A study of the donor properties of sialyl phosphites having an auxiliary 3-(S)-phenylseleno group

Two phosphite sialyl donors, each having an auxiliary 3-(S)-phenylseleno group, were prepared and evaluated. The phenylseleno group was introduced via a new mode of generating phenylselenenic acid (‘PhSeOH’). Although the sialyl donors provided fair yields (32–76%) of the desired sialosides in glycosylations of the reactive acceptor 1,2;3,4-di-O-isopropylidene-α-d-galactopyranose, no sialylated products could be obtained with less reactive acceptors. The presence of a 5-N-acetylacetamido group on the phosphite sialyl donor did not appear to improve its sialylating capability. The weak C-Se bond, possibly in combination with a steric hindrance, which disfavors α-nitrilium ion formation, seem to explain the unsuccessful sialylations of the less reactive acceptors.