A Route to Carbasugar Analogues - Jonathan Clayden - The ...

4.4 – Revised altrose synthesis
indicative of a strong interaction between the allylic alcohol and osmium tetroxide.
The substrate-metal interaction might be further strengthened by the presence of an
internal amine which would compete with the amine ligands of the catalytic systems
(N-methylmorpholine, trimethylamine or quinuclidine). Sharpless has previously
shown that osmate esters are unreactive to NMO when coordinatively saturated with
amines. 198
Since it is postulated that it is the catalytic cycle that is being inhibited, the problem
might be overcome under stoichiometric conditions. The most notable such reaction is
the application of the stoichiometric TMEDA/osmium tetroxide by Donohoe. 199 A
further advantage of this system is that it has been estimated to be 100 times more
reactive than the monodentate catalytic systems previously used, although conducting
the reaction at –78 °C to enhance selectivity might be expected to negate this. We
would not expect to obtain tetraol 311 as the major product from this reaction as
Donohoe conditions generally prefer syn-dihydroxylation.
Despite rapid consumption of starting material, no products could be identified either
before or after purification (Table 4.6, entry h). The reaction mixture remained rust
brown after all recommended hydrolysis conditions were employed, making analysis
of the reaction difficult. It is noted that osmate esters are normally detectable by 1 H
NMR and TLC, and no compounds could be identified by either method. Meanwhile,
enough of tetraol 311 had been isolated to investigate the forward reaction.
4.4.3.a Route B: oxazolidine hydrolysis
The isolated tetraol 311 and ketone 319 were subject to the hydrolysis conditions
previously developed (Scheme 4.39). The tetraol could be seen to go to a UV-inactive
product by TLC at both ambient and elevated temperatures, but upon workup no
aldehyde could be seen in the crude NMR. Reaction of ketone 319 was slow,
presumably since it lacked an available hydroxyl group to facilitate hydrolysis (section
4.2.9).
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