Norbert Krause

Leveraging the Micellar Effect: Gold-Catalyzed Dehydrative Cyclizations … in Water at Room Temperature

The first examples of gold-catalyzed cyclizations of diols and triols to the corresponding hetero- or spirocycles in an aqueous medium are presented. These reactions take place within nanomicelles, where the hydrophobic effect is operating, thereby driving the dehydrations, notwithstanding the surrounding water. By the addition of simple salts such as sodium chloride, reaction times and catalyst loadings can be significantly decreased.

Retinoide-V. Synthese acetylenischer retinoide durch cadiot-chodkiewicz-kupplung

Abstract The Cadiot-Chodkiewicz and related coupling reactions are shown to be valuable methods for the preparation of acetylenic (10, 15, 18) and diacetylenic retinoids (8).

Retinoide. VIII: Azidoretinoide

Abstract The addition of hydrazoic acid to the acetylenic aldehydes 2 and 11 provides an access to retinoids bearing an azido group In the 9- and 13-position, respectively.

Sustainable gold catalysis: synthesis of new spiroacetals

Abstract Various [O,O]- and [N,O]-spiroacetals were synthesized by gold-catalyzed spirocyclization of suitable functionalized alkynes. Whereas simple spiroacetals with two heteroatoms were readily obtained by regioselective cyclization of acetylenic diols or aminoalcohols, hitherto unknown spirocyclic isoxazolidines and pyrazolidines bearing three heteroatoms were formed by three-component coupling of alkynols, aldehydes, and protected hydroxylamine or hydrazine derivatives. The sustainability of these spirocyclizations was improved by using recyclable gold catalysts in water or nanomicelles as reaction medium.

Gold-catalyzed cycloisomerization of trifluoromethylated allenols: sustainability and mechanistic studies

Trifluoromethyl-substituted α-allenols are cyclized to the corresponding 2,5-dihydrofurans in the presence of neutral or cationic gold catalysts. The catalyst can be recycled if ionic liquids (in particular [BMIM][PF6]) are used as reaction medium. The allene forms droplets in the ionic liquid generating a heterogeneous two-phasic system. Kinetic studies in organic solvents using electronically different gold catalysts allowed to identify the protodeauration of a σ-gold-species as rate-determining step of typical allene cyclizations. Only if a CF3 group is present at C-5, π-complex formation is rate-limiting.