Oh

137b-succinate

137b

137b'-succinate

FIGURE 14.62 Asymmetric hydrolysis of racemic menthyl succinate (137b- and 137b'-succinates) to obtain pure (-)-menthol (137b). (Modified from Yamaguchi, Y. et al., 1977. J. Agric. Chem. Soc. Jpn., 51: 411-416,)

O-COCH2CH2COOH

(Asakawa et al., 1991). Aspergillus niger converted (-)-menthol (137b) to 1- (138b), 2- (140b), 6- (139b), 7- (143b), 9-hydroxymenthols (144b), and the mosquito repellent-active 8-hydroxymenthol (142b), whereas (+)-menthol (137b') was smoothly biotransformed by the same microorganism to 7-hydroxy-menthol (143b). The bioconversion of (+)- (137a') and (-)-neomenthol (137a) and (+)-isomenthol (137c') by Aspergillus niger was studied later by Takahashi et al. (1994), mainly giving hydroxylated products. Noma and Asakawa (1995) reviewed the schematic menthol hydroxylation in detail.

Incubation of (-)-menthol (137b) with Cephalosporium aphidicola for 12 days yielded 10-acetoxymenthol (144bb-Ac), 1a-hydroxymenthol (138b), 6a-hydroxy- menthol (139bb), 7-hydroxy-menthol (143b), 9-hydroxymenthol (144ba), and 10-hydroxymenthol (144bb) (Atta-ur-Rahman et al., 1998) (Figure 14.63).

Aspergillus niger TBUYN-2 converted (-)-menthol (137b) to 1a- (138b), 2a- (140b), 4ß - (141b), 6a- (139bb), 7- (143b)-, 9-hydroxymenthols (144ba), and the mosquito repellent-active 8-hydroxy-menthol (142b) (Figure 14.64). Aspergillus cellulosae M-77 biotransformed (-)-menthol (137b) to 4ß -hydroxymenthol (141b) predominantly. The formation of 141b is also observed in Aspergillus cellulosae IFO 4040 and Aspergillus terreus IFO 6123, but its yield is much less than that obtained from 137b by Aspergillus cellulosae M-77 (Asakawa et al., 1991) (Table 14.1).

FIGURE 14.63 Biotransformation of (-)-menthol (137b) by Cephalosporium aphidicola. (Modified from Atta-ur-Rahman, M. et al., 1998. J. Nat. Prod., 61: 1340-1342.)
FIGURE 14.64 Metabolic pathways of (-)-menthol (137b) by Aspergillus niger. (Modified from Asakawa, Y. et al., 1991. Phytochemistry, 30: 3981-3987.)

On the other hand, (+)-menthol (137b') was smoothly biotransformed by Aspergillus niger to give 1ß -hydroxymenthol (138b'), 6ß-hydroxymenthol (139ba'), 2ß-hydroxymenthol (140ba'), 4a-hydroxymenthol (141b'), 7-hydroxymenthol (143b'), 8-hydroxymenthol (142b'), and 9-hydroxy-menthol (144ba') (Figure 14.65) (Table 14.2).

Spodoptera litura converted (-)- and (+)-menthols (137b and 137b') gave the corresponding 10-hydroxy products (143b and 143b') (Miyazawa et al., 1997a) (Figure 14.66).

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