Recent Advances in Synthetic Routes to Azacycles

One-step syntheses of diaza-dioxa-fenestranes via the sequential (3 + 2) cycloadditions of linear precursors and their structural analyses

Fenestranes, in which four rings share one carbon atom, have garnered much attention because of their flattened quaternary carbon centers. In addition, the rigid and nonplanar structures of heteroatom-containing fenestranes are attractive scaffolds for pharmaceutical applications. We report one-step syntheses of diaza-dioxa-fenestranes via the sequential (3 + 2) cycloadditions. Our synthesis employs readily synthesizable, nonbranched acyclic allenyl precursors that facilitate sequential cycloaddition reactions. We report the synthesis of 22 heteroatom-containing and differently substituted fenestranes with rings of varying sizes. The prepared diaza-dioxa-fenestranes are subjected to X-ray crystallography and DFT calculations, which suggest that replacing the carbon atoms at the non-bridgehead positions in the fenestrane skeleton with nitrogen and oxygen atoms results in a slight flattening of the quaternary carbon center. Moreover, one of our synthesized c,c-[5.5.5.5]fenestranes containing two isoxazoline rings possesses the flattest quaternary carbon center among previously synthesized heteroatom-containing fenestrane versions.

Enantioselective propargylic amination and related tandem sequences to α-tertiary ethynylamines and azacycles

Chiral α-tertiary amines and related azacycles are sought-after compounds for drug development. Despite progress in the catalytic asymmetric construction of aza-quaternary stereocentres, enantioselective synthesis of multifunctional α-tertiary amines remains underdeveloped. Enantioenriched α-disubstituted α-ethynylamines are attractive synthons for constructing chiral α-tertiary amines and azacycles, but methods for their catalytic enantioselective synthesis need to be expanded. Here we describe an enantioselective asymmetric Cu(I)-catalysed propargylic amination (ACPA) of simple ketone-derived propargylic carbonates to give both α-dialkylated and α-alkyl-α-aryl α-tertiary ethynylamines. Sterically confined pyridinebisoxazoline (PYBOX) ligands, with a C4 shielding group and relaying groups, play a key role in achieving excellent enantioselectivity. The syntheses of quaternary 2,5-dihydropyrroles, dihydroquinines, dihydrobenzoquinolines and dihydroquinolino[1,2-α]quinolines are reported, and the synthetic value is further demonstrated by the enantioselective catalytic total synthesis of a selective multi-target β-secretase inhibitor. Enantioselective Cu-catalysed propargylic substitutions with O- and C-centred nucleophiles are also realized, further demonstrating the potential of the PYBOX ligand.