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Keywords = tandem sigmatropic rearrangements

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12 pages, 2419 KB  
Review
Recent Advances in Oxa-6π Electrocyclization Reactivity for the Synthesis of Privileged Natural Product Scaffolds
by Stéphane P. Roche
Organics 2021, 2(4), 376-387; https://doi.org/10.3390/org2040021 - 26 Oct 2021
Cited by 11 | Viewed by 6143
Abstract
The stunning advances in understanding the reactivity and selectivity principles of asymmetric pericyclic reactions have had a profound impact on the synthetic planning of complex natural products. Indeed, electrocyclizations, cycloadditions, and sigmatropic rearrangements enable synthetic chemists to craft highly functionalized scaffolds that would [...] Read more.
The stunning advances in understanding the reactivity and selectivity principles of asymmetric pericyclic reactions have had a profound impact on the synthetic planning of complex natural products. Indeed, electrocyclizations, cycloadditions, and sigmatropic rearrangements enable synthetic chemists to craft highly functionalized scaffolds that would not otherwise be possible with a similar atom-, step-, and redox-economy. In this review, selected examples from the last two decades of research (2003–2020) on tandem processes combining oxa-6π electrocyclic reactions are discussed in terms of reactivity challenges, inherent reversibility, and key structural bond formation in the assembly of natural products. A particular emphasis is given to the electrocyclic ring-closures in the tandem processes featuring Knoevenagel-type condensations, Diels–Alder cycloadditions, Stille couplings, and oxidative dearomatizations. The synthetic manifolds reviewed here illustrate how oxa-6π electrocyclizations are intimately linked to the construction of complex natural product scaffolds and have inspired a number of biomimetic syntheses in the laboratory. Full article
(This article belongs to the Special Issue Pericyclic Reactions in Organic Synthesis)
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13 pages, 1804 KB  
Article
The Cyclic Nitronate Route to Pharmaceutical Molecules: Synthesis of GSK’s Potent PDE4 Inhibitor as a Case Study
by Evgeny V. Pospelov, Ivan S. Golovanov, Sema L. Ioffe and Alexey Yu. Sukhorukov
Molecules 2020, 25(16), 3613; https://doi.org/10.3390/molecules25163613 - 8 Aug 2020
Cited by 8 | Viewed by 4213
Abstract
An efficient asymmetric synthesis of GlaxoSmithKline’s potent PDE4 inhibitor was accomplished in eight steps from a catechol-derived nitroalkene. The key intermediate (3-acyloxymethyl-substituted 1,2-oxazine) was prepared in a straightforward manner by tandem acylation/(3,3)-sigmatropic rearrangement of the corresponding 1,2-oxazine-N-oxide. The latter was assembled [...] Read more.
An efficient asymmetric synthesis of GlaxoSmithKline’s potent PDE4 inhibitor was accomplished in eight steps from a catechol-derived nitroalkene. The key intermediate (3-acyloxymethyl-substituted 1,2-oxazine) was prepared in a straightforward manner by tandem acylation/(3,3)-sigmatropic rearrangement of the corresponding 1,2-oxazine-N-oxide. The latter was assembled by a (4 + 2)-cycloaddition between the suitably substituted nitroalkene and vinyl ether. Facile acetal epimerization at the C-6 position in 1,2-oxazine ring was observed in the course of reduction with NaBH3CN in AcOH. Density functional theory (DFT) calculations suggest that the epimerization may proceed through an unusual tricyclic oxazolo(1,2)oxazinium cation formed via double anchimeric assistance from a distant acyloxy group and the nitrogen atom of the 1,2-oxazine ring. Full article
(This article belongs to the Special Issue Nitro Compounds and Their Derivatives in Organic Synthesis)
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13 pages, 6942 KB  
Article
Exploring the Scope of Tandem Palladium and Isothiourea Relay Catalysis for the Synthesis of α-Amino Acid Derivatives
by Jacqueline Bitai, Alexandra M. Z. Slawin, David B. Cordes and Andrew D. Smith
Molecules 2020, 25(10), 2463; https://doi.org/10.3390/molecules25102463 - 25 May 2020
Cited by 6 | Viewed by 4845
Abstract
The scope and limitations of a tandem N-allylation/[2,3]-rearrangement protocol are investigated through the incorporation of a variety of functional groups within an allylic phosphate precursor. This method uses readily accessible N,N-dimethylglycine aryl esters and functionalized allylic phosphates, forming quaternary ammonium salts in situ [...] Read more.
The scope and limitations of a tandem N-allylation/[2,3]-rearrangement protocol are investigated through the incorporation of a variety of functional groups within an allylic phosphate precursor. This method uses readily accessible N,N-dimethylglycine aryl esters and functionalized allylic phosphates, forming quaternary ammonium salts in situ in the presence of a palladium catalyst. Subsequent enantioselective [2,3]-sigmatropic rearrangement, promoted by the chiral isothiourea tetramisole, generates α-amino acid derivatives with two contiguous stereocenters. The incorporation of electron-withdrawing ester and amide groups gave the best results, furnishing the desired products in moderate to good yields (29–70%), with low diastereocontrol (typically 60:40 dr) but high enantioselectivity (up to 90:10 er). These results indicate that substrate–catalyst interactions in the proposed transition state are sensitive to the substitution pattern of the substrates. Full article
(This article belongs to the Special Issue New Synthetic Methods for Organic Compounds)
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18 pages, 102 KB  
Review
Claisen, Cope and Related Rearrangements in the Synthesis of Flavour and Fragrance Compounds
by Janusz Nowicki
Molecules 2000, 5(8), 1033-1050; https://doi.org/10.3390/50801033 - 23 Aug 2000
Cited by 66 | Viewed by 18090
Abstract
A review of the use of the Claisen, Cope and related [3,3]-sigmatropic rearrangements, sequential ("tandem") sigmatropic rearrangements and the "ene" reaction in the syntheses of flavour and fragrance compounds is presented. Full article
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