Pyridine-Derived Oxazolidines as Chiral 3-Alkyl-4,5-dihydropyridinium and 3-Alkyl-3,4,5,6-tetrahydropyridinium Salt Equivalents

Chiral oxazolidines acting as transient hydroxyalkyl-functionalized N-heterocyclic carbenes: an efficient route to air stable copper and gold complexes for asymmetric catalysis

Optically pure oxazolidines were synthesized in nearly quantitative yields from chiral hydroxyalkyl-functionalized imidazolinium salts. Acting as transient chiral diamino N-heterocyclic carbenes (NHCs), these oxazolidines allowed the efficient formation of well-defined copper(i) and gold(i) hydroxyalkyl-NHC complexes, which could be isolated, for the first time, as air stable complexes after silica gel chromatography. Interestingly, X-ray analysis of gold complexes revealed that the hydroxyl-function is not chelated to the metal. Computational studies suggested that both cyclisation to produce oxazolidine and O–H bond elimination to form the transient carbene (prior to coordination) occur through a concerted mechanism. The novel chiral copper-catalysts, as well as oxazolidines alone (copper free), demonstrated excellent performances in asymmetric conjugate addition and allylic alkylation with high regio- and enantio-selectivities (up to 99% ee).

Well-defined chiral Cu(i) and Au(i) hydroxyalkyl NHC complexes were synthesized from oxazolidines. The copper-catalysts and oxazolidines alone (copper free), demonstrated excellent performances in asymmetric conjugate addition and allylic alkylation.

Enantioselective synthesis of 2-substituted and 3-substituted piperidines through a bromoaminocyclization process

A catalytic enantioselective bromocyclization of olefinic amides using amino-thiocarbamates as the catalysts has been developed. The resulting enantioenriched 2-substituted 3-bromopiperidines can readily be transformed to 3-substituted piperidines through a silver salt-mediated rearrangement. This process has been applied to the synthesis of a dopaminergic drug, Preclamol.

Synthesis of 3-Arylpiperidines by a Radical 1,4-Aryl Migration

[reaction: see text] A route to 3-arylpiperidines, 3-arylpyridines, and 5-arylpiperidin-2-ones involving a radical 1,4-aryl migration has been explored. The sequence requires a xanthate addition to an N-allylarylsulfonamide, followed by acetylation and treatment with dilauroyl peroxide to give the 1,4-aryl transfer product, which upon acidic hydrolysis affords the desired piperidine derivative.