Tyrosine silylation

Albro and Fishbein [93] compared 11 different silylating mixtures for the derivatiza-tion of tyrosine and tryptophan metabolites. BSTFA with the addition of TMCS is the most suitable reagent as far as quantitative reaction is concerned a mixture of BSTFA with TMSDEAand TMCS in pyridine (99 1 30 100, v/v) is recommended as a universal silylating agent. 

The procedure, based on the phosphoramidite method, is illustrated in Scheme 7.11. Nn-Fmoc-L-tyrosine was temporarily protected as its ferf-butyldimethylsilyl ester (see section 6.6). Phosphitylation with dimethyl tyN-diethylphosphorami-dite followed by in situ oxidation with /erf-butyl hydroperoxide gave the phos-phodiester 11 3 The labile silyl ester hydrolysed during the sodium metabisulfite workup used to destroy excess /erf-butyl hydroperoxide to give 11.4 in 57% overall yield, By the same procedure, the dkerf-butyl phosphate 11.5, dibenzyl phosphate 11.6 and diallyl phosphate (not shown) were prepared. 

Similarly, (S)-fi-tyrosine hydrochloride is prepared by addition of the lithium amide of N-[(R)-l-phenylethyl]benzylamine to methyl ( )-3-[(4-benzyloxy)phenyl]-2-propenoate and subsequent hydrogenolysis and saponification24. Recently, a number of publications concerning related work has appeared139-146. The reaction of lithiated, mono-silylated 1-phenylethyl-amines with a-enones has also been studied130131. 

Example 4.6 In the previous example catalytic complex VIII is a derivative of tartaric acid. More easily accessible is boronate X, derived from V-tosyl-L-tyrosine IX. This catalyst proved highly effective in the synthesis of (35)-a-hydroxy ketones 26-30 starting from silyl enolates and aldehydes (Scheme 4.14) [14]. 

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