Horner-Wadsworth-Emmons reagents as azomethine ylide analogues: pyrrole synthesis via (3 + 2) cycloaddition

Amido-substituted Horner-Wadsworth-Emmons reagents can serve as precursors to 1,3-dipoles for use in cycloaddition. These compounds are assembled in one pot via the TMSOTf-catalyzed Arbuzov reaction of imines, acid chlorides, and phosphites. The coupling of this synthesis with alkyne cycloaddition provides a three-component synthesis of pyrroles. The dipoles can be prepared with a diverse range of imines and acid chlorides, and (3 + 2) cycloaddition with unsymmetrical alkynes is highly regiospecific, providing a modular approach to form substituted pyrroles.     

Metal‐Free Synthesis of Highly Substituted Pyridines by Formal [2+2+2] Cycloaddition under Mild Conditions

The synthesis of pyridines through direct intermolecular cycloaddition of alkynes and nitriles is a contemporary challenge in organic synthesis. A Brønsted acid mediated formal [2+2+2] cycloaddition of heteroalkynes and nitriles was developed that proceeds under mild conditions. This constitutes a modular approach to highly substituted pyridine cores.     

Visible‐Light Photoredox Catalysis Enables the Biomimetic Synthesis of Nyingchinoids A,B, and D,and Rasumatranin D

The total synthesis of nyingchinoids A and B has been achieved through successive rearrangements of a 1,2‐dioxane intermediate that was assembled using a visible‐light photoredox‐catalysed aerobic [2+2+2] cycloaddition. Nyingchinoid D was synthesised with a competing [2+2] cycloaddition. Based on NMR data and biosynthetic speculation, we proposed a structure revision of the related natural product rasumatranin D, which was confirmed through total synthesis. Under photoredox conditions, we observed the conversion of a cyclobutane into a 1,2‐dioxane through retro‐[2+2] cycloaddition followed by aerobic [2+2+2] cycloaddition.     

An efficient synthesis of peri-hydroxy aromatic compounds via a strong-base-induced

An efficient synthesis of peri-hydroxy aromatic compounds has been accomplished via a strong-base-induced [4+2] cycloaddition of homophthalic anhydrides with alpha-sulfinyl-substituted derivatives of enolizable enones. The unsubstituted enones did not undergo an efficient [4+2] cycloaddition reaction with homophthalic anhydrides, presumably due to their enolization under the basic reaction conditions. The sulfinyl group not only promotes the cycloaddition reaction but also undergoes in situ elimination under the reaction conditions to afford the peri-hydroxy aromatic compounds in a single step. The application of this methodology for the synthesis of a key intermediate of antitumor antibiotic fredericamycin A is described. PM3 calculations of various 2-substituted cyclopentenones as well as the mechanism of the cycloaddition are also discussed.     

Visible‐Light Photoredox Catalysis Enables the Biomimetic Synthesis of Nyingchinoids A,B, and D,and Rasumatranin D

The total synthesis of nyingchinoids A and B has been achieved through successive rearrangements of a 1,2‐dioxane intermediate that was assembled using a visible‐light photoredox‐catalysed aerobic [2+2+2] cycloaddition. Nyingchinoid D was synthesised with a competing [2+2] cycloaddition. Based on NMR data and biosynthetic speculation, we proposed a structure revision of the related natural product rasumatranin D, which was confirmed through total synthesis. Under photoredox conditions, we observed the conversion of a cyclobutane into a 1,2‐dioxane through retro‐[2+2] cycloaddition followed by aerobic [2+2+2] cycloaddition.     

Total synthesis of (+)-frondosin A. Application of the Ru-catalyzed [5+2] cycloaddition

The first total synthesis of (+)-frondosin A was accomplished in 19 longest linear and 21 total steps from commercially available materials. The key features of the synthesis include a Ru-catalyzed [5+2] cycloaddition, a Claisen rearrangement, and a ring expansion to construct the core of the frondosin A in a diastereoselective and regioselective fashion. This is the first application of a Ru-catalyzed [5+2] cycloaddition in the total synthesis of a natural product. Through this synthesis, the absolute configuration of (+)-frondosin A was established.     

In situ generation of vinyl allenes and its applications to one-pot assembly of cyclohexene, cyclooctadiene, 3,7-nonadienone, and bicyclo[6.4.0]dodecene derivatives with palladium-catalyzed multicomponent reactions

A novel tandem Pd-catalyzed cross-coupling and [4 + 4] cycloaddition sequence allows the rapid synthesis of eight-membered carbocycles starting from alpha-bromovinyl arenes and propargyl bromides in one reaction vessel. It is noteworthy that four components are assembled into one molecule via this procedure. In contrast to alpha-bromovinyl arenes, alpha-bromovinyl alkanes afforded tandem cross-coupling and homo [4 + 2] cycloaddition products. Subjecting an equimolar mixture of alpha-bromostyrene and 2-bromo-1-octene to propargyl bromides furnished the tandem Pd-catalyzed cross-coupling and hetero [4 + 2] cycloaddition product. Exposure of equimolar mixtures of alpha-bromovinyl arenes to allenylindium resulted in tandem a Pd-catalyzed cross-coupling and hetero [4 + 4] cycloaddition products. Synthesis of vinylallene from the reaction of vinyl triflate with allenylindium followed by Pd-catalyzed carbon monoxide insertion reaction gave the corresponding 3,7-nonadienone product via tandem Pd-catalyzed cross-coupling and [4 + 4 + 1] annulation. Tandem Pd-catalyzed cross-coupling, [4 + 4] cycloaddition, and [4 + 2] cycloaddition provided the rapid synthesis of bicyclo[6.4.0]dodecene derivatives starting from alpha-bromovinyl arenes, propargyl bromides, and dienophiles in one operation, in which five components were integrated into one molecule.     

The use of enantiomerically pure ketene dithioacetal bis(sulfoxides) in highly diastereoselective intramolecular nitrone cycloadditions. Application in the total synthesis of the beta-amino acid (-)-cispentacin and the first asymmetric synthesis of cis-(3R,4R)-4-amino-pyrrolidine-3-carboxylic acid

Intramolecular 1,3-dipolar nitrone cycloaddition onto an enantiomerically pure ketene dithioacetal dioxide using a three-carbon tether gave the corresponding 5,5-disubstituted isoxazolidine as a single diastereomer in good yield. This reaction has been used as the key step in an asymmetric synthesis of the naturally occurring antibiotic, (-)-cispentacin. An asymmetric synthesis of 4-amino-pyrrolidine-3-carboxylic acid has also been carried out using the intramolecular nitrone cycloaddition as the stereocontrolling step.     

Synthesis of dispirooxindolecycloalka[d]pyrimidino[2,3-b]-thiazole pyrrolidine/thiapyrrolizidine ring systems

The synthesis of a new class of spirooxindolo pyrrolidines and spirooxindolo thiapyrrolizidines has been accomplished by a three component, one-pot 1,3-dipolar cycloaddition reaction. The cycloaddition was found to be highly regioselective.     

Synthesis of (−)‐Hebelophyllene E: An Entry to Geminal Dimethyl‐Cyclobutanes by [2+2] Cycloaddition of Alkenes and Allenoates

The first synthesis of hebelophyllene E is presented, along with assignment of its previously unknown relative configuration through synthesis of epi‐ent‐hebelophyllene E. Development of a catalytic enantioselective [2+2] cycloaddition of alkenes and allenoates provides access to the required chiral geminal dimethylcyclobutanes. Key to its success is the identification of a novel oxazaborolidine catalyst which promotes the cycloaddition in high enantioselectivities with good functional‐group tolerance (9 examples, up to 97:3 e.r.). Thus, a late‐stage cycloaddition using a fully functionalized alkene, followed by a diastereoselective reduction allows a concise entry to this class of natural products.     

Highly Enantioselective Intramolecular 1,3‐Dipolar Cycloaddition: A Route to Piperidino‐Pyrrolizidines

Enantioselective catalytic intermolecular 1,3‐dipolar cycloadditions are powerful methods for the synthesis of heterocycles. In contrast, intramolecular enantioselective 1,3‐dipolar cycloadditions are virtually unexplored. A highly enantioselective synthesis of natural‐product‐inspired pyrrolidino‐piperidines by means of an intramolecular 1,3‐dipolar cycloaddition with azomethine ylides is now reported. The method has a wide scope and yields the desired cycloadducts with four tertiary stereogenic centers with up to 99 % ee. Combining the enantioselective catalytic intramolecular 1,3‐dipolar cycloaddition with a subsequent diastereoselective intermolecular 1,3‐dipolar cycloaddition yielded complex piperidino‐pyrrolizidines with very high stereoselectivity in a one‐pot tandem reaction.     

The cycloaddition strategy for the synthesis of natural products containing carbocyclic seven-membered rings

Highly substituted carbocyclic seven-membered rings are frequently found in natural products and their synthesis represents a significant challenge to the synthetic chemist. Direct intramolecular cyclization of these systems often proves difficult and this fact has catalyzed the development of a variety of strategies based on a convergent intermolecular cycloaddition strategy. This concept article discusses the major cycloaddition approaches utilized to access these types of structures and primarily focuses on examples employed in the synthesis of natural products.     

Synthetic studies toward highly functionalized 5beta-lanosterol derivatives: a versatile approach utilizing anionic cycloaddition

Stereoselective synthesis of the potentially biologically valuable 5beta-lanosteroidal-type backbone was achieved via anionic cycloaddition. Synthesis of the two new bicyclic Nazarov intermediates 14 and 40 and their cycloaddition with chiral cyclohexenone 25 and further functional group manipulations resulted in highly functionalized tetracyclic intermediates 28 and 44. These synthetic intermediates could lead to the total synthesis of new lanosterol-based inhibitors.     

An Efficient Synthesis of Benzazocines by Gold(I)‐Catalyzed Tandem 1,2‐Acyloxy Shift/[3+2] Cycloaddition of Terminal 1,9‐Enynyl Esters

An effective synthesis of structurally diverse benzazocines was accomplished in good to excellent chemical yields (55–82 %) through a gold(I)‐catalyzed cascade reaction involving tandem 1,2‐acyloxy shift/[3+2] cycloaddition of terminal 1,9‐enynyl esters. The reaction proceeds under extremely mild conditions and represents one of the relatively few transition‐metal‐catalyzed intramolecular cycloaddition reactions for the synthesis of benzazocines.     

Tandem cycloaddition reactions of allenyl diazo compounds forming triquinanes via trimethylenemethane diyls

A tandem reaction strategy for forming triquinanes from linear allenyl diazo compounds through an intramolecular 1,3-dipolar cycloaddition reaction of an allenyl diazo group that generates a trimethylenemethane (TMM) diyl followed by an intramolecular [2 + 3] TMM diyl cycloaddition reaction has been developed. The new tandem cycloaddition reaction is readily applicable to the synthesis of complex molecules with high versatility and efficiency.     

Parallel Synthesis of Tetraaryl-4,5-dihydro-1,2,4-triazoles via 1,3-Dipolar Cycloaddition on Soluble Polymer Support

The polymer-supported liquid-phase synthesis of small organic molecules has been a subject of intense research activity. 1,2,4-Triazoles are well known for their antifungal[1] and antibacterial[2] activities. This moiety was also found in potent agonist or antagonist receptor lingands. The synthesis of substituted 1,2,4-triazoles via 1,3-dipolar cycloaddition of imines with nitrilimines is well documented.[3] Herein, we would like to report the first liquid-phase synthesis of 1,3,4,5-tetraaryl-4,5-dihydro-1,2,4-triazoles through a 1,3-dipolar cycloaddition of imines with nitrilimines on PEG support.     

Asymmetric synthetic access to the hetisine alkaloids: total synthesis of (+)-nominine

A dual cycloaddition strategy for the synthesis of the hetisine alkaloids has been developed, illustrated by a concise asymmetric total synthesis of (+)-nominine (7). The approach relies on an early-stage intramolecular 1,3-dipolar cycloaddition of a 4-oxido-isoquinolinium betaine dipole with an ene-nitrile dipolarophile. Subsequent late-stage pyrrolidine-induced dienamine isomerization/Diels-Alder cascade allows for rapid construction of the carbon--nitrogen polycyclic skeleton within this class of C(20)-diterpenoid alkaloids.     

A Nature-Inspired Diels-Alder Reaction Facilitates Construction of the Bicyclo[2.2.2]octane Core of Andibenin B

A rapid synthesis of the bicyclo[2.2.2]octane core of andibenin B via a nature-inspired intramolecular [4+2] cycloaddition is described. This cycloaddition permits the construction of a sterically congested bicycle and simultaneously establishes three new all-carbon quaternary stereogenic centers in a highly efficient fashion.     

Iodine‐Promoted Synthesis of 1‐Alkyl‐3‐aroylindolizines from N‐(Aroylmethyl)pyridinium Salts and Aliphatic Aldehydes

An effective and practical method has been developed for the diversity‐oriented synthesis of 1‐alkyl‐3‐aroylindolizines via the 1,3‐dipolar cycloaddition of pyridinium ylides and aliphatic aldehydes in the presence of molecular iodine and a catalytic amount of MnO₂. The synthesis proceeds by tandem reactions involving [3+2] cycloaddition, dehydration of the cycloadduct, and dehydroaromatization. Molecular iodine served both as a catalyst and a dehydroaromatization reagent in the reaction.     

New strategy for convergent steroid synthesis

We published recently our results on a new and convergent synthesis of natural steroids. The strategy was based on a cycloaddition reaction of Nazarov reagents 2 and 5 with cyclohexenones 1 and 4. In this paper we report results that deal with the synthesis of two new bicyclic Nazarov reagents (13 and 19) and their cycloaddition with two cyclohexenones (1 and 4). These new results constitute an important improvement concerning the versatility of the strategy since tetracycles having the stereochemistry found in natural steroids are now available.