One-pot chemo-, regio-, and stereoselective double-differential glycosidation mediated by lanthanide triflates

Nuanced activation of n-pentenyl, thioglycoside, and trichloroacetimidate donors by lanthanide salts coupled with donor/acceptor matching can simplify oligosaccharide assembly. Thus, a one-pot, double-differential glycosidation process can be designed, in which an n-pentenyl acceptor-diol is chemo- and regioselectively glycosidated by using an n-pentenyl ortho ester under the agency of Yb(OTf)(3)/NIS followed by in situ addition of a 2-O-acylated trichloroacetimidate or ethyl thioglycoside to effect stereoselective glycosidation at the remaining OH.     

Highly efficient chemo- and regioselective silylation of -OH groups and cyanosilylation of aldehydes promoted by TiCl2(OTf)-SiO2 as a new recyclable catalyst

TiCl₂(OTf)-SiO₂ as an easy handling recyclable catalyst was applied for trimethylsilylation of diethyl α-hydroxyphosphonates, alcohols and phenols with high selectivity using HMDS as a silylating agent. Cyanotrimethylsilyl ethers were also obtained in excellent yields from treatment of aldehydes with TMSCN in the presence of this catalyst.     

Highly stereoselective epoxidation of O-protected 3-hydroxy-1-nitroalkenes

The diastereoselectivity of the nucleophilic epoxidation of O-protected 3-hydroxy-1-nitroalkenes was investigated. Epoxidation of the O-protected 3-hydroxy-1-nitroalkenes was highly stereoselective, giving rise to the anti isomer. The resulting nitroepoxides have been transformed into nitroaldols through hydrogenation. The nucleophilic epoxidation of nitroalkenes was found to be irreversible. Models to explain the observed stereoselectivities are proposed.     

A novel promoter, heteropoly acid, mediated chemo- and stereoselective sulfoxide glycosidation reactions

The chemo- and stereoselective glycosidations of sulfinylglycosides and alcohols using a heteropoly acid, H₃PW₁₂O₄₀, as a new promoter have been developed.     

Highly stereoselective beta-anomeric glycosidation of a 2'-deoxy syn-5'-configured 4'-spironucleoside

A direct enantioselective pathway that delivers exclusively the beta-anomer of a 4'-spironucleoside has been successfully developed. The key starting material is the enantiomerically pure dihydroxy lactone 19, which has proven amenable to conversion to glycal 22 via the chloro acetonide. This intermediate is then capped as the 3,5-O-(tetraisopropyldisiloxane-1,3-diyl) glycal. The latter can enter into N-iodosuccinimide-promoted glycosidation with persilylated thymine. Only the beta anomer is formed. Ensuing deiodination and desilylation proceed quantitatively to furnish the targeted, previously elusive anomer.     

AlCl3-NaI(NaBr)-t-BuOH: mild, chemo- and stereoselective reagents for hydrohalogenation of propiolic derivatives

(Z)-β-Iodo-propenamides and -β-iodo-propenoic esters were selectively prepared in high yields, at room temperature, through reaction of 2-propynamides and 2-propynoic esters, respectively, with AlCl₃ and NaI in the presence of t-BuOH in dichloromethane. These experimental conditions are compatible with the presence of acid sensitive acetal groups. Alternative use of EtOH or H₂O in place of t-BuOH was investigated. (Z)-Bromo-propenamides and corresponding esters were prepared according to a similar procedure using sodium bromide in refluxing acetonitrile.     

Chemoselective monobromination of alkanes promoted by unactivated MnO2

Reaction of alkanes with bromine promoted by unactivated MnO₂ gave the corresponding alkylbromides in excellent yield with good chemoselectivity. The MnO₂ could be easily recovered and reused.     

A mild and efficient synthetic protocol for Ferrier azaglycosylation promoted by ZnCl2/Al2O3

An improved method for the Ferrier sulfonamidoglycosylation of tri-O-acetyl-d-glucal with different N-nucleophiles has been developed. ZnCl₂ impregnated on activated alumina acts as an excellent reagent system for the conversion of 3,4,6-tri-O-acetyl-d-glucal into 2,3-unsaturated-N-pseudoglycals with good yields and preferential α-anomeric selectivity.     

Further studies in the acyl-type radical additions promoted by SmI2: mechanistic implications and stereoselective reduction of the keto-functionality

Attempts were made to promote the carbonyl coupling of cyclohexanone to 4-pyridylthioesters of N-carbamate-protected amino acids with the one electron reducing agent, samarium diiodide. Such reactions proved unsuccessful due to the inability of the ketyl-type radical anion intermediate to be reduced to the corresponding dianion at −78°C. Nevertheless, these results explain our recently published work on the high efficiency of the SmI₂-mediated acyl-type radical additions of the same thioesters with electron deficient alkenes [J. Am. Chem. Soc. 2003, 125, 4030]. A study was also undertaken to examine methods for the stereoselective reduction of N-carbamate-protected amino ketones to either the syn- or anti-vicinal amino alcohols. In most cases, LiAl(O-t-Bu)₃H and (S)-Alpine-Hydride were found to effectively provide the anti- and syn-amino alcohols, respectively. The SmI₂-promoted reduction of the same ketones afforded a majority of the syn-isomer with selectivities of approximately 5:1. However, in one case, the SmI₂-promoted reduction was found to be more effective than that of (S)-Alpine-Hydride.     

Oxidizing supported reagents, III. Oxidation of alcohols by new supported K2Cr2O7 reagents

Several new supported K₂Cr₂O₇ reagents are used in the oxidation of secondary alcohols. The influence of the number of active sites and the nature of solid support on the yield is discussed. The influence of the structure of alcohol on the yield of ketone is analyzed.

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K₂Cr₂O₇ . . .

     

Highly regio- and stereoselective PdCl2(MeCN)2-catalyzed cross coupling of 1,2-allenylic sulfoxides with allyl bromide

2-Allyl-1(E),3(E)-dienyl sulfoxides were prepared highly stereoselectively via the PdCl₂(MeCN)₂-catalyzed coupling reaction of 1,2-allenylic sulfoxides and allyl bromide. A rationale was proposed for this transformation.     

Highly chemo- and stereoselective intermolecular coupling of diazoacetates to give cis-olefins by using Grubbs second-generation catalyst

Highly stereoselective formation of cis-2-ene-1,4-diesters by homo- and heterocoupling of alpha-diazoacetates in the presence of Grubbs second-generation catalyst is demonstrated. The dual reactivity of the catalyst in alkene metathesis and diazocoupling has been exploited in the synthesis of 12-26-membered macrocyclic dienyl dilactones by one-pot carbene dimerisation/ring-closing metathesis.     

Facile and stereoselective synthesis of 25-hydroxyvitamin d2

25-Hydroxyvitamin D₂ as synthesized conveniently and stereoselectively for the first time by utilizing Sharpless' chiral epoxidation and the subsequent regio- and stereoselective methylation with lithium dimethylcuprate to introduce the desired chirality at C(24) and the reductive elimination of the vicinal hydroxy sulfone to introduce stereoselectively the $\text{E}$ double bond at C(22) of the target molecule.     

Alkaline hydrogen production promoted by small-molecule modification on flowerlike Co2(OH)2CO3

Developing a low-cost and high-efficiency nonprecious metal-based catalyst for hydrogen evolution reaction(HER) is of great significance for the utilization of hydrogen energy.In this work,we report a molecular-modification strategy to fabricate a self-supported hydrogen evolution electrode,specially by grafting the macrocyclic molecules(HHTP=2,3,6,7,10,11-hexahydroxytriphenylene) on the surface of a cobaltous dihydroxy carbonate(COC) seed layer.The HHTP-COC electrode is endowed with a rodlike s...     

A new synthesis of acetamido phenols promoted by Ce(SO4)2

A new method for the synthesis of acetamido phenols by a one-pot, three-component Ritter type reaction in moderate to good yields is described. Both electron donating and electron releasing substitution on aromatic rings are tolerated.     

State of components in Pd/C catalysts promoted by Ho2O3

Catalysts prepared through the interaction of C₃H₅PdC₅H₅ with the surface of evacuated Ho₂O₃/C after reduction by H₂ at 573 K contain Pd and Ho₂O₃ particles whose size is 30 and 20–40 Å, respectively. XPS and TEM data indicate that Pd particles have a positive charge and their surface is partly covered by Ho oxide.

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, C₃H₅PdC₅H₅ Ho₂O₃/C, H₂ 573 Pd 30 Å Ho₂O₃ 20–40 Å. XPS TEM Pd , Ho.

     

Synthesis of tri- and tetracyclic diterpenes. Cyclisations promoted by SmI2

Several tri- and tetracyclic diterpenes have been synthesised from zamoranic acid. The key step is the cyclisation of a dicarbonyl 13,14-secoderivative by SmI₂.     

Lewis acid-catalyzed stereoselective lactonization and subsequent glycosidation of 2-C-malonyl carbohydrates

Gold(III) bromide is a suitable catalyst for the stereoselective cyclization of 2-C-malonyl carbohydrates to the anomeric center under retention of one ester group. Reopening of the lactones with alcohols in the presence of TMSOTf affords allyl, propargyl and benzyl glycosides with high α-selectivity.     

高电化学活性SnO2/TiO2空心微球的制备及其性能

采用一种简便的无模板溶剂热法合成了尺寸在1μm左右、具有堆叠结构的SnO_2/TiO_2空心微球。合成过程的研究结果表明:SnO_2/TiO_2空心微球在形成过程中经历了空心、被填充、分裂到再次形成空心结构的过程。随后,SnO_2/TiO_2空心微球作为锂离子电池负极材料的电化学性能测试结果表明:SnO_2/TiO_2空心微球在0.1 A·g~(-1)的电流密度下,其首次放电容量达到1 484.9mAh·g~(-1),库伦效率为49.0%。经过600次循环后,其放电容量依然可以达到565.6 mAh·g~(-1),显示了高的容量和循环稳定性。