[4450-68-4] · C14H13NO5S · p-Nitrobenzyl p-Toluenesulfonate · (MW 307.35)
Physical Data: mp 103-104 °C.
Solubility: sol most organic solvents.
Form Supplied in: not commercially available.
Preparative Methods: by reaction of (a) p-Toluenesulfonyl Chloride with p-nitrobenzyl alcohol3 for 30 min in dry pyridine at -10 °C; or (b) dry Silver(I) p-Toluenesulfonate with p-nitrobenzyl bromide in refluxing dry CHCl3 for 24 h in the dark.2
Handling, Storage, and Precautions: mutagenic in the Ames Salmonella plate incorporation assay (using strain TA100);5 should be treated as a potential carcinogen; can induce skin sensitization in guinea pigs.6 Can be kept for long periods without apparent decomposition.3
p-Nitrobenzyl p-toluenesulfonate (PNB-OTs) is used as a reagent for protecting carboxylic acids. Formation of the ester involves conversion of the acid to a carboxylate salt (e.g. to the sodium salt using NaOMe in methanol,2 or to the Et3NH+ salt using Et3N7), followed by reaction with PNB-OTs in Me2CO2,7 or Me2CO-DMF (2:1).2,8 The PNB esters are stable to HBr/HOAc at room temperature (permitting cleavage of N-Cbz groups) and are readily cleaved by catalytic hydrogenation2 (or NaOH/aq acetone8); the conditions of protection and deprotection minimize racemization of amino acids and peptides. In the example in eq 1, the protection-deprotection sequence was used9 to avoid formation of a lactone along with the diacetate.
PNB-OTs has also been used as a UV labeling agent for detecting monocarboxylic acids in HPLC analyses.10
James F. King
The University of Western Ontario, London, Ontario, Canada