Cyclohexane, trans-1,2-disubstituted

Table 4.2 Axial and Equatorial Relationships in Cis- and Trans-Disubstituted Cyclohexanes...

C A 1,2-trans disubstituted cyclohexane must have either both groups axial or both groups equatorial. Explain. 

Which is more stable, a 1,4-trans disubstituted cyclohexane or its cis isomer ... 

Both the cis- and the trans-disubstituted spiranes resulted, in different ratios, depending on the reaction conditions. Clearly, the trans spiranes are chiral. The first conjugate addition to the Michael acceptors 75a-c is intermolecular in nature and defines the sense of chirality at the first chiral center. Subsequent intramolecular ring closure to the spiranes 76 defines the cis or trans configuration of the product. When cyclohexane-1,3-dione (74a) was reacted with dibenzalacetone (75a) in the presence of ca 5 mol% (—)-quinine (3a, Scheme 4.3), a 2.5 1 trans/cis mixture resulted, with the trans isomer 76 having optical purity of ca 30% (Scheme 4.37) [61] (the absolute configuration of the predominant enantiomer was not assigned). 

Intramolecular ene reaction of IJ-dienes. The ZnBrz-catalyzed reaction of the activated 1,7-diene 1 gives the trans-disubstituted cyclohexane 2 (>98% selectivity.) Under the same conditions, the chiral 1,7-diene 3 [from (R)-citronellal] is converted into two diastereomeric trflAw-disubstituted cyclohexanes, 4 and 5 in the ratio 96.5 3.5. Slightly higher diastereoselectivity obtains with (C2H5)2A1C1 (96% de), but the thermal ene reaction gives 4 and 5 in the ratio 86 14. 

Zefirov NS, Gurvich LG, Shashkov AS, Krirntr MZ, Vorob eva EA (1976) Stereochemical studies. XX. Cmdxmnations of 1,2-trans-disubstituted cyclohexanes. Tetrahedron 32 1211... 

How do we know two groups are cis or trans A general way to recognize a trans-disubstituted cyclohexane is to notice that one group is attached by the upper bond (of the two to its carbon) and one by the lower bond ... 

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... 

Disubstituted cyclohexanes can exist as cis-trans isomers as well as axiaEequatorial conformers. Two isomers are predicted for 1,4-dimethylcyclohexane (see Fig. 1.9). For the trans isomer the diequatorial conformer is the energetically favorable form. Only one cis isomer is observed, since the two conformers of the cis compound are identical. Interconversion takes place between the conformational (equatoriaEaxial) isomers but not configurational (cis-trans) isomers. 

A summary of the various axial and equatorial relationships among substituent groups in the different possible cis and trans substitution patterns for disubstituted cyclohexanes is given in Table 4.2. 

Why is a 1,3-cis disubstituted cyclohexane more stable than its trans isomer ... 

The chemoenzymatic synthesis of the analgesic U-(—)-50,488 [41] and new C2-symmetric bisaminoamide ligands derived from N,N-disubstituted trans-cyclohexane, ,2-diamine [41] has been possible by a CALB-catalyzed resolution using ethyl acetate as solvent and acyl donor [42]. 

DIASTEREOSELECTIVE FORMATION OF TRANS-1,2-DISUBSTITUTED CYCLOHEXANES FROM ALKYLIDENEMALONATES BY AN INTRAMOLECULAR ENE REACTION DIMETHYL (1 R,2 R,5 R)-2-(2 -ISOPROPENYL-5 -METHYLCYCLOHEX-1 -YL)-PROPANE-1,3-DIOATE... 

This is also true for 1,4 disubstituted cyclohexanes, but for 1,3 compounds, the reverse is true, the trans must have a e and cis either aa or ee conformation. But if the substituents are alkyl groups, the diequatorial predominates the diaxial. The trans 1, 2-e e conformations are thermodynamically more stable than cis 1, 2 isomers which therefore occurs as a e form. 

We now focus our attention on conformational isomerism in 1,2-disubstituted cyclohexanes. Our model system will be 1,2-difluorocyclohexane and we first examine the relative stabilities of the two trans 1,2-difluorocyclohexanes, i.e. axial-axial (aa) and equatorial-equatorial (ee). These two molecules are shown below along with a listing of the dominant stabilizing a—a interactions. 

The cis/trans ratio of 1,4-disubstituted cyclohexanes formed from activated cyclohexenes at a Hg cathode depends on the solvent and proton source and shows a low diastereoselectivity. Protonation of the first formed radical anion is kinetically... 

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