A practical and environmentally friendly strategy for generating alkoxycarbonyl radicals from readily available carbazates under metal‐free conditions has been developed. In the presence of tetrabutylammonium iodide and tert‐butyl hydroperoxide, 2‐isocyanobiphenyls smoothly underwent radical alkoxycarbonylation with carbazates to afford phenanthridine‐6‐carboxylates.
An asymmetric Michael addition of 5H‐oxazol‐4‐ones to vinyl sulfones has been developed. In the presence of 10 mol % of quinine‐based benzyl‐substituted thiourea as catalyst at 0 °C, the products could be obtained with excellent enantio‐ and diastereoselectivity (up to>99 % ee and>20:1 dr). 1.0 mol % of catalyst also produced the corresponding adducts with similar stereoselective results when the temperature was increased to 25 °C. The obtained adducts have been demonstrated as significant synthetic fragments to conveniently access the monofluorinated analogs of biologically important 2‐tertiary hydroxyl‐3‐methyl‐substituted carboxylic acid derivatives.
A facile one‐pot, catalyst‐free reaction has been developed for the synthesis of 2,3,6,7‐tetrahydro‐1H‐pyrrolo[3,2‐c]pyridin‐4(5H)‐ones from readily available 1‐acryloyl‐1‐N‐arylcarbamylcyclopropanes and amines using a domino ring‐opening/cyclization/aza‐addition sequence.
An unprecedented organocatalytic enantioselective cascade Michael/hemiketalization/retro‐aldol reaction of 2‐[(E)‐2‐nitrovinyl]phenols and 2,4‐dioxo‐4‐arylbutanoates is described. With a bifunctional squaramide catalyst incorporating (1R,2R)‐1,2‐diphenylethane‐1,2‐diamine, the reactions afford products in 75–99% yields with 80–98% ee. This process provides an enantioselective pathway for the synthesis of chiral α‐keto esters, precursors of 3‐arylproline derivatives, δ‐amino α‐keto acids or cyclic α‐keto lactams.
An umpolung approach to the synthesis of diaryl ketones has been developed based on in situ generation of acyl anion equivalents and their catalytic arylation. This method entails the base‐promoted, palladium‐catalyzed direct C‐H arylation of 2‐aryl‐1,3‐dithianes with aryl bromides. Use of MN(SiMe3)2 (M=Li, Na) base results in reversible deprotonation of the weakly acidic dithiane. In the presence of a Pd(NiXantphos)‐based catalyst and aryl bromide, cross‐coupling of the metallated 2‐aryl‐1,3‐dithiane takes place under mild conditions (2 h at rt) with yields as high as 96 %. The resulting 2,2‐diaryl‐1,3‐dithianes were converted into diaryl ketones by either molecular iodine, N‐bromo succinimide (NBS) or Selectfluor in the presence of water. The dithiane arylation/hydrolysis can be performed in a one‐pot procedure to yield a variety of diaryl ketones in good to excellent yields. This method is suitable for rapid and large‐scale synthesis of diaryl ketones. A one‐pot preparation of anti‐cholesterol drug fenofibrate (TriCor®) has been achieved on 10.0 mmol scale in 86 % yield.
Visible light irradiation of N‐bromosuccinimide serves as an effective means to convert methyl 2‐(azidomethyl)‐3‐arylpropenoates and 2‐(azidomethyl)‐3‐arylacrylonitriles to the corresponding iminyl radicals via α‐hydrogen abstraction and subsequent extrusion of dinitrogen. Thus formed iminyl radicals then undergo intramolecular ortho attack on the aryl ring, affording methyl quinoline‐3‐carboxylates and quinoline‐3‐carbonitriles respectively.
A concise synthesis of 1‐naphthols via cyclization of o‐iodoacetophenones and methyl ketones has been realized under very mild conditions. The cyclization process is initiated by a rare copper‐catalyzed arylation of simple methyl ketones with ortho‐iodoacetophenones.
A simple N‐heterocyclic carbene (NHC) derived from 1‐methyl‐3‐ethylimidazolium tetrafluoroborate was found to be an efficient ligand for a range of copper‐catalyzed cross‐coupling reactions, leading to the formation of aromatic ethers and thioethers.
The asymmetric aldol reaction of 3‐acetyl‐2H‐chromen‐2‐ones and isatins has been realized by using a bifunctional quinidine‐derived urea as the catalyst. The corresponding 3‐hydroxyoxindole derivatives containing a 2H‐chromen‐2‐one moiety were obtained in good yields and high enantioselectivities. When (Z)‐ethyl 2‐benzylideneacetoacetate was used as the substrate, a mixture of two diastereomers (both Z and E) was obtained due to isomerization of the double bond under the reaction conditions.
An enantioselective Friedel–Crafts alkylation reaction of indoles with cyclic N‐sulfonyl ketimino esters was developed. Under the optimized conditions using a chiral copper(II) triflate‐bisoxazoline complex as the catalyst, a range of N‐sulfonyl ketimino ester derivatives and indoles reacted smoothly to afford indole‐containing chiral cyclic α‐amino esters bearing tetrasubstituted α‐stereogenic centers [3‐ethoxycarbonyl‐3‐(3‐indolyl)‐2,3‐dihydrobenzo[d]isothiazole 1,1‐dioxides] in excellent yields and with high enantioselectivities (up to 99% ee). Pyrrole and N,N‐dimethylaniline were also investigated as aromatic substrates to afford the corresponding products with good results. An asymmetric induction model was then proposed on the basis of the observed absolute configuration of the product 3‐ethoxycarbonyl‐3‐(5‐bromo‐3‐indolyl)‐2,3‐dihydrobenzo[d]isothiazole 1,1‐dioxide. Synthetic transformations to convert the products into cyclic chiral N‐sulfonamido alcohols and the deprotection of the sulfonamides were performed. This study provides an efficient approach to chiral α‐tetrasubstituted indolic α‐amino acids as potential building blocks for peptides and biologically active molecules.
An efficient intramolecular arene‐alkene oxidative coupling of 1,4‐diaryl‐1,3‐butadienes has been developed involving the use of a 2,3‐dichloro‐5,6‐dicyano‐para‐benzoquinone (DDQ)/acid catalyst. The reaction involves the generation of a radical cation by abstraction of an electron from the substrate with DDQ, an intramolecular Friedel–Crafts‐type reaction, and the loss of hydrogen radical.
A simple and efficient catalytic system for the aerobic oxidation of primary amines into corresponding oximes has been developed, with 3‐methyl‐4‐oxa‐5‐azahomoadamantane as catalyst, acetaldoxime as co‐catalyst and water as solvent. This process, which uses oxygen (O2) as an economic and green oxidant and water as a green solvent, tolerates a wide range of substrates, affording the target oximes in moderate to excellent yields. It was found that high selectivity was achieved when 3‐methyl‐4‐oxa‐5‐azahomoadamantane was used, and E‐type oximes were the only detected products. A possible mechanism for this catalytic process is proposed.
Herein a practical and efficient protocol for preparing a range of aminoisoquinolines is reported. Various aminoisoquinolines were prepared in moderate to good yields from the corresponding 2‐methylbenzonitriles and benzonitriles upon treatment with potassium tert‐butoxide.
A novel and efficient domino process has been developed for the synthesis of 1,4‐benzoxazepine derivatives from a range of readily accessible N‐tosylaziridines, 2‐iodophenols and isocyanides. This process involves the aziridine ring‐opening reaction with 2‐iodophenol, followed by a palladium‐catalyzed isocyanide‐insertion reaction. This regioselective and high‐yielding transformation could be extended to its application for the synthesis of natural products and biologically interesting heterocycles.
We recently developed a completely new type of Rh‐catalyzed [5+2] cycloaddition by using 3‐acyloxy‐1,4‐enyne (ACE) as the 5‐carbon building block. In this update, we show that ACE can undergo intramolecular [5+2] cycloaddition with either an alkene or an allene in the presence of an appropriate rhodium catalyst and ligands to afford bicyclic compounds with multiple stereogenic centers. In most cases, cis‐fused bicyclo[5.3.0]decadienes are prepared highly diastereoselectively.
The iridium‐catalyzed highly regioselective transfer hydrogenation of a variety of 2‐substituted and 2,9‐disubstituted 1,10‐phenanthrolines under mild conditions with formic acid as the hydrogen source is described. In the presence of a catalytic amount of the iridium complex [Cp*IrCl2]2, the transfer hydrogenation proceeded smoothly in 1,4‐dioxane under ligand‐free conditions, exclusively leading to a range of 1,2,3,4‐tetrahydro‐1,10‐phenanthroline products in high yields. The catalyst generated in situ from [Cp*IrCl2]2 and (R,R)‐(CF3)2C6H3SO2‐dpen [N‐(2‐amino‐1,2‐diphenylethyl)‐3,5‐bis(trifluoromethyl)benzenesulfonamide] could efficiently catalyze the asymmetric transfer hydrogenation of these 1,10‐phenanthrolines in isopropyl alcohol (i‐PrOH) to afford chiral 1,2,3,4‐tetrahydro‐1,10‐phenanthrolines in high yields with up to >99% ee. The key to the success of the reduction is the choice of solvent and hydrogen source.
A convenient three‐component coupling reaction for the construction of conjugated enynes using rhodium catalysis is reported. Dimerization of a monosubstituted alkyne followed by trapping of the vinyl metal intermediate with an electron‐deficient alkene, such as methyl vinyl ketone, provided moderate to good yields of these enynes. The use of the hindered electron‐rich tris(ortho‐tolyl)phosphine as a ligand for the rhodium catalyst provided the best conversions to these complex products.
Organocatalyzed highly stereoselective 1,4‐thia‐Michael addition of mercaptans to linear 2,4‐dienones and 2‐en‐4‐ynones was developed using Cinchona alkaloid‐based squaramides. Application of only 0.5–1 mol % loading afforded products in up to 98:2 e.r. and above 99:1 after a single recrystallization. The adducts of allyl mercaptan can be conveniently further transformed to new chiral 2‐substituted 2,5‐dihydrothiophenes by ring‐closing metathesis.
A copper‐catalysed multicomponent coupling reaction between readily available (Z)‐3‐iodoacrylic acids, terminal alkynes, and primary amines was developed to smoothly access a small library of 5‐hydroxy‐1H‐pyrrol‐2(5H)‐ones in good yields. This practical and general process was applied to a short‐steps synthesis of the natural product pulchellalactam.
