An efficient dehydrogenative Diels–Alder (DHDA) reaction of prenyl derivatives with 2,3‐dichloro‐5,6‐dicyanobenzoquinone (DDQ) has been developed under mild conditions, leading to a series of cyclohexene derivatives with good to excellent yields and excellent diastereoselectivity.
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.
The formation of chiral γ‐butenolides has been achieved with good yields (up to 90%), high enantioselectivity (up to 91%) and diastereoselectivity (up to 9/1, anti‐selective) through an organocatalyzed vinylogous Mukaiyama aldol reaction of 2‐(trimethylsilyloxy)furan and aldehydes. A wide range of chiral γ‐butenolides was obtained under mild conditions by this methodology. 相似文献
Polar meso‐tetraarylporphyrins 2 – 4 were synthesized from tetrakis‐4‐hydroxyphenylporphyrin 1 as the central building block by consecutive base‐induced reactions with glycidol. The decorating units form a polar hydrogen‐bonded shell around the sensitizer core which is proposed as the binding site for polar substrates in photocatalyzed oxygenation reactions. As substrate, the polarity‐sensor mesitylol ( 5 ) was applied and the reaction constrained in a polystyrene matrix. Increasing shell dimensions lead to increased diastereoselectivities for the allylic hydroperoxides 6 and thus clearly demonstrate the concept of shell‐induced substrate stereoselectivity in singlet oxygen reactions. 相似文献
The first catalytic asymmetric 1,3‐dipolar cycloaddition of azomethine ylides with various sterically hindered α,α,β‐trisubstituted 2‐alkylidene‐cycloketones has been developed successfully with silver acetate/TF‐BiphamPhos complex for the construction of spiro heterocyclic compounds containing pyrrolidine motifs and a spiro quaternary stereogenic carbon center. The highly efficient catalytic system exhibited high reactivity, excellent diastereoselectivity, good enantioselectivity and broad substrate scope under mild conditions. Subsequent transformations led to the expedient preparation of synthetically useful spiro[pyrrolidine‐tetrahydropyranone] and spiro[pyrrolidine‐isochroman‐1‐one] without loss of the diastereo‐ and enantiomeric excesses. 相似文献
Enzymes often convert both physiological and non-physiological substrates with high stereoselectivity; yet, for some enzymes, opposite product chirality is observed. A possible explanation is the existence of hidden specificities becoming apparent when non-physiological substrates confer different substrate–enzyme interactions than the physiological substrate. To test this hypothesis, a series of α-methylated β-keto esters were converted with Tyl-KR1, a ketoreductase from polyketide synthesis in Streptomyces fradiae. The conversions of six substrates with different physicochemical properties exhibited enantioselectivities ranging from 84 % ee for R,R to 84 % ee for S,S, yet high and uniform diastereoselectivity (anti, d.r.>9:1). The exchange of a single atom, namely an oxygen ester instead of a thioester, led to almost complete loss of enantioselectivity (<5 % ee). An additional S,S-selective binding mode as a hidden specificity in Tyl-KR1 has been identified through molecular modeling and site-directed mutagenesis. 相似文献
A unique one‐pot synthesis of cyclic cis‐1,2‐amino alcohols from trans‐1,2‐azido alcohol precursors was developed. The key step is highlighted by the stereoselective reduction of the cyclic α‐alkoxy imines, which could be prepared from the corresponding azides by ruthenium catalysis under photolytic conditions. Remarkably, this unprecedented reaction pathway offers a stereodivergent access to structurally diverse cyclic 1,2‐amino alcohols.
The first catalytic endo‐selective 1,3‐dipolar cycloaddition of azomethine ylides and vinyl phenyl sulfone has been developed successfully. The highly efficient silver acetate (AgOAc)/TF‐BiphamPhos catalytic system exhibited high reactivity, excellent diastereoselectivity (>98:2), good enantioselectivity (67–92% ee) and broad substrate scope under mild conditions. 相似文献
A stereochemically promiscuous 2‐keto‐3‐deoxygluconate aldolase has been used as an efficient biocatalyst to catalyse the aldol reaction of pyruvate with C3‐ and C4‐aldoses to afford syn‐ and anti‐3‐deoxy‐2‐ulosonic acids in poor to good de. A continuous flow bioreactor containing immobilised aldolase has been developed that enables gram quantities of C6‐ and C7‐3‐deoxyhept‐2‐ulosonic acids to be produced in an efficient manner. 相似文献