Alkaloids Naturally Occurring Bases

Alkaloids vary widely in structure, from the simple to the enormously complex. The odor of rotting fish, for example, is caused largely by methylamine, CH3NH2, a simple relative of ammonia in which one of the NH3 hydrogens has been replaced by an organic CH3 group. In fact, the use of lemon juice to mask fish odors is simply an acid-base reaction of the citric acid in lemons with methylamine base in the fish. 

Many alkaloids have pronounced biological properties, and a substantial number of the pharmaceutical agents used today are derived from naturally occurring amines. As a few examples, morphine, an analgesic agent, is obtained from the opium poppy Papaver somniferum. Cocaine, both an anesthetic and a central nervous system stimulant, is obtained from the coca bush Erythroxylon coca, endemic to upland rain forest areas of Colombia, Ecuador, Peru, Bolivia, and western Brazil. Reserpine, an antianxiety agent and antihypertensive, comes from powdered roots of the semitropical plant Rauwolfia serpentina. Ephedrine, a bronchodilator and decongestant, is obtained from the Chinese plant Ephedra sinica. 

The coca bush Erythroxylon coca, native to upland rain forest areas of Colombia, Ecuador, Peru, Bolivia, and western Brazil, is the source ofthe alkaloid cocaine. 

A recent report from the U.S. National Academy of Sciences estimates than less than 1% of all living species have been characterized. Thus, alkaloid chemistry today remains an active area of research, and innumerable substances with potentially useful properties remain to be discovered. 

CHAPTER 2 POLAR COVALENT BONDS ACIDS AND BASES 

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For the purposes of this book, an alkaloid is regarded as a relatively complex, organic base, occurring naturally in a plant and usually possessing marked pharmacological activity. This excludes simple, naturally Occurring bases and the biological amines, which are adequately dealt with... 

The reaction has been extensively used for the determination of the structure of naturally occurring bases (e.g. the alkaloids). However it has rather limited preparative value, even though the elimination reaction occurs without any rearrangement of the carbon skeleton, and the regioisomer which predominates in the product is the less highly substituted alkene (Hofmann rule contrast the Saytzeff rule). Such alkenes are now more usually prepared by other procedures noted below. 

Chapter 3 by S. Prabhakar and M.R. Tavares summarizes recent advances in the synthesis of Amaryllidaceae alkaloids, an important class of naturally-occurring bases and neutral compounds. The increased activity in the synthesis of these alkaloids over the last decade is undoubtedly due to the fact that certain members of this family possess interesting and useful biological properties. Many elegant syntheses, chiral and otherwise, of structures incorporating many asymmetric centres are reviewed. 

Thus, although the yields in the oxidative cyclizations of diphenolic substances and their derivatives may sometimes be lower than might be desired, this biomimetic approach constitutes a rapid and concise entry to the crinine family of alkaloids. Continued research in this area should lead to the development of improved synthetic methodology and tactics for effecting this important conversion, resulting thereby in highly useful syntheses of members of this class of naturally occurring bases. 

Under indole alkaloids we include naturally occurring bases or their derivatives which contain a true indole nucleus or a modification thereof, indoline (dihydroindole), N-acyl-indoline, indolenine, oxindole etc. 

Although plants containing alkaloids have been used by man for at least 3000 years as medicines, teas, and potions (Cordell, 1978), the compounds responsible for activity were not isolated and characterized until the 19th century. Many proved to be challenging chemical problems and the structures were resolved only in recent years. At present, there are at least 10,000 characterized alkaloids (Dalton, 1991 Wink, 1993). The exact number is difficult to estimate, because many alkaloids listed in compendia are not naturally occurring. Based on the NAPRALERT Database, almost 16,000 alkaloids have been reported from plant, animal, and marine sources (Verpoorte et al., 1991). About 20-30% of higher plants accumulate alkaloids. Perhaps 60-70% of species of the Solanaceae and Apocynaceae contain alkaloids (Hartmann, 1991 Verpoorte et al., 1991 Wink, 1993). 

The quinazoline alkaloids form a small but important group of naturally occurring bases, which were isolated from a number of different families in the plant kingdom. Witt and Bergman [21] review the chemistry of quinazoline alkaloids, viz. chiysogine, luo-tonine A, tryptanthrin, febrifugine, and rataecarpine. The only non-alkaloid naturally... 

Quinine is an alkaloid, or naturally occurring base, used to treat malaria. A 0.0015 M solution of quinine has a pH of 9.84. The basicity of alkaloids is due to a nitrogen atom that picks up protons from water in the same manna- as ammonia does. What is K l... 

Hantzsch 2 gave the name pseudo bases to those carbinols that gave salts with acids by the elimination of water and a simultaneous change of constitution. Such carbinols are common among the nitrogen heterocyclic compounds and the naturally occurring alkaloids e.g., berberine, sanguinarine, chelerythrine. 

Alkaloid (Chapter 2 Focus On) A naturally occurring organic base, such as morphine. 

Some of the potential uses of the fats and oils found in plants have been reviewed and some uses of carbohydrate-based polymers briefly discussed. Plants contain a whole variety of other chemicals including amino acids, terpenes, flavonoids, alkaloids, etc. When the potential for these naturally occurring materials are combined with the secondary products that can be obtained by fermentation or other microbial processes or by traditional chemical transformations, the array of chemicals that can readily be created from renewable resources is huge. In this section a few of the more interesting examples are considered. 

For thousands of years, humans have used plants to make medicines and dmgs. Many of the biologically active ingredients in plant extracts contain at least one nitrogen atom with a lone pair of electrons, making them weak bases. Such naturally occurring weak bases are classified as alkaloids. Modem pharmaceutical chemists have built upon nature s chemistry by synthesizing new compounds that are important medicines. 

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