Alkylation of vanillin 112 with methyl 4-bromobutyrate, followed by nitration afforded 113. Addition of the methyl group to the aldehyde was performed with the commercially available AIMc, to yield 114; this could be crystallized directly from the crude, three-step reaction mixture. After hydrolysis, the corresponding acid may be attached to a hydroxymodi-fied support as an ester. After acylation, this results in the desired construct 111. Photocleavable Linker Based on Pivaloylglycol

Recently, a new type of photolabile linker 115 for the attachment of carboxylic acids was reported [107]. The linker is based on pivaloylglycol (the bold structure in Scheme 50). Cleavage of the silyl groups was followed by attachment of the iodobenzoic acid to give 116 ready for CC-couplings.The photolytic cleavage proceeded via a two-step process. The reaction was initiated by the formation of a radical center 117, which was followed by a spontaneous p-C,0-bond scission to yield the final product 118. The photolytic cleavage proceeded rapidly and produced high yields of released acids. The system proved to be compatible with many reagents and reactions, and its utility was demonstrated in peptide synthesis, in Stille and Suzuki couplings (Scheme 50), and epoxidations.

on xJ 1

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