Natural Acetylenic and Olefinic Compounds

BY C. M. SCRIMGEOUR

1 Introduction

This report covers the same topics as the corresponding chapter in the previous volume, but in addition includes acetylenic and olefinic compounds of marine origin. Acetylenic compounds form the bulk of the material, the olefinic compounds being those which are not covered by other chapters. The literature covered is that for 1980-1.

2 Natural Acetylenic Compounds

2.1 Introduction.- The results reported during this two-year spell continue the pattern previously noted. A number of new compounds are described and known compounds are recorded from new sources. Two tools are increasingly used - high performance liquid chromatography (h.p.l.c.) and high field (≥ 270 MHz) nuclear magnetic resonance (n.m.r.) spectroscopy. Very little new synthetic methodology has been reported.

2.2 New Polyacetylenic Compounds from Terrestrial Plants. - The majority of new compounds have been isolated from members of the large Compositae family, and many are reported as a result of Bohlmann's continuing study of this family. Most are closely related to known structural types and their structures therefore follow easily from spectral data. The growing use of high resolution n.m.r. for this purpose is apparent. Unless otherwise indicated the species mentioned belong to the Compositae.

Two C17 hydrocarbons (1) and (2) were isolated from Leucanthemum adustum. Three sources have revealed oxygenated C17 compounds of closely related structure. The epoxides (3) and (4) were isolated from Cacosmia Pugosa and Cirsium japonicum respectively, while the mixture of diacetates (5) and (6) was found in two Ptilostemon species.

A number of new C17 compounds related to falcarinol have also been reported. The acetate (7) occurs in Viguiera incana along with dehydrofalcarinol. Panax ginseng (Araliaceae) contains the epoxide (8). This compound could also be obtained from falcarinol by epoxidation and the structure was further confirmed by synthesis. P. ginseng also contains the dial (9), and mass spectral data are reported for both (9) and the saturated dial obtained by hydrogenation. The absolute configuration of falcarindiol (10) is 3R, 8S, which was shown by examination of the dial and mono -ol mixture obtained by hydrogenation. A C15 acetate (11) with a structure related to falcarinol has been isolated from Helianthus angustifolius.

Two C16 amides (12) and (13) occur in Achillea tomentosa along with a number of related olefinic amides.

The C14 angelate ester (14) occurs in Senecio clevelandii. Interestingly, the isolation of this compound is reported along with that of the first known acetylenic monoterpene (15) and the related olefin (16). It is suggested that the co-occurrence of (14), (15) and (16) may support the assumption that the acetylenic bond is formed by dehydrogenation of a cis double bond. Lycopersicon esculentum (Solanaceae) produces (17) along with falcarindiol and falcarinol after inoculation with Cladosporium fulvum. This is believed to be the first report of polyacetylenic phytoalexins from the Solanaceae. Compound (17) has subsequently been found in tomato plants infected with Verticillium albo-atrum.

A number of new C13 compounds are reported. The isovalerate (18) occurs in some Leucanthemum species along with many other acetylenic compounds. Carlina diae contains (19) and the carlina oxide derivative (20) in addition to other known compounds including carlina oxide. This is taken as confirmation of the taxonomic reclassification of this species to the genus Carlina. Three new thiophen chlorohydrins (21) - (23) are reported from Pterocaulon virgatum, along with two known compounds of this type. The dithienylacetylene (24) has been isolated from Porophyllum ruderale, and another dithienyl compound (25) occurs in Calea pilosa.

Two new phenols C12 (26) and (27) occur in Leuoanthemum segetum. A mixture of seven new aromatic amides (28) - (34) ranging in chain length from C9 - C12 was isolated from Spilanthes alba.

A number of C10 compounds related to matricaria ester are reported. Baccharis quitensis contains (35) and (36) in addition to related known compounds. Four new matricaria ester isomers (37) - (40) were isolated from the rabbit brush, Chrysothamnus nauseosus The rabbit brush is a desert plant, known to be resistant to attack by the Colorado beetle. The extracted mixture of (37) - (40) was shown to have an anti-feedant effect on the beetle larvae and this type of naturally occurring compound is suggested as an environmentally acceptable pest control agent. Dimerostemma asperatum contains two unusual lachnophyllum esters (41) and (42). The fungus Fayodia bisphaerigera produces the methyl ester (43) and the amide linked amino acid derivatives (44) and (45) which are the first reported compounds of this type. These structures were confirmed by synthesis.

Fuller details have now been published of the monoacetylenic γ-lactones described in the previous review. The original report described these as coming from Licaria mahuba (Lauraceae) , but this plant has now been reclassified as ciinostemon mahuba.

2.3 KnownPolyacetylenic Comeounds from Terrestrial Plants. - A large number of new sources of known polyacetylenic compounds have been reported, mainly by Bohlmann and co-workers, often as an offshoot of their studies on the Compositae terpenes. These new sources are recorded in Table 1, along with the major compound types isolated from them. Known compounds which were isolated along with the compounds described in the previous section are not included in the table.

A useful method of visualising polyacetylenes on TLC plates has been described. Vanillin and p-dimethylaminobenzaldehyde were found to be the best reagents for locating a large variety of polyacetylenes. The combination of RF and the colour produced by the two spray reagents allowed twenty-five acetylenic compounds to be distinguished.

2.4 Acetylenic Compounds of Marine Origin. - A number of acetylenic compounds have been isolated from a variety of marine organisms, including molluscs,sponges and seaweeds. The nudibranch mollusc Diaulula sandiegensis contains nine chlorinated C16 acetylenes, (46) - (54). These unstable compounds were easily extracted from the intact mollusc and were separated by repeated reverse-phase liquid chromatography. The structures are based on spectroscopic data, particularly 360 MHz 1H n.m.r. spectra, and...