P-Menth

Since the chemical addition of HCN always results in mixtures of cis/trans-isomers, the stereoselective HNL-catalyzed addition is of great advantage in the synthesis of natural products. The syntheses of the natural monoterpenes cis-p-menth-8-ene-l,7-diol and cA-/ -menthane-l,7,8-triol are interesting examples for the application of this methodology (Scheme 9). ... 

Scheme 9 Stereoselective synthesis of cw-p-menthane-l,7,8-triol (A) and cA-p-menth-8-ene-l,7-diol (B).

Heating of carvone in water at 210 °C for 10 min afforded 8-hydroxy-p-menth-6-en-2-one, the equilibrium position favoring the starting material by a factor of 4 1. The hydroxymenthenone, although in equilibrium with the starting material, also underwent elimination of water and isomerization to carvacrol, giving a 1 1 1 mixture of these three substances at 230 °C after 10 min. At 250 °C, for 10 min, carvone isomerized to carvacrol almost quantitatively (Scheme 2.11) [47]. 

Manganese(lll) acetate oxidation of (+)-p-menth-l-ene yields the two lactones (165 X=0, Y = CH2) and (165 X = CH2, Y = O) as major products together with anticipated acetates similar oxidation of (+)-pulegone yields the C-2 acetates in low yield and oxidation of isomenthone in the presence of isopropenyl acetate results in acetonylation at C-2 and C-4. Further examples of the rearrangement of epoxides with KOBu -aprotic solvents (pyridine is favoured) have been reported (c/. Vol. 6, p. 44), e.g. (166) to (167), although with the corresponding 1,2-epoxy-... 

Manganese(lil) acetate oxidation of a-pinene yields two lactones analogous to those reported earlier with (+)-p-menth-l-ene however, the major product consists of derived acetates [of a-terpineol, c/5-pin-3-en-2-ol, and myrtenol (224 ... 

The biotransformation of linalool by Botrytis cinerea has also been described [60]. After addition of linalool to botrytised must, a series of transformation products was identified (E)- (49) and (Z)-2,6-dimethyl-2,7-octadiene-l,6-diol (48), trans- (76) and cw-furanoid linalool oxide (77), trans- (78) and c/s-pyranoid linalool oxide (79) and their acetates (80, 81), 3,9-epoxy-p-menth-1 -ene (75) and 2-methyl-2-vinyltetrahydrofuran-5-one (66) (unsaturated lactone), Fig. (11). Quantitative analysis however, showed that linalool was predominantly (> 90%) metabolised to ( )-2,6-dimethyl-2,7-octadiene-l,6-diol (49) by B. cinerea. The other compounds were only found as by-products in minor concentrations. 

As mentioned before, a Pseudomonas incognita was isolated by enrichment technique on the monoterpene alcohol linalool that was also able to grow on geraniol, nerol and limonene [36]. The metabolism of limonene by this bacterium was also investigated [37]. After fermentation the medium yielded as main product a crystallic acid, perillic acid, together with unmetabolised limonene, and some oxygenated compounds dihydrocarvone, carvone, carveol, p-menth-8-en-1 -ol-2-one, p-menth-8-ene-1,2-diol or p-menth-1 -ene-6,9-diol (structure not fully elucidated) and finally / -isopropenyl pimelic acid. 

The fungal bioconversion of limonene was further studied [82]. Penicillium sp. cultures were isolated from rotting orange rind that utilised limonene and converted it rapidly to a-terpineol. Bowen [83] isolated two common citrus moulds, Penicillium italicum and P. digitatum, responsible for the postharvest diseases of citrus fruits. Fermentation of P. italicum on limonene yielded cis- and frans-carveol (93) (26%) as main products, together with cis- and from-p-mentha-2,8-dien-l-ol (110) (18%), (+)-carvone (94) (6%), p-mentha-1,8-dien-4-ol (111) (4%), perillyl alcohol (100) (3%), p-menth-8-ene-1,2-diol (98) (3%), Fig. (17). Conversion by P. digitatum yielded the same products in lower yields. The two alcohols />-mentha-2,8-dien-1 -ol (110) and p-mentha-1,8-dien-4-ol (111) were not described in the transformation studies where soil Pseudomonads were used [69]. 

The electrooxidation of alpha- and hefa-pinenes in an AcOH—Et4NOTs—(C) system affords ring-opened products53 . For example, the electrolysis of alpha-pinene 17 gives carveols 18 and p-menth-6-ene-2,8-diol derivatives 19 (Scheme 3-6). 

The electrooxidation of the enol acetate 23 of isopinocamphone generally leads to three different types of products 7-carvone 7, 8-acetoxy-p-menth-6-en-2-one 24, and 2-acetoxy-2,6,6-trimethylbicyclo[3.1.1]heptan-3-one. The product distribution is largely dependent on the combination of solvent and electrolyte. It is found that the enol acetate 23 leads to /-carvone 7 selectively, if electrolysis is performed in a CH2C12/ AcOH(8/l)—Et4NOTs—(C) system (Scheme 3-8)55>. 

Electrochemical reduction of the ozonization products from monoterpenes, i.e., />-meth-l-ene, (-l-)-limonene, (+ )-a// /ia-pinene, (+)-car-3-ene, provides the corresponding double-bond cleavage products in 45-70% yields57. The electrolysis of the acetyloxy hydroperoxide 28 derived from p-menth-l-ene 27 is carried out in an Ac0H/H20(6/1 v/v)— AcONa— (Pb/Pb) system at —1.1 to —1.4V vs. SCE, 2.0 to 2.2 A/dm2 in a divided cell to give the corresponding keto-alcohol 29 in 70% yield (Scheme 3-10). 

---