The Conservation of Orbital Symmetry

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

Die Erhaltung der Orbitalsymmetrie

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

The constitution of limonin

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

From the non-volatile, neutral fraction of Asa foedita (Umbelliferae) 3 isomeric compounds C24H30O4, designated farnesiferol A, B, and C, have been isolated. Farnesiferol A (I) is a mixed ether of umbelliferone with a sesquiterpene moiety of the bicyclofarnesylic type. It might arise biogenetically by oxidative cyclisation of the known umbelliprenine (XXXII).

Zur Kenntnis der Sesquiterpene und Azulene. 126. Mitteilung. Über die Inhaltsstoffe der Asa foetida I. Farnesiferol A

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).

D. Arigoni

Papers

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

The constitution of limonin

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

Zur Kenntnis der Sesquiterpene und Azulene. 126. Mitteilung. Über die Inhaltsstoffe der Asa foetida I. Farnesiferol A

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