Some structural and photochemical studies in triterpenoids

Abstract

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.