Chiral Squaramide Catalyzed Asymmetric Spiroketalization toward Aromatic [6,5] Spiroketals

Intramolecular Cascade Cyclization of Cyclobutanone: Asymmetric Construction of Cyclobutanone Fused Oxa-Spirocycles

The successful implementation of a cascade reaction involving a cyclobutyl unit has posed a significant challenge in achieving ring-retentive functionalization because of the ring's sacrificial tendency. Herein, we have accomplished a cinchona-derived squaramide-catalyzed cascade reaction sequence, encompassing the desymmetrization of cyclobutanone, followed by an aldol reaction and, subsequently, a 1,4-addition step. This overall process offers a viable strategy to access architecturally fascinating oxa-spirocycles fused with cyclobutanone motifs in good yields with high optical purity.

Regio-, Enantio-Controlled Cascade Oxa-Michael Addition of Hydroxyl Amine: Synthesis of Dihydro-1,2-Oxazines

Herein, we disclose a cinchona-alkaloid-based squaramide-catalyzed intermolecular oxa-Michael cascade addition cyclization of N-protected hydroxyl amine to formyl-tethered Michael acceptors toward the synthesis of enantioenriched 1,2-oxazine scaffolds. Generally, good yield (up to 78%), good to excellent enantiomeric ratio (up to 98:2 er), and excellent regioselectivity (>19:1) were observed. Furthermore, the scalability of this methodology and a successful demonstration of postsynthetic transformations have been accomplished.

Organocatalytic Desymmetric Spirocyclization of Enone-Tethered 2,5-Cyclohexadienone with Isoxazolines to Access Enantioenriched Spiro-Fused Scaffolds

Enantioselective desymmetric spirocyclization is a powerful tool for concisely generating intricate 3D molecular architectures from readily accessible substrates. Herein, we report an organocatalytic desymmetric spirocyclization of enone-tethered 2,5-cyclohexadienone with 3-arylisoxazol-5(4H)-one to access enantioenriched spiro-fused scaffolds featuring four consecutive stereogenic centers in high yields and excellent diastereo- and enantioselectivities. This protocol could proceed smoothly under parts per million-level organocatalyst loading. Furthermore, the reusability of the catalyst and various intriguing derivatizations of the title product highlight its synthetic utility.

Intramolecular Silanoxy-Michael Reactions with Pendant Nitroalkenes: Racemic and Enantioselective

We present the first racemic and scalemic examples of di-tert-butyl silanoxy-Michael additions. Our operationally simple protocol is selective for nitro-olefins and simply involves stirring the substrate with an appropriate hydrogen-bond donor catalyst without any special precautions to exclude air or moisture. For each substrate examined, we have developed complementary protocols that optimize yield and enantioselectivity. Our reactions scale well, and the products are valuable intermediates for further transformations, including for the preparation of enantioenriched vicinal amino alcohols.

Organocatalytic Desymmetric Double Aza-Michael Addition Cascade: Enantioselective Synthesis of Fused Morpholines

Double aza-Michael addition (DAM) has become an emerging strategy for the construction of two carbon-nitrogen bonds in a single step, which can significantly simplify the synthesis of N-heterocycles. Hitherto, their asymmetric catalytic genre remains unattempted. Herein, we describe the judicious design of an organocatalytic enantioselective desymmetric double aza-Michael addition cascade to access a series of functionalized fused morpholines with excellent yields and diastereo- and enantioselectivities. A one-pot telescopic synthesis was demonstrated for a bridged triheterocyclic compound. In addition, scale-up synthesis and various attractive postsynthetic modifications of the title products amplify the significance of the current methodology.

Brønsted acid catalyzed [4 + 2] cycloaddition for the synthesis of bisbenzannulated spiroketals with antifungal activities

The intermolecular [4 + 2] cycloaddition of o-hydroxy benzyl alcohols with isochroman ketals was realized by CF3CO2H catalysis. A broad range of bisbenzannulated [6,6]-spiroketals were formed under the metal-free mild conditions in moderate to excellent yields (45-98%) with mostly excellent diastereoselectivities (up to >20 : 1 dr). Furthermore, the enantioselective version was also preliminarily investigated and the bisbenzannulated [6,6]-spiroketal was obtained with 61% ee in the presence of Sc(OTf)3/Feng's chiral N,N'-dioxide ligand. Some of the bisbenzannulated [6,6]-spiroketal products showed good in vitro antifungal activities against Sclerotinia sclerotiorum and Rhizoctonia solani.

Stereodivergent Synthesis of Spiroaminals via Chiral Bifunctional Hydrogen Bonding Organocatalysis

Spiroaminals represent novel structural motifs prevalent in diverse natural products and biologically active molecules. Achieving their enantioselective synthesis is a highly desirable and challenging task in synthetic endeavors due to their intricate molecular frameworks. Herein, we accomplished the first stereodivergent construction of spiroaminals using chiral bifunctional organocatalyzed intramolecular 1,2-addition followed by an oxa-Michael addition cascade in a high atom and step economical pathway. A proper modulation of the cinchona-derived squaramide catalysts efficiently provided access to all the possible stereoisomers with high yield, diastereoselectivity, and excellent enantioselectivity while displaying a broad substrate tolerance. Additionally, we validated the scalability of the reaction and demonstrated the synthesis of variable spiroaminal scaffolds, confirming the viability of our protocol.

Organocatalytic Enantioselective Intramolecular Michael Addition by In Situ Generated Aminoisobenzofulvenes: Construction of Spiro Quaternary Carbon Stereocenters

An unprecedented enantioselective organocatalytic spirocyclization strategy is presented by in situ generation of aminoisobezofulvenes. The reaction sequence involves a reductive Michael/aldol-condensation/Michael addition cascade by iminium-enamine catalysis. The key success of this spirocyclization was the formation of intermediatory nucleophilic aminoisobenzofuvenes accountable for intramolecular Michael addition. Benzospirononanes featuring an all carbon qauternary spirocenter were obtained using proline-derived amino-organocatalyst in moderate to good yields and excellent diastereo- and enantioselectivities (up to >20 : 1 dr, and 99 % ee). Post-methodological manipulation of benzospirononanes was also demonstrated.

An Intramolecular Umpolung Cascade Kukhtin-Ramirez Reaction/Michael Addition-Initiated Cyclization: Stereoselective Synthesis of Tetrasubstituted Cyclopropane Fused 1-Indanones

Herein, we disclose a fascinating highly stereoselective P(NMe₂)₃ mediated intramolecular deoxygenative umpolung cascade Michael addition-initiated cyclopropanation with a diverse substrate adaptability. This methodology creates a new horizon for expedient access to valuable 6,5,3-fused scaffolds having an all-carbon quaternary stereocenter via Kukhtin-Ramirez (K-R) adduct formation, with excellent diastereoselectivity and yields under metal-free ambient conditions. A few functional group transformations have also been performed successfully. Additionally, an asymmetric catalytic attempt using (R)-(+)-H8-BINOL has delivered good enantioselectivity.

Enzyme-Catalyzed Dynamic Kinetic Resolution of 2-Formylbenzoic Acids for the Asymmetric Synthesis of Phthalidyl Esters and Related Prodrugs

An enzyme-catalyzed dynamic kinetic resolution strategy was applied for the asymmetric synthesis of phthalidyl esters in high yields (up to 95%) and enantiomeric purities (up to 99% ee) through a direct one-pot procedure. Preparation of phthalidyl ester prodrugs and a scale-up reaction demonstrated the potential of this method for practical applications.

Ir-Catalyzed Asymmetric Cascade Allylation/Spiroketalization Reaction for Stereoselective Synthesis of Oxazoline-Spiroketals

An asymmetric cascade allylation/spiroketalization reaction between 2-(1-hydroxyallyl)phenols and 5-methyleneoxazolines is accomplished by using a chiral Ir(I) catalyst derived from commercially available iridium precursor and the Carreira ligand. This protocol furnishes a class of structurally novel and unique oxazoline-spiroketals in up to 86% yield, >99% ee and >20:1 dr. Moreover, control experiments reveal that 4,4-disubstitution on 5-methyleneoxazolines is necessary to avoid the aromatization and for the spiroketalization to occur. On the basis of this, a plausible reaction mechanism is illustrated.