Cyclic compounds carbocyclic

In the construction of carbocycles, five-membered ring formation has been used for preparing fused cyclic compounds, such as functionalized diquinanes. ° The reaction of 36 with (TMSlsSiH furnished the expected product 37 in 80% yield and in a or.fi ratio of 82 18, as the result of a kinetic controlled reaction (Reaction 43). 

Alkenes and alkynes can also add to each other to give cyclic products in other ways (see 15-61 and 15-63). The first exclusive exo-dig carbocyclization was reported using HfCU as a catalyst. Alkynes also add to alkenes for form rings in the presence of a palladium catalyst or a zirconium catalyst. " Carbocyclization of an alkene unit to another alkene unit was reported using an yttrium catalyst and alkenes add to alkynes to give cyclic compounds with titanium catalysts. ... 

Several of the compounds identified in ISM have not so far been synthesized in the laboratory however, two of them have now been obtained. The cyclic compound cyclopropylidene (C3H2), first detected in ISM in 1985 and later more frequently, was considered to be too unstable to exist on Earth under laboratory conditions. A derivative of this carbocycle, stabilized by amino groups which serve as -donors, has now been reported. X-ray crystallography shows that the presence of the amino groups has little effect on the molecular geometry as calculated for the unsubstituted cyclopropylidene (Lavallo et al., 2006). 

Syntheses of Four-Membered Cyclic Compounds 3.1.2.1 Carbocyclic Compounds... 

Several indium-mediated intramolecular carbonyl allylation reactions have been investigated, and it has been found that these reactions provide an easy access to a variety of cyclic compounds. The intramolecular cyclization of 49a-c mediated by indium in aqueous media proceeds smoothly to afford carbocyclic systems containing y-hydroxy-Q -methylene esters 50a-c, which either spontaneously or readily cyclize to give fused o -methylene-y-butyrolactones 51a-c (Scheme 52). The same cyclization of 49d is too slow to compete with the side-reaction, in which the bromide is substituted by a hydroxy group. The ring junction stereochemistry of fused lactones 51 has been found to be cis in all cases. Of the two possible transition states, the one leading to the m-fused compounds is preferred, because the chair-chair conformation is favored over the chair-boat conformation.209... 

The intramolecular aldol reaction in the presence of a titanium Lewis acid is a viable means of preparation of cyclic compounds. The cyclization is most conveniently performed between an enol silyl ether and an acetal, because the former is a reactive enol derivative and is readily prepared by silylation of the corresponding ketone in the presence of the acetal moiety in the same molecule. Equation (12) exemplifies a substrate undergoing intramolecular ring closure mediated by TiCU [74]. The conversion of sugar derivatives to carbocycles (called the Perrier reaction [75,76]) has been reported to occur in the presence of a Lewis acid. This process involves the aldol reaction between the enol ether and acetal moieties in the same molecule promoted by a titanium salt, as illustrated in Eq. (13) [77]. The similar reaction of a different type of substrate was also reported [78]. 

A large number of polycyclic heterocycles containing two or more six-membered rings that share two or more carbon atoms are known. For the majority of these systems, only one of the six-membered rings contains the heteroatoms and the other rings are fused carbocyclic rings such as cyclohexane, benzene, naphthalene, and in rare examples, anthracene. Systems in which more than one of the six-membered rings contain heteroatoms are also briefly discussed these include spiro-cyclic compounds and substituted naphthalenes. 

Cyclic compounds in which the ring is made up of atoms of one element only are called isocyclic compounds. If the ring consists of C-atoms only, then we speak of a carbocyclic compound, e.g. ... 

To determine the stability and reactivity of heterocyclic compounds, it is useful to compare them with their carbocyclic analogues. In principle, it is possible to derive every heterocycle from a carbo-cyclic compound by replacing appropriate CH2 or CH groups by heteroatoms. If one limits oneself to monocyclic systems, one can distinguish four types of heterocycles as follows ... 

Six-membered carbocycles Propenone derivatives react with 10 to yield six-membered cyclic compounds. So 1,3-diarylpropenones yielded 4-cyanocyclohexanols and 4-... 

Five member ed carbocycles Although a number of cyclic compounds have been prepared from ylidenemalononitriles, only a few carbocycles have been discussed. So benzylidenemalononitrile (43) was dimerized in DMF/acetic acid to yield cis- and trans-169 44. 

The aim of this account was to show that cyclometallated complexes, the majority being palladium-containing derivatives, in stoichiometric or catalytic quantities, can certainly be part of the arsenal of the chemist seeking to synthesize cyclic compounds selectively and efficiently. One of the major themes of our research in this respect has been to study these reactions from an organometallic chemist s point of view, with the particular goal of defining the precise role of the metal in the hetero- or carbocyclization process. This has led to some unusual and fruitful mechanistic findings and some useful synthetic approaches to many novel hetero- and carbocyclic systems. 

There are many examples of preparing cyclopentane structures from enynes by gold-catalyzed carbocyclization reactions. Toste et al. have reported that Au(l)-phosphine complexes act as superior catalysts for isomerization of 1,5-enynes to bicyclo[3.1.0]hexenes [124]. For example, treatment of 1,5-enyne (86) with 1 mol% of PhsP-AuPFfi in dichloromethane at room temperature results in formation of cyclopropane-fused cyclopentene (87) in 99% yield (Scheme 18.30). 1,6-Enynes also undergo similar cycloisomerization to five-membered cyclic compounds under the influence of cationic gold(l) catalysts [125, 126], Hydroxylated enynes are versatile precursors for cyclopentenones by gold-catalyzed cycloisomerization... 

Compounds that have a chain of carbon atoms, some of which are attached to functional groups, are called acyclic compounds, meaning not cyclic. Compounds that contain rings of carbon atoms, and that may also contain functional groups, are carbocyclic compounds, commonly called cycloalkanes. One example is cyclobutane, which is shown above. Some cyclic compounds contain at least one atom in the ring other than carbon those atoms are called heteroatoms. Cyclic compounds containing one or more heteroatoms are called heterocyclic compounds. The structures of an acy-chc compound 2-heptanone, a carbocyclic compound, carvone, and the heterocyclic compound nicotinic acid are shown below. 

Carbocycles are important compounds in natural products and functional materials. The traditional preparative methods for cyclic compounds include the Diels-Alder addition reaction, and cyclization mediated by Lewis acid and base. With the aid of metallacycles, carbocycles are often prepared in high yields with high selectivity from different components (Scheme 1). Among many metallacycles, zirconacycles and titanacycles with Cp ligands have been most widely used in stoichiometric reactions to prepare carbocycles. 

Insertion of a ttvo-carbon unit into a five-membered metallacycle may afford formation of a six-membered carbocycle (Eq. 60), such as a benzene derivative via a formal [2+2+2] aromatization of three alkynes. Similarly, insertion of a C-X unit such as a nitrile into a five-membered metallacycle may afford formation of a six-membered heterocycle, such as a pyridine derivative (Eq. 60). In recent years, Takahashi laboratory and other laboratories have developed a number of synthetically useful methods for six-membered cyclic compounds by taking advantage of five-membered metallacycles of zirconocenes and ti-tanocenes [1-5]. 

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