Researches in polyacetylenes

More than five hundred polyacetylenes have now been isolated from plants and fungi. Bu'Lock has proposed that polyynes originate in vivo from oleate (1) via crepenynate (2).Those steps of polyacetylene biogenesis that have been established through study of the incorporation of labelled compounds into natural systems are summarised in Part I of this thesis. This section also includes a review of natural acetylenes which do not obviously evolve from oleate (1). Polyyne aldehydes (3) have often been utilised in syntheses of polyacetylenes, even though they are unstable in the condensed phase. Hexa-2,4-diynal (3; R = Me) was shown not only to polymerise but also to be converted into a C11 aldehyde of structure (4) (Part II). 3-Phenylprop-2-ynal 'dimerised' similarly on heating at 130anddeg; into both the stereoisomers of aldehyde (5). A scheme for this reaction of andalpha;,andbeta;-acetylenic aldehydes is proposed (Scheme A; R3 = H) which is consistent with other evidence, e.g. use of labelled compounds and the small yield (3%) of cross-dimer (6; R1 = Ph.Candequiv;C; R2 = Me.Candequiv;C; R3 = Me) obtained from a mixture of 5-phenylpenta-2,4-diynal and hepta-3,5-diyn-2-one. The ester (7) is a likely precursor of some short-chain polyacetylenes in line with Bu'Lock's hypothesis, and was to be prepared with radioactive label in the proximal half. Radioactive compounds which were to be isolated after administering this ester to a suitable fungus might throw light on the mode of chain shortening. Various routes suitable for the synthesis of a tritium-labelled proximal half are investigated in Part III of this thesis, for the C18 chain was to be built up by condensation of dehydromatricarianal and the Grignard reagent of an andomega;-iodoacetal (Scheme B). However, neither of the acetals (8) and (9) nor even the ether (10) would form a useful Grignard reagent. Possible reasons for this phenomenon, which prevented the synthesis of the ester (7), are discussed. A systematic search for polyacetylenes in previously untested plant families is in progress in this department. As part of this, examinations of two species of the Lobeliaceae, Lobelia syphilitica L. and Lobelia cardinalis L., and their hybrid Lobelia andtimes; vedariensis were carried out (Part IV). The last two contained the new (6-R,7-R) triol (11), but apart from this, only traces of other polyacetylenes were detected. Wyerone (12) is a natural acetylene, with antifungal properties, that is produced by the broad bean, Vicia faba L. It is of biosynthetic interest as present concepts of polyacetylene biogenesis (Bu'Lock's hypothesis) would not easily accommodate the presence of a cis-butenyl end group in such an acetylene. [1-14C] Acetate and the doubly-labelled fatty acid esters, oleate (1) and crepenynate (2) [tritium at C(9) and C(10), carbon-14 at C(9), C(10) or C(l)] were incorporated into sprouting beans. The isolations of active wyerone (12) and the closely related metabolites (13andndash;15) are described in part V. The incorporations were positive (ca. 0.04%), and tritium isotope effects were observed. Possible pathways for wyerone biosynthesis are discussed. [For the formulae, please consult the PDF.]