Cyclopropyl intermediate

The observed acid-catalysed conversion of complestatin (289) into chloropeptin L (291) has been envisioned" " as proceeding through a cyclopropyl intermediate (290) (see Scheme 94). An intramolecular oxygen-transfer reaction illustrated in Scheme 95 has been proposed" to explain hydroxylation of the aromatic nucleus, viz. formation of (292), during the course of a modified Polonovski reaction on galanthamine. 

A fourth mechanism has been suggested by several researchers in order to explain the formation of near-linear polyethylene type oligomers produced on modified[22] and unmodified1231 ZSM-5 zeolite from olefins such as propene, isobutene and 1-decene. They propose a mechanism which proceeds by losing methyl branches via protonated cyclopropyl intermediates as presented in Scheme 6.4. 

The rearrangement of 2-vinyl- and acyl-substituted 2,2-dimethylethyl radicals 2.69 and 2.72 to 2.71 and 2.74 proceeds through cyclopropyl intermediates 2.70 and 2.73, respectively. 

First, we will take up cyclopropyl group formation by the rearrangement of carbon skeletons via cationic intermediates encountered in various mono- and sesquiterpenes, and also examine the illudin biosynthesis where contraction of a cyclobutyl cation to a cyclopropane has been invoked. We will then discuss the head-to-head condensation of isoprenoid alcohols at the C15 or C20 level to generate the cyclopropyl intermediates, presqualene pyrophosphate and prephytoene pyrophosphate, on the way to the C30 and C40 polyene hydrocarbons, squalene and phytoene respectively. Conversion of 2,3-oxidosqualene via common intermediate protosterol cation to cycloartenol or lanosterol represents an important pathway in which the angular methyl group participates in the three-membered ring formation. The cyclopropanation outcome of this process has been carefully studied. 

NADPH. In a number of examples it has been demonstrated that cyclic sesquiter-penoids result from 2-trans-FFP by dephosphorylation and conversion into 2-cis-farnesol (and hence presumably 2-c/5-FPP) via a redox system. However, in the present case farnesol was not a substrate and no tracer could be demonstrated to be associated with the farnesals. The fact that the added hydrogen came from the opposite face of NADPH to that utilized in common redox systems e.g. liver alcohol dehydrogenase) may indicate that a totally different type of mechanism occurs from that previously proposed for the trans to cis conversion, and the occurrence of a novel cyclopropyl intermediate was suggested (Scheme 12). 

The rapid rearrangement of vinyl and acyl substituents can be explained as proceeding through cyclopropyl intermediates. 

The fourth mechanistic approach was suggested by various investigators, and it explains the linear structure of the products obtained by the olefinic oligomerization reaction.The authors have proposed a scheme for an oligomerization reaction in the propene HZSM-5 system, proceeding via protonated cyclopropyl intermediates as shown in Figure 3. " ... 

Synthetic oils with high viscosity indices were produced in the oligomerization of isobutene using HZSM-5 catalyst, surface-deactivated with 2,6-di-/ert-butyl-pyridine. Chen and Bridger proposed a protonated cyclopropyl intermediate for the isobutene-HZSM-5-system in the formation of linear oligomers. 

In general, cyclobutanones are synthesized by either ketene cycloadditions or by ring expansions of cyclopropyl precursors. For the synthesis of simple o-substituted monocyclic cyclobutanones, the latter method is usually employed, and a variety of approaches have been used to prepare the required cyclopropyl Intermediates. 

The ratio of the three possible products 39, 40 and 41, is highly influenced by the concentration of BusSnH. Note that in this case, the 6-endo product is predominant over the 5-exo, probably owing to the rearrangement of the generated alkyl radical A via cyclopropyl intermediate C. In a later paper (05S3067), the same group carried out... 

CRI Reaction of Diazo Compound 17a In our first attempts to perform the CRI reaction, the diazo compotmd 17a was treated with 1% CuOTf in freshly dried and degassed CH2CI2 tmder an N2 environment. To our delight, diazo decomposition of 17a proceeded smoothly at room temperature to yield the desired cyclopropyl intermediate 18a as a mixture of inseparable isomers (5.5 1) in 58% yield after 1 h the C-C insertion by-product 19 was also generated in 21% yield. The stability of 18a may reflect the lower ring strain in the 3/6 bicyclic ring system compared to that of... 

Where MH stands for methyl-hexane and DMP stands for dimethyl-pentane. The isoheptanes do not appear to be at thermodynamic equilibrium, nor are they fixed at some constant composition. Mango argued that it is difficult to explain the relationship between the isoheptanes by a mechanism involving the thermal decomposition of natural products or their respective kerogenous derivatives, and suggested a steady-state kinetics where certain product ratios are necessarily time-invariant. He proposed a catalytic process in which the four isoheptanes are formed pairwise through two cyclopropyl intermediates (Fig. 19). 

An alternative way in which p-keto-esters can be used to form 1,4-diketones is shown in Scheme 11 [21]. A cyclopropyl intermediate 9 is in equilibrium with organozinc species 10 which can then be trapped, best with an aldehyde followed by oxidation, to give a 1,4-diketone. Note that the thiophene formation (Scheme 11) involves the two ketone carbonyl groups and not the ester carbonyl, although in other cases, it can involve ester carbonyl groups (cf. Scheme 6). 

The authors suggest a new mechanism for the methyl-malonyl-sucdnyl rearrangement, catalysed by one of the Big enzymes, which is based on the cyclopropyl-intermediate hypothesis (Scheme 1). The details require retention of configuration during the H-transfer, (a feature of the enzyme system) and provide a clear role for the cobalt fragment during the reaction. 

Ciner, J.-L. and Djerassi, C. (1991) Biosynthetic studies of marine lipids. 31. Evidence for a protonated cyclopropyl intermediate in the biosynthesis of 24-propylidenecholesterol. I. Am. Chem. Soc., 113, 1386-1393. 

A mechanism for the origin of skeletal isomers observed in the cationic dimerization of 1-decene is proposed to involve a protonated cyclopropyl intermediate formed directly by reaction of a 2-decyl carbocation with 1-decene. This intermediate is said not to occur via ring closure of a branched secondary carbocation. Rapid repeated isomeriza-tions of the protonated cyclopropyl intermediates lead to a multitude of skeletal isomers (Scheme 77). ... 

Spirodienone 28 arose through decarboxylation of the corresponding P-keto acid 27, which was obtained by proton loss from C9 and concomitant cleavage of the 1,10-bond — a process that would be favored in the chelated conjugated cyclopropyl intermediate 26b (Scheme 4). 

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