1.4- Pentadiene structure

Sulphur was found through spark source mass spectrometry to be abundant in SRS-A [93], Also, incorporation of S into SRS has been reported [64,94-96], The observation that several thiols enhance SRS formation in different systems [97-100] and that arylsulfatase (an enzyme that cleaves sulfate monoesters of phenolic and other unsaturated hydroxylated systems) inactivates SRS-A [101,102] opened the view that a thiol is a constituent of the active compound. However, it has recently been shown that the SRS-inactivating action of commercially available arylsulfatase is not due to sulfate ester cleavage, but to a dipeptidase contaminant [103,104]. The destruction of different SRS compounds by hpoxygenase [67,105,106] shows that SRS contains a cij,c/s-1,4-pentadiene structure, since this is a prerequisite for a lipoxygenase substrate [107], For a review of the earlier structural work on SRS, see ref. 108. 

Lipoxygenases (E.C. 1.13.11.12) catalyze the peroxidation of compounds that possess a cis cis-1,4-pentadiene structure. Their principal substrates in higher plants are the di- and tri-unsaturated fatty acids, linoleic (CIS 2) and linolenic (Cl 8 3) acids. Lipoxygenases (LOXs) are also known to catalyze the cooxidation of chlorophylls and carotenoids (Axelrod et al, 1987 Vick and Zimmermann, 1987). 

NAME -Methyl-1, trans-3-pentadiene STRUCTURAL FORMULA... 

No mononuclear alkyne complexes containing PF3 have been reported to date but a number of complexes of the type [Rh2(PF3)6(alkyne)] listed in Table IX have been obtained by heating a solution of [Rh2(PF3)8] in n-hexane under reflux in an inert atmosphere with an equimolar amount of the alkyne (method A). Care must be taken in the case of MeC02C=CC02Me and HC=CC02Me since reactions occur below room temperature to give the red complexes [Rh2(PF3)5(alkyne)2] (see Section IX), while explosive polymerization of the alkyne occurs above room temperature. The [Rh2(PF3)5(alkyne)2] compounds have been assigned a metallocyclo-pentadiene structure. 

The criterion required of a compound to serve as a plant lipoxygenase substrate, i.e. a m,m-l,4-pentadiene structure [146], seems to be valid also for the mammalian lipoxygenases. All reports so far fit with this concept, and arachidonic acid (all cw-5,8,11-14-eicosatetraenoic acid) is by far the substrate most studied. Oxygen can be introduced at various positions of arachidonic acid (Fig. 4), and in Table 2 the different monohydroxy-eicosatetraenoic acids (HETE) obtained from arachidonic acid in different cells are summarized. 

Fig. 4.15. Proposed mechanism of the lipoxygenase reaction. Lipoxygenases recognize the f,4-cis,cis-pentadiene structure of fatty acid substrates, and catalyze the stereoselective elimination of one hydrogen atom, followed by incorporation of molecular oxygen. This results in the formation of a chiral hydroperoxy fatty acid.

The di-7r-methane rearrangement has been studied in a sufficient number of cases to develop some of the patterns regarding substituent effects. When the central sf carbon is unsubstituted, the di-7i-methane mechanism becomes less favorable. The case of 1,1,5,5-tetraphenyl-l,4-pentadiene is illustrative. Although one of the products has the expected structure for a product of the di-7t-methane rearrangement, labeling with deuterium proves that an alternative mechanism operates ... 

Consider 1,3-pentadiene and 1,4-pentadiene. Which, if either, would benefit from the type of resonance described above Draw appropriate resonance contributors for this isomer. Indicate the likely importance of different zwitterionic structures which you might draw. Compare the energies of 1,3-pentadiene and 1,4-pentadiene. Which one is more stable ... 

Plot the energy of E-l,3-pentadiene (vertical axis) vs. C1C2C3C4 torsion angle (horizontal axis). How many minima are there Do they correspond to structures that offer maximum 7t-type orbital overlap How can you account for differences in their energies ... 

Let us finally consider two Z-matrices for optimization to transition structures, the Diels-Alder reaction of butadiene and ethylene, and the [l,5]-hydrogen shift in Z-1,3-pentadiene. To enforce the symmetries of the TSs (Cj in both cases) it is again advantageous to use dummy atoms. 

Figure E.6 Atom numbering for the transition structure for the [l,5]-hydrogen shift in 1,3-pentadiene...

Problem 14,2 Give the structures of both 1,2 and 1,4 adducts resulting from reaction of 1 equiva-1 lent of HCI with 1,3-pentadiene. 

Draw the structure of 1,3-pentadiene. Use valence-bond and molecular orbital pictures to describe the bonding for the (T-framework and -n-orhitals, respectively. 

Complexes 17-19 can be written in one valence structure as a, /3-unsaturated carbonyl compounds in which the carbonyl oxygen atom is coordinated to a BF2(OR) Lewis acid. The C=C double bonds of such organic systems are activated toward certain reactions, like Diels-Alder additions, and complexes 17-19 show similar chemistry. Complexes 17 and 18 undergo Diels-Alder additions with isoprene, 2,3-dimethyl-1,3-butadiene, tram-2-methyl-l,3-pentadiene, and cyclopentadiene to give Diels-Alder products 20-23 as shown in Scheme 1 for complex 17 (32). Compounds 20-23 are prepared in crude product yields of 75-98% and are isolated as analytically pure solids in yields of 16-66%. The X-ray structure of the isoprene product 20 has been determined and the ORTEP diagram (shown in Fig. 3) reveals the regiochemistry of the Diels-Alder addition. The C-14=C-15 double bond distance is 1.327(4) A, and the... 

FIGURE 1. Molecular structure of 1,4-pentadiene (1,4-PD) presentation with thermal probability plots... 

An example is the structure of the derivative 18 formed by reacting the labelled oxypyrrolinyl with 2,4-pentadienal. 

According to the conformational energy minima, isotactic trans- 1,4-poly (1,3-pentadiene)73,74,90 - 94 and trans-, 4-poly(2-methyl-1,3-pcntadicnc)95 are characterized by chains in the conformation (A trans A+T)n (tl symmetry) and chain axes c values of 4.85 and 4.82 A, respectively. The conformation (A cisA 1 T) with s(2/l) symmetry characterizes the chains in the structures of isotactic m-l,4-poly(l,3-peiiladiene)96 98 and cis-1,4-poly (2-methyl-1,3-pentadiene).85... 

An analogous reaction applied to l,l-dichloro-4-methyl-l,3-pentadiene would lead to a selective synthesis of permethric acid, of which the cis isomer was more desirable. It is interesting that this seemingly simple structural change provided... 

This view also explains satisfactorily why a [1, 3] sigmatropic shift of a hydrogen in 1, 3 pentadiene will not be possible. In this case, a [1, 3] shift would have to be antarafacial and such a rearrangement would be structurally prohibited. 

Scheme 6.30 Structure ofthe trapping products of bicyclo[3.2.1]-octa-2,3-diene (123) with 1,3-cyclopentadiene, (Z)-l, 3-pentadiene and 2,3-dimethyl-l, 3-butadiene and ofthe dimers of123.

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