,3-dione resulted in the formation of the corresponding cyclopropanones, but in low yield a number of complex photoreactions occurred. The parent cyclopropanone was prepared in 90% yield on addition of a cold solution of diazomethane in dichloromethane to a four- to six-fold excess of ketene at — 130°C. ... 

Sodium cyanide in dimethyl sulfoxide is a less convenient source for the cyanation but it does allow the use of the stable 1-acetoxycyclopropanol (7) as cyclopropanone sourceX ... 

Bromopentan-3-one (46, X = Br) reacted in a highly stereoselective manner with sodium methoxide in methanol to provide cw-2,3-dimethylcyclopropanone methyl hemiacetal (47). ° This hemiacetal must be in equilibrium with the cyclopropanone in methanol, because dimethyl malonate in the presence of a catalytic amount of sodium methoxide converted the intermediate into cyclopropanol 48. ... 

Chlorocyclopropyl sulfides 71 are easily methanolyzed at ambient temperature into the corresponding 5,0-acetals of cyclopropanone. The methanolysis is accelerated by the introduction of methyl groups on the cyclopropane ring, with each geminal dimethyl pair increasing the rate by approximately fourfold. ... 

Much more reactive are cyclopropanone acetals 5 (X = O) and l-methoxy-2-phenylcyclo-propanes 5 (R = Me R = Ph XR" = H). Thus, methyl substituents at C2 (or C3) decrease, and a phenyl substituent increases the reaetion rate. Several facts are in agreement with a concerted [ 2, +, 2a] mode the reaction of cyclopropanone diethyl acetal proceeds ten times more rapidly in the less polar dichloromethane than in aeetonitrile, the molar volume of activation is large and negative, and l-methoxy-2,3-c -dimethyl-l-(methylsulfanyl)cyclopropane (5, R = R = R = Me XR = SMe) reacts with TCNE with inversion at C2 (or C3) to give the fra/15-dimethylcyclopentane derivative 6. 

Electron-donating silyl ethers such as ketene silyl acetals and cyclopropanone silyl acetals also can be used as latent carbon-functional groups. The photoreactions of 1,2,4,5-tetracyanobenzene with dimethyl ketene silyl acetal and 2,2-dimethylcyclopropanone silyl methyl acetal afford substituted products in good yields, respectively. 

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