Solid-phase synthesis equipment

Syntheses on solid phase can, in principle, be performed with standard equipment for organic synthesis. This is, however, only justifiable if large amounts of support are being handled. For small-scale preparations or for the development of solid-phase synthetic methodology, the simplicity of solid-phase synthesis should be fully exploited by conducting experiments in parallel. This requires more systematic planning of the experiments, but will be amply rewarded by the number of results obtained. 

Combinatorial chemistry has moved from specially centralized laboratories, often equipped with multimillion-dollar robots, onto the bench of individual medicinal chemists. This change in direction requires the availability of personal chemistry tools that are simple to operate, easy to arrange in the laboratory, and reasonably priced. Such instruments are now available for the effective synthesis of combinatorial libraries. The Encore synthesizer represents a simple and efficient personal chemistry tool that allows the execution of directed split-and-pool combinatorial synthesis. The current version of the Encore synthesizer is designed for solid-phase synthesis on SynPhase Lanterns however, it can be modified for synthesis on alternative solid supports such as resin plugs from Polymer Laboratories (e.g., StratoSpheres Plugs). 

The automated solid-phase synthesis of oligosaccharides was first reported in 2001 [38], A peptide synthesizer was modified to enable solid-phase oligosaccharide synthesis. The synthesizer was equipped with a cooling system for low-temperature glycosylations. As in the case with other automated platforms, a combination of new technologies and synthetic methods were developed, and distinctive breakthroughs were achieved. Many state-of-the-art synthetic and analytical methods were adopted and adjusted over the years. The first decade of automated solid-phase oligosaccharide synthesis relied on the development of new methods to access complex structures (Fig. 7.3). 

For the parallel synthesis of ureas based on amino acids, a solid-phase synthesis as well as a solution-phase synthesis were used (Scheme 5) [11]. Solution-phase synthesis gave the desired compounds 21 in yields ranging from 80-100% and purities in the range 71-97%. The work-up involved extraction of the benzotriazole formed in the coupling steps. An aqueous borax buffer (pH 9.2) was used and the separation of the CH2CI2 layer from the aqueous phase was performed in cartridges equipped with a PTFE frit. 

The differentiation between a combination of one of these reaction blocks and a magnetic stirplate and a manual synthesizer is largely arbitrary. Versatile manual synthesizers (Table 3) that can be used for both solution- and solid-phase synthesis range from the aapptec Labmate to the Biichi Syncore, Chemspeed MSW 500, and Heidolph Synthesis 1 (Fig. 5). The Biichi Syncore can be equipped with a concentrator cover, which converts it into a parallel evaporator, or -with a filtration unit that permits top filtration from the reaction mixture and collection of the filtrate (Fig. 5). 

Solution-phase reactions can be followed conveniently by simple means such as TLC, conventional NMR or UV, whereas solid-phase synthesis requires either prior cleavage from the support or special equipment, e.g. ATR (attenuated total reflection)-IR or MAS (magic angle spinning)-NMR. This difference affects primarily the development of new reactions but may also be relevant for the actual library synthesis, e.g., checking completion of a reaction or the quality of intermediates. 

Synthesis on solid supports was first developed by Merrifield [1] for the assembly of peptides. It has expanded to include many different applications including oligonucleotide, carbohydrate, and small-molecule assembly (see Chapters 11 and 14). The repetitive cycle of steps involved in the solid-phase synthesis of biopolymers can be performed manually using simple laboratory equipment or fully automated with sophisticated instrumentation. This chapter examines typical solid-phase reaction kinetics to identify factors that can improve the efficiency of both manual and automated synthesis. The hardware and software features of automated solid-phase instruments are also discussed. The focus of this discussion is not on particular commercial model synthesizers but on the basic principles of instrument operation. These considerations can assist in the design, purchase, or use of automated equipment for solid-phase synthesis. Most contrasting features have advantages and disadvantages and the proper choice of instrumentation depends on the synthetic needs of the user. 

Merrifield s revolutionary concept of solid-phase synthesis was not limited to peptides, and similar techniques have been developed for the synthesis of nucleic acids and carbohydrates on solid supports. For each application, specialized instrumentation that is computer-controlled is commercially available. Access to such equipment has enabled researchers in areas of biology, medicine, material science, and biomedical engineering to prepare thousands of peptides and polypeptides for study. In the pharmaceutical industry, for example, solid-phase synthesis has been used to prepare relatively large numbers of related molecules, so-called compound libraries, that... 

MultiBlock. In the late eighties, Krchnak and Vagner designed the MultiBlock, the first reaction block for parallel solid-phase synthesis (Figm-e 3.19). The MultiBlock consists of five parts (i) a Teflon block that holds 42 reactors, polypropylene syringes equipped with a plastic (ii) a vacuum adapter that connects each reactor to a vacuum line and allows... 

The explosion of combinatorial chemistry in the nineties triggered the constmction of a variety of reaction blocks for multiple parallel synthesis. Some of them are still in production, including the MiniBlock developed by Bohdan, Inc. (now part of Mettler-Toledo, www.mt.com/autochem) and Solid-Phase Synthesis Reaction blocks by J-KEM (www.jkem.com). The Bohdan MiniBlock reactor can hold up to 48 disposable polypropylene-fritted tubes in a 48-well format (6x8 array) and accommodates IRORI s MicroKans. The MiniBlock can be equipped with accessories for synthesis under inert atmosphere or at elevated temperatures. 

A method has been developed that enables solid phase organic synthesis to be performed in microtiter wells not equipped with any kind of porous material at the bottom to facilitate the separation of solid resin beads from a solvent. The concept of washing resin beads in the Don Cucna synthesizer was developed by the need for a reliable and fast operational cycle applicable to a hundred reaction vessels at the same time. The simplest compact reaction block for solid phase synthesis is the 96-well plate. The suction (aspirating) principle of the Don Cucna synthesizer is based on the fact that in most solvents used in solid phase synthesis the resin beads settle to the bottom of the wells of the plates. The settling of the resin is relatively fast (tens of seconds). After the resin beads have settled, stainless-steel needles connected to an evacuated waste container are slowly immersed into the wells of a plate (Fig. 8). The needles remove the liquid from above the surface of the resin without disturbing the resin bed. For washing the resin beads in 96-well microtiter plates, two... 

Memfield successfully automated all the steps m solid phase peptide synthesis and computer controlled equipment is now commercially available to perform this synthesis Using an early version of his peptide synthesizer m collaboration with coworker Bemd Gutte Memfield reported the synthesis of the enzyme ribonuclease m 1969 It took them only SIX weeks to perform the 369 reactions and 11 391 steps necessary to assemble the sequence of 124 ammo acids of ribonuclease... 

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