Some structural and photochemical studies in triterpenoids
Permanent link to Research Commons versionhttps://hdl.handle.net/10289/14905
This thesis describes the photochemical behaviour of 22-hydroxyhopan-7-one, hop-22(29)-en-7-one, 22-hydroxyhopan-15-one, hop-21-en-15-one, hop-22(29)-en-15-one, hopan-15-one and 21α-H-hopan-15-one in different solvents, and the isolation and structural elucidation of a new stictane and a new flavicane triterpene from the New Zealand lichen Pseudocyphellaria colensoi. Photolysis of 22-hydroxyhopan-7-one in benzene-methanol was reinvestigated and confirmed to afford methyl 22-hydroxy-7,8-secohopan-7-oate. The photochemical behaviour of 22-hydroxyhopan-7-one and hop-22(29)-en-7-one in the presence of benzene alone, and in benzene-isopropanol, were also investigated. Photolysis of hop-22(29)-en-7-one in benzene-methanol afforded methyl 7,8-secohop-22(29)en-7-oate while photolysis of 22-hydroxyhopan-7-one in methanol-isopropanol afforded isopropyl 22-hydroxy-7,8-secohopan-7-oate. Photolysis of 22-hydroxy-hopan-7-one in benzene alone afforded mainly unreacted starting material and some minor products, one of which was believed to be 22-hydroxy-7,8-secohop-8(26)-en-7-al. The stereochemistry at C-8 for these compounds was determined by analyses of ROESY or NOESY NMR spectral data which showed the C-8 methyl group to be β-oriented, and indicated that photochemical rearrangement reactions proceeded with retention of configuration at C-8. Modelling analyses indicated the 8β-methyl secoesters to have lower energy than the corresponding 8α-methyl isomers. Complete assignments of the ¹H and ¹³C NMR signals of methyl 22-hydroxy-7,8-secohopan-7-oate, isopropyl 22-hydroxy-7,8-secohopan-7-oate and methyl 7,8-secohop-22(29)-en-7-oate, were achieved using a combination of one- and two-dimensional NMR techniques. The three products obtained on photolysis of 22-hydroxyhopan-15-one, in benzene-methanol, benzene-isopropanol, or benzene alone, were identified as 14,15-seco-15,22-O-abeohop-14(27)-en-15α-ol, 14,15-seco-15,22-O-abeohop-14(27)-en-15-one, and 14,15-seco-15,22-O-abeohopa-14(27), 15-diene, respectively. These products can be envisaged as arising from participation of the 22-hydroxyl group in the photoreaction pathway, after the initial formation of a 14,15-secoaldehyde. Photolysis of hop-21-en-15-one in benzene-methanol afforded 14,15-secohopa-14(27),21-dien-15-al as the major product, while photolysis of hop-22(29)-en-15-one gave a complex mixture of products which was not further investigated. Photolysis of hopan-15-one yielded two products which were identified as the isomeric alcohols, 14,15-seco-15,22-abeohop-14(27)-en-15α-ol and 14,15-seco-15,22-abeohop-14(27)-en-15β-ol. Photolysis of 21α-H-hopan-15-one afforded two products, which were identified as methyl 14α,21α-H-14,15-secohopan-15-oate and methyl 21α-H-14,15-secohopan-15-oate. Complete assignments of the ¹H and ¹³C NMR signals of the foregoing series of 15-oxohopanoid photoreaction products, and where appropriate the stereochemistry at C-14 and/or C-15, were achieved using a combination of one- and two-dimensional ROESY, NOESY, COSY, HMBC, and HSQC NMR spectral data, and molecular modelling results. The results of the series of 15-oxohopanoid photoreactions can be interpreted indicating that in the presence of an α-oriented side chain (ie 22-hydroxyisopropyl group or isopropyl group) at C-21, the photoreaction proceeds with hydrogen transfer via an eight-membered transition state to afford a secoaldehyde or 14,15-secohop-4(27)-en-15-al which then affords 14,15-seco-15,22-O-abeohop-14(27)-en-15α-ol via participation of the 22-hydroxyl group and related analogues. Similarly the isomeric alcohols, 14,15-seco-15,22-abeohop-14(27)-en-15α-ol and 14,15-seco-15,22-abeohop-14(27)-en-15β-ol, obtained on photolysis of hopan-15-one can be viewed as 22-H participation products. On the other hand photolysis of a 15-oxohopanoid possessing a β-oriented side chain at C-21 was found to proceed with hydrogen transfer via a six-membered transition state to afford a pair of epimeric 14,15-secoesters. A new stictane triterpene, and a new flavicane triterpene were isolated from extracts of the New Zealand lichen Pseudocyphellaria colensoi, and identified by analyses of mass spectral and one- and two-dimensional NMR spectral data to be 3α,6β,22α-triacetoxystictane and 22-hydroxyflavicano-25,3β-lactone, respectively. Complete assignments of the ¹H ¹³C NMR signals of the new triterpenes, and revisions to previously reported ¹³C NMR signal assignments for 22α-hydroxystictano-25,3β-lactone are reported.
The University of Waikato
All items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.
- Higher Degree Theses