R1 R2 Och3

oh oh

och3

och3

FIGURE 4.4 Prenylated phenylpropanoids (compounds 29-36)

Bicycloillicinone asarone acetal (54) and bicycloillicinone (55), which was an enantiomer of the product 52 photochemically derived from 4R-illicinone A (37b), were isolated from the wood of I. tashiroi (Fukuyama et al., 1995, 1997) (Figure 4.7). Bicycloillicinone (55) should be photochemically transformed from 45-illicinone A (37a) by essentially the same way as that used for the preparation of 52. The unusual structure of 54 was elucidated by extensive two-dimensional nuclear magnetic resonance analysis and was identified by the acid-degraded product 56 and by 4,5-0-dimethyl-y-asarone (57). The relative configurations at C-2 and C-4 were set up by the bicyclo[3,2,1]octane framework, and the relative stereochemistry of the remaining chiral carbons at C-3, C-11 was proved by the NOE (nuclear Overhauser effect) results, as shown in Figure 4.8. These spectral data indicate that 54 consists of a unique stereostructure; the y-asarone acetal moiety takes an pseudoaxial orientation at the C-3 position, whereas the dimethylcarbinol group at the C-11 position lies inside the molecule. The formation of 57 may be presumably involved in a radical process, but its details have been a puzzle. It is worth noting that both compounds 54 and 55 could increase choline acetyltransferase activity at 30 pM in the culture of P10 rat septal neurons (Hatanaka et al., 1988).

Another example of prenylated C6-C3 compounds, illifunones 58-67, belongs to tetrahydrofuran type, as shown in Figure 4.9 (Yakusijin et al., 1984; Fukuyama et al., 1994; Kouno et al., 1991, 1997). Illifunones 58-67 are probably obtained from illicinones C (39) and D (40) having an epoxide ring on the prenyl group, either by concerted cyclization with cleavage of the epoxy group

SCHEME 4.1 Chemical correlation of feniculin (36a) with anisoxide (36) by consecutive thermal rearrangements

(Yakushijin et al., 1984) or by 1,4-addition of the secondary alcohol derived from ring opening of the epoxide followed by hydrolysis of the methylenedioxy ring. The relative spatial relationship between the hydroxyl group at C-4 and the dimethylcarbinol group at C-11 in illifunones can be easily predicted on the basis of the comparison of the chemical shift value for H-11, as summarized in Figure 4.9 (Fukuyama et al., 1994). Namely, H-11 in illifunones A (59) and B (61), and 12-0-methylillifunone A (60), and 2,3-dihydro-5,6-di-0-methylillifunone E (67), having an anti-hydroxyl group, appeared at a higher field than 5 4.4 ppm, whereas the presence of a syn-hydroxyl group as in illifunones C (62) and D (65), 12-chlroroillifunone C (63), 2,3-dehydroillifunone C (58), and 4,12-di-0-methylillifunone C (64) caused a significant low field shift (5 4.6~4.8 ppm) for H-11. Among them, illifunone E (66) was not consistent with this diagnostic difference value. Thus, the structure of illifunone E (66) was confirmed by dehydration of 66 with acid converting to illifunone A (59) (Fukuyama et al., 1994).

There are novel prenylated C6-C3 compounds such as illicinone E (68), as shown in Figure 4.10, which belong to a tetrahydrofurano type with a methylenedioxy group (Fukuyama et al., 1992, 1994; Kouno et al., 1997). These compounds occur so far in only I. tashiroi and I. anisatum. Taking their biosynthesis into consideration, however, other Illicium species should elaborate illicinone E-related compounds.

The absolute configuration of illicinone E (68) was established on the basis of CD exciton chirality rule (Harada and Nakanishi 1982) of allylic benzoates 81a and 81b, derived from 68 by the following ways, as shown in Scheme 4.5. Illicinone E was reduced with NaBH4-CeCl3 (Luche, 1978) to afford two alcohols, 79a and 79b, which were readily separated by silica gel chromatog-raphy. The allylic double bond in 79a and 79b was selectively hydrogenated over Wilkinson's

11 15

O
O
0 0

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