One‐Pot Synthesis of Tri‐ and Tetrasubstituted Pyridines by Sequential Ring‐Opening/Cyclization/Oxidation of N‐Arylmethyl 3‐Aziridinylpropiolate Esters

A novel strategy for the one‐pot synthesis of substituted pyridines from N‐arylmethyl 3‐aziridinylpropiolate esters is described. The method employs a three‐step procedure including the formation of allenyl imines, phosphine‐catalyzed cyclization, and subsequent oxidation of the dihydropyridines. Depending on the reaction conditions of the final oxidation step, tri‐ and tetrasubstituted pyridines can be selectively produced.     

Metal‐Free Decarboxylative Cyclization/Ring Expansion: Construction of Five‐, Six‐, and Seven‐Membered Heterocycles from 2‐Alkynyl Benzaldehydes and Cyclic Amino Acids

A one pot synthesis of 1H‐benzo[g]indoles, tetrahydrobenzo[h]quinolines, and naphtho[1,2‐b]azepines from 2‐alkynyl benzaldehydes and cyclic amino acids is reported. The salient feature of the strategy involves formation of three new bonds (one C? N and two C? C bonds) by a metal‐free decarboxylation/cyclization/one‐carbon ring expansion sequence in one pot.     

Rhodium(II)‐Catalyzed Annulation of Azavinyl Carbenes Through Ring‐Expansion of 1,3,5‐Trioxane: Rapid Access to Nine‐Membered 1,3,5,7‐Trioxazonines

The rhodium(II)‐catalyzed denitrogenative coupling of N‐alkylsulfonyl 1,2,3‐triazoles with 1,3,5‐trioxane led to nine‐membered‐ringed trioxazonines in moderate‐to‐good yields. 1,3,5‐Trioxane, acting as an oxygen nucleophile, reacted with the α‐aza‐vinylcarbene intermediate, giving rise to ylide formation, which was probably the key step in the reaction. Triazoles that contained aryl substituents with various electronic and steric features on the C4 carbon atom were well‐tolerated. The synthesis of trioxazonine derivatives was achieved through a one‐pot, two‐step procedure from 1‐mesylazide and a terminal alkyne by combining Cu^I‐catalyzed 1,3‐dipolar cycloaddition and rhodium‐catalyzed transformations.     

Gold‐Catalyzed 1,2‐Oxoarylations of Nitriles with Pyridine‐Derived Oxides

We report the first success in the gold‐catalyzed oxoarylations of nitriles with pyridine‐derived N‐oxides using gold carbenes as initiators. These oxoarylations were also achieved satisfactorily in intermolecular three‐component oxidations, including diverse alkenyldiazo esters, nitriles, and pyridine‐based oxides.     

N‐Heterocyclic Carbene Catalyzed Cyclocondensation of α,β‐Unsaturated Carboxylic Acids: Enantioselective Synthesis of Pyrrolidinone and Dihydropyridinone Derivatives

The catalytic cyclocondensation of in situ activated α,β‐unsaturated carboxylic acids was developed. N‐heterocyclic carbenes efficiently catalyzed the generation of α,β‐unsaturated acyl azolium intermediates from α,β‐unsaturated carboxylic acids via in situ generated mixed anhydrides for the enantioselective [3+2] and [3+3] cyclocondensation with α‐amino ketones and alkyl(aryl)imines, respectively. The corresponding pyrrolidinones and dihydropyridinones were isolated in good yields with high to excellent enantioselectivities.     

Medium‐Sized Rings versus Macrocycles through Rhodium‐Catalyzed Ring‐Expansion Reactions of Cyclic Acetals

α‐Diazo β‐ketoesters and diketones react with cyclic acetals under Rh^II catalysis to yield unprecedented polyoxygenated 8‐ and 9‐membered rings in one pot. The reactions occur under mild conditions with yields up to 90 %. A perfect regioselectivity is obtained, which can be rationalized through a mechanistic hypothesis that considers 1) the formation of an oxonium ylide, 2) its transformation into an unsaturated acyclic oxocarbenium electrophilic intermediate, and 3) an intramolecular nucleophilic attack in a direct application of Baldwin’s rules.     

Copper‐Catalyzed Heteroarylboration of 1,3‐Dienes with 3‐Bromopyridines: A cine Substitution

A method for the heteroarylboration of 1,3‐dienes is presented. The process involves an unusual cine substitution of 3‐bromopyridine derivatives to deliver highly functionalized heterocyclic products. Mechanistic studies are included that clarify the details of this unusual process.     

Catalytic Asymmetric Inverse‐Electron‐Demand 1,3‐Dipolar Cycloaddition of C,N‐Cyclic Azomethine Imines with Azlactones: Access to Chiral Tricyclic Tetrahydroisoquinolines

Reported herein is a bifunctional‐organocatalyst‐mediated enantioselective inverse‐electron‐demand 1,3‐dipolar cycloaddition of C,N‐cyclic azomethine imines with azlactones. The strategy provides concise access to enantioenriched C1‐substituted tetrahydroisoquinolines featuring a pyrazolidinone scaffold. Moreover, the scalability and practical utility of this protocol was well demonstrated by employing a gram‐scale reaction and some representative transformations.     

Asymmetric Cyclization Reactions of Allenoates with Imines or α,β‐Unsaturated Ketones Catalyzed by Organocatalysts Derived from Cinchona Alkaloids

Lewis base‐catalyzed cyclization reactions of allenoates with electron‐deficient olefins and imines have been demonstrated by the preparation of biologically active natural products and pharmaceutically interesting substances and have emerged as powerful synthetic tools in the rapid construction of cyclic molecular complexity. In contrast to phosphine‐containing Lewis bases, nitrogen‐containing Lewis base amines display markedly different reaction profiles; however, this area is not well‐developed. Herein we summarize the recent progress in this emerging field and outline the challenges ahead.