Transition-Metal Catalysis of Triene 6π Electrocyclization: The π-Complexation Strategy Realized

Triene 6π electrocyclization, wherein a conjugated triene undergoes a concerted stereospecific cycloisomerization to a cyclohexadiene, is a reaction of great historical and practical significance. In order to circumvent limitations imposed by the normally harsh reaction conditions, chemists have long sought to develop catalytic variants based upon the activating power of metal–alkene coordination. Herein, we demonstrate the first successful implementation of such a strategy by utilizing [(C₅H₅)Ru(NCMe)₃]PF₆ as a precatalyst for the disrotatory 6π electrocyclization of highly substituted trienes that are resistant to thermal cyclization. Mechanistic and computational studies implicate hexahapto transition-metal coordination as responsible for lowering the energetic barrier to ring closure. This work establishes a foundation for the development of new catalysts for stereoselective electrocyclizations.     

Transition‐Metal Catalysis of Triene 6π Electrocyclization: The π‐Complexation Strategy Realized

Triene 6π electrocyclization, wherein a conjugated triene undergoes a concerted stereospecific cycloisomerization to a cyclohexadiene, is a reaction of great historical and practical significance. In order to circumvent limitations imposed by the normally harsh reaction conditions, chemists have long sought to develop catalytic variants based upon the activating power of metal–alkene coordination. Herein, we demonstrate the first successful implementation of such a strategy by utilizing [(C₅H₅)Ru(NCMe)₃]PF₆ as a precatalyst for the disrotatory 6π electrocyclization of highly substituted trienes that are resistant to thermal cyclization. Mechanistic and computational studies implicate hexahapto transition‐metal coordination as responsible for lowering the energetic barrier to ring closure. This work establishes a foundation for the development of new catalysts for stereoselective electrocyclizations.     

Solvent Effects on Aza‐anionic Cycloaromatization of 2‐(2‐Substituted‐ethynyl)benzonitriles

The methanolysis of 2‐(2‐aryl ethynyl)benzonitriles is accelerated when the polar aprotic solvents are added, which could enhance the 6‐endo pathway and give isoquinolones, though the 5‐exo pathway is occupied mostly. The yields of products would also be in creased.     

Preparation,Structural Properties and Thermal Isomerization of Hex‐3‐ene‐1,5‐diyne Bridged [2.2]Paracyclophanes

The [2.2]paracyclophane moiety is used as a spacer to connect the ends of a hex‐3‐ene‐1,5‐diyne unit, a π‐system that on thermolysis usually cycloaromatizes to a benzene ring (Bergman cyclization). For the preparation of the pseudo‐geminally‐bridged system 9 , the diacetylene 3 was chain‐extended to the diol 16 , which after conversion to the pseudo‐geminal dibromide 17 was ring‐closed by treatment with LiHMDS/HMPA to the [2.2]paracyclophane enediyne 9 . Whereas the McMurry coupling of the pseudo‐ortho bisaldehyde 24 resulted in the formation of the hexadienyne‐bridged cyclophane 27 , the pseudo‐ortho‐bridged hydrocarbon 11 was obtained by preparing first the diol 28 from 24 , converting the latter into the dioxolane 29 , which in the last step furnished the olefin 11 by treatment with Tf₂O/EtN(iPr)₂. The authentic Bergman product 10 of the pseudo‐gem‐bridged hexenediyne 9 was synthesized by a conventional sequence starting from the ethynyl formyl substrate 18 . Since the pseudo‐ortho‐enediyne‐bridged hydrocarbon 11 is thermally labile, its benzannelated derivate 34 was prepared. No classical Bergman cyclization reactions could be observed for any of the [2.2]paracyclophane‐bridged hexenediynes prepared here. In the pseudo‐gem‐series the fulvenes 14 and 15 were the only products that could be identified under thermal conditions (McMurry coupling); the benzannelated substrate 34 gave the benzofulvene‐bridged cyclophane 36 on photolysis. Bergman cyclizations yielding fulvene derivatives are extremely rare. The mechanism of the cyclization of 9 and 34 is discussed, using compliance constants. The structure assignments of the hydrocarbons synthesized in this study are based on spectroscopic studies as well as X‐ray structural analyses for 9 , 10 , 11 , 27 , and 34 .     

Synthesis of Benzo[a]carbazoles through Visible Light-Induced Cycloaromatization

We report herein a synthesis of 5,6-diarylbenzo[a]carbazoles by a sequence of 6π-electrocyclization followed by β-elimination. The highly functionalized 2,3-disubstituted indoles used for this cycloaromatization process are prepared by Pd-catalyzed cyclizative cross-coupling of ortho-alkynylanilines with ortho-alkynylbenzamides. The combination of these two reactions allows us to develop a facile synthesis of benzo[a]carbazoles directly from two easily accessible internal alkynes without isolation of the indole intermediates.     

Synthesis, reactions and DNA damaging abilities of 10-membered enediyne-sulfone and related compounds

The synthesis and Myers-Saito type cycloaromatization reactions of 10-membered enediynes containing a sulfone or a sulfoxide moiety are described. These enediynes cycloaromatized smoothly under physiological conditions to generate bioactive α,3-didehydrotoluene biradicals, that showed potent DNA damaging abilities. Further, in the presence of a nucleophile such as amine or sulfide, cycloaromatization did not occur following the radical reaction pathway but ionically induced cycloaromatization took place.     

APEX Strategy Represented by Diels–Alder Cycloadditions—New Opportunities for the Syntheses of Functionalised PAHs

Diels–Alder cycloaddition of various dienophiles to the bay region of polycyclic aromatic hydrocarbons (PAHs) is a particularly effective and useful tool for the modification of the structure of PAHs and thereby their final properties. The Diels–Alder cycloaddition belongs to the single-step annulative π-extension (APEX) reactions and represents the maximum in synthetic efficiency for the constructions of π-extended PAHs including functionalised ones, nanographenes, and π-extended fused heteroarenes. Herein we report new applications of the APEX strategy for the synthesis of derivatives of 1,2-diarylbenzo[ghi]perylene, 1,2-diarylbenzo[ghi]perylenebisimide and 1,2-disubstituted-benzo[j]coronene. Namely, the so far unknown cycloaddition of 1,2-diarylacetylenes into the perylene and perylenebisimide bay regions was used. 1,2-Disubstituted-benzo[j]coronenes were obtained via cycloaddition of benzyne into 1,2-diarylbenzo[ghi]perylenes by using a new highly effective system for benzyne generation and/or high pressure conditions. Moreover, we report an unprecedented Diels–Alder cycloaddition–cycloaromatisation domino-type reaction between 1,4-(9,9-dialkylfluoren-3-yl)-1,3-butadiynes and perylene. The obtained diaryl-substituted core-extended PAHs were characterised by DFT calculation as well as electrochemical and spectroscopic measurements.     

Development of an Allenyne-Alkyne [4+2] Cycloaddition and its Application to Total Synthesis of Selaginpulvilin A

A new [4+2] cycloaddition of allenyne-alkyne is developed. The reaction is believed to proceed with forming an α,3-dehydrotoluene intermediate. This species behaves as a σπ-diradical to react with a hydrogen atom donor, whereas it displays a zwitterionic reactivity toward weak nucleophiles. The efficiency of trapping α,3-dehydrotoluene depends not only on its substituents but also the trapping agents. Notable features of the reaction are the activating role of the extra alkyne of the 1,3-diyne that reacts with the allenyne moiety and the opposite mode of trapping with oxygen and nitrogen nucleophiles. Oxygen nucleophiles result in the oxygen-end incorporation at the benzylic position of the α,3-dehydrotoluene, whereas with amine nucleophiles the nitrogen-end is incorporated into the aromatic core. Relying on the allenyne-alkyne cycloaddition as an enabling strategy, a concise total synthesis of phosphodiesterase-4 inhibitory selaginpulvilin A is realized.