Polyenes oxidation

Oxidative desulfonylation polyenes. Oxidation of the anion of a primary sulfone with MoOPH results in a p-hydroxy sulfone formed by condensation of... 

Tris(acetylacetonato) chromium has been shown to be of interest as a smoke retarder for poly(vinyl chloride)5 and it augments the inhibition of polyene oxidation by organic phosphites.29... 

Francke, W., Gries, G., Gries, R., Haeussler, D., Moeller, K. and Plass, E. (1999). Patent application. Preparation of polyenes and polyene oxides as insect pheromones. 

Miscellaneous Reactions of Oxirans.—The first successful enzymatic cyclization of a non-natural squalene has been disclosed. (18Z)-Oxidosqualene (188), which does not possess the naturally occurring a -trans stereochemistry, was caused to cyclize, in the presence of 2,3-epoxysqualene sterol cyclase, to (205)-epinorlanosterol (189). The polyene oxide (190) underwent an uncommon tricyclization in CH2CI2 containing Bp3-OEt2 to form the cw-fused A/B-ring 18-nor-steroid (191) (25%) this compound was found to be identical with a material derived by treatment of a naturally occurring steroid with BF3. 

Open-chain 1,5-polyenes (e.g. squalene) and some oxygenated derivatives are the biochemical precursors of cyclic terpenoids (e.g. steroids, carotenoids). The enzymic cyclization of squalene 2,3-oxide, which has one chiral carbon atom, to produce lanosterol introduces seven chiral centres in one totally stereoselective reaction. As a result, organic chemists have tried to ascertain, whether squalene or related olefinic systems could be induced to undergo similar stereoselective cyclizations in the absence of enzymes (W.S. Johnson, 1968, 1976). 

The achiral triene chain of (a//-rrans-)-3-demethyl-famesic ester as well as its (6-cis-)-isoiner cyclize in the presence of acids to give the decalol derivative with four chirai centres whose relative configuration is well defined (P.A. Stadler, 1957 A. Escherunoser, 1959 W.S. Johnson, 1968, 1976). A monocyclic diene is formed as an intermediate (G. Stork, 1955). With more complicated 1,5-polyenes, such as squalene, oily mixtures of various cycliz-ation products are obtained. The 18,19-glycol of squalene 2,3-oxide, however, cyclized in modest yield with picric acid catalysis to give a complex tetracyclic natural product with nine chiral centres. Picric acid acts as a protic acid of medium strength whose conjugated base is non-nucleophilic. Such acids activate oxygen functions selectively (K.B. Sharpless, 1970). 

Work at Rhc ne-Poulenc has involved a different approach to retinal and is based on the paHadium-cataly2ed rearrangement of the mixed carbonate (41) to the aHenyl enal (42). Isomerization of the aHene (42) to the polyene (43) completes the constmction of the carbon framework. Acid-catalyzed isomerization yields retinal (5). A decided advantage of this route is that no by-products such as triphenylphosphine oxide or sodium phenylsulfinate are formed. However, significant yield improvements would be necessary for this process to compete with the current commercial syntheses (25—27) (Fig. 9). 

Isoxazolmes are good preciusors of a,fi-unsatiuated ketones.This transformadon ii nsefid for synthesis of polyenes. For example, nitnle oxide cycloaddidon chemistry is used tc prepare4-oxo-2-alkenylphosphonates, which are useful to synthesize a long polyethylenic unii via Woodworth-Errunons olefinadon fEq. 8.66. ... 

Braun [22] showed from ozonolysis that for fractions of bulk PVC the number of internal double bonds and the rate of thermal degradation, although dependent on each other, were independent of the molecular weight. This clearly demonstrated the role of internal unsaturation on the stability of the polymer. After careful chlorination of the double bonds, an increase in thermal stability was observed and the number of double bonds as determined by oxidation with potassium permanganate were reduced. It was also shown that one polyene sequence was formed from each isolated double bond. 

Additions of sulfinic acids to polyenes ( hydrosulfonylation ), however, proceed with very strong acids80 or under catalysis of Pd complexes81 (equation 17). With copper(II) arenesulfinates, azulene has been oxidatively sulfonylated in the 1- and 2-positions of the five-membered ring82 (equation 18). The sulfonylmercuration has also been applied with success to conjugated dienes83 (equation 19). 

Cyclopentene, cyclohexene, and cycloheptene were obtained by intramolecular oxidative couplings of the bis-ylides (161 n = 3, 4, or 5) but oxidation of (161 n = 2) gave cyclo-octa-1,5-diene. Oxidation of the bis-ylide (161 n — 8) gave cyclic polyenes containing 20, 30, 40, 50, and 60 atoms. 

Polyene cyclizations are of substantial value in the synthesis of polycyclic terpene natural products. These syntheses resemble the processes by which the polycyclic compounds are assembled in nature. The most dramatic example of biosynthesis of a polycyclic skeleton from a polyene intermediate is the conversion of squalene oxide to the steroid lanosterol. In the biological reaction, an enzyme not only to induces the cationic cyclization but also holds the substrate in a conformation corresponding to stereochemistry of the polycyclic product.17 In this case, the cyclization is terminated by a series of rearrangements. 

Since chlorinated PVC is totally transparent in the near-UV and visible range, it will not absorb at 488 nm, the emission line of the argon ion laser that we intended to use to perform the carbonization. Therefore C-PVC films were first exposed to the UV radiation of a medium pressure mercury lamp in order to produce the strongly absorbing polyenes. This irradiation was carried out at room temperature in the absence of oxygen, thus preventing the formation of undesirable oxidation products. 

The reductive coupling/silylation reaction was extended to more complicated polyenes, such as the triene-substituted cyclopentanol 73, which cyclizes to provide 74 with a 72% yield and 6 1 dr after oxidation (Eq. 10) [44], The reaction is chemoselective the initial insertion occurs into the allyl substituent, which then inserts into the less hindered of the two remaining olefins, leaving the most hindered alkene unreacted. 

EPR techniques have also been used to detect and establish the structure of the carotenoid I3 complexes formed upon oxidation of carotenoids with I2 (Ding et al. 1988). At 77 K the equilibrium is shifted so that Car" , forms where n=5,7, or 9, and the polymeric l resides over the polyene chain in a n-n interaction giving rise to a detectable shift in the g-value. 

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