Stereochemistry cyclohexane

The details of cyclohexane stereochemistry are important enough to warrant a lengthy discussion (Section 5.4, p. 197), but we can do a few things here in preparation. First of all, it is necessary to learn to draw a decent cyclohexane. No person can truly be described as educated unless he or she can do this, and anyone can. 

How To Name an Alkane Using the lUPAC System 121 How To Name a Cycloalkane Using the lUPAC System 125 How To Draw a Newman Projection 132 How To Draw the Chair Form of Cyclohexane 139 How To Draw the Two Conformations for a Substituted Cyclohexane How To Draw Two Conformations for a Disubstituted Cyclohexane Stereochemistry... 

C, b.p. 81"C. Manufactured by the reduction of benzene with hydrogen in the presence of a nickel catalyst and recovered from natural gase.s. It is inflammable. Used as an intermediate in the preparation of nylon [6] and [66] via caprolactam and as a solvent for oils, fats and waxes, and also as a paint remover. For stereochemistry of cyclohexane see conformation. U.S. production 1980 1 megatonne. 

Disubstituted cyclohexanes present us with a challenging exercise in stereochemistry Con sider the seven possible dichlorocyclohexanes 1 1 as and trans 1 2 as and trans 1 3 and as and trans 1 4 Which are chiral Which are achiral Four isomers—the ones that are achiral be cause they have a plane of symmetry—are relatively easy to identify... 

Stereochemistry. Cyclohexane can exist ia two molecular conformations the chair and boat forms. Conversion from one conformation to the other iavolves rotations about carbon—carbon single bonds. Energy barriers associated with this type of rotation are low and transition from one form to the other is rapid. The predominant stereochemistry of cyclohexane has no influence ia its use as a raw material for nylon manufacture or as a solvent. 

Preferential elimination of hydrogen fluonde from vicinal halofluoro compounds occurs also in the cyclohexane series [55 56, 57], acenaphthene series [55], and benzodihydrofuran series [59 60] Here, the strength of the base and the stereochemistry play important roles... 

The Diels-Alder reaction is one of the most useful synthetic reactions for the construction of the cyclohexane framework. Four contiguous stereogenic centers are created in a single operation, with the relative stereochemistry being defined by the usually ewdo-favoring transition state. 

The stereochemistry of the carboxylation of 4-substituted ( + )-(/ S)-fra ,v-1-(4-mcthylphcnyl-sulfinylmethyl)cyclohexane after metalation with methyllithium and quenching with carbon dioxide was reported64. The results listed in Table 1 show that the d.r. of around 75 25 under kinetic control changes to 25 75 under thermodynamic control. This is the result of the equilibration of the two diastereomeric metalated species. As shown by the experiment in hexamethylphosphoric Iriamide (IIMI A) (d.r. = 57 43 under kinetic control) an electrophilic assistance of the lithium cation to the electrophilic approach is probably involved. 

Table 2. Effect of Lithium Salt on the Stereochemistry of the Carboxylation of ( + )-(/ S)-/ra s-l-(4-Methylphcnyl-sulfinylmcthyl)-4-(methoxymethyl)cyclohexane in THF...

In considering the retrosynthetic analysis of juvabione, two factors draw special attention to the bond between C(4) and C(7). First, this bond establishes the stereochemistry of the molecule. The C(4) and C(7) carbons are stereogenic centers and their relative configuration determines the diastereomeric structure. In a stereocontrolled synthesis, it is necessary to establish the desired stereochemistry at C(4) and C(7). The C(4)-C(7) bond also connects the side chain to the cyclohexene ring. As a cyclohexane derivative is a logical candidate for one key intermediate, the C(4)-C(7) bond is a potential bond disconnection. 

Addition of Bu3SnLi or McsSnI.i to 4-t-butylcyclohexanone affords mixtures of trans and cis adducts in ratios that depend on reaction conditions (Table ll)68. In THF, a 93 7 mixture is obtained with both reagents. This ratio is thought to represent the thermodynamic distribution—the axial stannane being favored. In ether, the cis isomer predominates, suggesting a kinetic preference for equatorial addition. Each of the two isomers can be lithiated with BuLi. Subsequent treatment with alkyl halides or carbonyl compounds affords the substituted alkoxy cyclohexanes with retention of stereochemistry. 

Structurally novel /3-lactams were obtained using enantiopure 4-oxoazetidine-2-carbaldehydes and methylene cyclohexane and a-methyl styrene (Equation (4)).7 Boron trifluoride diethyletherate and tin(iv) chloride produced the products in the highest yields, and all ene products possessed yy/z-stereochemistry. 

Incorporation of the carboxylic acid group into the substrate also had an effect on the stereochemistry of the Alder-ene products. Trost and Gelling60 observed diastereoselectivity in the palladium-catalyzed cycloisomerization of 1,7-enynes when the reactions were conducted in the presence of A,A-bis(benzylidene)ethylene diamine (BBEDA, Figure 2). They were able to synthesize substituted cyclohexanes possessing vicinal (Equation (53)) and... 

Table 10.2 (From book Topics in Stereochemistry by J. Hirsch, Interscience Publishers, New York) Conformational free energies of substituents in substituted cyclohexanes.

Allenylsilanes undergo intramolecular additions to appropriately positioned aldehydes, imines, conjugated esters and alkenes to afford various alkynylcyclopentane and cyclohexane derivatives (Eqs. 9.66-9.70) [66]. The reactions are promoted by SnCl4 or by thermolysis. The stereochemistry of these cyclization reactions is consistent with a concerted sigmatropic process as illustrated in Scheme 9.17. 

Fig. 10.29. Stereochemistry of the EH-catalyzed hydration of 1,2-epoxycyclohexane (10.5) to the chiral tiansfR,R)-cyclohexane-l,2-diol (10.124). The substrate in the catalytic site adopts the conformation shown (3,4M helicity), and enzymatic attack occurs at the (,S )-con figured C-atom with /ra .v-diaxial opening [185][186]. 

It is worth noting that in this synthesis of Cecropia juvenile hormone a strategy which is the reverse of the one developed by W.S. Johnson [8] for the synthesis of steroids and other fused polycyclic systems bearing cyclohexane rings is used. This method involves a non-enzymatic cyclisation of a polyunsaturated intermediate with the appropriate stereochemistry (all-trans) (Scheme 13.3.6). Such cyclisation occurs with a really amazing stereoselectivity and several new chiral centres with the correct stereochemistry are created in one single step ... 

Synthesis, Stereochemistry and Transformations of Cyclopentane-, Cyclohexane-, Cycloheptane-, and Cyclooctane-Fused 1,3-Oxazines, 1,3-Thiazines, and Pyrimidines... 

The final chapter in Volume 69 is concerned with the synthesis, stereochemistry, and transformation of cyclopentane-, cyclohexane-, cyclohep-tane-, and cyclooctane-fused 1,3-oxazines, 1,3-thiazines, and pyrimidines and is authored by Professors Ferenc Ftilop, Gabor Bernath, and Kalevi Pihlaja from the Universities of Szeged in Hungary and Turku in Finland. This is a field which has shown rapid development over the last dozen years because of the increased availability of spectroscopic and other analytical methods allowing definition of the precise steric chemistry of these compounds. 

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