N-Bis-trimethylsilylation of alpha-amino acids using the powerful trimethylsilyl triflate reagent is difficult, and is rendered impossible in the case of bulky side-chains (valine). However, favorable entropy changes resulting from a cyclization reaction allow the formation of "benzostabase" N-diprotections regardless of the side-chain bulk.
Reaction of „Jordan′s Cation” [Cp2ZrCH3(thf)]+ with Amino Acid-Derived Isocyanates—A First Synthetic Route to Oligopeptide-Modified Group 4 Bent Metallocene Cation Systems
作者:Markus Oberhoff、Gerhard Erker、Roland Fröhlich
DOI:10.1002/chem.19970030921
日期:1997.9
AbstractThe organometallic cation [Cp2ZrCH3(thf)]+, employed as the tetraphenylborate salt (1), reacts cleanly in 1:1 stoichiometry with the isocyanates 2 derived from valine methyl ester or valylvaline methyl ester, respectively. In each case addition of the Zr–CH3 group to the isocyanate sp‐carbon center is observed with formation of a functionalized zirconocene cation derivative containing a chelating N‐metallated N‐acetylvaline methyl ester (3a) or N‐acetylvalylvaline methyl ester (3b) moiety, respectively, coordinated in the bent metallocene s̀‐ligand plane. The spectroscopic data of 3, supported by an X‐ray crystal structure analysis of the zirconated dipeptide derivative 3b, have revealed the presence of chelating (η1‐O:η1‐N)‐coordination of the terminal N‐acetyl groups in addition to a Zr–O=C interaction with the adjacent valyl amido group.