Recent Developments in the [5+2] Cycloaddition

The rapid generation of molecular complexity in a relatively easy manner has made the [5+2] cycloaddition approach a highly useful tool in the synthesis of a wide number of complex natural products and important biologically active products containing seven‐membered rings. Besides the common and highly efficient [5+2] cycloadditions of oxidopyrylium and oxidopyridinium ions with π‐systems which offer an easy access to a wide range of novel heterocyclic seven‐membered ring molecules with a oxygen or nitrogen bridge, and the less employed [5+2] photocycloadditions, the metal‐catalyzed [5+2] cycloadditions have attracted a lot of attention and have become one of the most popular ways of constructing seven‐membered ring products. All these cycloadducts can be synthetically manipulated easily toward a number of interesting molecular structures with high potential applications. Abbreviations: Ac: acetyl; Ar: aryl; Bar F : tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate; BINAP: 2,2′‐bis(diphenylphosphanyl)‐1,1′‐binaphthyl; Bn: benzyl; Boc: tert ‐butoxycarbonyl; Bu: butyl; Bz: benzoyl; Cbz: benzyloxycarbonyl; cod: cyclooctadiene; Cp: cyclopentadienyl; DBU: 1,8‐diazabicyclo[5.4.0]undec‐7‐ene; DCE: 1,2‐dichloroethane; de : diastereomeric excess; DFT: density functional theory; DMF: dimethylformamide; DMSO: dimethyl sulfoxide; dppb: 1,4‐bis(diphenylphosphino)butane; dppe: 1,2‐bis(diphenylphosphine)ethane; dppp: 1,3‐bis(diphenylphosphino)propane; ee : enantiomeric excess; Et: ethyl; HFIP: hexafluoroisopropyl alcohol; L: ligand; M: metal; Me: methyl; MOM: methoxymethyl; Ms: mesyl; Ph: phenyl; PMB: para ‐methoxybenzoyl; Pr: propyl; Segphos: 5,5′‐bis(diphenylphosphino)‐4,4′‐bi‐1,3‐benzodioxole; TBDPS: tert ‐butyldiphenylsilyl; TBS: tert ‐butyldimethylsilyl; TEA: triethylamine; Tf: trifluoromethanesulfonyl; TFE: trifluoroethanol; THP: tetrahydropyran; TIPS: triisopropylsilyl; TMS: trimethylsilyl; Tol: tolyl; Tp: hydridotris(pyrazolyl)borate; Ts: 4‐toluenesulfonyl (tosyl); VCP: vinylcyclopropane.