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- Intermolecular alkyl radical addition to methyl 2-(2,6-Dichlorophenyl)-2H-azirine-3-carboxylatePublication . Lemos, A.; Guimaraes, Emanuel; Fortes, Gil; Alves, Maria José ChãoThe 2H-azirine 1 acts as an effective radical acceptor for secondary and tertiary alkyl iodides mediated by triethylborane. The addition proceeds with high regio- and diastereo-selectivity.
- A hetero-Diels–Alder approach to functionalized 1H-tetrazoles: synthesis of tetrazolyl-1,2-oxazines, -oximes and 5-(1-aminoalkyl)-1H-tetrazolesPublication . Lopes, Susana M. M.; Lemos, A.; Melo, Teresa M. V. D. Pinho eThis work describes the first and unprecedented examples of inverse electron demand Diels–Alder reactions of 5-(1-nitrosovinyl)-1-phenyl-1H-tetrazole, generated in situ from the corresponding bromooxime, with electron rich alkenes and heterocycles, providing in good overall yields tetrazolyl- 1,2-oxazines and -oximes. Upon subsequent reduction these allowed the access to 5-(1-aminoalkyl)- 1H-tetrazoles, paving the way for a new entry into this important class of compounds, bioisosteres of a-amino acids.
- Reactions of Azovinylphosphonates with Nucleophilic Alkenes and Heterocycles: synthesis of Tetrahydropyridazine-3-phosphonate and 2-Substituted-1-hydrazonoethylphosphonate derivativesPublication . Lemos, A.; Lopes, MartaTransient azovinylphosphonates, generated in situ by base induced dehydrohalogenation of the corresponding 2-bromo- and 2-chloro-acetylphosphonate- tertbutoxycarbonyl hydrazones are intercepted by electron rich alkenes and heterocycles in hetero Diels-Alder reactions, producing tetrahydopyridazine-3-phosphonates or open chain α-hydrazono phosphonates.
- Asymmetric neber reaction in the Synthesis of Chiral 2-(Tetrazol-5-yl)-2H-AzirinesPublication . Alves, Claudia; Grosso, Carla; Barrulas, Pedro; Paixao, Jose A.; Cardoso, Ana L.; Burke, Anthony J.; Lemos, Americo; Pinho e Melo, Teresa M. V. D.A successful one-pot methodology for the synthesis of chiral 2-tetrazolyl-2H-azirines has been established, resorting to organocatalysis. The protocol involves the in situ tosylation of beta-ketoxime-1H-tetrazoles followed by the Neber reaction, in the presence of chiral organocatalysts. Among the organocatalysts studied a novel thiourea catalyst derived from 6 beta-aminopenicillanic acid afforded excellent enantioselectivities.
- Regioselectivity in hetero diels-alder reactionsPublication . Grosso, Carla; Liber, Marta; Brigas, Amadeu; Pinho e Melo, Teresa M. V. D.; Lemos, AmericoRegioselectivity in hetero Diels-Alder reactions can be observed in a simple reaction between a nonsymmetrical heterodiene and an unsymmetrical heterodienophile. A 9 h and easy to implement laboratory experiment is described, in which students can observe the regioselectivity of inverse "electron-demand" hetero Diels-Alder reactions of an azoalkene with furan or 2,3-dihydrofuran acting as dienophile. This experiment combines synthesis, structural analysis (IR spectroscopy and NMR spectroscopy), and dry- and vacuum-flash isolation methods.
- Reactions of azovinylphosphonates with nucleophilic alkenes and heterocycles: synthesis of Tetrahydropyridazine-3-phosphonate and 2-Substituted-1-hydrazonoethylphosphonate derivativesPublication . Lemos, A.; Lopes, MartaTransient azovinylphosphonates, generated in situ by base induced dehydrohalogenation of the corresponding 2-bromo- and 2-chloro-acetylphosphonate- tertbutoxycarbonyl hydrazones are intercepted by electron rich alkenes and heterocycles in hetero Diels-Alder reactions, producing tetrahydopyridazine-3-phosphonates or open chain α-hydrazono phosphonates.
- [4+2] Cycloadditions of 3-Tetrazolyl-1,2-diaza-1,3-butadienes: synthesis of 3-Tetrazolyl-1,4,5,6-tetrahydropyridazinesPublication . Lopes, Susana M. M.; Brigas, Amadeu; Palacios, Francisco; Lemos, A.; Melo, Teresa M. V. D. Pinho eThe synthesis of new 3-tetrazolyl-1,4,5,6-tetrahydropyridazines by Diels–Alder reactions of ethyl 3-(tetrazol-5-yl)-1,2- diaza-1,3-butadiene-1-carboxylates is reported. These 1,2- diaza-1,3-dienes reacted with electron-rich heterocycles, nucleophilic olefins and cumulenes, as well as with electrondeficient dienophiles, to give the target compounds regioselectively. Computational studies corroborated the rationalization of the observed reactivity and selectivity.
- Reactions of nitrosovinylphosphonates with electron-rich alkenes and heterocyclesPublication . Lemos, A.; Guimaraes, Emanuel; Lopes, MartaIn the presence of a base, chlorohydroxyiminophosphonates are converted in situ into the corresponding transient nitrosovinylphosphonates, which react as heterodienes with electron-rich alkenes and heterocycles producing oxazines and open-chain oximes in moderate yields and good selectivity. This approach may be regarded as a new strategy for the synthesis of precursors of α-amino phosphonic acids.
- Natural deep eutectic solvents in the hetero-Diels-Alder approach to bis(indolyl)methanesPublication . Grosso, Carla; Brigas, Amadeu; de los Santos, Jesus M.; Palacios, Francisco; Lemos, Americo; Pinho e Melo, Teresa M. V. D.For the first time, the use of natural deep eutectic solvents (NADES) as reaction media to carry out hetero-Diels-Alder reactions is disclosed. This allowed to improve the efficiency and sustainability of the synthetic approach to bis(indolyl)methanes (BIMs) based on bis-hetero-Diels-Alder/conjugate addition reactions of nitroso- and azoalkenes with indoles. The ternary mixture of H2O with choline chloride/glycerol allowed the tuning of the physical properties of this NADES leading to a better solvent system, affording the target hydroxyiminomethyl- and hydrazonomethyl-BIMs, in much shorter reaction times, higher efficiency and easier isolation procedures. Furthermore, the direct access to carbonyl-BIMs was possible when 3-methyl-1-tert-butoxycarbonyl azoalkenes were used. [GRAPHICS] .
- Diels–Alder reactions of 3-(1H-tetrazol-5-yl)-nitrosoalkenes: synthesis of functionalized 5-(substituted)-1H-tetrazolesPublication . Lopes, Susana M. M.; Palacios, Francisco; Lemos, A.; Melo, Teresa M. V. D. Pinho eA general route to 1,2-oxazines and open chain oximes bearing a 1H-tetrazolyl substituent via DielseAlder reaction of 3-(tetrazol-5-yl)-nitrosoalkenes is reported. It was also demonstrated that reduction of these adducts followed by deprotection of the tetrazolyl group give 5-(1-aminoalkyl)-1H-tetrazoles, a-amino acid analogues.
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