Recent Developments in the [5+2] Cycloaddition

The [5+2] cycloaddition allows the synthesis of a diversity of complex highly functionalized seven‐membered products in a single step. These cycloadducts can readily be further manipulated synthetically for use in the synthesis of a number of complex natural products and important biologically active products containing seven‐membered rings. In addition to the common and highly efficient [5+2] cycloadditions of (oxido)pyrylium and (oxido)pyridinium ions with various π‐systems, providing an easy access to a wide range of novel heterocyclic seven‐membered rings exhibiting an oxygen or nitrogen bridge, the metal‐catalyzed [5+2] cycloadditions still attract a great attention and have become one of the most popular ways of constructing seven‐membered compounds. Among the most important reactions are metal‐catalyzed (hetero) [5+2] cycloadditions of vinyl‐substituted three‐membered rings, rhodium‐catalyzed [5+2] cycloadditions of 3‐acyloxy‐1,4‐enynes, and metal‐catalyzed [5+2] cycloadditions of ortho ‐vinylphenols and ortho ‐vinyl/arylanilines. Abbreviations : Ar: aryl; Bipy: bipyridine; Bn: benzyl; Boc: tert ‐butoxycarbonyl; cod: cyclooctadiene; coe: cyclooctene; Cp: cyclopentadienyl; Cp*: pentamethylcyclopentadienyl; DABCO: 1,4‐diazabicyclo[2.2.2]octane; dbcot: dibenzocyclooctatetraene; DBU: 1,8‐diazabicyclo[5.4.0]undec‐7‐ene; DCE: 1,2‐dichloroethane; de : diastereomeric excess; DIPEA: diisopropylethylamine; DMAP: 4‐( N,N ‐dimethylamino)pyridine; DMF: N,N‐ dimethylformamide; dppb: 1,4‐bis(diphenylphosphino)butane; dppe: 1,2‐bis(diphenylphosphino)ethane; ee : enantiomeric excess; Hept: heptyl; Hex: hexyl; HFIP: hexafluoroisopropyl alcohol; L: ligand; M: metal; MCPBA: 3‐chloroperoxybenzoic acid; MOM: methoxymethyl; Mes: mesyl; MS: molecular sieves; Naph: naphthyl; NBD: norbornadiene; Ns: nosyl (4‐nitrobenzenesulfonyl); Pent: pentyl; Phth: phthaloyl; Piv: pivaloyl; PMB: p ‐methoxybenzyl; rr : regioselectivity ratio; r.t.: room temperature; TBS: tert ‐butyldimethylsilyl; TES: triethylsilyl; Tf: trifluoromethanesulfonyl; TFA: trifluoroacetic acid; TFE: 2,2,2‐trifluoroethanol; THF: tetrahydrofuran; TIPS: triisopropylsilyl; TMP: 2,2,6,6‐tetramethylpiperidine; TMS: trimethylsilyl; Tol: tolyl; Ts: 4‐toluenesulfonyl (tosyl) magnified image