Gassman’s Cationic [2 + 2] Cycloadditions Using Temporary Tethers (original) (raw)
Angewandte Chemie International Edition, 2012
cooperative catalysis · cycloaddition · cyclobutanes · dienamine · organocatalysis Cyclobutene derivatives have become increasingly important as molecular building blocks (a result of their ring strain, which allows their easy cleavage to other structures), resulting in the development of new methods for their synthesis. However, most existing methods only give access to racemic cyclobutanes, and only a few examples of asymmetric metalcatalyzed reactions have been reported. [1b] In this context, asymmetric organocatalysis has become extremely attractive over the last decade. In particular, aminocatalysis has proven to be a powerful synthetic tool and has found applications in a wide range of cycloadditions. In this field, only one example has been reported (by Ishihara in 2007), in which an enantioselective [2+2] cycloaddition of unactivated alkenes (3) and a-acyloxyacroleins (1) was studied under iminium catalysis (Scheme 1). [4a] In addition, Miranda and Bach recently reported an intramolecular [2+2] photocycloaddition catalyzed by a chiral sensitizer, but it was restricted to a very specific transformation. [4b] Therefore, the problems associated with [2+2] asymmetric cycloadditions still present an important challenge for the development of new approaches, catalysts, and activation modes.
Proceedings of The 17th International Electronic Conference on Synthetic Organic Chemistry, 2013
We herein report on a new class of intramolecular [3+2] cycloaddition reactions potentially occurring in Naryl heterocumulenes ortho-substituted by a cyclopropylidenemethyl unit. The required isothiocyanates, ketenimines and carbodiimides were prepared following aza-Wittig processes of common phosphazene intermediates. To date, only carbodiimides seem to behave as suitable substrates for the desired cyclization.
Chemistry (Weinheim an der Bergstrasse, Germany), 2014
Solid-state [2+2] photochemical cycloaddition reactions have been extensively studied after the classical work of Schmidt in the 1960s. Of these, trans-1,2-bis(4'-pyridyl)ethylene (bpe) is one of the well-studied alkenes to synthesize tetrakis(4-pyridyl)cyclobutane (tpcb). However, almost all the solid-state [2+2] cycloaddition reactions of bpe yielded, almost exclusively, one of the four possible isomers, namely, the rctt-tpcb (r=regio c=cis and t=trans). Here we describe a stereoselective synthesis of the tetrahedrally disposed rtct-tpcb by the solid-state thermal isomerization of the rctt-isomer in atmospheric air. We propose that this isomerization occurs through a topochemical unimolecular mechanism by a radical chain pathway, initiated by molecular oxygen. This is supported by the nature of products formed in air and nitrogen, detection of a radical in ESR spectral studies, ESI-MS crossover experiments, VT PXRD studies along with QM, MD and docking calculations. The format...
Annulation of Oxime‐Ether Tethered Donor–Acceptor Cyclopropanes
Chemistry – A European Journal, 2019
Novel oxime-ether tethered cyclopropanes when exposed to Yb(OTf)3 and heat, annulate to generate hydropyrrolo-oxazines products that can be taken to their respective pyrrolidines via hydrogenative NO bond cleavage. The hydropyrrolo-oxazines are generated in a diastereoselective manner isolating the cis or trans product based on the temperature of the reaction. 20 examples of selective cis and trans hydropyrrolo-oxazines are generated in high yields by temperature control.
A facile entry into a stable functionalized bis(methylene)cyclobutene system
Tetrahedron Letters - TETRAHEDRON LETT, 2000
A parameter for the stabilization of the 3,4-bis(methylene)cyclobutene system has been identified. Appropriate functionalized derivatives, stable even in solution, have been conveniently prepared by tandem double [2,3]-sigmatropic rearrangement of conjugated dipropargylic sulfenates to diallenic disulfoxides and spontaneous cyclization of the latter. Stereochemical aspects are presented.
The Journal of Organic Chemistry, 1986
Chlorophenylcyclobutanone 11, prepared by chlorophenylketene addition to cyclohexene, reacts readily with simple and hindered carboxylic acids in a cine substitution to produce keto esters 14. The acyloxy and phenyl substituents in 14c are shown by X-ray diffraction to be cis oriented; nevertheless 14 reacts with NaOMe at 20 O C by an unusual ester-ether interchange to produce 15 and the released carboxylate RC02-. These reactions apparently proceed via an oxyallyl cation intermediate. The behaviors of related cyclobutanones 4, 6, and 11 with methoxide are contrasted.
Journal of Heterocyclic Chemistry, 2014
A new class of container molecules is described and the first steps in producing protypes are reported. Central to the approach is the formation of polynorbornanes with cyclobutene-1,2-difurfuryl esters at the terminus or similar functionality at the bridgehead of a central norbornane subunit. The synthesis of the furfuryl starting materials is described as well as their anthracenyl counterparts. Conversion to the container systems involved the intermolecular linking of the furfuryl or anthracene by treatment with dimethyl acetylene dicarboxylate (DMAD) in a Diels-Alder (DA) protocol under thermal or high-pressure (HP) conditions. In practice, no intermolecular linking occurred between the norbornane substrates and only products from DA 1:1-addition with DMAD were produced. Intramolecular addition of one of the furfuryl units onto the cyclobutene p-bond was detected under HP conditions, and this intermolecular product was capable of isolation and characterization by working at room temperature or below, but reverted to starting material above room temperature. When conducted in the presence of DMAD, a single 1:1-adduct was obtained in which one furfuryl moiety was intramolecularly cyclized and the other present as the DMAD adduct; again this product underwent retro-DA reaction at 40 C. Similar intermolecular cyclization was observed with the bis-anthracenyl esters. The stereoselectivity of the intermolecular attack of the furfuryl diene with the dienophilic cyclobutene gave a single adduct by endo-face attack in which the oxa-bridge is endo-positioned. Quantum chemical DFT calculations (B3LYP) predict that the formation of the endo-isomer is kinetically favored and that relief of ring strain enhances the rate of retro-Diels-Alder in the tethered system.