Phosphine‐Catalyzed Enantioselective Dearomative [3+2]‐Cycloaddition of 3‐Nitroindoles and 2‐Nitrobenzofurans

Over the past years, the metal‐catalyzed dearomative cycloaddition of 3‐nitroindoles and 2‐nitrobenzofurans have emerged as a powerful protocol to construct chiral fused heterocyclic rings. However, organocatalytic dearomative reaction of these two classes of heteroarenes has become a long‐standing challenging task. Herein, we report the first example of phosphine‐catalyzed asymmetric dearomative [3+2]‐cycloadditio of 3‐nitroindoles and 2‐nitrobenzofurans, which provide a new, facile, and efficient protocol for the synthesis of chiral 2,3‐fused cyclopentannulated indolines and dihydrobenzofurans by reacting with allenoates and MBH carbonates, respectively through a dearomative [3+2]‐cycloaddition.     

A reinvestigation of the reaction of allylsilanes with N-phenyltriazolinedione: stereoselective synthesis of substituted urazoles by [3+2] cycloaddition

A stereoselective synthesis of hydroxy substituted urazoles of potential biological significance has been developed via the [3+2] annulation of α-substituted allylic silanes with N-phenyltriazolinedione. The need for complete blocking of the α-CH₂ group of allylsilanes for successful [3+2] annulation, as reported previously, is found to be unnecessary.     

FeCl3 promoted highly regioselective [3 + 2] cycloaddition of dimethyl 2-vinyl and aryl cyclopropane-1,1-dicarboxylates with aryl isothiocyanates

A FeCl(3) promoted [3 + 2] annulation of dimethyl 2-vinyl and arylcyclopropane-1,1-dicarboxylate with aryl isothiocyanates has been developed to give pyrrolidine-2-thiones in good yields with high regioselectivity.     

Enantioselective formal [3+3] annulation for the direct construction of bicyclic skeletons with four stereogenic centers

An enantioselective formal [3+3] annulation reaction of cyclic ketones with enones has been developed. In the presence of 20 mol % of pyrrolidine-thiourea 1a or N-(pyrrolidin-2-ylmethyl)trifluoromethanesulfonamide 1i, the reactions afford bicyclo [3.3.1] adducts in moderate to good yields with good to high enantioselectivities under mild conditions.     

Divergent synthesis of spirocyclopentene-pyrazolones and pyrano[2,3-c]-pyrazoles via Lewis base controlled annulation reactions

An effective Lewis base catalyst-controlled α-regioselective annulation reaction of γ-substituted allenoates with unsaturated pyrazolones has been developed, affording various spirocyclopentene-pyrazolones and pyrano[2,3-c]pyrazoles. The combination of PPh₃ and K₂CO₃ promoted the [3+2] annulation to access the spirocyclopentene-pyrazolones in moderate to good yields (up to 90%) with excellent diastereoselectivities (dr >19:1), while [4+2] annulations to generate pyrano[2,3-c]pyrazoles were successfully achieved by employing DBU as catalyst (in up to 92% yield). Of importance, the asymmetric synthesis of spirocyclopentene-pyrazolone was realized by using our previously reported chiral ferrocenylphosphine catalyst.     

Electrooxidation Enables Selective Dehydrogenative [4+2] Annulation between Indole Derivatives

Dearomative annulation of indoles has emerged as a powerful tool for the preparation of polycyclic indoline‐based alkaloids. Compared with well‐established methods towards five‐membered‐ring‐fused indolines, the six‐membered‐ring‐fused indolines are rarely accessed under thermal conditions. Herein, a dearomative [4+2] annulation between different indoles is developed through an electrochemical pathway. This transformation offers a remarkably regio‐ and stereoselective route to highly functionalized pyrimido[5,4‐b]indoles under oxidant‐ and metal‐free conditions. Notably, this electrochemical approach maintains excellent functional‐group tolerance and can be extended as a modification tactic for pharmaceutical research. Preliminary mechanism studies indicate that the electrooxidation annulation proceeds through radical–radical cross‐coupling between an indole radical cation and an N‐centered radical generated in situ.     

Aza- and carbo-[3+3] annulations of exo-cyclic vinylogous amides and urethanes. Synthesis of tetrahydroindolizidines and an unexpected formation of hexahydroquinolines

[3+3] Annulations of exo-cyclic vinylogous amides and urethanes with vinyl iminium salts are described here. We observed an intriguing dichotomy in their reaction pathways. For pyrrolidine- and azepane-based vinylogous amides or urethanes, aza-[3+3] annulation would dominate to give tetrahydroindolizidines, whereas, unexpectedly, for piperidine-based vinylogous amides or urethanes, carbo-[3+3] annulation was the pathway, leading to hexahydroquinolines. The origin for such a contrast is likely associated with a switch in the initial reaction pathway between C-1,2-addition and C-1,4-addition.     

Efficient synthesis of highly functionalized dihydropyrido[2,3-d]pyrimidines by a double annulation strategy from alpha-alkenoyl-alpha-carbamoyl ketene-(S,S)-acetals

A convenient and efficient synthesis of highly functionalized dihydropyrido[2,3-d]pyrimidines via a double [5 + 1] annulation strategy starting from easily available alpha-alkenoyl-alpha-carbamoyl ketene-(S,S)-acetals 1 and cheap reagents (NH4OAc, DMF, and POCl3) has been developed. In the first step of the double annulation route, 2-amino-3-carbamoyl-5,6-dihydro-4-pyridones 2 were created in high to excellent yields by a formal [5C + 1N] annulation reaction of ketene-(S,S)-acetals 1 with ammonia (from ammonium acetate). In the second step of the double annulation strategy, the highly functionalized dihydropyrido[2,3-d]pyrimidine derivatives, 7,8-dihydropyrido[2,3-d]pyrimidin-4(3H)-ones 3 (when R1 = aryl) and 7,8-dihydropyrido[2,3-d]pyrimidines 4 (when R1 = H), were constructed, respectively, in fair to good yields by reacting 2 with excessive Vilsmeier reagent (DMF/POCl3). A mechanism involved in the second [5 + 1] annulation step, including a formal [5 + 1] annulation and accompanied chlorovinylation, chloroformylation, amination, and aromatization reactions, is proposed.     

Access to 2′,3′-dihydro-1′H-spiro[indoline-3,4′-pyridin]-2-ones via amino acid derived phosphine-catalyzed asymmetric [4+2] annulation with easily available oxindole-derived α,β-unsaturated imines

The phosphine-catalyzed asymmetric [4+2] annulation of vinyl ketones with more easily available oxindole-derived α,β-unsaturated imines has been further developed in the presence of an easily available multifunctional thiourea-phosphine catalyst derived from natural amino acid, providing the enantioselective synthesis of 2′,3′-dihydro-1′H-spiro[indoline-3,4′-pyridin]-2-ones in good yields and moderate de values with higher enantioselectivities under mild conditions.     

Asymmetric [3+2] annulation of allenes with maleimides catalyzed by dipeptide-derived phosphines: facile creation of functionalized bicyclic cyclopentenes containing two tertiary stereogenic centers

D-Threonine-L-tert-leucine-derived bifunctional phosphine, Cat. 11, catalyzed highly enantioselective [3+2] annulation of maleimides with allenes has been disclosed, allowing the synthesis of optically active functionalized bicyclic cyclopentenes containing two tertiary stereogenic centers in good to high yields along with good to high enantioselectivities.     

Bifunctional N‐Heterocyclic Carbene Catalyzed [3+4] Annulation of Enals and Aurones

Bifunctional N‐heterocyclic carbenes with a free hydroxy group are demonstrated as efficient catalysts for the [3+4] annulation of enals with aurones to give the corresponding benzofuran‐fused ε‐lactones in good yields with good diastereoselectivities and excellent enantioselectivities. Control experiments reveal that the [3+4] cycloadducts are kinetically favored and could be transformed to the thermodynamically favored [3+2] cycloadducts with a non‐bifunctional NHC catalyst.     

Selective construction of indeno[1,2-b]phenothiazine and indeno[2,1-c]phenothiazine via tandem annulation reaction

AcOH promoted annulation reaction of 2-arylideneindane-1,3-diones with methyl 2-(benzo[b][1,4]thiazin-3-ylidene)acetate in refluxing ethanol afforded pentacyclic tetrahydroindeno [1,2-b]phenothiazine in satisfactory yields and with high diastereoselectivity according to the unexpected tandem annulation process. When the above reaction was carried out in refluxing acetic acid, isomeric dihydroindeno [1,2-b]phenothiazines and dihydroindeno [2,1-c]phenothiazines were obtained in comparable yields according to alternate tandem annulation process.     

Cyclopentannulation on 3-phospholenes: an expedient route to the 2-phosphabicyclo[3.3.0]octene ring system

Treatment of 1-phenyl-3-phospholene derivatives with 2 equiv. of LDA followed by quenching the metallated intermediates with 1,3-dihaloalkanes affords 2-phosphabicyclo[3.3.0]oct-3-ene derivatives in good yield. The annulation reactions are highly regio- and stereoselective and lead to the formation of exo-Ph-P substituted products exclusively. Reduction of the resulting bicyclic phosphine oxides by phenylsilane gives the corresponding phosphines with complete retention of configuration at P. Application of this annulation procedure to acyclic allylic substrates leads to the corresponding monocyclic annulation products.     

Organocatalytic asymmetric [3 + 3] annulation of isatin N,N'-cyclic azomethine imines with enals: Efficient approach to functionalized spiro N-heterocyclic oxindoles

An unprecedented chiral secondary amine-catalyzed [3 + 3] annulation of isatin N,N'-cyclic azomethine imines with α,β-unsaturated aldehydes was developed. This strategy allowed the construction of structurally novel spiro N-heterocyclic oxindole derivatives in good yields (up to 91%) and good to excellent enantioselectivities (up to >99% ee), albeit with modest diastereoselectivities (up to 3.1:1 dr).     

Asymmetric synthesis of tetrahydropyran[3,2-c]quinolinones via an organocatalyzed formal[3 + 3] annulation of quinolinones and MBH 2-naphthoates of nitroolefin

An efficient asymmetric and enantio-swithchable organocatalytic[3+3]annulation reaction using MBH-2-naphthoates of nitroalkenes and 4-hydroxyquinolin-2(1H)-ones has been developed.Densely substituted tetrahydropyrano[3,2-c]qui noli nones scaffolds with two adjacent stereogenic centers are obtained with high yield(up to 95%yield)and good stereoselectivities(up to>20:1 dr and 96%ee)in an enantio-switchable manner.Furthermore,gram scale synthesis was achieved and the nitro group could easily transform into an amino group without any appreciable loss in the diastereo-and enantioselectivity.     

Annulation reactions of allene-derived 1,3-dipole with 3-substituted-chromones: unusual recognition of 4pi-component in 3-(N-Aryliminomethyl)chromones through

All-carbon dipole derived by the interaction of triphenylphosphine with allenic ester is able to locate the polarized 2pi-component in 3-formylchromones through a regioselective [2 + 3] addition to the C2-C3 pi-bond, which is followed by deformylation leading to novel 3a,9a-dihydro-1-ethoxycarbonyl-1-cyclopenteno[5, 4-b]benzopyran-4-ones. On the contrary, the dipole recognizes azadiene in 3-(N-aryliminomethyl)chromones through [4 + 3] annulation and initially formed adducts undergo tandem rearrangements to afford novel N-aryl-2, 3-dihydro-4-ethoxycarbonylchromano[2,3-b]azepine-6-ones in good yield.     

Palladium-catalyzed one-pot synthesis of benzo[b][1,6]naphthyridines via Sonogashira coupling and annulation reactions from 2-chloroquinoline-3-carbonitriles

Palladium-catalyzed one-pot synthesis of 1,3-disubstituted benzo[b][1,6]naphthyridines and [1,6]naphthyridines has been described from easily accessible precursors, 2-chloroquinoline-3-carbonitriles and 2-chloropyrido-3-carbonitrile via sequential additions of palladium-catalyst for Sonogashira-coupling and the following annulations in good to excellent yields. A plausible mechanism for annulation is discussed.     

Constructing the architecturally distinctive ABD-tricycle of phomactin A through an intramolecular oxa-[3+3] annulation strategy

Our efforts in constructing the ABD-ring of phomactin A through an intramolecular oxa-[3+3] annulation strategy is described. This struggle entailed finding a practical and efficient preparation of annulation precursor, and a realization of the unexpected competing regioisomeric pathway. The success entailed accessing the A-ring through Diels-Alder cycloaddition of Rawal’s diene. Furthermore, the discovery that the regioisomers from the annulation existed as atropisomers with respect to the D-ring olefin and that they could be equilibrated to the desired ABD-tricycle, allowing large quantities of tricycle to be accessed.     

Regioselective intramolecular annulations of ambident β-enamino esters: A diversity-oriented synthesis of nitrogen-containing privileged molecules

Diversity-oriented, regioselective, intramolecular annulation of β-enamino esters is described under solvent-free, calcium-catalysis. 2-aminoaryl ketones and alkyl propiolates undergone a [4+2] annulation to yield substituted quinolines; with an excess of alkyl propiolates, benzodiazepines were formed via a [4+2+1] annulation. We also described a one-pot, 3-component synthesis of quinoline derivatives via a [4+2+2] annulation. Interestingly, 2-aminoaryl ketones undergone a self-condensation [4+4] and gave the dibenzodiazocines.     

Rh(III)-Catalyzed annulative aldehydic C-H functionalization for accessing ring-fluorinated benzo[b]azepin-5-ones

A new Rh(III)-catalyzed aldehydic C-H activation/[4 + 3] annulation cascade of N-sulfonyl-2-aminobenzaldehydes with gem-difluorocyclopropenes is reported for the first time, and used to produce a range of hitherto unreported precedented β-monofluorinated benzo[b]azepin-5-ones with good yields and complete regioselectivity. This approach features a broad substrate scope, good functional group tolerance, and high regioselectivity, which may include Rh(III)-catalyzed aldehydic C−H activation, tandem site-/regioselective insertion, defluorinated ring-scission, and 1,2-elimination.