In Situ Conformational Fixation of the Amide Bond Enables General Access to Medium-Sized Lactams via Ring-Closing Metathesis

Synthesis of Phenanthrenes and 1-Hydroxyphenanthrenes via Aromatization-Assisted Ring-Closing Metathesis: toward Polynuclear Aromatic Hydrocarbons

This study presents an efficient synthetic strategy for phenanthrenes and 1-hydroxyphenanthrenes through aromatization-assisted ring-closing metathesis (RCM). It involves vinylation of 1-bromo-2-naphthaldehyde derivatives, Barbier allylation, and subsequent one-pot RCM/dehydration of the diene precursors to yield phenanthrene derivatives. Further, the corresponding keto analogues of diene precursors produce 1-hydroxyphenanthrenes through RCM and aromatization-driven keto-enol tautomerism. This pathway enables rapid access to a diverse array of functionalized phenanthrenes and 1-hydroxyphenanthrenes, including synthetically challenging derivatives containing both -OH and -OMe groups via the sequential construction of the terminal phenanthrene ring.

Heterogeneous Photocatalytic Ring Expansion of Cyclic Ketones for the Construction of Medium-Sized Lactams

Construction of medium-sized ring compounds remains challenging in synthetic chemistry. Herein, we describe the synthesis of medium-sized lactams via a photoinduced ring expansion of benzo-fused cyclic ketones using graphitic carbon nitride (g-C3N4) as a photocatalyst. The ring expansion protocol provided an efficient access to 8-10-membered lactams in good yields and displayed good tolerance to a range of functional groups. The mechanism studies revealed that the photochemical reaction proceeds via an intermediary of a nitrogen radical, which is generated through an oxidative hydrogen atom transfer (HAT) process.

FeCl3-catalyzed regioselective ring-opening of aryl oxirane with 4-hydroxycoumarin for the synthesis of furo[3,2-c]coumarins

The regioselective ring-opening of aryl oxiranes was investigated with various 4-hydroxycoumarins in dimethyl sulfoxide in the presence of 20 mol% FeCl3 as a catalyst at 110 °C. This approach provided a short and concise synthetic route for the regioselective synthesis of 2-aryl-4H-furo[3,2-c] coumarin derivatives. Product formation occurred through regioselective ring-opening of the aryl oxirane at a less hindered site, followed by dehydration and concomitant cyclization. The salient features of our protocol were: cost-effectiveness; short reaction time; step- and atom economy; easy handling; broad scope of substrates; regioselectivity; good-to-excellent yields; non-requirement of dry solvents, co-catalysts, ligands, or any other additives; inert atmospheric conditions.

pH-Dependent Conformational Switching of Amide Bonds─from Full trans to Full cis and Vice Versa

Strategies enabling the pH-dependent conformational switching of amide bonds from trans to cis, and vice versa, are yet limited in the sense that, in a suitable pH range, one rotamer may be stabilized to a large extent while the complementary pH range only leads to a mixture of isomers. By exploiting the effects of steric demand and the interaction of the amide carbonyl with a positive charge, we herein present the first examples for reversible pH-dependent switching from full trans to full cis.

Synthesis and development of seven-membered constrained cyclic urea based PSMA inhibitors via RCM

Intramolecular ring-closing metathesis on an N,N-diallyl Glu-urea-Gly substrate affords 7-membered cyclic ureas as inhibitors of prostrate specific membrane antigen (PMSA).

Cyclopropene-Templated Assembly of Medium Cycles via Ru-Catalyzed Ring-Closing Metathesis

An expeditious click-click cyclize strategy for the assembly of medium-sized heterocyclic rings is described. The sequence involves the reaction of cycloprop-2-ene carboxylic acids with unsaturated amines to furnish amides, which are further subjected to a Cu-catalyzed directed carbomagnesiation and a ring-closing olefin metathesis reaction. This methodology allows for the efficient preparation of lactams with ring sizes up to 10.

Total Synthesis of (+)-Jatrophalactam

The first asymmetric total synthesis of (+)-jatrophalactam was reported, which unambiguously determined the absolute configuration of the titled natural product. The key features entail a conformationally controlled cyclopropanation, a Meldrum's acid adduct-engaged macrolactam formation, and a Pd(II)-mediated oxidative cyclization.

Nitromethane as a nitrogen donor in Schmidt-type formation of amides and nitriles

The Schmidt reaction has been an efficient and widely used synthetic approach to amides and nitriles since its discovery in 1923. However, its application often entails the use of volatile, potentially explosive, and highly toxic azide reagents. Here, we report a sequence whereby triflic anhydride and formic and acetic acids activate the bulk chemical nitromethane to serve as a nitrogen donor in place of azides in Schmidt-like reactions. This protocol further expands the substrate scope to alkynes and simple alkyl benzenes for the preparation of amides and nitriles.

Modulation of Conformational Preferences of Heteroaromatic Ethers and Amides through Protonation and Ionization: Charge Effect

Multiple approaches reveal the strong effects of a positive charge introduced by protonation or ionization on the conformation of o‐heteroaromatic ethers and amides. The ethers and amides containing an ortho‐N heteroatom are syn‐preferring while those containing an ortho‐O or ortho‐S heteroatom are mostly anti‐preferring. However, for all the monocyclic o‐heteroaromatic ethers and amides, the protonated ones are all anti‐preferring while the ionized ones are all syn‐preferring. Interestingly, although both the protonation and ionization introduce a positive charge, they have such different effects on molecular conformation, very informative for understanding the origin of conformational preferences. Detailed analysis shows that the population of the introduced positive charge dictates the conformational preferences via electrostatic and orbital interactions. Compared to ortho‐heteroatoms, meta‐heteroatoms have weaker effect on conformational preference. Achieved by complete inductive method, the regularity of conformational preferences and switching provides easy ways to modulate conformers (by pH or redox), and makes this kind of ether or amide bond a conformational hinge applicable to design of functional molecules (drugs and materials) and modulation of molecular biological processes.