Cyclopropanones ring opening

In the symmetrical cyclopropanone intermediate, the two a-carbons are equivalent and ring opening via route a or route b gives the same carbanion. However, unsymmetrical cyclopropanone ring opens in such a way so that the more stable carbanion of the two possible carbanions is formed. 

The cyclopropanone ring opens so as to give the more stable carbanion, which is rapidly protonated by the solvent "" . Elimination of halide ion is first-order in both the substrate and the base. 

In cyclohexanyl haloketones 21, 22, 23, and 24 the very bulky equatorial substituents 1,3 to the halogen prohibit the existence of any chair-chair equilibration. However, these derivatives can adopt a boat conformation in which the axial halogen is not sterically hindered, except in the case of derivative 24, which does not ring contract in the presence of sodium methoxide in methanol.When zwitterion formation is favored under such conditions, its disrotatory in or out cyclization can give rise to both a- and cyclopropanones. Ring opening of the latter will lead to esters with configurations opposite to those of the initial carbon-bromine bond. Such a hypothesis is in accord with the reactivity observed in the case of 23. ... 

Cyclopropanone ring opening to give the ester is generally considered to be rapid and not to have any influence on the overall rate of the reaction. Similarly, direct substitution of the zwitterion is not considered to be the slow step in the reaction. As the reaction rate of both cyclopropanone and zwitterion are comparable, it is reasonable to assume that the ratio of the products formed reflects the ratio of the intermediates from which they are issued. 

Methylideneoxiranes (allene oxides Section 5.05.3.2.1) react with nucleophiles as if ring opening occurs to give a zwitterion (e.g. 51 or 52), which may be captured by the nucleophile before (Scheme 42) or after (Scheme 43) isomerization to a cyclopropanone. 

Nucleophilic addition of the base to the intermediate 2 leads to ring opening. With a symmetrically substituted cyclopropanone, cleavage of either Ca-CO bond leads to the same product. With unsymmetrical cyclopropanones, that bond is broken preferentially that leads to the more stable carbanion 5 ... 

In some cases, the two stereoisomers can interconvert. In cis- and trans-disubstituted cyclopropanones, for example, there is reversible interconversion that favors the more stable trans isomer. This fluxional isomerization occurs via ring opening to an unseen oxyallyl valence bond isomer. ... 

The cyclopropanone intermediate (81) undergoes subsequent addition of eOH, followed by ring-opening to yield the more stable of the two possible carbanions (83, benzyl > primary), followed by proton exchange to yield the rearranged carboxylate anion end-product (84). 

The analogous calculations for the ring-opening of cyclopropanone to form the oxyallyl intermediate (11) give more pronounced minima in both the ground and excited configuration surfaces, and the A and S levels do not cross (see Fig. 2b). This requires, on orbital symmetry grounds, a disrotatory reclosure for oxyallyl. 

Ethyl phenyl cyclopropenone (14) on reduction with NaBH4 gave rise to prod ucts 289-291, which can be ascribed to a common cyclopropanone intermediate 288 ring-opened by further reduction or attack of solvent209 ... 

Product distribution in the reaction of 4 with furan depends on the reaction conditions as well as on the oxy group of the acetal substrates 4a-c. The diverse products formed in the reaction of 4a-c with furan are rationalized by the reaction pathways illustrated in Scheme 13. All products arise from nucleophilic addition of furan to alkylideneallyl cation intermediate 5M (5S), which is generated by acid-mediated ring opening of cyclopropanone acetals 4a-c (Scheme 5). The [4 + 3] cycloadduct 23 is simply formed via 27, and the furanyl... 

Semiempirical calculations have been used to study the mechanism of the ring opening of cyclopropanone and substituted analogues in a range of solvents of varying polarity. Transition states and oxyallyl intermediates have been characterized, as have the effects of solvents on their stability. The results are also compared with kinetic data in the literature. 

The molecular mechanisms for the ring openings of various cyclopropanone systems in the gas phase have been studied at the PM3 semiempirical level and shown to be disrotatory processes, while an experimental study of the stereomutation of 1,1-difluoro-2-ethyl-3-methylcyclopropane has confirmed the predicted preference for disrotatory ring opening and ring closure for this system. 

A common intermediate is required in this Favorskii rearrangement to set the same product. Removal of an aH by OH- is followed by an SN2 displacement of Cl- to give a cyclopropanone ring. Ring opening occurs to give the more stable benzylic carbanion. 

Although the spectral properties of cyclopropanones and the easy formation of hydrates and hemiketals are inconsistent with the dipolar form, some reactions of cyclopropanones do indicate that the ring carbons are much more electrophilic than in other cyclic or acyclic ketones. For example, nucleophilic ring opening often occurs easily ... 

Another useful route to cyclopentanes is the ring contraction of 2-bromo-cyclohexanones by a Favorskii rearrangement to give cyclopentanecarboxylic acids. If a,fi-dibromoketones are used, ring opening of the intermediate cyclopropanone leads selectively to /J,y-unsaturated carboxylic acids (S.A. Achmad, 1963, 1965 J. Wolinsky, 1965). 

The spiro ketal derivatives studied by Giusti 105> are remarkably uniform in their behavior toward acid hydrolysis (Table 17). Ring opening usually occurs at the site of the less-substituted carbon and, in all cases, the ester is the sole product. The overwhelming preference shown by cyclopropanone ethylene ketals for path b) in Scheme 28 may be attributed to (a) the reversibility of path a) due to rapid intramolecular ketalization at the incipient Ci-carbonium ion 131 (R = CH2CH2OH) and (b) the stability of the intermediate dioxocarbonium ion 131 a generated in path 6).106>... 

Table 17. Ring opening reactions of cyclopropanone acetals under acidic conditions... 

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