1-bromo-1-lithioethene: a practical reagent for the efficient preparation of 2-bromo-1-alken-3-ols

A reliable preparative-scale synthesis of 1-bromo-1-lithioethene is reported. This reagent undergoes clean 1,2-addition with a range of aldehydes and ketones at -105 degrees C to afford the corresponding 2-bromo-1-alken-3-ols in moderate to excellent yield. Efficient diastereoselective addition to alpha-siloxy and alpha-methylcyclohexanones, as well as protected 3-keto furanose sugars, is achieved in the presence of 10 mol % CeBr(3). The resulting bromoallylic alcohol adducts have considerable potential as synthetic building blocks. [reaction: see text]     

1-Bromo-1-lithioethene: a practical reagent in organic synthesis

[reaction: see text] A reliable preparative scale synthesis of 1-bromo-1-lithioethene is reported. This reagent undergoes clean 1,2-addition with a range of aldehydes and ketones at -110 degrees C to afford the corresponding 2-bromo-1-alken-3-ols in moderate to excellent yield. Trapping with other electrophiles (acylsilanes, chlorosilanes, tributyltin chloride, iodine) cleanly provides practically useful yields of various 1-substituted 1-bromoethene products. Unexpectedly high diastereoselectivities were observed during the addition of 1-bromo-1-lithioethene to alpha-siloxy aldehydes (typically 10:1, Felkin-Ahn control) and protected ketopyranose and ketofuranose sugars (> or = 10:1, addition from the less-hindered face). The title organolithium reagent possesses relatively low basicity at low temperature, and is compatible with a variety of common protecting groups. We believe that these unusual properties of 1-bromo-1-lithioethene may originate from the specific crystalline structure of the reagent in which lithium is coordinatively saturated and thus unavailable for chelation.     

Novel synthesis of cyclobutanone derivatives via dimetalation of iminium ions with the TiCl4-trialkylamine reagent system

Iminium salts are generated in situ, react with TiCl4-trialkylamines, and diaryl ketones to produce 3,3-diaryl-cyclobutanones in moderate to good yields.     

36-Dimethoxybenzocyclobutenone: a reagent for quinone synthesis

3,6-Dimethoxybenzocyclobutenone 4 is prepared in four efficient steps from 2,5-dimethoxybenzoic acid 8. The derived benzocyclobutenol 13 undergoes electrocyclic ring opening at 110–115°C to give the hydroxy-o-quinone dimethide 21, which reacts with dienophiles to give 5,8-dimethoxy-1,2,3,4-tetrahydro-1-naphthol derivadves stereoselecdvely. Since the ketone 4 can be functionalised at C-5 using electrophiles and at C-2 via hoimolytic bromination, the ring opening and cycloaddition sequence offers a flexible route to linear fused hydroquinone and quinone derivatives. In model studies, the benzocyclobutenol derivative 48 underwent thermal electrocyclic ring opening and intramolecular cycloaddition to give 49, while the analogous reaction with 52 failed due to adverse steric effects during the cycloaddition step. In photochemical experiments, attempts to generate the silyi ether 57 by in situ silylation of the dienol 55 and to prepare the benzocyclobutenol 62 via irradiation of the o-phthalaldehyde monoacetal 60 were unsuccessful.     

N-Methylpyrrolidin-2-one hydrotribromide (MPHT) a mild reagent for selective bromination of carbonyl compounds: synthesis of substituted 2-bromo-1-naphtols

The reaction of the N-methylpyrrolidin-2-one hydrotribromide complex (MPHT) with substituted-1-tetralones has been investigated. This safety reagent proved to be successful for selective α,α-dibromination of tetralones. Moreover, under base-free conditions, several 2-bromo-1-naphtols were obtained from tetralones in a ‘one pot’ sequence in good to excellent yields.     

Facile and efficient synthesis of 4-azidotetrafluoroaniline: a new photoaffinity reagent

p-Azidotetrafluoroaniline (1) was synthesized in 65-73% yield by two different methods employing a stable carbamate intermediate. The first method trapped the intermediate isocyanate generated via a modified Curtius rearrangement with 2-methyl-2-propanol or 2-(trimethylsilyl)ethanol to form the stable carbamates 2d and 2e, respectively. Benzoic acid 2c was first converted to its acid chloride with PCl(5). Displacement of the chloride by NaN(3) in acetone/water formed the acyl azide. Thermal rearrangement followed by the addition of the appropriate alcohols provided the carbamates. The acid labile carbamate 2d was deprotected with HCl/AcOH to provide 1, while trifluoroacetic acid was required to deprotect 2e and afford 1. In the second path, 1 was synthesized in five steps from pentafluoronitrobenzene (3a) in 65% overall yield. Compound 3a was converted into 4-azidotetrafluoronitrobenzene (3b) with NaN(3) in 93% yield and was used without further purification to form 1, 4-diaminotetrafluorobenzene (3c) by Sn/HCl reduction in 85% yield. The mono-9-fluorenylmethoxycarbonyl (FMOC) derivative 3d was formed from 3c with FMOC-Cl and pyridine in EtOAc in 92% yield. Diazotization of 3d under anhydrous conditions with TFA/NaNO(2) and NaN(3) gave 3e in 87% yield. The aryl azide was formed with concurrent nitration of the 2-position of the fluorenyl system. The protecting group was removed with piperidine to afford 1 in 93% yield. Irradiation of 1 with 254 nm light in cyclohexane gave cyclohexylamine 11, diamine 3c, and azobenzene 12 as the primary products. The formation of C-H insertion product 11 indicates that 1 forms a singlet nitrene upon photolysis. Two heterobifunctional photoaffinity reagents iodoacetamide 9 and dansyl derivative 10 were prepared.     

Strained silacycles in organic synthesis: a new reagent for the enantioselective allylation of aldehydes

A new reagent for the enantioselective allylation of aliphatic aldehydes has been developed. The reagent is easily prepared in a single step from commercially available materials and may be stored without significant decomposition. The reactivity of the reagent is attributed to strain-release Lewis acidity.     

1-溴-3-甲苯氧基-2-丙醇的Jones试剂氧化反应

本文采用易得的1-溴-3-甲苯氧基-2-丙醇(1a,1b和1c)经Jones试剂氧化反应,合成了1-溴-3-(4-甲基苯氧基)-2-丙酮(2a)、1-溴-3-(3-甲基苯氧基)-2-丙酮(2b)、1-溴-3-(2-甲基苯氧基)-2-丙酮(2c)化合物,并研究了这类反应的副产物。反应中,产物均经硅胶柱分离得到,IR,^1HNMR,MS确定其结构。产物酮(2a)、(2b)、(2c)非常不稳定,影响了元素分析的准确测定。     

Dipyrromethane as a new organic reagent for the synthesis of gold nanoparticles: preparation and application

Condensation reaction of several ketones with pyrrole in the presence of ferric hydrogen sulfate as a green homogenous acidic catalyst furnished the corresponding pure dipyrromethanes in good yields. Gold nanoparticles were produced through reduction of HAuCl₄ with substituted dipyrromethanes as new reducing agents at room temperature with the exclusion of any capping agent or surfactant. Gold nanoparticles were characterized by transmission electron microscopy, scanning electron microscopy, XRD and UV–visible absorption spectroscopic measurements. It is proposed that in situ formed oxidative products of dipyrromethane, such as polydipyrromethane could serve effectively as a capping agent to preferably adsorb the {111} facets of gold crystals during the reduction process, which leads to the formation of gold nanoparticles.     

Trimethylsilyl trichloroacetate: a new reagent for salt-free silylations

Trimethylsilyl trichloroacetate ($\text{1}$) is a convenient reagent for the silylation of phenols, carboxylic acids, mercaptans, amides, acetylenes, and β-keto esters, while the reaction of $\text{1}$ with aldehydes and ketones affords silylated trichloromethyl carbinols ($\text{5}$).     

The Vilsmeier reagent: a useful and versatile reagent for the synthesis of 2-azetidinones

(Chloromethylene)dimethylammonium chloride (Vilsmeier reagent), prepared easily from N,N-dimethylformamide and oxalyl chloride or thionyl chloride, works as a versatile acid activator reagent for the direct [2+2] ketene-imine cycloaddition of substituted acetic acid and imines in one-pot synthesis under mild conditions. Monocyclic, spirocyclic and 3-electron-withdrawing group β-lactams were synthesized by this method and optimization of conditions were performed.     

Polymer-supported Mukaiyama reagent: a useful coupling reagent for the synthesis of esters and amides

Polymer-supported N-alkyl-2-chloro pyridinium triflate was synthesized in one step from Wang resin. This reagent proved to be a very effective coupling reagent for the synthesis of esters or amides from carboxylic acids and alcohols or amines (primary and secondary). [reaction: see text]     

Tetraethylammonium tetraselenotungstate: a new and efficient selenium transfer reagent for the chemoselective synthesis of functionalised diselenides

A variety of functionalised organodiselenides were prepared in excellent yields from the corresponding halides and activated alcohols on treatment with tetraethylammonium tetraselenotungstate 1 under very mild conditions.     

Improved procedure for the synthesis of thiazolium-type peptide coupling reagents: BMTB as a new efficient reagent

An efficient scalable synthesis of 2-halothiazolium-type peptide coupling reagents has been developed. The key step is the formation of the 2-bromothiazole scaffold through cyclization of α-thiocyanato ketones with hydrogen bromide. Using this method, the new coupling reagent 2-bromo-N-methylthiazolium bromide (BMTB) was synthesized. BMTB was tested in a difficult model coupling reaction of two sterically hindered N-methylated amino acids and showed higher activity than the well-established peptide coupling reagent HATU.     

A new polymer-supported reagent for the synthesis of beta-lactams in solution

A modified Mukaiyama reagent was prepared on a PS-DVB resin. This reagent was used for the preparation of beta-lactams, using the Staudinger reaction. The products were obtained by generating the ketene from a carboxylic acid under sonication with the resin followed by reaction with the imine. Excess of the imine was removed by reduction followed by acid scavenging.