Alkaloids biosynthetic origins

Isolation of Alkaloids Biosynthetic Origin of Alkaloids Some General Reactions Involved in the Formation of Alkaloids... 

In the last ten years, research in the field of insect defensive chemistry has made remarkable breakthroughs which would not have been possible without the advances in separation techniques, structure determination methods, and synthetic methodology. The structures of structurally complex compounds can now be determined on less than 1 mg of material, as exemplified by the hexa- and heptacyclic coccinellid alkaloids. Moreover, in-depth investigations on the biosynthetic origins of some of the defensive compounds are now made possible and will surely bring interesting data in the future. 

Of the four major classes of biochemicals (carbohydrates, proteins, nucleic acids and lipids), experiments have shown that the first three classes could have arisen through prebiotic chemistry. Although the biosynthesis of many natural products can be traced back to acetate (e.g. fatty acids, terpenes and polyketide biosynthesis) or amino acids (e.g. alkaloid biosynthesis), there are many whose biosynthetic origins are either obscure or result from a complex combination of pathways (Fig. 2). 

Parry conducted experiments that demonstrated the biosynthetic origin of cephalotaxin (1) from one molecule each of tyrosine and phenylalanine, indicating that the compound should be regarded as a modified 1-phenethyltetrahydroisoquinoline alkaloid.s... 

Alkaloids are compounds of biosynthetic origin that contain nitrogen, usually in a heterocyclic ring. These compounds are produced by plants in which they are usually present as salts of organic acids. They tend to be basic and to have a variety of physiological effects. One of the more notorious alkaloids is cocaine, and alkaloidal strychnine is a deadly poison. The structural formulas of these compounds and three other alkaloids are given in Figure 15.10. 

QUINOLIZIDINE ALKALOIDS Fig. 1 The biosynthetic origin of secondary metabolites. 

Pyridine and piperidine bases occur widely in Nature, both as alkaloids in plants and in the defence secretions of certain insects and amphibians. They frequently co-occur with tetrahydroquinolines and quinolizines with which they share common biosynthetic origins. 

The biosynthetic origin of these alkaloids is of some interest. One non-alkaloidal constituent of E. polydactylus is 2-hydroxy-6-methylacetophenone, a structural unit which may be clearly discerned in the alkaloids. This unit is very probably derived from acetate, hence the alkaloids may be formed by the condensation of a polyketomethylene chain with ornithine, as depicted in (19). If so, it would appear that elaeocarpiline and its stereoisomers precede the other alkaloids in the biosynthetic sequence aromatisation and racemisation then constitute the final steps. ... 

C. Alkaloids of Unknown Biosynthetic Origin from a Colonial Zoanthid. 237... 

Chemical investigations on a new species of a colonial zoanthid of the genus Zoanthus collected in the Bay of Bengal and from the Karachi coast of the Arabian Ocean have resulted in the isolation of a new class of alkaloids of unknown biosynthetic origin. The colonial zoanthids occur as dense mats on intertidal rocks along the coast of the Indian subcontinent. The animals can eject jets of water when they are disturbed. If the spray comes in contact with a victim s eyes, it causes tears followed by prolonged redness and pain. 

Alkaloids from colonial zoanthids such as zoanthamine, zoanthamide, 28-deoxyzoanthenamine, and others form a new class of alkaloids of unknown biosynthetic origin, although some structural features suggest a triterpenoid origin (20-23). 

Alkaloid L21, originally isolated from Lycopodium lucidulum, has been reinvestigated and assigned the trivial name, luciduline. Its composition (CigHgiNO) has been verified and its structure has been elucidated by both a degradative and an X-ray study (8). The structure of luciduline shown in 69 is unique among the alkaloids of this family and intriguing from a biosynthetic viewpoint. 

The biosynthetic origin of the styrylamine unit in 4(14)- and 5(14)-type cyclopeptide alkaloids is obvious (tyrosine). However, in the 4(13)-and 5(13)-type cyclopeptide alkaloids the P-(2-alkoxy-5-hydroxy)styr-ylamine unit cannot be attributed to tyrosine. Studies on the biosynthetic origin of the styrylamine unit would be of interest. 

Figure 7.1 Stemona alkaloid classification based on biosynthetic origins.

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