Trans stereoisomers

We 11 begin with cis and trans 1 4 dimethylcyclohexane A conventional method uses wedge and dash descriptions to represent cis and trans stereoisomers m cyclic systems... 

Probably the most interesting observation in Table 3 2 concerns the 1 3 dimethylcyclohexanes Unlike the 1 2 and 1 4 dimethylcyclohexanes in which the trans stereoisomer is more stable than the cis we find that cis 1 3 dimethylcyclo hexane is 7 kJ/mol (17 kcal/mol) more stable than trans 1 3 dimethylcyclohexane Why" ... 

Thus the trans stereoisomer with one axial methyl group is less stable than cis 1 3 dimethylcyclohexane where both methyl groups are equatorial... 

Structural drawings (molecular models too) can be deceiving For example the chlonne atoms in 1 2 dichlorocyclohexane seem much closer to each other in a drawing of the trans stereoisomer than in the cis Make a molecular model of each and measure the distance between the chlorines What do you find" ... 

A similar steric effect was seen in Section 3 11 where van der Waals strain be tween methyl groups on the same side of the ring made as 1 2 dimethylcyclopropane less stable than its trans stereoisomer... 

In addition to being regioselective alcohol dehydrations are stereoselective A stereo selective reaction is one m which a single starting material can yield two or more stereoisomeric products but gives one of them m greater amounts than any other Alcohol dehydrations tend to produce the more stable stereoisomer of an alkene Dehydration of 3 pentanol for example yields a mixture of trans 2 pentene and cis 2 pentene m which the more stable trans stereoisomer predominates... 

Reactions such as catalytic hydrogenation that take place at the less hindered side of a reactant are common m organic chemistry and are examples of steric effects on reactivity Previously we saw steric effects on structure and stability m the case of CIS and trans stereoisomers and m the preference for equatorial substituents on cyclo hexane rings... 

Step 2 of the mechanism m Figure 6 12 is a nucleophilic attack by Br at one of the carbons of the cyclic bromonium ion For reasons that will be explained m Chapter 8 reactions of this type normally take place via a transition state m which the nude ophile approaches carbon from the side opposite the bond that is to be broken Recall mg that the vicinal dibromide formed from cyclopentene is exclusively the trans stereoisomer we see that attack by Br from the side opposite the C—Br bond of the bromonium ion intermediate can give only trans 1 2 dibromocyclopentane m accordance with the experimental observations... 

A second isomer of [lOJannulene (the cis trans cis cis trans stereoisomer) can have bond angles close to 120° but is destabilized by a close contact between two hydro gens directed toward the interior of the ring To minimize the van der Waals strain between these hydrogens the nng adopts a nonplanar geometry which limits its ability to be stabilized by tt electron delocalization It too has been prepared and is not very stable Similarly the next higher (4n + 2) system [14]annulene is also somewhat desta bilized by van der Waals strain and is nonplanar... 

In the companion experiment carried out on the trans stereoisomer of the ketone only the trans acetate was formed... 

The preparation of cis 4 tert butylcyclohexanol from its trans stereoisomer was earned out by the following sequence of steps Write structural formulas including stereochemistry for com pounds A and B... 

The intermediate m hydrogenation formed by reaction of the unsaturated ester with the hydrogenated surface of the metal catalyst not only can proceed to the saturated fatty acid ester but also can dissociate to the original ester having a cis double bond or to its trans stereoisomer Unlike polyunsaturated vegetable oils which tend to reduce serum cholesterol levels the trans fats produced by partial hydrogenation have cholesterol raising effects similar to those of saturated fats... 

H2C=CHCH(CH2)4CH3 as well as the cis and trans stereoisomers of BrCH2CH=CH(CH2)4CH3 Br... 

Feeding experiments utilizing C-, N-, and H-labeled cadaverine (44) and lysine (24) in l upinus augustifolius a source of the lupine alkaloids (—)-sparteine (50, R = H,H) and (+)-lupanine (50, R = O), have been reported which lend dramatic credence to the entire biosynthetic sequence for these and the related compounds discussed above (41). That is, the derivation of these bases is in concert with the expected cyclization from the favored aH-trans stereoisomer of the trimer expected on self-condensation of the 1-dehydropiperidine (45). 

PApo-8 -carotenal. The specifications of this colorant. (38) were discussed earlier. P-Apo-8 -carotenal has provitamin activity with 1 g of the colorant equal to 1,200,000 lU of vitamin A. Like all crystalline carotenoids, it slowly decomposes ia air through oxidatioa of its coajugated double boads and thus must be stored ia sealed coataiaers uader an atmosphere of iaert gas, preferably under refrigeration. Also like other carotenoids P-apo-8 -carotenal readily isomerizes to a mixture of its cis and trans stereoisomers when its solutions are heated to about 60°C or exposed to ultraviolet... 

Canthaxanthin crystallines from various solvents as brownish violet, shiny leaves that melt with decomposition at 210°C. As is the case with carotenoids in general, the crystals are sensitive to light and oxygen and, when heated in solution or exposed to ultraviolet light or iodine, form a mixture of cis and trans stereoisomers. Consequentiy, crystalline canthaxanthin should be stored under inert gas at low temperatures. Unlike the carotenoid colorants P-carotene and P-apo-8 -carotenal, canthaxanthin has no vitamin A activity. It is chemically stable at pH 2—8 (the range normally encountered in foods) and unaffected by heat in systems with a minimal oxygen content. 

Examine the geometry of norcarane. What is the conformation of the cyclohexane ring Choose a name (chair, twist boat, half-chair, etc. see Chapter 5, Problem 4) that accurately describes its shape. The bridgehead hydrogens in norcarane are cis. Do you think a trans stereoisomer is possible Explain. 

A conveniently short synthesis of a1prostadi1 begins with a mixed aldol assembly of the requisite cyclopentenone 13. This product is then oxidatively cleaved with periodate-permanganate and the alcohol moiety is protected as the tetra-hydropyranyl ether (14). Aqueous chromous sulfate satisfactorily reduces the olefinic linkage and the trans stereoisomer 15 predominates after work-up. The remainder of the synthesis of involves the usual steps, through 16 to with the exception that thexyl tetrahydrolimonyllithium borohydride is used to reduce the C-15 keto moiety so as to produce preferentially the desired C-15S stereochemistry. 

A Draw 1,3,5-triniethvlcyclohexane using a hexagon to represent the ring. How many cis-trans stereoisomers are possible ... 

How many cis-trans stereoisomers of ///yo-inositol (Problem 4.49) are there Draw the structure of the most stable isomer. 

Since bond rotation can t occur, the two 2-butenes can t spontaneously interconveTt they are different, isolable compounds. As with disubstituted cycloalkanes, we call such compounds cis-trans stereoisomers. The compound with substituents on the same side of the double bond is called c/3-2-butene, and the isomer with substituents on opposite sides is fra/iS-2-butene (Figure 6.3). 

Trienone 124 underwent intramolecular cycloaddition affording hydrobenzo-suberone 125. Thermal reaction was poorly diastereoselective (62 38 cis trans stereoisomers). When the cycloaddition was carried out in the presence of LiBF4, trienone 124 was converted into cA-adduct 125 quantitatively and stereo selectively [118] (Equation 2.35). 

Diene-based polymers such as polybutadiene have other structural distinctions. The linear versions of these polymers have one residual double bond for each mer. When the double bonds are in the polymer chain, the cis and trans stereoisomers are possible. The double bonds can appear as pendant vinyl groups, which can then exhibit tacticity. 

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