Cyclopropanone, ring-opening reaction

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

Ring-opening reactions of cyclopropanone hemiketals are well known. Under appropriate conditions, cleavage of trimethylsilyl protected hemiketals can provide a synthetically useful route to homoenolate anions as noted earlier (Scheme 20). The reaction of an isopropoxy-titanium homoenolate (128) derived from 127 with an aldehyde has recently been used as the key step in the stereocontrolled construction of the steroidal side-chain of depresosterol (Scheme 49) ... 

Bicyclic cyclopropanone aminals with a succinimido moiety as iV-substituent allow displacement of succinimide by hydrogen by heating with triethylammonium formate giving 1. A ratio 3 1 of formate to aminal gave the best results. This reduction could not be applied to bicy-clo[3,1.0]hexane or bicyclo[9.1.0]dodecane systems due to ring-opening reactions under these conditions. ... 

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. 

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... 

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 ... 

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 ... 

Displacement reactions on 1,1-disubstituted cyclopropanes have been used to prepare other cyclopropanone equivalents. The most readily available 1,1-disubstituted cyclopropanes are geminal dihalo derivatives prepared by the addition of dihalocarbenes to olefins. Unfortunately, these materials do not undergo direct displacement easily and therefore do not provide a general route to other cyclopropanone derivatives. Solvolysis usually leads to ring-opened products, although dibromocyclopropanes with a barrier to... 

As discussed in an earlier section, displacement reactions on appropriately 1,1-disubstituted cyclopropanes may give rise to new cyclopropanone equivalents (see Schemes 43, 44). These reactions occur by S 1 processes involving carbenium ions of structure 136. In addition to being susceptible to ring-opening (Scheme 18) and trapping... 

With bulkier alkoxides, e. g. potassium ter/-butoxide in fcrr-butyl alcohol or potassium (4-chlorophenyl)dimethylcarbinolate, a-bromo ketone 16 is converted into rra x-2,3-di-/cr/-butyl-cyclopropanone (17). Addition of methanol or 2-propanol to cyclopropanone 17 at 25 °C gives a fast addition reaction resulting in hemiacetal 21, which slowly undergoes ring opening at 80 °C in methanol to give a-methoxy ketone 22 (R = Me). ... 

The reaction of lithium aluminum hydride with cyclopropanones, e. g. trans-2, i-6 -tert-h xiy -cyclopropanone (l)A 2-(trimethylsilyl)cyclopropanone (3) ° and 2-(trialkylgermyl)cyclo-propanone (5, Me = Ge)A ° gave rise to the 1,2-addition products which, upon aqueous workup, delivered cyclopropanols 2, 4, and 6. Under forcing conditions the addition reaction may be followed by ring opening. 

Both ring-opened and ring-closed products result when cyclopropanones are treated with nucleophiles or electrophiles. Which of these reaction types predominates depends on the structure of the cyclopropanones and also on the structure of the acid or base employed. In the preceding sections most of the examples dealt with the addition of nucleophiles across the carbonyl moiety of cyclopropanones. However, as briefly noted, acids also add to cyclopropanones. The addition of hydrogen chloride to cyclopropanone in dichloromethane at 25 °C in the presence of inhibitors, or at — 78°C in the absence of inhibitors, resulted in 1-chlorocyclopropanol (1) in 65% yield.Acetic acid also added to cyclopropanone under similar reaction conditions to give adduct 2 in variable yield.Both compounds, 1 and 2, slowly decompose at room temperature. 

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