Stereoselective access to fluorinated and non-fluorinated quaternary piperidines: synthesis of pipecolic acid and iminosugar derivatives

Cyclisations and Strategies for Stereoselective Synthesis of Piperidine Iminosugars

This personal account focuses on synthesis of polyhydroxylated piperidines, a subset of compounds within the iminosugar family. Cyclisations to form the piperidine ring include reductive amination, substitution via amines, iminium ions and cyclic nitrones, transamidification (N-acyl transfer), addition to alkenes, ring contraction and expansion, photoinduced electron transfer, multicomponent Ugi reaction and ring closing metathesis. Enantiomerically pure piperidines are obtained from chiral pool precursors (e. g. sugars, amino acids, Garner's aldehyde) or asymmetric reactions (e. g. epoxidation, dihydroxylation, aminohydroxylation, aldol, biotransformation). Our laboratory have contributed cascades based on reductive amination from glycosyl azide precursors as well as Huisgen azide-alkene cycloaddition. The latter's combination with allylic azide rearrangement has given substituted piperidines, including those with quaternary centres adjacent to nitrogen.

Synthesis of enantiopure α-Tfm-proline and α-Tfm-pipecolic acid from oxazolo-pyrrolidines and -piperidines

Enantiopure α-Tfm-proline and α-Tfm-pipecolic acid were synthesized starting from commercially available diesters and ethyl trifluoroacetate. A Strecker type reaction on intermediate chiral Tfm-oxazolo-pyrrolidine and -piperidine provided the corresponding nitrile precursor of enantiopure (R) and (S) α-Tfm-proline and α-Tfm-pipecolic acid. The C-terminal peptide coupling reaction of α-Tfm-pipecolic acid has been successfully achieved.

Synthesis of complex unnatural fluorine-containing amino acids

The area of fluorinated amino acid synthesis has seen rapid growth over the past decade. As reports of singly fluorinated natural amino acid derivatives have grown, researchers have turned their attention to develop methodology to access complex proteinogenic examples. A variety of reaction conditions have been employed in this area, exploiting new advances in the wider synthetic community such as photocatalysis and palladium cross-coupling. In addition, novel fluorinated functional groups have also been incorporated into amino acids, with SFX and perfluoro moieties now appearing with more frequency in the literature. This review focuses on synthetic methodology for accessing complex non-proteinogenic amino acids, along with amino acids containing multiple fluorine atoms such as CF3, SF5 and perfluoroaromatic groups.

Enantioselective synthesis of α-perfluoroalkylated prolines, their 6,7-membered homologues and derivatives

The highly enantioselective Strecker reaction with alpha-perfluoroalkylated cyclic ketimines was reported. The corresponding asymmetric cyclic nitriles are a useful tool for obtaining amino acids and their derivatives.

Synthesis of Substituted α-Trifluoromethyl Piperidinic Derivatives

A comprehensive survey of pathways leading to the generation of α-trifluoromethyl monocyclic piperidinic derivatives is provided (65 references). These compounds have been synthesized either from 6-membered rings e.g., pipecolic acid or lactam derivatives by introduction a trifluoromethyl group, from pyridine or pyridinone derivatives by reduction, and from 5-membered rings e.g., prolinol derivatives by ring expansion, from linear amines by cyclization or from dienes/dienophiles by [4 + 2]-cycloaddition.

Enantioselective synthesis of trifluoromethyl substituted piperidines with multiple stereogenic centers via hydrogenation of pyridinium hydrochlorides

An enantioselective iridium-catalyzed hydrogenation of trifluoromethyl substituted pyridinium hydrochlorides is explored, providing chiral piperidines with three stereogenic centers and up to 90% ee.

Catalytic asymmetric α-Iminol rearrangement: new chiral platforms

A series of 19 different asymmetric catalysts were screened in an effort to identify the first chiral catalyst for the rearrangement of α-hydroxy imines to α-amino ketones involving a 1,2-carbon shift. Although aluminate complexes of VAPOL, VANOL, and 7,7'-(t)Bu2VANOL were quite effective catalysts giving up to 88% ee, the ne plus ultra catalyst for this reaction was found to be a zirconium complex of VANOL which gives 97 to >99% ee for the majority of the substrates examined. An X-ray diffraction study of the catalyst reveals that the zirconium exists as a homoleptic complex with three VANOL ligands and two protonated N-methyl imidazoles.

Lewis acid promoted diastereoselective addition of TMSCN and TMSCF3 to isatin-derived N-sulfinyl ketimines: synthesis of optically active tetrasubstituted 3-aminooxindoles

A practical and efficient method for preparation of highly enantiomerically enriched 3-cyano-3-aminooxindoles and 3-trifluoromethyl-3-aminooxindoles with up to 99% optical purity by a Lewis acid promoted diastereoselective Strecker reaction and trifluoromethylation of isatin-derived N-tert-butanesulfinyl ketimines has been developed. This protocol allows direct use of N-free isatin substrates under mild conditions.