Jonathan Minshall

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 N 2 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.

Nucleophilic displacement reactions in carbohydrates. Part XXII. Formation of 1,4-anhydropyranoses from 1-O-acetyl-6-deoxy-2,3-O-isopropylidene-4-O-methylsulphonyl-α-L-manno- and -talo-pyranose with sodium azide

1-O-Acetyl-6-deoxy-2,3-O-isopropylidene-4-O-methylsulphonyl-α-L-mannopyranose (10; R = Ac) and the epimeric L-talopyranose sulphonate (12; R = Ac) yielded 1,4-anhydro-6-deoxy-2,3-O-isopropylidene-β-L-talopyranose (11) and 1,4-anhydro-6-deoxy-2,3-O-isopropylidene-α-L-mannopyranose (13), respectively, on treatment with sodium azide in NN-dimethylformamide at 140°. A mechanism involving initial deacetylation is proposed for these and related reactions.

The deamination of methyl 5-amino-5,6-dideoxy-2,3-O-isopropylidene-α-L-talo- and -β-D-allo-furanosides with nitrous acid

Abstract Deamination of methyl 5-amino-5,6-dideoxy-2,3- O -isopropylidene-α- L -talofuranoside ( 6 ) with sodium nitrite in 90% acetic acid at ≈0° gave methyl 6-deoxy-2,3- O -isopropylidene-α- L -talofuranoside ( 8a ) and methyl 6-deoxy-2,3- O -isopropylidene-β- D -allofuranoside ( 9a ) (combined yield, 12.3%), the corresponding 5-acetates 8b (2.9%) and 9b (26.4%), and the unsaturated sugar methyl 5,6-dideoxy-2,3- O -isopropylidene-β- D - ribo -hex-5-enofuranoside ( 10 ) (43.5%). Similar deamination of methyl 5-amino-5,6-dideoxy-2,3- O -isopropylidene-β- D -allofuranoside ( 7 ) gave 8a and 9a (combined yield, 20.4%), 8b (12.5%), 9b (25.8%), 10 (7.7%), and the rearranged products 6-deoxy-2,3- O -isopropylidene-5- O -methyl- L -talofuranose ( 13a , 17.5%) and the corresponding 1-acetate 13b (14.1%). A synthesis of 13a was accomplished by successive methylation and debenzylation of the conveniently prepared benzyl 6-deoxy-2,3- O -isopropylidene-α- L -talofuranoside ( 15b ). Differences between the two sets of deamination products can be rationalized by assuming that the carbonium-ion intermediate reacts in the initial conformation assumed, before significant interconversion to other conformations occurs.

Branched-chain sugars. Part IV. Synthesis of derivatives of aldgarose, a component of aldgamycin E

Methyl 4,6-dideoxy-3-C-[(S)-1-hydroxyethyl]-β-D-ribo-hexopyranoside 3,31-cyclic carbonate (methyl aldgaro-side B)(14) has been synthesised and shown to be identical with one of the anomeric glycosides obtained from aldgamycin E by methanolysis. The key step in the synthesis involved the epoxidation of methyl 4,6-dideoxy-2,3-O-isopropylidene-3-C-vinyl-α-D-ribo-hexopyranoside (4) to yield principally the (S)-epoxide (10).