Terminal methyls, 24 cyclic double bonds

The oxidation of simple internal alkenes is very slow. Clean selective oxidation of the terminal double bond in 30 to the methyl ketone 31 in the presence of the internal double bond is possible under normal conditions [36]. The oxidation of cyclic alkenes is difficult. Addition of strong mineral acids such as HC104,H2S04 or HBF4 accelerates the oxidation of cyclohexene and cyclopentene [25,37]. PdCl2-CuCl2 in EtOH is used for the oxidation of cyclopentene and cyclohexene [38]. 

It should be noted that when an R group, e.g. a methyl group, is present at the internal unsaturated carbon atom in a terminal diene molecule, as in the CH2=C(Me)—(CH2)x-C(Me)=CH2 monomer, the cyclic diene metathesis ceases [7,8]. The steric effect exerted by the R substituent can even be important at the a position to the double bond in the monomer. Sterically encumbering this position hinders polymer formation [9]. 

Migration of the double bond of the cyclic olefin formed by the RCM has also been observed [35]. RCM of the diene 77 by 69 in refluxing dichloromethane resulted in the formation of considerable amounts of the unexpected cyclic olefin 79 in addition to 78 (Eq. 12.31) [35a[. It was also noted that the formation of 79 was effectively suppressed by the addition of amine to the dichloromethane solution or employment of diethylether as solvent, which implies participation of proton in the isomerization reaction. It was also noted that a terminal vinyl group with a free tertiary allylic hydroxyl group accelerates RCM, rather than its methyl ether derivative. These results suggested some interaction between the alkylidene complex 69 with hydroxyl proton in situ. 

Most C. contain an unsatuiated C22 chain with methyl branches and a 9-membered unit on each end, which may be cyclic as in a- and P-carotene, or acyclic as in lycopenes. When present, the terminal rings are usually 6-membered a- and P-ionone systems. The median 22 C atoms are generally constant, and oxidations to xanthophylls usually occur on the two C9 end sections. Most of the 700 known C. are xanthophylls, with 1, 2 or many hydroxyl groups, and ether, aldehyde, ketone or add substituents. Stereochemical differences arise from the ds-lram isomerism of the C s C double bonds, and from chiral centers that... 

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