Selective benzylic lithiation of N-Boc-2-phenylpiperidine and pyrrolidine: expedient synthesis of a 2,2-disubstituted piperidine NK1 antagonist

Recent Developments in Enantio- and Diastereoselective Hydrogenation of N-Heteroaromatic Compounds

The enantioselective and diastereoselective hydrogenation of N-heteroaromatic compounds is an efficient strategy to access chirally enriched cyclic heterocycles, which often possess highly bio-active properties. This strategy, however, has only been...

Modular synthesis of chiral 1,2-dihydropyridines via Mannich/Wittig/cycloisomerization sequence that internally reuses waste

1,2-Dihydropyridines are valuable and reactive synthons, and particularly useful precursors to synthesize piperidines and pyridines that are among the most common structural components of pharmaceuticals. However, the catalytic enantioselective synthesis of structurally diverse 1,2-dihydropyridines is limited to enantioselective addition of nucleophiles to activated pyridines. Here, we report a modular organocatalytic Mannich/Wittig/cycloisomerization sequence as a flexible strategy to access chiral 1,2-dihydropyridines from N-Boc aldimines, aldehydes, and phosphoranes, using a chiral amine catalyst. The key step in this protocol, cycloisomerization of chiral N-Boc δ-amino α,β-unsaturated ketones recycles the waste to improve the yield. Specifically, recycling by-product water from imine formation to gradually release the true catalyst HCl via hydrolysis of SiCl₄, whilst maintaining a low concentration of HCl to suppress side reactions, and reusing waste Ph₃PO from the Wittig step to modulate the acidity of HCl. This approach allows facile access to enantioenriched 2-substituted, 2,3- or 2,6-cis-disubstituted, and 2,3,6-cis-trisubstituted piperidines.

Organocatalytic enantioselective synthesis of 2,5,5-trisubstituted piperidines bearing a quaternary stereocenter. Vinyl sulfonamide as a new amine protecting group

An organocatalytic desymmetrizing IMAMR with vinyl sulfonamides as nitrogen nucleophiles has been devised with good levels of diastereo- and enantioselectivity.

Stereocontrolled synthesis of vicinally functionalized piperidines by nucleophilic β-addition of alkyllithiums to α-aryl substituted piperidine enecarbamates

Substituted piperidines are emerging as important medicinally-active structural motifs. Here, we report highly stereoselective carbolithiation reactions of α-aryl piperidine enecarbamates that offer direct access to vicinally-substituted piperidine compounds. We have also demonstrated that the carbanion intermediates can be trapped with a carbon electrophile.

Regiocontrolled synthesis of (hetero)aryl and alkenyl dehydropyrrolidines, dehydropiperidines and azepenes by Ru-catalyzed, heteroatom-directed α-C–H activation/cross-coupling of cyclic enamides with boronic acids

The synthesis of α-aryl and alkenyl pyrrolidine-, piperidine-, and azepane derivatives by ruthenium-catalyzed, carbonyl-assisted, site-selective sp² C–H activation of cyclic enamides and concomitant cross-coupling with boronic acids is described.

α- and α'-Lithiation-Electrophile Trapping of N-Thiopivaloyl and N-tert-Butoxythiocarbonyl α-Substituted Azetidines: Rationalization of the Regiodivergence Using NMR and Computation

(1)H NMR and computational analyses provide insight into the regiodivergent (α- and α'-) lithiation-electrophile trapping of N-thiopivaloyl- and N-(tert-butoxythiocarbonyl)-α-alkylazetidines. The magnitudes of the rotation barriers in these azetidines indicate that rotamer interconversions do not occur at the temperature and on the time scale of the lithiations. The NMR and computational studies support the origin of regioselectivity as being thiocarbonyl-directed lithiation from the lowest energy amide-like rotameric forms (cis for N-thiopivaloyl and trans for N-tert-butoxythiocarbonyl).

Trapping of carbolithiation-derived tertiary benzylic α-lithio piperidines with carbon electrophiles: Controlling the formation of α-amino quaternary and vicinal stereocenters

The interception of carbolithiation-derived tertiary benzylic α-lithio piperidines with carbon electrophiles has led to the diastereoselective synthesis of vicinally functionalized piperidines bearing α-amino quaternary stereocenters.

Synthesis and kinetic resolution of N-Boc-2-arylpiperidines

An efficient kinetic resolution of 2-arylpiperidines was developed using a chiral base.

Synthetic applications and inversion dynamics of configurationally stable 2-lithio-2-arylpyrrolidines and -piperidines

In diethyl ether, N-Boc-2-lithio-2-arylpiperidines have been found to be configurationally stable at -80 °C, whereas N-Boc-2-lithio-2-arylpyrrolidines are configurationally stable at -60 °C. Several tertiary benzylic carbanions derived from enantioenriched 2-aryl heterocycles have been successfully alkylated or acylated with little to no loss of enantiopurity. The scope of the reactions has been explored. The enantiomerization dynamics of N-Boc-2-lithio-2-phenylpyrrolidine and N-Boc-2-lithio-2-phenylpiperidine have been studied in the presence of different solvents and achiral ligands.

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

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.

Unusual carbamate-directed CH-activation at an annulated ferrocenophane framework

The tetrahydroazepine-annulated [3]ferrocenophane carbamate (4) was synthesized by two different linear routes starting from the readily available α-dimethylamino[3]ferrocenophane-ortho-carbaldehyde rac-6. The carbamate directed lithiation of 4 resulted in a selective attack at a (Cp)C-H bond at the higher substituted "lower" [3]ferrocenophane Cp-ring to eventually yield the respective ester (18) after treatment with ClCO(2)Me.

The barrier to enantiomerization and dynamic resolution of N-Boc-2-lithiopiperidine and the effect of TMEDA

The kinetics of enantiomerization and dynamic thermodynamic resolution (DTR) of N-Boc-2-lithiopiperidine have been measured, revealing significant differences in enthalpy and entropy for these processes and a role for the achiral ligand TMEDA; the racemization and the DTR are catalytic and first order in [TMEDA], and this has implications for asymmetric synthesis with chiral organolithiums.

Direct sp3 C-H bond activation adjacent to nitrogen in heterocycles

Activation of sp(3) C-H bonds adjacent to nitrogen in heterocycles is an attractive transformation that is emerging as a practical method in organic synthesis. This tutorial review aims to summarize the key examples of direct functionalization of nitrogen-containing heterocycles via metal-mediated and metal-catalyzed processes, which is meant to serve as a foundation for future investigations into this rapidly developing area of research. The review covers functionalization of N-heterocycles via alpha-lithiation with alkyllithium/diamine complexes, alpha-amino radical formation, metal-catalyzed direct C-H activation, C-H oxidations and oxidative couplings, and metal-catalyzed carbene insertions.