Trans configurations

CH3-[CHi]5.CH CH-[CH2]g-C02H. A white solid, m.p. 43-44°C, which is present in small quantities in animal fats and in milk. It is the only naturally occurring fatty acid with the trans configuration. 

The chemist can try to separate the two isomers by careful fractional distillation but it will be next to impossible to do because both the cis and the trans have practically the same boiling point. There are a few things that the chemist can do or hope for to get rid of that cis isomer. The cis configuration is less stable than the trans and may switch over to the trans configuration with a little help. The chemist can gently heat the isosafrole oil to about 150°C for an hour or so. She can also try the same heating except have the oil mixed with some acetic acid. Also, the isosafrole can be fractionally distilled to ultra purity and then be allowed to simply sit for a couple of days. Trans isosafrole may spontaneously crystalize out while the cis form remains as an oil. They can then be separated by filtration. 

Measurement of Unsaturation. The presence of double bonds in a fatty acid side chain can be detected chemically or through use of instmmentation. Iodine value (IV) (74) is a measure of extent of the reaction of iodine with double bonds the higher the IV, the more unsaturated the oil. IV may also be calculated from fatty acid composition. The cis—trans configuration of double bonds may be deterrnined by infrared (59) or nmr spectroscopy. Naturally occurring oils have methylene-intermpted double bonds that do not absorb in the uv however, conjugated dienes maybe deterrnined in an appropriate solvent at 233 nm. 

Maleic and fiimaric acids have physical properties that differ due to the cis and trans configurations about the double bond. Aqueous dissociation constants and solubiUties of the two acids show variations attributable to geometric isomer effects. X-ray diffraction results for maleic acid (16) reveal an intramolecular hydrogen bond that accounts for both the ease of removal of the first carboxyl proton and the smaller dissociation constant for maleic acid compared to fumaric acid. Maleic acid isomerizes to fumaric acid with a derived heat of isomerization of —22.7 kJ/mol (—5.43 kcal/mol) (10). The activation energy for the conversion of maleic to fumaric acid is 66.1 kJ/mol (15.8 kcal/mol) (24). 

A considerable number of experiments have shown that symmetrical PMDs in the ground state have an aH-trans configuration and are nearly planar with practically equalized carbon—carbon bonds and slightly alternating valence angles within the polymethine chain (1,3,5,22,23). This is caused by some significant features of the PMD electron stmcture. 

Other Complexes. The reaction of TYZOR TPT with two equivalents of a semicarba2one produces complex (17), the stmcture of which has been assigned the trans-configuration (148) ... 

Butyl Rubber. In butyl mbber, isoprene is enchained by 1,4-addition ia the trans configuration (74). 

The coupling constant of the aldehyde doublet 7.8 Hz) is repeated in the doublet of doublets signal at Sh = 6.3. Its larger splitting of 15.6 Hz is observed also in the doublet at Sh = 7.3 and indicates a CC double bond with a trans configuration of the vicinal protons. 

First the trans configuration of the C-2-C-3 double bond is derived from the large coupling constant Jhh = 15 Hz) of the protons at % = 5.10 and 7.11, whereby the middle CH proton (Sh = 5.10) appears as a doublet of doublets on account of the additional coupling ([Pg.189]

In decoupling the methyl protons, the NOE difference spectrum shows a nuclear Overhauser enhancement on the cyclopropane proton at = 1.60 and on the terminal vinyl proton with trans coupling at <5// = 5.05 and, because of the geminal coupling, a negative NOE on the other terminal proton at Sh= 4.87. This confirms the trans configuration G. In the cis isomer H no NOE would be expected for the cyclopropane proton, but one would be expected for the alkenyl-// in the a-position indicated by arrows in H. 

The perpendicular geometry for the excited state permits the possibility for returning to either the cis or the trans configuration of the ground state. 

The stereochemistry of the product resulting from the reaction of a 17-keto steroid with ethylidenetriphenylphosphorane is different from that of the 17-ethylidene steroids obtained by dehydration of 17a-ethyl-17/ -hydroxy compounds, Wolff-Kishner reduction of A -20-keto steroids or by sodium-alcohol or sodium-ammonia " reductions of 17-ethynyl carbinols. These latter products have generally been assumed to possess the trans configuration (C-21 methyl away from the bulk of the ring system) because of anticipated greater stability. The cis configuration for... 

Dulou et al. (66) have assigned the trans configuration to the pyrrolidine examine of desoxybenzoin (133) on the basis of the comparison of its UV SDectrum with that of tran -stilbene and triphenylethylene. This assignment is open to question in the light of the work of Munk and Kim (63). 

Some of the fatty acids found in the diets of developed nations (often 1 to 10 g of daily fatty acid intake) are trans fatty acids— fatty acids with one or more double bonds in the trans configuration. Some of these derive from dairy fat and ruminant meats, but the bulk are provided by partially hydrogenated vegetable or fish... 

Thermal decomposition of [N(PPh3)2]" -[S4N5] in MeCN yields sequentially the corresponding salts of S3N3 and S4N (50% yield). An X-ray crystallographic analysis of the dark-blue air-stable product [N(PPh3)2] [S4N] revealed the presence of the unique acyclic anion [SSNSS] whose structure is in Fig. 15.42c. The anion is planar with cis-trans configuration. 

Methylindole and acetylenedicarboxylic acid is reported to give two unidentified products, but with the dimethyl ester the crs and trans (61) adducts were obtained. It was suggested that the major product had the trans configuration but this was not proved. Hydrogenation of both adducts gave the corresponding succinic ester, which was also obtained from 2-methylindole and maleic anhydride, followed by esterification. 

The cyclometallated palladium and platinum derivatives of trimesityl phosphine or arsine react with pyrazole or 3,5-dimethylpyrazole to form metal chelates 243 (E = P, As M = Pd, Pt R = H, Me) having the trans configuration (81TMC24). 

This occurs because 2-butene, itself, can exist in two different structures, the cis-or the trans- configurations, depending on whether the methyl groups are situated on the same side or on opposite sides of the main chain. 

Due to the presence of residual double bonds, the polymer could be cross inked with regular agents. TPR is a linear polymer with a high trans configuration. It is highly amorphous at normal temperatures and has a Tg of about 90°C and a density of 0.85. 

Hydrogenolysis of the acetylene-cumulenes 19 can be achieved by means of a partially poisoned palladium catalyst to yield the regular [22]porphyrin(2.2,2.2) 20 with cisjrans.cis,-trans configuration of the two-carbon bridges. 

Polymerization of butadiene and of isoprene confronts us with still another configurational problem. The addition may take place in either the 1,2 or 1,4 positions (with an additional possibility of 3,4 addition in the case of isoprene), and, moreover, in the 1,4 addition the new unit may acquire a cis or a trans configuration. It is known that by proper choice of a catalyst and by judicious adjustment of polymerization conditions processes can be developed which yield polymers of high stereospecificity, namely all 1,4 cis, all 1,4 trans, all 1,2 isotactic, or all 1,2 syndiotactic polymers. 

The addition of various a-lithio ethers to benzaldehyde leads to a mixture of the anti- and. syn-epimers. However, with the substituted 2-lithiotetrahydropyrans, the products with a trans configuration of the ring substituents are formed almost exclusively, independent of the risj trans ratio of the parent (phenylthio) ethers. 

Chrysanthemumic acid may exist in four stereoisomers, because of the two asymmetric carbon atoms in the cyclopropane ring. The natural acid has the D-trans configuration and this has been shown to be more insecticidally active than any of the other isomers or the racemic form. Harper et al, (4,18) have synthesized, separated, and optically resolved all of the isomers of this acid. 

Total Sn0 for transversely isotropic structural units in trans configuration + 0.00424 —0.00227 + 0.00276... 

In this classic41 and general method, an acylated glycosyl halide reacts with a thiolate anion to produce a 1-thioglycoside, usually with 1,2-trans configuration. With alkyl thiolates, re-acylation is normally required following this treatment. 

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