Carbocycles cyclohexanes

Cyclohexane is a benchmark of conformational analysis, and it is an important building block of many natural and synthetic products. Ironically, though, cyclohexane is much more the exception than the rule for carbocycles. Cyclohexane is unique in that it is predominantly found in a single, relatively rigid conformation, the chair form (Figure 2.11 A). 

The annulation of 4//-thiopyran and cyclohexane rings in 50a results in the planarity of the heterocycle and a half-chair conformation of the carbocycle (81KGS1342). On the other hand, a boat conformation of the 2//-thiopyran ring was found in the crystal of224b [91JCS(P2)2061], Other geometrical parameters were within the limits of the expected values (Fig. 2). 

Furthermore, intramolecular cyclization of acyclic chiral imines, in which the imine and the enone groups are separated by alkyl chains, yield optically active cyclohexane and cyclopentane derivatives. /ra .v-l,2-Disubstitutcd carbocyclic compounds are exclusively or predominantly formed with diastereomeric ratios in the range 80 20 100 0, strongly dependent on the conditions used to induce cyclization, i.e. heat, pressure or Lewis acid (MgBr2) catalysis227. 

In the heteroatom O-substituted cyclohexane 22 equatorial attack, to give 23a, is stereoelec-tronically even more favored than in the analogous carbocyclic case [d.r. (eq/ax) 85 15]81. 

The NMR spectra of cyclopropanes are unique among carbocycles. The HNMR chemical shift (c>H) of cyclopropane is 0.12, considerably upheld from cyclohexane (1.44), whereas the hydrogens of cyclobutane resonate further downheld (1.96).110 Similarly, upheld 13C chemical shifts (<>c) are found for cyclopropane (— 2.9), compared with cyclohexane (27) and cyclobutane (23.). 11... 

Hydrogenative ring opening of cycloalkanes is also a well-studied area.16 252 253 289-292 Mainly cyclopropanes and cyclopentanes were studied, since three- and five-membered adsorbed carbocyclic species are believed to be intermediates in metal-catalyzed isomerization of alkanes (see Section 4.3.1). Ring-opening reactivity of different ring systems decreases in the order cyclopropane > cyclobutane > cyclopentane > cyclohexane.251 Cyclopropane and its substituted derivatives usually react below 100°C. 

Review coverage of the chemistry involved in the synthesis of enantiomerically pure natural products from carbohydrates is now extensive [2]. It is relevant to note that whereas, in 1983, the numbers of carbocyclic target compounds were relatively limited, as outlined in an authoritative monograph written by Hanessian [3], a comprehensive review of the methods available for making cyclopentane and cyclohexane derivatives from carbohydrates published 10 years later [4] cited 338 references, more than 80% of which were dated 1980 or later. The attention afforded the synthesis of carbocyclic products also features prominently iu a 1993 review of the use of sugars in the preparation of enantiomerically pure natural products [2],... 

Cycloaddition reactions can provide excellent means of producing carbocyclic compounds, and there are many good examples of the application of this approach to the synthesis of functionalized cyclohexanes from carbohydrates [4],... 

When radical induced carbocyclisations of 8-bromo-2,3-unsaturated octonolactones i.e. 5 or 8 (Scheme 15) were performed, cis-fused cyclohexane-lactones 6 or 9 were formed with stereospecific generation of one or two new chiral centers (35) (Scheme 15). Again the configuration of the new chiral centers were determined by the configuration at C-4 in the unsaturated lactone. Deprotection followed by reduction of the lactone function gave the carbocyclic analogues of heptopyranoses, 7 or 10. 

In 1937 Isbell published an important paper on the conformational analysis of aldopyranoses, in which several of the forms were depicted (see Fig. 2). They comprise a 4C, chair (I), afl03 boat (II), two half-chairs (III and IV), and a coplanar pyranose (V). He correctly favored the chair form (I) and predicted that, in the case of saccharides, the chair would tend to assume a somewhat flatter conformation than that of carbocycles such as cyclohexane because of the smaller bond angles of oxygen as compared to those of carbon (105° instead of 109.5°). We now know that such coplanarity is strongly avoided because of the strain that would be produced and because of the repulsive forces between the substituents. 

Raman spectra of saturated unsubstituted carbocycles exhibit not only bands which are typical of CH2 groups, but also bands representing characteristic in-phase vibrations of C-C bonds cyclopropane at 1188 cm (Fig. 4.1-10), cyclobutane at 1001 cm, cyclopentane at 886 cm, and cyclohexane at 802 cm. These vibrations are al.so found in many substituted ring systems C3 between 1220 and 1200 cm, C4 between 930 and 850 cm C5 between 810 and 800 cm , and Q between 800 and 700 cm . 

Carbocyclic compounds can be formed by the nucleophilic intramolecular capture of a seleniranium intermediate of an olefinic bond. The carbonium ion which is formed as intermediate can react with another nucleophile or with the solvent. The first examples of these carbocyclization reactions were observed with dienes. Clive [105] reported that the reaction of the diene 203 with phenyl-selenyl chloride in acetic acid afforded the intermediate 204 which reacted with the solvent to give the bicyclic compound 205 (Scheme 31). Carbocyclization reactions were efficiently promoted by phenylselenyl iodide produced by diphenyl diselenide and iodine. As indicated in Scheme 31,Toshimitsu reported that the reaction of 1,5-hexadiene 206, in acetonitrile and water, afforded the acetamido cyclohexane derivative 209, derived from the cyclization of the seleniranium intermediate 207 followed by the reaction of the carbocation 208 with acetonitrile [106]. In several cases, carbocyclization reactions can be more conveniently effected by independently generating the seleniranium intermediates. A simple procedure consists of the reaction of trifluoromethane-sulfonic acid with j9-hydroxyselenides, which can be easily obtained from the... 

The NMR spectra of cyclopropane derivatives are unique among carbocycles. The NMR chemical shift of cyclopropane is 3 0.12, considerably upheld from cyclohexane... 

Barton deoxygenation, 548, 549 [Functionalized carbocycles] bicyclic compounds, 551 caiba-D-fructofuranose, 547 Corey lactone, 549 Cp2TiCl catalyst, 553, 563 cyclic transition state, 548, 549 cyclohexanes, 554 cyclopentanes, 546 1,2-dialkylcyclopentanes, 548 electron spin resonance, 552 eneynes, 556... 

Cyclohexane is the most important of the carbocycles its structural unit is widely encountered in various natural products. It can adopt a chair conformation that is essentially strain free. The chair form of cyclohexane has two distinct types of hydrogens equatorial and axial. 

So far, the only example of an asymmetric hydrosilylation/carbocyclization sequence of a,co-hexadienes and heptadienes utilize the (i )-BINOL-derived yttrocene catalyst 16 to produce cyclopentanes and cyclohexanes in high yields but only low to moderate enantioselectivities of up to 50% ee (25) [75]. 

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