The allyl ether as a protecting group in carbohydrate chemistry. Part 11. The 3-methylbut-2-enyl (‘prenyl’) group

Abstract

The ‘prenyl’ group in ally 3,4,6-tri-O-benzyl-2-O-(3-methylbut-2-enyl)-α-D-galactopyranoside was stable to the action of chlorotris(triphenylphosphine)rhodium(I) during 24 h, whilst the allyl group was isomerised within 1 h. Previous work has shown that the but-2-enyl group is completely isomerised within 24 h by the rhodium catalyst. Thus an allyl group can be removed without affecting a ‘prenyl’ group in the same molecule. The ‘prenyl’ group is cleaved at about the same rate as a but-2-enyl group by potassium t-butoxide in dimethyl sulphoxide. The isomerisation of the allyl group by the rhodium catalyst gives a mixture of cis- and trans-prop-1-enyl ethers.