Original basic or acidic organic compounds derived from guanidine or phenyl phosphonic acid were specifically designed and tested as new catalysts for the bulk synthesis of polyurethane prepolymers from a precursor system with particularly low reactivity (secondary alcohol + aliphatic isocyanate at low temperature). Both families showed interesting catalytic activities at 60–80 °C, but must nevertheless be used in much higher amounts (1 mol%, i.e. between 0.15 and 0.50 wt%) than traditional metal-based catalysts. The efficiency of guanidine derivatives seems to be related to their nucleophilicity, whereas that of phosphonic acid derivatives depends on their acidity. However, the solubility of the considered species in the reactive medium also plays a major role. The water/alcohol selectivity of the catalysts, especially at room temperature, was then examined as an additional criterion. Guanidines are not selective and favor the reaction of isocyanate groups with water as much as that with alcohols. Phenyl phosphonic acid derivatives are more selective, and particularly pentafluorophenyl phosphonic acid displays a remarkable catalytic activity together with an acceptable selectivity and could represent an interesting and safer alternative to toxic tin and mercury derivatives for many industrial polyurethanes. 相似文献
For the synthesis of multisubstituted dispirocyclohexane derivatives, a Rauhut–Currier‐initiated organocascade reaction between 2‐arylideneindan‐1,3‐diones and activated alkenes in the presence of diazabicyclo[2.2.2]octane (DABCO) was demonstrated. The functionalized dispirocyclohexanes with a wide range of substituents were obtained in satisfactory to excellent chemical yields (58–99%) with high chemo‐ and diastereoselectivities (>95:5 dr). This organocatalytic reaction proceeded smoothly through Michael/Michael cyclization, which involves the formation of three new carbon‐carbon bonds with simultaneous construction of two all‐carbon spiro quaternary centers.
A novel synthetic method to access fused indolin‐3‐ones with a tetrasubstituted carbon stereocenter has been developed via NHC‐catalyzed umpolung formal [3+3] cycloaddtion of enals with isatogens. This methodology could be also applied for the quick construction of the 6‐5‐5 tricyclic pyrrolo[1,2‐a]indole skeleton which is frequently found as a core structure of many indole alkaloids.
In the presence of a Cinchona alkaloid‐based squaramide organocatalyst, the [3+2] cycloaddition of isatin‐derived azomethine ylides with maleimides proceeded readily, thus delivering the desired pyrrolidine‐fused spirooxindoles in 61–89% yields with >20:1 dr and 12 to >99 % ee. The absolute configuration of 5‐chloro‐1,5′‐dimethyl‐3′‐phenyl‐3′,3a′‐dihydro‐2′H‐spiro[indoline‐3,1′‐pyrrolo[3,4‐c]pyrrole]‐2,4′,6′(5′H,6a′H)‐trione was unambiguously determined by means of X‐ray single crystal structure analysis. The reaction mechanism was hypothesized to account for the enantioselective formation of 5‐chloro‐1,5′‐dimethyl‐3′‐phenyl‐3′,3a′‐dihydro‐2′H‐spiro[indoline‐3,1′‐pyrrolo[3,4‐c]pyrrole]‐2,4′,6′(5′H,6a′H)‐trione.
We disclose herein an efficient enantioselective conjugate addition reaction between coumarin‐3‐carboxylic acids and malonic acid half thioesters (MAHTs). The reaction was catalyzed by N‐heteroarenesulfonyl Cinchona alkaloid amides to afford double‐decarboxylative conjugate addition products in good yield with high enantioselectivity. The reaction of various coumarin‐3‐carboxylic acids with MAHTs gave products in high yield with high enantioselectivity.
The first catalytic asymmetric construction of the cyclic enaminone‐based 3‐substituted 3‐amino‐2‐oxindole scaffold with potential bioactivity has been developed via chiral phosphoric acid‐catalyzed enantioselective addition reactions of cyclic enaminones to isatin‐derived imines, which afforded a series of cyclic enaminone‐based 3‐substituted 3‐amino‐2‐oxindoles in high yields and excellent enantioselectivities (up to 99% yield, 97% ee). The investigation of the reaction mechanism suggested that it was facilitated by a dual hydrogen‐bonding activation mode between the two substrates and the chiral phosphoric acid. Besides, this method could be utilized for a large‐scale synthesis with maintained enantioselectivity. This approach will not only offer a useful method for enantioselective construction of the cyclic enaminone‐based 3‐substituted 3‐amino‐2‐oxindole scaffold, but also enrich the research on catalytic asymmetric addition reactions of isatin‐derived imines by using electron‐rich olefins as nucleophiles. More importantly, a preliminary evaluation on the cytotoxicity of some selected products revealed that two of the enantio‐pure compounds exhibited moderate to strong cytotoxicity to A549, 786‐0, ECA109 and BT474 cancer cell lines.
An enantioselective one‐pot Michael/Michael/Henry/hemiacetalization reaction between α,β‐unsaturated aldehydes, α‐ketoamides, and nitroalkenes under mild conditions catalyzed by a diarylprolinol silyl ether has been developed. The sequential methodology provides a direct approach to a wide range of fully substituted chiral oxabicyclo[2.2.2]octanes with seven contiguous stereocenters in moderate to excellent yields (up to 99%), high to excellent diastereoselectivities (up to >25:1 dr), and high to excellent enantioselectivities (up to 99% ee).
Site‐selectivity, differentiating members of the same functional group type on one substrate, is an emerging tactic for shortened advanced building block and biomolecule synthesis. Despite its potential, site‐selectivity remains less studied and especially so for ketone‐based substrates. During this work ketone site‐selectivity has been coupled with the chiral amine‐catalyzed aldol desymmetrization of 4‐keto‐substituted cyclohexanones, allowing three stereogenic centers to form in the aldol product while leaving the acyclic ketone unreacted. Unique here, compared to all previous 4‐substituted cyclohexanone desymmetrizations, is the first access to synthetically useful quantities of an epimeric (remote stereogenic center) aldol product. To demonstrate the value of these aldol products, we show their elaboration into eight keto‐acetonide and one keto‐lactone products. All compounds were isolated as single diastereomers and in high ee (≥96%). These efforts represent the first full characterization of aldol products with type III, Figure 2, relative stereochemistry, regardless of the enantiomer formed.
下载免费PDF全文