Traceless tethers

Equation 12.97 illustrates use of catalyst 104 in the synthesis of a bicyclic ring system.207 Note that the presence of a catalyst also allows the transformation to occur under a CO pressure of 1 bar. Equation 12.98 demonstrates a clever application of the so-called traceless tether method for running a Ru-catalyzed P-K reaction.208 Although this is an intermolecular P-K reaction, the alkene is tethered to a pyridylsilyl group (compound 105), which directs regioselective reaction of the alkene fragment with the alkyne. The presence of residual H20 in the reaction mixture removes the silyl group, which can be recycled. 

Hackenberger and Kleineweischede reported a traceless Staudinger ligation for the head-to-tail macrocyclization of peptides without a deprotection step (Fig. 8) [63]. In this strategy, a phosphine tethered to a thioester at the C-terminus of a peptide reacts intramolecularly with an azide at the N-terminus to form the cyclic peptide. 

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