AbstractTreatment of the readily accessible spiro-cyclohexadienones from the PhI(OCOCF3)2-mediated spiro-cyclization of N-substituted benzanilides, with BF3⋅Et2O initiates a tandem ring opening/closure reaction leading to the formation of the biologically interesting 8-hydroxy-phenanthridin-6(5H)-one compounds. This unique rearrangement pattern involves the ‘migration’ of the electron-deficient N-methyl carbamoyl moiety rather than the electron-rich aryl group as observed and reported previously in all other similar transformations.

Combined experimental and theoretical investigations into the phenyliodine bis(trifluoroacetate) (PIFA)-mediated reaction of N-arylcinnamamide to produce 3-arylquinolin-2-one derivatives have been conducted. High regioselectivity during the aryl migration process was observed in 3,3-disubstituted acrylamides. Density functional theory calculation was conducted in an attempt to understand the mechanism and the origin of the regioselectivity. On the basis of both the experimental and the theoretical results, a mechanism involving an oxidative annulation, followed by an aryl migration, has been proposed. The annulation is the regioselectivity determining step.

An efficient ring-contraction reaction of isochromeno[4,3-b]indol-5(11H)-ones via a nucleophile-induced disproportionation/spirocyclization cascade process has been developed under mild conditions. The process realized the conversion of isochromeno[4,3-b]indol-5(11H)-ones into N-unsubstituted spiro[indoline-2,1′-isobenzofuran]-3,3′-diones and spiro[indoline-2,1′-isoindoline]-3,3′-diones in the absence of a transition-metal catalyst or oxidant. Gram-scale reaction further demonstrated the practicability of the protocol.

A series of substituted indoloquinolinones were readily synthesized, via two steps, from indole-2-carboxylic acid and its derivatives: a 3-arylation of indole-2-carboxamides and a subsequent PIDA-mediated oxidative C–N bond formation. The underpinning strategy involves a Cu(I)-promoted C(sp2)–C(sp2) coupling and a PIDA-mediated oxidative C(sp2)–N coupling.

Various chromeno[2,3-b]indol-11(6H)-ones were conveniently constructed via phenyliodine(III) diacetate (PIDA)-mediated intramolecular oxidative annulation. This method, while realizing a direct oxidative C–N bond formation between an aromatic ring and a pendent free-NH2 moiety, features a metal-free protocol, mild reaction conditions, simple workup, and the ready availability of the starting substrates.

The synthesis of an undocumented skeleton of 3-hydroxy-2,3-dihydroisoquinoline-1,4-diones has been discovered and reported. The reaction consists of an intramolecular cyclization of o-(1-alkynyl)benzamides in MeCN/H2O, mediated by metal-free, hypervalent reagent of PhI(OCOCF3)2, followed by an oxidative hydroxylation reaction. The mechanism consisting of two pathways has been proposed and discussed.

PhICl2 in wet DMF was found to form an efficient system for realizing difunctionalization of various alkenes and olefinic derivatives possessing a wide range of functional groups. This novel methodology provides convenient access to either regioselective chloroformyloxylated products or α-chlorinated olefinic products, depending on the type of structure of the original unsaturated starting material. The mechanism of the reaction is proposed and discussed.

A novel tandem metal-free oxidative aryl migration/C–C bond-cleavage reaction, mediated by hypervalent iodine reagent, has been discovered. The presented transformation provided straightforward access to important α-ketoamide and α-ketoester derivatives from readily available acrylic derivatives via a concerted process of 1,2-aryl shift concomitant with C–C bond cleavage.

A series of biologically important 4(3H)-quinazolinones were readily synthesized in good to excellent yields from 2-amino-N-methoxybenzamides and aldehydes via a cascade reaction consisting of AcOH-promoted cyclocondensation and elimination. The current method sets itself apart from the conventional approach utilizing anthranilamide derivatives and aldehydes as building blocks, by its unique features, other than the high yields and one-pot procedure, including the absence of an oxidant, the elimination of a heavy-metal catalyst, and the formation of a non-toxic ester byproduct.

An intramolecular C(sp3)–O bond formation has been achieved via PhI(OAc)2/NaN3-mediated oxygenation of N,N-diaryl tertiary amines. The appealing features of this method include mild reaction conditions, absence of heavy-metal catalysts, and the direct intramolecular functionalization of sp3 C–H bonds adjacent to nitrogen.

A metal-free protocol for direct aryl-aldehyde Csp2–Csp2 bond formation via a PhI(OAc)2-mediated intramolecular cross-dehydrogenative coupling (CDC) of various 2-(N-arylamino)aldehydes was developed. The novel methodology requires no need of preactivation of the aldehyde group, is applicable to a large variety of functionalized substrates, and most of all provides a convenient approach to the construction of biologically important acridone derivatives.

New compounds involving the biologically important 1,4-benzodiazepine skeleton were conveniently constructed from 2-(arylamino)benzamides through PhI(OAc)2-mediated oxidative C–N bond formation. The attractive features of this new synthetic strategy include mild reaction conditions, the heavy-metal-free characteristic of the oxidative coupling process, and the flexibility to tolerate a broad scope of substrates.

Hypervalent iodine(III) reagents have been vastly used in many useful organic transformations. In this review article, we highlight the strategies that used the common hypervalent iodine(III) reagents as oxidants to synthesize the heterocyclic compounds, based on the patterns of bond formation during the construction of the heterocyclic backbones.

The series of 3-monofunctionalized 2-oxindoles 2 were conveniently synthesized from reactions between anilide 1 and phenyliodine(III) diacetate (PIDA) through hypervalent iodine mediated C(sp2)–C(sp2) bond formation followed by a subsequent deacylation reaction. This metal-free method, shown to provide direct access to an important oxindole intermediate, could be applied to the total synthesis of naturally occurring horsfiline.

The biologically important 1,2,4-triazolo[1,5-a]pyridines were readily synthesized from N-(pyridin-2-yl)benzimidamides via phenyliodine bis(trifluoroacetate)-mediated intramolecular annulation. This novel strategy allows for the convenient construction of a 1,2,4-triazolo[1,5-a]pyridine skeleton through direct metal-free oxidative N–N bond formation, featuring a short reaction time and high reaction yields.

A variety of enaminones and enamine carboxylic esters were converted to trifluoroethoxylated 2H-azirines through reactions with PhIO in trifluoroethanol (TFE). The cascade reaction is postulated to proceed via a PhIO-mediated oxidative trifluoroethoxylation and a subsequent azirination of the α-trifluoroethoxylated enamine intermediates.

The reaction of the readily available N-methyl-N-phenylcinnamamides with phenyliodine bis(trifluoroacetate) (PIFA) in the presence of Lewis acids provides a general and efficient assembly of a variety of 3-arylquinolin-2-one compounds. This novel approach features not only metal-free oxidative C(sp2)–C(sp2) bond formation but also an exclusive 1,2-aryl migration.

A novel Pd-catalysed C–H activation reaction for the synthesis of biaryl imino/keto carboxylic acids is developed. This reaction underwent aryl amide directed C–H activation ortho-acylation followed by ring closing and ring opening processes to give a range of biaryl imino/keto carboxylic acids. Our methodology features the utilization of a cheap and green oxidant (TBHP) as well as readily available aldehydes.

The synthetically and biologically important 4-methyl and 4-methoxy tetrahydro-γ-carboline compounds were readily synthesized in high yields from an aryl amine and a 5-amino-3-oxopentanoate derivative through a series of reactions of enamination, oxidative annulation, deprotection/lactamization and the final reduction reaction of the carbonyl group. The underpinning strategy involves the oxidative C(sp2)–C(sp2) bond formation realized by either Pd(OAc)2/Cu(OAc)2 or a hypervalent iodine reagent.

A variety of functionalized coumarins were synthesized from substituted phenylacrylic acids via PIDA/I2-mediated and irradiation-promoted oxidative carbon–oxygen bond formation. Our studies show that the oxygen in the pendant carboxylic acid group cyclizes favorably to the aryl ring that is cis to it. The main advantages of this method include good functional group tolerance and the transition-metal-free characteristic.

We report an application of FeCl2 as an inexpensive, nontoxic, and efficient catalyst in a clean ring expansion reaction of 2H-azirine derivatives, an intermediate formed through the PhI(OAc)2-mediated azirination of readily available enaminones. An alternative one-pot protocol for the synthesis of various substituted isoxazoles from their corresponding enaminones has been further established based on this reaction, and herein described.

A clean, one-pot synthesis of the biologically important 3-hydroxyquinolin-2(1H)-one compounds has been realized from the readily available N-phenylacetoacetamide derivatives through a PhI(OCOCF3)2-mediated α-hydroxylation and a H2SO4-promoted intramolecular condensation. The hydroxyl group in the generated α-hydroxylated intermediate can be well tolerated in the second H2SO4-promoted cyclization step.

Treatment of N-alkoxyamides with NBS in toluene was found to conveniently afford the corresponding carboxylic esters, including those bearing a bulky or long-chain substituent, in satisfactory to excellent yields. This approach not only represents a convenient and economic approach to a direct transformation of an alkoxyamide moiety into the carboxylic ester functional group, via oxidative homocoupling and the subsequent thermal denitrogenation, but also facilitates the synthesis of sterically hindered carboxylic esters.

A series of indoloquinolinones bearing different aromatic substitutents were readily synthesized starting from an aryl amine, a methyl 3-oxo-3-phenylpropanoate derivative, and methoxylamine through a series of reactions of coupling/enamination, oxidative annulation, a one-pot sequence of N-alkylation, saponification and methoxyamidation, and final intramolecular oxidative C–N bond formation. The underpinning of the strategy entails Pd(OAc)2/Cu(OAc)2-mediated oxidative C(sp2)–C(sp2) bond formation and I(III)-mediated oxidative C(sp2)–N bond formation.

The reaction of phenyliodine bis(trifluoroacetate) (PIFA) with a series of anilides 1 (E = CO2Et) in CF3CH2OH was found to give 3-hydroxy-2-oxindole derivatives 2, while that with various anilides 1′ (E = CON(R4)Ar) afforded the C2-symmetric or unsymmetric spirooxindoles 3. These processes feature a metal-free oxidative C(sp2)–C(sp3) bond formation, followed by oxidative hydroxylation or spirocyclization.

A direct β-acyloxylation of enamine compounds has been achieved by using iodosobenzene (PhIO) as an oxidant to realize the intermolecular oxidative C(sp2)-O bond formation between enamines and various carboxylic acids, including N-protected amino acids. The transformation tolerates a wide range of functional groups and furnishes a variety of β-acyloxy enamines that can be conveniently converted to oxazole compounds via cyclodehydration.

A variety of N-aryl and N-alkyl carbazolones were conveniently achieved in good to high yields via Pd2(dba)3-mediated intramolecular coupling of N-substituted α-iodo enaminones under microwave irradiation. The Pd(0)-catalyzed cyclization was found to proceed favorably with the more electron-deficient phenyl ring during the reactions involving unsymmetrical N,N-diaryl α-iodo enaminones. This unique property enables the construction of carbazolone skeleton containing nitro substituted benzenoid ring.

A group of functionalized oxazoles were synthesized in moderate to good yields from enamides via phenyliodine diacetate (PIDA)-mediated intramolecular cyclization. The main advantageous features of the present method include its broad substrate scope and the heavy-metal-free characteristic of the oxidative carbon–oxygen bond formation process.

A novel Pd-catalysed C–H activation reaction for the synthesis of biaryl imino/keto carboxylic acids is developed. This reaction underwent aryl amide directed C–H activation ortho-acylation followed by ring closing and ring opening processes to give a range of biaryl imino/keto carboxylic acids. Our methodology features the utilization of a cheap and green oxidant (TBHP) as well as readily available aldehydes.

The synthetically and biologically important 4-methyl and 4-methoxy tetrahydro-γ-carboline compounds were readily synthesized in high yields from an aryl amine and a 5-amino-3-oxopentanoate derivative through a series of reactions of enamination, oxidative annulation, deprotection/lactamization and the final reduction reaction of the carbonyl group. The underpinning strategy involves the oxidative C(sp2)–C(sp2) bond formation realized by either Pd(OAc)2/Cu(OAc)2 or a hypervalent iodine reagent.