With a cis double bond

Although s-trans is the more favorable conformer, reaction occurs with s-cis because this conformation has its double bonds on the same side of the single bond connecting them hence, the stable form of cyclohexene with a CIS double bond is formed. Reaction of the s-trans conformer with ethene would give the impossibly strained /ranj-cyclohexene [Problem 9.12(a)]. As the s-cis conformer reacts, the equilibrium between the two conformers shifts toward the s-cis side, and in this way all the unreactive s-trans reverts to the reactive s-cis conformer. 

Oxygen Requirement for I)esat,iirases The biosynthesis of palmitoleate (see Fig. 21-12), a common unsaturated fatty acid with a cis double bond in the A9 position, uses palmitate as a precursor. Can this be carried out under strictly anaerobic conditions Explain. 

The Diels-Alder reaction is highly stereospecific. The diene reacts in an unfavorable conformation in which its double bonds lie in a plane on the same side (cis) of the single bond connecting them. This s-cis (or cisoid) conformation is required to give a stable product with a cis double bond. Addition of ethene to the alternate and more stable (transoid) conformation would give an impossibly strained Pimv-cyclohexene ring. Possible transition states for reaction in each conformation follow, and it will be seen that enormous mo-... 

Because the lipid components of membranes must be in a fluid state to function as membranes in living cells, it is reasonable to assume that primitive membranes in the first forms of cellular life must also have had this property. Straight-chain hydrocarbons have relatively high melting points due to the ease with which van der Waals interactions can occur along the chains. Any discontinuity in the chains interrupts these interactions and markedly decreases the melting point. As an example, stearic acid contains 18 carbons in its alkane chain and melts at 68 °C, while oleic acid, with a cis-double bond between carbons 9 and 10, has a melting point near 14 °C. If cellular life today requires fluid membranes, it is reasonable to assume that the earliest cell membranes were also composed of amphiphilic molecules in a fluid state. 

Fatty acids (the carboxylic acids produced by saponification of fats) usually have an even number of carbons and may also have one or more cis double bonds. The fatty acid that has 18 carbons with a cis double bond at the 9 position, (Z)-9-octadecenoic acid, is called oleic acid. Catalytic hydrogenation of oleic acid produces stearic acid, or octadecanoic acid, another fatty acid. One of these fatty acids melts at 13.4°C and the other melts at 69.6°C. Explain which fatty acid melts at the higher temperature. 

It is not necessary to have an all-carbon ring to preserve the cis geometry of a double bond. Lactones (cyclic esters) and cyclic anhydrides are useful too. A double bond in a five- or six-member ed compound must have a cis configuration and compounds like these are readily made. Dehydration of this hydroxylactone can give only a cis double bond and ring-opening with a nucleophile (alcohol, hydroxide, amine) gives an open-chain compound also with a cis double bond. The next section starts with an anhydride example. 

The presence and the number of double bonds. Crystallization of paraffinic compounds is difficult but occurs easiest if the chains can assume a linear zigzag conformation. A chain with a cis double bond cannot do this it tends to form a bend (see Figure 15.19b, SOS). This makes crystallization more difficult. The effect will be greater for more double bonds. Cf. the series SSS-OOO-LiLiLi-LnLnLn. 

Oleate is an abundant 18-carbon monounsaturated fatty acid with a cis double bond between C-9 and C-10 (denoted A ). In the first step of oxidation, oleate is converted to oleoyl-CoA and, like the saturated fatty acids, enters the mitochondrial matrix via the carnitine shuttle (Fig. 17-6). Oleoyl-CoA then undergoes three passes through the fatty acid oxidation cycle to yield three molecules of acetyl-CoA and the coenzyme A ester of a A, 12-carbon unsaturated fatty acid, cis-A -dodecenoyl-CoA (Fig. 17-9). This product cannot serve as a substrate for enoyl-CoA hydratase, which acts only on trans double bonds. The auxiliary enzyme A, A -enoyl-CoA isomerase isomerizes the ci5-A -enoyl-CoA to the fra/J5-A -enoyl-CoA, which is converted by enoyl-CoA hydratase into the corresponding L-/3-hydroxyacyl-CoA (fra/75-A -dodecenoyl-CoA). This intermediate is now acted upon by the remaining enzymes of /3 oxidation to yield acetyl-CoA and the coenzyme A ester of a 10-carbon saturated fatty acid, decanoyl-CoA. The latter undergoes four more passes through the pathway to yield five more molecules of acetyl-CoA. Altogether, nine acetyl-CoAs are produced from one molecule of the 18-carbon oleate. 

In each case, enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase converts an intermediate with a cis double bond into an intermediate with a trans double bond. Moreover, the trans double bond has the appropriate structure to be handled by the enzymes involved in the / -oxidation of saturated fatty acids. 

The conversion of a monounsaturated fatty acid to acetyl-GoA requires a reaction that is not encountered in the oxidation of saturated acids, a cis-trans isomerization (Figure 21.9). Successive rounds of P-oxidation of oleic acid (18 1) provide an example of these reactions. The process of P-oxidation gives rise to unsaturated fatty acids in which the double bond is in the trans arrangement, whereas the double bonds in most naturally occurring fatty acids are in the cis arrangement. In the case of oleic acid, there is a cis double bond between carbons 9 and 10. Three rounds of P-oxidation produce a 12-carbon unsaturated fatty acid with a cis double bond between carbons 3 and 4. The hydratase of the P-oxidation cycle requires a trans double bond between carbon atoms 2 and 3 as a substrate. A cis-trans isomerase produces a trans double bond between carbons 2 and 3 from the cis double bond between carbons 3 and 4. From this point forward, the fatty acid is metabolized the same as for saturated fatty acids. When oleic acid is P-oxidized, the first step (fatty acyl-GoA dehydrogenase) is skipped, and the isomerase deals with the cis double bond, putting it into the proper position and orientation to continue the pathway. 

The sex attractant of fhe common housefly (Musca domestica) is a 23-carbon alkene with a cis double bond between atoms 9 and 10 called muscalure ... 

Of the remaining acids there were several with a cis double bond in the straight chain at C-9 or C-12 and either cyclopentyl [from C-13 to C-17... 

Polymers of this type generally give rise to carbon-13 NMR spectra which show sensitivity to at least dyad cc, tc, and tt sequences (where t = trans, c = cis% therefore allowing the distribution of c and t bonds to be determined. From an examination of his own results, and those of others, Ivin found that ROMP polymers with less than 35% cis double bonds had a random distribution of cis/trans bonds. In contrast, polymers with a cis double bond fraction greater than 50% show a marked tendency towards a blocky distribution. Ivin concluded that for polymers with high cis content, there are at least two kinetically distinct propagating species, whereas for low cis polymers there is only one. Explanations for this behaviour have been proposed which are based on steric constraints about the transition metal to which the growing chain end is coordinated. 

FIGURE 17.62 The triesters with no double bonds can stack much better than the triesters with a cis double bond. Trans fats also hne up well and have higher melting points than cis fats. 

How does the structure of a fatty acid with a cis double bond differ from the structure of a fatty acid with a trans double bond ... 

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