Gold-Catalyzed Cycloisomerization of 1,7-Enyne Esters to Structurally Diverse cis-1,2,3,6-Tetrahydropyridin-4-y1 Ketones

A synthetic method that relies on gold(I)-catalyzed cycloisomerization of 1,7-enyne esters to prepare highly functionalized cis-1,2,3,6-tetrahydropyridin-4-y1 ketone derivatives in good to excellent yields and as a single regio-, diastereo-, and enantiomer is described. By taking advantage of the distinctive differences in the electronic and steric properties between an NHC (NHC = N-heterocyclic carbene) and phosphine ligand in the respective gold(I) complexes, a divergence in product selectivity was observed. In the presence of [PhCNAuIPd+SbF6- (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) as the catalyst, tandem 1,3-acyloxy migration/6-exo-trig cyclization/1,5-acyl migration of the substrate was found to selectively occur to give the delta-diketone-substituted 1,2,3,6-tetrahydropyridine adduct. In contrast, reactions with the gold(I) phosphine complex [MeCNAu(JohnPhos)]+SbF6- (johnPhos = (1,1'-biphenyl-2-yI)-di-tert-butylphosphine) as the catalyst was discovered to result in preferential 1,3-acyloxy migration/6-exotrig cyclization/hydrolysis of the 1,7-enyne ester and formation of the cis-1,2,3,6-tetrahydropyridin-4-y1 ketone derivative. The utility of this piperidine forming strategy as a synthetic tool that makes use of 1,7-enyne esters was exemplified by its application to the synthesis of an enantiopure analogue of the bioactive 2,3,4,4a,5,9b-hexahydroindeno[1,2-c]pyridine family of compounds.