Double bond configuration

PerfluoroaUcenes are converted to vicinal diols when no fluonne atom is present at the double bond Configurational isomers of perfluoroalkenes [29] (equations 20 and 21) are oxidized stereospecifically Perfluorbicyclo[4 3 0]non-1(6) ene gives the corresponding 1,6 diol m a 24% yield upon oxidation with potassium permanganate at 18 °C for 1 h [29]... 

In Lewis acid catalyzed carbonyl additions of allylsilanes and -stannanes, syn diastereoselectiv-ity predominates, irrespective of the double-bond configuration, indicating that open-chain transition states are involved. 

Since the double-bond configuration is established in the final elimination step from a /t-silicon-(or tin-) substituted carbenium ion in a conformation of lowest energy, often high E selectivity is observed. In reactions of allylstannanes, catalyzed by tin(TV) chloride or titanium(IV) chloride, occasionally a metal exchange occurs, followed by the pericyclic addition pathway leading to the iwti-diastereomers17 19. A more detailed discussion is given in Section D.1.3.3.3.5. 

One of the major advantages of 2-alkenylpotassium derivatives over the appropriate lithium and magnesium analogs is the possibility of controlling the double-bond configuration in the allylie anion. For instance, the geometry of ( )-2-butene is retained in the e.vu-2-butenyl anion below — 50 0C, but equilibration above - 25 C leads to the endo-crotyl anion with >97% selectivity12. 

Methodology for the preparation of preformed enolates with well-defined double bond configuration was developed and it was shown that simple diastereoselection can be controlled, in many instances, through the use of these enolates 3. 

The stereochemistry of acyclic anionic oxy-Cope rearrangements is consistent with a chair TS having a conformation that favors equatorial placement of both alkyl and oxy substituents and minimizes the number of 1,3-diaxial interactions.214 For the reactions shown below, the double-bond configuration is correctly predicted on the basis of the most stable TS available in the first three reactions. In the fourth reaction, the TSs are of comparable energy and a 2 1 mixture of E- and Z-isomers is formed. 

These reactions proceed with retention of double-bond configuration in both the boron derivative and the alkenyl halide. The oxidative addition by the alkenyl halide, transfer... 

In addition, the results indicated that the efficiency of cis —> trans increased as the initial cis double bond configuration is shifted from the center of the polyenic chain, consistent with the 7j, triplet excited state potential curve that has a very shallow minimum at the 15-cis position compared to the deep minima at the all-trans position. The results strongly suggest that isomerization takes place via the 7j state of the carotenoid even in the case of direct photoexcitation, with their photosensitized process because of the very low intersystem crossing quantum yield, isc ([Pg.246]

The specific behaviour of unsaturated fatty acids under oxidation is determined by the position and the number of double bonds in the fatty acid molecule. The stepwise oxidation of an unsaturated acid to the position of a double bond in it proceeds in a manner similar to that of saturated acid oxidation. If the double bond retains the same configuration (trans-configuration) and position (A2,3) as those of the enoyl-CoA, which is produced during the oxidation of saturated fatty acids, the subsequent oxidation proceeds via conventional route. Otherwise, the oxidation reaction proceeds with the involvement of an accessory enzyme, A3,4-CiS-A2,3jrans-enoyl-CoA isomerase this facilitates the translocation of the double bond to an appropriate position and alters the double-bond configuration from cis to trans. 

Abbreviations for pheromone molecules follows the following format example (Z)-7-do-decen-l-yl acetate is shortened to Z7-12 OAc where Z denotes the double bond configuration, 7 the double bond position, 12 the number of carbons in the chain OAc indicates the functional group as an acetate ester. 

Some pheromone components are dienes and these can be produced by either the action of two desaturases or one desaturase and isomerization around the double bond. Some dienes with a 6,9-double bond configuration are produced using linoleic acid. Desaturases that utilize monounsaturated acyl-CoA substrates include A5 [39], A9 [36,40], All [41],A12 [42],andA13 [43].These can act sequentially to produce the diene [41,42] or conjugated dienes could be produced by the action of one desaturase followed by isomerization [44-47]. 

Most moth sex pheromones that are straight chain hydrocarbons also usually have an odd number of carbons. Most of these are polyunsaturated with double bonds in the 3,6,9- or 6,9-positions, indicating that they are derived from linolenic or linoleic acid, respectively [49,51]. Iinolenic and linoleic acid cannot be biosynthesized by moths so they must be obtained from the diet [75]. A few even chain-length hydrocarbon sex pheromones have been identified that also have 3,6,9- or 6,9-double bond configurations [49], indicating they too are derived from linolenic or linoleic acids however, it is not known how these even chain hydrocarbons are formed. 

The method enables conversion of substituted alkynes to (fc)-2-methyl-1 -alkenylalumi-num species, and, by subsequent iodinolysis, to the corresponding iodoalkenes with retention of the double-bond configuration. Depending on the substitution pattern of the starting alkyne, many useful products emerge from this reaction, which themselves can serve as building blocks for transition metal-mediated or -catalyzed coupling reactions [59—62]. 

As already noted, lower order cyanocuprates are more SN2 -selective reagents. On treatment with acetate 163, however, a mixture of the two regioisomers was obtained (entry 2) [81]. In addition, y-alkylation had taken place with ca. 25% loss of double bond configuration [82]. 

Better results were obtained for the carbamate of 163 (entry 3) [75, 80). Thus, deprotonation of the carbamate 163 with a lithium base, followed by complexation with copper iodide and treatment with one equivalent of an alkyllithium, provided exclusive y-alkylation. Double bond configuration was only partially maintained, however, giving 164 and 165 in a ratio of 89 11. The formation of both alkene isomers is explained in terms of two competing transition states 167 and 168 (Scheme 6.35). Minimization of allylic strain should to some extent favor transition state 167. Employing the enantiomerically enriched carbamate (R)-163 (82% ee) as the starting material, the proposed syn-attack of the organocopper nucleophile could then be as shown. Thus, after substitution and subsequent hydrogenation, R)-2-phenylpentane (169) was obtained in 64% ee [75]. 

The selective Te/Li transmetallation and the subsequent functionalization reactions occur with total retention of the double bond configuration. 

With regard to the arrangement of substituents at double bonds (double-bond configuration), it is recommended that E and Z stereodescriptors rather than cis and trans stereodescriptors be used throughout. 

Cozzi and co-workers (243,263) studied the influence of the double-bond configuration on the stereochemical course of the intramolecular cycloaddition of chiral alkenes, where the stereocenter is located outside the isoxazoline ring (Table 6.15). On the basis of experimental results as well as theoretical calculations, two models were proposed for the reaction with (Z)- and ( )-aIkenes, in accord with the model proposed for a-X-substituted alkenes (see Section 6.2.3.1). 

This technique (see Section 4.1.1.4.) is well suited to the exploration of double-bond configurations. This is because these compounds arc often rigid and therefore there are no conformational rearrangements to complicate lanthanide induced shift (LIS) interpretations. However, if present, mobile groups within a molecule must be taken into consideration. 

A double-bond configuration can have a strong impact on the conformational behavior of a molecule, and this may be monitored by dynamic NMR (DNMR) spectroscopy. 

The conversion of (4S,5S)-dihydro-5-[(S)-l-hydroxyethyl]-4-trimethylsilyl-2(3//)-furanone (7, see p 416) into the (Z)-alkene 8 served to establish the relative configuration at C-4 and C-5 in 7 on the basis of the established double bond configuration in 8 and the anticipated (anti) stereochemistry of the elimination reaction (see also p474)81. 

Thienylmagnesium or zinc derivatives can be coupled with vinyl halides, bromo- or iodo-benzene, or ethynyl bromide under Ni or Pd catalysis (Equation 8). The reaction can be extended to the synthesis of (3-styrylthiophene in this case the double-bond configuration is retained in the product. 

Copper(I) salts of enamines have been allylated with the 2-allyloxybenzimidazoles to give y,5-unsaturated ketones upon hydrolysis (79CL957). Primary allylic ethers react preferentially at their a-carbon with retention of double bond configuration whereas secondary allylic ethers react mainly at the y-carbon to afford alkenes of predominantly (E)-stereochemistry. 

The conjugated 1,3,5,7,9-cyclodecapentaene with the double-bond configuration as in 53 is far less stable than either azulene, 54, or bicyclo[4.4.1 J-1.3.5,7.9-undecapentaene, 55. Explain why this is so on the basis of the VB method (molecular models will be helpful). 

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