Mechanism, cyclopropane ring

Karthikeyan S, Q Zhou, Z Zhao, C-L Kao, Z Tao, H Robinson, H-w Liu, H Zhang (2004) Structural analysis of Pseudomonas 1-aminocyclopropane-l-carboxylate deaminase complexes insight into the mechanism of a unique pyridoxal-5 -phosphate dependent cyclopropane ring-opening reaction. Biochemistry 43 13328-13339. 

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. 

Schiller JG, AE Chung (1970) Mechanism of cyclopropane ring cleavage in cyclopropanecarboxylic acid. J Biol Chem 245 6553-6557. 

Quinone methides 156 and 157 were generated on irradiation of the cis and trans forms of 155, respectively (Eqs. 1.42 and 1.43).99 100 The mechanism is believed to involve a radical ring opening of the cyclopropane ring, followed by hydrogen or methyl migration. 

The [5 + 2]-cycloadditions of tethered alkyne-VCPs that are 1,2-disubstituted on the cyclopropane ring 5j—1 have been studied and a mechanism has been advanced to explain the regio- and stereoselectivities of the reactions.37 In most cases, the product resulting from cleavage of the less-substituted (sterically less encumbered) carbon-carbon bond is obtained. The [5 + 2]-reaction is stereospecific in that a /ram-rclationship of the substituents on the cyclopropane leads to a m-relationship of the substituents in the product and vice versa (Equations (4) and (5)). For some tethered alkyne-VCPs which contain a functional group that weakens the carbon-carbon bond of the cyclopropane system, the more substituted (weaker) carbon-carbon bond can be cleaved selectively depending on the choice of catalyst. Thus far, the rhodium(l)-catalysts are more selective catalysts than the mthenium(0)-catalysts in the [5 + 2]-reaction of these substituted alkyne-VCPs (Scheme 7).38... 

These processes constitute a chain reaction. There are many possible chain-ending steps, which could account for some of the minor products observed in the pjorolysis. Certainly the pyrolyses of 3-ethylpent-l-ene and 3-ethylpent-2-ene do yield ethylbutadiene, but the rate of production of the diene is considerably slower than in the pyrolysis of the cyclopropane. Thus reaction (4) represents a sensitization of these latter processes. Further, unlike Chesick, we envisage the formation of the methyl radical from the cyclopropane to involve the rupture of the cyclopropane ring, and believe that this concerted process should be energetically favoured over the two-step mechanism. There are several simple experimental tests that could be used to decide between these mechanisms. 

For the sake of simplicity, carbenium ions, carbonium ions or protonated cyclopropane rings were used as reaction intermediates, omitting the anionic zeolite framework in the illustrahon of the reaction mechanisms for the reactions discussed here. Furthermore, it is conceivable that many such reachon paths involve alkoxide intermediates, instead of carbenium and carbonium ions. 

The unimolecular mechanism involves formation of a protonated cyclopropane ring first, which avoids the formahon of a primary carbenium ion until after skeletal rearrangement has taken place. Such reachon intermediates were first... 

Most of the cyclopropane ring forming reactions can be accommodated in the mechanism that involves a carbanion in 8 2 displacement of the second halogen... 

From a formal point of view, the one-pot heterocyclizations discussed in Sect. 4.2 may be also called MIRC reactions, in the present section this term implies the formation of a cyclopropane ring in accordance with the generally accepted MIRC mechanism [1]... 

Two steps, shown in Figure 17-13, are required to prepare the cyclopropane ring. Even though not normally required in a retrosynthetic analysis problem. Figure 17-13 also shows a partial mechanism. 

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