Strategies for the construction of γ-spirocyclic butenolides in natural product synthesis

Substrate-Controlled Regioselective Cascade Reactions of Deconjugated Butenolides and Cinnamaldehydes: Access to Structurally Diverse Spirobutenolides

Two different cascade pathways to access spirobutenolides were achieved based on the substrate-controlled regioselectivity of deconjugated butenolides. A new class of functional deconjugated butenolides was designed and exhibited superior γ-regioselectivity in the vinylogous Michael/Michael cascade reactions with cinnamaldehydes. The aryl-substituted deconjugated butenolides and cinnamaldehydes underwent a Michael/Michael/aldol/dehydration cascade process induced by double α-regioselectivities. Both conjugated and deconjugated spirobutenolides could be obtained in good yields with excellent enantioselectivities.

Concise synthesis of chiral γ-butenolides via an allylation/lactonization cascade reaction

A highly enantioselective allylation/lactonization cascade was developed, which provides a concise and efficient route to chiral γ-butenolides under mild conditions. Most of the reaction examples can be completed in 10 minutes with high selectivity.

Utilizing Allenic Acids and Heterocyclic Diazo Compounds in the Synthesis of Polysubstituted Spirocyclic Butenolides and β-Methylidene 2-Furanones

Herein, we report a novel approach for the assembly of spirocyclic Δα,β-butenolides and β-methylidene 2-furanones via Rh(II)-catalyzed O-H insertion of heterocyclic diazo compounds into allenic acids followed by base-promoted cyclization. Utilizing various diazo heterocycles, including α-diazo homophthalimides, 3-diazo tetramic acids, and diazo oxindoles, diverse spirocyclic scaffolds were produced. The research revealed that the allenic acid substitution pattern is decisive for the product type, enabling extraordinary target compound switching between two types of spirocyclic 2-furanones with exo- and endocyclic C═C bonds.

Regioselective Synthesis of Benzannulated [5,6]-Oxaspirolactones via Cu(II)-Catalyzed Cycloisomerization of 2-(5-Hydroxyalkynyl)benzoates

Spiroketals and oxaspirolactones are widely found in biologically active natural products, serving as important structural motifs. In this study, we present a Cu(II)-catalyzed cascade cycloisomerization of 2-(5-hydroxyalkynyl)benzoates, enabling the regioselective synthesis of benzannulated [5,6]-oxaspirolactones containing an isochromen-1-one moiety. This strategy offers a rapid and efficient approach to access a diverse array of benzannulated [5,6]-oxaspirolactones. The methodology presented here showcases a broad substrate scope, delivering good yields and scalability up to gram scale. The structures of the oxaspirolactones were unequivocally confirmed through single-crystal X-ray analysis and by analogy using 1H and 13C{1H} NMR data.

Oxidative Spirocyclization of β-Furyl Amides for the Assembly of Spiro-γ-butenolide-γ-butyrolactones

The synthesis of heretofore unknown γ-spirobutenolides has been achieved via an m-CPBA-mediated oxidation of β-furyl amides. The reaction employs a tethered amide, ostensibly a poorly reactive carbonyl, as a nontraditional nucleophile resulting in spirolactone formation and concurrent amide cleavage. The transformation exhibits functional group tolerance and compatibility with complex compounds. In situ 1H NMR spectroscopic studies reveal the identities of key intermediates in the oxidation-spirolactonization-oxidation cascade, suggesting a plausible mechanistic pathway. The distinct diastereofaces of the electrophilic butenolide product may be used for diastereoselective cycloaddition and conjugate addition reactions.

General acid-mediated aminolactone formation using unactivated alkenes

Spirocyclic butyrolactones and butenolides are widespread structural motifs in bioactive substances. Despite their prevalence, a simple method ensuring their direct preparation from exocyclic alkenes, ideally in a late-stage context, remains elusive. Herein, we report direct aminolactone formation using unactivated alkenes which addresses this gap, employing cheap and readily available reactants. The method relies on the hijacking of a cationic aminoalkylation pathway and affords (spiro)aminolactones with excellent functional group tolerance and chemoselectivity. The synthetic versatility of the products is demonstrated through a range of transformations, notably exploiting stereospecific rearrangement chemistry to produce sterically congested scaffolds.

Access to Spiropyrazolone-butenolides through NHC-Catalyzed [3 + 2]-Asymmetric Annulation of 3-Bromoenals and 1H-Pyrazol-4,5-diones

The stereoselective synthesis of spirocyclic pyrazolin-5-ones by N-heterocyclic carbene (NHC) organocatalysis has been less studied so far. For this reason and considering the interest of this class of compounds, here, we present the NHC-catalyzed [3 + 2]-asymmetric annulation of β-bromoenals and 1H-pyrazol-4,5-diones that achieves to produce chiral spiropyrazolone-butenolides. The synthesis is general for aryl and heteroaryl β-bromo-α,β-unsaturated aldehydes and 1,3-disubstituted pyrazolones. The spirobutenolides have been obtained in good yields (up to 88%) and enantioselectivities (up to 97:3 er). This constitutes the first described example using pyrazoldiones as the starting materials for this class of spiro compounds.

Visible-Light-Mediated Vicinal Difunctionalization of Activated Alkynes with Boronic Acids: Substrate-Controlled Rapid Access to 3-Alkylated Coumarins and Unsaturated Spirocycles

A visible-light-mediated difunctionalization of activated alkynes with boronic acid is unveiled to synthesize 3-alkylated coumarins and unsaturated spiro-lactones. The substituent at the para-position of the aryl ring of aryl alkynoate plays a pivotal role in the selective formation of chain-alkylated coumarins or spirocyclic compounds under mild conditions. The reaction employs a hypervalent iodine reagent and ruthenium photocatalyst. The spiro-lactones thus obtained were subjected to another novel mode of visible-light-mediated radical addition cascade cyclization (RACC) to access various new fused spirocyclic compounds.

Metal-free synthesis of dihydrofuran derivatives as anti-vicinal amino alcohol isosteres

Dihydrofuran cores are commonly incorporated into synthetically and pharmacologically significant scaffolds in natural product and drug discovery chemistry. Herein, we report a concise and practical strategy to construct spiro-dihydrofuran and amino dihydrofuran scaffolds as anti-vicinal amino alcohol isosteres. Hypervalent iodine (PhI(OAc)(NTs₂))-mediated C-H activation of alkynes resulted in two-bond formations with one pi bond cleavage: (i) C(sp²)-N(sp³) and O(sp³)-C(sp²); (ii) C(sp²)-N(sp³) and C(sp³)-C(sp²). The metal-free 5-endo-dig oxidative cyclization provided versatile amino 2,3- and 2,5-dihydrofurans bearing the C₅ quaternary carbon. The non-toxicity of all synthesised dihydrofurans was verified via in vitro cell viability assay.

Organobase-Catalyzed Umpolung of Amides: The Generation and Transfer of Carbamoyl Anion

A novel organobase-catalyzed umpolung reaction of amides was disclosed. This method provides an efficient method to generate and transfer carbamoyl anions. In this transformation, some of the inherent disadvantages of carbamoyl metal were avoided. The mechanistic analysis revealed that the reaction proceeds through polarity inversion of amide, and various carbamoyl anions were applied in the reaction. Moreover, a wide range of substrates was achieved with moderate to excellent yield.

Multiphotocatalyst Cascades: From Furans to Fused Butyrolactones and Substituted Cyclopentanones

High value oxygenated polycycles have been rapidly and efficiently accessed from simple precursors in one pot processes. The reported methodology relies on a new and mild method for butenolide synthesis mediated by thiols. The initial photooxygenation and butenolide synthesis have been merged with subsequent photoredox reactions to achieve rare dual-photocatalyst cascades affording various fused butyrolactones. Ground state Lewis acid activity for methylene blue has been unveiled and then exploited in the synthesis of substituted cyclopentanones.

Multicomponent Approach to the Synthesis of 4-(1H-indol-3-yl)-5-(4-methoxyphenyl)furan-2(5H)-one

A simple one-pot approach was developed for the synthesis of furan-2(5H)-one derivative containing indole fragments. This method includes the telescoped multicomponent reaction of indole, 4-methoxyphenylglyoxal, and Meldrum’s acid. The synthetic utility of the prepared furan-2(5H)-one was demonstrated by condensation with 4-methoxybenzaldehyde. The advantages of this method include the employment of readily accessible starting materials, atom economy, process simplicity, and the easy isolation of the target products. The structure of the synthesized furanones was confirmed by 1H and 13C-NMR spectroscopy and high-resolution mass spectrometry with electrospray ionization (ESI-HRMS).

Synthesis of Spirolactones via a BF3·Et2O-Promoted Cascade Annulation of α-Keto Acids and 1,3-Enynes

A novel and effective method for the synthesis of spirolactones from readily available α-keto acids and 1,3-enynes is developed via a BF₃·Et₂O-promoted cascade annulation. This sequential process is conducted at room temperature, and it provides the functionalized spirolactones in good to excellent yield under metal-free conditions.

Tailored Synthesis of Skeletally Diverse Stemona Alkaloids through Chemoselective Dyotropic Rearrangements of β-Lactones

The collective synthesis of skeletally diverse Stemona alkaloids featuring tailored dyotropic rearrangements of β-lactones as key elements is described. Specifically, three typical 5/7/5 tricyclic skeletons associated with stemoamide, tuberostemospiroline and parvistemonine were first accessed through chemoselective dyotropic rearrangements of β-lactones involving alkyl, hydrogen, and aryl migration, respectively. By the rational manipulation of substrate structures and reaction conditions, these dyotropic rearrangements proceeded with excellent efficiency, good chemoselectivity and high stereospecificity. Furthermore, several polycyclic Stemona alkaloids, including saxorumamide, isosaxorumamide, stemonine and bisdehydroneostemoninine, were obtained from the aforementioned tricyclic skeletons through late-stage derivatizations. A novel visible-light photoredox-catalyzed formal [3+2] cycloaddition was also developed, which offers a valuable tool for accessing oxaspirobutenolide and related scaffolds.

Rh(II)-catalyzed Denitrogenative Cascade of 1,2,3-Triazolyl Propiolates and Indoles: Access to Butenolide Tethered Homotryptamines

An efficient Rh(II)-catalyzed denitrogenative reaction of indoles with 1,2,3-triazolyl propiolates has been developed. This methodology provides facile access to butenolide tethered homotryptamines in good to excellent yields under operationally simple conditions and features a broad substrate scope. Overall, the reaction sequence involves the formation of three new bonds (two C-C and one C-O) in a nucleophilic cascade manner. Additionally, an intramolecular rearrangement of these derivatives to thermodynamically more stable butenolides is also demonstrated.

Synthesis of spiro[4.4]thiadiazole derivatives via double 1,3-dipolar cycloaddition of hydrazonyl chlorides with carbon disulfide

An operationally simple and convenient synthesis method toward a series of diverse spiro[4.4]thiadiazole derivatives via double [3 + 2] 1,3-dipolar cycloaddition of nitrilimines generated in situ from hydrazonyl chlorides with carbon disulfide has been achieved under mild reaction conditions.

Natural-Like Spirocyclic Δα,β -Butenolides Obtained from Diazo Homophthalimides

α-Diazo homophotalimides were reacted with various propiolic acids on Rh₂ (esp)₂ catalysis. The resulting propiolate esters were transformed into novel, heterocyclic Δ^α,β -spirobutenolides in good to excellent product yields. The approach represents a fundamentally novel entry into natural-like Δ^α,β -spirobutenolides present in many biologically active natural products as well as fully synthetic compounds endowed with diverse biological activities. The Δ^α,β -spirobutenolides thus obtained were shown to inhibit thioredoxin reductase, a selenocysteine enzyme target for cancer. Moreover, for the best compound in the series (TrxR IC₅₀ 1.49±0.08 μM), by using MALDI-TOF mass-spectrometry it was shown that it selectively binds selenocysteine in the presence of a 10-fold excess of cysteine. This validates the new compound as a promising lead for anticancer therapy development.

Synthesis of the Hexahydropyrrolo-[3,2-c]-quinoline Core Structure and Strategies for Further Elaboration to Martinelline, Martinellic Acid, Incargranine B, and Seneciobipyrrolidine

Natural products are rich sources of interesting scaffolds possessing a plethora of biological activity. With the isolation of the martinella alkaloids in 1995, namely martinelline and martinellic acid, the pyrrolo[3,2-c]quinoline scaffold was discovered. Since then, this scaffold has been found in two additional natural products, viz. incargranine B and seneciobipyrrolidine. These natural products have attracted attention from synthetic chemists both due to the interesting scaffold they contain, but also due to the biological activity they possess. This review highlights the synthetic efforts made for the preparation of these alkaloids and formation of analogues with interesting biological activity.