1,2-ACYLSPIROORTHOESTERS OF 3,4,6-TRI-O-ACETYL-α-D-GLUCOPYRANOSE

Article abstract of DOI:10.1016/S0008-6215(00)85697-9

The 1,2-O-(2-oxa-3-oxocyclopentylidene) derivative of 3,4,6-tri-O-acetyl-α-D-glucopyranose was prepared in both the exo (4) and endo (5) forms.The compounds were prepared by bromide-ion promoted cyclization of 3,4,6-tri-O-acetyl-2-O-(3-carboxypropanoyl)-α-D-glucopyranosyl bromide.The similar acylorthoester derivatives of phthalic acid were prepared from 3,4,6-tri-O-acetyl-2-O-(2-carboxybenzoyl)-α-D-glucopyranosyl bromide.The cyclizations produced a much higher ratio of the endo forms than would have been expected from their relative thermodynamic stabilities.The configurations were established by nuclear Overhauser enhancement studies and their conformations deduced from (1)H-n.m.r. parameters.The greater stability of the exo isomers appears to have a stereoelectronic origin.Preliminary efforts to engage the acylorthoesters in reactions with isopropyl alcohol to form glycosides are reported.It was discovered that a carboxylic acid provides powerful catalysis for the β to α anomerization of O-acetylatyed glucopyranosides by stannic chloride.