86 Rengyolおよびその類縁化合物の構造と反応

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Structure determination and biogenetic-type transformations of a group of novel nonaromatic C_6-C_2 natural products have been described. A crude drug, the fruits of Forsythia suspensa Vahl (Oleaceae), afforded, besides the antibacterial glycosides forsythoside A,C-E, a number of new natural alcohols and their glucosides containing the cyclohexylethane skeleton. The structures have been established by their spectral data and chemical transformations. Rengyol 1 yielded mono-, di-, and triacetate depending on the reaction conditions proving the presence of a primary, a secondary, and a tertiary hydroxyl groups. ^1H and ^<13>C NMR data indicate that 1 is symmetric and the secondary hydroxyl group is equatorial. Confirmingly, the Reformatsky reaction of 4-acetoxycyclohexanone 19 afforded two epimeric acetates 20 and 21, and the one having an equatorial acetoxyl group 20 yielded on LAH reduction a triol identical to 1. Further, dehydration of the esters 20 and 21 followed by pyrolytic deacetoxylation afforded the cyclohexa-1,3-diene derivative 27, which on photosensitized cis-dioxygenation with rose bengal, followed by reduction reactions yielded 1, establishing its stereostructure. Rengyoxide 2 afforded rengyol 1 on NaBH_4 reduction, though it existed primarily as a hemiketal form which was indicated by its IR and ^<13>C NMR spectra. Anomaly of the carbonyl group is further demonstrated by very easy dimethylketal formation on dissolving simply in MeOH. These characteristics are possibly common to 4-hydroxycyclohexanones. Rengyoside A 3 yielded rengyol 1 and glucose on hydrolysis with crude hesperidinase and the structure assigned by its NMR. Rengyolone 4, cornoside 5, and salidroside 6 of known constitutions have been characterised by their physical and spectral data. Resembrance in the structures of these constituents indicate their close biogenetic relationship, which is substantiated as follows. Condensation of 4'-hydroxyphenylethanol 42 and acetobromoglucose yielded, after alkaline hydrolysis, salidroside 6. On photosensitized oxygenation with rose bengal, in MeOH, followed by reduction with Me_2S, salidroside 6 yielded cornoside 5 in a good yield. Similar reactions with 42 or its 1-acetate 44 gave rengyolone 4 and hallerone 7, respectively. Catalytic hydrogenation of 5 with Pd-C under a forcing condition yielded rengyoxide-glucoside 9 and its dimethylketal 45. On reduction of either the former with NaBH_4, or of the latter with NaBH_3 CN, both afforded rengyoside A 3. Enzymatic hydrolysis of the glucosides 9 and 3 yielded rengyoxide 2 and rengyol 1, respectively.
天然有機化合物討論会の論文
1985-09-07

86 Rengyolおよびその類縁化合物の構造と反応

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