Stereoisomers cis-trans

Stereoisomers are isomers having the same molecular formula and the same connectivity, but different spatial arrangements. There are three classes of stereoisomer cis-trans isomers, conformational isomers and enantiomers. 

The procedure reported here (parts B and C) has been applied with minor modifications to the syntheses of the cis,cis,cis isomers of the 1,3,2-dioxaborole and perhydro-9b-phenalenol.7 The two other stereoisomers, cis,trans,trans and trans,trans,trans, have been prepared from cis, cis, fran,v-perhydro-9b-phenalenol via the cis and trans isomers of A3a,9b-perhydrophenalene.5 In addition, a few isomers of perhydrophenalenol and of perhydrophenalene and cis, cis, frans,-9b-chloroperhydrophenalene have also been prepared from cis, cis, fraw.s-perhydro-9b-phenalenol.5 Some of the representative transformations are summarized in Scheme 1. (The numbers in parentheses refer to references.)... 

Stereoisomers have bonds attached in the same order but with different spatial arrangements. Stereoisomerism is also known as spatial isomerism, and there are two types of stereoisomers cis-trans isomers and enantiomers. Detailed naming rules for stereoisomers are an advanced topic. 

Unlike the atoms in constitutional isomers, the atoms in stereoisomers are connected in the same way. Stereoisomers (also called configurational isomers) differ in the way their atoms are arranged in space. Stereoisomers are different compounds that do not readily interconvert. Therefore, they can be separated. There are two kinds of stereoisomers cis-trans isomers and isomers that contain chirality (ky-RAL-i-tee) centers. 

There are two kinds of stereoisomers cis-trans isomers and isomers that contain asymmetric centers. 

The CIS and trans forms of 1 2 dimethylcyclopropane are stereoisomers Stereoisomers are isomers that have their atoms bonded m the same order—that is they have the same constitution but they differ m the arrangement of atoms m space Stereoiso mers of the cis-trans type are sometimes referred to as geometric isomers You learned m Section 2 18 that constitutional isomers could differ m stability What about stereoisomers We can measure the energy difference between as and trans 1 2 dimethylcyclo propane by comparing their heats of combustion As illustrated m Figure 3 20 the two compounds are isomers and so the difference m their heats of combustion is a direct measure of the difference m their energies Because the heat of combustion of trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) less than that of its cis stereoisomer it follows that trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) more stable than as 1 2 dimethylcyclopropane... 

Cis-trans stereoisomerism m alkenes is not possible when one of the doubly bonded carbons bears two identical substituents Thus neither 1 butene nor 2 methyl propene can have stereoisomers... 

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... 

The poly(vinyl alcohol) made for commercial acetalization processes is atactic and a mixture of cis- and /n j -l,3-dioxane stereoisomers is formed during acetalization. The precise cis/trans ratio depends strongly on process kinetics (16,17) and small quantities of other system components (23). During formylation of poly(vinyl alcohol), for example, i j -acetalization is more rapid than /ra/ j -acetalization (24). In addition, the rate of hydrolysis of the trans-2iQ. -A is faster than for the <7 -acetal (25). Because hydrolysis competes with acetalization during acetal synthesis, a high cis/trans ratio is favored. The stereochemistry of PVF and PVB resins has been studied by proton and carbon nmr spectroscopy (26—29). 

The cis and trans forms of 1,2-dimethylcyclopropane are stereoisomers. Stereoisomers are isomers that have then- atoms bonded in the sane order—that is, they have the sane constitution, but they differ in the anangernent of atoms in space. Stereoisomers of the cis-trans type are sometimes refened to as geometric isomers. You learned in Section 2.18 that constitutional isomers could differ in stability. What about stereoisomers ... 

The 1,2-diinethylcyclopropanes are members of a subclass of stereoisomers called cis-trans isomers. The prefixes cis- (Latin "on the same side") and tmns-(Latin "across") are used to distinguish between them. Cis-trans isomerism is a common occurrence in substituted cycloalkanes. 

A cycloalkane is a saturated cyclic hydrocarbon with the general formula C H2 . In contrast to open-chain alkanes, where nearly free rotation occurs around C, -C bonds, rotation is greatly reduced in cycloalkanes. Disubstituted cycloalkanes can therefore exist as cis-trans isomers. The cis isomer has both substituents on the same face of the ring the trans isomer has substituents on opposite faces. Cis-trans isomers are just one kind of stereoisomers—isomers... 

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). 

R Stereoisomers (Section 4.2) are compounds whose atoms are connected in the same order but with a different geometry. Among the kinds of stereoisomers we ve seen are enantiomers, diastereomers, and cis-trans isomers (both in alkenes and in cycloalkanes). Actually, cis-trans isomers are just another kind of diastereomers because they are non-mirror-image stereoisomers. 

Cis-trans isomers (Sections 4.2, 6.4) Stereoisomers that differ in their stereochemistry about a double bond or ring. 

Pyrethrolone and cinerolone make up the keto alcohol moiety of the pyrethrins. Both of these keto alcohols have one asymmetric carbon at the 4-position and a double bond in the side chain which is capable of cis-trans isomerism in the 2-position. It is possible, therefore, to have four stereoisomers for each keto alcohol. Katsuda et al. (22) show that only the ( + ) form occurs in the natural esters. Elliott (8) has shown recently, by a new procedure developed to obtain pure ( + ) pyrethrolone, that the hitherto unidentified prye-throlone C is in reality pyrethrolone contaminated with thermally isomerized material. (+) Pyrethrolone forms a crystalline monohydrate from which the pure alcohol is obtained. The natural configurations of the keto alcohols in the esters are insecticidally more active, as is the case with the acid moiety. 

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). 

The cyclodecapentaene shown here is the cis-trans-cis-cis-trans form. For other stereoisomers, see page 62. 

This system of identifying double-bond stereoisomers is very limited, because you need two groups that are the same to use the cis/trans system of naming. So what do you do if you have four different groups on a double bond There are still two possible stereoisomers ... 

Both stereoisomers (cis and trans) are produced, but the trans product predominates. This specific example is said to be stereoselective, because the substrate produces two stereoisomers in unequal amounts. 

Mancini et al. (2004) reported the use of achiral-chiral LC-LC to separate the various forms of allethrin. A silica gel column was used to separate the allethrin into cis, trans isomers. Then, the cis or trans peak was switched onto a Chiralcel OJ column to separate the stereoisomers. The other set of isomers was analyzed in a subsequent run on the achiral-chiral system. 

Heats of formations (A//f) of the possible two trans- and four cA-fused stereoisomers of the 9-hydroxyperhydropyr-imido[2,1 / 1,4]oxazine 287 derived from 3-(2-hydroxyethyl)-l,3-propanediamine and glyoxal were calculated by the AMI program. Experimentally, 287 was obtained as 3 1 cis—trans mixture the isomers were not separated < 1996H(43)1991 >. 

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