Total, Asymmetric-Synthesis of (+)-Castanospermine, (+)-6-Deoxycastanospermine, and (+)-6-Deoxy-6-Fluorocastanospermine

Bromination of the dibenzyl acetal of (-)-(1S,4S)-7-oxabicyclo[2.2.1]hept-5-en-2-one ((-)-5) led to (+)-(1S,5S,6S,7S)-6-endo-(benzyloxy)-5-exo-bromo-7-oxabicyclo[2.2.1]hept an-2-one (25). Baeyer-Villiger oxidation of 25 gave 2-O-benzyl-3-bromo-3,5-dideoxy-beta-L-arabino-hexofuranosidurono-6,1-lac tone (26). Methanolysis of 26 afforded the corresponding methyl (methyl alpha-beta-L-arabinofuranosid)uronates (27 + 28). The alpha anomer 27 was reduced with DIBAH into methyl 2-O-benzyl-3-bromo-3,5-dideoxy-beta-L-arabino-hexofuranoside (29). Mesylation of the primary alcohol, followed by treatment with NH3 gave methyl 2-O-benzyl-3,5-6-trideoxy-3,6-imino-beta-L-lyxo-hexofuranoside (32). Acetylation of the amine with ClCH2COCl, acetolysis of the methyl furanoside followed by Arbuzov condensation with (EtO)3P, and then intramolecular Horner-Emmons reaction led to (5S,6S,7S)-7-hydroxy-5-(benzyloxy)-1-azabicyclo[4.3.0]non-3-en-2-one (37). Base-catalyzed hydrolysis of the corresponding epoxide 43 ((1S,6S,7S,8R,8aS)-8-(benzyloxy)-6,7-epoxy-1-hydroxyoctahydroindolizidin -5-one) followed by reduction of the lactam and deprotection of the alcoholic functions afforded (+)-castanospermine ((+)-1). The conversion of (-)-5 into (+)-1 was highly stereoselective, requiring the isolation of 10 synthetic intermediates and with an overall yield of 15.2%. Reduction of 43 with BH3.Me2S or its treatment with HF.Et3N allowed one to prepare readily (+)-6-deoxycastanospermine ((+)-2 and 6-deoxy-6-fluorocastanospermine ((+)-3). The crystal structure of (+)-3 is also reported.