Dynamic Kinetic Spiroketalization/Oxa-Michael Addition Cascade of Alkoxyboronates and Peroxyacetals: Enantio- and Diastereoselective Synthesis of Benzannulated Spiroketals

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.

An Organocatalytic Highly Enantioselective Stereospecific Synthesis of 1,1-Disubstituted-1,3-Dihydroisobenzofurans

Herein, we disclosed the asymmetric construction of an oxa-quaternary stereocenter via an intramolecular oxa-Michael (IOM) reaction in β-substituted ortho-hydroxymethyl chalcone by the formation of 1,1-disubstituted-1,3-dihydroisobenzofuran using cinchona alkaloid-based chiral amino-squaramide catalyst. Both the (E- and Z)-β-substituted ortho-hydroxymethyl chalcone provide (S)- and (R)-enantiomers of the 1,1-disubstituted-1,3-dihydroisobenzofuran with excellent stereospecificity. In general, excellent yields (up to 95 %) and enantioselectivity (up to 98 % ee) were obtained. Furthermore, the resulting 1,1-disubstituted isobenzofuran or phthalan was converted to corresponding chiral 3,3-disubstituted phthalides without losing the enantioselectivity. This methodology provides the core moiety of the (S)-citalopram drug.

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.

Unravelling the Development of Non-Covalent Organocatalysis in India

This review is devoted to underpinning the contributions of Indian researchers towards asymmetric organocatalysis. More specifically, a comprehensive compilation of reactions mediated by a wide range of non-covalent catalysis is illustrated. A detailed overview of vividly catalogued asymmetric organic transformations promoted by hydrogen bonding and Brønsted acid catalysis, alongside an assortment of catalysts is provided. Although asymmetric organocatalysis has etched itself in history, we aim to showcase the scientific metamorphosis of Indian research from baby steps to large strides within this field.

1 Introduction

2 Non-Covalent Catalysis and Its Various Activation Modes

3 Hydrogen-Bonding Catalysis

3.1 Urea- and Thiourea-Derived Organocatalysts

3.1.1 Thiourea-Derived Organocatalysts

3.1.2 Urea-Derived Organocatalysts

3.2 Squaramide-Derived Organocatalysts

3.2.1 Michael Reactions

3.2.2 C-Alkylation Reactions

3.2.3 Mannich Reactions

3.2.4 [3+2] Cycloaddition Reactions

3.3 Cinchona-Alkaloid-Derived Organocatalysts

3.3.1 Michael Reactions

3.3.2 Aldol Reactions

3.3.3 Friedel–Crafts Reactions

3.3.4 Vinylogous Alkylation of 4-Methylcoumarins

3.3.5 C-Sulfenylation Reactions

3.3.6 Peroxyhemiacetalisation of Isochromans

3.3.7 Diels–Alder Reactions

3.3.8 Cycloaddition Reactions

3.3.9 Morita–Baylis–Hilman Reactions

4 Brønsted Acid Derived Organocatalysts

4.1 Chiral Phosphoric Acid Catalysis

4.1.1 Diels–Alder Reactions

4.1.2 Addition of Ketimines

4.1.3 Annulation of Acyclic Enecarbamates

5 Conclusion

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

Herein is disclosed an efficient enantio- and diastereoselective spiroketalization of aromatic ketone tethered to ortho-homoformyl and enone moiety via in situ enol formation using quinine derived squaramide organocatalyst to access aromatic [6,5] spiroketals with complete atom economy. Furthermore, aromatic spiroketals undergo Brønsted acid catalyzed Piancatelli type rearrangement to provide dihydronaphtho[1,2-b]furans with retention of the enantioselectivities.

Copper-catalyzed enantioselective synthesis of bridged bicyclic ketals from 1,1-disubstituted-4-methylene-1,6-hexanediols and related alkenols

Bridged bicyclic ketals display a range of bioactivities. Their catalytic enantioselective synthesis from acyclic 1,1-disubstituted alkene diols is disclosed. This reaction combines asymmetric catalysis with a distal radical migration. Alkynes and arenes undergo the group transfer.

Enantioselective, Organocatalytic, Dissymmetric 1,4- and 1,2-Addition of Malononitrile to a Keto-bisenone Followed by an Oxa-Michael Addition Cascade

An unprecedented enantioselective dissymmetric 1,4- and 1,2-addition of malononitrile to a keto-bisenone followed by an oxa-Michael addition cascade to trap the in situ generated unstable tertiary alcohol have been developed. The quinine-derived amino-squaramide bifunctional organocatalyst worked efficiently and provides the oxa-spiro-[4,4]-nonanes in good yields and excellent diastereo- and enantioselectivities (up to 99:1 dr and 99% ee). Notably, a complete chemoselective addition of a methylene unit to an aliphatic-tethered enone over the aromatic-tethered enone was observed.

Enantioselective Synthesis of Cyclohexadienone Containing Spiroketals via DyKat Ketalization/oxa-Michael Addition Cascade

An oxidative dearomatization of phenol followed by a dynamic kinetic (DyKat) ketalization/oxa-Michael addition cascade using cinchona alkaloid-based chiral bifunctional amino-squaramide catalysts is reported. A broad array of sterically hindered [5,5]-spiroketals attached to a cyclohexadienone moiety in spiro-fashion is synthesized in an enantiopure form. Further, the methodology was optimized and extended to the corresponding benzannulated [5,5]-spiroketals attached to a cyclohexadienone moiety in spiro-fashion. In general, good yields and excellent diastereoselectivies and enantioselectivities (up to 20:1 dr and up to 99% ee) were obtained.

Organocatalytic Asymmetric Synthesis of Bridged Acetals with Spirooxindole Skeleton

The first highly diastereo- and enantioselective synthesis of bridged O, O-acetals embedded with spirooxindoles has been developed. Dioxindoles and 2-hydroxy cinnamaldehydes were employed as the reaction partners in this method. The desired products were obtained via diaryl prolinol TBS ether catalyzed Michael reaction followed by acetal formation with TFA.

Organocatalytic, Chemoselective Hydrophosphenylation/oxa-Michael Addition Cascade toward Diastereo- and Enantioenriched 1,3-Dihydroisobenzofuryl Phosphonates

An efficient method for the construction of chiral C-P bond via an enantioselective 1,2-hydrophosphenylation followed by an oxa-Michael addition cascade of ortho-formyl chalcones has been developed. This provides the diastereoenriched ( cis)-1,3-dihydroisobenzofuryl phosphonates with excellent enantioselectivities (up to >99%). The origin of enantio- and diastereoselectivity is induced by using a chiral bifunctional organocatalyst. Further, functionalization to highly enantioselective 3-substituted phthalides has also been demonstrated.

Primary Aminothiourea-Catalyzed Enantioselective Synthesis of Rauhut-Currier Adducts of 3-Arylcyclohexenone with a Tethered Enone on the Aryl Moiety at the Ortho-Position

An enantioselective synthesis of Rauhut-Currier (RC) adducts from 3-aryl cyclohexenone with a tethered enone moiety at the ortho-position on the aryl group is accomplished. This method provides a wide range of valuable synthetic building blocks having a unique [6-5-6] all-carbon-fused tricyclic skeleton. A primary amine-containing thiourea, a bifunctional organocatalyst, was found to be an efficient catalyst for this transformation. The primary amine counterpart of the catalyst possibly activates the aliphatic enone via dienamine formation (HOMO activation), whereas the thiourea counterpart activates the tethered enone (LUMO activation). Considering the difficulty in achieving an RC reaction of β,β-disubstituted (alkyl and aryl) enones, this method would be significantly rewarding.

Synthesis of benzannulated spiroketals with gold-catalyzed cycloisomerization/spiroketalization cascade

An unprecedented gold-catalyzed cascade reaction of alkynyl alcohol and quinone monoimines to diverse 5,5-benzannulated spiroketals has been successfully developed.

Diastereoselective synthesis ofcis-1,3-disubstituted isoindolinesviaa highly site-selective tandem cyclization reaction

A highly site-selective tandem reaction involving regioselective ring opening of aziridines and Michael addition of electron-deficient alkenes has been described.