R1 H R OnBu R3 OAc

123a o o oh

oh

oh oh 123b

FIGURE 4.20 Minwanensin-type sesquiterpenes 123 and 133-136

to a biogenetic precursor A of all the Illicium sesquiterpenes; that is, anisatin, pseudoanisatin, majucin, and pseudomajucin types. It is generally accepted that anisatin could be biosynthesized from an acrorane through a tricarbocyclic precursor A after breaking the bond between C-6 and C-11, as shown in Scheme 4.10 (Devon and Scott 1972). Thus, the isolation of tashironin, anisatin, and its related sesquiterpenes from the same source is of considerable significance and throws light on the biogenesis of Illicium sesquiterpenes.

As discussed previously, anisatin (107) and noranisatin (108) are convulsive toxic principles in I. anisatum and are regarded as picrotoxin-like potent phytotoxins. The neuropharmacological study of anisatin demonstrates that their convulsive toxicity is probably caused by a potent non-competitive GABA antagonist. However, it has remained equivocal, at the present time, which structural part of anisatin is able to cause convulsive activity. A systematic study of the structure and toxicity relationship has not been carried out because of the limited available quantity of compounds, although a number of various anisatin-related compounds have been known as natural products. The toxicity of representative compounds to mice (i.p.) was examined and compared with those of anisatin (107) and neoanisatin (108) (Kouno et al., 1989; Niwa et al., 1991) (Table 4.1). Another anisatin-type 2-oxo-6-dehydroxyneoanisatin (115) and another majucin-type neomajucin (138) are also very toxic (Yang et al., 1990; Kouno et al., 1990), whereas majucin (137) and its analogues 139, 142, 144, 145, and 146 could not produce any appreciable behavioral changes at a dose of up to 40 mg/kg (Kouno et al., 1990). As neomajucin (138) was recognized as a toxic compound, the presence of a spiro P-lactone moiety in anisatin is not likely to be absolutely

HO/ 142

R2,r

R2,r

O
O

FIGURE 4.21 Majucin-type sesquiterpenes (compounds 137-147)

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