Menthol menthyl succinate

DL-menthyl succinate L - specific L-menthol + D-menthyl succinate... 

The process uses cells of Rhodotorula minuta entrapped in polyurethane. These cells selectively hydrolyse the L-ester. The remaining D-menthyl succinate is then hydrolysed and the 15-menthol racemised via D-menthone and then recycled. We have represented this process in Figure 9.9. 

FIGURE 19.62 Asymmetric hydrolysis of racemic menthyl succinate (137b- and 137b -succinates) to obtain pure (-)-menthol (137b). (Modi ed from Yamaguchi, Y. et al., J. Agric. Chem. Soc. Jpn., 51, 411,1977.)... 

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 this example, synthetic DJ.-menthol racemate is initially chemically esterified with acetate, formate, propionate, myristate, benzoate, or succinate. The ester is then selectively hydrolyzed by steriospecilic microbial enzymes to produce L-menthol that is readily isolated from the D-menthol ester. The low water solubility of the menthyl esters and L-menthol has made this process particularly suitable to non-aqueous systems. Omata et al. [90] immobilized cells of Rhodotorula minuta in photo-cross-linked or polyurethane resin gels and used a water saturated hexane eluant to obtain a product of 100% optical purity. The immobilized cells had an estimated half-life of 55 to 63 days in this solvent. 

Since the introduction of WS-3, many other coolants have appeared on the market, mostly based on the menthol structure and, in fact, synthesized from menthol. These are shown in Fig. 8.45. Three menthyl esters are produced. The lactate (201) is made by Symrise and sold under the trade name Frescolat ML, while the succinate (202) is available from Advanced Biotech as Koolada. Menthyl pyrrohdone carboxylate (203) is sold by Quest as Questice. Questice serves as a sustained release precursor for menthol, the ester function being cleaved by lipases. It therefore finds use in cosmetics since the release rate is affected by the moisture levels in the skin. Thus as the body tries to lose heat, e.g., on exercise, Questice provides the desirable sensation of cooling. Some higher menthyl esters have been patented as coolants by Firmenich (259). Two menthyl ethers are also of interest as coolants. The mono- ether of propylene glycol (204) has been added to the FEMA/GRAS list and the monoglyceryl ether (205) is sold as... 

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