Cyclopropane fission

As part of a study of ring construction through transposition of activated cyclo-propanes, Danishefsky s group has scrutinized the feasibility of intramolecular homoconjugate nucleophilic additon.64 Two of the substrates examined were 34 and 35 and these were found to undergo rather efficient closure to bicyclo[3.3.0]octane derivatives when treated with strong base. The somewhat elevated temperatures allow both the cyclopropane fission and subsequent Dieckmann condensation to proceed at respectable rates. 

Cyclopropane fission. Cyclopropane and this process can be used to generate esu... 

Cyclopropane carboxylic acid is degraded via 3-hydroxybutyrate by both the bacterium Rhodococcus rhodochrous (Toraya et al. 2004) (Figure 7.33) and by fungi (Schiller and Chung 1970), although the mechanism for ring fission has not been determined. 

A mechanistic interpretation for the formation of 35 is depicted in Scheme 5. Deprotonation of an allylic proton yields ylide intermediate 36. This then adds to methyl acrylate to give intermediate 37, which cyclizes to construct a cyclopropane ring together with the fission of the S-C bond to afford the final adduct 35. 

The reported proposed sequence also offers two additional alternative mechanisms for the cyclodimerization of BCP (3), involving either intermediate 463 or 464 [6a, 13b]. However, they appear less likely, requiring successive three-membered ring fissions and formations. Alternatively, a thermally allowed concerted [jt2s + rt2a -I- pericyclic reaction involving the Walsh type molecular orbital of cyclopropane [124] has been proposed (Fig. 4) [13b]. 

The [Rh(CO)2Cl]2-induced ring fission of substituted cyclopropanes 8a-b affords the rhodium complexes 9a-b via carbonylation [8]. The regioselectivity of carbonyl group insertion depends on the substituent. Reduction with NaBH4 leads to the corresponding alcohol. (Scheme 4)... 

The coupling reaction of the cyelopropylcuprates 87 with the 4-chloro-cyclobutenones 75 or their ethylene acetals 86 is useful for preparing the 4-cyclopropyl-2-cyclobutenones 88. The ring fission of 88 to the cyclohep-tadienones 89 is performed by a Rh(I)-catalyst. The less substituted cyclopropane ring bond is cleaved selectively. Cyclooctadienones are obtained by using 4-cyclobutyl-2-cyclobutenones [43]. (Scheme 31)... 

Fig. 4.22. Formation of 1,3-dipoles by ring fission of donor-acceptor-substituted cyclopropanes.

The reaction of heteroatom-substituted alkenes with electrophilic carbene complexes can lead to the formation of highly reactive, donor-acceptor-substituted cyclopropanes. This type of cyclopropane usually undergoes ring fission and rearrangement reactions under milder conditions than do unsubstituted cyclopropanes (Figure 4.22). 

Irradiation of methylcyclopropylketone (Formula 311) causes isomerization to 3-penten-2-one (Formula 312) (124). This reaction appears to be limited to acyl cyclopropanes, since acyl cycloalkanes with larger rings undergo photochemical fission to acyl radicals and cycloalkyl radicals (105). The photoisomerization of the bicyclic ketone (Formula 313) to Formula 314 (125) is analogous to the isomerization of Formula 311. 

On the basis of the general mechanistic concept used in Charts 11 and 12, the transformations of the steroid ketones 21 and 22 (Chart 3) can be rationalized as follows photoproducts 23, 24, and 26 are derived from ketone 21 via the common intermediate species 93, which may be produced upon 1,5-bridging in the excited state 91 and subsequent homolytic fission of the 1,10-cyclopropane bond in the resulting diradical 92 (Chart 13). Bonding between C-1 and C-6 in the key intermediate 93 (->- 94) and opening of the three-membered ring gives access to struc-... 

MO calculations have been carried out on the isomerization of cyclopropane to propene, and the MNDO method has been used to study the reaction pathway and to optimize the structure of reactant, transition structure, and product of the ring opening reaction of bicyclo[1.1.0]butane. Various methods have been employed to estimate the rate constants for ring opening of the 2-cyclopropyl-2-propyl radical. 1-Acceptor-1-sulfenyl-substituted 2-vinylcyclopropanes of the type (430) have been found to afford 6-sulfenyl-a,jS y, -unsaturated carboxylic esters and nitriles (431) upon treatment with acid, by a process which involves C(l)—C(2) bond fission and a novel 1,5-sulfenyl rearrangement (see Scheme 110). It has been shown that the benzophenone-sensitized photolysis of vinyl norcaradiene derivatives, such as 5-(2-methylprop-l-enyl)-3-oxatricyclo[4.4.0.0 ]deca-7,9-dien-4-ones (432), results in the regioselective cleavage of only one of the cyclopropyl c-bonds to afford isochroman-3-one derivatives (433). It has been reported that the major product obtained from the reaction of structurally diverse a-diazo ketones with an electron-rich alkene in the... 

In the spectrum of cyclobuxine-D, the 3-methylamino group is characterized by the peak at m/e 44. The fragmentation process (d) probably commences by the fission of the 2,3-linkage and transfer of the hydrogen atom from position 1 (16). Thus, the presence of the exomethylene double bond influences the course of the fragmentation more strongly than does the cyclopropane ring. 

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