Pentadiene group

Lipoxygenase is highly specific and attacks the cis-cis-1,4-pentadiene group contained in the fatty acids linoleic, linolenic, and arachidonic, as follows ... 

Figure 10-19 Essential Steps in the Mechanism of the Lipoxygenase-Catalyzed Oxidation of the 1,4-Pentadiene Group...

Hamberg and Samuelsson (111) had tried a number of polyunsaturated fatty acids earlier and concluded that the structural requirement for lipoxygenase attack was a cis,ci5-l,4-pentadiene group whose methylene carbon was at position a>-8. In other words the requirement was a distal double bond in position w-6. These authors also examined the modified products of the reaction by mass spectroscopy and found that in all cases O2 was inserted at position w-6 except for linoleic acid. In... 

Analogous materials with cyclopentadienyliron units coordinated to cyclopentadie-nyl ligands in the main chain, but with shorter C2 spacers, were described in Chapter 3, Section 3.2.2. Polymers with cyclopentadienyl cobalt groups in the main chain have been produced by the treatment of precursor materials containing cobaltacyclo-pentadiene groups (e.g. 4.28) first with butyl isocyanide and then with Mel [79]. 

Figure 14-26 The two possible distinct products resulting from a shift of a hydrogen from position 1 to position 5 in a substituted 1,3-pentadiene. Groups A, B, C, D are deuterium atoms, methyl groups, etc., enabling us to distinguish the products.

Another protecting group of amines is 1-isopropylallyloxycarbonyl, which can be deprotected by decarboxylation and a /3-elimination reaction of the (tt-l-isopropylallyl)palladium intermediate under neutral conditions, generating CO2 and 4-methyl-1,3-pentadiene. The method can be applied to the amino acid 674 and peptides without racemization[437]. 

A large number of pyridazines are synthetically available from [44-2] cycloaddition reactions. In one general method, azo or diazo compounds are used as dienophiles, and a second approach is based on the reaction between 1,2,4,5-tetrazines and various unsaturated compounds. The most useful azo dienophile is a dialkyl azodicarboxylate which reacts with appropriate dienes to give reduced pyridazines and cinnolines (Scheme 89). With highly substituted dienes the normal cycloaddition reaction is prevented, and, if the ethylenic group in styrenes is substituted with aryl groups, indoles are formed preferentially. The cycloadduct with 2,3-pentadienal acetal is a tetrahydropyridazine derivative which has been used for the preparation of 2,5-diamino-2,5-dideoxyribose (80LA1307). 

Example 17 Estimate liquid heat capacity at 293.15 K of 1,4-pentadiene, CH2 =CH-----CH2---CH =CH2- The atomic groups are ... 

The groups at the termini of the 1,4-pentadiene system also affect the efficiency and direction of the the di-7c-methane reaction. The general trend is that cyclization oceurs at the diene terminus that best stabilizes radical character. Thus, a terminus substituted with aryl groups will cyclize in preference to an unsubstituted or alkyl-substituted terminus ... 

Chiral allenes are exanples of a small group of molecules that are chiral, but don t have a chirality center. What they do have is a chirality axis, which in the case of 2,3-pentadiene is a line passing through the three carbons of the allene unit (carbons 2, 3, and 4). 

For the synthesis of permethric acid esters 16 from l,l-dichloro-4-methyl-l,3-pentadiene and of chrysanthemic acid esters from 2,5-dimethyl-2,4-hexadienes, it seems that the yields are less sensitive to the choice of the catalyst 72 77). It is evident, however, that Rh2(OOCCF3)4 is again less efficient than other rhodium acetates. The influence of the alkyl group of the diazoacetate on the yields is only marginal for the chrysanthemic acid esters, but the yield of permethric acid esters 16 varies in a catalyst-dependent non-predictable way when methyl, ethyl, n-butyl or f-butyl diazoacetate are used77). 

Addition reactions depend on the metal used for the transmetalation of the zirconacyclo-pentadiene. After transmetalation to copper, an addition reaction occurs to the carbon-carbon double bond or to a carbon—carbon triple bond bearing electron-withdrawing groups (Michael addition reactions). On the other hand, transmetalation to Ni allows the use of carbon—carbon triple bonds bearing electron-donating groups. 

Schafer and coworkers devised a useful synthetic procedure based on this reaction [26]. Thus, the Diels-Alder reaction of /i-silylacrylic acid with cyclo-pentadiene gave the adduct which was oxidized anodically with the elimination of the carboxyl and the silyl groups. Successful formation of norbonadiene indicates that /J-silylacrylic acid can be used an a synthon of acetylene in Diels-Alder reactions with dienes (Scheme 33). 

A combination of RuH2(PPh3)4 and ferrocenylphosphines catalyzes the crosscoupling reaction of a-hydroxyallenes and 1-alkynes to give exo-enynes selectively (Scheme 16.96) [102]. From the observation that O-protected allenes and 5-phenyl-1,2-pentadiene are inert to the cross-coupling reaction, the a-hydroxyl group in allenes plays an important role in this exo-selective reaction. 

Furthermore, in the addition to the 3,4-bond of 1,3-pentadienes, the anti stereoselectivity observed with both bromine and chlorine has been attributed to a tightly bridged bromonium ion intermediate involving less charge dispersal in the vinyl group. In support of this hypothesis, it has been noted that bromine addition to the terminal double bond of the 1,3-pentadienes occurs without isomerization of the internal cis or trans double bond15. 

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