In studying the evolution of organic chemistry and grasping its essence, one comes quickly to the conclusion that no other type of reaction plays as large a role in shaping this domain of science than carbon-carbon bond-forming reactions. The Grignard, Diels-Alder, and Wittig reactions are but three prominent examples of such processes, and are among those which have undeniably exercised decisive roles in the last century in the emergence of chemical synthesis as we know it today. In the last quarter of the 20th century, a new family of carbon-carbon bond-forming reactions based on transition-metal catalysts evolved as powerful tools in synthesis. Among them, the palladium-catalyzed cross-coupling reactions are the most prominent. In this Review, highlights of a number of selected syntheses are discussed. The examples chosen demonstrate the enormous power of these processes in the art of total synthesis and underscore their future potential in chemical synthesis.

Palladium-catalyzed cross-coupling reactions in total synthesis.

The design and implementation of cascade reactions is a challenging facet of organic chemistry, yet one that can impart striking novelty, elegance, and efficiency to synthetic strategies. The application of cascade reactions to natural products synthesis represents a particularly demanding task, but the results can be both stunning and instructive. This Review highlights selected examples of cascade reactions in total synthesis, with particular emphasis on recent applications therein. The examples discussed herein illustrate the power of these processes in the construction of complex molecules and underscore their future potential in chemical synthesis.

Cascade reactions in total synthesis.

Introduction Cationic domino reactions Anionic domino reactions Radical domino reactions Pericyclic domino reactions Photochemically induced domino processes Transition metal catalysis Domino reactions initiated by oxidation or reduction Enzymes in domino reactions Multicomponent reactions Special techniques in domino reactions

Domino Reactions in Organic Synthesis

Asymmetric Organocatalytic Cyclization and Cycloaddition Reactions

Saponins occur widely in plant species and exhibit a range of biological properties, both beneficial and deleterious. This review, which covers the literature to mid 1986, is concerned with their occurrence in plants and their effects when consumed by animals and man. After a short discussion on the nature, occurrence, and biosynthesis of saponins, during which the distinction between steroidal and triterpenoid saponins is made, the structures of saponins which have been identified in a variety of plants used as human foods, animal feedingstuffs, herbs, and flavorings are described. Many of these compounds have been characterized only during the last 2 decades, and modern techniques of isolation, purification, and structural elucidation are discussed. Particular consideration is given to mild chemical and enzymatic methods of hydrolysis and to recent developments in the application of NMR and soft ionization MS techniques to structural elucidation. Methods currently used for the quantitative analysis of saponins, sapogenols, and glycoalkaloids are critically considered; advances in the use of newer methods being emphasized. The levels of saponins in a variety of foods and food plants are discussed in the context of the methods used and factors affecting these levels, including genetic origin, agronomic, and processing variables, are indicated. Critical consideration is given to the biological effects of saponins in food which are very varied and dependent upon both the amount and chemical structure of the individual compounds. The properties considered include membranolytic effects, toxic and fungitoxic effects, adverse effects on animal growth and performance, and the important hypocholesterolemic effect. A final section deals briefly with the pharmacological effects of saponins from ginseng, since use of this plant is increasing in certain sections of western society as well as being traditional in the Orient.

The chemistry and biological significance of saponins in foods and feedingstuffs.

The biogenetic isoprene rule in its application to the triterpenes is discussed from a stereochemical standpoint. On the basis of a well defined system of arbitrary assumptions a scheme has been developed leading from squalene to the formulae of the basic representatives of all known cyclic triterpene groups - i.e. euphol, tirucallol, lupeol, taraxasterol, germanicol, β-amyrin, taraxerol, friedelin, α-amyrin, lanosterol - in their full structural and configurational detail. This result is considered to support the squalene hypothesis of the biogenesis of cyclic triterpenes.

Zur Kenntnis der Triterpene. 190. Mitteilung. Eine stereochemische Interpretation der biogenetischen Isoprenregel bei den Triterpenen

Treatment of 3β-acetoxy-20β-hydroxy-5α-pregnane (11) with lead tetraacetate in benzene allows the selective introduction of an oxygen function at the no11 activated angular methyl group C-18. The main product of this reaction is the 18,20β-oxido compound I11 as proved by its conversion into the lactone V and the lactol VI.

Über Steroide und Sexualhormone. 211. Mitteilung. Direkte Einführung einer Sauerstoffunktion in die Methylgruppe C-18 im intakten Steroidgerüst

Structures III and XXV have been derived on the basis of degradative work and physical evidence for Farnesiferol B and C, two isomeric C24H30O4 compounds previously isolated from Asa foetida (Umbelliferae).

Zur Kenntnis der Sesquiterpene und Azulene. 127. Mitteilung. Über die Inhaltsstoffe der Asa foetida II. Farnesiferol B und C

The influence of the environment of the hydroxyl group on the course of the oxidation of 20-hydroxy-steroids with lead tetraacetate has been investigated. It could be shown that, whenever the normal reaction — ether formation between C-18 and C-20 — is inhibited by steric factors, fragmentation processes occur, resulting in the formation of 17α-acetoxy compounds and of saturated ring D unsubstituted compounds respectively.

Über Steroide und Sexualhormone. 221. Mitteilung. Weitere Oxydationsversuche von 20-Hydroxysteroiden mit Blei(IV)-acetat

Es wird ein einfacher, praparativ ergiebiger und allgemein anwendbarer Weg zur Synthese von 11-Keto-Steroiden beschrieben. Dieser ist auch auf solche Steroide anwendbar, die im Ring C keine Sauerstoffunktion vorgebildet enthalten. So ist es zum Beispiel gelungen, ausgehend von Δ7;22-3β-Acetoxy-ergostadien („α”-Dihydro-ergosterinacetat, VII) mit Hilfe einfacher und ubersichtlicher Reaktionen (VII VIII IX XIII XIV bzw. XV) das Δ22-3β-Acetoxy-7,11-diketo-ergosten (XVI) zu bereiten. In diesem liess sich die reaktionsfahige Carbonyl-Gruppe in der Stellung 7 muhelos selektiv entfernen. Als Endprodukte der Versuche in der Ergosterin-Reihe werden das Δ22-3β-Acetoxy-11-keto-ergosten (XVII) und das gesattigte 3β-Acetoxy-11-keto-ergostan (XIX) beschrieben.

O. Jeger

Papers

Zur Kenntnis der Triterpene. 190. Mitteilung. Eine stereochemische Interpretation der biogenetischen Isoprenregel bei den Triterpenen

Über Steroide und Sexualhormone. 211. Mitteilung. Direkte Einführung einer Sauerstoffunktion in die Methylgruppe C-18 im intakten Steroidgerüst

Zur Kenntnis der Sesquiterpene und Azulene. 127. Mitteilung. Über die Inhaltsstoffe der Asa foetida II. Farnesiferol B und C

Über Steroide und Sexualhormone. 221. Mitteilung. Weitere Oxydationsversuche von 20-Hydroxysteroiden mit Blei(IV)-acetat

Über Steroide und Sexualhormone. 176. Mitteilung. Ein neuer Weg zur Synthese von 11‐Keto‐Steroiden