Dimerisation of tri-O-acetyl-D-glucal
The dimerisation of tri-O-acetyl-D-glucal (1) using acetyl perchlorate1 as initiator has been investigated.
Initiation via Ferrier rearrangement and insertion of a second monomer 1 affords a 50:50 mixture of anomers 2 which can be reduced to a single enantiomer 3 on treatment with a Lewis acid and triethylsilane. Diagnostic NOE enhancements confirm the stereochemistry as shown, which is consistent with a precident reported by Ferrier2 who isolated a single anomer in only 10% yield.
Dimerisation of tri-O-acetyl-D-galactal
Analogous dimerisation with tri-O-acetyl-D-galactal 4 proceeds at -78 °C to afford only the axially substituted anomer 5 in 69% yield. This is successfully converted to its deoxy analogue 6 on treatment with triethylsilane/Lewis acid.
Other substrates
Tri-O-benzyl-D-glucal (7) proceeds at -78°C, affording an inseparable mixture of products in 81% yield.
In analogous fashion di-O-acetyl-D-xylal 8 affords a single axial anomer which is yet to be fully characterised.
The racemic 5-deoxyglucal 9 analogue also proceeds at -78 °C to afford one diastereoisomer in 58% yield and several minor products, none of which are isolable.
Higher oligomers
Trimerisations and higher oligomerisations are currently under investigation.
