Phytoalexins and Antifungal Substances in the Mulberry Tree

Prohibitins isolated so far include eleven flavonoid derivatives, such as albanins A-E (64-68) (71-73), F (25) (74) [ = kuwanon G (55)=mor-acenin B (75)], G (26) (74) [(=kuwanon H (5 )=moracenin A (76)], H (69) (7, morusin (24) (72), kuwanons C (33) (72), and E (42) (73), and three 2-arylbenzofuran derivatives such as albafurans A-C (70-72) (78, 79) (Fig. 16). These prohibitins are obtained from the epidermis and not from the xylem tissue of the shoot, which suggests that the epidermis of the mulberry shoot plays an important role not only as a physical defense structure but also as a chemical defense against pathogens. Details concerned with the Diels-Alder type adducts albanins F (25), G (26), and H (69), will be presented later in this article. 

About thirty phytoalexins have so far been isolated from the mulberry tree. A few differences are observed in the structures of phytoalexins isolated from different parts of the mulberry tree. From the fungus-infected xylem tissue of the shoot, two stilbene phytoalexins were isolated and characterized as oxyresveratrol (73) and 4 -prenyloxyresvera-trol (74) (80). From the acetone extracts of cortex and phloem tissues of the shoot infected with Fusarium solani f. sp. mori, the following phytoalexins have been isolated, twenty-six 2-arylbenzofuran derivatives, moracins A-Z (75-100) (71, 81-83), as well as dimoracin (101) (84), a Diels-Alder type adduct composed of two 2-arylbenzofuran derivatives with isoprenoid substituents. Biogenetically these phytoalexins seem to be derived from moracin M (87) by hydroxylation, methyla-tion, and isoprenylation (Fig. 17). Structures were determined on the 

From an acetone extract of infected mulberry leaves a new phytoalexin, chalcomoracin (103) (57), was isolated. Compound 103 can be viewed biogenetically as a Diels-Alder type adduct between a chalcone and a dehydroprenyl-2-arylbenzofuran derivative. The isolation of 103 was the first report on a series of optically active Diels-Alder type adducts obtained from the mulberry tree (72). Details of its structure determination will be presented later. 

Comparison of the structures of the 2-arylbenzofuran and stilbene derivatives suggests that the latter compounds are biogenetic precursors of the former. The skeletons of the 2-arylbenzofuran and stilbene derivatives could formally be regarded as a 1,2-diphenylethane biogeneti-cally the skeleton would be derived from cinnamoylpolyketide produced from cinnamic acid and acetate (70). 

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