Contributors: Kühl, Jochen, Reggelin, Michael
... Abstract In this study, two synthetic methods for the synthesis of a hitherto unknown class of heterocyclic diastereo- and enantiopure S,O-acetals are described. Method A involves a chemoselective monohalogenation of sulfoximines and method B a stereoselective ring opening of sulfonimidates with a carbenoid as the key step, both followed by a base-induced cyclization of the S-(halomethyl)sulfoximine intermediates. The absolute configuration of the resulting 3-oxo-1,3λ6,4-oxathiazines has been confirmed by X-ray structural analysis. Furthermore, the first experiments exploring the reactivity of the new compounds are described.
Conditions and Edges for the Photochemical Generation of Short-Lived Aryl Cations: A Computational Approach
Contributors: Raviola, Carlotta, Ravelli, Davide, Protti, Stefano, Albini, Angelo, Fagnoni, Maurizio
... Abstract A DFT investigation on the conditions required for the photochemical generation of triplet aryl cations from differently para-substituted aryl halides has been carried out. The results showed a correlation between the energy change associated with the Ar–X bond heterolytic cleavage and the substituent Hammett–Brown constant (σp +). The method provides a means to predict the viability of the ArSN1 photosubstitution reaction.
Bifunctional (Thio)urea–Phosphine Organocatalysts Derived from d-Glucose and α-Amino Acids and Their Application to the Enantioselective Morita–Baylis–Hillman Reaction
Contributors: Gergelitsová, I., Tauchman, J., Císařová, I., Veselý, J.
... Abstract Novel (thio)urea–tertiary phosphines were developed for use as bifunctional organocatalysts readily available from naturally occurring molecules: saccharides and amino acids. The efficiency of the organocatalysts was demonstrated in the asymmetric Morita–Baylis–Hillman (MBH) reaction of aromatic aldehydes with acrylates. The MBH products were obtained in good yields (up to 85%) and with high enantioselectivities (up to 87% ee).
Bisallylation of Zirconacyclopentenes and Ring-Closing Metathesis: A Route to Eight-Membered-Ring Compounds
Contributors: Topolovčan, Nikola, Panov, Illia, Kotora, Martin
... Abstract The cyclization of various enynes with the Negishi reagent provides the corresponding bicyclic zirconacyclopentenes, which after exposure to allyl chloride in the presence of a catalytic amount of copper(I) chloride, undergo bisallylation to furnish 1,9-decadienes in good yields (42–89%). The dienes are subjected to ring-closing metathesis to afford bicyclic compounds with [6.3.0]bicycloundecane (8,5-fused ring system) or [6.4.0]bicyclododecane (8,6-fused ring system) frameworks in good to excellent yields (52–92%). Selective monoallylation of a selected zirconacyclopentene followed by carboxyethylation with ethyl chloroformate gives rise to the corresponding ester, which after metalation and reaction with allyl bromide, furnishes a 1,9-decadiene. Ring-closing metathesis then yields the expected eight-membered cyclic product in 92% yield. This procedure constitutes a new pathway to bicyclic carbocyclic systems starting from 1,ω-enynes.
Novel One-Pot Multicomponent Strategy for the Synthesis of Pyrrolo[1,2-a]benzimidazole and Pyrrolo[1,2-a]quinoxaline Derivatives
Contributors: Georgescu, Emilian, Nicolescu, Alina, Georgescu, Florentina, Shova, Sergiu, Teodorescu, Florina, Macsim, Ana-Maria, Deleanu, Calin
... Abstract A simple one-pot multicomponent procedure has been developed for the synthesis of pyrrolo[1,2-a]benzimidazole and pyrrolo[1,2-a]quinoxaline derivatives by heating a 1:2:1 molar mixture of a benzimidazole unsubstituted on the imidazole ring, a 2-bromoacetophenone derivative, and a nonsymmetrical activated alkyne in refluxing 2-ethyloxirane. The reaction can be performed under conventional reflux conditions or with microwave irradiation.
Synthesis of (+)-Methyl Dihydropalustramate and of the Pyrido[1,2-a]azepine Core of Stemona Alkaloids
Contributors: Mayer, Camilla, Romek, Alexandra, Bach, Thorsten
... Abstract Starting from a readily available, enantiomerically pure 2,6-disubstituted piperidine the synthesis of pyrido[1,2-a]azepines was accomplished. Key reactions for the ring closure were a photochemically induced acyl radical addition or a SmI2-promoted ketyl radical addition to an α,β-unsaturated ester. En route to the cyclization precursor an epoxidiation/ring opening sequence led to an undesired oxazolidinone which turned out to be useful for the configuration assignment. The compound was successfully converted into (+)-methyl dihydropalustramate.
Contributors: Dobrydnev, Alexey, Popova, Maria, Saffon-Merceron, Nathalie, Listunov, Dymytrii, Volovenko, Yulian
... Abstract A strategy for the construction of spiro[cycloalkane-1,3′-1′λ6-isothiazolidine]-1′,1′,4′-triones through the sulfonylation of 1-aminocyclopropane- and 1-aminocyclobutanecarboxylates with methanesulfonyl chloride followed by alkylation with methyl iodide and subsequent cyclization in the presence of potassium tert-butoxide in N,N-dimethylformamide is reported. An efficient synthesis of starting 1-aminocyclopropane- and 1-aminocyclobutanecarboxylic acids was developed. The reaction of spiro[cycloalkane-1,3′-1′λ6-isothiazolidine]-1′,1′,4′-triones with N,N-dimethylformamide dimethyl acetal (DMFDMA) gives 5′-[(Z)-(dimethylamino)methylene]spiro[cycloalkane-1,3′-1′λ6-isothiazolidine]-1′,1′,4′-triones, the structure of which was confirmed by X-ray diffraction study.
One-Pot, Highly Regioselective 1,3-Dipole Cycloaddition Promoted by Montmorillonite for the Synthesis of Spiro[indole-pyrrolizine], Spiro[indole-indolizine], and Spiro[indole-pyrrolidine] gem-Bisphosphonates
Contributors: Li, Guozhu, Wu, Mingshu, Liu, Fengjiao, Jiang, Jie
... Abstract Various spiro[indole-pyrrolizine], spiro[indole-indolizine], and spiro[indole-pyrrolidine] gem-bisphosphonates were prepared by multicomponent reactions between isatins, tetraethyl vinylidenebis(phosphonate), and amino acids in the presence of montmorillonite. The one-pot reactions proceeded by 1,3-dipole cycloadditions of azomethine ylides formed in a decarboxylative manner. The proposed mechanism is in line with experimental data that confirmed that the azomethine ylide is formed in a decarboxylative manner; this provides new insight into the underlying mechanisms of such cycloadditions. The method has many notable features, such as a broad substrate scope, high efficiency, and high regioselectivity.
Photocycloaddition and Rearrangement Reactions in a Putative Route to the Skeleton of Plicamine-Type Alkaloids
Contributors: Rimböck, Karl-Heinz, Pöthig, Alexander, Bach, Thorsten
... Abstract Two isoquinolones were prepared, to which an allenyl side chain was linked at position C4 via a stereogenic silyloxy-substituted carbon atom. Intramolecular [2+2] photocycloaddition reactions of these substrates proceeded with high diastereoselectivity and delivered the respective cyclobutanes with an exocyclic methylene group (83% and 49% yield). With the 5,6-dioxoloisoquinolone precursor an unprecedented meta-photocycloaddition was observed as a significant side reaction, which occurred at positions C4 and C8a of the isoquinolone skeleton. The cyclobutane products were, after N-alkylation and transformation into the respective cyclobutanones (22–57%), subjected to various rearrangement reactions. In detail, a direct photochemical rearrangement, thermal and photochemical Beckmann rearrangements, and Baeyer–Villiger oxidation reactions were studied. In all cases, products were found, which resulted from cleavage of the amino-substituted cyclobutane bond, but not from the desired cleavage of the alternative alkyl-substituted cyclobutane bond.
An Azide-Functionalized Nitronyl Nitroxide Radical: Synthesis, Characterization and Staudinger–Bertozzi Ligation Reactivity
Contributors: Barbon, Stephanie, Gobbo, Pierangelo, Luo, Wilson, Price, Jacquelyn, Biesinger, Mark, Workentin, Mark, Gilroy, Joe
... Abstract An azide-functionalized nitronyl nitroxide was synthesized and its reactivity towards the Staudinger–Bertozzi ligation was explored. Whereas a model reaction in solution showed the conversion of nitronyl nitroxide to an imino nitroxide radical, the same reaction at the interface of gold nanoparticles allowed for successful covalent incorporation of the nitronyl nitroxide radical onto the nanoparticles.