Nucleophilic displacements of methyl 2,3- O-isopropylidene-4- O-toluene- p-sulphonyl-α- d-lyxo- and -β- l-ribo-pyranosides, and deamination of the corresponding 4-amino sugars

Abstract

Reinvestigation of the reaction of methyl 2,3- O-isopropylidene-4- O-toluene- p-sulphonyl-α- d-lyxopyranoside ( 4) with azide ion has shown that methyl 4-deoxy-2,3- O-isopropylidene-β- l- erythro-pent-4-enopyranoside ( 8, ∼51.5%) is formed, as well as the azido sugar 7 (∼48.5%) of an S N2 displacement. The unsaturated sugar 8 was more conveniently prepared by heating the sulphonate 4 with 1,5-diazabicyclo-[5.4.0]undec-5-ene. An azide displacement on methyl 2,3- O-isopropylidene-4- O-toluene- p-sulphonyl-β- l-ribopyranoside ( 12) furnished methyl 4-azido-4-deoxy-2,3- O-isopropylidene-α- d-lyxopyranoside ( 13, ∼66%) and the unsaturated sugar 14 (∼28.5%), which was also prepared by heating the sulphonate with 1,5-diazabicyclo[5.4.0]undec-5-ene. Deamination of methyl 4-amino-4-deoxy-2,3- O-isopropylidene-α- d-lyxopyranoside ( 5), prepared by reduction of 13, with sodium nitrite in 90% acetic acid at ∼0°, yielded methyl 2,3- O-isopropylidene-α- d-lyxopyranoside ( 10a, 26.2%), methyl 2,3- O-isopropylidene-β- l-ribofuranoside ( 21a, 18.4%), and the corresponding acetates 10b (34.5%) and 21b (21.3%). These products are considered to arise by solvolysis of the bicyclic oxonium ion 29, formed as a consequence of participation by the ring-oxygen atom in the deamination reaction. Similar deamination of methyl 4-amino-4-deoxy-2,3- O-isopropylidene-β- l-ribopyranoside ( 6) afforded, exclusively, the products 10a (34.4%) and 10b (65.6%) of inverted configuration. Deamination of methyl 5-amino-5-deoxy-2,3- O-isopropylidene-β- d-ribofuranoside ( 20) gave 22ab, but no other products. An alternative synthesis of the amino sugars 5 and 6 is available by conversion of 10a into methyl 2,3- O-isopropylidene-β- l- erythro-pentopyranosid-4-ulose ( 11), followed by reduction of the derived oxime 15 with lithium aluminium hydride.