Menthol solid support

Mayoral and colleagues210 studied the same reaction catalyzed by a menthoxyaluminum catalyst supported on silica gel and alumina. The catalyst was prepared by treatment of the solid support with diethylaluminum chloride and (—)-menthol. The silica-supported catalyst proved more active than the alumina-supported catalyst. The reaction rates and enantioselectivities depended strongly on the amount of (—)-menthol used. The highest ee obtained was 31% at 81% conversion (endo/exo = 10/90). 

The procedure is commendable for its sinq>licity, reduced toxicity (chromium in all its oxidation states is carcinogenic) and achieves good yields of ketones from alcohol, for example, octan-2-ol is oxidized into octan-2-one (92%), cyclohexanol into cyclohexanone (90%) and menthol into menthone (98%). Pyridinium chromate is also a well-known oxidant for allylic oxidations. As a silica gel supported reagent, this is turned into an efficient alcohol oxidant that will leave acid-labile functions unscathed. Another advantage of the reagent is the long shelf-life of more than a year. These solid-supported oxidants also greatly facilitate pr uct work-up, when compared with their solution counterparts. 

The first successful examples of enantioselective Diels-Alder reactions catalyzed by chirally modified Lewis acids were reported by Koga [85]. The catalysts were prepared from menthol and AlEt2Cl [86]. Alumina-supported chiral menthoxy aluminum derivatives (64, 65, 66, 67) have been prepared by simple mixing of (-)-menthol, AlEt2Cl, and alumina in toluene under reflux. The reaction of methacrolein with cyclopentadiene (Eq. 20) was conducted with 67 as catalyst at -50 °C and afforded 81 % conversion with 31 % ee [87] Koga reported 57 % ee at -78 °C by use of an homogeneous catalyst [85]. Solid catalyst 69, prepared from silica gel-supported proli-nol 68 and AlEt2Cl (Eq. 21) is also an active catalyst in the same reaction, but with low enantioselectivity [87]. When the same catalyst was attached to crosslinked polystyrene (70) the ee in the reaction was lower [88]. 

The monomers with relatively small N-substituents such as N-methyl methacrylamide afford polymers rich in syndiotacti-city. The bulkier N-isopropyl, N-t-butyl, and N-phenyl derivatives result in polymers with a slightly reduced syndio-tacticity. However, radical polymerization of bulky N-[di (4-butylphenyl)phenylmethyl] methacrylamide (DBuTrMAM, 67) and N-triphenylmethylmethacrylamide (TrMAM, 68) produces a nearly complete isotactic polymer due to helix formation of the polymer chain. When the radical polymerization of these monomers was carried out in the presence of optically active menthol, single-handed helical, optically active polymers were produced. Because poly(TrMAM) was insoluble in solvents, its CD spectra were measured in the solid state to support the chiral structure. 

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