Cyclopropanones 1,1-disubstituted

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. ... 

Reactions with cyclopropene.11 Lithium organocuprates react with the cyclo-propenone ketal 1 (12, 152-154) to form a copper species (a) that behaves as an enolate of a cyclopropanone. Thus it reacts with alkyl halides to form cis-2,3-disubstituted derivatives of 1. 

In connection with the above discussion, formation of 3,3-disubsti-tuted 2 (3 H)-oxepinones (73) in the dye-sensitized photooxygenation of 6,6-disubstituted fulvenes is of special interest. 57>58> The reaction may be pictured in terms of an allene oxide intermediate which, as shown in Scheme 11, isomerizes to a cyclopropanone, followed by intramolecular rearrangement. 

Aside from the ready preparation of some a,p-disubstituted cyclopentanones, the utility of the cyclopropanone hemiacetal approach has been illustrated by the total synthesis of the methyl ester of 11-deoxyprostaglandin E2 342 15). Towards this end, 1-trimethylsilylbutadiynylcyclopropanol 13, readily available from the cyclopropanone hemiacetal 3 (vide supra, Sect. 2.1, Eq. (6)) was successively treated with dihydro-pyran in CH2C12 in the presence of 10% mol. equiv. of PPTS 101). Desilylation by potassium fluoride in DMF 138), formation of the lithium salt with n-BuLi and condensation with hexanal gave the propargylic alcohol 339 in 64 % overall yield. The... 

In this chapter, we will review methods for preparing cyclopropanones, their physical and spectroscopic properties, and the nature of their reactions with nucleophiles, electrophiles and in cycloaddition processes. Another part of the chapter will deal with cyclopropanone equivalents, 1,1-disubstituted systems which under certain conditions may provide carbonyl-related derivatives of the parent ketones. We will also discuss the role of cyclopropanones in biological phenomena and cite specific examples of the use of cyclopropanone intermediates as key units in the synthesis of natural products. 

An unusual example of the allene oxide-cyclopropanone isomerization is found in the results of singlet oxygen oxygenation of 6,6-disubstituted fulvenes (18) The production of 3,3-disubstituted 2(3//)-oxepinones (20) in this process can be rationalized by the sequence of transformations outlined in equation 8. The initially formed endoperoxide (19) undergoes a well-precedented rearrangement to the epoxy ketone (19a) which contains the allene oxide residue. Rearrangement to the cyclopropanone form is... 

Among the systems which have played the role of cyclopropanone equivalents have been cyclopropane derivatives containing both a secondary amino group and a hydroxyl substituent on the same carbon atom. These 1,1-disubstituted carbinol amines appear to be in equilibrium with the corresponding iminium salts and thereby act as reactive electrophilic sites for attack by a variety of nucleophiles. 

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... 

The addition of alcohols across 2,3-disubstituted cyclopropanones also occurs from the less hindered side. CK-2,3-Bis(trimethylsilyl)cyclopropanone (31) reacts with benzyl alcohol to yield adduct 32 in a stereospecific way. ... 

Upon thermolysis, l.l -disubstituted dicyclopropyl ethers expel cyclopropanone, which oligomerizes rapidly (major pathway). This process is accompanied by some ring opening to a 2-hydroxyallyl cation, thus accounting for the ultimate formation of substituted acetones. 

Cyclopropanones are also reactive toward certain types of cycloadditions. Theoretical modeling indicates that a dipolar species resulting from reversible cleavage of the cyclopropanone ring is the reactive species.cw-Disubstituted cyclopropanes with bulky substituents exhibit NMR features that indicate a barrier of 10-13 kcal/mol for... 

---