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      A Short Synthesis of (±)‐3‐Demethoxyerythratidinone by Ligand‐Controlled Selective Heck Cyclization of Equilibrating Enamines

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          Abstract

          A short, 5‐step total synthesis of (±)‐3‐demethoxyerythratidinone from a simple pyrrole derivative is described. Features include the formation of gram quantities of a key tricylic aziridine from a challenging photochemical cascade reaction through the use of flow photochemistry. The final step involved a highly unusual Heck cyclization whereby ligand control enabled efficient formation of the natural product in 69 % yield from the minor isomer present in an equilibrating mixture of labile enamines.

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          Most cited references 37

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          Batch versus flow photochemistry: a revealing comparison of yield and productivity.

          The use of flow photochemistry and its apparent superiority over batch has been reported by a number of groups in recent years. To rigorously determine whether flow does indeed have an advantage over batch, a broad range of synthetic photochemical transformations were optimized in both reactor modes and their yields and productivities compared. Surprisingly, yields were essentially identical in all comparative cases. Even more revealing was the observation that the productivity of flow reactors varied very little to that of their batch counterparts when the key reaction parameters were matched. Those with a single layer of fluorinated ethylene propylene (FEP) had an average productivity 20% lower than that of batch, whereas three-layer reactors were 20% more productive. Finally, the utility of flow chemistry was demonstrated in the scale-up of the ring-opening reaction of a potentially explosive [1.1.1] propellane with butane-2,3-dione.
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            In situ generated bulky palladium hydride complexes as catalysts for the efficient isomerization of olefins. Selective transformation of terminal alkenes to 2-alkenes.

            Application of an in situ generated bulky palladium(II) hydride catalyst obtained from a 1:1:1 mixture of Pd(dba)(2), P(tBu)(3), and isobutyryl chloride provides an efficient protocol for the isomerization and migration of a variety of olefins. In addition to the isomerization of (Z)- to (E)-olefins, the conjugative migration of allylbenzenes, allyl ethers, and amines was effectively achieved in near-quantitative yields and with excellent functional group tolerance. Catalyst loadings in the range of 0.5-1.0 mol % were typically applied, but even loadings as low as 0.25 mol % could be achieved when the reactions were performed under neat conditions. More interestingly, the investigated catalyst proved to be selective for converting terminal alkenes to 2-alkenes. This one-carbon migration process for monosubstituted olefins provides an alternative catalyst, which bridges the gap between the allylation and propenylation/vinylation protocols. Several substrates, including homoallylic alcohols and amines, were selectively transformed into their corresponding 2-alkenes, and examples using enantiomerically enriched substrates provided products without epimerization at the allylic stereogenic carbon centers. Finally, some mechanistic investigations were undertaken to understand the nature of the active in situ generated Pd-H catalyst. These studies revealed that the catalytic system is highly dependent on the large steric demand of the P(tBu)(3) ligand. The use of an alternative ligand, cataCXium PinCy, also proved effective for generating an active catalyst, and it was demonstrated in some cases to display better selectivity for the one-carbon shifts of terminal olefins. A possible intermediate involved in the preparation of the active catalyst was characterized by its single-crystal X-ray structure, which revealed a monomeric tricoordinated palladium(II) acyl complex, bearing a chloride ligand.
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              Concise synthesis of the Erythrina alkaloid 3-demethoxyerythratidinone via combined rhodium catalysis.

              The total synthesis of the erythrina alkaloid 3-demethoxyerythratidinone has been achieved via a strategy based on combined rhodium catalysis. The catalytic tandem cyclization effected by the interplay of alkynyl and vinylidene rhodium species allows for efficient access to the A and B rings of the tetracyclic erythrinane skeleton in a single step. The synthesis also features rapid preparation of the requisite precursor for the double ring closure and thus has been completed in only 7 total steps in 41% overall yield.
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                Author and article information

                Contributors
                http://www.chm.bris.ac.uk/org/bmilburn/index2.htm
                k.booker-milburn@bristol.ac.uk
                Journal
                Angew Chem Int Ed Engl
                Angew. Chem. Int. Ed. Engl
                10.1002/(ISSN)1521-3773
                ANIE
                Angewandte Chemie (International Ed. in English)
                John Wiley and Sons Inc. (Hoboken )
                1433-7851
                1521-3773
                04 May 2017
                01 June 2017
                : 56
                : 23 ( doiID: 10.1002/anie.v56.23 )
                : 6613-6616
                Affiliations
                [ 1 ] School of ChemistryUniversity of Bristol Cantock's Close Bristol BS8 1TSUK
                Article
                ANIE201701775
                10.1002/anie.201701775
                5488232
                28470985
                © 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

                This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

                Page count
                Figures: 5, Tables: 1, References: 46, Pages: 4, Words: 0
                Product
                Funding
                Funded by: Engineering and Physical Sciences Research Council
                Award ID: EP/G036764/1
                Categories
                Communication
                Communications
                Natural Product Synthesis
                Custom metadata
                2.0
                anie201701775
                June 1, 2017
                Converter:WILEY_ML3GV2_TO_NLMPMC version:5.1.2 mode:remove_FC converted:28.06.2017

                Chemistry

                cycloaddition, total synthesis, photochemistry, ligands, heck reaction

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