Applications in Combinatorial Chemistry

Characterisation of the structure and conformation of small biologically active molecules is part of the standard approach to lead generation in drug design studies. In particular, it is now possible to automatically synthesise many thousands of small molecules and then rapidly measure their effects in a given pharmacological test system. The power of such techniques comes from the immense number of compounds which can be generated and screened for activity. Two studies have evaluated HPLC-NMR in the field, one based on a mixture of 27 closely related tripeptides [24] and the other on two separate mixtures of four aromatic compounds and three pentapeptides [25]. 

The tripeptide application [24] comprised a synthetic mixture of the 27 combinations of the tripeptide formed from alanine (A), methionine (M) and tyrosine (Y), as the C-terminal amide, H2N.CH(Ri).CONH.CH(R2). CONH.CH(R3).CONH2, where Ri, R2 and R3 take all combinations of A, M and Y. 

HPLC analysis was carried out using a 250 x 4.6 mm id Spherisorb ODS-2 column at 35°C with elution using isocratic D20/phosphate buffer at pH 2.5 for 5 min, followed by a linear gradient of acetonitrile to 50% after 50 min with UV monitoring of the eluent peaks at 225 nm. The NMR measurements were carried out with solvent suppression at a H NMR frequency of 600 MHz. The assignment of the resonances were based on those of standard compounds such as A-A-A-OH and Y-Y-Y-OH using standard 1-dimensional (ID) spectroscopy and 2-dimensional (2D) heteronuclear correlation methods such as and H-15N HMQC spectra. 

Based on chemical shifts and peak multiplicities, the on-flow HPLC-NMR characterisation of the majority of the components in the mixture of 27 tripeptides was achieved and demonstrated that this approach is likely to be an effective method for compound mixtures. The elution positions of all of the alanyl-containing peptides were determined, with the exception of A-M-M-NH2, which may have co-eluted with another peptide or may have been synthesised in a much smaller quantity. The only other tripeptides for which assignments have not been obtained are the MY2-NH2 isomers and two of the three M2Y-NH2 isomers. These eluted towards the end of the gradient run and are not as well resolved under these HPLC conditions. Additionally, with changes in the relative chemical shifts of the solvent signals, the intensities of the non-TV-terminal a-CH protons and the methionyl [3-methylene signals from these peptides may have been reduced by the effects of the solvent suppression irradiation of the water and acetonitrile resonances, respectively. With further optimisation of the elution conditions, it is possible that all 27 analytes could have been resolved and characterised. 

More recently, significant effort has been put into using direct-flow NMR probes such as HPLC-NMR probes for analysing the components of combinatorial chemistry libraries but without the need for an HPLC separation first [26], In this case, 88 samples from a combinatorial chemistry synthesis were placed in a 96-well plate and after the NMR spectra were measured the samples could be returned to the sample plate for further analysis or 

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The major impetus for the development of solid phase synthesis centers around applications in combinatorial chemistry. The notion that new drug leads and catalysts can be discovered in a high tiuoughput fashion has been demonstrated many times over as is evidenced from the number of publications that have arisen (see references at the end of this chapter). A number of )proaches to combinatorial chemistry exist. These include the split-mix method, serial techniques and parallel methods to generate libraries of compounds. The advances in combinatorial chemistry are also accompani by sophisticated methods in deconvolution and identification of compounds from libraries. In a number of cases, innovative hardware and software has been developed tor these purposes. 

Lewell XQ, Judd DB, Watson SP, Hann MM. RECAP—retrosynthetic combinatorial analysis procedure a powerful new techniqne for identifying privileged molecular fragments with useful applications in combinatorial chemistry. J Chem Inf Comput Sci 1998 38 511-22. 

The preparation of resin-bound nitroalkenes via a microwave-assisted Knoevenagel reaction of resin-bound nitroacetic acid with aryl and alkyl substituted aldehydes is reported. The potential of these resin-bound nitroalkenes for application in combinatorial chemistry is demonstrated by a Diels-Alder reaction with 2,3-dimethylbutadiene (Scheme 8.9). It is also used for one-pot three-component tandem [4+2]/[3+2] reactions with ethyl vinyl ether and styrene 46... 

Laser heating, case hardening by, 16 200 Laser heterodyne radiometry, 23 142 Laser-induced breakdown spectroscopy (LIBS), archaeological materials, 5 743 Laser-induced EDC cracking, 25 646 Laser-induced fluorescence, 23 127 Laser-induced fluorescence imaging, application in combinatorial chemistry, 7 404... 

Microreactor technology, application in combinatorial chemistry, 7 401, 422 Micro RNAs (miRNA), 17 620-621 Micro-routing, in waste collection, 25 869 Microsampling methods, in infrared spectroscopy, 14 232-233 Microscale microbial cultures, shaken, 16 406... 

Lewell, X.Q. and Smith, R. Dmg-motif-based diverse monomer selection method and application in combinatorial chemistry. J. Mol. Model. 1997, 15, 43-48. 

Dankwardt, S. M>. Newman, S. R. Krstenansky, J. L. Solid Phase Synthesis of Aryl and Benzylpiperazines and their Application in Combinatorial Chemistry, Tetrahedron Lett. 1995, 36, 4923. 

Undoubtedly, NMR is the most informative method for characterization of organic compounds. However, it has limited application in combinatorial chemistry due to several factors. NMR is a relatively insensitive and slow method, requires homogeneous samples, and consumes quite expensive deuterated solvents. Here we will discuss the most recent developments of this method that overcome the major limitations and make NMR one of the promising techniques in combinatorial chemistry. It relates to the application of NMR, not only for analyzing compounds attached to polymer support and for monitoring reactions on a solid phase, but also as a detector for liquid chromatography (LC/NMR). For the most recent review, see [10]. 

High-Pressure Promoted Cycloadditions for Application in Combinatorial Chemistry... 

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