| 1. |
- Bah, Juho, et al.
(författare)
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Carbocations as Lewis Acid Catalysts : Reactivity and Scope
- 2015
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Ingår i: Advanced Synthesis and Catalysis. - : Wiley. - 1615-4150 .- 1615-4169. ; 357:1, s. 148-158
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Tidskriftsartikel (refereegranskat)abstract
- One class of potential Lewis acids that has received negligible attention as a catalyst is the carbocation. Here we show the potential of triarylmethylium ions as highly powerful Lewis acid catalysts for organic reactions. The Lewis acidity of the triarylmethylium ion can be easily tuned by variation of the electronic properties of the aromatic rings and the catalytic activity of the carbocation is shown to correlate directly to the level of stabilization of the empty p(C)-orbital at the cationic carbon. The versatility of triarylmethylium ions as efficient Lewis acid catalysts for organic reactions is demonstrated in Diels-Alder, aza-Diels-Alder, conjugate addition, halogenation, epoxide rearrangement and intramolecular hetro-ene reactions.
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| 2. |
- Bah, Juho, et al.
(författare)
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Carbocations as Lewis Acid Catalysts in Diels-Alder and Michael Addition Reactions
- 2014
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 20:4, s. 1066-1072
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Tidskriftsartikel (refereegranskat)abstract
- In general, Lewis acid catalysts are metal-based compounds that owe their reactivity to a low-lying empty orbital. However, one potential Lewis acid that has received negligible attention as a catalyst is the carbocation. We have demonstrated the potential of the carbocation as a highly powerful Lewis acid catalyst for organic reactions. The stable and easily available triphenylmethyl (trityl) cation was found to be a highly efficient catalyst for the Diels-Alder reaction for a range of substrates. Catalyst loadings as low as 500ppm, excellent yields, and good endo/exo selectivities were achieved. Furthermore, by changing the electronic properties of the substituents on the tritylium ion, the Lewis acidity of the catalyst could be tuned to control the outcome of the reaction. The ability of this carbocation as a Lewis acid catalyst was also further extended to the Michael reaction.
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| 3. |
- Bah, Juho, 1979-
(författare)
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Metal-Free Catalysis for Efficient Synthesis
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The strength of efficient metal-free catalysis will be examined in this thesis. Efforts towards more sustainable processes will be demonstrated through implementation of strategies that meet several of the 12 principles of Green Chemistry.In the first part, a stereoselective total synthesis of multiple alkaloids from the Corynantheine and Ipecac families together with their non-natural analogues will be disclosed. A highly efficient, common synthetic strategy is applied leading to high overall yields starting from easily available starting material. Overall operational simplicity and sustainability have been the main focus. Time-consuming and waste-generating isolations and purifications of intermediates have been minimized, as well as the introduction of protection-group chemistry. Moreover, the first example of the total synthesis of Hydroxydihydrocorynantheol together with its non-natural epimer has been accomplished in multi-gram scale without protection groups and without a single isolation or purification step in high overall yield and diastereoselectivity.In the second part, carbocations will be presented as highly effective and versatile non-metal Lewis acid catalysts. Lewis acidity-tuning of carbocations will be introduced and applied in several reactions to suppress competing reactions. Finally, the broad scope of carbocation catalyzed transformations will be exposed.At large, evident progress has been made towards more sustainable chemistry.
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| 4. |
- Naidu, Veluru Ramesh, et al.
(författare)
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Direct Organocatalytic Oxo-Metathesis, a trans-Selective Carbocation-Catalyzed Olefination of Aldehydes
- 2015
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Ingår i: European Journal of Organic Chemistry. - : Wiley. - 1434-193X .- 1099-0690. ; :8, s. 1834-1839
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Tidskriftsartikel (refereegranskat)abstract
- A direct organocatalytic carbonyl/olefin oxo-metathesis has been developed. The reaction is catalyzed by trityl tetrafluoroborate (TrBF4) and utilizes unactivated alkenes for the olefination of aromatic aldehydes to give trans -alkylstyrenes in yields of 44-85% with only acetone as the byproduct. The pronounced Lewis acidity of the carbocation results in unusual reactivity that is proposed to catalyze a stepwise [2+2] cycloaddition to give an oxetane intermediate. Fragmentation of the latter in a formal retro [2+2] reaction gives the oxo-metathesis product.
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| 5. |
- Zhang, Wei, et al.
(författare)
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A Stereodivergent Strategy for the Preparation of Corynantheine and Ipecac Alkaloids, Their Epimers, and Analogues: Efficient Total Synthesis of ()- Dihydrocorynantheol, ()-Corynantheol, ()-Protoemetinol, ()- Corynantheal, ()-Protoemetine, and Related Natural and Nonnatural Compound
- 2011
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 17:49, s. 13814-13824
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Tidskriftsartikel (refereegranskat)abstract
- Abstract: Here we present a general and common catalytic asymmetric strategy for the total and formal synthesis of a broad number of optically active natural products from the corynantheine and ipecac alkaloid families, for example, indoloACHTUNGTRENUNG[2,3-a]- and benzo[a]quinolizidines. Construction of the core alkaloid skeletons with the correct absolute and relative stereochemistry relies on an enantioselective and diastereodivergent one-pot cascade sequence followed by an additional diastereodivergent reaction step. This allows for enantio- and diastereoselective synthesis of three out of four possible epimers of the quinolizidine alkaloids that begin from common and easily accessible starting materials by using a common synthetic route. Focus has been made on excluding protecting groups and limiting isolation and purification of synthetic intermediates. This methodology is applied in the total synthesis of the natural products ()-dihydrocorynantheol, ()-hirsutinol, ()-corynantheol, ()-protometinol, ()-dihydrocorynantheal, ()-corynantheal, ()-protoemetine, ()-(15S)-hydroxydihydrocorynantheol, and an array of their nonnatural epimers. The potential of this strategy is also demonstrated in the synthesis of biologically interesting natural product analogues not accessible through synthetic elaboration of alkaloid precursors available from nature, for example, thieno[3,2-a]quinolizidine derivatives. We also report the formal synthesis of (+)-dihydrocorynantheine, ()-emetine, ()-cephaeline, ()-tubulosine, and ()-deoxytubulosine.
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