Copper Catalyzed [3+2] Annulation Reaction of Exocyclic Sulfonyl Enamides for the Synthesis of N,O-Spiroketal and Spiroketal

A copper-catalyzed [3+2] annulation reaction of exocyclic enamines/enol ethers with 1,4-benzoquinone esters has been developed, providing facile access to N,O-spiroketals and spiroketals under mild conditions with broad substrate scope (26 examples, 71-94 % yields). Gram scale synthesis and chemical transformations demonstrated that this method is potentially useful in the synthesis of natural products and drugs containing a N,O- spiroketal moiety. The chiral N,O-spiroketal could be obtained with 98 % ee after recrystallization, when a chiral SaBOX ligand was employed.

Gold-catalyzed cyclization and cycloaddition in natural product synthesis

Covering: 2016 to mid 2023Transition metal catalysis, known for its remarkable capacity to expedite the assembly of molecular complexity from readily available starting materials in a single operation, occupies a central position in contemporary chemical synthesis. Within this landscape, gold-catalyzed reactions present a novel and versatile paradigm, offering robust frameworks for accessing diverse structural motifs. In this review, we highlighted a curated selection of publications in the past 8 years, focusing on the deployment of homogeneous gold catalysis in the ring-forming step for the total synthesis of natural products. These investigations are categorized based on the specific ring formations they engender, accentuating the prevailing gold-catalyzed methodologies applied to surmount intricate challenges in natural products synthesis.

An Efficient and Scalable "Second Generation" Total Synthesis of the Marine Polyketide Limaol Endowed with Antiparasitic Activity

The cluster of four skipped exo-methylene substituents on the "northern" wing of limaol renders this dinoflagellate-derived marine natural product unique in structural terms. This arguably non-thermodynamic array gains kinetic stability by virtue of populating local conformations which impede isomerization to a partly or fully conjugated polyene. This analysis suggested that the difficulties encountered during the late stages of our first total synthesis of this polyketide had not been caused by an overly fragile character of this unusual substructure; rather, an unfavorable steric microenvironment about the spirotricyclic core was identified as the likely cause. To remedy the issue, the protecting groups on this central fragment were changed; in effect, this amendment allowed all strategic and practical problems to be addressed. As a result, the overall yield over the longest linear sequence was multiplied by a factor of almost five and the material throughput increased more than eighty-fold per run. Key-to-success was a gold-catalyzed spirocyclization reaction; the reasons why a Brønsted acid cocatalyst is needed and the origin of the excellent levels of selectivity were delineated. The change of the protecting groups also allowed for much improved fragment coupling processes; most notably, the sequence of a substrate-controlled carbonyl addition reaction followed by Mitsunobu inversion that had originally been necessary to affix the southern tail to the core could be replaced by a reagent controlled asymmetric allylation. Finally, a much-improved route to the "northern" sector was established by leveraging the power of asymmetric hydrogenation of a 2-pyrone derivative. Limaol was found to combine appreciable antiparasitic activity with very modest cytotoxicity.

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.

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.

Total Syntheses of Nominal and Actual Prorocentin

The dinoflagellate-derived polyether prorocentin is a co-metabolite of the archetypical serine/threonine phosphatase inhibitor okadaic acid. Whereas a structural relationship cannot be missed and a biosynthetic link was proposed, it is currently unknown whether there is any parallel in the bioactivity profile of these natural products. However, it was insinuated in the past that the structure assigned to prorocentin might need to be revised. Indeed, re-examination of the published spectra cast doubts as to the constitution of the fused/spirotricyclic BCD-ring system in the core. To clarify this issue, a flexible synthesis blueprint was devised that allowed us to obtain the originally proposed structure as well as the most plausible amended structure. The key to success was late-stage gold-catalyzed spirocyclization reactions that furnished the isomeric central segments with excellent selectivity. The lexicon of catalytic transformations used to make the required cyclization precursors comprised a titanium-mediated ester methylenation/metathesis cascade, a rare example of a gold-catalyzed allylic substitution, and chain extensions via organocatalytic asymmetric aldehyde propargylation. A wing sector to be attached to the isomeric cores was obtained by Krische allylation, followed by a superbly selective cobalt-catalyzed oxidative cyclization of the resulting di-unsaturated alcohol with the formation of a 2,5-trans-disubstituted tetrahydrofuran; the remaining terminal alkene was elaborated into an appropriate handle for fragment coupling by platinum-catalyzed asymmetric diboration/oxidation. The assembly of the different building blocks to the envisaged isomeric target compounds proved that the structure of prorocentin needs to be revised as disclosed herein.

Cationic Gold(II) Complexes: Experimental and Theoretical Study

Gold(II) complexes are rare, and their application to the catalysis of chemical transformations is underexplored. The reason is their easy oxidation or reduction to more stable gold(III) or gold(I) complexes, respectively. We explored the thermodynamics of the formation of [Au^II (L)(X)]⁺ complexes (L=ligand, X=halogen) from the corresponding gold(III) precursors and investigated their stability and spectral properties in the IR and visible range in the gas phase. The results show that the best ancillary ligands L for stabilizing gaseous [Au^II (L)(X)]⁺ complexes are bidentate and tridentate ligands with nitrogen donor atoms. The electronic structure and spectral properties of the investigated gold(II) complexes were correlated with quantum chemical calculations. The results show that the molecular and electronic structure of the gold(II) complexes as well as their spectroscopic properties are very similar to those of analogous stable copper(II) complexes.

Enantioselective Synthesis of Spiroketals and Spiroaminals via Gold and Iridium Sequential Catalysis

The enantioselective cascade reaction between racemic 2-(1-hydroxyallyl)phenols and alkynols/alkynamides was realized by using a gold and iridium sequential catalytic system. In this procedure, the in situ generated exocyclic vinyl ethers or enamides undergo the asymmetric allylation/spiroketalization with π-ally-Ir amphiphilic species, which provides an efficient and straightforward access to spiroketals and spiroaminals with excellent enantioselectivities. Moreover, racemic 2-(1-hydroxyallyl)anilines were also suitable in this reaction along with a kinetic resolution process, affording enantioenriched spiroaminals and 2-(1-hydroxyallyl)anilines in good yields. The synthetic utility of this method has been demonstrated by efficient enantioselective synthesis of the analogue of Paecilospirone.

Accessing Complex Tetrahydrofurobenzo-Pyran/Furan Scaffolds via Lewis-Acid Catalyzed Bicyclization of Cyclopropane Carbaldehydes with Quinone Methides/Esters

Herein, we report a straightforward one-pot synthesis of tetrahydrofurobenzopyran and tetrahydrofurobenzofuran systems via an in situ ring-expansion of the cyclopropane carbaldehydes followed by a [2 + n] cycloaddition with the quinone derivatives. The transformation not only unveils a new reaction mode of cyclopropane carbaldehydes with quinone methides/esters, but also promotes a step-efficient diastereoselective route to the sophisticatedly fused oxygen tricycles that can be further dehydrogenated to access the valued dihydro-2H-furo[2,3-b]chromene frameworks.

Mechanistic Study on Gold(I)-Catalyzed Unsaturated Spiroketalization Reaction

Abstract:

The mechanism of metal-catalyzed spiroketalization of propargyl acetonide is explored by employing DFT with the B3LYP/6-31+G(d) method. Acetonide is used as a regioselective regulator in the formation of monounsaturated spiroketal. The energies of transition states, intermediates, reactants and products are calculated to provide new insight into the mechanism of the reaction. The energetic features, validation of the observed trends in regioselectivity are conferred in terms of electronic indices via FMO analysis. The presence of acetonide facilitates a stepwise spiroketalization as it masks the competing nucleophile, and thus hydroxyl group present, exclusively acts as a nucleophile. The vinyl gold intermediate 3 is formed from 2 via activation barrier TS1. This is the first ring formation, which is 6-exo-dig cyclization. The intermediate 3 is converted into allenyl ether 4, which isomerizes to the intermediate oxocarbenium ion 5 via activation barrier TS2. The intermediate 5 cyclizes to 6 via TS3. This is the second ring formation. The intermediate 6 on protodeauration turns into 6,6-monounsaturated spiroketal 7. It is concluded that acetonide as a protecting group serves the purpose, and thus a wide range of spiroketals can be prepared, regioselectivity.

Gold-catalyzed alkyne-diol bicycloketalization enables enantioselective divergent total syntheses of attenols

Gold-catalyzed bicycloketalization of alkyne-diols provides synthetic advantages when compared to that of keto-diols under acidic conditions.

Indium-catalyzed synthesis of benzannulated spiroketals by intramolecular double hydroalkoxylation of ortho-(hydroxyalkynyl)benzyl alcohols

The novel indium-catalyzed synthesis of benzannulated spiroketals by a double intramolecular hydroalkoxylation reaction of o-(hydroxyalkynyl)benzyl alcohols is reported.

Copper-Catalyzed One-Pot Synthesis of Quinazolinones from 2-Nitrobenzaldehydes with Aldehydes: Application toward the Synthesis of Natural Products

A novel, efficient, and atom-economical approach for the construction of quinazolinones from 2-nitrobenzaldehydes has been unveiled via copper-catalyzed nitrile formation, hydrolysis, and reduction in one pot for the first time. In this reaction, urea is used as a source of nitrogen for nitrile formation, hydrazine hydrate is used for both the reduction of the nitro group and the hydrolysis of nitrile, and atmospheric oxygen is used as the sole oxidant. The method portrays a wide substrate scope with good functional group tolerances. Moreover, this method was applied for the synthesis of schizocommunin, tryptanthrin, phaitanthrin-A, phaitanthrin-B, and 8H-quinazolino[4,3-b]quinazolin-8-one.

Synthesis and biological evaluation of novel steroidal pyrazole amides as highly potent anticancer agents

A series of thirty-six steroidal pyrazole amides, divided into two categories based on their main skeletons were designed and synthesized via a five-step synthetic route. The final product is obtained through Pinnick oxidation of pyrazole aldehydes to yield the corresponding acids, which then underwent amidation to afford the target products efficiently under mild reaction conditions. Structures of the desired compounds were confirmed by ¹H NMR, ¹³C NMR, high resolution mass spectrometry; X-ray structural characterization of compound 16n was also obtained. The synthesized compounds were screened for their antiproliferative activity against four cancer cell lines (Pc-3 A549, Hela, HepG2) using the SRB method. Amides 10n, 16n, and 16p-16t exhibited moderate to high cytotoxic activities with IC₅₀ values ranging from 2.05 to 8.73 μM. Of note, the hydrochloride derivative 16p displayed the highest activity towards PC-3 cells with IC₅₀ values of 2.05 μM. Analysis of structure-activity relationships indicated that the presence of the diamine moiety and the aqueous solubility of the derivatives were vital factors for antiproliferative potency. Furthermore, molecule 16p induced PC-3 cells apoptosis and arrested cell cycle at G1 phase in a dose-dependent manner.

Transition Metal-Catalyzed Hydroalkoxylation of Alkynes: An Overview

Oxygen-bearing motifs, mainly the congener heterocycles are ubiquitous due to their presence in various natural products and bioactive scaffolds. Although in literature, several strategies have been developed for their synthesis, hydroalkoxylation of alkynes has come forward as a method of choice and has been used extensively. In particular, hydroalkoxylation of alkynes has gained enormous attention from the synthetic community due to the rapid access to a very useful and reactive synthetic intermediate like 'enol ether'. Furthermore, to manifold the utility of these methods, reports have been developed elaborating the generation of 'enol ether' using hydroalkoxylation and their usage in various reactions in cascade or tandem manner. This review focuses on recent development on the hydroalkoxylation of alkynes for the synthesis of oxygen-containing entities.

Gold-Catalyzed Reactions Towards Diversity: From Simple Substrates to Functionalized Carbo- and Heterocycles

The field of gold catalysis has been in constant expansion during the last twenty years. Based on the precept of π-activation of unsaturated simple substrates, several new rearrangements have been discovered, implying aryl, alkyne, alkene or keto derivatives as key partners. In this personal account, the main contributions in the field of gold catalysis from our group will be highlighted, emphasizing the recent reports, starting from 1,6- and 1,5-enynes and then moving to keto-ynes derivatives. The gold-catalyzed reactions will be presented starting from classical skeletal rearrangements (cycloisomerization) and then domino processes. In each part, the presentation of asymmetric versions will be highlighted.

Lewis Base Catalyzed, Sulfenium Ion Initiated Enantioselective, Spiroketalization Cascade

A Lewis base catalyzed, enantioselective sulfenocyclization of alkenes to afford [6,6]spiroketals has been developed. The method uses a chiral Lewis base catalyst with an electrophilic sulfur source to generate enantioenriched thiiranium ion with alkenes. Upon formation, the thiiranium ion is subsequently captured in a cascade-type reaction, wherein a ketone oxygen serves as the nucleophile to open the thiiranium ion and an alcohol provides the secondary cyclization to form biorelevant spiroketals. A variety of electron-rich and electron-neutral E-substituted styrenes form the desired spiroketals in good yields with excellent enantio- and diastereoselectivities. Alkyl-substituted and terminal alkenes participate in the cascade reaction, but with a limited scope compared to the styrenyl substrates. This method allows for rapid formation of highly substituted spiroketals in good yield and excellent enantioselectivity.

1,5-Hydrogen Atom Transfer/Surzur-Tanner Rearrangement: A Radical Cascade Approach for the Synthesis of 1,6-Dioxaspiro[4.5]decane and 6,8-Dioxabicyclo[3.2.1]octane Scaffolds in Carbohydrate Systems

The 1,5-HAT–1,2-(ester)alkyl radical migration (Surzur–Tanner rearrangement) radical/polar sequence triggered by alkoxyl radicals has been studied on a series of C-glycosyl substrates with 3-C-(α,β-d,l-glycopyranosyl)1-propanol and C-(α-d,l-glycopyranosyl)methanol structures prepared from chiral pool d- and l-sugar. The use of acetoxy and diphenoxyphosphatoxy as leaving groups provides an efficient construction of 10-deoxy-1,6-dioxaspiro[4.5]decane and 4-deoxy-6,8-dioxabicyclo[3.2.1]octane frameworks. The alkoxyl radicals were generated by the reaction of the corresponding N-alkoxyphthalimides with group 14 hydrides [n-Bu₃SnH(D) and (TMS)₃SiH], and in comparative terms, the reaction was also initiated by visible light photocatalysis using the Hantzsch ester/fac-Ir(ppy)₃ procedure. Special attention was devoted to the influence of the relative stereochemistry of the centers involved in the radical sequence on the reaction outcome. The addition of BF₃•Et₂O as a catalyst to the radical sequence resulted in a significant increase in the yields of the desired bicyclic ketals.

Geminal Diheteroatomic Motifs: Some Applications of Acetals, Ketals, and Their Sulfur and Nitrogen Homologues in Medicinal Chemistry and Drug Design

Acetals and ketals and their nitrogen and sulfur homologues are often considered to be unconventional and potentially problematic scaffolding elements or pharmacophores for the design of orally bioavailable drugs. This opinion is largely a function of the perception that such motifs might be chemically unstable under the acidic conditions of the stomach and upper gastrointestinal tract. However, even simple acetals and ketals, including acyclic molecules, can be sufficiently robust under acidic conditions to be fashioned into orally bioavailable drugs, and these structural elements are embedded in many effective therapeutic agents. The chemical stability of molecules incorporating geminal diheteroatomic motifs can be modulated by physicochemical design principles that include the judicious deployment of proximal electron-withdrawing substituents and conformational restriction. In this Perspective, we exemplify geminal diheteroatomic motifs that have been utilized in the discovery of orally bioavailable drugs or drug candidates against the backdrop of understanding their potential for chemical lability.

Studies on gold(I) and gold(III) alcohol functionalised NHC complexes

Five pairs of novel chiral alcohol functionalised gold(i) and gold(iii) NHC complexes derived from chiral amino alcohols, were synthesized and characterised (NMR, IR, HRMS). Single crystal X-ray diffraction data of gold(i) and gold(iii) complexes are reported and discussed. The chiral imidazolium preligands were readily synthesized via the oxalamides, subsequent reduction and final orthoformate condensation. An improved method was used for generation of gold(i) NHC complexes (up to 92%) and further oxidation afforded the corresponding gold(iii) NHC complexes (up to 99%). All the Au(i) and Au(iii) NHC complexes proved far more catalytically active in a 1,6-enyne alkoxycyclization test reaction than our previously tested N,N- and P,N-ligated Au(iii) complexes. Comparative gold(i) and gold(iii) catalytic studies demonstrated different catalytic ability, depending on the NHC ligand flexibility and bulkiness. Excellent yields (92-99%) of target alkoxycyclization product were obtained with both gold(i) and gold(iii) complexes with the bulky N1-Mes-N2-ethanol based NHC ligand.

[Development of Gold-catalyzed Reaction Utilizing Electron Acceptability of Z-type Ligand]

The interaction between transition metals and ligands is important for catalytic reactions. The ligands are largely dominated by the covalent X-type (hydride, alkyl and halogen) and/or dative L-type ligands (e.g., P, N, CO, olefin, etc.). Therefore, the interaction of the Z-type ligands (B, Al and Si, etc.) with transition metals is emerging as a new concept for the reactivity of the metal center. Recently, we developed the synthesis of the gold complex Au(DPB)X (DPB=diphosphine-borane) featuring the Z-type ligand, and their catalytic reaction. The gold catalysts showed a high activity compared to the general catalysts (without Z-ligand) for the various cyclization reactions due to the electron-withdrawing effect of the Z-ligand on the coordinating gold center. In this review, first the structure analysis of the synthesized Au→Z complex is introduced in detail, and second, the catalytic reactions based on the alkyne activation are described.

seco-Labdanes: A Study of Terpenoid Structural Diversity Resulting from Biosynthetic C-C Bond Cleavage

The cleavage of a C-C bond is a complexity generating process, which complements oxidation and cyclisation events in the biosynthesis of terpenoids. This process leads to increased structural diversity in a cluster of related secondary metabolites by modification of the parent carbocyclic core. In this review, we highlight the diversifying effect of C-C bond cleavage by examining the literature related to seco-labdanes-a class of diterpenoids arising from such C-C bond cleavage events.

Total Synthesis of Limaol

A nonthermodynamic array of four skipped methylene substituents on the hydrophobic tail renders limaol, a C40-polyketide of marine origin, unique in structural terms. This conspicuous segment was assembled by a two-directional approach and finally coupled to the polyether domain by an allyl/alkenyl Stille reaction under neutral conditions. The core region itself was prepared via a 3,3′-dibromo-BINOL-catalyzed asymmetric propargylation, a gold-catalyzed spirocyclization, and introduction of the southern sector via substrate-controlled allylation as the key steps.

The Formosalides: Structure Determination by Total Synthesis

Total synthesis allowed the constitution of the cytotoxic marine macrolides of the formosalide family to be confirmed and their previously unknown stereostructure to be assigned with confidence. The underlying blueprint was inherently modular to ensure that each conceivable isomer could be reached. This flexibility derived from the use of strictly catalyst controlled transformations to set the stereocenters, except for the anomeric position, which is under thermodynamic control; as an extra safety measure, all stereogenic centers were set prior to ring closure to preclude any interference of the conformation adopted by the macrolactone rings of the different diastereomers. Late-stage macrocyclization by ring-closing alkyne metathesis was followed by a platinum-catalyzed transannular 6-exo-dig hydroalkoxylation/ketalization to craft the polycyclic frame. The side chain featuring a very labile unsaturation pattern was finally attached to the core by Stille coupling.

On the Structure of Intermediates in Enyne Gold(I)-Catalyzed Cyclizations: Formation of trans-Fused Bicyclo[5.1.0]octanes as a Case Study

The nature of cyclopropyl gold(I) carbene-type intermediates has been reexamined as part of a mechanistic study on the formation of cis- or trans-fused bicyclo[5.1.0]octanes in a gold(I)-catalyzed cascade reaction. Benchmark of DFT methods together with QTAIM theory and NBO analysis confirms the formation of distinct intermediates with carbenic or carbocationic structures in the cycloisomerizations of enynes.

Total Synthesis of (±)-Leonuketal

Leonuketal is an 8,9-seco-labdane terpenoid with a unique tetracyclic structure, owing to a diversity-generating biosynthetic C-C bond cleavage event. The first total synthesis of leonuketal is reported, featuring a Ti(III)-mediated reductive cyclization of an epoxy nitrile ether, an unusual ring-opening alkyne formation as part of an auxiliary ring strategy, and the previously undescribed Au(I)-catalyzed cyclization of a β-keto(enol)lactone to assemble the core spiroketal motif.

Gold-Catalyzed Cycloisomerization of 1,6-Cyclohexenylalkyne: An Efficient Entry to Bicyclo[3.2.1]oct-2-ene and Bicyclo[3.3.1]nonadiene

An efficient and mild synthetic route for the preparation of functionalized bicyclo[3.2.1]oct-2-ene and bicyclo[3.3.1]nonadiene via gold-mediated cycloisomerization of 1,6-enynes has been developed. This atom-economical catalytic process was optimized and relied on the efficiency of IPrAuNTf₂ allowing the formation of functionalized bicyclic adducts in 55-91% isolated yields (18 products). The reliable access to bicyclic derivatives was demonstrated on a 3 g scale with a low catalyst loading. The process occurred on a 5-exo versus 6-endo pathway depending on the substitution of the alkynyl moiety. Density functional theory (DFT) calculations were performed on the stability of intermediates, and this study corroborated the endo/exo ratio and the mechanistic pathway with key intermediates. Reduction of the ester moiety and hydrogenation of the exo-methylene double bond of the bicyclo[3.2.1]oct-2-ene adduct illustrated the potential postfunctionalization of bicyclic derivatives.

Total Synthesis of the Plant Growth Promoter Auxofuran Featuring a Gold(I) Catalyzed Furan Formation

A concise synthesis of auxofuran (1) was developed. Starting with a Sonogashira cross-coupling reaction, enynol (10) was prepared. A gold(I) catalyzed cycloisomerization led to disubstituted furan 12. Via an intramolecular Friedel-Crafts cyclization, a dihydrobenzofuranone was obtained. Functional group manipulations, including benzylic oxidation, led to the target molecule.

Au(I)-Catalyzed Cyclization of Epoxyalkynes to Allylic Alcohol Containing Spiroketals and Application to the Total Synthesis of (-)-Alotaketal A

A gold-catalyzed tandem spiroketalization of epoxyalkynes accompanied by epoxide rearrangement into the corresponding allylic alcohol was developed for the construction of functionalized spiroketals. This new synthetic methodology for unsaturated spiroketals warranted a facile total synthesis of alotaketal A from carvone via a corresponding epoxyalkyne precursor.

Acetylene as a Dicarbene Equivalent for Gold(I) Catalysis: Total Synthesis of Waitziacuminone in One Step

The gold(I)-catalyzed reaction of acetylene gas with alkenes leads to (Z,Z)-1,4-disubstituted 1,3-butadienes and biscyclopropanes depending on the donor ligand on gold(I). Acetylene was generated in situ from calcium carbide and water in a user-friendly procedure. Reaction of acetylene with 1,5-dienes gives rise stereoselectively to tricyclo[5.1.0.02,4 ]octanes. This novel double cyclopropanation has been applied to the one step total synthesis of the natural product waitziacuminone from acetylene and geranyl acetone.

Gold-catalyzed domino reactions of alkynol and p-quinone methides: divergent synthesis of fused- and spiro-ketals

A gold-catalyzed domino reaction of alkynyl alcohols and p-quinone methides to obtain divergent fused- and spiro-ketals has been developed.