C([NH:11][C@H:12]([C:23]([OH:25])=[O:24])[CH2:13][C:14]1[CH:19]=[CH:18][C:17]([N+:20]([O-:22])=[O:21])=[CH:16][CH:15]=1)(OCC1C=CC=CC=1)=O.CC(=C)C.S(=O)(=O)(O)O.C(=O)([O-])[O-].[Na+].[Na+]>O1CCOCC1.C(=O)=O.C(OCC)(=O)C>[N+:20]([C:17]1[CH:16]=[CH:15][C:14]([CH2:13][C@@H:12]([C:23]([OH:25])=[O:24])[NH2:11])=[CH:19][CH:18]=1)([O-:22])=[O:21]The Cbz-4-nitrophenylalanine (75 g, 0.22 m) is dissolved in dioxane (300 ml). The resulted stirred solution is cooled in Dry Ice bath to −20° C. (internal). The liquefied isobutylene (approx. 290 ml) is added followed by conc. sulfuric acid (35 ml) added in three equal portions, 30 min apart. The addition of acid is a very exothermic process, accompanied by substantial degree of polymerization. Efficient mechanical stirring is essential at this stage. Resulted mixture is stirred for 20 hr, allowing to warm up to ambient temperature then is cautiously poured into sat. aq. sodium carbonate solution (2 L) and diluted with ethyl acetate (600 ml). Organic layer is separated and aqueous layer is extracted with ethyl acetate (2×200 ml). Combined extracts are washed with water and dried with sodium sulfate. The solution is filtered and evaporated to dryness. The residue is taken up in ethyl acetate/hexane mixture (500 ml; 1:1) and filtered through plug of silica gel (ca. 2×2 in). The silica is rinsed with an additional amount of the same solvent (2 L total) and the filtrates are evaporated to give fully protected 4-nitrophenylalanine as a viscous oil, 73 g (83% after two steps). 1H-NMR, CDCl3, (δ): 8.12 (d, 2H, J=8.4 Hz), 7.36 (m, 7H), 5.35 (m, 1H), 5.10 (m, 2H), 4.57 (m, 1H), 3.31 (m, 2H), 1.43 (s, 9H). 13C-NMR, CDCl3, (δ): 169.7, 155.3, 146.9, 143.9, 136.0, 130.2, 128.4, 128.2, 128.0, 123.3, 82.9, 66.9, 54.7, 38.2, 31.4, 27.8, 13.9. MS (m/z): 423.1 [M+23].
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