The natural pyrethrins and carotenoid pigments

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.