Highly Stereoselective Synthesis of Tricyclic Chromenoisoxazolidines by Intramolecular 1,3-Dipolar Cycloadditions

Eutectic Media Open a Synthetic Route to Oligocitrazinic Acid Fluorophores of Purple Hue

Under isochoric and solvent-free conditions, the reaction between ammonium formate and citric acid results in a deeply purple reaction product with fluorescent properties. This brings this reaction in the realm of bio-based fluorophores and bottom-up carbon nanodots from citric acid. The reaction conditions are optimized in terms of UV-vis spectroscopic properties and, subsequently, the main reaction product is separated. While the structural analysis does not give any indication for carbon nanodots in a general sense, it points towards the formation of molecular fluorophores that consist of oligomerized citrazinic acid derivatives. Furthermore, EPR spectroscopy reveals the presence of stable free radicals in the product. We hypothesize that such open-shell structures may play a general role in molecular fluorophores from citric acid and are not yet sufficiently explored. Therefore, we believe that analysis of these newly discovered fluorophores may contribute to a better understanding of the properties of fluorophores and CND from citric acid in general.

Nitronate-aryne cycloaddition as a concise route to stereochemically complex fused benzisoxazolines and amino alcohols

The reaction of cyclic nitronates (isoxazoline N-oxides and 5,6-dihydro-4H-1,2-oxazine N-oxides) with Kobayashi's aryne precursors affords tricyclic benzene-fused nitroso acetals as a result of [3 + 2]-cycloaddition. The process is regio- and stereoselective in most cases and produces the target cycloadducts possessing up to four contiguous stereogenic centers. These nitroso acetals were shown to be convenient precursors of valuable polysubstituted aminodiols through catalytic hydrogenolysis of the N-O bonds. Also, the action of protic acids resulted in an unusual fragmentation of the cyclic nitroso acetal moiety through heterolytic N-O bond cleavage and Beckmann-type reaction. Using this acid-mediated reaction, the synthesis of a hitherto unknown hexahydrobenzo[4,5]isoxazolo[2,3-a]azepine scaffold was accomplished.

Domino ring opening and selective O/S-alkylation of cyclic ethers and thioethers

Chemoselective domino ring opening and selective O/S-alkylation of ethers/thioethers over enol C/O alkylation has been observed. Various 2-aryl chromanones/thiochromanones with alkyl/allyl bromides were smoothly converted into the corresponding highly functionalized ethers and thioethers in excellent yields with high selectivity. An unusual, chemoselective domino ring opening and selective ether/thioether O/S-alkylation over. An unusual, chemoselective domino ring opening and selective ether/thioether O/S alkylation over enol C/O alkylation has been observed.

Intramolecular [3 + 2] nitrone cycloaddition reaction: highly regio and diastereoselective synthesis of bicyclo[3.2.1]octane scaffolds

A regio and diastereoselective strategy has been developed for the synthesis of complex bicyclo[3.2.1]octane scaffolds from the reaction of readily accessible vinylogous carbonates with N-substituted hydroxylamine hydrochlorides via intramolecular 1,3-dipolar nitrone cycloaddition reaction. Wide varieties of bicyclic isoxazolidines were synthesized in high yields under catalyst free conditions through a highly efficient and operationally simple protocol.

Rhodium-Catalyzed Diastereoselective [3 + 2] Cycloaddition of Carbonyl Ylide: An Access to the Core Ring System of Cordigol and Lophirone H

This paper describes a new synthetic strategy for the construction of tricyclic chromeno/quinolino furan frameworks via creation of two new rings and three contiguous stereogenic centers with high diastereoselectivity through a rhodium-catalyzed intramolecular carbonyl ylide cycloaddition reaction for the first time. This protocol allows the synthesis of the core ring system of natural products such as cordigol and lophirone H.

Cascade annulation reaction (CAR): highly diastereoselective synthesis of pyranopyrazole scaffolds

An unprecedented domino protocol for the novel synthesis of highly diverse and functionalized tetrahydro pyranopyrazole scaffolds using chalcone epoxide has been reported for the first time. This synthetic protocol generates three consecutive stereogenic centres in a highly diastereoselective manner with the formation of vicinal diol and a quaternary carbon centre. A wide range of substrates were utilized for the scope of this methodology and provided very good yields of pyranopyrazoles. The pyranopyrazoles were also transformed into densely functionalized tetrasubstituted olefins.

An unprecedented domino protocol for the novel synthesis of highly diverse and functionalized tetrahydro pyranopyrazole scaffolds using chalcone epoxide has been reported.

A novel multicomponent quadruple/double quadruple domino reaction: highly efficient synthesis of polyheterocyclic architectures

A novel multicomponent quadruple domino reaction (MCQDR) for the assembly of structurally complex molecular architectures via the formation of three rings and three contiguous stereogenic centers has been accomplished with high regio- and diastereoselectivity. Solvents, catalysts and work-up were not required to obtain the target molecules. In addition, this new protocol is also extended for the multicomponent double quadruple domino reaction (MCDQDR) to create novel polyheterocyclic architectures in an orthogonal manner.

A rearrangement involving a solid-state melt reaction for the synthesis of multifunctional tetrasubstituted olefins

A new catalyst- and solvent-free intramolecular rearrangement sequence leading towards benzimidazole-tethered tetrasubstituted olefins through a solid-state melt reaction (SSMR).

Solvent and catalyst free ring expansion of indoles: a simple synthesis of highly functionalized benzazepines

Highly functionalized benzazepines have been synthesized from indoles via ring expansion. Ring contraction of benzazepines to indole derivatives has also been acheived in excellent yields.

Highly regio- and diastereo-selective synthesis of novel tri- and tetra-cyclic perhydroquinoline architectures via an intramolecular [3 + 2] cycloaddition reaction

A facile and efficient synthetic protocol was established for the construction of novel tri- and tetra-cyclic pyrrolo/pyrrolizinoquinoline architectures via the in situ formation of azomethine ylide followed by an intramolecular [3 + 2] cycloaddition reaction strategy. This protocol leads to the creation of two/three new rings and three/four contiguous stereocentres, in which one of them is a tetra-substituted carbon center, in a highly diastereoselective fashion with excellent yields.

Diastereoselective synthesis of novel tetra-and pentacyclic annulated coumarino-δ-sultam pyrrolidine, pyrrolizidine, pyrrolothiazole and isoxazolidine derivatives via intramolecular 1,3-dipolar cycloadditions

A facile and efficient strategy towards the novel tetra- and pentacyclic annulated coumarino-δ-sultam pyrrolidine, pyrrolizidine, pyrrolothiazole and isoxazolidine via intramolecular 1,3-dipolar cycloadditions of is described.

A novel protocol for the facile construction of tetrahydroquinoline fused tricyclic frameworks via an intramolecular 1,3-dipolar nitrile oxide cycloaddition reaction

An efficient method towards the synthesis of quinoline fused tricyclic compounds involving an intramolecular 1,3-dipolar nitrile oxide cycloaddition reaction utilizing Baylis–Hillman derivatives in good yields has been described for the first time.

A new route for the synthesis of highly substituted 4-aminoquinoline drug like molecules via aza hetero–Diels–Alder reaction

A new route has been developed for the synthesis of 4-aminoquinoline drug like moleculesviaaza hetero–Diels–Alder reaction starting from 2H-indazole as a diene for the first time.

G, Malik A, Kumar V, Chatterjee A, Roy BG, Banerjee M. In situ mechanochemical synthesis of nitrones followed by 1,3-dipolar cycloaddition: a catalyst-free, “green” route to cis-fused chromano[4,3-c]isoxazoles

An efficient, catalyst free mechanochemical route to cis-fused chromano[4,3-c]isoxazoles has been developed via a simple mortar-pestle grinding method.

A facile synthesis of isoxazolo[3,4-a]pyrrolizine and isoxazolo[4,3-c]pyridine derivatives via intramolecular nitrone cycloaddition reaction

Isoxazolo [3,4-a]pyrrolizine and isoxazolo[4,3-c]pyridine are formed by a facile and efficient intramolecular [3 + 2] nitrone cycloaddition, forming two new rings and three contiguous stereocentres with high diastereoselectivity and good yields.

Methyl 1-phenyl-3-p-tolyl-1,9b-dihydro-3H-chromeno[4,3-c]isoxazole-3a(4H)-carboxylate

In the title compound, C25H23NO4, the pyran ring of the chroman moiety has an envelope conformation with the methylene C atom as the flap. The isoxazole ring has a twist conformation on the O—C bond. The dihedral angle between their mean planes is 57.87 (9)°. The attached phenyl and benzene rings are twisted away from its mean plane by 56.19 (10) and 50.57 (10)°, respectively. These two rings are normal to each other, subtending a dihedral angle of 89.2 (1)°. In the crystal, there are no classical hydrogen bonds; the molecules are linkedviaC—H...π interactions, forming a two-dimensional network lying parallel to (10-1).

(E)-3-(2-Chlorophenyl)-2-[(2-formylphenoxy)methyl]prop-2-enenitrile

In the title compound, C17H12ClNO2, the dihedral angle between the two benzene rings is 42.9 (1)°. There are no sgnificant intermolecular interactions.

(2S,5S,6R)-5-(4-Methyl-phen-yl)-3-phenyl-4,8-dioxa-3-aza-tricyclo-[7.4.0.0]trideca-1(13),9,11-triene-6-carbonitrile

In the title compound, C(24)H(20)N(2)O(2), the six-membered pyran ring adopts a half-chair conformation with one C atom deviating from the mean plane of the remaining ring atoms by 0.654 (6) Å. The five-membered isoxazole ring adopts an N-envelope conformation with the N atom displaced by 0.742 (5) Å from the mean plane formed by the remaining ring atoms. The carbonitrile side chain is almost linear, with a C-C-N angle of 178.6 (5)°. The crystal packing is stabilized by inter-molecular C-H⋯N inter-actions, through bifurcated acceptor hydrogen bonds formed between the carbonitrile N atom and two alternate C atoms in the unsubstituted benzene ring. The mol-ecular structure and crystal packing are further stabilized by intra-molecular and inter-molecular C-H⋯π inter-actions.

(E)-2-[(2-Formylphenoxy)methyl]-3-(4-methylphenyl)prop-2-enenitrile

In the title compound, C₁₈H₁₅NO₂, the dihedral angle between the two benzene rings is 74.8 (1)°. The carbonitrile chain is almost linear, the C—C—N angle being 176.2 (2)°. In the crystal, π–π inter­actions [centroid–centroid distance = 3.842 (1) Å] are observed.

1-Methyl-3-p-tolyl-3,3a,4,9b-tetrahydro-1H-chromeno[4,3-c]isoxazole-3a-carbonitrile

In the title compound, C₁₉H₁₈N₂O₂, the dihedral angle between the mean planes of the fused chromeno and isoxazole units is 43.71 (7)°. The isoxazole and pyran rings exhibit envelope and half chair conformations, respectively. The crystal packing is stabilized by inter­molecular C—H⋯π inter­actions.

1-Methyl-3-(2-methyl-phen-yl)-3,3a,4,9b-tetra-hydro-1H-chromeno[4,3-c][1,2]oxazole-3a-carbonitrile

In the title compound, C(19)H(18)N(2)O(2), the five-membered isoxazole ring adopts an envelope conformation and the deviation of the N atom from the mean plane of the isoxazole ring is -0.3256 (11) Å. The pyran ring adopts a half-chair conformation. The isoxazole ring forms dihedral angles of 44.07 (7) and 84.23 (7)° with the pyran and methyl-benzene rings, respectively. The mol-ecular structure is stabilized by weak C-H⋯π inter-actions.

1-Methyl-3-(naphthalen-1-yl)-3,3a,4,9b-tetra-hydro-1H-chromeno[4,3-c]isoxazole-3a-carbonitrile

In the title compound, C(22)H(18)N(2)O(2), the pyran ring of the chromene unit is fused with an isoxazole ring, which adopts an N-envelope conformation with the N atom lying 1.3291 (14) Å from the mean plane of the remaining ring atoms [maximum deviation = 0.341 (2) Å]. The dihedral angle between the isoxazole and chromene units is 43.74 (8)° and that between the iosxazole ring and the naphthalene ring system is 58.82 (8)°. In the crystal, the molecules are linked by weak C-H⋯π inter-actions.

3-(3-Chloro-phen-yl)-1-methyl-3,3a,4,9b-tetra-hydro-1H-chromeno[4,3-c]isoxazole-3a-carbonitrile

In the title compound, C(18)H(15)ClN(2)O(2), the five-membered isoxazole ring adopts an envelope conformation [the deviation of the N atom is 0.3154 (15) Å] and the six-membered pyran ring adopts a half-chair conformation. The mean plane through all atoms of the isoxazole ring forms dihedral angles of 47.98 (8)° with the mean plane of the chromene ring system and 75.10 (9)° with the chloro-benzene ring.