Ohira-Bestmann reagent

Formation of Triazoles Using the Bestmann-Ohira Reagent. 174... 

In another series of experiments, the Ley research group [42] has used a Bestmann-Ohira reagent (dimethyl 1-diazo-2-oxopropylphosphonate [43]) to synthesize various alkynes and triazoles starting from aldehydes in the same Vapourtec system [38] combined with Omnifit glass columns [41]. 

Scheme 17 Flow synthesis of terminal alkynes using the Bestmann-Ohira reagent [42]...

Scheme 19 A three-step synthesis of triazole using Bestmann-Ohira reagent starting from an alcohol [42]...

Baxendale IR, Ley SV, Mansfield AC et al (2009) Multistep synthesis using modular flow reactors Bestmann-Ohira reagent for the formation of alkynes and triazoles. Angew Chem Int Edit 48(22) 4017 021... 

As seen in the last example, the configuration of the double bond is conserved in the reaction. When Mn02 is used in combination with the Bestmann-Ohira reagent, activated alcohols are converted into terminal alkynes. ... 

Another attractive TOP sequence involves use of a combination of the Bestmann-Ohira reagent and Mn02. In these examples, a range of activated alcohols are oxidized to the corresponding aldehydes, which in turn are efficiently trapped by the subsequently added alkynylating reagent (eq 103). 

The Bestmann-Ohira Reagent for the Conversion of Aldehydes into Terminal Alkynes Zanatta, S.D. Anstr. J. Chem. 2007, 60,963. 

Keywords Aldehydes, active methylene compounds, Bestmann-Ohira reagent [dimethyl(dia-zomethyl)phosphonate], potassium hydroxide, methanol, room temperamre, Knoevenagel condensation, one-pot multicomponent reaction, click chemistry, cycloaddition, phospho-nates, regioselective synthesis, phosphonyl pyrazoles... 

To a stirred solution of aldehyde (1 1 mmol), active methylene cyano derivative (2 1.2 mmol) and Bestmann-Ohira reagent (3 1.5 mmol) in distilled methanol (4 mL), was added molecular sieves followed by powdered KOH (2 mmol) and the reaction mixture was stirred at room temperature for one hour. Upon completion of the reaction (TLC monitoring), methanol was distilled off under reduced pressure. The crude residue was dissolved in ethyl acetate (50 mL) and washed with saturated ammonium chloride (2 x 20 mL). Finally the organic layer was washed with saturated brine (20 mL) and dried over anhydrous sodium sulphate. After removal of the solvent, column chromatographic purification was carried out using 0-30% acetone-dichloromethane as eluent to afford phosphonyl pyrazole 4. All the products were characterized based on spectral studies. 

Keywords Aldehydes, Bestmann-Ohira reagent [diethyl(diazomethyl)phosphonate], diazomethyl sulfone, cesium carbonate, potassium hydroxide, copper(I) iodide, ethanol, room temperature, Homer-Wadsworth-Emmons (HWE) reaction, click chemistry, cycloaddition, phosphonates, one-pot synthesis, phosphonylpyrazoles, sulfonylpyrazols... 

Kumar, R., Verma, D., Mobin, S. M., and Namboothiri, I. N. N. (2012). One-pot, two-step conversion of aldehydes to phosphonyl- and sulfonylpyrazoles using Bestmann-Ohira reagent. Org. Lett., 14, 4070-4073. 

Dess-Martin oxidation of alcohol III-26 gave aldehyde III-27, which was used in the next step without further purification. Aldehyde III-27 reacted with the Bestmann-Ohira reagent [96-101] to give alkyne III-28, from which alkynylindole III-29 was obtained by Boc cleavage after a brief exposure to trifluoroacetic acid. This four step procedure can be routinely carried out without purification of any intermediate in 14—23 % overall yield. Alternatively, alkyne III-29 could be prepared from aldehyde III-27 by the Corey-Fuchs procedure, although the overall yield was lower using this procedure [102],... 

From the chiral epoxide 56, the TMM cycloaddition precursor 55 for construction of the tetraquinane stmcture was prepared in a straightforward manner. Oxidation of epoxy alcohol 56 by the Swem protocol followed by treatment with Bestmann-Ohira reagent (E) produced alkyne 70. Iron-catalyzed Sisr2 -type reaction of 70 afforded an allene moiety as a diastereo-meric mixture in a 1 1 ratio and this mixture was used in the next step without the separation of isomers. Allenyl alcohol 71 was protected as a TBDPS ether to give 0-silylated allene 72. Deprotection of the acetal of 72 was delicate as the allenyl moiety was not stable under acidic condition. Fortunately, treatment of 72 with p-toluenesulfonic acid monohydrate in presence of formaldehyde afforded aldehyde 73, and subsequent treatment with p-toluene sulfonyUiyrazide furnished the precursor 55 for the tandem cycloaddition reaction (Scheme 21). 

Novel carbonylated pyrazole phosphonates (596-599) have been synthesised as single regioisomers by treating conjugated enones (592), dienones (593), tropone (594), and quinone (595) with the Bestmann-Ohira reagent (591) under KOH/EtOH conditions at room temperature (Scheme 147). ... 

A one-pot domino Claisen-Schmidt/l,3-dipolar cycloaddition/oxidation reaction sequence involving an aldehyde (801), a methyl ketone (802), and the Bestmann Ohira reagent (803), has been developed for the synthesis of variously substituted 3-carbo-5-phosphonylpyrazoles (804) (Scheme 204). ... 

A Wittig reaction of the diformyl[2.2]paracyclophane with methyltriphenyl-phosphonium bromide affords the corresponding divinyl[2.2]paracyclophane (Fig. 16c) [92,93]. Treatment of the dialdehyde with the Bestmann-Ohira reagent (CH3C0CN2P0(0CH3)2) provides the corresponding diethynyl[2.2]paracyclophane [94-96]. 

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