Synthesis of highly enantioenriched 3,4-dihydroquinolin-2-ones by 6-exo-trig radical cyclizations of axially chiral alpha-halo-ortho-alkenyl anilides

Metal-free synthesis of carbamoylated dihydroquinolinones via cascade radical annulation of cinnamamides with oxamic acids

We report a metal-free procedure for the sustainable synthesis of carbamoylated dihydroquinolinones via tandem addition-cyclization of carbamoyl radicals to cinnamamides. Readily accessible, non-toxic and inexpensive oxamic acids are used as carbamoyl radical precursors. This highly straightforward method provides a mild and environmentally friendly route showing good atom economy and excellent functional group tolerance to obtain diverse medicinally important carbamoylated dihydroquinolinones in one pot. The cascade cyclization is also modular and step-economical with a wide substrate scope and the products were obtained in good to excellent yields. Additionally, the tolerance to air and water, operational simplicity, low cost and scalability enhance the practical value of the proposed synthetic strategy. Preliminary mechanistic studies reveal that cheap and environment-friendly ammonium persulfate acts as a radical initiator in the cascade process and generates carbamoyl radicals from oxamic acids. The synthetic utility of this method is further demonstrated by late stage functionalization of drug molecules with good yields.

Visible-Light-Induced Metal- and Photocatalyst-Free Radical Cascade Cyclization of Cinnamamides for Synthesis of Functionalized Dihydroquinolinones

Visible-light-promoted metal- and photocatalyst-free radical cascade cyclization of cinnamamides with α-oxocarboxylic acids is described for sustainable synthesis of diverse pharmaceutically important dihydroquinolinone scaffolds in one pot under mild conditions. The decarboxylative cascade cyclization proceeded efficiently at room temperature without the need for expensive photocatalysts such as Ir or Ru complexes, which indicates the practicability and environmentally benign nature of this protocol. Preliminary mechanistic studies reveal that the blue LED irradiation efficiently cleaves the I-O bond of the hypervalent iodine reagent PhI(O₂CCOAr)₂ formed through ligand exchange between iodobenzene diacetate and arylglyoxylic acid to initiate the cascade reaction. The synthetic value of this operationally simple and energy-efficient method is further demonstrated by late-stage functionalization of drug molecules in excellent yields.

Attrition-Enhanced Asymmetric Transformation of Axially Chiral Nicotinamides by Dynamic Chiral Salt Formation

Atroposelective resolution for axially chiral nicotinamides was achieved by dynamic chiral salt formation with L-DBTA using six types of nicotinamides that could not be optically resolved by the preferential crystallization method. Kinetic studies of their racemization indicated that the chiral conformation was retained for a significant period of time. Two methods of crystallization-induced asymmetric transformation were examined by dynamic diastereomeric salt formation: solvent evaporation from a supersaturated solution, and attrition-enhanced asymmetric transformation. The attrition method was more effective for asymmetric amplification of diastereomeric salts of axially chiral materials. Attrition of equimolar amount of the nicotinamide salts with L-DBTA converged to one diastereomer salts, and the corresponding enantiomers in 87-99 % ee were obtained after the chiral acid was removed. Changing the ratio of two of the nicotinamides with L-DBTA to 1 : 2 inverted the axial chirality.

Meso bromination and derivatization of synthetic bacteriochlorins

Twelve bacteriochlorin building blocks featuring meso-substitution have been prepared including a set with finely tuned long-wavelength absorption (725–757 nm) for studies in photonics.

Characterising Supramolecular Architectures in Crystals Featuring I⋯Br Halogen Bonding: Persistence of X⋯X’ Secondary-Bonding in Their Congeners

The Cambridge Structural Database was surveyed for crystals featuring I⋯Br secondary-bonding in their supramolecular assemblies occurring independently of other obvious supramolecular synthons and devoid of other halogen bonding interactions. In all, 41 crystals satisfied these criteria, with nine examples of zero-dimensional aggregation (uniformly two-molecule aggregates) and 30 one-dimensional chains of varying topology (linear, zigzag and helical). There is one example each of two- and three-dimensional patterns. Type-I, type-II and intermediate bonding situations are apparent; for type-II bonding, the ratio of iodide:bromide functioning as the electrophile is 2:1. Most molecules participated, on average, in one I⋯Br contact, although smaller numbers of half (zero-dimensional) or two contacts (two- and three-dimensional) were observed. The propensity of the formation of related halogen bonding interactions in congeners of the 41 investigated crystals was also studied. Congeners were apparent for 11 crystals, with seven of these exhibiting isostructural relationships, in terms of space-group symmetry and unit-cell parameters. Isostructural relationships do not ensure the formation of analogous aggregation patterns, particularly and in accord with expectation, for the lighter halides. When formed, often distinct aggregation patterns are observed despite the isostructural relationships. Hetero-atomic halogen bonding offers surprises and opportunities in crystal engineering endeavours.

H· Transfer-Initiated Synthesis of γ-Lactams: Interpretation of Cycloisomerization and Hydrogenation Ratios

A cobaloxime/H2 system used to synthesize valuable γ-lactams from acrylamide molecules is described. In addition to cycloisomerized lactams, linear hydrogenated products were also observed. The amounts of the hydrogenation product were observed to correlate with the bulk of the substituent on the acrylamide nitrogen. Further analysis of the product distributions with experimental and computational studies suggested that while cyclization can occur from one C=C acrylamide rotamer, hydrogenation can occur from both. This observation was further evinced through calculation of the hydrogenation rate constant, which was observed to be ca. 102 faster than previously determined for a related system using n Bu3SnH.

Atroposelective Catalytic Asymmetric Allylic Alkylation Reaction for Axially Chiral Anilides with Achiral Morita-Baylis-Hillman Carbonates

A highly efficient method to access axially chiral anilides through asymmetric allylic alkylation reaction with achiral Morita-Baylis-Hillman carbonates by using a biscinchona alkaloid catalyst was reported. Through the atroposelective approach, a broad range of axially chiral anilide products with different acyl groups, such as substituted phenyl, naphthyl, alkyl, enyl, styryl, and benzyl, were generated with very good yields, moderate to excellent cis: trans ratios, and good to excellent enantioselectivities. The reaction can be scaled up, and the synthetic utility of axially chiral anilides was proved by transformations. Moreover, the linear free energy relationship analysis was introduced to investigate the reaction.

A combined density functional theory and numerical simulation investigation of levels of chirality transfer and regioselectivity for the radical cyclizations of N-methyl-, N-ethyl- and N-isopropyl-substituted ortho-halo-N-acryloylanilides

There is a high chirality transfer ratio when the N-alkyl substituent is larger in volume than isopropyl.

A phosphine mediated sequential annulation process of 2-tosylaminochalcones with MBH carbonates to construct functionalized aza-benzobicyclo[4.3.0] derivatives

A novel phosphine mediated sequential annulation process to construct functionalized aza-benzobicyclo[4.3.0] derivatives has been developed involving a one-pot process.

Axially Chiral Enamides: Substituent Effects, Rotation Barriers, and Implications for their Cyclization Reactions

The barrier to rotation around the N-alkenyl bond of 38 N-alkenyl-N-alkylacetamide derivatives was measured (ΔG(⧧) rotation varied between <8.0 and 31.0 kcal mol(-1)). The most important factor in controlling the rate of rotation was the level of alkene substitution, followed by the size of the nitrogen substituent and, finally, the size of the acyl substituent. Tertiary enamides with four alkenyl substituents exhibited half-lives for rotation between 5.5 days and 99 years at 298 K, sufficient to isolate enantiomerically enriched atropisomers. The radical cyclizations of a subset of N-alkenyl-N-benzyl-α-haloacetamides exhibiting relatively high barriers to rotation round the N-alkenyl bond (ΔG(⧧) rotation >20 kcal mol(-1)) were studied to determine the regiochemistry of cyclization. Those with high barriers (>27 kcal mol(-1)) did not lead to cyclization, but those with lower values produced highly functionalized γ-lactams via a 5-endo-trig radical-polar crossover process that was terminated by reduction, an unusual cyclopropanation sequence, or trapping with H2O, depending upon the reaction conditions. Because elevated temperatures were necessary for cyclization, this precluded study of the asymmetric transfer in the reaction of individual atropisomers. However, enantiomerically enriched atropsiomeric enamides should be regarded as potential asymmetric building blocks for reactions that can be accomplished at room temperature.

A simple approach to a new T8-POSS based MRI contrast agent

A simple synthesis of a new nanoglobular T₈-silsesquioxane based contrast agent for the application in MRI is reported.

Development of CINPA1 analogs as novel and potent inverse agonists of constitutive androstane receptor

Constitutive androstane receptor (CAR, NR1I3) and pregnane X receptor (PXR, NR1I2) are master regulators of endobiotic and xenobiotic metabolism and disposition. Because CAR is constitutively active in certain cellular contexts, inhibiting CAR might reduce drug-induced hepatotoxicity and resensitize drug-resistant cancer cells to chemotherapeutic drugs. We recently reported a novel CAR inhibitor/inverse agonist CINPA1 (11). Here, we have obtained or designed 54 analogs of CINPA1 and used a time-resolved fluorescence resonance energy transfer (TR-FRET) assay to evaluate their CAR inhibition potency. Many of the 54 analogs showed CAR inverse agonistic activities higher than those of CINPA1, which has an IC50 value of 687 nM. Among them, 72 has an IC50 value of 11.7 nM, which is about 59-fold more potent than CINPA1 and over 10-fold more potent than clotrimazole (an IC50 value of 126.9 nM), the most potent CAR inverse agonist in a biochemical assay previously reported by others. Docking studies provide a molecular explanation of the structure-activity relationship (SAR) observed experimentally. To our knowledge, this effort is the first chemistry endeavor in designing and identifying potent CAR inverse agonists based on a novel chemical scaffold, leading to 72 as the most potent CAR inverse agonist so far. The 54 chemicals presented are novel and unique tools for characterizing CAR's function, and the SAR information gained from these 54 analogs could guide future efforts to develop improved CAR inverse agonists.

Domino Acylation/Diels-Alder Synthesis of N-Alkyl-octahydroisoquinolin-1-one-8-carboxylic Acids under Low-Solvent Conditions

The development of the domino reaction between an aminoethyl-substituted diene and maleic anhydride to afford an N-substituted octahydroisoquinolin-1-one is described. A typical procedure involves the treatment of a 1-aminoethyl-substituted butadiene with maleic anhydride at 0 °C to room temperature for 20 min under low-solvent conditions, which affords a series of isoquinolinone carboxylic acids in moderate to excellent yields. NMR monitoring suggested that the reaction proceeded via an initial acylation step followed by an intramolecular Diels-Alder reaction. For the latter step, a significant rate difference was observed depending on whether the amino group was substituted by a phenyl or an alkyl (usually benzyl) substituent, with the former noted by NMR to be substantially slower. The Diels-Alder step was studied by density functional theory (DFT) methods, leading to the conclusion that the degree of preorganization in the starting acylated intermediate had the largest effect on the reaction barriers. In addition, the effect of electronics on the aromatic ring in N-phenyl substrates was studied computationally and experimentally. Overall, this protocol proved considerably more amenable to scale up compared to earlier methods by eliminating the requirement of microwave batch chemistry for this reaction as well as significantly reducing the quantity of solvent.

Radical [3 + 2]-annulation of divinylcyclopropanes: rapid synthesis of complex meloscine analogs

A radical [3 + 2]-divinylcyclopropane annulation cascade has been extended to encompass five D-ring variants of the meloscine/epimeloscine core structure. Representative ABCD tetracyclic intermediates were further elaborated with novel substituted E-rings through subsequent transformations of advanced intermediates that provided opportunities for late-stage variation of the B-ring (lactam) N-substituents which were also developed.

Deracemization of axially chiral nicotinamides by dynamic salt formation with enantiopure dibenzoyltartaric acid (DBTA)

Dynamic atroposelective resolution of chiral salts derived from oily racemic nicotinamides and enantiopure dibenzoyltartaric acid (DBTA) was achieved by crystallization. The absolute structures of the axial chiral nicotinamides were determined by X-ray structural analysis. The chirality could be controlled by the selection of enantiopure DBTA as a chiral auxiliary. The axial chirality generated by dynamic salt formation was retained for a long period after dissolving the chiral salt in solution even after removal of the chiral acid. The rate of racemization of nicotinamides could be controlled based on the temperature and solvent properties, and that of the salts was prolonged compared to free nicotinamides, as the molecular structure of the pyridinium ion in the salts was different from that of acid-free nicotinamides.

Rotational isomers of N-methyl-N-arylacetamides and their derived enolates: implications for asymmetric Hartwig oxindole cyclizations

The rotational preferences of N-(2-bromo-4,6-dimethylphenyl)-N-methyl 2-phenylpropanamide were studied as a model of precursors for Hartwig asymmetric oxindole cyclizations. The atropisomers of this compound were separated by flash chromatography, and then the enantiomers were resolved and the interconversions of the stereocenter and the N-Ar axis were studied. Under thermal conditions, the axis is very stable. Under the basic conditions of the Hartwig cyclization, both the stereocenter and the chiral axis equilibrate via enolate formation. The N-Ar rotation barrier of a 2-phenylacetamide analogue was reduced from 31 kcal mol(-1) in the precursor to 17 kcal mol(-1) in the enolate. Reasons for this dramatic barrier reduction and implications of both N-Ar and amide C-N rotations for Hartwig cyclizations are discussed.

A short total synthesis of (±)-epimeloscine and (±)-meloscine enabled by a cascade radical annulation of a divinylcyclopropane

The first stereoselective synthesis of epimeloscine has been accomplished in 13 total steps with a longest linear sequence of 10 steps. The core of the synthesis takes only five steps, the key ones being acylation, stereoselective tandem radical cyclization of a divinylcyclopropane to make two rings, and group-selective ring-closing metathesis of the resulting divinylcyclopentane to make the last ring.

Diverging chemoselective reactions of separable amide rotational isomers

Diverging chemoselective reactions of a pair of amide rotamers have been observed by separating the rotamers and then reacting them individually. Reduction of (Z)-N-allyl-2-(phenylselanyl)-N-(2,4,6-tri-tert-butylphenyl)acetamide with tributyltin hydride at room temperature provides only the product of 5-exo cyclization, 4-methyl-1-(2,4,6-tri-tert-butylphenyl)pyrrolidin-2-one. In contrast, reduction of the corresponding (E) amide rotational isomer under identical conditions provides only the reduced product, (E)-N-allyl-N-(2,4,6-tri-tert-butylphenyl)acetamide. Such diverging reactions of rotamers may be common in transformations involving reactive intermediates (carbenes, radicals, organometallic intermediates) that have low barriers to onward reactions relative to amide rotation.