Water as catalyst and solvent: Tetrahydropyranylation of alcohols in an aqueous medium

Water is found to catalyze the tetrahydropyranylation of alcohols at elevated temperature. Interestingly, tetrahydropyranylation of alcohols works under a wide range of pH, 6.5 to 2 and does not work beyond pH 7.5 in an aqueous medium. Hydrophobic interactions between the substrate alcohol and Dihydropyran ether (DHP) and favorable pKa’s of water, alcohols, and protonated hydroxyl and protonated ethereal functions are the driving forces for the reaction. Thus the reaction, which is carried out in anhydrous aprotic solvents, can be carried out in aqueous medium in an environmentally benign way

Tetrabutylammonium tribromide mediated condensation of carboxylic acids with alcohol

The direct condensation of various carboxylic acids and alcohols was achieved efficiently at reflux temperature under a solvent free condition using a catalytic amount of Tetrabutylammonium Tribromide (TBATB). Chemoselective acylation of primary alcohols in presence of secondary alcohols and phenols has been achieved. Steric factors in carboxylic acids played a crucial role during chemoselective acylation of diols. Reaction under a solvent free condition, absence of any dehydrating agent or use of any special techniques for removal of water and higher yields are the important features of this protocol

Reinvestigation of themechanism ofgem-diacylation: chemoselective conversion of aldehydes to variousgem-diacylates and their cleavage under acidic and basic conditions

The mechanism of gem‐diacylate formation has been studied extensively using Tetrabutylammonium tribromide (TBATB) as the catalyst. The reaction proceeds by a nucleophilic attack of an anhydride on an aldehydic carbonyl group, nucleophilic attack of the hemiacylate intermediate on a second molecule of the anhydride, followed by an intermolecular attack of a second acetate group to regenerate the anhydride. gem‐Diacylates of various aliphatic and aromatic aldehydes were obtained directly from the reaction of a variety of aliphatic and aromatic acid anhydrides in the presence of a catalytic quantity of Tetrabutylammonium Tribromide (TBATB) under solvent‐free conditions. A significant electronic effect was observed during its formation as well as deprotection to the corresponding aldehyde. Chemoselective gem‐diacylation of the aromatic aldehyde containing an electron‐donating group has been achieved in the presence of an aldehyde containing an electron‐withdrawing group. Deprotection of the gem‐diacylate to the parent carbonyl compound can be accomplished in methanol in presence of the same catalyst. Here again, chemoselective deprotection of the gem‐diacylate of a substrate containing an electrondonating group has been achieved in the presence of a substrate containing an electron‐withdrawing group. Both the acid and base stability order of the various gem‐diacylates examined follow a similar order. The stability order determined from the present study is: gem‐dibenzoate > gemdipivalate > gem‐diisobutyrate > gem‐diacetate > gem‐dipropionate. All the gem‐diacylals are more stable under basic conditions than acidic condition. No correlation was found between the stability order and the pKa’s of the corresponding acids; rather, the stability order is directly related to the steric crowding around the carbonyl carbon

A new recyclable ditribromide reagent for efficient bromination under solvent free condition

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Bromineless bromine as an efficient desulfurizing agent for the preparation of cyanamides and 2-aminothiazoles from dithiocarbamate salts

n a one-pot procedure, bromineless brominating reagent 1,1′-(ethane-1,2-diyl)dipyridinium bistribromide (EDPBT) has been used as a desulfurizing agent in the preparation of organic cyanamides and substituted thiazoles starting from dithiocarbamic acid salts. In this approach, alkyl/aryl isothiocyantes were first obtained by the desulfurization of dithiocarbamic acid salts with EDPBT. The in situ–generated isothiocyanates reacts with an aqueous ammonia, forming alkyl or aryl thioureas, which on subsequent oxidative desulfurization with EDPBT led to the formation of corresponding cyanamides in good yields. Alternatively, an efficient one-pot synthesis of substituted thiazoles has been achieved by the condensation of the in situ–generated 1-aryl thioureas with the in situ–generated α-bromoketones from ketones, again using EDPBT. The reagent EDPBT can be easily prepared from the readily available reagents. Desulfurizing ability dominates over its brominating ability for substrates amenable to bromination

1,1'-(Ethane-1,2-diyl)dipyridinium bistribromide (EDPBT) as a recyclable catalyst for acylation

1,1′-(Ethane-1,2-diyl)dipyridinium bistribromide (EDPBT) catalyzes the acylation of structurally diverse alcohols, amines, thiols, and phenols with a variety of aliphatic and aromatic anhydrides. Steric factors in substrates as well as anhydrides and solvent play significant role during the formation of acylates. Chemoselective mono acetylation of symmetrical diols, primary hydroxy group over secondary and phenolic group and amines over phenols has been achieved. The compatibility of the protocol has been shown by the survival of different acid sensitive functionalities under the present reaction condition. The solvent, acetone, reacts with EDPBT giving bromoacetone and HBr, thus suppressing the bromination of substrates otherwise amenable to bromination. The reagent EDPBT being devoid of phase transfer property and owing to the high solubility of its precursor 1,1′-(ethane-1,2-diyl)dipyridinium dibromide (EDPDB) in water, it was possible to isolate pure acylates by an aqueous work-up circumventing the need for further purification. The process is superior owing to the recyclability of the reagent. The spent reagent can be recovered, regenerated, and reused without any significant loss

Self-assembled superstructure of xanthene derivatives

Various multi carbon homologations of 9-phenyl-9H-xanthen-9-ol (1) were obtained through a C-C bond formation by reacting it with various enolisable ketones in the presence of 1,1′-(ethane-1,2-diyl)dipyridinium bistribromide (EDPBT). All the ten derivatives along with the starting xanthen-9-ol have been characterized by single crystal X-ray diffraction. They all form self-assembled superstructure in the solid state. The self-assembling patterns in these supramolecular architectures were explained based on steric and electronic nature of the pendant arm

It Is “Thiazolidene-2-imine” and not imidazole-2-thione as the reaction product of 1-benzoyl-3-phenylthiourea with Br₂/enolizable ketone

The products obtained by the reaction of benzoyl-3-phenylthioureas with bromine and enolizable ketones in the presence of triethylamine are not imidazole-2-thione derivatives as reported

Syntheses and regiochemistry of enol addition to 9-phenyl-9H-xanthen-9-ol

Regioselective C–C bond formation of 9-phenyl-9H-xanthen-9-ol 1 with various enolizable ketones I–X in an acidic (HBr) medium, obtained by the reaction of 1,1′-(ethane-1,2-diyl)dipyridinium bistribromide (EDPBT) with ketone is observed. Except for ketone, 4-methyl-pentan-2-one VII in all other cases examined the attack to xanthenyl carbocation is from the thermodynamically stable enolizable side of the unsymmetrical ketones. In the case of 3-methyl-butan-2-one VIII the equilibrium is in favor of the more stable enolizable ketone, which has large steric factor, hence no reaction was observed during its addition to alcohol 1

A convenient one-pot synthesis of thiazol-2-imines: application in the construction of pifithrin analogues

For the first time a reaction intermediate has been isolated giving further insight into the mechanism of thiazol-2-imine formation. The first step of the reaction requires a basic medium, while the second step is an acid mediated E1 elimination reaction. An efficient one-pot synthesis of substituted thiazol-2-imines have been achieved by the condensation of carbonyl compounds with thioureas and 1,3-disubstituted thioureas using 1,1′-(ethane-1,2-diyl)dipyridinium bistribromide (EDPBT). Unsymmetrical 1,3-disubstituted thioureas give regioselective products with symmetrical ketones, which are mainly governed by the p K a s of NH protons of thiourea, whereas symmetrical 1,3-disubstituted thioureas give regioselective products with symmetrical carbonyl compounds owing to the regioselective bromination of ketones. The methodology is extended to access novel neurodegenerative drug candidate pifithrin-α analogues in good yields in shorter reaction time. This method is simple, versatile and is applicable for different 1,3-disubstituted thioureas as well as a range of carbonyl compounds