Studies related to carba-pyranoses: synthesis of acetylated derivatives of 4-amino-2,4-dideoxy-3-O-(β-d-glucopyranosyl)-β-l-(and β-d-) altrocarba-pyranose from ad-glucose template

Abstract

Methyl (E)-3-nitroacrylate 15, the X-ray analysis of which is reported here, is prepared from methyl acrylate by a new route involving sequential reactions with mercury(II) chloride-sodium nitrite, bromine and sodium acetate. The dienophile 15 reacts with Danishefsky's diene 17 to give, after acidic hydrolysis, a 67 : 33 mixture of the ketones rac-18 and rac-19. With (E)-1-(2',3',4',6'-tetra-O-acetyl-beta-D-glucopyranosyloxy)-3-(trimethylsiloxy)buta-1,3-diene 1, it affords, after hydrolysis, a 42 : 18 : 28 : 12 mixture of the ketones 30, 31, 32 and 33. The alcohol 36, obtained by sodium borohydride reduction of the ketone 30, is converted into the monocarba-disaccharide 35 by the action of aluminium amalgam, lithium aluminium hydride and acetic anhydride. Similarly, the alcohol 41, derived from the ketone 32, is transformed into the monocarba-disaccharide 40; the structure of the alcohol 41 is secured by an X-ray analysis. The isolation of a mixture of the acetoxyamino and acetylamino derivatives 38 and 39 from the reaction of the alcohol 36 with aluminium amalgam and acetic anhydride indicates that the nitro function is converted into the hydroxylamino and amino groups by the reducing agent. The cyclohexane rings of the ketones rac-18, rac-19, 30, 31 and 33 adopt the expected chair conformations. Thus, the methoxycarbonyl, nitro and oxy substituents are equatorially orientated in the ketones rac-18 and 30; in the ketones rac-19, 31 and 33, the methoxycarbonyl and nitro groups occupy equatorial dispositions and the oxy substituent is axially orientated. The cyclohexane ring of the ketone 32 (which bears a diastereomeric relationship to that of the ketone 30) displays unexpected conformational properties, that are attributed to a significant population of the chair conformer with axial arrangements of the methoxycarbonyl, nitro and oxy groups.