A mild, catalytic, atom economical synthesis of imidazo[1,2‐a]pyridines has been developed: catalytic dichloro(2‐pyridinecarboxylato)gold [PicAuCl2] in the presence of an acid produces a range of imidazo[1,2‐a]pyridines in good yields starting from alkynes and 2‐aminopyridine N‐oxides. This strategy is mild and foreseen to be of particular use for the installation of stereogenic centers adjacent to the imidazo[1,2‐a]pyridine ring without loss of enantiomeric excess.
Heterosubstituted aminoallenes underwent smooth ruthenium‐catalyzed intramolecular exo‐hydroamination reactions yielding the corresponding five‐, six‐, or seven‐membered 1,3‐diaza‐ or 1,3‐oxaza‐heterocyclic structures. This procedure is a valuable and less expensive alternative to the already known transition metal‐catalyzed hydroamination reactions of aminoallenes.
A direct diastereoselective synthesis approach of important 9H‐pyrrolo[1,2‐a]azepin‐9‐amines was established via base‐promoted [4+3] annulation between donor–acceptor reagents derived from 1H‐pyrrole‐2‐carbaldehydes and alkyl 2‐aroyl‐1‐chlorocyclopropanecarboxylates. This transition metal‐free domino reaction proceeded quickly under mild basic conditions, affording potentially bioactive azepine derivatives in moderate to high yields with high diastereoselectivities (up to >20:1).
An efficient and practical copper‐catalyzed domino synthesis of benzo[4,5]imidazo[1,2‐a]pyrimidin‐4(10H)‐ones has been developed. The protocol uses N‐(2‐halophenyl)‐3‐alkylpropiolamides and cyanamide as the starting materials, inexpensive copper(I) iodide and pipecolinic acid as the catalyst and ligand, and the corresponding products were obtained in moderate to good yields.
A triphenylphosphine‐catalyzed cascade reaction of unsaturated pyrazolones with dialkyl acetylenedicarboxylates or but‐3‐yn‐2‐one to synthesize the bioactive pyrano[2,3‐c]pyrazoles in moderate to excellent yields has been developed. Meanwhile, spiro‐cyclopentanone‐pyrazolones were also first constructed by the triphenylphosphine‐catalyzed cascade reaction of unsaturated pyrazolones with 4‐phenylbut‐3‐yn‐2‐one in moderate to good yields.
A palladium(0)‐catalyzed reaction of 2‐allyl‐3‐indolyl boronates/propargylic carbonates was observed to afford dihydrocycloocta[b]indoles highly efficiently via carbon‐carbon coupling, [1,5]‐hydrogen migration involving dearomatization, and electrocyclization involving rearomatization. Their thermal reactions with the addition of dienophiles involving eletrocyclization and [4+2] cycloaddition have been studied.
A copper‐catalyzed cascade reaction involving trifluoromethylation of acrylamides coupled with ring closure and indole dearomatization is reported. This facile transformation was highly regioselective and proceeded under mild conditions, allowing efficient access to trifluoromethyl‐substituted spiro[indole‐3,3′‐pyrrolidine] derivatives, which are of increasing interest to the pharmaceutical industry.
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.
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.
Cyclometallated NHC palladium complexes prepared from palladium(II) acetate [Pd(OAc)2] and unsymmetrical 1,3‐diarylimidazolinium salts catalyzed the hydroxymethylation of (hetero)arylboronic acids using an excess amount of formalin to afford (hetero)arylmethanols in good to satisfactory yields.
A unique one‐pot synthesis of cyclic cis‐1,2‐amino alcohols from trans‐1,2‐azido alcohol precursors was developed. The key step is highlighted by the stereoselective reduction of the cyclic α‐alkoxy imines, which could be prepared from the corresponding azides by ruthenium catalysis under photolytic conditions. Remarkably, this unprecedented reaction pathway offers a stereodivergent access to structurally diverse cyclic 1,2‐amino alcohols.
A direct method for the arylation of 1,2‐azolo[1,5‐a]pyridines has been developed. In the process, the fused pyridines react with aryl halides in the presence of the palladium complex Pd(OAc)2(Phen) as a catalyst and copper(I) chloride (CuCl) as a Lewis acid to form arylated derivatives. While pyrazolo[1,5‐a]pyridines and [1,2,4]triazolo[1,5‐a]pyridines are arylated at ortho‐positions of their pyridine rings using this method, in situ ring‐opening of the formed C‐7 arylated [1,5‐a]pyridine takes place to generate the 2,6‐disubstituted pyridine. Also, upon treatment with lithium diisopropylamide (LDA), C‐7 arylated pyrazolo[1,5‐a]pyridine‐3‐carboxylates react to produce diversely substituted 2,6‐disubstituted pyridines. Finally, a sequential C‐3 arylation was accomplished through a two‐step sequence involving hydrolysis of pyrazolo[1,5‐a]pyridine‐3‐carboxylates followed by the bimetallic Pd/Cu‐catalyzed decarboxylative coupling reaction with aryl bromide.
Under zinc Lewis acid catalysis, terminal alkynes coupled dehydrogenatively with 1,8‐naphthalenediaminatoborane [HB(dan)]. It is important to note that the resulting alkynylboranes with an C(sp) B(dan) bond are isolable by column chromatography on silica gel (SiO2) and are usable as coupling partners for palladium‐ and copper‐catalyzed cross‐coupling reactions with (hetero)aryl halides.
A new transition metal‐free oxidative coupling of unactivated terminal alkenes with aldehydes and hydroperoxides in the presence of 10 mol% potassium tert‐butanolate (t‐BuOK) is described thereby realizing trifunctionalization of alkenes toward 2,3‐epoxy ketones. This method is applicable to a wide range of aldehydes, including aryl and alkyl aldehydes, with excellent functional group tolerance, and provides for the one‐step assembly of 2,3‐epoxy ketones.
The iron(III) chloride‐multicatalyzed dioxygenation of enamides with TEMPO in the presence of alcohols has been developed. This multicomponent domino process affords efficient new strategies for the synthesis of α‐oxy‐N‐acylhemiaminals or α‐oxyimides in good to excellent yields under mild conditions.
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.
Diphenyl disulfide (PhSSPh), a typical boryl monosulfide (diiPr‐Imd‐BH2SPh) and a typical boryl bis‐sulfide [diMe‐Imd‐BH(SPh)2] all serve as both initiators and precatalysts in the reduction of alkyl and aryl halides by readily available 1,3‐dimethylimidazol‐2‐ylidene⋅borane (diMe‐Imd‐BH3). The reactions are suggested to occur by a polarity reverse catalysis mechanism where in situ generated thiophenol is the active catalyst.
The efficient simultaneous activation of cyclohexenones or simple alkyl acyclic enones and 2‐hydroxynitrostyrenes was realized by using a prolinol thioether catalyst. Thus, a highly chemo‐, diastereo‐ and enantioselective synthesis of functionalized tetrahydroxanthenones and chromanes with up to three contiguous stereocenters was developed.
