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

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Helvetica Chimica Acta ± Vol. 83 2000)
The two prearranged glycosides tethered with a malonate spacer via position 6 of
the mannosyl donor to position 2 and 6, respectively, of the glucosyl acceptor were
prepared from ethyl 1-thiomannoside 34 [41]. Thus, condensation of the latter with
benzyl 3-O-benzoyl-4,6-O-benzylidene-a-d-glucopyranoside 35) [52], followed by
regioselective opening of the benzylidene group of intermediate 36, afforded the
prearranged glycoside 37 Scheme 4). Cyclization of compound 37 gave exclusively the
tethered a-d-linked disaccharide 38 see below, Table), which was deacylated and
benzoylated to afford 39 in 73% yield. Both disaccharides showed C,H-coupling
constants typical for an a-d-linkage 1 J ˆ 168.8 38) and 171.8 Hz 39)). Similarly, 34
was condensed regioselectively with benzyl 2,3-di-O-benzoyl-a-d-glucopyranoside 40)
[53] to give the prearranged glycoside 41 65%), cyclization of which see below, Table)
and subsequent deacylation and benzoylation afforded a-d-linked disaccharides 42 and
43, respectively 1 J ˆ 172.542) and 172.0 Hz 43)).
Scheme 4
Discussion. ±TheTable summarizes the results for the cyclizations of tethered
disaccharides 5, 17, 23, 28, 31, 37, and 41. All cyclizations described here were performed
in CH 2Cl 2 or MeCN with N-iodosuccinimide NIS) for phenyl and ethyl 1-thioglycosides
and with methyl trifluoromethanesulfonate MeOTf) for ethyl 1-thioglycosides,
as previously described [41]. The results of the cyclizations of prearranged glycosides 5
Table, Entries 1 ± 3) having the succinate tether at position 2 of the mannosyl donor
and at position 3 of the glucosyl acceptor are in contrast to previously performed
glycosylations with glucosyl or galactosyl instead of mannosyl donors [31] [39] [42]. In
the case of 5, solely a-d-1 ! 4)-linked disaccharides were found. It was originally
expected for compounds 5 to give at least significant amounts of the corresponding b-dlinked
disaccharides. That this was not the case may be explained by the results of forcefield
calculations [54] performed for the a-d-linked product 6 and its b-d-linked