Approaches to the C-B-A trisaccharide of dihydroaclacinomycin by extending the chain from either side

Selective benzylation of L-fucal (1) under phase-transfer conditions gave the 3- and 4-monoethers 2 and 3, respectively. Two routes, the "tail" or the "head" addition are presented, both leading to the target molecule 9, a mimic of the C-B-A trisaccharide component of dihydroaclacinomycin. Addition of glycals 2 and 3 respectively, to the acetylated glycal (7) of amicetose used as glycosyl donor gave the disaccharide glycals 6 and 8. Alternatively, glucosylation of the 4-acetate (4) of 2 with the benzyl hex-2-enopyranoside derivative 10 gave the disaccharide derivative 11. In the first case, the final glycosylation step involves the addition of 10 to disaccharide glycal 8. In the second procedure, the disaccharide alcohol 12 is obtained by O-deacetylation of 11, and serves as the glycosyl acceptor for glycal derivative 7 to give the C-B-a precursor trisaccharide derivative.