1. Rhodium(I)-Catalyzed
B Witulski, T Stengel Angew Chem Int Ed Engl. 1999 Aug;38(16):2426-2430.
The regiospecific introduction of substituents into the 4-position of 2,3-dihydroindoles (indolines), which is significant for the synthesis of various natural products and pharmaceuticals, was achieved by rhodium(I)-catalyzed cyclotrimerizations of 1 with acetylene to give 2. Up to four substituents can be introduced simultaneously into the indoline core by using this novel strategy. R=OH, OBzl, Otetrahydropyranyl, NHSO(2)C(6)H(4)CH(3); Ts=H(3)CC(6)H(4)SO(2).
2. Synthesis and crystal structures of Boc-L-Asn-L-Pro-OBzl.CH3OH and dehydration side product, Boc-beta-cyano-L-alanine-L-Pro-OBzl
A N Stroup, L B Cole, M M Dhingra, L M Gierasch Int J Pept Protein Res. 1990 Dec;36(6):531-7. doi: 10.1111/j.1399-3011.1990.tb00992.x.
Boc-L-Asn-L-Pro-OBzl: C21H29O6N3.CH3OH, Mr = 419.48 + CH3 OH, monoclinic, P2(1), a = 10.049(1), b = 10.399(2), c = 11.702(1) A, beta = 92.50(1)degrees, V = 1221.7(3) A3, dx = 1.14 g.cm-3, Z = 2, CuK alpha (lambda = 1.54178 A), F(000) = 484 (with solvent), 23 degrees, unique reflections (I greater than 3 sigma(I)) = 1745, R = 0.043, Rw = 0.062, S = 1.66. Boc-beta-cyano-L-alanine-L-Pro-OBzl: C21H27O5N3, Mr = 401.46, orthorhombic, P2(1)2(1)2(1), a = 15.741(3), b = 21.060(3), c = 6.496(3) A, V = 2153(1) A3, dx = 1.24 g.cm-3, Z = 4, CuK alpha (lambda = 1.54178 A), F(000) = 856, 23 degrees, unique reflections (I greater than 3 sigma(I)) = 1573, R = 0.055, Rw = 0.078, S = 1.86. The tert.-butyloxycarbonyl (Boc) protected dipeptide benzyl ester (OBzl), Boc-L-Asn-L-Pro-OBzl, prepared from a mixed anhydride reaction using isobutylchloroformate, Boc-L-asparagine, and HCl.L-proline-OBzl, crystallized with one methanol per asymmetric unit in an extended conformation with the Asn-Pro peptide bond trans. Intermolecular hydrogen bonding occurs between the methanol and the Asn side chain and between the peptide backbone and the Asn side chain. A minor impurity due to the dehydration of the Asn side chain to a beta-CNala crystallized with a similar extended conformation and a single intermolecular hydrogen bond.
3. Amino acids and peptides. XVIII. Dipeptide formation during the synthesis of Z-Asp(OBzl)-OH
S Iguchi, K Kawasaki, Y Okada Int J Pept Protein Res. 1987 Nov;30(5):695-700. doi: 10.1111/j.1399-3011.1987.tb03382.x.
During the benzyloxycarbonylation of H-Asp(OBzl)-OH by the Schotten-Bauman reaction with benzyloxycarbonyl chloride in the presence of NaHCO3 or Na2CO3, besides Z-Asp(OBzl)-OH, Z-Asp(OBzl)-Asp(OBzl)-OH was formed as side product, although the extent of the dipeptide formation differed depending on the base used (10% and 20% respectively). It was found that melting point, rotation value and Rf values upon thin-layer chromatography of Z-Asp(OBzl)-Asp(OBzl)-OH were quite similar to those of Z-Asp(OBzl)-OH.
