Concise Approach to Cyclohexyne and 1,2-Cyclohexadiene Precursors

Access to Complex Scaffolds Through [2 + 2] Cycloadditions of Strained Cyclic Allenes

We report the strain-induced [2 + 2] cycloadditions of cyclic allenes for the assembly of highly substituted cyclobutanes. By judicious choice of trapping agent, complex scaffolds bearing heteroatoms, fused rings, contiguous stereocenters, spirocycles, and quaternary centers are ultimately accessible. Moreover, we show that the resulting cycloadducts can undergo thermal isomerization. This study provides an alternative strategy to photochemical [2 + 2] cycloadditions for accessing highly functionalized cyclobutanes, while validating the use of underexplored strained intermediates for the assembly of complex architectures.

Strain-promoted reactions of 1,2,3-cyclohexatriene and its derivatives

Since 18251, compounds with the molecular formula C6H6-most notably benzene-have been the subject of rigorous scientific investigation2-7. Of these compounds, 1,2,3-cyclohexatriene has been largely overlooked. This strained isomer is substantially (approximately 100 kcal mol-1) higher in energy compared with benzene and, similar to its relatives benzyne and 1,2-cyclohexadiene, should undergo strain-promoted reactions. However, few experimental studies of 1,2,3-cyclohexatriene are known8-12. Here we demonstrate that 1,2,3-cyclohexatriene and its derivatives participate in a host of reaction modes, including diverse cycloadditions, nucleophilic additions and σ-bond insertions. Experimental and computational studies of an unsymmetrical derivative of 1,2,3-cyclohexatriene demonstrate the potential for highly selective reactions of strained trienes despite their high reactivity and short lifetimes. Finally, the integration of 1,2,3-cyclohexatrienes into multistep syntheses demonstrates their use in rapidly assembling topologically and stereochemically complex molecules. Collectively, these efforts should enable further investigation of the strained C6H6 isomer 1,2,3-cyclohexatriene and its derivatives, as well as their application in the synthesis of important compounds.

Three-Component Difunctionalization of Cyclohexenyl Triflates: Direct Access to Versatile Cyclohexenes via Cyclohexynes

Difunctionalization of strained cyclic alkynes presents a powerful strategy to build richly functionalized cyclic alkenes in an expedient fashion. Herein we disclose an efficient and flexible approach to achieve carbohalogenation, dicarbofunctionalization, aminohalogenation and aminocarbonation of readily available cyclohexenyl triflates. We have demonstrated the novel use of zincate base/nucleophile system for effective formation of key cyclohexyne intermediates and selective addition of various carbon and nitrogen nucleophiles. Importantly, leveraging the resulting organozincates enables the incorporation of a broad range of electrophilic partners to deliver structurally diverse cyclohexene motifs. The importance and utility of this method is also exemplified by the modularity of this approach and the ease in which even highly complex polycyclic scaffolds can be accessed in one step.

Origins of Endo Selectivity in Diels-Alder Reactions of Cyclic Allene Dienophiles

Strained cyclic allenes, first discovered in 1966 by Wittig and co-workers, have recently emerged as valuable synthetic building blocks. Previous experimental investigations, and computations reported here, demonstrate that the Diels-Alder reactions of furans and pyrroles with 1,2-cyclohexadiene and oxa- and azaheterocyclic analogs proceed with endo selectivity. This endo selectivity gives the adduct with the allylic saturated carbon of the cyclic allene endo to the diene carbons. The selectivity is very general and useful in synthetic applications. Our computational study establishes the origins of this endo selectivity. We analyze the helical frontier molecular orbitals of strained cyclic allenes and show how secondary orbital and electrostatic effects influence stereoselectivity. The LUMO of carbon-3 of the allene (C-3 is not involved in primary orbital interactions) interacts in a stabilizing fashion with the HOMO of the diene in such a way that the carbon of the cyclic allene attached to C-1 favors the endo position in the transition state. The furan LUMO, allene HOMO interaction reinforces this preference. These mechanistic studies are expected to prompt the further use of long-avoided strained cyclic allenes in chemical synthesis.

Synthesis of multisubstituted cycloalkenes through carbomagnesiation of strained cycloalkynes

An efficient synthetic method of seven- and six-membered cycloalkenes through the generation of strained cycloalkynes and following carbomagnesiation is described. Further bond formations of the resulting cycloalkenylmagnesium intermediates with a wide variety of electrophiles enabled us to prepare diverse cycloalkene derivatives including benzoxepine analogs having a fully substituted alkene structure.

1,2-Bis(arylthio)arene synthesis via aryne intermediates

Lithium 1,1-diadamantylamide (LDAM) base-promoted insertion of arynes into disulfide S-S bonds is described. After generation of arynes from readily available aryl halides and triflates, reactions with diaryl and diisopropyl disulfides afford the insertion products in moderate to excellent yields. Use of 1-cyclohexenyl triflate gave an excellent yield of 1,2-bis(phenylthio)cyclohexene.