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.

A series of (NHC)Pd(R-allyl)Cl complexes [NHC:  IPr = N,N‘-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, SIPr = N,N‘-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene; R = H, Me, gem-Me2, Ph] have been synthesized and fully characterized. When compared to (NHC)Pd(allyl)Cl, substitution at the terminal position of the allyl scaffold favors a more facile activation step. This translates into higher catalytic activity in the Suzuki−Miyaura and Buchwald−Hartwig reactions, allowing for the coupling of unactivated aryl chlorides at room temperature in minutes. In the Suzuki−Miyaura reaction, aryl triflates, bromides, and chlorides react with boronic acids using very low catalyst loading. In the N-aryl amination reaction, a wide range of substrates has been coupled efficiently; primary-, secondary-, alkyl-, or aryl-amines react in high yields with unactivated, neutral, and activated aryl chlorides and bromides. In both reactions, extremely hindered substrates such as tri-ortho-substituted biaryls and tetr...

Modified (NHC)Pd(allyl)Cl (NHC = N-heterocyclic carbene) complexes for room-temperature suzuki-miyaura and buchwald-hartwig reactions

Die Entwicklung und Umsetzung von Kaskadenreaktionen hat zu beachtlichen neuartigen, eleganten und effizienten Synthesestrategien gefuhrt. Als besonders anspruchsvoll erwies sich die Anwendung von Kaskadenreaktionen in der Naturstoffsynthese, doch gerade hier winken verbluffende und zugleich aufschlussreiche Ergebnisse als Belohnung. In diesem Aufsatz werden ausgewahlte Kaskadenreaktionen in der Totalsynthese erlautert, wobei neuere Anwendungen besonders hervorgehoben werden. Die erorterten Beispiele sollen die Leistungsfahigkeit dieser Prozesse beim Aufbau von komplexen Molekulstrukturen veranschaulichen und ihr Potenzial fur zukunftige chemische Synthesen verdeutlichen.

Kaskadenreaktionen in der Totalsynthese

A main issue in modern synthetic organic chemistry, which deals with the preparation of natural products, pharmaceuticals, diagnostics, agrochemicals, and other important materials, is the improvement of efficiency, the avoidance of toxic reagents, the reduction of waste, and the responsible treatment of our resources. One of the ways to fulfill these goals is the development and use of domino processes, which consist of several bond-forming reactions and which allow the highly efficient synthesis of complex molecules starting from simple substrates. Herein, the combination of several catalytic bond-forming transformations is clearly most appropriate. The synthesis of the enantiopure alkaloid (-)-hirsutine 22, which has a strong inhibitorial effect on influenza A viruses, was accomplished using a biomimetic domino Knoevenagel-hetero-Diels Alder-solvolysis-hydrogenation process. In a similar way the alkaloids (+)-dihydrocorynantheine 23 and (-)-dihydroantirhine 24 as well as heterosteroids 62, D-homosteroids 65 and 68, and azasteroids 25 are prepared. In addition, novel steroid alkaloids 26 are accessible by a combination of the formation of an iminium salt, a hydride shift, and an alkylation. The anti-leukemic pentacyclic (-)-cephalotaxine 27 is obtained by a combination of two Pd-catalyzed reactions.

Multicomponent domino reactions for the synthesis of biologically active natural products and drugs.

Bifunctional α-amino nitriles are not only versatile 
intermediates in organic synthesis but also exhibit a valuable dual 
reactivity, which has been utilized in a broad range of synthetic 
applications. This review highlights recent developments in the chemistry 
of α-amino nitriles, including asymmetric synthesis of α-amino 
acids via Strecker reactions using chiral auxiliaries and 
catalysts, α-amino nitriles as masked iminium ion equivalents in 
cationic reactions and the synthesis of natural products and heterocycles, 
and α-metallation to provide nucleophilic acyl anion equivalents and 
applications to asymmetric Umpolung reactions.

Some recent applications of α-amino nitrile chemistry

Preparation and conformational study of deformyl-z-and deformyl-e-geissoschizine epimers and na-boc derivatives, and their nb-oxides

Facts, based on the measurements of the distance between C-5 and C-16, are presented to support that in the transformation of strictosidine-derived intermediates to the sarpagan and ajmalan skeletons, the bond between C-5 and C-16 has to be established before the D-ring formation.

Biogenetic formation of sarpagine/ajmaline type indole alkaloids

Publisher Summary This chapter discusses an area of indole alkaloids, which has been neglected in the series for over 30 years—namely, the ajmaline group of alkaloids, where numerous advances in chemistry and biosynthesis have been made recently. The number of known ajmaline structures has grown markedly in recent years to a present count of 77. Some of these might be artefacts and a few structures have not been convincingly determined. In addition, seven bisindole alkaloids containing at least one monomeric ajmalan unit have been isolated, increasing the total number to 84. Ajmaline alkaloids contain the polycyclic ajmalan ring system. The “biogenetic numbering” of Le Men and Taylor is used throughout the chapter. The priority sequence for the C-17 substituents in the Cahn–Ingold–Prelog system is different in the absence and presence of the COOCH, substituent at C-16. All of the ajmaline alkaloids found thus far occur in the plant family Apocynaceae. They have been recognized in the following genera Alstonia , Aspidospenna , Cabucala , Melodinus , Rauvolfia , Tonduzia (Alstonia), and Vinca .

Pirjo Hanhinen

Papers

Preparation and conformational study of deformyl-z-and deformyl-e-geissoschizine epimers and na-boc derivatives, and their nb-oxides

Biogenetic formation of sarpagine/ajmaline type indole alkaloids

The Ajmaline Group of Indole Alkaloids

Preparation and Determination of Stereochemical Properties of Epimeric Z-Geissoschizine and 19,20-Dihydrogeissoschizine Acetals

Synthesis of ()-cathenamine and ()-19-epicathenamine, isolated as their c-21-alpha -cyano derivatives