Carboxyl protection agent giving the 4-methoxybenzyl (PMB; 4-methoxyphenylmethyl; MPM) ester, e.g. by reaction with the 4-nitrophenyl ester. The group is stable to 0.5N HCl in MeOH which readily cleaves Boc groups, but can be removed by TFA, leaving Cbz-groups unaffected: Austr. J. Chem., 21, 2543 (1968). For cleavage using AcOH at 90o, see: Synth. Commun., 24, 1151 (1994). For cleavage with Cerium(III)- chloride heptahydrate, in the presence of NaI, see: J. Org. Chem., 64, 5696 (1999). Has also been used, in the presence of TFA, to protect thiols: Tetrahedron Lett., 35, 1631 (1994). Cleavage is by refluxing with TFA: Bull. Chem. Soc. Jpn., 37, 433 (1964); Bioorg. Chem. Med. Lett., 3, 739 (1993), or Hg(II) salts: Chem. Pharm. Bull., 26, 1576 (1978); J. Med. Chem., 31, 2199 (1988); J. Org. Chem., 52, 4420 (1987).
