A two-step method
was adopted to produce furfural from the selective dissolution and
conversion of hemicellulose in pubescens. First,
in GVL(γ-valerolactone)-H2O co-solvent at 160 °C,
H2O promoted the cleavage of chemical bonds linking hemicellulose,
lignin, and cellulose, and GVL further helped the co-dissolution of
hemicellulose (93.6 wt %) and lignin derivatives (80.2 wt %), leaving
a high purity cellulose (83.3 wt %). Heating to 200 °C, the liquid
system obtained with NaCl and THF added, achieved the maximum yield
of 76.9 mol % with 82.2% selectivity to furfural based on the moles
of converted hemicellulose using a 5 wt % pubescens to solvent ratio. It was demonstrated that NaCl with GVL promoted
the depolymerization of oligomers to small molecular products (Mw
< 150 Da), while the co-contribution of NaCl and co-solvent improved
the selectivity to furfural. Cl– could interact
strongly with C-OH-2,3,4 of the xylose unit, and the dehydration of
xylose to form furfural might first occur on C-OH-4 of xylose, then
on C-OH-2,3 of xylose, which enhanced the dehydration and ring open
reaction via the cleavage of C1–O6 bonds,
then promoted the formation of furfural by inhibiting the retro-aldol
reaction to form lactic acid. The co-contribution of NaCl and co-solvent
was benefical not only for the selective conversion of the mixture
containing hemicellulose-derived monomers and oligomers to furfural
but also for obtaining a lower molecular weight lignin derivatives
(150–500 Da) that could be further used.
The unique steric effect of geminal bis(silane) [(R3Si)2CH] allows an exo-selective intermolecular Diels-Alder reaction of geminal bis(silyl) dienes with α,β-unsaturated carbonyl compounds. The approach shows good generality to form ortho-trans cyclohexenes in good yields with high exo-selectivity and high enantioselectivity in some asymmetric cases. The excellent exo-stereocontrol aptitude of (R3Si)2CH group is highlighted by comparing with R3SiCH2 and R3Si groups, which leads to endo-selectivity predominantly. The conformational analysis of dienes suggests that (R3Si)2CH group effectively shields both sides of the diene moiety, ensuring the desired exo-selectivity. Moreover, the geminal bis(silane) can be further functionalized to transform the resulting ortho-trans cycloadducts into useful synthons, which makes the approach hold great potential for organic synthesis.
The enantioselective tandem reaction of b,g-unsaturated a-ketoesters with b-alkynyl ketones was realized by ab imetallic catalytic system of achiral Au III salt and chiral N,N'-dioxide-Mg II complex. The cycloisomerization of balkynyl ketone and asymmetric intermolecular [4+ +2] cycloaddition with b,g-unsaturated a-ketoesters subsequently occurred, providing an efficient and straightforwarda ccess to chiral multifunctional 6,6-spiroketals in up to 97 %yield, 94 % ee and > 19/1 d.r.Besides,acatalytic cycle was proposed based on the results of control experiments. Scheme 2. Substrate scope for b-alkynyl ketones. Unless otherwise noted, all reactions were performed with Mg(OTf) 2 (2.5 mol %), AuCl 3 (2.5 mol %), l-RaPr 3 (5 mol %), 1a (0.10 mmol), 2b-2o (0.15 mmol), 5 M. S. (30 mg) in DCM (1.0 mL) at 40 8 8Cfor 2d ays. [a] In DCM (0.5 mL) at 40 8 8Cf or 4d ays. Scheme 3. The transformations of the chiral product 3a.Scheme 4. Control experiments and plausible mechanism.
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