Shining light on clay-chromophore hybrids: layered templates for accelerated ring closure photo-oxidation.
ABSTRACT: Templates with specific microenvironments have been long employed to facilitate specialized reactions. From enzymes to metal organic frameworks (MOFs), various systems have exerted their prowess to affect specific chemical reactions. Here we report, for the first time, the acceleration of a ring closure photo-oxidation reaction due to the specific structural constraints provided by layered materials. A stilbene derivative has been used as a prototype reactant and the di-hydrophenanthrene intermediate has been isolated and characterized en route to the complete photo-oxidation. Combining the gathered evidence, a possible mechanism for the chemical transformation has been proposed. Kinetic analysis showed that layered materials help to manipulate the rate of the electrocyclic ring closure and, in turn, accelerate the complete reaction sequence. The structural microenvironment induced by layered materials could be a unique platform to probe and stabilize a plethora of photo-oxidative reactions and intermediates.
Project description:An approach to the total synthesis of the antimicrotubule agent welwistatin is described. Key transformations include (1) a 7-endo intramolecular conjugate addition reaction of enone 6 to deliver the strained bicyclo[4.3.1]decanone 5; (2) a 6pi electrocyclic ring closure of trienecarbamate 16 followed by oxidation to afford protected aniline 17; and (3) an intramolecular cyclization of acetic acid derivative 18 to give rise to indole 19. [reaction: see text]
Project description:A diastereoselective 6?-electrocyclic ring closure employing halogen-substituted 3-amidotrienes via a 1,6-remote asymmetric induction is described. This new asymmetric manifold for pericyclic ring closure further underscores the significance of the allenamide chemistry.
Project description:Preparations of de novo acyclic 2-amido-dienes and 3-amido-trienes through 1,3-hydrogen shifts from allenamides are described. These 1,3-hydrogen shifts could be achieved thermally or they could be promoted by the use of Brønsted acids. Under either condition, these processes are highly regioselective in favour of the ?-position, and highly stereoselective in favour of the E-configuration. In addition, 6?-electron electrocyclic ring-closure could be carried out with 3-amido-trienes to afford cyclic 2-amido-dienes, and such electrocyclic ring-closure could be rendered in tandem with the 1,3-hydrogen shift.
Project description:A new method for the preparation of indoles from readily available alpha-haloenones and alpha-(trialkylstannyl)enecarbamates is described. Following a Stille coupling, trienecarbamate 2 is electronically activated to undergo a facile 6pi-electrocyclic ring closure and subsequent oxidation to afford protected aniline 4. Upon deprotection and reductive amination, acid 5 underwent clean cyclization to N-acetylindole 6 (Ac2O, NEt3, 130 degrees C). This method has been used to construct a variety of substituted indoles that are not easily prepared by conventional indole annelation methods.
Project description:A regio- and stereoselective isomerization of allenamides is described, leading to preparations of de novo 2-amido-dienes and a tandem isomerization-6pi-electron electrocyclic ring-closure.
Project description:M06-2X/6-31+G(d,p) computations are reported for the 8?-6? electrocyclization cascades of 1,3,5,7-tetraenes. The rate-determining step for these cascades is typically the second (6?) ring closure. According to experiment and theory, un- and monosubstituted tetraenes readily undergo 8? electrocyclic ring closure to form 1,3,5-cyclooctatrienes; however, the 6? electrocyclizations of these cyclooctatriene intermediates are slow and reversible, and mixtures of monocyclic and bicyclic products are formed. Computations indicate that di- and trisubstituted tetraenes undergo facile but less exergonic 8? electrocyclization due to a steric clash that destabilizes the 1,3,5-cyclooctatriene intermediates. Relief of this steric clash ensures the subsequent 6? ring closures of these intermediates are both kinetically facile and thermodynamically favorable, and only the bicyclic products are observed for the cascade reactions of naturally occurring tri- and tetrasubstituted tetraenes (in agreement with computations). The 6? electrocyclization step of these cascade electrocyclizations is also potentially diastereoselective, and di- and trisubstituted tetraenes often undergo cascade reactions with high diastereoselectivities. The exo mode of ring closure is favored for these 6? electrocyclizations due to a steric interaction that destabilizes the endo transition state. Thus, theory explains both the recalcitrance of the unsubstituted 1,3,5,7-octatetraene and 1-substituted tetraenes toward formation of the bicyclo[4.2.0]octa-2,4-diene products, as well as the ease and the stereoselectivity with which terminal di- and trisubstituted tetraenes are known to react biosynthetically.
Project description:The total synthesis of (±)-isophellibiline is described. This represents the first synthesis of a member of the nonaromatic homoerythrinan family of alkaloids. The tetracyclic ring system of the natural product was quickly assembled by a strategy that features a retrocycloaddition/cycloaddition reaction of an amidodioxin, an intramolecular Heck reaction and a 6?-electrocyclic ring closure of a dienoic acid.
Project description:On-surface synthesis is a promising strategy for engineering heteroatomic covalent nanoarchitectures with prospects in electronics, optoelectronics and photovoltaics. Here we report the thermal tunability of reaction pathways of a molecular precursor in order to select intramolecular versus intermolecular reactions, yielding monomeric or polymeric phthalocyanine derivatives, respectively. Deposition of tetra-aza-porphyrin species bearing ethyl termini on Au(111) held at room temperature results in a close-packed assembly. Upon annealing from room temperature to 275 °C, the molecular precursors undergo a series of covalent reactions via their ethyl termini, giving rise to phthalocyanine tapes. However, deposition of the tetra-aza-porphyrin derivatives on Au(111) held at 300 °C results in the formation and self-assembly of monomeric phthalocyanines. A systematic scanning tunnelling microscopy study of reaction intermediates, combined with density functional calculations, suggests a [2+2] cycloaddition as responsible for the initial linkage between molecular precursors, whereas the monomeric reaction is rationalized as an electrocyclic ring closure.
Project description:tert-Butoxyacetylene is shown to undergo Sonogashira coupling with aryl iodides to yield aryl-substituted tert-butyl ynol ethers. These intermediates participate in a [1,5]-hydride shift, which results in the extrusion of isobutylene and the generation of aryl ketenes. The ketenes are trapped in situ with multiple nucleophiles or undergo electrocyclic ring closure to yield hydroxynaphthalenes and quinolines.
Project description:We report the first experimental evidence for the generation of highly strained cis,trans-cycloheptadienones by electrocyclic ring opening of 4,5-fused cyclobutenamides. In the presence of AlCl3, the cyclobutenamides rearrange to [2.2.1]-bicyclic ketones; DFT calculations provide evidence for a mechanism involving torquoselective 4?-electrocyclic ring opening to a cis,trans-cycloheptadienone followed by a Nazarov-like recyclization and a 1,2-alkyl shift. Similarly, 4,6-fused cyclobutenamides undergo AlCl3-catalyzed rearrangements to [3.2.1]-bicyclic ketones through cis,trans-cyclooctadienone intermediates. The products can be further elaborated via facile cascade reactions to give complex tri- and tetracyclic molecules.