Insights on the Pseudo-Enantiomeric Properties of Bifunctional Cinchona Alkaloid Squaramide-Derived Organocatalyst

The use of pseudo enantiomers is a well-known method of achieving products of complementary stereochemistry. Only rarely can different enantiomers of a product be accessed without modulation of the catalyst. Recently, a system was reported wherein two different enantiomers of spirocycles were obtained by a cascade reaction of unsaturated pyrrolin-4-ones with mercaptoacetaldehyde catalyzed by a single optimized cinchona alkaloid squaramide-derived organocatalyst. It was originally proposed that the E/Z geometry of the unsaturated pyrrolin-4-one dictated the stereochemistry of the spirocycle product, but this was not investigated further. In the present work, we have investigated the nature of a pseudo-enantiomeric organocatalyst conformation applying density functional theory calculations for investigating the transition states for the reaction. Furthermore, the influence of the double-bond geometry of the pyrrolin-4-one has been studied beyond what is possible to test experimentally. The results provide a greater understanding for this class of reactions that may be applicable in future methodology development.     

Organocatalytic Asymmetric Addition of Naphthols and Electron‐Rich Phenols to Isatin‐Derived Ketimines: Highly Enantioselective Construction of Tetrasubstituted Stereocenters

A quinine‐derived thiourea organocatalyst promoted the highly enantioselective addition of naphthols and activated phenols to ketimines derived from isatins. The reaction afforded chiral 3‐amino‐2‐oxindoles with a quaternary stereocenter in high yields (up to 99 %) with excellent enantioselectivity (up to 99 % ee). To the best of our knowledge, this transformation is the first highly enantioselective addition of naphthols to ketimines.     

金鸡钠碱催化烯胺的不对称C-亚磺酰化反应

以金鸡钠碱催化烯胺的不对称C-亚磺酰化反应,产物(E,Z)-N-苄氧羰基-2-苯亚磺酰基-1-苯基丙烯胺[(E,Z)-4],收率71%,立体选择性E:Z=3:2,其中(E)-4和(Z)-4的对映选择性分别为75%ee和93%ee。     

Using Natural Cinchona Alkaloids to Promote the Enantioselective Addition of Dialkylzinc to N－Diphenylphosphinylimines

Cinchona alkaloids are utilized as chiral ligands to promote the enantioselective addition of dialkylzinc to N-diphenyiphosphinylirnlnes affording enantiomerically enriched N-diphenyiphosphinylamines in up to 91% ee.     

Bioinspired Total Synthesis of the Dimeric Indole Alkaloid (+)‐Haplophytine by Direct Coupling and Late‐Stage Oxidative Rearrangement

A bioinspired convergent total synthesis of (+)‐haplophytine, a dimeric indole alkaloid with diazabicyclo[3.3.1]nonane and hexacyclic aspidosperma segments, is described. This synthesis involves the direct coupling of the two segments in a AgNTf₂‐mediated Friedel–Crafts reaction and construction of the diazabicyclo[3.3.1]nonane skeleton through late‐stage chemoselective aerobic oxidation of the 1,2‐diaminoethene moiety and a sequential semipinacol‐type rearrangement.     

Total Synthesis of (−)‐Platensimycin by Advancing Oxocarbenium‐ and Iminium‐Mediated Catalytic Methods

(?)‐Platensimycin is a potent inhibitor of fatty acid synthase that holds promise in the treatment of metabolic disorders (e.g., diabetes and “fatty liver”) and pathogenic infections (e.g., those caused by drug‐resistant bacteria). Herein, we describe its total synthesis through a four‐step preparation of the aromatic amine fragment and an improved stereocontrolled assembly of the ketolide fragment, (?)‐platensic acid. Key synthetic advances include 1) a modified Lieben haloform reaction to directly convert an aryl methyl ketone into its methyl ester within 30 seconds, 2) an experimentally improved dialkylation protocol to form platensic acid, 3) a sterically controlled chemo‐ and diastereoselective organocatalytic conjugate reduction of a spiro‐cyclized cyclohexadienone by using the trifluoroacetic acid salt of α‐amino di‐tert‐butyl malonate, 4) a tetrabutylammonium fluoride promoted spiro‐alkylative para dearomatization of a free phenol to assemble the cagelike ketolide core with the moderate leaving‐group ability of an early tosylate intermediate, and 5) a bismuth(III)‐catalyzed Friedel–Crafts cyclization of a free lactol, with LiClO₄ as an additive to liberate a more active oxocarbenium perchlorate species and suppress the Lewis basicity of the sulfonyloxy group. The longest linear sequence is 21 steps with an overall yield of 3.8 % from commercially available eugenol.     

Asymmetric Cyclization Reactions of Allenoates with Imines or α,β‐Unsaturated Ketones Catalyzed by Organocatalysts Derived from Cinchona Alkaloids

Lewis base‐catalyzed cyclization reactions of allenoates with electron‐deficient olefins and imines have been demonstrated by the preparation of biologically active natural products and pharmaceutically interesting substances and have emerged as powerful synthetic tools in the rapid construction of cyclic molecular complexity. In contrast to phosphine‐containing Lewis bases, nitrogen‐containing Lewis base amines display markedly different reaction profiles; however, this area is not well‐developed. Herein we summarize the recent progress in this emerging field and outline the challenges ahead.     

Direct Asymmetric Mannich‐Type Reaction of α‐Isocyanoacetates with Ketimines using Cinchona Alkaloid/Copper(II) Catalysts

The enantioselective direct Mannich‐type reaction of ketimines with α‐isocyanoacetates has been developed. Excellent yields and enantioselectivity were observed for the reaction of various ketimines and α‐isocyanoacetates using cinchona alkaloid/Cu(OTf)₂ and a base. Both enantiomers of the products could be obtained by using pseudoenantiomeric chiral catalysts. This process offers an efficient route for the synthesis of α,β‐diamino acids.     

Direct Enantioselective Vinylogous Mannich Reaction of Ketimines with γ‐Butenolide by Using Cinchona Alkaloid Amide/Zinc(II) Catalysts

A direct enantioselective vinylogous Mannich reaction of ketimines with γ‐butenolide has been developed. Good yields and enantioselectivities were observed for the reaction of various ketimines by using a cinchona alkaloid amide/Zn(OTf)₂ catalyst and Et₃N. Both enantiomers of the products could be obtained by using pseudoenantiomeric chiral catalysts.     

Diastereodivergent Asymmetric 1,4‐Addition of Oxindoles to Nitroolefins by Using Polyfunctional Nickel‐Hydrogen‐Bond‐Azolium Catalysts

Diastereodivergency is a challenge for catalytic asymmetric synthesis. For many reaction types, the generation of one diastereomer is inherently preferred, while the other diastereomers are not directly accessible with high efficiency and require circuitous synthetic approaches. Overwriting the inherent preference by means of a catalyst requires control over the spatial positions of both reaction partners. We report a novel polyfunctional catalyst type in which a Ni^II‐bis(phenoxyimine) unit, free hydroxy groups, and an axially chiral bisimidazolium entity participate in the stereocontrol of the direct 1,4‐addition of oxindoles to nitroolefins. Both epimers of the 1,4‐adduct are accessible in excess on demand by changes to the ligand constitution and configuration. As the products have been reported to be valuable precursors to indole alkaloids, this method should allow access to their epimeric derivatives.     

Linking Conformational Flexibility and Kinetics: Catalytic 1,4‐Type Friedel–Crafts Reactions of Phenols Utilizing 1,3‐Diamine‐Tethered Guanidine/Bisthiourea Organocatalysts

Herein, we present details of our conformationally flexible, 1,3‐diamine‐tethered guanidine/bisthiourea organocatalysts for chemo‐, regio‐, and enantioselective 1,4‐type Friedel–Crafts reactions of phenols. These organocatalysts show a unique stereo‐discrimination governed by the differential activation entropy (ΔΔS^≠), rather than by the differential activation enthalpy (ΔΔH^≠). Extensive kinetic analyses using Eyring plots for a series of guanidine/bisthiourea organocatalysts revealed the key structural motif in the catalysts associated with a large magnitude of differential activation entropy (ΔΔS^≠). A plausible guanidine–thiourea cooperative mechanism for the enantioselective Friedel–Crafts reaction is proposed.     

Carbocations as Lewis Acid Catalysts in Diels–Alder and Michael Addition Reactions

In general, Lewis acid catalysts are metal‐based compounds that owe their reactivity to a low‐lying empty orbital. However, one potential Lewis acid that has received negligible attention as a catalyst is the carbocation. We have demonstrated the potential of the carbocation as a highly powerful Lewis acid catalyst for organic reactions. The stable and easily available triphenylmethyl (trityl) cation was found to be a highly efficient catalyst for the Diels–Alder reaction for a range of substrates. Catalyst loadings as low as 500 ppm, excellent yields, and good endo/exo selectivities were achieved. Furthermore, by changing the electronic properties of the substituents on the tritylium ion, the Lewis acidity of the catalyst could be tuned to control the outcome of the reaction. The ability of this carbocation as a Lewis acid catalyst was also further extended to the Michael reaction.