Menthol, chirality structure

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. 

In y-alkoxyfuranones the acetal functionality is ideally suited for the introduction of a chiral auxiliary simultaneously high 71-face selectivity may be obtained due to the relatively rigid structure that is present. With ( + )- or (—(-menthol as auxiliaries it is possible to obtain both (5S)- or (5/ )-y-menthyloxy-2(5//)-furanones in an enantiomerically pure form293. When the auxiliary acts as a bulky substituent, as in the case with the 1-menthyloxy group, the addition of enolates occurs trans to the y-alkoxy substituent. The chiral auxiliary is readily removed by hydrolysis and various optically active lactones, protected amino acids and hydroxy acids are accessible in this way294-29s-400. 

In 1990, Choudary [139] reported that titanium-pillared montmorillonites modified with tartrates are very selective solid catalysts for the Sharpless epoxidation, as well as for the oxidation of aromatic sulfides [140], Unfortunately, this research has not been reproduced by other authors. Therefore, a more classical strategy to modify different metal oxides with histidine was used by Moriguchi et al. [141], The catalyst showed a modest e.s. for the solvolysis of activated amino acid esters. Starting from these discoveries, Morihara et al. [142] created in 1993 the so-called molecular footprints on the surface of an Al-doped silica gel using an amino acid derivative as chiral template molecule. After removal of the template, the catalyst showed low but significant e.s. for the hydrolysis of a structurally related anhydride. On the same fines, Cativiela and coworkers [143] treated silica or alumina with diethylaluminum chloride and menthol. The resulting modified material catalyzed Diels-Alder reaction between cyclopentadiene and methacrolein with modest e.s. (30% e.e.). As mentioned in the Introduction, all these catalysts are not yet practically important but rather they demonstrate that amorphous metal oxides can be modified successfully. 

The first report of an asymmetric Diels-Alder reaction with chiral Lewis acids (252) was made by Russian chemists in 1976 (253). Koga was probably the first to report a meaningful enantioselective Diels-Alder reaction (Scheme 105) in which the cyclopentadiene-methacro-lein exo adduct was obtained in 72% ee with the aid of 15 mol % of a menthol-modified aluminum chloride (254). The ee is highly dependent on the structures of the substrates, and asymmetric induction has not been observed with methyl acrylate as dienophile. Disproportionation... 

Less than 5% enantiomeric excess was obtained in the osmium-catalyzed oxyamination of 2-phenyl-l-propene when chiral carbamates derived from 1-menthol and 1-borneol were used96, but the structures of the resulting products were not reported. 

Extremely high diastereomeric excesses are observed with chiral phenylglyoxylates derived from 8-phenylmenthol and are independent of the nature of the olefin (Table 7) however, de values for the corresponding menthol derivatives vary with the structure of the olefin. Remarkable is the high selectivity obtained with 2,2-dimethyl-1,3-dioxole in both cases (entry 2). The tWo-phenyl selective formation of oxetanes with cyclic olefins is of note and resembles the results reported by Griesbeck32 34 In addition various olefins demonstrate the influence of C2 symmetry and the stereoselectivity with respect to regiomeric oxetanes. 

Other examples of chiral dopants with a very high twisting power (like 1-menthol and some steroid structures) are given in [76]. 

Hu JS, Li D, Zhang WC, Meng QB. Synthesis, structure, and properties of chiral hquid crystal monomers and polymers based on menthol. J Polym Sci A Polym Chem 2012 50 5049-59. 

Al-Phenyl-iV-allylmethacrylamide (23) gives a polymer with a five-membered ring structure with true asymmetric centers in the main chain by free-radical cyclopolymerization [63]. When the polymerization is carried out in the presence of SnCl4/(-)-menthol, the resulting polymer was optically active ([a]] -5.6°). Chiral induction was also observed in the copolymerization of 23 with MMA. Cationic cyclopolymerization of 24 using a ZnClJ 10-camphorsulfonic acid (25) initiator system gives an optically active polymer having a 1,3-dioxane structure in the main chain([a]435-17°)[64]. 

The first example of appreciable enantioselectivity in a catalytic asymmetric Diels-Alder reaction was reported in 1979 by Koga (Equation 14) [73]. This was achieved by use of 14 mol % of an aluminum complex resulting from reaction of menthol and EtAlCl2, as formulated in the proposed catalyst structure 133. In the presence of this catalyst, cycloadduct 134 was obtained in 57 % ee and 69 % yield. This result provided an important proof of principle that catalytic enantioselective Diels-Alder reactions can indeed be achieved with chiral Lewis acids [73, 74). 

---