Regioselective synthesis of indoles via reductive annulation of nitrosoaromatics with alkynes

[reaction: see text] Indoles are produced regioselectively and in moderate yields by two new processes: (a) from the [CpRu(CO)2]2-catalyzed reaction of nitrosoaromatics (ArNO) with alkynes under carbon monoxide and (b) in a two-step sequence involving the (uncatalyzed) reaction of ArNO with alkynes, followed by reduction of the intermediate adduct.     

Direct synthesis of 3-arylindoles via annulation of aryl hydroxylamines with alkynes

3-Arylindoles are produced in moderate to excellent yields from the reaction between aryl hydroxylamines and alkynes catalyzed by 10 mol % iron(II) phthalocyanine [Fe(Pc)]. Terminal and internal alkynes afford 3-aryl substituted indoles exclusively. Electron-donating and -withdrawing groups are tolerated on the aryl hydroxylamine. A few bioactive indoles are synthesized as well as several new indoles using this one-step intermolecular annulation procedure.     

An expedient synthesis of 3-substituted indoles via reductive alkylation with ketones

3-Alkylindoles were prepared in one step from indoles and ketones via a convenient reductive alkylation procedure using triethylsilane and trichloroacetic acid. Under this particular condition, unsubstituted indoles could be tolerated to afford good yields of 3-sec-alkylation products.     

Ruthenium-catalyzed annulation of alkynes with amides via formyl translocation

The first example of Ru-catalyzed intramolecular annulation of alkynes with amides via formyl translocation has been developed, which provides an efficient approach for the synthesis of 1H-indole-3-carbaldehydes.     

Visible-light-induced synthesis of benzothiophenes and benzoselenophenes via the annulation of thiophenols or 1,2-diphenyldiselane with alkynes

An effective metal-free photoredox-mediated tandem addition/cyclization reaction of thiophenols or 1,2-diphenyldiselane with alkynes leads to 2,3-disubstituted benzothiophenes and benzoselenophenes. Blue light irradiation of the organic dye, Mes-Acr-Me⁺, initiates the photoredox catalysis. A series of functional groups could be tolerated under ambient conditions, and good to excellent yields were generated.     

Synthesis of tryptamine derivatives via a direct, one-pot reductive alkylation of indoles

An efficient, one-pot reductive alkylation of indoles with N-protected aminoethyl acetals in the presence of TES/TFA is reported. It represents the first general method for the direct synthesis of tryptamine derivatives from indoles and nitrogen-functionalized acetals. This convergent and versatile approach employs safe and inexpensive reagents, proceeds under mild conditions, and tolerates several functional groups. The new procedure was efficiently applied to a gram-scale synthesis of both luzindole, a reference MT2-selective melatonin receptor antagonist, and melatonin.     

Highly enantioselective direct reductive coupling of conjugated alkynes and alpha-ketoesters via rhodium-catalyzed asymmetric hydrogenation

Catalytic hydrogenation of 1,3-enynes 1a-7a in the presence of ethyl pyruvate and related activated ketones using chirally modified cationic rhodium catalysts results in reductive coupling to afford dienylated alpha-hydroxy esters 1b-7b and 3c-3f with exceptional levels of regio- and enantiocontrol. These studies represent the first highly enantioselective direct catalytic reductive couplings of alkynes to ketones. As illustrated by the conversion of 6b to 6c-6h, the diene containing the side chain of the coupling products is subject to diverse chemo- and regioselective manipulation. Reductive coupling of enyne 6a and ethyl pyruvate using elemental deuterium provides the monodeuterated product deuterio-6b, consistent with a catalytic mechanism involving alkyne-carbonyl oxidative coupling followed by hydrogenolytic cleavage of the resulting oxametallacycle, as corroborated by ESI-MS analysis.     

Highly selective catalytic intermolecular reductive coupling of alkynes and aldehydes

Alkynes (internal and terminal) and aldehydes (aromatic and aliphatic) are reductively coupled in a single catalytic reaction to yield di- and trisubstituted allylic alcohols with high stereoselectivity and regioselectivity. In most cases, a 1:1 ratio of alkyne to aldehyde is sufficient for efficient coupling. The yield and regioselectivity are strongly dependent on the phosphine ligand, but the allylic alcohols formed are invariably the products of cis addition to the alkyne.     

A catalytic reaction of alkynes via multiple-site functionalization

The first multiple-site activation of alkynes with amine/halogen functionalities has been established. The reaction was performed by treating alkyne with N,N-dichlorobenzenesulfonamide at 80 degrees C in the presence of palladium acetate catalyst. A new mechanism was proposed which involves the novel formation of beta-halovinyl palladium and pi-allylpalladium species. Excellent regio- and stereoselectivities were achieved with the absolute structure determined by X-ray structural analysis.     

A first regioselective synthesis of 3-fluoroalkylated benzofurans via palladium-catalyzed annulation of fluorine-containing internal alkynes with variously substituted 2-iodophenol

The palladium-catalyzed annulation reaction of a variety of fluorine-containing internal alkynes with 2-iodophenol derivatives was investigated. The use of P(t-Bu)₃ as a ligand on palladium was found to be crucial in this annulation reaction, resulting in the exclusive formation of 3-fluoroalkylated benzofurans in high yields. ¹⁹F NMR analysis of the reaction mixture revealed that the addition of phenol to the fluoroalkylated alkynes was followed by intramolecular Heck reaction, giving to the corresponding 3-fluoroalkylated benzofurans.     

Palladium-catalyzed, modular synthesis of highly functionalized indoles and tryptophans by direct annulation of substituted o-haloanilines and aldehydes

One-pot synthesis of indoles by a palladium-catalyzed annulation of ortho-haloanilines and aldehydes has been developed. Coupling of ortho-iodoaniline with aldehyde is realized under mild ligandless conditions [Pd(OAc)2, DABCO, DMF, 85 degrees C], whereas X-Phos is found to be the ligand of choice for coupling reactions involving ortho-chloroanilines/ortho-bromoanilines and aldehydes. A variety of ortho-haloanilines with different electronic properties are suitable substrates, and aldehydes including chiral ones participated in this reaction without racemization. Coupling of (S)-2-N,N-di-tert-butoxycarbonyl-5-oxopentanoate, derived from L-glutamic acid, with ortho-haloanilines provides a rapid access to the ring-A-substituted tryptophans in good to excellent yields.     

Cobalt-catalyzed annulation of aryl iodides with alkynes

A cobalt-catalyzed approach for the concise synthesis of naphthalenes by the annulation of aryl iodides with alkynes is disclosed. In the reaction, manganese reductant and MeCN solvent are necessary to proceed efficiently, which tolerates various functional groups, for example, boronate ester. Mechanistic investigation indicates that the transformation employs electrophilic aromatic substitution (S_EAr) in the aromatic C–H bond replacement step.     

Convenient synthesis of 1,2,3,4-tetrahydroquinolines via direct intramolecular reductive ring closure

A simple and convenient procedure for the synthesis of 3-aryl-1,2,3,4-tetrahydroquinolines is reported. 3-Aryl-1,2,3,4-tetrahydroquinolines are directly obtained by reductive ring closure of 2-phenyl-3-(2-nitrophenyl)-propionitrile derivatives in moderate to high yields.     

A novel three-step synthesis of Celecoxib via palladium-catalyzed direct arylation

A novel three linear step synthesis of Celecoxib was achieved in 33% overall yield via a key regioselective direct C-H arylation reaction between a 1,3-disubstituted pyrazole and an aryl bromide.     

anti-1,2-Diols via Ni-catalyzed reductive coupling of alkynes and alpha-oxyaldehydes

[reaction: see text] Ni-catalyzed reductive coupling of aryl alkynes (1) and enantiomerically enriched alpha-oxyaldehydes (2) afford differentiated anti-1,2-diols (3) with high diastereoselectivity and regioselectivity, despite the fact that the methoxymethyl (MOM) and para-methoxybenzyl (PMB) protective groups typically favor syn-1,2-diol formation in carbonyl addition reactions of this family of aldehydes.     

One-pot synthesis of 2,4-disubstituted quinolines via silver-catalyzed three-component cascade annulation of amines,alkyne esters and terminal alkynes

Described herein is a new and general method for one-pot synthesis of 2,4-disubstituted quinolines via silver-catalyzed [3?+?1?+?2] annulation of simple amines, alkyne esters and terminal alkynes. The versatile transformation might initiate with the facile formation of the enamine species with the feature of good to excellent yields, exclusive regioselectivity, and excellent substrate/functional group tolerance.     

Synthesis of coumarins via palladium-catalyzed carbonylative annulation of internal alkynes by o-iodophenols

A variety of substituted coumarins have been prepared in good yields by the palladium-catalyzed coupling of o-iodophenols with internal alkynes and 1 atm of carbon monoxide. Unlike most of the previous work on the palladium-catalyzed carbonylation of alkynes, the insertion of the internal alkyne occurs in preference to the insertion of CO.     

Palladium-catalyzed carbonylative annulation of internal alkynes: synthesis of 3,4-disubstituted coumarins

The palladium-catalyzed annulation of internal alkynes by o-iodophenols in the presence of CO results in exclusive formation of coumarins. No isomeric chromones have been observed. The best reaction conditions utilize the 2-iodophenol, 5 equiv of alkyne, 1 atm of CO, 5 mol % Pd(OAc)2, 2 equiv of pyridine, and 1 equiv of n-Bu4NCl in DMF at 120 degrees C. The use of a sterically unhindered pyridine base is essential to achieve high yields. A wide variety of 3,4-disubstituted coumarins containing alkyl, aryl, silyl, alkoxy, acyl, and ester groups have been prepared in moderate to good yields. Mixtures of regioisomers have been obtained when unsymmetrical alkynes are employed. 2-iodophenols with electron-withdrawing and electron-donating substituents and 3-iodo-2-pyridone are effective in this annulation process. The reaction is believed to proceed via (1) oxidative addition of the 2-iodophenol to Pd(0), (2) insertion of the alkyne triple bond into the aryl-palladium bond, (3) CO insertion into the resulting vinylic carbon-palladium bond, and (4) nucleophilic attack of the phenolic oxygen on the carbonyl carbon of the acylpalladium complex with simultaneous regeneration of the Pd(0) catalyst. This annulation process is the first example of intermolecular insertion of an alkyne occurring in preference to CO insertion.