Aluminium triflate mediated reactions of cyclic enol ethers

• Main Author: Simelane, Sandile Bongani
• Published: 2012
• Subjects: Aluminium triflate
• Online Access: http://hdl.handle.net/10210/4688

M.Sc. === The objective of the project described in this dissertation was to develop environmentally benign organic transformations using aluminium triflate as a Lewis acid catalyst. The activity of aluminium triflate had already been investigated in our labs in other reactions. The present project was directed towards protection and deprotection of hydroxyl functions and glycosidation of differently protected glycal substrates. A range of alcohols and phenols was successfully protected to their corresponding tetrahydropyranyl (THP) and tetrahydrofuranyl (THF) ethers using 3,4-Dihydro-2H-pyran (DHP) and 2,3-Dihydrofuran (DHF) respectively. Catalytic amounts of Al(OTf)3 were employed to drive the reaction through in the presence of dichloromethane (DCM) as a solvent. This process allowed the preparation of a structural variety of these protected ethers, including those of a protected D-ribose derivative. The THP and THF ethers so generated were hydrolysed back to their corresponding alcohols using Al(OTf)3 in the presence of methanol. In all cases, the free alcohols were isolated in excellent yields. Importantly, other acid-sensitive groups (e.g. the ring acetal and acetonide on the ribose derivative) were retained on the more complex systems. Aluminium triflate was also employed to catalyse the O-glycosidation of 3,4,6-tri-O-acetyl-Dglucal, 3,4,6-tri-O-acetyl-D-galactal and 3,4,6-tri-O-benzyl-D-glucal. Different alkyl and aromatic nucleophiles were used as glycosyl acceptors in these reactions. This aspect of the study allowed a focus on O-glycosidation leading to the synthesis of 2,3-unsaturated glycosides via Ferrier rearrangement and 2-deoxy glycosides by direct addition of alcohol nucleophiles. Interestingly, the work discovered a temperature-switched selective Ferrier rearrangement or direct addition with alcohols in certain instances. This important innovation led to the idea that this protocol may be employed in the synthesis of unsymmetrical glycal based bolaforms. Progress in this direction shall also be detailed.