Synthesis of Indolizines and Benzoindolizines by Annulation of Donor-Acceptor Cyclopropanes with Electron-Deficient Pyridines and Quinolines

Conformation- and Coordination Mode-Dependent Stimuli-Responsive Salicylaldehyde Hydrazone Zn(II) Complexes

Luminescent Zn(II) complexes that respond to external stimuli are of wide interest due to their potential applications. Schiff base with O,N,O-hydrazone shows excellent luminescence properties with multi-coordination sites for different coordination modes. In this work, three salicylaldehyde hydrazone Zn(II) complexes (1, 2a, 2b) were synthesized and their stimuli-responsive behaviors in different states were explored. Only complex 1 exhibits reversible and self-recoverable photochromic and photoluminescence properties in solution. This may be due to the configuration eversion and the excited-state intramolecular proton transfer (ESIPT) process. In the solid state, 2a has obvious mechanochromic luminescence property, which is caused by the destruction of intermolecular interactions and the transformation from crystalline state to amorphous state. 2a and 2b have delayed fluorescence properties due to effective halogen bond interactions in structures. 2a could undergo crystal-phase transformation into its polymorphous 2b by force/vapor stimulation. Interestingly, 2b shows photochromic property, which can be attributed to the electron transfer and generation of radicals induced by UV irradiation. Due to different conformations and coordination modes, the three Zn(II) complexes show different stimuli-responsive properties. This work presents the multi-stimuli-responsive behaviors of salicylaldehyde hydrazone Zn(II) complexes in different states and discusses the response mechanism in detail, which may provide new insights into the design of multi-stimuli-responsive materials.

Tandem Dearomative/Rearomative (3 + 2) Annulation of Aroyl-Substituted Donor-Acceptor Cyclopropanes with Benzothiazoles

A series of benzo[d]pyrrolo[2,1-b]thiazoles was synthesized by (3 + 2) annulation of aroyl-substituted donor-acceptor cyclopropanes with benzothiazoles. The annulation, promoted by a substoichiometric amount of Sc(OTf)₃, takes place through the formation of the respective dearomatized (3 + 2) adducts, followed by unexpected decarbethoxylative and dehydrogenative rearomatization to afford fully aromatized products. The unusual reactivity is attributed to the presence of an extra aroyl group in the donor-acceptor cyclopropanes.

Donor-Acceptor Cyclopropanes: Activation Enabled by a Single, Vinylogous Acceptor

A novel class of highly activated donor-acceptor cyclopropanes bearing only a single, vinylogous acceptor is presented. These strained moieties readily undergo cycloadditions with aldehydes, ketones, thioketones, nitriles, naphth-2-ols and various other substrates to yield the corresponding carbo- and heterocycles. Diastereocontrol can be achieved through the choice of catalyst (Brønsted or Lewis acid). The formation of tetrahydrofurans was shown to be highly enantiospecific when chiral cyclopropanes are employed. A series of mechanistic and kinetic experiments was conducted to elucidate a plausible catalytic cycle and to rationalize the stereochemical outcome.

A regioselective [4 + 2] annulation approach to 5-acylindolizine-7-carbonitriles: generation of poly-substituted pyridines

The reaction of β-ketonitrile and N-substituted pyrrole-2-carboxaldehyde in the presence of piperidinium acetate in CH₃CN permitted regioselective access to 5-acylindolizine-7-carbonitrile through a domino Knoevenagel condensation-intramolecular aldol cyclization sequence. A wide range of new indolizine derivatives with a poly-functionalized pyridine moiety were readily synthesized in an atom-economical fashion, thereby expanding the indolizine chemical space for further applications.

Catalytic (3 + 2) annulation of donor-acceptor aminocyclopropane monoesters and indoles

The efficient catalytic activation of donor-acceptor aminocyclopropanes lacking the commonly used diester acceptor is reported here in a (3 + 2) dearomative annulation with indoles. Bench-stable tosyl-protected aminocyclopropyl esters were converted into cycloadducts in 46-95% yields and up to 95 : 5 diastereomeric ratio using catalytic amounts of triethylsilyl triflimide. Tricyclic indoline frameworks containing four stereogenic centers including all-carbon quaternary centers were obtained.

Assembly of functionalized π-extended indolizine polycycles through dearomative [3+2] cycloaddition/oxidative decarbonylation

Reported herein is an unexpected construction of functionalized π-extended indolizine polycycles through a one-pot two-step cascade process comprising the base-promoted dearomative [3+2] cycloaddition of quinilinium salts and 3-alkenyl oxindoles, followed by a DDQ-mediated oxidative decarbonylation. Moreover, we could achieve the substrate-controlled diverse synthesis of structurally strained cyclopropane spirooxindole by using pyridinium salts as starting materials.

DBU-promoted ring-opening reactions of multi-substituted donor–acceptor cyclopropanes: access to functionalized chalcones with a quaternary carbon group

A strategy to synthesize highly stereoselective chalcones with alkylcyanoacetate subunits via DBU-promoted ring-opening reactions of multi-substituted D–A cyclopropanes has been developed without the requirement of a transition metal catalyst and extra solvent.

Chemoselective asymmetric dearomative [3 + 2] cycloaddition reactions of purines with aminocyclopropanes

Chemoselective asymmetric [3 + 2] cycloaddition reactions of purines with aminocyclopropanes for the dearomatization of purines have been successfully developed.

Dearomatization of electron poor six-membered N-heterocycles through [3 + 2] annulation with aminocyclopropanes

Dearomative [3 + 2] annulation of N-heterocycles with donor–acceptor aminocyclopropanes: fast access to indolizidine building blocks.

Many abundant and highly bioactive natural alkaloids contain an indolizidine skeleton. A simple, high yielding method to synthesize this scaffold from N-heterocycles was developed. A wide range of pyridines, quinolines and isoquinolines reacted with donor–acceptor (DA)-aminocyclopropanes via an ytterbium(iii) catalyzed [3 + 2] annulation reaction to give tetrahydroindolizine derivatives. The products were obtained with high diastereoselectivities (dr > 20 : 1) as anti-isomers. Additionally, the formed aminals could be easily converted into secondary and tertiary amines through iminium formation followed by reduction or nucleophile addition. This transformation constitutes the first example of dearomatization of electron-poor six-membered heterocycles via [3 + 2] annulation with DA cyclopropanes.

Formal [3+2] cycloaddition of 1-cyanocyclopropane 1-ester with pyridine, quinoline or isoquinoline: a general and efficient strategy for construction of cyanoindolizine skeletons

An efficient and straightforward preparation of cyanoindolizine derivatives via a cycloaddition reaction between 1-cyanocyclopropane 1-ester and pyridine or benzopyridine.

[1-Meth-oxy-3-(pyridin-2-yl)indolizin-2-yl](pyridin-2-yl)methanone

Methyl-ation of [1-hy-droxy-3-(pyridin-2-yl)indolizin-2-yl](pyridin-2-yl)methanone was performed via metalation with potassium tert-butano-late in toluene and a subsequent metathesis reaction with methyl iodide yielded the yellow title compound, C(20)H(15)N(3)O(2). The substituents at the indolizine unit are twisted [the indolizine ring system makes dihedral angles of 34.67 (7) and 77.49 (5)°, respectively, with the pyridyl and pyridinoyl rings] with single bonds between the central unit and the attached pyridine ring [1.459 (3) Å] and the pyridinoyl group [1.483 (3) Å]. There are no classical hydrogen bonds in the crystal structure.

Synthesis and reactivity of bicyclo[3.2.1]octanoid-derived cyclopropanes

Photochemical oxa-di-π-methane rearrangement of bicyclo[3.2.1]octanoid scaffolds affords multifunctional, donor-acceptor cyclopropanes. A related photochemical reaction of an iminium ether substrate uncovered an unprecedented aza-di-π-methane rearrangement of a β,γ-unsaturated iminium. Donor-acceptor cyclopropanes have been evaluated as substrates for reactions generating several new chemotypes.

Regioselective synthesis of 3-acylindolizines and benzo- analogues via 1,3-dipolar cycloadditions of N-ylides with maleic anhydride

3-Acylindolizines (5a-5f) and their benzo- analogues 1-acylpyrrolo[1,2-a]quinolines (6a-6f) and 1-acylpyrrolo[2,1-a]isoquinolines (7a-7i) are regioselectively synthesized by a convenient one pot reaction of the corresponding pyridinium (quinolinium, isoquinolinium) ylide with maleic anhydride (MA) in the presence of the mild oxidant tetrakispyridinecobalt(ii) dichromate (TPCD). These reactions proceed via a tandem reaction sequence of 1,3-dipolar cycloaddition of azomethine ylide with MA, anhydride hydrolysis and oxidative bisdecarboxylation of the primary cycloadducts followed by dehydrogenative aromatization of the dihydroindolizines. TPCD serves as both decarboxylation and dehydrogenation reagent in the reactions. These results show that TPCD is a promising new reagent for bisdecarboxylation of aliphatic carboxylates.