1364783-09-4Relevant articles and documents
An experimental and in situ IR spectroscopic study of the lithiation-substitution of N-Boc-2-phenylpyrrolidine and -piperidine: Controlling the formation of quaternary stereocenters
Sheikh, Nadeem S.,Leonori, Daniele,Barker, Graeme,Firth, James D.,Campos, Kevin R.,Meijer, Anthony J. H. M.,O'Brien, Peter,Coldham, Iain
, p. 5300 - 5308 (2012/05/05)
A general and enantioselective synthesis of 2-substituted 2-phenylpyrrolidines and -piperidines, an important class of pharmaceutically relevant compounds that contain a quaternary stereocenter, has been developed. The approach involves lithiation-substitution of enantioenriched N-Boc-2-phenylpyrrolidine or -piperidine (prepared by asymmetric Negishi arylation or catalytic asymmetric reduction, respectively). The combined use of synthetic experiments and in situ IR spectroscopic monitoring allowed optimum lithiation conditions to be identified: n-BuLi in THF at -50 °C for 5-30 min. Monitoring of the lithiation using in situ IR spectroscopy indicated that the rotation of the tert-butoxycarbonyl (Boc) group is slower in a 2-lithiated pyrrolidine than a 2-lithiated piperidine; low yields for the lithiation-substitution of N-Boc-2-phenylpyrrolidine at -78 °C can be ascribed to this slow rotation. For N-Boc-2-phenylpyrrolidine and -piperidine, the barriers to rotation of the Boc group were determined using density functional theory calculations and variable-temperature 1H NMR spectroscopy. For the pyrrolidine, the half-life (t1/2) for rotation of the Boc group was found to be ～10 h at -78 °C and ～3.5 min at -50 °C. In contrast, for the piperidine, t1/2 was determined to be ～4 s at -78 °C.
