A synthetic method to prepare a wide range of 2H‐indazoles was developed via a tandem palladium‐catalyzed deacylative cross‐coupling reaction of 2‐iodoazoarenes and 2‐iodoaryltriazenes with acyldiazoacetates and denitrogenative cyclization reaction of in situ generated diazoacetates having azoaryl and triazenylaryl moieties in one‐pot. Additionally, azoaryl‐substituted diazoacetates underwent palladium‐catalyzed denitrogenative cyclization to produce 2H‐indazoles. The present reaction is a good example in which a Pd(0)‐catalyst is involved in two catalytic cycles in one‐pot.
Recent progress in asymmetric organocatalysis has led to the development of several asymmetric transformations that employ various substrates. Among these, cyanoacetates have emerged as excellent nucleophiles in conjugate addition, alkylation, Mannich and α‐heterofunctionalization reactions. In this review we discuss the enantioselective functionalization of 2‐cyanoacetates through organocatalytic reactions.
A four‐electron electrocyclic ring‐opening/intermolecular [4+2] cycloaddition of α‐hydroxycyclobutenones is reported. The reaction represents the first example for the intermolecular cycloaddition of the extensively studied enol‐ketene intermediate, and provides a new synthetic route to multiply substituted δ‐lactams in high stereoselectivity.
We have developed an electrophilic addition of allylic carbocations to 2‐cyclopropylidene‐2‐arylethanols constructing carbon‐carbon bonds with excellent regio‐ and stereoselectivities. The reaction affords 3‐oxabicyclo[3.2.0]heptanes in moderate to good yields via the electrophilic addition/ring‐opening/vinyl group shift/intramolecular cyclization sequence.
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 novel methodology has been developed for the synthesis of chiral phosphoryl‐1,4‐dihydropyridazine derivatives based on an asymmetric allylic alkylation/intramolecular 1,3‐dipolar cycloaddition/rearrangement reaction sequence, various chiral 3‐phosphoryl‐4‐aryl‐1,4‐dihydropyridazine‐5‐carboxylates were achieved in high yields with high enantioselectivities. Moreover, chiral 4,5‐dihydropyridazinone derivatives were also facilely furnished by the reduction of the allylic alkylation products of Morita–Baylis–Hillman carbonates and α‐diazophosphonates.
Water‐soluble gold(III/I) N‐heterocylic carbene complexes behave as efficient catalysts for the hydration of terminal alkynes in neat water. The transformation proceeds in the absence of Brønsted acids or halide scavengers and is suitable for sensitive substrates. Kinetic profiles and DFT studies provide a clear picture of intermediates present during catalysis.
An efficient approach for the synthesis of 1,2,3,4,5,6‐hexahydrobenzo[f]isoquinolines via a tandem aza‐Prins/Friedel–Crafts cyclization from 2‐arylethyl‐2,3‐butadienyl tosylamides and aldehydes has been developed. This iron(III) chloride‐catalyzed cascade cyclization at the room temperature with different types of aldehydes, such as aromatic and heteroaromatic aldehydes, alkyl aldehydes, and α,β‐unsaturated aldehydes, affords the products in moderate to excellent yields (up to 97 %). In this reaction, chlorotrimethylsilane was applied to activate the aldehydes. The cheap catalyst, the mild reaction conditions, and the broad scope of the substrates make this method highly useful.
Silver‐catalyzed three‐component, tandem reactions of 4‐alkynyl‐2‐oxo‐2H‐chromene‐3‐carbaldehydes, amines and various nucleophiles result in the formation of highly functionalized chromeno[3,4‐c]pyridin‐5‐ones in high yields. Gold‐catalyzed [4+2] cycloadditions of 4‐alkynyl‐2‐oxo‐2H‐chromene‐3‐carbaldehydes with alkynes or alkenes have also been achieved to afford benzo[c]chromen‐6‐ones efficiently.
Herein, we disclose the N‐heterocyclic carbene (NHC)‐catalyzed [3+3] annulation of challenging esters with methyl ketoimines for the highly enantioselective synthesis of intriguing δ‐lactams featuring various substituent patterns. The annulation occurs under mild conditions and offers good tolerance, good yields and excellent enantioselectivities. The six‐membered heterocyclic products are valuable for the synthesis of bioactive molecules.
An asymmetric monoborylation of alkenylsilanes catalyzed by a copper(I) complex with the chiral bisphosphine ligand BenzP* is reported. The reaction proceeded with excellent regioselectivity and high enantioselectivity to afford the corresponding optically active organoboronate esters with a stereogenic C B bond containing a vicinal silyl group. The synthetic utility of the product is demonstrated through stepwise transformations to multifunctional optically active compounds in a stereospecific manner.
The paper describes the oxidative Heck arylation of various allylic amines using arylboronic acids for the preparation of tetrasubstituted alkenes. As oxidant the commercially available 2,2,6,6‐tetramethylpiperidine‐N‐oxyl (TEMPO) is used and coupling reactions occur under very mild conditions at room temperature. The densely substituted alkenes are formed in good to excellent yields with complete control of the diastereoselectivity. Substrate scope with respect to the allylic amine and the arylboronic acid is investigated.
Copper chloride‐catalyzed aerobic oxidative annulation of N‐furfuryl‐β‐enaminones provides access to polysubstituted pyrroles and indoles. This protocol involves an unprecedented copper chloride‐catalyzed oxidative chlorination of furan and pyrrole rings with oxygen as the terminal oxidant.
A transition metal‐free and efficient method for the synthesis of 3‐alkynylpyrrole‐2‐carboxylates from diynones and glycine esters or 2‐aminoacetophenone hydrochloride has been developed. This transformation provides a large range of substituted pyrroles in good to excellent yields with the elimination of water as the only by‐product. The detailed mechanistic studies elucidated that this transformation involves a Michael addition/intramolecular cyclodehydration process.
A series of 1,2‐ and 1,4‐dihydroquinolines has been successfully prepared. The Pd‐catalyzed intramolecular N‐arylation of Z‐enamines, formally prepared by the Horner–Wadsworth–Emmons olefination, proceeded efficiently to furnish the cyclized products. Depending on the cyclization conditions, substituted 1,4‐dihydroquinolines and further isomerized 1,2‐dihydroquinolines were independently obtained in high yields with an excellent control of isomerization of the double bond.
An efficient synthesis of polycyclic indole and pyrrole derivatives via a ruthenium‐catalyzed intramolecular allylic dearomatization/migration reaction has been developed. This method features wide substrate scope, mild reaction conditions, and an operationally simple procedure. The capture of the spiro intermediate provides solid evidence for the dearomatization/migration reaction pathway.
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.
An efficient route to 4‐cyanoquinolines via a palladium‐catalyzed cyanative reaction of 2‐alkynylbenzaldimines with isocyanides has been developed. The transformation proceeds through 6‐endo cyclization, isocyanide insertion, and cyanation. 4‐Amidylisoquinolines can be generated as well if water is involved in the reaction.
