Catalytic Conversion of Biomass-Derived Polyols into Para-xylene over SiO2-Modified Zeolites

This work proved that biomass-based polyols (sorbitol, xylitol, erythritol, glycerol and ethanediol) were able to be converted into high-value chemical (p-xylene) by catalytic cracking of polyols, alkylation of aromatics, and the isomerization of xylenes over the SiO₂-modified zeolites. Compared to the conventional HZSM-5 zeolite, the SiO₂-containing zeolites considerably increased the selectivity and yield of p-xylene due to the reduction of external surface acidity and the narrowing of pore entrance. The influences of the methanol additive, reaction temperature, and types of polyols on the selectivity and yield of p-xylene were investigated in detail. Catalytic cracking of polyols with methanol significantly enhanced the production of p-xylene by the alkylation of toluene with methanol. The highest p-xylene yield of 10.9 C-mol% with a p-xylene/xylenes ratio of 91.1% was obtained over the 15wt%SiO₂/HZSM-5 catalyst. The reaction pathway for the formation of p-xylene was addressed according to the study of the key reactions and the characterization of catalysts.     

Stereoselective Synthesis of the Proposed C79–C104 Fragment of Symbiodinolide

Stereoselective and streamlined synthesis of the proposed C79–C104 fragment 2 of symbiodinolide ( 1 ), a polyol marine natural product with a molecular weight of 2860, was achieved. In the synthetic route, the proposed C79–C104 fragment 2 was synthesized by utilizing a Julia–Kocienski olefination and subsequent Sharpless asymmetric dihydroxylation as key transformations in a convergent manner. Detailed comparison of the ¹³C NMR chemical shifts between the natural product and the synthetic C79–C104 fragment 2 revealed that the stereostructure at the C91–C99 carbon chain moiety of symbiodinolide ( 1 ) should be reinvestigated.     

General,Highly Selective Synthesis of 1,3‐ and 1,4‐Difunctionalized Building Blocks by Regiodivergent Epoxide Opening

We describe a regiodivergent epoxide opening (REO) featuring a catalyst‐controlled synthesis of enantiomerically and diastereomerically highly enriched or pure syn‐ and anti‐ 1,3‐ and 1,4‐difunctionalized building blocks from a common epoxide precursor. The REO is attractive for natural product synthesis and as a branching reaction for diversity‐oriented synthesis with epoxides.     

Biology-oriented synthesis of stereochemically diverse natural-product-derived compound collections by iterative allylations on a solid support

A strategy aiming at the introduction of stereocenters into polymer-bound natural-product-derived and -inspired compound collections is presented. Treatment of immobilized aldehydes with Brown's pinene-derived allylboranes results in the stereoselective formation of homoallylic alcohols with up to 89 % ee (ee=enantiomeric excess). Subsequent iterative ozonolysis-allylation sequences with up to three allylations on a solid support give access to 1,3-polyols with different relative configurations. Esterification with acryloyl chloride and final ring-closing metathesis yields alpha,beta-unsaturated delta-lactones with multiply oxygenated side chains, a substructure found in a group of natural products with a broad range of biological activity. The flexibility of the approach is exemplified by the parallel synthesis of all eight diastereomers of cryptocarya diacetate on a solid support. The individual isomers are obtained in overall yields of 40-60 % over 10 steps and with 63-85 % diastereoselectivity for the major isomer.     

新型的承载双金属氰化物络合催化剂

用双金属氰化物(DMC)络合催化剂可制得相对分子质量高、相对分子质量分布窄和不饱和度低的聚醚多元醇。为提高DMC催化剂的利用率,降低催化剂和聚醚的生产成本,国外开发了承载的DMC催化剂。它不仅可简化聚醚的后处理工艺,而且使聚醚的连续化大规模生产得以实现,因此代表了DMC催化剂的一个重要发展方向。     

Diastereoselective synthesis of highly functionalized tetrahydroxanthenols--unprecedented access to privileged structural motifs

Tetrahydroxanthenones, which can be easily prepared by a domino oxa-Michael aldol condensation, offer various possibilities for diastereoselective functionalization, giving access to the stereocontrolled synthesis of stereochemical triades or tetrades, which represent privileged structural motifs. In most cases, the relative stereochemistry was unequivocally established by crystal structure analysis.     

A simple and efficient approach to 1,3-polyols: application to the synthesis of cryptocarya diacetate

A highly enantio- and stereoselective synthetic strategy for both syn- and anti-1,3-polyols has been developed. The sequence involves iterative Jacobsen's hydrolytic kinetic resolution (HKR), diastereoselective iodine-induced electrophilic cyclization, and ring-closing metathesis (RCM). This protocol has subsequently been utilized for the synthesis of cryptocarya diacetate, a natural product with broad range of biological activity.     

Pd‐Catalyzed Reaction of Allyl Carbonate with Polyols: The Role of CO2 in Transesterification versus Etherification of Glycerol

An intermolecular Pd/PPh₃‐catalyzed transesterification of diallyl carbonate with glycerol to generate glycerol carbonate has been developed. Analysis of the reaction kinetics in THF indicates a first‐order dependence on Pd and diallyl carbonate, that the Pd bears two phosphines during the turnover limiting event, and that increasing the glycerol concentration inhibits reaction, possibly via change in the polarity of the medium. ¹³C isotopic labeling studies demonstrate that the Pd‐catalyzed transesterification requires at least one allyl carbonate moiety and that there is rapid equilibrium of the allyl carbonate with CO₂ in solution, even when present only at low concentrations. A mechanism that is consistent with these results involves oxidative addition of the allyl carbonate to Pd followed by reversible decarboxylation, with the intermediate η¹‐ and η³‐allyl Pd alkoxides mediating direct and indirect transesterification reactions with the glycerol. Using this model, successful simulations of the kinetics of reactions conducted under atmospheres of N₂ or CO₂ could be achieved, including switching in selectivity between etherification and transesterification in the early stages of reaction. Reactions with the higher polyols threitol and erythritol are also efficient, generating the terminal (1,2) monocarbonates with high selectivity.     

Interaction of triols with formaldehyde and acetone: Experimental and theoretical study

Experimental and theoretical aspects of the condensation of glycerol and its homologs (1,2,3- and 1,2,4-butanetriols) with formaldehyde and acetone are studied under conditions of acid catalysis. Calculation of the thermodynamic parameters of the resulting products by the composite method CBS-QB3 shows that the six-membered heterocycles, the products of the interaction of triols with formaldehyde, are thermodynamically more stable than the five-membered acetals, while the reaction of the same triols with acetone is preferable for the formation of the five-membered acetals. This is due to the fact that the regioselectivity of the studied reactions is determined by the structural features and reactivity of the carbocations formed in a condensed medium during the course of the reaction. According to the theoretical data obtained experimentally, during the condensation of glycerol and 1,2,4-butanetriol with formaldehyde in the most stable form of the six-membered cyclic carbocation, intramolecular hydrogen bonding and anomeric stabilization due to the axially oriented hydroxyl group take place. As a result, cation 1b–1 is 1.2–1.6 kJ/mol more stable than its five-membered isomers ( 1a–1 and 1b–2 ). It leads to the predominant formation of 1,3-dioxane ( 3b ). However, upon condensation of butanetriol-1,2,3 with formaldehyde, the intermediate cation 4a–1 turns out to be significantly more stable than the other isomers due to the strong intramolecular hydrogen bond in the six-membered ring with the participation of the hydroxyl group of the substituent and the hydroxyl group of the cationic center, leading to the predominant formation of the dioxolane 6a .     

DMC聚醚的降解研究

研究DMC聚醚 (双金属催化剂的聚环氧丙烷 )的降解现象 .讨论了各种因素对降解的影响 ,以及降解中不饱和度的变化情况 .实验表明 ,DMC聚醚的降解与自由基有关 ,降解在常温下 1 5天后开始发生 ,在抗氧剂存在下受到抑制 .加入抗氧剂 1 0 1 0和 61 8可使聚醚得以稳定储存