Carboxylic acid derivatives chlorotrimethylsilane

Due to the extremely low nucleophilicity of the imino group, 187,188 acylation of thiazolidine-4-carboxylic acid (11) is not a trivial procedure. In fact, N-protected Thz derivatives can be prepared by standard procedures, but strong acylating conditions are required. The related N-Boc derivative is obtained only by prolonged reaction times with Boc-N3 U2,189 or with Boc20. 113 For preparation of the N-Z derivative, silylation of Thz with, for example, chlorotrimethylsilane is recommended prior to the reaction benzyl chloroformate. 200 Due to the stability of thiazolidine-4-carboxylic acid to acids its methyl ester is obtained by HC1 catalyzed reaction with methanol, 190 whereas the amide is formed by reacting Thz N-carboxyanhydride with ammonia.1 89 Derivatives of thiazolidine-4-carboxylic acid are listed in Table 8. 

Procedures A and B illustrate the two current methods for preparation of N-9-phenylfluoren-9-yl derivatives of amino acids and amino acid esters. Free carboxylate (as in alanine in Step A) or free hydroxyl (e.g., serine7) functions can be blocked for the duration of the reaction as trimethylsilyl (TMS) esters or ethers, respectively, by treatment with chlorotrimethylsilane and triethylamine. The TMS group(s) are then removed by methanolysls from carboxylic acids (as in Step A) and mild acidic hydrolysis from hydroxyl groups, both being accomplished during product isolation. In addition to 2, the N-9-phenylfluoren-9-yl derivatives of serine,7 glutamic add y-methyl ester,8 and aspartic acid 3-methyl ester3 9 have been prepared in this manner. 

This difference in chemical reactivity can be converted into a differential etch rate by treating the exposed film with an appropriate silylating reagent. For example, chlorotrimethylsilane, hexamethyldisilazane and bis(trimethylsilyl)acetamide are well know to react with phenolic hydroxyls, carboxylic acids and other nucleophilic species to form the corresponding trimethylsilyl derivatives. 

When l,l,2-triphenylethane-l,2-diol-derived esters are submitted to a monodeprotonation and subsequently treated with Chlorotrimethylsilane, the formation of 2-trimethylsilyloxy-l,3-dioxolanes results. The orthoester moiety thus obtained serves as a protecting group for carboxylic acids (eq 3) it is stable towards alkyllithium reagents and can be cleaved under nonacidic conditions by alkaline hydrolysis. ... 

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