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Mo-catalyzed asymmetric olefin metathesis in target-oriented synthesis: enantioselective synthesis of (+)-africanol.

ABSTRACT: Catalytic asymmetric ring-opening metathesis (AROM) provides an efficient method for the synthesis of a variety of optically enriched small organic molecules that cannot be easily prepared by alternative methods. The development of Mo-catalyzed AROM transformations that occur in tandem with ring-closing metathesis are described. The utility of the Mo-catalyzed AROM/ring-closing metathesis is demonstrated through an enantioselective approach to the synthesis of (+)-africanol.

SUBMITTER: Weatherhead GS

PROVIDER: S-EPMC395989 | biostudies-literature |

REPOSITORIES: biostudies-literature

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The significance of degenerate processes to enantioselective olefin metathesis reactions promoted by stereogenic-at-Mo complexes.

Project description:The present study provides spectroscopic and experimental evidence demonstrating that degenerate metathesis is critical to the effectiveness of this emerging class of chiral catalysts. Isolation and X-ray characterization of both diastereomeric complexes, as well as an examination of the reactivity and enantioselectivity patterns exhibited by such initiating neophylidenes in promoting ring-closing metathesis processes, are disclosed. Only when sufficient amounts of ethylene are generated and inversion at Mo through degenerate processes occurs at a sufficiently rapid rate is high enantioselectivity achieved, irrespective of the stereochemical identity of the initiating alkylidene (Curtin-Hammett kinetics). With diastereomeric metal complexes that undergo rapid interconversion, stereomutation at the metal center becomes inconsequential, and stereoselective synthesis of a chiral catalyst is not required.

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Enantioselective olefin metathesis with cyclometalated ruthenium complexes.

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| S-EPMC4183615 | biostudies-other

Enantioselective total synthesis of clavirolide C. Applications of Cu-catalyzed asymmetric conjugate additions and Ru-catalyzed ring-closing metathesis.

Project description:The first enantioselective total synthesis of clavirolide C, a member of the dolabellane family of diterpenes isolated from Pacific soft coral Clavularia viridis, is disclosed. The total synthesis features the application of chiral amino acid based ligands in Cu-catalyzed asymmetric conjugate addition (ACA) reactions and a relatively rare application of catalytic ring-closing metathesis to access an 11-membered ring structure. The total synthesis effort has spawned the development of a new protocol for NHC.Cu-catalyzed ACA of alkylaluminum reagents to beta-substituted cycloalkenones. The enantioselective clavirolide C synthesis requires 17 steps (longest linear sequence), affords the target molecule in 3.5% overall yield, and confirms the stereochemical assignment for the natural product.

| S-EPMC2628296 | biostudies-literature

Synthesis of 1,2-Dihydroquinolines via Hydrazine-Catalyzed Ring-Closing Carbonyl-Olefin Metathesis.

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Visible-Light-Controlled Ruthenium-Catalyzed Olefin Metathesis.

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| S-EPMC7097883 | biostudies-literature

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Project description:The first highly Z- and enantioselective class of ring-opening/cross-metathesis reactions is presented. Transformations are promoted in the presence of <2 mol % of chiral stereogenic-at-Mo monoaryloxide complexes bearing an adamantylimido ligand that are prepared and used in situ. Reactions involve meso oxabicyclic substrates and afford the desired pyrans in 50-85% yield and up to >98:<2 enantiomer ratio. Importantly, the desired chiral pyrans thus obtained bear a Z olefin either exclusively (>98:<2 Z/E) or predominantly (>or=87:13 Z/E).

| S-EPMC2681245 | biostudies-other

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| S-EPMC2600727 | biostudies-literature

Catalyst Behavior in Metal-Catalyzed Carbonyl-Olefin Metathesis.

Project description:Iron(III)-catalyzed carbonyl-olefin ring-closing metathesis employs reactivity not typically observed in Lewis acid-catalyzed reactions. In converting a ketone with a pendant olefin into a cycloalkene and a simple carbonyl byproduct, the reaction requires the Lewis acid catalyst to differentiate between the carbonyl of the substrate and that of the byproduct. It is necessary to determine how this solution interaction imparts the desired reactivity to best employ this method. Herein, we report detailed kinetic, spectroscopic, and colligative measurements applied toward the identification of the solution structures of the active Fe(III) and Ga(III) carbonyl-olefin metathesis catalysts. These data are consistent with formation of Lewis acid-carbonyl pairs for both metal systems under stoichiometric conditions. However, they diverge in the presence of higher equivalents of carbonyl, with Fe(III) forming highly ligated complexes, and no observed change for Ga(III). These findings are consistent with the resting state identity of the Fe(III) metathesis catalyst changing over the course of the reaction.

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Project description:Synthesis of complex bioactive molecules is substantially facilitated by transformations that efficiently and stereoselectively generate polyfunctional compounds. Designing such processes is hardly straightforward, however, especially when the desired route runs counter to the inherently favored reactivity profiles. Furthermore, in addition to being efficient and stereoselective, it is crucial that the products generated can be easily and stereodivergently modified. Here, we introduce a catalytic process that delivers versatile and otherwise difficult-to-access organoboron entities by combining an allenylboronate, a hydride, and an allylic phosphate. Two unique selectivity problems had to be solved: avoiding rapid side reaction of a Cu-H complex with an allylic phosphate, while promoting its addition to an allenylboronate as opposed to the commonly utilized boron-copper exchange. The utility of the approach is demonstrated by applications to concise preparation of the linear fragment of pumiliotoxin B (myotonic, cardiotonic) and enantioselective synthesis and structure confirmation of netamine C, a member of a family of anti-tumor and anti-malarial natural products. Completion of the latter routes required the following noteworthy developments: (1) a two-step all-catalytic sequence for conversion of a terminal alkene to a monosubstituted alkyne; (2) a catalytic SN2'- and enantioselective allylic substitution method involving a mild alkylzinc halide reagent; and (3) a diastereoselective [3+2]-cycloaddition to assemble the polycyclic structure of a guanidyl polycyclic natural product.

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Catalytic, enantioselective synthesis of 1,2-anti-diols by asymmetric ring-opening/cross-metathesis.

Project description:An enantioselective method for the synthesis of 1,2-anti-diols has been developed. A cyclometalated chiral-at-ruthenium complex catalyzes the asymmetric ring-opening/cross-metathesis of dioxygenated cyclobutenes, thus resulting in functionally rich synthetic building blocks. Syntheses of the insect pheromone (+)-endo-brevicomin and monosaccharide ribose demonstrate the synthetic utility of the 1,2-anti-diol fragments generated in the title reaction.

| S-EPMC4037230 | biostudies-literature

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