Enantioselective, palladium-catalyzed α-arylation of N-Boc pyrrolidine: in situ react IR spectroscopic monitoring, scope, and synthetic applications.
ABSTRACT A comprehensive study of the enantioselective Pd-catalyzed α-arylation of N-Boc pyrrolidine has been carried out. The protocol involves deprotonation of N-Boc pyrrolidine using s-BuLi/(-)-sparteine in TBME or Et(2)O at -78 °C, transmetalation with ZnCl(2) and Negishi coupling using Pd(OAc)(2), t-Bu(3)P-HBF(4) and the aryl bromide. This paper reports several new features including in situ React IR spectroscopic monitoring of the process; use of (-)-sparteine and the (+)-sparteine surrogate to access products with opposite configuration; development of a catalytic asymmetric lithiation-Negishi coupling reaction; extension to a wide range of heteroaromatic bromides; total synthesis of (R)-crispine A, (S)-nicotine and (S)-SIB-1508Y via short synthetic routes; and examples of α-vinylation of N-Boc pyrrolidine using vinyl bromides exemplified by the total synthesis of naturally occurring (+)-maackiamine (thus establishing its configuration as (R)). In this way, the full scope and limitations of the methodology are delineated.
- SourceAvailable from: Haishen Yang[Show abstract] [Hide abstract]
ABSTRACT: A series of trityl-based photolabile hydroxyl protecting groups have been examined. These PPGs evolve from the traditional acid-labile trityl protecting group with proper electron-donating substituents. Structure-reactivity relationships have been explored. A m-dimethylamino group is crucial to achieve high photochemical deprotection efficiency. The o-hydroxyl group in 8 greatly improves the yield of the photochemical deprotection reaction, compared with the corresponding o-methoxyl-substituted counterpart 7. However, comparison between the photoreactions of 9 and 11 does not show similar structural relevance. The PPG in ether 1 (i.e., DMATr group) is structurally simple and easy to prepare and install. Its deprotection can be successfully carried out with irradiation of sunlight without requirement of photochemical devices.The Journal of Organic Chemistry 06/2011; 76(15):5873-81. DOI:10.1021/jo200692c · 4.64 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: This study describes a very efficient strategy for the synthesis of two new bridged-nicotine analogues. Starting from either 4- or 3-chloropyridine the desired tricyclic ring systems are accessed in just three steps in 23% and 40% overall yield, respectively.Tetrahedron 02/2012; 68(5-5):1417-1421. DOI:10.1016/j.tet.2011.12.029 · 2.82 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: 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 (1)H NMR spectroscopy. For the pyrrolidine, the half-life (t(1/2)) 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, t(1/2) was determined to be ∼4 s at -78 °C.Journal of the American Chemical Society 03/2012; 134(11):5300-8. DOI:10.1021/ja211398b · 11.44 Impact Factor