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plied as outlined in Scheme 40 to result in compounds of type 83. The driving force for the reaction is the energetically favorable formation of the five-membered ring [78].

Scheme 40

Prodecure for metathesis [78]

Resin (400 mg; loading 0.52 mmol/g) was suspended in 3 mL of dry DCM (Argon, glove-box), and 5 mg (6.08 (mol, 3 mol %) of catalyst 72 was added.The mixture was stirred for 12 h at rt and then passed through a glass filter. The resin was washed with DCM. The crude product was obtained by evaporation of the filtrate and purified by silica gel chro-

matography, whereupon 23 mg (44 %) of 83 were obtained. When the resin was exposed a second time to the same conditions, a second batch of 83 (6 mg, 11 %) was obtained.

The metathesis concept on solid support was extended to the so-called cross-metathesis, whereby one of the reacting olefins was attached to the solid support and a terminal olefin was present in solution [85], During metathesis, this terminal olefin becomes immobilized on the resin. The reaction conditions were optimized in such a way that the possible formation of macrocycles could be prevented. The allyl-dimethylsilyl polystyrene 85 used in the reaction was synthesized according to Scheme 41. The release after metathesis is possible by scission of the Si-C bond mediated by appropriate nucleophiles (Sakurai conditions) to yield terminal olefins 86.

Me Me

Ru catalyst 72

The reaction was carried out with an entire range of olefins carrying various functional groups. Besides obtaining terminal olefins, this linker can also be used to obtain hydroxyl or carboxyl functions in the final product. As the electrophilic attack may also occur intramole-cularly, the synthesis of cyclic compounds was also possible using this route. Recently, the methodology was extended to cross-coupling metathesis between terminal alkynes and linker 85 (Scheme 42). Dienes 87 are obtained by such a process after electrophilic cleavage [86].

3.1.3.4 Trace less Synthesis by Using Polymer-Bound Triphenylphosphine

The Wittig reaction on solid support offers the advantage that the phosphine oxide formed as byproduct remains bound to the solid support and can thus be separated from the olefinic product by filtration.

A phosphonium salt was prepared from commercially available polymer-bound triphenylphosphine. This phosphonium salt is compatible with a whole range of functionalities and reaction conditions. Depending on the conditions, different types of final product can be synthesized from the same precursor molecules (Scheme 43).

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