Catalytic Asymmetric Inverse-Electron-Demand Oxa-Diels-Alder Reaction of In Situ Generatedortho-Quinone Methides with 3-Methyl-2-Vinylindoles

Catalytic Asymmetric Synthesis of Inherently Chiral Eight-Membered O-Heterocycles through Cross-[4+4] Cycloaddition of Quinone Methides

Inherently chiral eight-membered rings embedded in tetraphenylene derivatives and hetero-analogues exhibit unique properties and allow diverse applications. A conceptually viable and straightforward approach to these frameworks is [4+4] cycloaddition, which still remains elusive. Herein, we describe the stereoselective cross-[4+4] cycloaddition of quinone methides (QMs), leading to the formation of oxa-analogues of tetraphenylene with exceptional chemo-, diastereo-, and enantioselectivity. The structures of these novel rigid eight-membered O-heterocycles were explored by single-crystal X-ray diffraction, and their stereochemical stability was elaborated through both density functional theory (DFT) calculations and thermal racemization experiments. The developed methodology exhibited broad substrate scope and the resulting cross-[4+4] cycloadducts could be readily transformed into valuable chiral building blocks. Our findings expand the library of inherently chiral medium-sized rings and also contribute to the advancement of asymmetric cross-[4+4] cycloadditions of quinone methides.

Catalytic, Enantioselective Cycloaddition of Pyrrole-2-methides with Aldehydes toward a Synthesis of 2,3-Dihydro-1H-pyrrolizin-3-ols

An organocatalytic, highly enantioselective [6 + 2]-cycloaddition of 2-methide-2H-pyrroles with aryl acetaldehydes represents a novel and straightforward route toward densely substituted 2,3-dihydro-1H-pyrrolizin-3-ols, which were generated with good yields and high enantio- and diastereoselectivity. This one-step process involves a BINOL-phosphoric acid catalyzed reaction of 1H-pyrrole-2-carbinols with aryl acetaldehydes via the corresponding hydrogen-bonded, chiral 2-methide-2H-pyrroles.

Catalytic Enantioselective [4+2] Cycloadditions of Salicylaldehyde Acetals with Enol Ethers

A readily accessible conjugate-base-stabilized carboxylic acid (CBSCA) catalyst facilitates highly enantioselective [4+2] cycloaddition reactions of salicylaldehyde-derived acetals and cyclic enol ethers, resulting in the formation of polycyclic chromanes with oxygenation in the 2- and 4-positions. Stereochemically more complex products can be obtained from racemic enol ethers. Spirocyclic products are also accessible.

Synthesis of Chiral Diarylmethylamides via Catalytic Asymmetric Aza-Michael Addition of Amides to ortho-Quinomethanes

Chiral diarylmethylamides are a privileged skeleton in many bioactive molecules. However, the enantioselective synthesis of such molecules remains a long-standing challenge in organic synthesis. Herein, we report a chiral bifunctional squaramide catalyzed asymmetric aza-Michael addition of amides to in situ generated ortho-quinomethanes, affording enantioenriched diarylmethylamides in good yields with excellent enantioselectivities. This work not only provides a new strategy for the construction of the diarylmethylamides but also represents the practicability of amides as nitrogen-nucleophiles in asymmetric organocatalysis.

Catalytic asymmetric oxa-Diels-Alder reaction of acroleins with simple alkenes

The catalytic asymmetric inverse-electron-demand oxa-Diels-Alder (IODA) reaction is a highly effective synthetic method for creating enantioenriched six-membered oxygen-containing heterocycles. Despite significant effort in this area, simple α,β-unsaturated aldehydes/ketones and nonpolarized alkenes are seldom utilized as substrates due to their low reactivity and difficulties in achieving enantiocontrol. This report describes an intermolecular asymmetric IODA reaction between α-bromoacroleins and neutral alkenes that is catalyzed by oxazaborolidinium cation 1f. The resulting dihydropyrans are produced in high yields and excellent enantioselectivities over a broad range of substrates. The use of acrolein in the IODA reaction produces 3,4-dihydropyran with an unoccupied C6 position in the ring structure. This unique feature is utilized in the efficient synthesis of (+)-Centrolobine, demonstrating the practical synthetic utility of this reaction. Additionally, the study found that 2,6-trans-tetrahydropyran can undergo efficient epimerization into 2,6-cis-tetrahydropyran under Lewis acidic conditions. This structural core is widespread in natural products.

Asymmetric Synthesis of Remotely Chiral Naphthols and Naphthylamines via Naphthoquinone Methides

Quinone methides are well-established intermediates in asymmetric synthesis. In contrast, their extended analogues with the carbonyl and methide units distributed across two different rings have not been exploited in asymmetric synthesis. Herein, we achieved the first asymmetric process involving such intermediates. Specifically, the use of suitable chiral phosphoric acids enabled in situ generation of 2-naphthoquinone 8-methides and the corresponding aza counterparts for mild one-pot asymmetric nucleophilic addition. These processes provided rapid access to a wide range of previously less accessible remotely chiral naphthols and naphthylamines with both high efficiency and excellent enantioselectivity. Control experiment and DFT calculations provided important insights into the reaction mechanism, which likely involves two phosphoric acid molecules in the enantiodetermining transition states. This work serves as a proof of concept for the exploitation of new types of extended quinone methides as versatile intermediates for asymmetric synthesis, thus providing a new platform for the efficient construction of remote benzylic stereogenic centers of aromatic compounds.

Access to Chiral Tetrahydroquinazolines/1,3-Benzoxazines via Iridium-Catalyzed Asymmetric [4 + 2] Cycloaddition

An iridium-catalyzed asymmetric [4 + 2] cycloaddition of 1,3,5-triazinanes with 2-(1-hydroxyallyl)anilines/2-(1-hydroxyallyl)phenols has been developed, providing a straightforward and efficient approach to a wide range of tetrahydroquinazolines in good yields and excellent enantioselectivities (up to >99% ee). Typically, chiral 1,3-benzoxazines, which are challenging substrates in asymmetric [4 + 2] cycloaddition, could be obtained in excellent enantioselectivities via this protocol.

Rh(III)-catalyzed (5 + 2)-cycloaddition reactions of ortho-hydroxyethyl phenols with internal alkynes: efficient synthesis of benzoxepines

An unprecedented (5 + 2)-cycloaddition reaction of ortho-hydroxyethyl phenol and internal alkyne was developed. This Rh(III)-catalyzed reaction provided benzoxepine derivatives which have very high biological significance. A wide range of ortho-hydroxyethyl phenols and internal alkynes were studied to provide the benzoxepines in high yields.

Catalytic Electrophilic Halogenation of Arenes with Electron-Withdrawing Substituents

The electrophilic halogenation of arenes is perhaps the simplest method to prepare aryl halides, which are important structural motifs in agrochemicals, materials, and pharmaceuticals. However, the nucleophilicity of arenes is weakened by the electron-withdrawing substituents, whose electrophilic halogenation reactions usually require harsh conditions and lead to limited substrate scopes and applications. Therefore, the halogenation of arenes containing electron-withdrawing groups (EWGs) and complex bioactive compounds under mild conditions has been a long-standing challenge. Herein, we describe Brønsted acid-catalyzed halogenation of arenes with electron-withdrawing substituents under mild conditions, providing an efficient protocol for aryl halides. The hydrogen bonding of Brønsted acid with the protic solvent 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) enables this transformation and thus solves this long-standing problem.

Selective radical cascade (4+2) annulation with olefins towards the synthesis of chroman derivatives via organo-photoredox catalysis

Due to the importance of chroman frameworks in medicinal chemistry, the development of novel synthetic methods for these structures is gaining increasing interest of chemists. Reported here is a new (4 + 2) radical annulation approach for the construction of these functional six-membered frameworks via photocatalysis. Featuring mild reaction conditions, the protocol allows readily available N-hydroxyphthalimide esters and electron-deficient olefins to be converted into a wide range of valuable chromans in a highly selective manner. Moreover, the present strategy can be used in the late-stage functionalization of natural product derivatives and biologically active compounds, which demonstrated the potential application. This method is complementary to the traditional Diels–Alder [4 + 2] cycloaddition reaction of ortho-quinone methides and electron-rich dienophiles, since electron-deficient dienophiles were smoothly transformed into the desired chromans.

We have developed a (4 + 2) radical annulation approach for the synthesis of diverse chromans. This method is complementary to the traditional Diels–Alder [4 + 2] annulation of ortho-quinone methides and electron-rich dienophiles.

Copper Catalyzed Inverse Electron Demand [4+2] Cycloaddition for the Synthesis of Oxazines

A copper catalyzed tandem CuAAC/ring cleavage/[4+2] annulation reaction of terminal ynones, sulfonyl azides, and imines has been developed to synthesize the functionalized oxazines under mild conditions. Particularly, the intermediate N-sulfonyl acylketenimines undergo cycloaddition of an inverse electron demand Diels–Alder reaction with imines and a series of 1,3-oxazine derivatives were obtained successfully in good yields.

Stereoselective Synthesis of Atropisomeric Acridinium Salts by the Catalyst-Controlled Cyclization of ortho-Quinone Methide Iminiums

Quinone methides are fundamental intermediates for a wide range of reactions in which catalyst stereocontrol is often achieved by hydrogen bonding. Herein, we describe the feasibility of an intramolecular Friedel-Crafts 6π electrocyclization through ortho-quinone methide iminiums stereocontrolled by a contact ion pair. A disulfonimide catalyst activates racemic trichloroacetimidate substrates and imparts stereocontrol in the cyclization step, providing a new avenue for selective ortho-quinone methide iminium functionalization. A highly stereospecific oxidation readily transforms the enantioenriched acridanes into rotationally restricted acridiniums. Upon ion exchange, the method selectively affords atropisomeric acridinium tetrafluoroborate salts in high yields and an enantioenrichment of up to 93 : 7 e.r. We envision that ion-pairing catalysis over ortho-quinone methide iminiums enables the selective synthesis of a diversity of heterocycles and aniline derivatives with distinct stereogenic units.

Organocatalytic Diastereodivergent Enantioselective Formal oxa-Diels-Alder Reaction of Unsaturated Ketones with Enoates Under Liquid-Assisted Grinding Conditions

Chiral heterocycles occur in many compounds of interest, but their efficient synthesis is challenging. This study concerns the enantioselective and diastereoselective synthesis of densely substituted chiral pyran derivatives. Diastereodivergence of the oxa-Diels-Alder reaction is achieved by using either a bifunctional amino-thiourea or a monofunctional quinine organocatalyst under ball-milling conditions. Liquid-assisted grinding proves a highly efficient means of affording pyrans in high yield, with high enantiomeric purities and short reaction times.

Catalytic Diastereoselective Hetero-Diels-Alder Reaction of α-Haloacroleins with Alkenes: Construction of 3,4-Dihydropyran

In this Letter, a catalytic diastereoselective hetero-Diels-Alder reaction of α-haloacroleins with less polarized alkenes was developed, and the resulting 3,4-dihydropyrans were produced in high yields with a broad substate scope. Mechanism studies showed that 3,4-dihydropyran was produced from the ring expansion of cyclobutane, which was generated in the ring contraction of the initially formed unstable 3,4-dihydropyran conformer.

Acid-promoted formal [3 + 2] cyclization/N,O-ketalization of in situ generated ortho-alkynyl quinone methides: access to bridged 2,3-cyclopentanoindoline skeletons

An acid-catalyzed formal [3 + 2] cyclization/N,O-ketalization of in situ formed ortho-alkynyl quinone methides (o-AQMs) and tryptophol derivatives was developed.

A nickel(ii)-catalyzed enantioselective all-carbon-based inverse-electron-demand Diels–Alder reaction of 2-pyrones with indenes

An asymmetric IEDDA reaction of 2-pyrones with indenes catalyzed by a chiral N,N′-dioxide/Ni(OTf)2 complex has been disclosed to construct highly functionalized hexahydrofluorenyl bridged-lactone scaffolds in high yields and ee.

Chemistry of 2-Vinylindoles: Synthesis and Applications

As a class of compounds, 2-vinylindoles have demonstrated a wide range of biological properties. Due to the general interest in these synthons, new divergent protocols of chemical synthesis have been...

Palladium-Catalyzed [4+4] Cycloadditions for Highly Diastereo- and Enantioselective Synthesis of Functionalized Benzo[b]oxocines

Asymmetric cycloaddition reactions represent a powerful strategy for building up complex molecular architectures, especially those with medium-sized rings. Herein, we disclose a highly diastereo- and enantioselective cycloaddition strategy that involves...

Enantioselective access to tricyclic tetrahydropyran derivatives by a remote hydrogen bonding mediated intramolecular IEDHDA reaction

Inverse-electron-demand-hetero-Diels-Alder reactions of alkenes with α,β-unsaturated keto compounds allow rapid access to the tetrahydropyran ring found in numerous natural products and bioactive molecules. Despite its synthetic interest, catalytic asymmetric versions of this process remain underdeveloped, especially regarding the use of non-activated alkenes reacting with α,β-unsaturated ketone or aldehyde, for which no report can be found in the literature. Herein, we describe the catalytic inverse-electron-demand-hetero-Diels-Alder reactions between neutral alkenes and an α,β-unsaturated ketones or aldehydes to produce a variety of trans-fused [5,6,8] tricyclic structures containing a central, chiral tetrahydropyran ring. This complex transformation, which is achieved using a chiral phosphoric acid, allows for the formation of four stereogenic centers in a single step with high regio-, diastereo- and enantioselectivity (up to 99% ee). Such level of stereocontrol could be achieved by a key remote double hydrogen atom bonding interaction between the linear substrate and the catalyst.

Although the hetero-Diels–Alder reaction is a staple of organic chemistry, catalytic asymmetric versions of the inverse-electron demand variant often require specially engineered substrates for the reaction to work. Here the authors cyclize non-activated alkenes with α,β-unsaturated ketones or aldehydes to form chiral fused heterocycles using a chiral phosphoric acid catalyst.

Organocatalytic Asymmetric [2 + 4] Cycloadditions of 3-Vinylindoles with ortho-Quinone Methides

Catalytic asymmetric [2 + 4] cycloadditions of 3-vinylindoles with ortho-quinone methides and their precursors were carried out in the presence of chiral phosphoric acid to afford a series of indole-containing chroman derivatives with structural diversity in overall high yields (up to 98%), good diastereoselectivities (up to 93:7 dr) and moderate to excellent enantioselectivities (up to 98% ee). This approach not only enriches the chemistry of catalytic asymmetric cycloadditions involving 3-vinylindoles but is also useful for synthesizing chiral chroman derivatives.

Organocatalytic Asymmetric Addition of Aromatic α-Cyanoketones to o-Quinone Methides: Synthesis of 3,4-Dihydrocoumarins and Tetrasubstituted Chromans

The first organocatalytic asymmetric addition of aromatic α-cyanoketones to in situ-generated o-quinone methides has been developed. The products 3,4-dihydrocoumarin and tetrasubstituted chroman were obtained via addition of aromatic α-cyanoketones to in situ-generated o-quinone methides followed by treatment with 0.7 N HCl. With 10 mol % catalyst, the desired products were obtained in high enantio- and diastereoselectivities.

Organocatalytic asymmetric formal oxidative coupling for the construction of all-aryl quaternary stereocenters

A new catalytic asymmetric formal cross dehydrogenative coupling process for the construction of all-aryl quaternary stereocenters is disclosed, which provides access to rarely explored chiral tetraarylmethanes with excellent enantioselectivity. The suitable oxidation conditions and the hydrogen-bond-based organocatalysis have enabled efficient intermolecular C–C bond formation in an overwhelmingly crowded environment under mild conditions. para-Quinone methides bearing an ortho-directing group serve as the key intermediate. The precise loading of DDQ is critical to the high enantioselectivity. The chiral products have also been demonstrated as promising antiviral agents.

A one-pot oxidation of racemic triarylmethanes to form para-quinone methides followed by enantioselective construction of all-aryl quaternary stereocenters has been developed.

Organocatalytic enantioselective reactions involving prochiral carbocationic intermediates

Since the discovery of carbocations in 1901, the past 120 years have witnessed many marvelous advances in the chemistry of carbocations. The state-of-the-art research in this field is to overcome the intrinsic instability and high reactivity of the prochiral carbocationic intermediates to develop catalytic asymmetric reactions. Such transformations enable the facile synthesis of structurally diverse value-added products from readily available starting materials such as alkenes, alcohols, and carbonyl derivatives, and enjoy high and even perfect atom-economy in most cases. Notably, such allows catalytic stereoconvergent synthesis from racemic substrates and can realize regioselectivity in olefin functionalization reactions complementary to radical processes. With the rapid developments in modern asymmetric organocatalysis, a variety of highly enantioselective protocols evolving prochiral carbocationic intermediates have been achieved by employing three strategies, namely chiral ion-pairing, chiral nucleophile, or chiral carbenium ion strategy. This feature article aims to summarize the exciting advances in this emerging area and briefly showcase the possible mechanisms. The advantages and limitations of each strategy are presented as well as their synthetic applications in the synthesis of natural products or bioactive compounds.

Organocatalytic Strategies for the Development of the Enantioselective Inverse-electron-demand Hetero-Diels-Alder Reaction

Cycloaddition reactions, in particular Diels-Alder reactions, have attracted a lot of attention from organic chemists since they represent one of the most powerful methodologies for the construction of carbon-carbon bonds. In particular, inverse-electron-demand hetero-Diels-Alder reactions have been an important breakthrough for the synthesis of heterocyclic compounds. Among all their variants, the organocatalytic enantioselective version has been widely explored since the asymmetric construction of diversely functionalized scaffolds under reaction conditions encompassed within the green chemistry field is of great interest. In this review, a profound revision on the latest advances on the organocatalytic asymmetric inverse-electron demand hetero-Diels-Alder reaction is shown.

Application of 3-Alkyl-2-vinylindoles in Catalytic Asymmetric Dearomative (2+3) Cycloadditions

The first application of 3-alkyl-2-vinylindoles in catalytic asymmetric dearomative cycloadditions was established by chiral phosphoric acid (CPA)-catalyzed (2+3) cycloaddition with azoalkenes, leading to the generation of chiral pyrroloindolines bearing two tetrasubstituted stereogenic centers in good yields (61-96%) and excellent stereoselectivities (all >95:5 dr, 86-99% ee). This reaction has realized the first enantioselective dearomative cycloaddition of 3-alkyl-2-vinylindoles, which brings a new reactivity to this class of vinylindoles and will enrich the chemistry of 3-alkyl-2-vinylindoles. In addition, this approach has provided a useful strategy for the construction of enantioenriched pyrroloindoline skeletons bearing two tetrasubstituted stereogenic centers. More importantly, the bioassay of these chiral pyrroloindolines has revealed that some compounds exhibit strong anti-cancer activity against Hela and MCF-7 cell lines, which will be helpful for discovering anti-cancer drug candidates.

Regio- and diastereoselective synthesis of trans-3,4-diaryldihydrocoumarins via metal-free [4+2] annulation of ynamides with o-hydroxybenzyl alcohols

An efficient regio- and diastereoselective method for the construction of valuable trans-3,4-diaryldihydrocoumarins via metal-free [4+2] annulation of ynamides with o-hydroxybenzyl alcohols has been developed. Ynamides are first treated as 2-π partners to react with o-hydroxybenzyl alcohols via traceless sulfonamide directing groups, affording trans-3,4-diaryldihydrocoumarins in good yields with high regio- and diastereoselectivities. This metal-free methodology is also characterized by a wide substrate scope, good functional group tolerance, and efficiency on a gram scale.

Catalytic Asymmetric Conjugate Addition/Hydroalkoxylation Sequence: Expeditious Access to Enantioenriched Eight-Membered Cyclic Ether Derivatives

A sequential enantioselective conjugate addition/hydroalkoxylation between in situ generated ortho-quinomethanes and ynones by combining bifunctional squaramide and DBU catalysis has been developed. A variety of eight-membered cyclic ethers with two contiguous tertiary stereocenters were obtained in high yields with excellent stereoselectivities. This reaction not only provides a new strategy for constructing enantioenriched eight-membered cyclic ethers but also demonstrates the practicability of ynones as C4-syntons for the synthesis of chiral medium-membered rings.

o-Quinone Methides and o-Quinone Sulfides via Arynes: Synthesis of Ortho-Disubstituted Arenes and Heterocycles

In recent decades, o-quinone methides and o-quinone sulfides have been extensively highlighted as reactive intermediates for the synthesis of diversely functionalized ortho-disubstituted arenes and heterocycles. Additionally, ortho-disubstituted arenes offer a constructive path for the synthesis of fused carbocycles, heterocycles, natural products, and drug candidates. In the lieu of that, this Synopsis highlights a comprehensive overview on the potential applications of in situ generated o-quinone methides and o-quinone sulfides for one-pot synthesis of ortho-disubstituted arenes and heterocycles via arynes.

Cooperative Photoinduced/Brønsted Acid Catalyzed Cycloaddition of Transient Thioaldehydes and ortho-Quinone Methides toward a Synthesis of Benzo[e][1,3]oxathiines

A cooperative, one-pot approach for the in situ generation and ensuing cycloaddition of thioaldehydes and ortho-quinone methides transiently formed under irradiation with UV-A light and Brønsted acid catalysis, respectively, has been developed giving direct access to benzo[e][1,3]oxathiines in good to excellent yields and diastereoselectivity. Both electron-rich and electron-poor thioaldehydes easily react with a broad range of ortho-quinone methides at ambient temperature in a short reaction time to furnish a wide variety of S,O-heterocycles.

Design and application of intramolecular vinylogous Michael reaction for the construction of 2-alkenyl indoles

A base-mediated transformation based on a designed intramolecular vinylogous Michael addition (intra-VMA) is presented to access 3-substituted 2-alkenyl indole derivatives. The reaction represents the first example of the intra-VMA for the construction of indoles. A one-pot N-allylation of ortho-tosylamidocinnamates/congeners with γ-bromocrotonates followed by intra-VMA has been described to provide access to a diverse range of 2-alkenyl indole derivatives in reasonable to high yields. The synthetic value of the developed intra-VMA has been demonstrated by gram-scale synthesis of a representative indole derivative and also by the formal synthesis of MK-7246: a Merck's clinical CRTH2 antagonist.

Brønsted Acid-Catalysed Dehydrative Substitution Reactions of Alcohols

The direct, catalytic dehydrative substitution of alcohols is a challenging, yet highly desirable process in the development of more sustainable approaches to organic chemistry. This review outlines recent advances in Brønsted acid-catalysed dehydrative substitution reactions for C-C, C-O, C-N and C-S bond formation. The wide range of processes that are now accessible using simple alcohols as the formal electrophile are highlighted, while current limitations and therefore possible future directions for research are also discussed.

Advances in organocatalytic asymmetric reactions of vinylindoles: powerful access to enantioenriched indole derivatives

This review summarizes advances in vinylindole-based organocatalytic asymmetric reactions since 2008 and includes the applications of some methodologies in the total synthesis of natural products, points out remaining challenges in this research area.

Facile access to benzofuran-fused tetrahydropyridines via catalytic asymmetric [4 + 2] cycloaddition of aurone-derived 1-azadienes with 3-vinylindoles

A highly enantioselective [4 + 2] cycloaddition reaction of 1-azadienes with 3-vinylindoles, catalyzed by chiral phosphoric acid has been developed to furnish a range of benzofuran-fused tetrahydropyridines with three contiguous stereogenic centers.

Copper(i)-catalyzed [4 + 2] cycloaddition of aza-ortho-quinone methides with bicyclic alkenes

An efficient copper(i)-catalyzed [4 + 2] cycloaddition of aza-ortho-quinone methides (ao-QMs) and bicyclic alkenes to prepare tetrahydroquinoline-fused bicycles bearing multiple stereocenters in good yields is reported.

Hydroquinine-catalyzed asymmetric 1,4-hydrophosphination of in situ generated aza-o-quinone methides with H-phosphine oxides

An organocatalytic enantioselective 1,4-addition of H-phosphine oxides to in situ generated aza-o-quinone methides has been successfully established using hydroquinine.

A silver-catalyzed stereoselective domino cycloisomerization-vinylogous aldol reaction of ortho-alkynylbenzaldehydes with 3-alkylidene oxindoles: an entry to functionalized isochromenes

A silver tetrafluoroborate catalyzed domino cycloisomerization-vinylogous aldol addition sequence on a multifunctional substrate such as ortho-alkynylbenzaldehydes yielding functionalized 1H-isochromenes in a single step with high yield and excellent diastereoselectivity (>19 : 1) is described. The reaction was well tolerated by alkyl, aryl, and unsubstituted alkynylbenzaldehydes, and furnished selective 6-endo-dig adducts exclusively without loss in the regio- as well as diastereoselectivity.

Modular Design of Chiral Conjugate-Base-Stabilized Carboxylic Acids: Catalytic Enantioselective [4 + 2] Cycloadditions of Acetals

Readily available 1,2-amino alcohols provide the framework for a new generation of chiral carboxylic acid catalysts that rival the acidity of the widely used chiral phosphoric acid catalyst (S)-TRIP. Covalently linked thiourea sites stabilize the carboxylate conjugate bases of these catalysts via anion-binding, an interaction that is largely responsible for the low pK_a values. The utility of the new catalysts is illustrated in the context of challenging [4 + 2] cycloadditions of salicylaldehyde-derived acetals with homoallylic and bishomoallylic alcohols, providing polycyclic chromanes in a highly enantioselective fashion.