A Short Synthesis of L‐Acosamine Based on Nitroaldol Addition (Henry Reaction). Analysis of the Key Step Concerning Solvent and Temperature Effects
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Of several routes explored for the synthesis of 3-amino2,3,6-trideoxyhexoses such as L-acosamine (L-arabino), two versions of the nitroaldol strategy were completed. The silyl nitronate approach with use of "anhydrous" tetrabutylammonium fluoride (Bu4NF) turned out to be rather complex, since the key step led to a moderately diastereoselective, though still efficient solution. In a simpler mode, with Bu4NF trihydrate as a catalyst, the two C3 building blocks (2-O-benzyl-L-lactaldehyde 1 and 3-nitropropanal dimethyl acetal 2) are combined to give the respective 3-nitrohexoses 4– 7 in ratios ranging from 37:22:22:19 (i-PrOH, 22 °C) to 29:62:1:8 (t-BuOMe, –30 °C) for the four diastereomers L-ribo/L-arabino/Lxylo/L-lyxo. In this context efficient analyses (HPLC) and separations (MPLC) of nitroaldol stereoisomer mixtures were sought and found. This permitted the systematic study of the effects of temperature and solvents on such Henry reactions and allowed the conclusion that higher reaction temperatures (above –10 °C) would alter the kinetic diastereomer ratio (d.r.), due to ensuing retro-nitroaldol reaction and, possibly, some epimerization at the nitromethine (CHNO2) stereocentre. – The L-arabino compound 5 was obtained according to optimized conditions and converted to the N,O-diacetyl derivative 10 of L-acosaminide by acid-catalyzed formation of the nitropyranosides a/b-8, hydrogenation of the nitro group and N,O-diacetylation of the intermediate methyl a-L-acosaminide 9. Overall, methyl N,O-diacetyl-a-L-acosaminide 10 is obtained from 2-O-methoxymethyl-L-lactaldehyde 3 and b-nitropropionaldehyde dimethyl acetal 2 in 4 steps with 36% yield.
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