Traceless release

Chemistry on solid support has gained tremendous importance during the last few years, mainly driven by the needs of the pharmaceutical sciences. Due to the robust and tolerable nature of the available catalysts, metathesis was soon recognized as a useful technique in this context. Three conceptually different, RCM-based strategies are outlined in Fig. 11. In the approach delineated in Fig. 1 la, a polymer-bound diene 353 is subjected to RCM. The desired product 354 is formed with concomitant traceless release from the resin. This strategy is very favorable, since only compounds with the correct functionality will be liberated, while unwanted by-products remain attached to the polymer. However, as the catalyst is captured in this process by the matrix (355), a higher catalyst loading will be required, or ancillary alkenes have to be added to liberate the catalyst. 

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. 

Scheme 69 Traceless release of epoC derivative 361 from solid support by RCM [152]...

The soluble polymer support was dissolved in dichloromethane and treated with 3 equivalents of chloroacetyl chloride for 10 min under microwave irradiation. The subsequent nucleophilic substitution utilizing 4 equivalents of various primary amines was carried out in N,N-dimethylformamide as solvent. The resulting PEG-bound amines were reacted with 3 equivalents of aryl or alkyl isothiocyanates in dichloromethane to furnish the polymer-bound urea derivatives after 5 min of micro-wave irradiation (Scheme 7.75). After each step, the intermediates were purified by simple precipitation with diethyl ether and filtration, so as to remove by-products and unreacted substrates. Finally, traceless release of the desired compounds by cyclative cleavage was achieved under mild basic conditions within 5 min of micro-wave irradiation. The 1,3-disubstituted hydantoins were obtained in varying yields but high purity. 

The traceless release of TBS-protected epoC 432 by RCM of resin-bound precursor... 

For the attachment and the traceless release of electron-deficient aromatics, germanium-based hnkers of type 89 (Fig. 10) have been developed [103, 104]. This linker has the advantage that it is fairly stable towards strongly basic conditions and strong nucleophiles. 

The hnker is stable to a great variety of reaction conditions. In the example illustrated in Scheme 46, the alkynes are loaded indirectly onto the phosphine-modified polymer as their hexacarbonyldicobalt(O) complexes, but they can also be attached to a Co-coated polymer. Instead of traceless release of the alkynes after modification, they can also be apphed to Pauson-Khand reactions to form cyclo-pentenones [119]. 

Other possibility for a traceless release of combinatorial compounds from solid supports. As only the desired products are cleaved during fhe reaction, these are obtained in high purity. The method is compatible wifh a whole variety of different functionalities, such as carboxylic acids, carboxylic acid anhydrides, amides, aldehydes, ketones, alcohols, and sulfonamides. 

An impressive example using this strategy is the recently published synthesis of analogues of dysidiolide [137], After a rriullislep synthesis pathway involving a diverse set of demanding chemical transformations, the desired compounds were obtained after a traceless release by metathesis using 101, as illustrated for one specific example in Scheme 54. 

The traceless anchors based on silicon just described are limited by the need to synthesize a preformed handle consisting of the first building block and the silicon linker. Moreover, traceless release from conventional aryldialkylsilanes is complicated by their low hydrolytic stability and loss of silicon from the resin in the cleavage process. The silyl chloride resin 8h can be used to anchor aryl rings by reaction with aryllithium reagents. The anchor is stable to concentrated TFA and bases, but treatment with TBAF cleaves the final products [353]. 

More recently published procedures describe the traceless release of aliphatic structures from simple benzylthiol resins 422 via oxidation with oxone and following treatment with Sml2 (Scheme 67) [269]. 

Sdieme 101 Germanium-based linkers for traceless release and formation of haloarenes... 

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