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.

Asymmetric 1,3-Dipolar Cycloaddition Reactions

Organic chemists have always been fascinated by the cyclopropane subunit.1 The smallest cycloalkane is found as a basic structural element in a wide range of naturally occurring compounds.2 Moreover, many cyclopropane-containing unnatural products have been prepared to test the bonding features of this class of highly strained cycloalkanes3 and to study enzyme mechanism or inhibition.4 Cyclopropanes have also been used as versatile synthetic intermediates in the synthesis of more functionalized cycloalkanes5,6 and acyclic compounds.7 In recent years, most of the synthetic efforts have focused on the enantioselective synthesis of cyclopropanes.8 This has remained a challenge ever since it was found that the members of the pyrethroid class of compounds were effective insecticides.9 New and more efficient methods for the preparation of these entities in enantiomerically pure form are still evolving, and this review will focus mainly on the new methods that have appeared in the literature since 1989. It will elaborate on only three types of stereoselective cyclopropanation reactions from olefins: the halomethylmetal-mediated cyclopropanation reactions (eq 1), the transition metal-catalyzed decomposition of diazo compounds (eq 2), and the nucleophilic addition-ring closure sequence (eqs 3 and 4). These three processes will be examined in the context of diastereoand enantiocontrol. In the last section of the review, other methods commonly used to make chiral, nonracemic cyclopropanes will be briefly outlined.

Stereoselective Cyclopropanation Reactions

Light-Emitting Self-Assembled Materials Based on d(8) and d(10) Transition Metal Complexes.

2.4. Tandem sp-sp Coupling/Cyclization 3418 3. Cycloaddition Reactions 3418 3.1. [3+2] Cycloaddition 3418 3.1.1. Triazole/Isoxazole-Forming Reactions 3418 3.1.2. Reactions of Azomethine Ylides 3419 3.1.3. Cycloaddition of Nitrones 3422 3.1.4. Other [3+2] Reaction 3422 3.2. [4+2] Heterocycloaddition 3422 3.3. [2+2] Cycloaddition 3424 4. Cycloisomerization of Enynes/Diynes 3425 5. Intramolecular Friedel-Crafts-Type Reactions 3428 6. Reactions of R-Diazocarbonyl Compounds 3429 7. Aziridination of Olefins 3431 8. N/O-Vinylation/Arylation 3431 9. Radical Cyclization of Haloalkenes and Haloalkynes 3433

Coinage Metal-Assisted Synthesis of Heterocycles

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Metal-Assisted Cycloaddition Reactions in Organotransition Metal Chemistry

An alternative top-down concept for searching for homogeneous catalysts is introduced. In this approach, three multi-dimensional spaces are considered. These represent the catalysts, the descriptor values (e.g., cone angle, lipophilicity indices), and the figures of merit (e.g., turnover frequency, enantiomeric excess, or product selectivity), respectively. By generating and connecting these spaces, it is possible to screen virtual catalyst libraries and indicate regions in the catalyst space where good catalysts are likely to be found. The generation of the catalyst space from simple building blocks is presented and the application of this approach is demonstrated for two cases: predicting the bite angle in a library of 600 ligand-Rh complexes, and predicting the linear : branched aldehyde product ratio that these 600 catalysts would give in the hydroformylation of 1-octene. In the latter case, the model is first trained on a set of 39 octene hydroformylation reactions. The limitations and applications of this concept are discussed.

Design and Assembly of Virtual Homogeneous Catalyst Libraries –Towards in silico Catalyst Optimisation

Organotransition metal [3+2] cycloaddition reactions

Comparing structures and reactivity in analogous fe and ru complexes. (ipr-dab)fe(co)2i2 and (ipr-dab)fei2 : a perfectly reversible co-carrier system. (r-dab = n, n'-r2-1,4-diaza-1,3-butadiene)

Hans-Werner Frühauf

Papers

Metal-Assisted Cycloaddition Reactions in Organotransition Metal Chemistry

Design and Assembly of Virtual Homogeneous Catalyst Libraries –Towards in silico Catalyst Optimisation

Organotransition metal [3+2] cycloaddition reactions

Comparing structures and reactivity in analogous fe and ru complexes. (ipr-dab)fe(co)2i2 and (ipr-dab)fei2 : a perfectly reversible co-carrier system. (r-dab = n, n'-r2-1,4-diaza-1,3-butadiene)

Oxidative addition reactions of [Rhi(Br)(Tpy)] (Tpy = 4'-(4-tert-butylphenyl)-2,2':6', 2 -terpyridine) with alkyl bromides

Hans-Werner Frühauf

Papers

Metal-Assisted Cycloaddition Reactions in Organotransition Metal Chemistry

Design and Assembly of Virtual Homogeneous Catalyst Libraries –Towards in silico Catalyst Optimisation

Organotransition metal [3+2] cycloaddition reactions

Comparing structures and reactivity in analogous fe and ru complexes. (ipr-dab)fe(co)2i2 and (ipr-dab)fei2 : a perfectly reversible co-carrier system. (r-dab = n, n'-r2-1,4-diaza-1,3-butadiene)

Oxidative addition reactions of [Rhi(Br)(Tpy*)] (Tpy* = 4'-(4-tert-butylphenyl)-2,2':6', 2 -terpyridine) with alkyl bromides

Oxidative addition reactions of [Rhi(Br)(Tpy)] (Tpy = 4'-(4-tert-butylphenyl)-2,2':6', 2 -terpyridine) with alkyl bromides