Natural Acetylenic and Olefinic Compounds excluding Marine Natural Products

BY V. THALLER

1 Introduction and Scope of the Chapter

This Report covers developments in the chemistry of natural acetylenes and olefins during 1976 – 7 and can be regarded as part-successor to G. Pattenden's Chapters 3 in the Specialist Periodical Reports on Aliphatic Chemistry Volumes 2 – 5 for the years 1972 – 5. Whilst the scope of the acetylene part is reduced only by the omission of the relevant marine products, most of the olefin part is covered now by other chapters of this volume. The material discussed has been collected for ease of orientation under several sub-headings, although this sometimes leads to repetition of references and double citations.

2 Natural Acetylenic Compounds

Introduction. — The very rapid expansion which has occurred in this field since the early fifties (7 compounds were known in 1950, over 700 are known today) has slowed down considerably. By far the greatest number of natural acetylenes belongs to the group of secondary metabolites known as natural polyacetylenes. They have been shown to originate in Nature from C18 fatty acids by pathways which include a combination of desaturation, chain shortening, oxidation, rearrangement, cyclization, and other funtionalization processes. The many imaginable permutations have certainly not been exhausted, but their detection and isolation are more sporadic. Although the general principles of polyacetylene biogenesis are thought to be understood, many details still wait to be confirmed, not least the biogenesis of the triple bond itself. The reason for polyacetylene formation, their role and fate in the economy of the plants, higher and lower, which produce them, still needs elucidation.

Bohlmann, who with his co-workers contributed more than any other single group to the development of the natural acetylene field, always conscious of the chemotaxonomic implications of his work, has moved even further in this direction. He is now involved in a more general analysis of the secondary metabolites of species, mostly Compositae, the classification of which leaves open some doubts and investigates the contribution chemotaxonomy can make to their reclassification. Thus Bohlmann's long polyacetylene series increased during the two years under review by only eight publications (parts 237 – 244), but some information on polyacetylenes appears continuously in his series on natural terpenes, coumarins, etc.

The book on 'Naturally Occurring Acetylenes' forms an invaluable basis for the study of this field. Pattenden's reports have been keeping it up-to-date, and this Report continues this trend.

New Polyacetylenes from Plants. — With the exception of the Anonaceae species and some Korean Umbelliferae species, which were screened for polyacetylenes, all plants investigated belong to various tribes of the Compositae family. The isolation of the sixteen C21 diynes and diynenes (1) – (4) has been reported from the roots and, to a lesser extent, the bark of the tree Alphonsea ventricosa Hook F. and Th. (Anonaceae family), a native of Assam. The discovery of C21 poly acetylenes, the longest poly acetylene chain found thus far in Nature, in the Anonaceae species has already been mentioned, but details of the structures were not available. These consist of all possible permutations of four distal C6 fragments (R1) and four proximal oxygenated C4 fragments (R2) around a central C11 diyne. The root extract was readily separated into the four groups (la – d), (2a – d), (3a – d), and (4a – d), and of these only the first could be further separated into (1a,b) and (1c,d). 1H n.m.r., aided by shift reagents, and low and high resolution mass spectra indicated structures for the metabolites present in each group. A series of chemical transformations which were carried out on the mixtures enabled the separation and identification of the four distal C6 fragments, and interconversions of the mixtures established their relationships firmly. Pathways for the formation of these polyacetylenes have been proposed: the preferred one is a C18 + C3 [right arrow] C21 chain elongation involving the condensation of the known (Santalaceae and Olacaceae) C18 acids (5a – d) with pyruvate. The analogy in the structure of the oxygenated proximal part of the C21 metabolites with those of the C17 metabolites from Persea gratissima seeds, the Lauraceae and Anonaceae being relatively closely related, stimulated the authors to speculate about a similar C14 + C3 pathway for the C17 compounds.

The presence of thirty nine C13 – C17 poly acetylenes was established in the roots and green parts of eight representatives of the genus Dahlia. All contained varying amounts of poly acetylenes, but only the C13 hydrocarbon (6) was found to be present in all eight of them. The new C16 alcohol (7) and its acetate (8) were isolated from D. scapigera (A. Dietr.) Link and Otto var. scapigera f. scapigera; the alcohol was synthesized. The new C13 diol (9) and its diacetate (10), the C14 diol (11) and the diacetate (12), and the C17 alcohol (13) and the corresponding acetate (14) were isolated from the garden varieties. The spectra of the metabolites and simple transformation products established their structures. The alcohol (13) was synthesized.

Two more benzenoid polyacetylenes have been isolated from Artemisia capillaris Thunb. Norcapillen (15) was found to represent 0.1% of its essential oil and neocapillen (16) was identified as a minor component in the root extract. Spectral analysis, hydrogenation, and synthesis were used in both instances.

Known polyacetylenes were isolated from A. monosperma (from Israel) and A. dracunculus L. (seeds from Tashkent). In the roots of the latter, the acetylenic isocoumarin (17) isolated from other A. dracunculus specimens gave place to the olefin (18), an occurrence which might be of biogenetic interest.

A few of Bohlmann's publications appear to represent the rounding off of his intensive investigation of South African Compositae. Thus, amongst several known thiophene acetylenes, the new thiophene diol (19) was isolated from Platicarpa glomerata: its structure followed from spectral data obtained from the metabolite and its MnO2-oxidation product. The majority of species Bohlmann has analysed recently seem, however, to originate in the Americas. Several new thiophene acetylenes (terthienyls are included on biogenetic grounds) were found here too and identified by...