(720d) Regio- and Stereo- Selective Synthesis of Hexane-Tetrols and -Triols from Methyl Glycosides over Supported Metal Catalysts

Partially
reduced C₆ polyols are potentially valuable precursors to polymers,
surfactants, and solvents [1]. Sugars are a renewable feedstock for the
production of C₆ polyols, but typical catalytic hydrogenolysis
processes are unselective towards C-OH bond cleavage at different positions and
do not preserve the sugar’s stereocenters. In mso-themecolor:text1">contrast, Tomishige and co-workers recently reported on
the selective cleavage of cis-vicinal OH groups of a variety of methyl
glycosides over a ReO_x-Pd/CeO₂ catalyst [2]. This
reaction step produces dideoxy sugars in which the chirality of the remaining
C-OH groups is preserved.

Here, we demonstrate the regio- and stereo-
selective synthesis of hexane-tetrols and hexane-triols from methyl glycosides over
supported metal catalysts 107%;font-family:" times new roman>(Scheme 1). First, the above method
is used to synthesize dideoxy sugars from sugar feedstocks. Next, these intermediates
are hydrolyzed and hydrogenated to produce hexane-tetrols and hexane-triols. The
reaction mechanism proceeds through initial methoxy bond hydrolysis to form a mixture
of intermediates. Different reaction intermediates are formed depending on the structure of the
feedstock. Hydrogenation of these intermediates over Pt/SiO₂ produces
the final hexane-tetrols and -triols. 2,3-Dideoxy substrates are more reactive
than 3,4-dideoxy substrates due to the electron withdrawing nature of the C₂-OH
group. 3,4-Dideoxy substrates also undergo aldose-ketose isomerization
at higher temperatures which decreases the stereoselectivity due to scrambling
of the C₂-OH stereocenter. Stereoselectivity can be increased by introducing
an acid catalyst to facilitate hydrolysis at lower temperature.

normal">

font-family:" times new roman>

Scheme
1. 12.0pt;line-height:107%;font-family:" times new roman> Reaction network
for synthesis of hexane-tetrols
and –triols from methyl glycosides with cis-vicinal OH groups.

This
work demonstrates a novel catalytic route to produce valuable chemical
precursors from sugars, and provides insight into controlling stereoselectivity
in biomass conversion reactions.

107%;font-family:" times new roman>References.

normal;mso-layout-grid-align:none;text-autospace:none"> 12.0pt;font-family:" times new roman>1. Corma, A, et al. Chem. Rev. 2007,
107 (6), 2411-2502.

normal;mso-layout-grid-align:none;text-autospace:none"> 12.0pt;font-family:" times new roman>2. Tamura, M., et al.  Angew.
Chem. Int. Ed, 2018, 57 (27), 8058-8062.

normal;mso-layout-grid-align:none;text-autospace:none"> 12.0pt;font-family:" times new roman>