An Efficient and Short Route for the Synthesis of Reverse Pyrrole Ribonucleosides

The synthesis of reverse pyrrole ribonucleosides methyl 5-C-(4-acetyl-5-methyl-pyrrol-1-yl)-2,3-O-isopropylidene-5-deoxy- beta-D-ribofuranoside (10), methyl 5-C-(4-ethoxycarbonyl-5-methyl-pyrrol-1-yl)-2,3-O-isopropylidene-5-deoxy- beta-D-ribofuranoside (11), methyl 5-C-(4-acetyl-5-methyl-pyrrol-1-yl)-5-deoxy-beta-D-ribofuranoside (12), methyl 5-C-(4-ethoxycarbonyl-5-methyl-pyrrol-1-yl)-5-deoxy- beta-D-ribofuranoside (13), methyl 5-deoxy-5-C-(3'-formyl-4'-hydroxypropyl-pyrrol-1'-yl)-2,3-O-isopropylidene- beta-D-ribofuranoside (16) and methyl 5-deoxy-5-C-(3'-formyl-pyrrol-1'-yl)-2,3-O-isopropylidene- beta-D-ribofuranoside (18) are described starting from readily available methyl 5-amino-5-deoxy-2,3-O-isopropylidene-beta-D-ribofuranoside (9). The synthetic strategy for the construction of the heterocyclic ring was based on the nucleophilic attack of (9) to 4-acetyl-2-n-butoxy-5-methyl-4,5-dihydrofuran (4), 4-carbetoxy-2-n-butoxy-5-methyl-4,5-dihydrofuran (5), 4-formyl-2-n-butoxy-4,5-dihydrofuran (6) and 4-formyl-1-methyl dioxabyciclo[3.3.0]oct-3-en (8, in situ). The later compounds were obtained from reaction between 3-diazo-2,4-pentadione (1), ethyl 2-diazoacetoacetate (2) or diazomalonaldehyde (3) and enol ethers using dirhodium tetraacetate as a catalyst.