L- -Menthyl ester

The kinetic resolution using a chiral zirconocene-imido complex 286 took place with high enantioselectivity to result in chiral allenes 287 (up to 98% ee) (Scheme 4.74) [116]. However, a potential drawback of these methods is irreversible consumption of half of the allene even if complete recovery of the desired enantiomer is possible. Dynamic kinetic resolutions avoid this disadvantage in the enantiomer-differentiating reactions. Node et al. transformed a di-(-)-L-menthyl ester of racemic allene-l,3-dicarboxylate [(S)- and (RJ-288] to the corresponding chiral allene dicarbox-ylate (R)-288 by an epimerization-crystallization method with the assistance of a catalytic amount of Et3N (Scheme 4.75) [117]. 

The chemically induced asymmetric photocyclization of the l-(—)-menthyl ester shows a striking temperature dependence 76), a completely reversed ratio is obtained at low temperature. When the chiral group is placed at C(ll) of the benzo[c]phen-anthryl group (77) the effect is of the same order (about 5 % diastereomeric excess) as when the same group is placed at the paraposition of the phenyl group, or when... 

XVII Methacrylsaure- L-menthyl- ester Cx4Ha4Os 224,3 Benzol 2,24 25 589 —90,0... 

An influence of the size of the substituent on the probabilities p8/% and p8/8 is hardly detectable (with the possible exception of the L-menthyl ester). From the present data, it cannot be said with certainty that the probability pi/ runs through a minimum (and consequently p8/i through a maximum) with increasing bulldness of the substituent. However, it is clear that there is no monotonic increase or decrease of each of the four probabilities with increasing size of the substituent. 

Enantiomers of carboxylic acids may sometimes be separated by GC as methyl esters, but special derivatives are mostly prepared for this purpose. Ackman et al. [188] resolved enantiomers of isoprenoid fatty acids after their conversion into L-menthyl esters. The acids under analysis were chlorinated by refluxing with distilled freshly prepared thionyl chloride and the chlorides produced were treated with L-menthol in the presence of pyridine under strictly anhydrous conditions. GC separation was carried out in a capillary column coated with butanediol succinate polyester. Annett and Stumpf [189] made use of L-menthyloxycarbonyl derivatives for the separation of enantiomers of methyl esters of hydroxy acids. The derivatization reagent, L-menthyl chloroformate, was prepared by the reaction of L-menthol with phosgene, with cooling with ice. Diastereoisomers of different hydroxy acids were thus separated on 1.5% OV-210. 

In the Pd-catalyzed polymerization of aryl isonitriles, the screw-sense of the helical structure was controlled by chiral groups introduced to the aryl moiety of the isoni-triles.f Thus, polymerization of (L)-menthyl ester of p- or m-isocyanobenzoic acid (13)... 

Shknada, Y, et ah, Enzymatic synthesis of L-menthyl esters in organic solvent-free system, J. Amer. Oil Chem. Soc., 76, 10, p. 1139, 1999. 

There has been a further report on the use of optically-pure (61), prepared emqmiically, to make carbanucleosldes, (-)-carbocyclic-ddT and (-)-carbocyclic-2, 3 -dideoxy-3 -fluorothymidlne being the targets thus prepared. A synthon for carbocycUc -nucleosides, previously available as a racemate (Vol.20, p.214. Scheme 16), has now been made in chiral form by use of a di-l-menthyl ester as a chiral auxiliary. 

---