Carbocycles five-membered

Conversion of the five-membered metallacycles into five-membered carbocycles, where the metal is eventually replaced with one carbon atom, is an attractive method for the construction of five-membered carbocycles (Eq. 38). In fact, such transformation is very popular using five-membered metallacycles of titanocenes and zirconocenes [1-5]. 

Organic Chemistry Section, National Institute of Interdisciplinary Science and Technology (NIIST), Kerala, India 

Department of Chemistry, C.M.S. College, Kottayam-686001, Kerala, India 

Stereoselective Multiple Bond-Forming Transformations in Organic Synthesis, First Edition. Edited by Jean Rodriguez and Damien Bonne. 

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The effectiveness of various sources of fluoride ion in the displacement of the trifluoromethanesulfonic group has been demonstrated while introducing a fluorine atom into the five-membered carbocyclic ring of a prostaglandin precursor Treat tnent of the corresponding triflyl derivative with potassium fluoride in acetonitrile or with cesium fluonde in refluxing diinethylformamide or hexamethylphosphoric... 

Asymmetric Pauson-Khand reaction in syntheses of heterocycles fused with five-member carbocyclic fragment 980PP121. 

An interesting strategy for the diastereoselective synthesis of five-membered carbocycles was achieved by the reaction of alkenylcarbene complexes and lithium enolates derived from simple methyl ketones [79]. The use of more or less coordinating solvents (THF or Et20) or the presence of cosolvents such as PMDTA allows the selective synthesis of one or the other diastereoisomer of the final cyclopentene derivative (Scheme 32). 

The following syntheses of five-membered carbocyclic systems involve radical-induced epoxide fragmentation with radical translocation and cyclization. The resulting bicyclic alcohols are formed as a mixture of two epimeric esters with cw-fused rings.[71]... 

The Pauson-Khand reaction (PKR) [96] consists of the synthesis of cyclopen-tenones by reaction of an alkene with a dicobalthexacarbonyl complexed alkyne (Scheme 57) and has recently emerged as one of the methods of choice for the obtainment of five-membered carbocyclic rings [97]. Its unique atom connectivity, which involves the two unsaturated carbons of the reagents and the carbon atom of a carbon monoxide ligand of cobalt usually in a regioselective manner (Scheme 57), has brought to refer to PKR as a [2 -I- 2 + 1] cycloaddition. 

Under a pressure (20 bar) of carbon monoxide, carbonylative silylcarbocyclization of enyne 92 was examined in the presence of a cationic rhodium-BINAP catalyst (Scheme 31).86 Although the enantioselectivity is low, the five-membered carbocycle functionalized with an alkenylsilane moiety and a formyl group was obtained with high selectivity. 

Five-membered carbocycles are the most easily formed [45, 107, 196, 200, 202,203]. Five-membered carbocyclic rings can be formed (with 2% MoF6 as the catalyst) even when the double bond is tetrasubstituted (Eq. 24) [200]. The stability of the catalyst toward the free OH group in this case is noteworthy. Evidently this particular t-butoxide-like alcohol does not react with this particular catalyst for steric reasons. Six-membered carbocyclic rings are also formed readily (Eq. 25) [200], as are seven-membered rings, especially if one takes advantage of a Thorpe-Ingold effect (e.g., Eq. 26) [20] or a similar conformational predisposition for the double bonds to remain near one another. 

Over the last twenty years there has been considerable interest in the application of the cyclobutylmethyl rearrangement to complex organic molecule synthesis. This attention is a result of the quest for intriguing and varied frameworks of organic compounds containing five-membered carbocycles.1 6 However, due to the enormous amount of literature, as well as the lack of systematization in connection with such rearrangements, a comprehensive coverage of the activities in this area is not feasible in this context. For this reason, the examples selected in this section reflect the personal interests of the author. 

Versatile [3 + 2]-cycloaddition pathways to five-membered carbocycles involve the trimethylenemethane (= 2-methylene-propanediyl) synthon (B.M. Trost, 1986). PaIladium(0)-induced 1,3-elimination at suitable reagents generates a reactive n - -methylene-1,3-propa-nediyl complex which reacts highly diastereoselectively with electron-deficient olefins. The resulting methylenecyclopentanes are easily modified, e.g., by ozonolysis, hydroboration etc., and thus a large variety of interesting cyclopentane derivatives is accessible. 

The syntheses of l,2,3,5,6,8a-hexahydroindolizine (1) and (-)-swainsonine were successfully carried out via a ROM-RCM strategy, and final CM with ethylene to free the ruthenium. Two different synthetic routes were implemented to achieve a similar compound. In the case of l,2,3,5,6,8a-hexahydroindolizine 1, the six-membered ring was established first using RRM and in the case of (-)-swainsonine, the five-membered ring was established first. In both cases, the ring rearrangement precursors were enantiomerically pure five-membered carbocycles, which can be synthesised efficiently from racemic starting materials. 

Dicarbonate 14 was used as the starting material to achieve both cis-and trans- ring rearrangement precursors 15 and 16. ROM-RCM of the five-membered carbocycles 15 and 16 leads to 17 and 18. These compounds contain a terminal double bond at position 9, which can be easily functionalised and are set up to form the tricyclic tetraponerines. 

This section follows the content of Section 9.02.8 in CHEC-II(1996) without repetition of data presented in the latter but refers to several papers omitted in CHEC-II(1996) along with new publications. Below are presented various synthetic methods, which are classified by the nature and size of the starting ring. Peculiar differences with CHEC-11(1996) should be marked (1) the absence of paragraphs devoted to formation of oxepine derivatives from four- and five-membered carbocycles because new syntheses of these kinds were not found in the literature (2) Sections 13.02.8.2.2, 13.02.8.2.5, and 13.02.8.2.6, which are new, appear concerning with formation of oxepine derivatives from four-, (other) seven-, and eight-membered heterocycles, respectively. 

The work reported by Eaton and coworkers can be summarized as reactions where an allene system in conjugation with a further unsaturated functionality reacts with carbon monoxide in the presence of an iron-carbonyl complex such as Fe(CO)5 under photochemical and thermal conditions when Fe2(CO)9 is used. When diallenes are used (X=R2C=C, Scheme 9.24), five-membered carbocyclic products are obtained [51, 52], whereas when allenyl ketones (X = O) are applied, five-membered lactones are generated [53, 54]. The use of allenylimines (X = NR) leads to five-membered lactams under these conditions [55]. 

Although a number of multistep procedures are available for the introduction of five-membered carbocycles, their direct formation in a thermal cycloaddition is rare.6 Interest in the potential application of such a three-carbon + two-carbon cyclopentane cycloaddition has been derived from the expectation that such a process could prove to be an effective complement to the four-carbon + two-carbon Diels-Alder reaction which is used extensively in the regio- and stereocontrol led preparation of functionalized six-membered carbocycles. 

Synthesis of Five-Membered Carbocyclic and Heterocyclic Ring Systems... 

The synthetic value of the photoinduced electron-transfer reactions of ketones has been expanded (Scheme 8.62) by generating a five-membered carbocyclic framework, as shown in the preparation of compound 226. 

With the radical controversy solved, Bailey and others set about the detailed investigation of anionic cyclisation as a synthetic method for the formation of five-membered carbocyclic rings, and we shall divide the remainder of this section, which covers work published between 1987 and 2000, into the main classes of compounds produced by the cyclisation cyclopentanes, furans, pyrrolidines, and polycyclic products. It turns out that organolithiums are unique among the alkali metals in their ability to undergo anionic cyclisation to give cyclopentanes, particularly where primary organometallics are concerned.121... 

Oligounsaturated Five-Membered Carbocycles -Aromatic and Antiaromatic Compounds in the Same Family... 

The chemistry of oligounsaturated five-membered carbocycles has fascinated chemists for more than 100 years, since Thiele published the first synthesis of the aromatic cyclopentadienyl (Cp) anion 1 [1], In the last couple of decades, knowledge about fully unsatu-... 

As far as purely five-membered carbocyclic systems are concerned, spiro[4.4]nonatetraene (31) is the prototypical example of two -systems experiencing electronic interaction through their common central carbon atom. Based on symmetry considerations, it was proposed that spiroconjugation in molecules such as 31 should result in a splitting of the cyclopentadienyl HOMOs, whereas the LUMOs should be degenerate [22, 23]. As a consequence, the first electronic absorption (EA) band of cyclopentadiene in 31 was predicted to be split into a lower and a higher lying component of quite different absorptivities. 

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