摘要:
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-alpha -D-galactopyranose, no sialylated products could be obtained with less reactive accepters. 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 alpha -nitrilium ion formation, seem to explain the unsuccessful sialylations of the less reactive accepters. (C) 2001 Elsevier Science Ltd. All rights reserved.