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Organic Letters

Motohiro Hirai, Shumpei Terada, Hiroaki Yoshida, Kenki Ebine, Tomoaki Hirata, Osamu Kitagawa
In the presence of (R)-DTBM-SEGPHOS-Pd(OAc)2 catalyst, treatment of various 3-(2,6-dibromophenyl)quinazolin-4-ones with NaBH4 gave optically active N-C axially chiral quinazolinone (mebroqualone) derivatives through reductive asymmetric desymmetrization (enantioselective monohydrodebromination) followed by kinetic resolution of the resulting monobromophenyl products (up to 99% ee). The enantioselectivity strongly depended on the substituent (R(2)) at the C4'position, amount of NaBH4, and reaction temperature...
October 26, 2016: Organic Letters
Eliška Matoušová, Petr Koukal, Bedřich Formánek, Martin Kotora
A synthesis of the unsaturated side chain of callyspongiolide has been accomplished from two chiral building blocks prepared by catalytic asymmetric procedures applied on simple starting materials. The synthesis of the chiral benzylic alcohol was based on an enantioselective aldol reaction of a substituted benzaldehyde catalyzed by a chiral amine, whereas the chiral homoallyl alcohol was prepared by the enantioselective crotylboration of iodomethacryl aldehyde catalyzed by a chiral phosphoric acid. Both fragments were joined together by using standard Sonogashira coupling conditions...
October 26, 2016: Organic Letters
Ming Wang, Qiaoling Fan, Xuefeng Jiang
A facile, straightforward protocol was established for diarylannulated sulfide and selenide construction through S-I and Se-I exchange without transition metal assistance. Elemental sulfur and selenium served as the chalcogen source. Diarylannulated sulfides were systematically achieved from a five- to eight-membered ring. A trisulfur radical anion was demonstrated as the initiator for this radical process via electron paramagnetic resonance (EPR) study. OFET molecules [1]benzothieno[3,2-b][1]benzothiophene (BTBT) and [1]benzothieno[3,2-b][1]benzoselenophene (BTBS) were efficiently established...
October 26, 2016: Organic Letters
A Leela Siva Kumari, Alla Siva Reddy, K C Kumara Swamy
An approach to both 1,3- and 1,4-oxazines is developed by the cascade cyclization of epoxy ynamides under transition metal-free conditions. Thus, while 1,3-oxazines are obtained in a regio- and stereoselective manner by using base, cyclization of epoxy ynamides with sodium azide as a nucleophile results in 1,4-oxazines. Deuterium-labeling experiment demonstrates the role of water as a proton source in the formation of 1,4-oxazines. Epoxy tethered alkyne precursors provide 1,4-oxazines that undergo click reaction...
October 26, 2016: Organic Letters
Lixin Li, Peipei Liu, Yijin Su, Hanmin Huang
Thanks to the facile imine-enamine tautomerization, the β,γ-unsaturated hydrazones have been successfully utilized as surrogates of aminodienes for realizing the Pd-catalyzed tandem aminomethylamination/aromatization reaction with aminals via C-N bond activation. This direct and operationally simple protocol provides a fundamentally novel strategy to synthesize aromatic heterocycles from alkenes in the absence of external oxidant and base. Mechanistic studies suggested that aminal not only functioned as an aminomethyl source but also acted as formal oxidant and inner base to promote the aromatization...
October 26, 2016: Organic Letters
Kalisankar Bera, Christoph Schneider
An efficient formal [3 + 2]-cycloaddition toward the highly diastereo- and enantioselective synthesis of indolo[1,2-a]indoles is disclosed. A chiral BINOL-derived phosphoric acid catalyzed the highly enantioselective conjugate addition of cyclic enamides to in situ generated 2-methide-2H-indoles and subsequent aminalization to give rise to acetamide-substituted indolo[1,2-a]indoles carrying three contiguous stereogenic centers. Importantly, these products were formed as single diastereomers and with excellent yields and enantioselectivities...
October 26, 2016: Organic Letters
Ramulu Akula, Patrick J Guiry
An enantioselective Pd-catalyzed DAAA of α-aryl-β-oxo esters has been developed employing the (R,R)-ANDEN-phenyl Trost ligand to prepare a series of α-aryl-α-allyldihydrocoumarins and 3-isochromanones. A variety of aryl groups were successfully employed to afford the dihydrocoumarin and 3-isochromanone products in high yields up to 95% and ee's up to 96%. Under these conditions, substrates containing di- and mono-ortho-substituted aryl groups gave the highest levels of enantioselectivities. This work represents the first example of the enantioselective preparation of all-carbon quaternary α-allyl-α-aryl dihydrocoumarins and 3-isochromanones...
October 26, 2016: Organic Letters
Anaïs Bouisseau, John Glancy, Michael C Willis
β'-Thio-substituted-enones, assembled from the combination of β-tert-butylthio-substituted aldehydes and alkynes, using rhodium catalysis, are shown to smoothly undergo in situ intramolecular S-conjugate addition to deliver a range of S-heterocycles in a one-pot process. Aryl, alkenyl, and alkyl aldehydes can all be employed, to provide thiochroman-4-ones, hexahydro-4H-thiochromen-4-ones, and tetrahydrothiopyran-4-ones, respectively. A variety of in situ oxidations are also performed, allowing access to S,S-dioxide derivatives, as well as unsaturated variants...
October 25, 2016: Organic Letters
Huiqiao Wang, Zhong Wang, Hongchun Huang, Jiajing Tan, Kun Xu
An efficient and practical (hetero)benzylic sp(3) C-H oxidation method has been developed. Notably, this user-friendly protocol employs inexpensive potassium tert-butoxide (KO(t)Bu) as a promoter and proceeds under mild conditions using oxygen as the oxidant. A large variety of oxidation products were prepared in good to excellent yields. The utility of this "green" methodology was further demonstrated by the scale-up preparation of a biologically valued molecule.
October 25, 2016: Organic Letters
Tadashi Ema, Yoshiko Nanjo, Sho Shiratori, Yuta Terao, Ryo Kimura
The intermolecular or intramolecular asymmetric benzoin reaction was catalyzed by a small amount of N-heterocyclic carbene (NHC) (0.2-1 mol %) under solvent-free conditions. The solvent-free intramolecular asymmetric Stetter reaction also proceeded efficiently with NHC (0.2-1 mol %). In some cases, even solid-to-solid or solid-to-liquid conversions took place with low catalyst loading (0.2-1 mol %).
October 25, 2016: Organic Letters
Rajeev K Dubey, Nick Westerveld, Stephen J Eustace, Ernst J R Sudhölter, Ferdinand C Grozema, Wolter F Jager
Nucleophilic aromatic substitution reactions on 1,7-dibromoperylene-3,4,9,10-tetracarboxylic monoimide dibutylester, using phenol and pyrrolidine reagents, have been exploited to synthesize perylenes with four different substituents at the perylene core. The first substitution is always regiospecific at the imide-activated 7-position. A second substitution reaction does not always replace the bromine at C-1, but may replace a phenol substituent at the highly activated 7-position. Exploiting this reactivity pattern, a "mixed" 1,7-diphenoxy, 1,7-dipyrrolidinyl, and two 1-phenoxy-7-pyrrolidinyl derivatives have been synthesized...
October 25, 2016: Organic Letters
Zu-Feng Xiao, Ting-Hui Ding, Sheng-Wei Mao, Zaher Shah, Xiao-Shan Ning, Yan-Biao Kang
A heterogeneous iridium-complex-catalyzed N-O-cleaving rearrangement/cyclization of 2,3-dihydroisoxazoles with alkenes has been developed. It provides divergent access to multiple substituted pyrrolidines, pyrroles, and carbazoles. The iridium catalyst remains highly catalytic active after seven cycles. The gram-scale synthesis afforded a carbazole with strong bluish-violet fluorescence.
October 25, 2016: Organic Letters
Su Chen, Pengbo Zhang, Wanyun Shu, Yuzhen Gao, Guo Tang, Yufen Zhao
Pyrrolo[1,2-a]indole is a common structural motif found in many natural products and pharmaceuticals. A silver-mediated oxidative phosphinoylation of N-propargyl-substituted indoles was used to construct a variety of 2-phosphinoyl-9H-pyrrolo[1,2-a]indoles under mild conditions. This transformation offers a straightforward route to the formation of the C-P bond, cyclization, and isomerization in one step.
October 25, 2016: Organic Letters
Jing Guo, Lili Lin, Yangbin Liu, Xiangqiang Li, Xiaohua Liu, Xiaoming Feng
The enantioselective α-vinylation of β-keto amides/esters using hypervalent iodine salts has been accomplished via a chiral N,N'-dioxide-nickel(II) complex promoted electrophilic addition and reductive elimination process. A wide range of vinyl-substituted all-carbon quaternary β-keto amides/esters were obtained in high yields and ee values (up to 99% yield and 99% ee). Moreover, the catalytic system has been applied to the enantioselective alkynylation/arylation of β-ketoamides with good results.
October 25, 2016: Organic Letters
Jingfu Li, Wenju Chang, Wenlong Ren, Jie Dai, Yian Shi
An effective Pd-catalyzed regio- and enantioselective hydroesterification of aryl olefins with phenyl formate is described. A variety of phenyl 2-arylpropanoates can be obtained in good yields with high b/l ratios and ee's without using toxic CO gas.
October 25, 2016: Organic Letters
Carlos Vila, Lucia Quintero, Gonzalo Blay, M Carmen Muñoz, José R Pedro
An efficient organocatalytic asymmetric synthesis of α-hydroxyketones has been developed. Quinine-derived thiourea catalyzed the enantioselective Friedel-Crafts alkylation of naphthols and activated phenols with aryl- and alkylglyoxal hydrates, providing the corresponding chiral α-hydroxyketones with high yields (up to 97%) and excellent enantioselectivities (up to 99% ee).
October 25, 2016: Organic Letters
Xiangmin Li, He Huang, Chenguang Yu, Yueteng Zhang, Hao Li, Wei Wang
A metal-free catalytic approach to tryptanthrins has been achieved for the first time. The unique process is realized by an organocatalytic and indole and anthranilic acid substrate co-catalyzed photochemical oxidative condensation with visible light and O2. The truly environmentally friendly reaction conditions enable various reactants to participate in the process to deliver structurally diverse tryptanthrins.
October 25, 2016: Organic Letters
Naoto Esumi, Kensuke Suzuki, Yoshihiro Nishimoto, Makoto Yasuda
We report the visible-light-induced radical coupling reaction of silyl enol ethers with α-bromocarbonyl compounds to give 1,4-dicarbonyls. The reaction was effectively accelerated using an inexpensive organic dye (eosin Y) as a photoredox catalyst. 1,4-Dicarbonyl compounds alone were afforded, without the generation of carbonyl adducts of the α-halocarbonyls, which are usually generated in the presence of fluoride anions or Lewis acids. A variety of silyl enol ethers, α-bromoketones, α-bromoesters, and α-bromoamides were applied to this system to produce the coupling compounds...
October 25, 2016: Organic Letters
Hao Yuan, Jianxian Gong, Zhen Yang
A novel synthetic strategy for the diastereoselective synthesis of structurally diverse oxabicyclo[2.2.1]heptenes has been developed, featuring a tandem reaction combining a Rh-catalyzed triazole denitrogenation and a novel type of [3 + 2] cycloaddition reaction. This tandem reaction was thought to proceed via a five-membered oxonium ylide intermediate, which was formed by the intramolecular nucleophilic attack of the carbonyl group on the α-imino metallocarbene followed by an inter- or intramolecular [3 + 2] dipolar cycloaddition with a range of alkynes and alkenes...
October 25, 2016: Organic Letters
Artur K Mailyan, Kyle Young, Joanna L Chen, Bradley T Reid, Armen Zakarian
A method for a directed stereoselective guanidinylation of alkenes is described. The guanidine unit can be delivered as an intact fragment by a hydroxy or carboxy group, usually with a high level of stereocontrol. After the guanidine delivery, the directing group can be cleaved under exceptionally mild conditions, typically by alcoholysis in the presence of acetic acid. Broad functional group tolerance and mild reaction conditions for the cycloguanidilation suggest applications in medicinal chemistry and natural products synthesis...
October 25, 2016: Organic Letters
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