8-Hydroxyquinoline-4-carbaldehyde oxime

Only three constituents have been reported from Broussonetia zeylanica, all by a group at the University of Peradeniya in Sri Lanka [89-91]. A major alkaloid, 8-hydroxyquinoline-4-carbaldehyde (55), was identified as an antimicrobial agent active against Staphyllococus aureus and Candida albicans (the levels of activity were not specified) [89] and then two minor compounds, 3,4 -dihydroxy-2,3 -bipyridine (56) and 3,4-bis(8-hydroxyquinolin-4-yl)-y-butyrolactone (broussonetine, 57), were reported, Fig. (10) [90,91]. However, the structure of 3,4 -dihydroxy-2,3 -bipyridine (56) was revised to 8-hydroxyquinoline-4-carbaldehyde oxime (58) by synthesis [92,93]. Also, it was noted that an artefactual origin of this oxime (58) could not be ruled out due to the presence of the corresponding aldehyde (55) [93]. 

Two minor alkaloids were also isolated from B. zeylanica timber. The structure of one of these, C1oN8N202, mp 223-224°C, was revised from 3,4-dihydroxy-2,2 -bipyridine (18) to 8-hydroxyquinoline-4-carbaldehyde oxime (2) based on synthetic, NMR and nuclear overhauser effect (NOE) difference NMR spectroscopic evidence (19). Natural occurrence of oximes, although rare in higher plants, is not without precedence, and the essential oil of Ruta montana L. has been reported to contain the bis-oxime of 3,4-hexanedione (20). The structure of the nonpolar minor alkaloid broussonetine, C22H16N2O4, mp 238-239°C, was elucidated as 3,4-bis(8-hydroxyquinolin-4-yl)-y-butyrolactone (4) (21). 

Hydroxyquinoline-4-carbaldehyde (1) 8-Hydroxyquinoline-4-carbaldehyde oxime (2) 10-Hydroxystrictamine (105)... 

Quinoline-4-carbaldehyde derivatives also occur in higher plants (Table Vlll). For example, 8-hydroxyquinoline-4-carbaldehyde oxime (J.3.1 Scheme 9) was isolated from Broussonetia zeylanica (Moraceae) (151-153). (+)-Tuberosine B (J.1.2), an unprecedented tetrahydroquinoline alkaloid, has been reported in the inaccessible Chinese literature as a new metabolite of Allium tuberosum (Alliaceae) (157). Eichhornia crassipes (Pontederiaceae) is an invasive plant and often jams rivers and lakes with uncounted thousands of tons of floating plant matter. It has yielded 1,4-dimethylquinolinium iodide (J.4) (142). 

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