Pyridines 2-cyclohexenones

When submitted to oxidation by a 2 per cent, solution of permanganate, pinononic acid, CgHj Og, melting at 128° to 129° C., the semi-carbazone of which melts at 204° C. Lastly the constitution of verbenone, as expressed by the above formula, is further confirmed by the fact that the bicyclic system is convertible into a monocyclic system by boiling with 25 per cent, sulphuric acid, with the formation of acetone and 3-methylcyclohexene-(2)-one-(l). This cyclohexenone has been characterised by its semi-carbazone (melting-point 198° C.) and by its conversion into y-acetobutyric acid (melting-point 36° C). The oily liquid, which did not react with sulphite, was submitted to benzoylation after dilution with pyridine. It thus gave rise to a benzoate from which was... 

Another class of new ligands was prepared in quantitative yields by Feringa et al., in 1997, by reaction between a-mercapto acids, aniline and 2-pyridine-carboxaldehyde." These pyridyl-substituted thiazolin-4-one ligands were further involved in the copper-catalysed conjugate addition of ZnEt2 to 2-cyclohexenone,... 

A number of chiral phosphine ligands has also been reported (Figure 4). Zhang and co-workers described binaphthalene phosphine 44 with a pyridine moiety to afford the addition product with up to 92% ee.50,50a With chiral bisphosphine 45, Imamoto et al. got only moderate enantioselectivity for the addition of cyclohexenone,... 

An improved route to the key intermediate 326 was also developed (165). Namely, 322 was converted to the monoprotected 1,4-dione 327 by sequential addition of the Grignard reagent derived from 2-(2-bromoethyl)-2-methyl-l,3-dioxolane followed by oxidation of the resulting benzylic alcohol with pyridin-ium dichromate (PDC). The ketone 327 was then smoothly transformed to the 2-azadiene 328 by olefination with BAMP. The regioselective addition of n-butyllithium to 328 as before followed by alkylation of the resulting metalloenamine with benzyl A-(2-bromoethyl)-A-methylcarbamate and acid-catalyzed hydrolysis furnished 325, which was converted to the cyclohexenone 326 by base-induced cycloaldolization and dehydration. 

Amino-Claisen rearrangement of propargylamino-cyclohexenone and cyclopentenone is reported to proceed with ring closure to quinoline and pyridine derivatives (equation 82). The isomeric 2-propynylenaminone gave an indolone in good yield118 (equation 83). 

In unsubstituted pyridine ligand the selectivity for the formation of 2-cyclohexenone was almost 100 %. Substitution of methyl groups in the beta and gamma positions of the pyridine ring decreases the selectivity for formation of 2-cyclohexenone and increases that of 2-cyclohexen-l-ol. Product yield also increases with methyl substitution in tbe pyridine ring. The oxidation reaction did not occur in protic solvents like methanol or in mixtures of solvent like chloroform or benzene. Product yield was very low compared to CHP as the oxidising agent (Table-3). In all these reactions with NMO, cyclohexene oxide was not detected. Chloroform was found to be the suitable solvent for tbe reaction. 

Olefins are subject to oxidation both at the double bond and at the allylic position. The CrOs-pyridine reagent in methylene chloride appears to be the most satisfactory reagent for allylic oxidation among the Cr(VI) reagents.Several pieces of mechanistic information indicate that allylic radicals or cations are intermediates in these oxidations. Thus, in cyclohexene is distributed in the product cyclohexenone in a manner indicating that a symmetrical allylic intermediate is... 

A study of oxidation of cyclohexene with O2, catalysed by meso-tetra(hydroxyphenyl) porphyrin metal complexes (M = Co(II), Mn(II)), to 2-cyclohexenol (26%) and 2-cyclohexenone (70%) addressed the effect of added pyridine on the conversion and selectivity of cyclohexene. ... 

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