Structural and synthetic studies of sesquiterpenoids and flavonoids isolated from Helichrysum species
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Date
2008
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Abstract
The genus Helichrysum (Asteraceae) consists of approximately 500 species worldwide,
with 245 indigenous to South Africa. As a result of the large number of species, the
chemistry and biological activity of several species have not yet been investigated. The aim
of this project was to investigate the phytochemistry of three species and propose a
synthetic route to one of the antibacterial compounds isolated.
An extensive literature review regarding the widespread traditional uses, biological activity
and phytochemistry of the South African Helichrysum species is provided.
From Helichrysum splendidum, a plant used traditionally to treat rheumatism, two
monomeric guaianolides and a dimeric guaianolide, helisplendidilactone, were isolated.
The stereochemistry of these known compounds was confirmed and the NMR assignments
for certain peaks of helisplendidilactone were corrected. An X-ray structure for
helisplendidilactone was obtained for the first time.
The phytochemistry of Helichrysum montanum was investigated for the first time and new
diastereoisomers of known guaianolides were isolated. The phytochemistry of H.
splendidum and H. montanum is remarkably similar and supports their morphological
classification in the same taxonomic group. The chloroform:methanol extract of H.
montanum yielded a new dimeric guaianolide, 13’-epihelisplendidilactone, which is related
to helisplendidilactone, as well as three monomeric guaianolides (of which one is a new
diastereomer of a known compound). The extract also yielded spathulenol (a
sesquiterpene), umbelliferone (a coumarin) and 4’,5,7-trihydroxy-3,3’,8-trimethoxyflavone
(a flavonoid).
Thirty-five Helichrysum species were screened for antimicrobial activity against six microorganisms
and a preliminary cytotoxic assay, which included the use of “normal” and
cancer cell lines, was performed. H. excisum was selected for further study based on the
fact that it exhibited promising antimicrobial activity and relative low toxicity.
Furthermore, with the exception of the essential oil, the phytochemistry of this species has
not been investigated. From the aerial parts of H. excisum, five flavonoids, identified as pinocembrin, gnaphaliin,
lepidissipyrone, 5-hydroxy-7,8-dimethoxyflavone and isoscutellarein 7-O-b-glucoside
were isolated. Four of these flavonoids have an unsubstituted B-ring, a phenomenon often
observed in flavonoids isolated from Helichrysum species. The active antimicrobial
component of H. excisum has been identified as lepidissipyrone.
Owing to the interesting biological activities reported for phloroglucinol a-pyrones and the
synthetic challenges associated with these molecules, lepidissipyrone was selected for a
synthetic study. Both the flavanone and pyrone moieties present in lepidissipyrone have
been successfully synthesised. A successful strategy towards the CH2 linker between the
two units has been illustrated. The strategy could be used to synthesise similar
phloroglucinol-derived pyrones.
Description
Thesis (Ph.D.) - University of KwaZulu-Natal, Pietermaritzburg, 2008.
Keywords
Organic compounds--Synthesis., Medicinal plants., Hellichrysum., Natural products., Theses--Chemistry., Flavonoids.