Lignans

Illicium was classified in the family Magnoliaceae in much of the early taxonomic literature. The chemical constituents of the genus of Magnolia are characterized by the presence of a series of unusual biphenyl-type neolignans. Thus, it is natural to search for biphenyl-type neolignans in ho.

SCHEME 4.3 Thermal reaction of (4Ä)-ilIicmone A (37b)

52 (24%)

37b hv

30 min o

SCHEME 4.4 Photochemical reaction of (4Ä)-ilIicinone A (37b)

Illicium plants. In fact, Illicium species were found to elaborate biphenyl-type neolignans as follows: magnolol (82), 3-methoxymagnolol (83), and dehydrodieugenol (84) from the pericarps of I. dunnianum (Sy et al., 1997; Kouno et al., 1991); isomagnolol (85) and isomagnolone (86) from the barks of I. simonsii (Kouno et al., 1994); and verimol G (87) and verimol H (88) from I. verum o o

HO

ohc VA O

ohc VA O

och3

och3

FIGURE 4.7 Bicycloillicinone asarone acetal (54) and bicycloillicinone (55) isolated from I. tashiroi

(Sy and Brown 1998) (Figure 4.13). In addition to these neolignans, another neolignan and its glycosides (89-98) were obtained from the pericarps of I. majus and the barks of I. difengpi (Kouno et al., 1991, 1993; Huang et al., 1996) (Figure 4.14). The configurations of C-7 and C-8 of 90-98 have not been determined, and Kouno et al. (1991) suggested that the configurations of 97 and 98 might be the same as that of Theander's compound (Theander et al., 1985) because of their nearly similar optical rotations. Moreover, some small signals in the :H-NMR of 91 and 92 indicated that they could be obtained as a mixture of diastereoisomers, respectively.

The most fascinating lignans, regarded as characteristic constituents of Illicium plants, are a series of triphenyl-type neolignans, called sesquineolignans (Figure 4.15). The first example is macranthol (99), from the pericarps of I. macranthum, which is presumably formed by both ortho, ortho- and ortho, para-oxidative coupling of three 4-allylphenol chavicol (Kouno et al., 1989). Because macranthol (99) was found in Illicium macranthum, some triphenyl-type neolignans, dunnianol (100), isodunnianol (101), and simonsinol (102), were isolated from the bark of I. dunnianum (Kouno et al., 1991) and I. simonsii (Kouno et al., 1994). Another sesquineolignan (103) has been isolated from the aerial part of I. dunnianum (Sy and Brown, 1996; Sy et al., 1997).

The compound 103 is of biosynthetic interest because it is derived from three 4-allylphenol-chavicol units linked together by both ortho, ortho-coupling and ortho, para-coupling. The first ortho, ortho-linkage occurs by standard oxidative coupling of the two molecules' chavicol radical (105), yielding magnolol (82). The second ortho, para-coupling (Scheme 4.7) then arises from oxidative coupling of magnolol radical (104), with a further molecule of chavicol leading to the formation of a trimer, which is unable to regain aromaticity at the central ring, thereby resulting in the formation of the second ether linkage between the central and right-hand rings by intramolecular Michael-type addition of the hydroxyl anion to the 5,6-double bond of the central ring (Sy and Brown, 1996).

Chinese Ladder

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