Scheme Library

Figure 17.2 An example of a lead generation design and testing cascade including diversity schemes, library enumeration, predicdve models, panels of in vitro assays, and ultimately in vivo pharmacokinetic and efficacy tests in animal models.

While true parallel and multi-processor computers use much faster connections than IP protocol lines for internal communication, it is possible to simulate such systems using the Internet. Some spectacular computational feats have been achieved by coupling thousands of Internet-connected computers and using their idle cycles for the factorization of large numbers or, closely related, for the cracking of encryption schemes. Libraries for the setup of virtual parallel computers and cooperating distributed systems such as PVM (parallel virtual machine ) are readily available. 

Eda and Kurth applied a similar solid-phase combinatorial strategy for synthesis of pyridinium, tetrahydropyridine, and piperidine frameworks as potential inhibitors of vesicular acetylcholine transporter. One member of the small library produced was prepared from amino-functionalized trityl resin reacting with a 4-phenyl Zincke salt to give resin-bound product 62 (Scheme 8.4.21). After ion exchange and cleavage from the resin, pyridinium 63 was isolated. Alternatively, borohydride reduction of 62 led to the 1,2,3,6-tetrahydropyridine 64, which could be hydrogenated to the corresponding piperidine 65. 

The improvements in resolution achieved in each deconvolution step are shown in Figure 3-3. While the initial library could only afford a modest separation of DNB-glutamic acid, the library with proline in position 4 also separated DNP derivatives of alanine and aspartic acid, and further improvement in both resolution and the number of separable racemates was observed for peptides with hydrophobic amino acid residues in position 3. However, the most dramatic improvement and best selectivity were found for c(Arg-Lys-Tyr-Pro-Tyr-(3-Ala) (Scheme 3-2a) with the tyrosine residue at position 5 with a resolution factor as high as 28 observed for the separation of DNP-glutamic acid enantiomers. 

The potential of such reaction sequences for the generation of molecular diversity was also demonstrated by the synthesis of a library of heterocycles. Epoxide ring-opening with hydrazine and subsequent condensation with (3-diketones or other bifunctional electrophiles gave rise to a variety of functionalized heterocyclic structures in high purity [34]. A selection based on the substrate derived from cyclohexene oxide is shown in Scheme 12.12. 

Scheme 12.22 Synthesis of a library of diamines from sulfonyl...

The feasibility of multistep natural product total synthesis via solid-phase methodology, and its application to combinatorial chemistry, was first demonstrated by Nicolaou and coworkers in epothilone synthesis and in the generation of an epothilone library [152]. The traceless release of TBS-protected epoC 361 by RCM of resin-bound precursor 360 (Scheme 69) is an early and most prominent example for the strategy outlined in Fig. 11a. 

To generate molecular libraries, a series of 5-oxo-2-azabicyclo[2.2.2]octane and triaza analogs were prepared via a stereospecific Diels-Alder reaction by reacting Wang-resin-bound diene 35 with a variety of dienophiles [28]. After removing the solid support with a strong acid, adducts 36 were isolated examples of reactions that have furnished the best yields are reported in Scheme 4.6. 

Linking the ketone and carboxylic acid components together in an Ugi reaction facilitates the synthesis of pyrrolidinones amenable to library design. The three-component condensation of levulinic acid 30, an amine and isocyanide proceeds under microwave irradiation to give lactams 31 [65]. The optimum conditions were established by a design of experiments approach, varying the equivalents of amine, concentration, imine pre-formation time, microwave reaction time and reaction temperature, yielding lactams 31 at 100 °C in poor to excellent yield, after only 30 min compared to 48 h under ambient conditions (Scheme 11). 

This transformation can also be carried out under solvent-free conditions in a domestic oven using acidic alumina and ammoniiun acetate, with or without a primary amine, to give 2,4,5-trisubstituted or 1,2,4,5-tetrasubstituted imidazoles, respectively (Scheme 15A) [69]. The automated microwave-assisted synthesis of a library of 2,4,5-triarylimidazoles from the corresponding keto-oxime has been carried out by irradiation at 200 ° C in acetic acid in the presence of ammonium acetate (Scheme 15B) [70]. Under these conditions, thermally induced in situ N - O reduction occurs upon microwave irradiation, to give a diverse set of trisubstituted imidazoles in moderate yield. Parallel synthesis of a 24-membered library of substituted 4(5)-sulfanyl-lff-imidazoles 40 has been achieved by adding an alkyl bromide and base to the reaction of a 2-oxo-thioacetamide, aldehyde and ammonium acetate (Scheme 15C) [71]. Under microwave-assisted conditions, library generation time was dramatically re-... 

Fewer procedures have been explored recently for the synthesis of simple six-membered heterocycles by microwave-assisted MCRs. Libraries of 3,5,6-trisubstituted 2-pyridones have been prepared by the rapid solution phase three-component condensation of CH-acidic carbonyl compounds 44, NJ -dimethylformamide dimethyl acetal 45 and methylene active nitriles 47 imder microwave irradiation [77]. In this one-pot, two-step process for the synthesis of simple pyridones, initial condensation between 44 and 45 under solvent-free conditions was facilitated in 5 -10 min at either ambient temperature or 100 ° C by microwave irradiation, depending upon the CH-acidic carbonyl compound 44 used, to give enamine intermediate 46 (Scheme 19). Addition of the nitrile 47 and catalytic piperidine, and irradiation at 100 °C for 5 min, gave a library of 2-pyridones 48 in reasonable overall yield and high individual purities. 

Scheme 32 Microwave-assisted synthesis of a [3-1-2] cycloaddition library...

Despite that the thiophene ring is considered as a bioisoster of the benzene ring, the synthesis and chemistry of thiophene analogs of heterocycles with therapeutic interest remain poorly studied. One of the most recent examples concerns the synthesis of new substituted thioisatoic anhydrides (6 and 7-arylthieno[3,2-d] [1,3]oxazine-2,4-diones), which were prepared on a large scale under microwave irradiation conditions. A small library of thiophene ureidoacids was easily performed using a Normatron microwave reactor (500 W) with high yields and good purity [4,5] (Scheme 4). 

Heterocyclic compounds are of great interest to the pharmaceutical industry, as they make up most of the known pharmacophores. As a result, a number of libraries of various heterocycles have been prepared using polymer-supported reagents. While an exhaustive list of all the heterocychc cores that have been prepared using PSRs is beyond the scope of this chapter, some selected examples are depicted in Scheme 3. 

Scheme 23 Preparation of a library of 3-oxopiperazinium salts using microwave-assisted scavenging of excess amines...

Inhibitors for proteases plasmepsin I and II of the malaria parasite Plasmodium falciparum, with a good plasmepsin/human protease cathepsin D selectivity, have been identified via library construction involving rapid microwave-accelerated Suzuki reactions [57]. The phenyl ring of the biphenyl unit in the lead compound M-((lS)-l- [((lS,2S)-3- [(lS)-2-amino-l-(4-phenyl-benzyl)-2-oxoethyl]amino -2-hydroxy-l-phenoxypropyl)amino]carbonyl -2-methylpropyl)pyridine-2-carboxamide has been altered by performing Suzuki reactions on N-((lS)-l- [((lS,2S)-3- [(lS)-2-amino-l-(4-bromobenzyl)-2-oxoethyl]amino -2-hydroxy-l-phenoxypropyl)amino]carbonyl -2-methyl-propyl)pyridine-2-carboxamide (Scheme 37). In particular, a 2-benzofuryl moiety proved to be interesting since a Ki value of 13 nM for plasmepsin I and... 

Organ et al. from York University demonstrated that a diarylated IH-pyrazole-based library, based on the structure of the potent COX II inhibitor Celecoxib [4-(3-trifluoromethyl-5-(4-methylphenyl)-lH-pyrazol-l-yl)benzenesulfonamide], could be rapidly prepared using MAOS [59]. Microwave-accelerated Suzuki reaction on 4-(5-iodo-3-methyl-lH-pyrazol-l-yl)-benzenesulfonamide using heterogeneous Pd/C was the principal diversification step investigated (Scheme 41). The interest of the team in microwave... 

The cyclization of 1,2-dicarbonyl compounds with aldehydes in the presence of NH4OAC to give imidazoles was employed in a combinatorial study that compared conventional and microwave heating in the preparation of a library of sulfanyl-imidazoles (Scheme 15). The study employed an array of expandable reaction vessels that could accommodate a pressure build-up system for heating without loss of volatile solvents or reagents. A 24-membered library of imidazoles (48 and 49) was prepared in 16 min instead of the 12 h required using conventional heating [45]. 

A small library of highly functionalized pyrrolines 95 was synthesized by reaction of allylic and propargylic isocyanides 94 with thiols followed by radical cyclization (Scheme 33). The radical reaction was carried out using a radical initiator (AIBN) under flash heating microwave irradiation [67]. 

A library of 800 substituted prolines of type 112 was described using a similar synthetic approach. The [3 + 2] cycloaddition occurred via a multicomponent reaction of a-amino esters, aldehydes, and maleimides (Scheme 38). 

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