Prearranged Glycosides, Part 12, Intramolecular Mannosylations of ...

Helvetica Chimica Acta ± Vol. 83 2000) 2659
Prearranged glycosides tethered via position 4 of the respective mannosyl residues
were prepared as follows. First, a suitably protected mannoside was needed to allow the
introduction of the succinyl tether. Thus, phenyl 1-thiomannoside 25 was first prepared
by opening of the benzylidene ring [46] of phenyl 2,3-di-O-benzyl-4,6-O-benzylidene-1thio-a-d-mannopyranoside
24) [51] Scheme 3). Next, succinylation at position 4
afforded monosaccharide 26 89%), which was again condensed with glucoside 3. The
thus-obtained succinyl-tethered glycoside 27 74%) was converted as described above
to compound 28 78%) and cyclized see below, Table) to the tethered disaccharides 29.
As previously observed for the tethered disaccharides 18, no assignment of the anomer
configuration of saccharides 29a and 29b could be made by NMR spectroscopy C1) of
the mannosyl residue at 100.4 29a) and 100.8 ppm 28a); corresponding 1 J ˆ 175.0
29a) and 173.7 Hz 29b)). Therefore, disaccharides 29 were each deacylated and
benzoylated to give saccharides 30, which now showed characteristic C,H-coupling
constants 1 J ˆ 174.1 Hz 30a) and 156.9 Hz 30b)). For comparison reasons, the Table
see below) also shows the already known results of the cyclization of compound 31,
which established the formation of anomer mixtures of tethered disaccharides 32 and
33, respectively, depending on the reaction conditions [41].
Scheme 3