Five-membered Carbocycles Research Articles

Recent Developments in Asymmetric Nazarov Reactions

Abstract: The Nazarov reaction involves the cyclization of divinyl ketones into cyclopentenones under the influence of strong acids. The prevalence of five-membered carbocycles in a multitude of natural and bioactive products has triggered an intense development of efficient methods for their construction. In particular, asymmetric versions of the Nazarov reaction are achieved by using either a chiral auxiliary or a chiral catalyst, which can be an organocatalyst, a metal catalyst, or a multicatalytic system. This review aims to update the field of asymmetric Nazarov reactions published since 2017. It is divided into four sections, dealing successively with Nazarov reactions of chiral auxiliaries, organocatalytic enantioselective Nazarov reactions, metal/boron-catalyzed enantioselective Nazarov reactions, and multicatalytic enantioselective Nazarov reactions. Each section of the review is subdivided into simple asymmetric Nazarov reactions and Nazarov-based domino/tandem reactions, which have allowed numerous more complex functionalized chiral molecules to be synthesized in one-pot procedures.

Construction of Cyclopentanes Consisting of Five Stereocenters via NHC-Catalyzed Cascade Reactions of Enals with Oxindole-Dienones.

Despite the widespread presence of the chiral cyclopentane motif, the asymmetric synthesis of cyclopentanes containing five stereocenters remains a formidable challenge. Here, we present an N-heterocyclic carbene (NHC)-catalyzed cascade reaction of enal and oxindole-dienone, which allows access to spiroxindole cyclopentanes featuring a complete set of chiral centers on the five-membered carbocycle. This strategy, characterized by the formation of multiple bonds and chiral centers, demonstrates a broad substrate scope, exclusive diastereoselectivity, and up to 99:1 er.

Recent Advances in the Catalytic Synthesis of the Cyclopentene Core.

Five-membered carbocycles are ubiquitously found in natural products, pharmaceuticals, and other classes of organic compounds. Within this category, cyclopentenes deserve special attention due to their prevalence as targets and as well as key intermediates for synthesizing more complex molecules. Herein, we offer an overview summarizing some significant recent advances in the catalytic assembly of this structural motif. A great variety of synthetic methodologies and strategies are covered, including transition metal-catalyzed or organocatalyzed processes. Both inter- and intramolecular transformations are documented. On this ground, our expertise in the application of C-H functionalization reactions oriented towards the formation of this ring and its subsequent selective functionalization is embedded.

Pogocablanols A–G, a new class of sesquiterpenoids with an unprecedented bicyclic sesquiterpenoid skeleton from Pogostemon cablin (patchouli) and their activities

Pogocablanols A–G (1–7), seven new compounds with an unprecedented sesquiterpenoid skeleton, were isolated from the aerial parts of Pogostemon cablin. This novel type of sesquiterpenoids, named pogocablane-type sesquiterpenoids, possesses a bicyclic nonisoprenoid system, including a five-membered carbocycle with a methyl group, a six-membered carbocycle with two methyl groups, and a methylene connecting the two carbocycles. Particularly, three isomers have an interesting 1-oxaspiro[4,4]nonane moiety. Their structures and absolute configurations were determined using extensive spectroscopic evidence, single-crystal X-ray diffraction, and electronic circular dichroism calculations. Pogocablanols A–D (1–4) exhibited a concentration-dependent inhibitory effect on rat jejunum smooth muscle contraction induced by acetylcholine, with Emax values varying from 48.87 ± 4.01% to 69.02 ± 7.54%. Preliminary structure–activity relationship analysis suggested that the oxaspirocycle and the pyran ring in pogocablanols are important structural features for the relaxing effects on jejunum smooth muscle. This study offered crucial support for more investigation and development of P. cablin by showing that these pogocablane-type sesquiterpenoids had the potential to be applied in pharmaceutical and functional food formulations.

Cobalt-Catalyzed Chemo- and Stereoselective Arylative Carbocyclization of 1,6-Allenynes.

Different from the well-investigated enynes, transition-metal-catalyzed carbocyclization reactions of allenynes are more attractive as a result of the unique structure and versatile reactivity of allenes. Herein, we report the first cobalt-catalyzed highly chemo- and stereoselective arylative carbocyclization of 1,6-allenynes with arylboronic acids, affording five-membered carbocycles and heterocycles with moderate to high yields, broad substrate scope, and wide functional group compatibility. Moreover, several mechanistic experiments were conducted to gain insight into the reaction process.

Rh-Catalyzed [4 + 1] Reaction of Cyclopropyl-Capped Dienes (but not Common Dienes) and Carbon Monoxide: Reaction Development and Mechanistic Study.

Transition-metal-catalyzed [4 + 1] reaction of dienes and carbon monoxide (CO) is the most straightforward and easily envisioned cyclization for the synthesis of five-membered carbocycles, which are ubiquitously found in natural products and functional molecules. Unfortunately, no test of this reaction was reported, and consequently, chemists do not know whether such kind of reaction works or not. Herein, we report that the [4 + 1] reaction of common dienes and CO cannot work, at least under the catalysis of [Rh(cod)Cl]2. However, using cyclopropyl-capped dienes (also named allylidenecyclopropanes) as substrates, the corresponding [4 + 1] reaction with CO proceeds smoothly in the presence of [Rh(cod)Cl]2. This [4 + 1] reaction, with a broad scope, provides efficient access to five-membered carbocyclic compounds of spiro[2.4]hept-6-en-4-ones. The [4 + 1] cycloadducts can be further transformed into other molecules by using the unique chemistry of cyclopropyl groups present in these molecules. The mechanism of this [4 + 1] reaction has been investigated by quantum chemical calculations, uncovering that cyclopropyl-capped dienes are strained dienes and the oxidative cyclization step in the [4 + 1] catalytic cycle can release this (angular) strain both kinetically and thermodynamically. The strain release in this step then propagates to all followed CO coordination/CO insertion/reductive elimination steps in the [4 + 1] catalytic cycle, helping the realization of this cycloaddition reaction. In contrast, common dienes (including cyclobutyl-capped dienes) do not have such advantages and their [4 + 1] reaction suffers from energy penalty in all steps involved in the [4 + 1] catalytic cycle. The reactivity of ene-allenes for the [4 + 1] reaction with CO is also discussed.

Reactions of α-Functionally Substituted Enals with Terminal Alkynes: Unexpected Assembly of 2-Amino-2-Cyclopentenones.

The reactions of α-chalcogen, α-halo, or α-amino functionally substituted enals with terminal alkynes leading either to corresponding propargyl alcohols (for O-, S-, Cl-, and Br-bearing substrates) or unexpected 2-amino-2-cyclopentenones (for aminoenals) are described. The key feature of these reactions is the rearrangement of adducts bearing amino groups on silica gel that triggered further cyclization to five-membered carbocycles.

Silver Ions Promoted Palladium-Catalyzed Inactive β-C(sp3)-H Bond Arylation in Batch and Continuous-Flow Conditions.

A palladium(II)-catalyzed protocol for inactive β-C(sp3)-H bond functionalization has been first accomplished. The reaction proceeds through five-membered carbocycles for the formation of C-C bonds via the Pd(II)/Pd(IV) cycle. This reaction was carried out with various aryl iodides and benzothiazoles/benzoxazoles/benzimidazoles, which were well-tolerated in this reaction and successfully generated β-C(sp3)-H arylated products. Further implementation of this batch protocol to continuous flow by utilizing a PTFE (polytetrafluoroethylene) capillary reactor enhanced the reaction efficiency and decreased the reaction time (18.4 min) as compared to batch conditions (8 h). Even on the gram scale, the process produced excellent yield with negligible diarylations. Functional group tolerance, a continuous-flow approach, and easy-to-handle reaction conditions make this inactive β-C(sp3)-H bond functionalization protocol very attractive.

Five-membered carbocycle construction in the synthesis of Daphniphyllum alkaloids: recent strategic and methodological advances

In this review article, we summarize novel or non-standard strategies and methods for the five-membered carbocycle construction in recent Daphniphyllum alkaloid synthesis.

Photocatalytic Sulfonylcarbocyclization of Alkynes Using SEt as a Traceless Directing Group: Access to Cyclopentenes and Indenes.

Cyclopentenes and indenes are important structural scaffolds in synthetic, medical, and material chemistry. Cyclization of alkynes via remote C-H functionalization is an appealing approach to construct these motifs due to its high efficiency and step-economy. Herein, a traceless directing group strategy was designed to reverse the regioselectivity of radical addition which enabled an unprecedented photocatalytic sulfonylcarbocyclization of terminal alkynes by forming C-C bond on inert C(sp3 )-H bond. It offers a facile access to decorated cyclopentenes and indenes under mild conditions. The resultant products could be converted into a set of valuable molecular scaffolds, including a key intermediate of AM-6226. Mechanistic experiments suggest a radical cascade pathway comprising a Markovnikov-type sulfonylation, 1,5-hydrogen atom transfer, 5-endo-trig cyclization, and β-elimination. This study lays further groundwork for the use of anti-Baldwin 5-endo-trig radical cyclization in rapidly assembling five-membered carbocycles.

Co-Catalyzed Asymmetric Intramolecular [3+2] Cycloaddition of Yne-Alkylidenecyclopropanes and its Reaction Mechanism.

Developing new transition metal-catalyzed asymmetric cycloadditions for the synthesis of five-membered carbocycles (FMCs) is a research frontier in reaction development due to the ubiquitous presence of chiral FMCs in various functional molecules. Reported here is our discovery of a highly enantioselective intramolecular [3+2] cycloaddition of yne-alkylidenecyclopropanes (yne-ACPs) to bicyclo[3.3.0]octadiene and bicyclo[4.3.0]nonadiene molecules using a cheap Co catalyst and commercially available chiral ligand (S)-Xyl-BINAP. This reaction avoids the use of precious Pd and Rh catalysts, which are usually the choices for [3+2] reactions with ACPs. The enantiomeric excess in the present reaction can be up to 92 %. Cationic cobalt(I) species was suggested by experiments as the catalytic species. DFT calculations showed that this [3+2] reaction starts with oxidative cyclometallation of alkyne and ACP, followed by ring opening of the cyclopropyl (CP) group and reductive elimination to form the cycloadduct. This mechanism is different from previous [3+2] reactions of ACPs, which usually start from CP cleavage, not from oxidative cyclization.

Lewis-Acid-Promoted Ligand-Controlled Regiodivergent Cycloaddition of Pd-Oxyallyl with 1,3-Dienes: Reaction Development and Origins of Selectivities.

For nearly 30 years, considerable research effort has been focused on the development of methods for catalytic (3 + 2) cycloaddition reactions of palladium-oxyallyl species with alkenes. However, because C-O bond formation is kinetically favored, the (3 + 2) cycloadditions achieved to date have involved C-O reductive elimination. We herein report a method of lithium triflate-promoted (3 + 2) cycloaddition reactions of palladium-oxyallyl species with 1,3-dienes that proceed via a pathway terminated with C-C bond formation to give a five-membered carbocycle. Coordination of the lithium ion with the alkoxide moiety disrupts the C-O reductive elimination and forms a metal-enolate tethered π-allyl-Pd. The π-allyl-Pd moiety then accepts intramolecular allylic attack from the enolate moiety to form carbocyclic products. Furthermore, by tuning the steric properties of the palladium ligand, we could also accomplish the competing (4 + 3) cycloadditions, and thus this method provides regiodivergent access to both cyclopentanones and cycloheptanones. The reaction mechanism was investigated by DFT calculation and the origins of the regioselectivities of the cycloaddition were rationalized.

Kinetically Controllable Pd-Catalyzed Decarboxylation Enabled [5 + 2] and [3 + 2] Cycloaddition toward Carbocycles Featuring Quaternary Carbons.

A decarboxylative protocol has been developed toward a range of carbocycles. The key success is based on the use of a batch of newly designed cyclic carbonates as substrates that can provide carbon-carbon zwitterion intermediate under palladium catalysis. The kinetics of the reactions are controllable toward either strained seven- or thermodynamically more favored five-membered carbocycles. The release of this chemistry will shed light on the synthesis of complex and valuable cyclic structures.

All-carbon [3 + 2] cycloaddition in natural product synthesis.

Many natural products possess interesting medicinal properties that arise from their intriguing chemical structures. The highly-substituted carbocycle is one of the most common structural features in many structurally complicated natural products. However, the construction of highly-substituted, stereo-congested, five-membered carbocycles containing all-carbon quaternary center(s) is, at present, a distinct challenge in modern synthetic chemistry, which can be accessed through the all-carbon [3 + 2] cycloaddition. More importantly, the all-carbon [3 + 2] cycloaddition can forge vicinal all-carbon quaternary centers in a single step and has been demonstrated in the synthesis of complex natural products. In this review, we present the development of all-carbon [3 + 2] cycloadditions and illustrate their application in natural product synthesis reported in the last decade covering 2011–2020 (inclusive).

Synthesis and Rh-catalyzed reductive cyclization of 1,6-enynes and 1,6-diynes containing alkynylboronate termini

Ir-catalyzed dehydrogenative borylation of terminal alkynes (DHBTA) has been used to borylate the C(sp)-H bond in a series of 1,6-enynes and 1,6-diynes. Ir catalysts based on the diarylamido/bis(phosphine) PNP ligands proved to be capable of performing DHBTA on these substrates. The alkynylboronate products were then explored as substrates in reductive cyclization reactions designed to yield five-membered carbocycles and heterocycles with preservation of the carbon-boron bond. The reductive cyclization of enynes with H2 was carried out using cationic Rh catalysts (after the work of Krische et al.) supported by bidentate phosphine ligands, of which BINAP proved to be the generally best performing. Reductive cyclization of borylated enynes typically resulted in mixtures of isomeric products. Attempts to optimize the reductive cyclization of borylated 1,6-enynes to yield a single product were not successful; in addition, the distribution of products in these reactions was poorly reproducible. On the other hand, reductive cyclization of 1,6-diynes with the same Rh catalysts was better behaved, allowing for selectivity and reproducibility. In particular, alkynylboronate derivatives of bis(propargyl)amine underwent smooth reductive cyclization to 3,4-substituted pyrroles. The influence of substituents and conditions on the product distribution was explored and representative reactions utilizing new borylated heterocycles in Suzuki coupling reactions were carried out.

Enantioselective Desymmetrization of Prochiral Cyclopentene-1,3-diones Triggered by Remote C(sp2)-N Bond Formation.

The enantioselective desymmetrization via remote C(sp2)-H amidation of the prochiral 2,2-disubstituted cyclopentene-1,3-dione with N-methoxybenzamide has been developed. The overall process was catalyzed by a chiral bifunctional thiourea catalyst through a sequential conjugate-addition-elimination-tautomerization. This strategy provides rapid access to highly functionalized five-membered carbocycles, bearing an all-carbon quaternary stereogenic center through remote stereocontrol in high yields with moderate to excellent enantioselectivities.

Key Factors for High Diastereo- and Enantioselectivity of Umpolung Cyclizations of Aldehyde-Containing Allylpalladium Intermediates

Abstract Two palladium/chiral diphosphine-catalyzed umpolung cyclizations of aldehyde-containing allylic acetates and allenes with arylboronic acid are fully investigated to establish key factors in their high stereoselectivities. Both cyclization reactions afford cis-disubstituted pyrrolidine and tetrahydrofuran. These occur in high diastereo- and enantioselectivities through a common cationic (Z)-η1-allylpalladium, toward which a ring strain generated in the cyclization step leading to trans-isomers biases the equilibrium through η3-η1-η3-complex in the former cyclization. Varied diastereoselectivities were observed in the formation of five-membered carbocycles and six-membered heterocycles. These reflect release of a ring strain generated in the cyclization step leading to trans-isomers and a different distribution of the (Z)- and the (E)-η1-allylpalladium intermediates generated by the oxidative addition of allylic acetates to Pd(0) or carbopalladation of allenes, respectively. A sterically demanding substituent at the center of the allyl moiety is necessary for high diastereo- and enantioselectivity. The enantioselectivity of the former cyclization was lowered by the presence of organometallic reductants or reagents, possibly causing the formation of neutral η1-allylpalladium species. We used a chiral allylic acetate containing (E)-deuterium-labeled alkene to demonstrate that the electrophilic attack of the aldehyde to the allyl ligand occurred on the side where the palladium existed, consistent with the Zimmerman-Traxler transition state.

Formation of Five-Membered Carbocycles from d-Glucose: A Concise Synthesis of 4-Hydroxy-2-(hydroxymethyl)cyclopentenone

Abstract A concise synthesis of 4-hydroxy-2-(hydroxymethyl)cyclopentenone (1) has been accomplished from d-glucose by a three-step sequence that features a catalyst-free hydrothermal reaction of d-glucal, which is readily obtained from d-glucose. Optimization of the reaction conditions for synthesizing 1 was performed by changing the temperature and reaction time. The treatment of d-glucal under the optimal conditions, i.e., at 120 °C for 24 h, provided 1 in the highest isolated yield of 61%. 1 would become a versatile intermediate for the synthesis of various fine chemicals having a cyclopentenone structure from cellulosic biomass.

Catalytic (3+2) Palladium-Aminoallyl Cycloaddition with Conjugated Dienes.

We describe the design and application of tailored aminoallyl precursors for catalytic (3+2) cycloaddition with conjugated dienes via a Pd-aminoallyl intermediate. The new cycloaddition reactions override the conventional (4+3) selectivity of aminoallyl cation cycloaddition through a sequence of Pd-allyl transfer and ring closure. A variety of highly substituted or fused pyrrolidine rings were synthesized using the cycloaddition, and can further undergo [1,3] N-to-C rearrangement to five-membered carbocycles with a different palladium catalyst. The utility of the (3+2) cycloaddition is also demonstrated by the preparation of various derivatives from the bicyclic pyrrolidine products.

Atom- and Step-Efficient Construction of Five-Membered Carbocycles with Alkenes and Alkynes Catalyzed by AgSbF6

We report a cationic intermolecular cyclization between simple alkenes and alkynes catalyzed by AgSbF6 with great selectivity, involving alkyl C–H bond cleavage. This methodology could supply important multisubstituted cyclopentene scaffolds. The remarkable tolerance of functional groups in the reaction allows great possibility for further transformations.