Direct construction of bicyclic heterocycles by palladium-catalyzed domino cyclization of propargyl bromides

The palladium-catalyzed domino cyclization of propargyl bromides having two nucleophilic functional groups is described. Treatment of 1,7-diamino-5-bromohept-3-yne derivatives with catalytic Pd(PPh3)4 in the presence of NaH in MeOH gives the 2,7-diazabicyclo[4.3.0]non-5-enes in good yields. Interestingly, the regioselectivity of the reaction is completely controlled by the relative reactivity of the amine functional groups, irrespective of the position of the nucleophiles. The malonate derivative also undergoes domino cyclization to produce a hexahydroindole derivative.     

Synthesis of tetrasubstituted alkenes through a palladium-catalyzed domino carbopalladation/C-H-activation reaction

Helical tetrasubstituted alkenes (7) were obtained in a highly efficient way through a palladium-catalyzed domino-carbopalladation/CH-activation reaction of propargylic alcohols 6 in good to excellent yields. Electron-withdrawing- and electron-donating substituents can be introduced onto the upper and lower aromatic rings. The substrates (6) for the domino process were synthesized by addition of the lithiated alkyne (20) to various aldehydes (19); moreover, the substrates were accessible enantioselectively (in 95% ee) by reduction of the corresponding ketone using the Noyori procedure.     

Synthesis of indolines via a domino Cu-catalyzed amidation/cyclization reaction

A highly efficient one-pot procedure for the synthesis of indolines and their homologues based on a domino Cu-catalyzed amidation/nucleophilic substitution reaction has been developed. Substituted 2-iodophenethyl mesylates and related compounds afforded the corresponding products in excellent yields. No erosion of optical purity was observed when transforming enantiomerically pure mesylates under the reaction conditions.     

Synthesis of 2-substituted indoles via a palladium-catalyzed domino Heck reaction and dealkylation

A palladium-catalyzed domino Heck reaction and dealkylation for the preparation of 2-substituted indoles is described. This novel transformation is based on an intramolecular Heck reaction followed by dealkylation.     

Synthesis of dithienothiophenes,cyclopentadithiophene and silacyclopentadithiophenes using palladium-catalyzed cyclization

The intramolecular cyclization of bromothiophene derivatives with hexamethylditin in the presence of tetrakis(triphenylphosphine)palladium(0) gave dithienothiophenes, cyclopentadithiophene, and silacyclopentadithiophenes in moderate to good yields.     

Synthesis of dibenzofurans via palladium-catalyzed phenol-directed C-H activation/C-O cyclization

A practical, Pd(0)/Pd(II)-catalyzed reaction was developed for phenol-directed C-H activation/C-O cyclization using air as an oxidant. The turnover-limiting step of the process was found to be C-O reductive elimination instead of C-H activation. This reaction can tolerate a variety of functional groups and is complementary to the previous methods for the synthesis of substituted dibenzofurans.     

Total synthesis of (+/-)-lysergic acid, lysergol, and isolysergol by palladium-catalyzed domino cyclization of amino allenes bearing a bromoindolyl group

Ergot alkaloids and their synthetic analogs have been reported to exhibit broad biological activity. We investigated direct construction of the C/D ring system of ergot alkaloids based on palladium-catalyzed domino cyclization of amino allenes. With this biscyclization as the key step, total synthesis of (+/-)-lysergic acid, (+/-)-lysergol, and (+/-)-isolysergol was achieved.     

Enantioselective palladium-catalyzed total synthesis of vitamin e by employing a domino Wacker-Heck reaction

An enantioselective total synthesis of vitamin E in which a novel palladium-catalyzed domino reaction was employed as the key step is described. This reaction allows the formation of the chiral chroman framework and the concurrent introduction of part of the side chain of vitamin E. The sequence comprises an enantioselective Wacker cyclization and a subsequent Heck reaction. Accordingly, reaction of alkenylphenol 12 with methyl vinyl ketone (13) in the presence of catalytic amounts of Pd(OTFA)(2) (TFA = trifluoroacetate), the enantiopure ligand (S,S)-Bn-BOXAX (8 b; Bn = benzyl, BOXAX = 2,2'-bis(oxazolyl)-1,1'-binaphthyl, and p-benzoquinone (9) as an oxidant gives access to chiral chroman 10 with an enantioselectivity of 97 % ee in 84 % yield. Chroman 10 is then converted into 24 by an aldol condensation reaction with (3R)-3,7-dimethyloctanal (11). Subsequent 1,2-addition of methyllithium, elimination of water, and hydrogenation yields vitamin E.     

Synthesis of polyhydroxylated bicyclic tetrahydrofurans and tetrahydropyrans via a stereoselective domino cyclization/reduction reaction

A novel reaction cascade involving a Lewis acid-induced migration of an isopropylidene protecting group followed by the formation of a pyranose or furanose ring and subsequent reduction of the hemiacetal is described. Depending on the reaction conditions, as well as, the stereochemistry of the substrate, polyhydroxylated tetrahydrofurans or tetrahydropyrans can be obtained in reasonable yields. The synthons used in this transformation were prepared via a highly stereoselective one-pot tandem reaction, consisting of a 1,4-Michael addition of vinylmagnesium bromide to d-glucose-derived cyclohexenone followed by aldol reaction with 2,3-O-isopropylidene-d-glyceraldehyde.     

Efficient synthesis of the structural core of tetracyclines by a palladium-catalyzed domino Tsuji-Trost-Heck-Mizoroki reaction

The Pd-catalyzed domino Tsuji-Trost-Heck-Mizoroki reactions of compounds 18, 27, and 34, respectively, each containing an allyl acetate and a halogen aryl moiety, lead to the formation of hexahydronaphthacenes 2 and 3 and octahydroanthracene 4 in 62-81 % yield. The octahydroanthracene and hexahydronaphthacene motifs are found in many natural products, for example, the tetracycline antibiotics.     

Synthesis of 3-(diarylmethylenyl)oxindole by a palladium-catalyzed domino carbopalladation/C-H activation/C-C bond-forming process

[reaction: see text] A highly efficient palladium-catalyzed synthesis of unsymmetrically substituted 3-(diarylmethylenyl)indolinones from readily accessible starting materials is developed. The domino reaction involves a sequence of intermolecular carbopalladation, C-H activation, and C-C bond formation. A plausible mechanistic pathway for the reaction is discussed on the basis of the kinetic isotope effect [K(H)/K(D) (intermolecular) = 1, K(H)/K(D) (intramolecular) = 2.7] as well as the electronic effect.     

Synthesis of trisubstituted imidazoles by palladium-catalyzed cyclization of O-pentafluorobenzoylamidoximes: application to amino acid mimetics with a C-terminal imidazole

1-Benzyl-4-methylimidazoles with a range of substituents at the 2-position are prepared from O-pentafluorobenzoylamidoximes on treatment with catalytic amounts of Pd(PPh3)4 and triethylamine. The sequence provides access to optically active amino acid mimetics with a C-terminal imidazole. [structure: see text]     

Synthesis of trans,trans,cis-fused tetracyclic skeleton via radical domino cyclization

Limonoids are characterized by a polycyclic structure and show a wide variety of bioactivities. In particular, mesendanin L, 12-hydroxyamoorastatone, and meliatoosenin F have unique structures containing a trans-A/B/C and cis-C/D-fused tetracyclic skeleton. We synthesized the core structure of these limonoids via Mn(OAc)₃ and Cu(OAc)₂-mediated radical domino cyclization of an acyclic tetraene precursor having a terminal β-keto ester. To the best of our knowledge, this is the first example of the radical-mediated construction of a 6/6/6/5-membered tetracyclic skeleton.     

Synthesis of 2-substituted-benzothiazoles by palladium-catalyzed intramolecular cyclization of o-bromophenylthioureas and o-bromophenylthioamides

2-Amino-, and 2-alkyl-benzothiazoles have been efficiently prepared by palladium catalyzed cyclization of o-bromophenylthioureas and o-bromophenylthiamides. Results were best with the Pd₂(dba)₃/monophosphine catalytic system.     

Uncommon intramolecular palladium-catalyzed cyclization of indole derivatives

A novel synthetic strategy based on the intramolecular palladium-catalyzed oxidative cyclization reaction, allows the formation of C-C bond and the synthesis of β-carbolinones. The reaction has been performed in the presence of catalytic amount of PdCl₂(CH₃CN)₂ and benzoquinone as a reoxidant.     

Solid-phase synthesis of indolines via palladium-catalyzed cyclization

The solid-phase synthesis of indolines from resin-bound 2-bromophenylacetylated amino acids is described. The exhaustive reduction of solid-support bound amides with borane afforded the requisite secondary amines that, following the palladium-catalyzed intramolecular cyclization and cleavage, provided the corresponding disubstituted indolines.