Cis/trans stereochemistry

Any of the combinations s + s, s + a, a + s, a + a is conceivable for a cycloaddition of two components. Comparison of cis-trans stereochemistry of substituents in product to that in reactants establishes which occurred. In additions of relatively short chains, the antarafacial interaction is difficult for the molecule to attain, but when systems with appropriate geometry are contrived, it is found that reactions in which one component acts in the antarafacial manner exhibit an inverted preference with respect to ring size The 2 + 2 additions are now favorable and the 4 + 2 not. 

Formation of the cyclic osmate, which occurs with syn stereochemistry, retains the cis-trans stereochemistry of the double bond because osmate formation is a single-step reaction. Oxidation of the osmate does not affect the stereochemistry of the carbon-oxygen bond, and the diol produced from czs-2-butene is a stereoisomer of the diol produced from trans-2-butene. We ll study this type of isomerism in Chapter 9. 

The cis/trans stereochemistry of diphosphiranes is investigated by MNDO calculations of dipole moments in terms of substituent effects on intracyclic phosphorus and carbon atoms. These results compared to experimental dipole moment measurements show the presence of preferential tram isomers <92CJC2834>. 

Note how the cis, trans stereochemistry is retained in each case. 

The cis/trans stereochemistry of a series of trivalent amino acid derived chiral phosphorus compounds has been established on the basis of Jhp couplings all of the cis isomers exhibited for the ring hydrogen Vhp = 3.4-3.T Hz while all of the trans isomers revealed Vhp = 1.4-1.8 Hz. It should be mentioned that the... 

The most studied case is that of a cyclic enone reacting with an olefin. In these systems several trends are observed. The reaction proceeds from T, which may be either n,iT or -IT,IT, and triplet biradicals are likely involved. Electron-rich olefins react more rapidly and add with predictable regiochemistry. Cis-trans stereochemistry in the olefin is lost in the cycloaddition, and trans fused rings can form. 

Table 4-4 Relation of Cis-Trans Stereochemistry in Substituted Cyclohexanes to Equatorial-Axial Positions in the Two Chair Forms ...

The formation of ion-dipole complexes from bifunctional compounds can yield suppression (or attenuation) of stereochemical effects. Indeed, from the cis- and trans-l-alkoxy-l-methyl-4-(2-aIkoxy-2-propyl)cyclohexanes, the fact that regioselective methanol loss induced by a 1,2-hydride transfer is not stereospecific suggests a possible isomerization of the protonated precursor ion into a common ion-neutral complex (Scheme 17.11). From this complex, long-distance proton transfer can be considered to be independent of the initial cis/trans stereochemistry. Such isomerization is especially favored from long lifetime precursor ions such as those that exist during ion storage in an ion trap instrument. ... 

Nitrogen J couplings are peculiarly valuable in structure determination because of their sensitivity to the presence and orientation of a lone pair on the nitrogen. Thus they are usually diagnostic of coordination, chelation, protonation, or hydrogen bonding of the nitrogen, and reflect departures from planarity, conformational relationships, syn-anti or cis-trans stereochemistry, solvent effects and solution equilibria, and so on. 

Double bonds m the mam chain are signaled by the ending enow acid and their position IS designated by a numerical prefix Entries 6 and 7 are representative carboxylic acids that contain double bonds Double bond stereochemistry is specified by using either the cis-trans or the E-Z notation... 

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 stereochemistry of pyrazolines and pyrazolidines has already been discussed (Section 4.04.1.4.3). Optically active A - and A -pyrazolines have seldom been described (77JA2740, 79CJC360), but cis-trans isomeric pairs are common. The C-4 acid-catalyzed epimerization involves the mechanism shown in Scheme 38 (70TL3099), but in spite of some inconclusive arguments the C-5 epimerization has never been established with certainty. 

Decomposition of the /rtin -decalyl perester A gives a 9 1 ratio of trans cis hydroperoxide product at all oxygen pressures studied. The product ratio from the cis isomer is dependent on the oxygen pressure. At 1 atm O2, it is 9 1 trans cis, as with the trans substrate, but this ratio decreases and eventually inverts with increasing O2 pressure. It is 7 3 cis trans at 545 atm oxygen pressure. What deduction about the stereochemistry of the decalyl free radical can be made from these data ... 

Trans stereochemistry of the alkene product is established during the second reduction step when the less hindered trans vinylic anion is formed from the vinylic radical. Vinylic radicals undergo rapid cis-trans equilibration, but vinylic anions equilibrate much less rapidly. Thus, the more stable trans vinylic anion is formed rather than the less stable cis anion and is then protonated without equilibration. 

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

The non-preservation of cis stereochemistry of dienophiles 24 and 26 in the adducts 25 and 27 is due to a cis-trans photoisomerization of the double bond and to the concerted suprafacial Diels-Alder cycloaddition of diene to the ground state of trans dienophiles. 

Scotter, M.J., Characterization of the coloured thermal degradation products of bixin from annatto and a revised mechanism for their formation, Food Chem., 53, 111, 1995. Zechmeister, L., Cis-trans isomerization and stereochemistry of carotenoids and diphenylpolyenes, Chem. Rev., 34, 267, 1944. 

In Entry 3, the trans stereochemistry arises at the stage of the protonation of the enolate. Entry 4 gives rise to a cis ring juncture, as does the corresponding carbocyclic compound.59 Models suggest that this is the result of a steric differentiation arising from the axial hydrogens on the a-face of the molecule. 

Zechmeister, L. 1944. Cis-trans isomerization and stereochemistry of carotenoids and diphenylpolyenes. Chem. Rev. 34 267-344. 

The structures and relative stereochemistry of epi-eudesmanes 25-27 from Axinella cannabina were formulated from 2D NMR and chemical correlation experiments [36]. The 500 MHz HNMR spectrum of 1 l-formamido-7/ H-eudesm-5-ene (27) measured in CDC13 showed a 2 3 cis.trans ratio of the -NHCHO group (cis 3 8.06 d, J = 2 Hz trans 5 8.20 d, J — 12 Hz). Isothiocyanate 26 was one of several sesquiterpenes reported from Acanthella pulcherrima [20]. 

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