Synthesis and Reactions of Highly Strained 2,3-Bridged 2H-Azirines

Nitrene C-H Bond Insertion Approach to Carbazolones and Indolones, and a Reactivity Departure for 7-Membered Analogues

A modular platform for facile access to 1,2,3,9-tetrahydro-4H-carbazol-4-ones (H4 -carbazolones) and 3,4-dihydrocyclopenta[b]indol-1(2H)-ones (H2 -indolones) is described. The requisite 6- and 5-membered 2-arylcycloalkane-1,3-dione precursors were readily obtained through a Cu-catalyzed arylation of 1,3-cyclohexanediones or by a ring expansion of aryl succinoin derivatives. Enolization of one carbonyl group in the diones, conversion to a leaving group, and subsequent azidation gave 2-aryl-3-azidocycloalk-2-en-1-ones. This two-step, one-pot azidation is highly regioselective with unsymmetrically substituted 2-arylcyclohexane-1,3-diones. The regioselectivity, which is important for access to single isomers of 3,3-disubstituted carbazolones, was analyzed mechanistically and computationally. Finally, a Rh-catalyzed nitrene/nitrenoid insertion into the ortho C-H bond of the aryl moiety gave the H4 -carbazolones and H2 -indolones. One carbazolone was elaborated to an intermediate reported in the total synthesis of N-decarbomethoxychanofruticosinate, (-)-aspidospermidine, (+)-kopsihainanine A. With 2-phenylcycloheptane-1,3-dione, prepared from cyclohexanone and benzaldehyde, the azidation reaction was readily accomplished. However, the Rh-catalyzed reaction unexpectedly led to a labile but characterizable azirine rather than the indole derivative. Computations were performed to understand the differences in reactivities of the 5- and 6-membered 2-aryl-3-azidocycloalk-2-en-1-ones in comparison to the 7-membered analogue, and to support the structural assignment of the azirine.

Direct Access to Quinone-Fused 5-Substituted-1,4-Benzodiazepine Scaffolds from Azidoquinones with/without [1,2]-Azide-Nitrogen Migration: Mechanistic Insights

Seven-membered nitrogen heterocycles have a strong influence in drug discovery due to their inherent 3D character, which allows the ability to explore a vast conformational space with a biological target. Notably, the privileged 1,4-benzodiazepine scaffold is dominant in treating the central nervous system due to its binding affinity with the GABAA receptor. Herein, we report a protocol for the transformation of azidoquinones to p-quinone fused 5-substituted-1,4-benzodiazepines (p-QBZDs) from InCl3-catalyzed intermolecular tandem cycloannulation of azidoquinones with amines and aldehydes. Detailed mechanistic studies reveal that the EDA complex between azidoquinones and InCl3 is crucial in determining the reaction pathway. In the absence of EDA complex formation, the reaction proceeds via the intermediacy of 2,3-bridged-2H-azirine followed by regiospecific addition of an amine to C═N/ring opening/cyclization to deliver p-QBZD with 1,2-azide-nitrogen migration. In the case of EDA complex formation, the reaction proceeds through regioselective aza-Michael addition/nitrene insertion with aldehyde and subsequent cyclization to deliver p-QBZD and p-quinone fused imidazole as a secondary product without 1,2-azide-nitrogen migration. This protocol provides straightforward access to redox-active quinone embedded 5-substituted-1,4-benzodiazepines from azidoquinones with diverse substrate scopes that would find potential applications in medicinal chemistry and drug discovery.

Formation and Reactivity of Triplet Vinylnitrenes as a Function of Ring Size

The photoreactivity of cyclic vinyl azides 1 (3-azido-2-methyl-cyclopenten-1-one) and 2 (3-azido-2-methyl-2-cyclohexen-1-one), which have five- and six-membered rings, respectively, was characterized at cryogenic temperature with electron paramagnetic resonance (EPR), IR, and UV spectroscopy. EPR spectroscopy revealed that irradiating (λ > 250 nm) vinyl azides 1 and 2 in 2-methyltetrahydrofuran at 10 K resulted in the corresponding triplet vinylnitrenes ³1N (D/hc = 0.611 cm^-1 and E/hc = 0.011 cm^-1) and ³2N (D/hc = 0.607 cm^-1 and E/hc = 0.006 cm^-1), which are thermally stable at cryogenic temperature. Irradiation of vinyl azides 1 (310 nm light-emitting diode at 12 K) and 2 (xenon arc lamp through a 310-350 nm filter at 8 K) in argon matrices showed that in competition with intersystem crossing to form vinylnitrenes ³1N and ³2N, vinyl azide 1 formed a small amount of ketenimine 3, whereas vinyl azide 2 formed significant amounts of azirine 7 and ketenimine 6. Thus, vinyl azide 1 undergoes intersystem crossing more efficiently than vinyl azide 2. Similarly, vinylnitrene ³1N is much more photoreactive than vinylnitrene ³2N. Quantum chemical calculations were used to support the mechanisms for forming vinylnitrenes ³1N and ³2N and their reactivity.

Ultra-low formation of octahydrophenazine in the Beckmann rearrangement of cyclohexanone oxime using a microreactor

An ultra-low content of octahydrophenazine (<1 ppm) was reached in the Beckmann rearrangement of cyclohexanone oxime to yield ε-caprolactam.

Synthesis of 2 H-Azirines via Iodine-Mediated Oxidative Cyclization of Enamines

A facile and practical oxidative cyclization reaction of enamines to 2 H-azirines has been developed, employing molecular iodine. The features of the present synthetic approach include no use of transition metals, mild reaction conditions, and simplicity of operation. Under the optimal reaction conditions, a variety of 2 H-azirine derivatives were synthesized from simple and readily accessible enamine precursors in an efficient and scalable fashion.

Synthesis and biological evaluation of cyanoaziridine phosphine oxides and phosphonates with antiproliferative activity

This work reports an efficient diastereoselective synthetic methodology for the preparation of phosphorus substituted cyanoaziridines through the nucleophilic addition of TMSCN, as cyanide source, to the C-N double bond of 2H-azirine derivatives. The aziridine ring, in these novel cyanoaziridines, can be activated by simple N-tosylation or N-acylation. In addition, the cytotoxic effect on cell lines derived from human lung adenocarcinoma (A549) and human embryonic kidney (HEK293) was also screened. N-H and N-Substituted cyanoaziridines showed excellent activity against the A549 cell line in vitro. Moreover, selectivity towards cancer cell (A549) over (HEK293), and non-malignant cells (MCR-5) has been observed.

Domino reactions of 2H-azirines with acylketenes from furan-2,3-diones: Competition between the formation of ortho-fused and bridged heterocyclic systems

3-Aryl-2H-azirines react with acylketenes, generated by thermolysis of 5-arylfuran-2,3-diones, to give bridged 5,7-dioxa-1-azabicyclo[4.4.1]undeca-3,8-diene-2,10-diones and/or ortho-fused 6,6a,12,12a-tetrahydrobis[1,3]oxazino[3,2-a:3',2'-d]pyrazine-4,10-diones. The latter compounds, with a new heterocyclic skeleton, are the result of the coupling of two molecules of azirine and two molecules of acylketene and can be prepared only from 3-aryl-2H-azirines having no electron-withdrawing groups in the aryl substituent. Calculations at the DFT B3LYP/6-31G(d) level for the various routes of bis[1,3]oxazino[3,2-a:3',2'-d]pyrazine skeleton formation revealed a new domino reaction of 3-aryl-2H-azirines occurring in the presence of furandiones: acid-catalyzed dimerization to dihydropyrazine followed by consecutive cycloaddition of the latter to two molecules of acylketenes.

β-Elimination of an Aziridine to an Allylic Amine: A Mechanistic Study

The base-induced rearrangement of aziridines has been examined using a combination of calculations and experiment. The calculations show that the substituent on nitrogen is a critical feature that greatly affects the favorability of both α-deprotonation, and β-elimination to form an allylic amine. Experiments were carried out to determine whether E2-like rearrangement to the allylic amine with lithium diisopropyl amide (LDA) is possible. N-Tosyl aziridines were found to deprotonate on the tosyl group, preventing further reaction. A variety of N-benzenesulfonyl aziridines having both α- and β-protons decomposed when treated with LDA in either tetrahydrofuran or hexamethylphosphoramide. However, when α-protons were not present, allylic amine was formed, presumably via β-elimination.

Organocatalytic asymmetric Neber reaction for the synthesis of 2H-azirine carboxylic esters

The first enantioselective Neber reaction of β-ketoxime sulfonates catalyzed by a bifunctional thiourea has been developed. The reaction proceeds stereoselectively with 5 mol % of the catalyst to give the 2H-azirine carboxylic esters in good yields with up to 93% ee. In addition, the resulting azirines can be successfully employed in the stereoselective synthesis of di- and trisubstituted aziridines.