Ring opening of cyclopropanones

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. 

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. 

Related to TME is TMM 34. The lale of TMM also entails a synergistic interplay of theory and experiment similar to that of TME. This story is nicely retold in the review article of Lineberger and Borden. Instead of reviewing this TMM story, we present the case of the analogous oxyallyl diradical (OXA) 40. This diradical has been proposed as an intermediate in the ring opening of cyclopropanone " and other reactions. 

An allylidene cyclopropane synthesized via phenylthiocyclopropyl lithium has served as a potent diene in a Diels-Alder reaction. Earlier, several methods for ring-opening of cyclopropanone dithioketals—one leading to ketones (equation 115)—had been reported ... 

Tishchenko and coworkers obtained dihydrofuran derivatives when 2-benzoyl-1,1-dichlorocyclopropane (102) was treated with alkoxides (Scheme 40) . Product (107) is highly reminiscent of butenolide (70) obtained by the addition of chloromethoxycarbene to acrolein as discussed earlier (Scheme 29) and may result from ring-opening of cyclopropanone ketal (104). Perhaps a more likely mechanism involves addition of alkoxide to intermediate 103 followed by cyclization 106 - 107. When treated with thiophenoxide, 102 gave cyclopropanone dithioketal (108). A similar result has been obtained by Ban well (Scheme 41) . Treatment of 109 with thiophenoxide gave dithioketal 110 in 98 % yield. In contrast to the results of Tishchenko, however, keto ester products (112) were obtained from 2,2-dichloro-l-acylcyclopropanes (111) and alkoxides (Scheme 42) ... 

The regiochemistry of the ring opening of cyclopropanone adducts is determined by the stability of the intermediate carbanions. 

Upon treatment with sodium methoxide ring opening of cyclopropanone hemiacetals 19 occurred to give the linear rearrangement products 20, exclusively. ... 

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. 

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). 

Giusti s studies on ring-opening of a series of spiro ketal derivatives showed that, on acid hydrolysis (Table 10), ring-opening usually occurs at the site of the less-substituted carbon. The preference shown by cyclopropanone ethylene ketals for path b in Scheme 18 may be attributed to (i) the reversibility of path a due to rapid intramolecular ketalization at the incipient C(l)-carbenium ion (45) (R = CH2CH2OH) and (ii) the stability of the intermediate dioxocarbenium ion (46) generated in path b. 

For example, irradiation of 3,5-di-fert-butyl-6,6-dimethoxycyclohexa-2,4-dienone (5) furnished l,4-di- ert-butyl-6,6-dimethoxybicyclo[3.1.0]hex-3-en-2-one (6). When a small amount of p-toluenesulfonic acid was added to the irradiated solutions, the cyclopropanone acetal 6 underwent protonation followed by ring opening of the three-membered ring and hydrolysis to give methyl 2,5-di-terr-butyl-4-oxocyclopent-2-enecarboxylate (7) in 72% overall yield. ... 

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. ... 

Bis()8-carboalkoxyalkyl)zinc reagents can also be prepared by ring opening of the cyclopropane ring of mixed cyclopropanone acetals ... 

Also related to a Favorskii reaction is the base-induced ring-opening of the cyclopropanone hemiacetal (275). The protonation at C-3 in this ring-opening has been estimated by an n.m.r. technique to occur with ca. 95 % retention of configuration. ... 

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