Sucrose and KF quenching system for solution phase parallel synthesis

The KF, sucrose (table sugar) exploited as quenching system in solution phase parallel synthesis. Excess of electrophiles were covalently trapped with hydroxyl functionality of sucrose and due to polar nature of sucrose derivative was solubilize in water. Potassium fluoride used to convert various excess electrophilic reagents such as acid chlorides, sulfonyl chlorides, isocyanates to corresponding fluorides, which are less susceptible for hydrolysis and subsequently sucrose traps these fluorides and dissolves them in water thus removing them from reaction mixture. Various excess electrophilic reagents such as acid chlorides, sulfonyl chlorides, and isocyanates were quenched successfully to give pure products in excellent yields.


Background
The generation and use of combinatorial chemical libraries for the identification of novel chemical leads or for optimization of a promising lead candidate has emerged as a potentially powerful tool for acceleration of the drug discovery process (Terrett et al. 1995;Gallop et al. 1994;Gordon et al. 1994;Janda 1994;Pavia et al. 1993). The combinatorial chemistry has already yielded several compounds that are currently undergoing clinical trials. The pharmaceutical industry needs large and diverse molecular libraries. The screening of large number of compounds can be quickly lead to early structure-activity relationships (SARs), and may provide a practical starting point for drug discovery program, where little or no information is known about the target.
The amide and sulfonamide functionalities are the key structural moieties in many pharmaceutically active compounds such as paracetamol (analgesic and antipyretic), nateglinide (for treatment of type 2 diabetes), probenecid (uricosuric drug), and sotalol (for cardiac arrhythmias) (Fig. 1). Similarly, urea functionality act as a non hydrolysable surrogate of amide bonds in many pharmaceutically active molecules (Majer and Randad 1994;Kruijtzer et al. 1997;Decieco et al. 1997). Therefore practical method for rapid synthesis of amide, sulfonamide and urea containing molecules are of great interest in drug discovery and lead optimization.
Solution-phase parallel synthesis is an excellent way to form libraries of small molecules containing amide, sulfonamide and urea functionalities. However, during the synthesis of library, the chemistry for solution phase parallel synthesis require complete conversion of reactants with little or no formation of by-products or impurities to simplify the tedious purification processes. The solid phase synthesis offers benefit of easy and fast purification to separate excess reagents and side products from the desired compounds attached to the insoluble carrier. However, due to heterogeneous reaction conditions, salvation of bound species, and mass transfer of reagents are the limitations of solid phase synthesis. The range of chemistry applied to the solid-phase synthesis has limitations.
In earlier reports, the solid phase quenchers in the form of reagents on the solid phase or ion exchange resins have been used in quenching reactions to eliminate the reactive entites (Thompson and Ellman 1996). Potassium sarcosinate and fluorous-tethered as quencher were also reported to achieve final compounds in good purity and quantities (Nikam et al. 1998;Lindsley et al. 2002;Curran 2001;Zhang et al. 2003;Zhang et al. 2002). Here, we report sucrose as readily available, environment friendly and cost effective novel quenching agent for the solution phase parallel synthesis.

Results and discussion
In medicinal chemistry program we need rapid synthesis of various acylated and sulfonated derivatives of benzyl amines. During the parallel synthesis, for the complete conversion of the substrate a little excess of an electrophilic reagent was always added, this resulted the impure products and required tedious column chromatographic purifications. Therefore, a new and rapid process for eliminating these excess electrophiles is required. To overcome this issue, KF and sucrose treatment was found to be a novel quenching system in these reactions. As a prototype we chose benzylamine as substrate (Fig. 2). Thus, in a typical reaction of benzyl amine (1.0 equivalent) was treated with an excess of electrophile 1.4 equivalents, e.g. acid chloride, sulfonyl chloride, or isocyanate in presence of triethylamine in DMF. After stirring for 6 h, potassium fluoride (1.5 equivalents) was added to the reaction and the reaction mixture was stirred for an additional 0.5 h. Further sucrose (1.5 equivalents) was added and reaction mixture stirred for additional 0.5 h. Water was then added to the reaction mixture under stirring and the final product was obtained by simple filtration or extraction with EtOAc. The reaction and the purity of the product were monitored by TLC and 1 H-NMR, which indicated pure product with no evidence of electrophile used. The products were isolated in excellent yields with high purity. The excess of acid chlorides can also be quenched using aqueous Sodium carbonate, while few aromatic acid chlorides do not get quenched in aqueous base and remain intact; but can be effectively quenched by KF sucrose quenching system ( Table 1).
The excess of electrophiles were covalently trapped with hydroxyl functionality of sucrose and dissolve in water. Role of potassium fluoride is to convert various excess electrophilic reagents such as acid chlorides, sulfonyl chlorides, isocyanates to corresponding fluorides (Ishikawa et al. 1981;Kimura and Suzuki 1989;Dang and Olofson 1990), which are less reactive than acid chlorides. Acid fluorides, on the other hand, are known to be more stable to hydrolysis than acid chlorides and hence avoid the formation of by-products. KF is readily soluble in water (92 g/100 ml at 18 °C, 102 g/100 ml at 25 °C) and the reaction in absence of KF yields impure product, which further required column purification. The role of KF is to convert chlorides to corresponding fluorides making them less susceptible to hydrolysis and generation of difficult-to-remove impurities and subsequently sucrose traps these fluorides and dissolves them in water thus removing them from reaction mixture.
After the aqueous quench no sucrose or its derivatives were seen on TLC or in 1  methodology worked with a larger excess of electrophiles in the reaction, which needed to be quenched with excess amounts of potassium fluoride and sucrose. This methodology can be easily automated on a synthesizer for synthesizing the desire array of compounds in several gram quantities of acylated products.

Conclusion
In summary, we have developed a simple solution-phase parallel synthesis using a novel quencher for the excess electrophiles. The quencher system KF, sucrose is readily available, environment friendly, cost effective and was found to be very efficient in trapping excess acid chlorides, sulfonyl chlorides and isocyanates to give water soluble by products which could be removed by an aqueous workup.

Reagents used
Reactions were monitored by thin layer chromatography (TLC), carried out on 0.2 mm silica gel 60 F 254 (Merck) plates using UV light (254 and 366 nm) for detection and compounds were purified by column chromatography by using silica gel of 5-20 µm (Merck, 60-120 mesh). Column dimension 39 × 2 cm and elution volume used is about 200-400 ml for each product. Common reagent grade chemicals are either commercially available and were used without further purification or were prepared by standard literature procedures.

Characterization
The 1 H spectra were recorded on a Bruker XL 300 spectrometer (300 MHz). Chemical shifts were reported in parts per million using tetramethylsilane as an internal standard and were given in δ units. The solvent for NMR spectra was DMSO-d 6 unless otherwise stated. Elemental analyses were performed on a Hosli CH-Analyzer and are within ±0.4 of the theoretical percentage. All reactions were monitored by thin layer chromatography, carried out on 0.2 mm silica gel 60 F 254 (Merck) plates using UV light (254 and 366 nm) for detection. Common reagents grade chemicals are either commercially available and were used without further purification or prepared by standard literature procedures.

Synthesis of sulfonamides derivatives 3(a-h)
The mixture of benzylamine (0.268 g, 2.5 mmol), sulfonyl chloride (3.5 mmol) and triethylamine (0.695 ml, 5 mmol) in DMF (5 ml) was stirred at room temperature for 5-6 h (TLC Check, 40 % ethyl acetate/hexane). To the stirring solution KF (88 mg, 1.5 mmol) was added and the reaction mixture was stirred for 0.5 h. Sucrose (0.514 g, 1.5 mmol) was added and stirring was continued for next half an hour. Water (50 ml) was then added to the reaction mixture under stirring. The product was isolated by filtration and washed with 10 ml water.