| dc.description.abstract | The thesis describes the chemistry of some commercial
natural products obtained from plants indigenous to Kenya. The
natural products are the insecticidal 'pyrethrins' obtained from
flower heads of Chrysanthemum cinerariaefolium, and the food
colourL~g pigment 'bixin', the main constituent of 'annatto'
extracted from seeds of Bixa orellana.
Chapter I first summarises the more important previous
synthetic approaches to the 'rethrolones' the alcohol portions
of natural pyrethrin esters. This is followed by a discussion
of a new synthetic approach we have developed to the rethrolones
and their analogues. The new approach has as its central theme,
a novel and facile 4-ylidenebutenolide-tocyc10pentene-1-,4-:"dione rearrangement. Thus,the 4-ylidenebutenolides (152a - c, 15Ja - c) in the presence of sodium methoxide in methanol were found to r: smoothly rearrange to the cyclopent-2-ene-l ,4-diones (154a - c).
The latter, when heated in the presence of sodium chloride L~
aqueous dimethyl sulphoxide gave rise to the 1,4-diones (155a - c),
which were then reduced usL~g zinc powder in acetic acid to the
required dihydrorethrolones (15a - c). In a similar manner, the
rearrangement of 2-methyl-J-methoxy-4-methoxycarbonylmethylidene
butenolides (16Ja - b) produced the 1,4-dione (164) which was
demethoxycarbonylated to give the 1,4-dione (165). The latter
dione (165) was protected selectively by methoxylation using methyl
orthoformate leading to the ketal (167) which was then alkenylated
by Grignard addition leading to aIlethrone (174). The allethrone
was reduced selectively to allethrolone (175).
Chapter II briefly presents the uses, chemistry and a possible
biogenetic sou=ce of bixin, the main pigment of 'a~natto'. Although
a number of chemical LDvestigations of bixin have been c~~ied out,
very little is known about the origin of this interesting 9'
cis-apocarotenoid in Nature. As a contribution to this problem
an extensive chromatographic examination of the components of the
extracts of seeds of Bixa orellana has been carried out, resulting in
theisolation and structural elucidation of the following interesting
compounds: methyl 9'-cis-apo-1-bixinal ester (218), all-trans
geranylgeraniol (219), farnesylacetone (220), all-trans geranyl
geranyl formate (221), all-trans geranylgeranyl octadecanoate (222),
and o-tocotrienol (223).
Quantitative gas liquid chromatographic analysis of the hexane
extract of Bixa seeds has incidentally revealed that the seeds are
the richest known natural source of all-trans geranylgeraniol. | en_US |
