Protecting groups migration

CF3SO3H, PhSMe, 0-25°. ° In this case O-methyltyrosine was depro-tected without evidence for O C migration, which is often a problem when removing protective groups from tyrosine. 

Na/NH3, -30°, 3 min, 1(X)% yield. This protective group is stable to acidic hydrolysis (4.5 N HBr/HOAc 1 N HCV, CF3CO2H, reflux). There is no evidence of S N acyl migration in 5-(A-ethylcarbamates) (RS = cysteinyl). Oxidation of 5-(A-ethylcarbamoyl)cysteine with performic acid yields cysteic acid. ... 

Silyl groups have found broad appeal as protective groups because their reactivity and stability can be tailored by varying the nature of the substituents on the silicon. Their ability to migrate from one hydroxyl to another is a property that can be used to advantage, but more often, it is a nuisance. The migratory apti-... 

FIGURE 8.7 Synthesis of a protected tripeptide containing a 2-hydroxy-4-methoxybenzyl-protected peptide bond.38 (A) Acylation of the carboxy-terminal residue, (B) removal of both protecting groups, (C) O-acylation of the benzyl-protector by the symmetrical anhydride of the amino-terminal residue, and (D) migration of the protected amino-terminal residue from the oxygen atom to the amino group of the dipeptide ester. 

On the other hand, since no migration resnlted with whether acetic anhydride or benzoyl chloride at room temperature, it is apparent that the results controlled by acyl reagents, where the 3,6-position protected product 37 was obtained with acetic anhydride whereas the 2,6-position protected product 38 was obtained with benzoyl chloride at room temperature, were not brought about by the organotin acyl group migration mentioned above (Scheme 9). 

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