trans-Cyclooctenes and trans-cycloheptenes have long been the subject of physical organic study, but the broader application had been limited by synthetic accessibility. This account describes the development of a general, flow photochemical method for the preparative synthesis of trans-cycloalkene derivatives. Here, photoisomerization takes place in a closed-loop flow reactor where the reaction mixture is continuously cycled through Ag(I) on silicagel. Selective complexation of the trans-isomer by Ag(I) during flow drives an otherwise unfavorable isomeric ratio toward the transisomer. Analogous photoreactions under batch-conditions are low yielding, and flow chemistry is necessary in order to obtain trans-cycloalkenes in preparatively useful yields. The applications of the method to bioorthogonal chemistry and stereospecific transannulation chemistry are described.The unusual reactivity and well-defined chiral structure of trans-cycloalkenes has made them attractive targets for synthesis for nearly 70 years. [1][2][3] For example, trans-cyclooctene possesses planar chirality and displays a high barrier to racemization (E a = 35.6 kcal/mol), [4] and the most stable "crown" conformer has an alternating sequence of equatorial and axial hydrogens that is akin to chair cyclohexane. [5][6] The double bond of trans-cyclooctene is twisted severely in the crown conformation, [7] and as a consequence the HOMO of trans-cyclooctene is relatively high in energy. [8] trans-Cycloheptene also has a rigid structure with a distorted alkene. [7] The double bonds of medium-ring trans-alkenes are twisted. [7] trans-Cycloalkenes display unusual reactivity in HOMOalkene controlled cycloaddition reactions with dienes, [9] 1,3dipoles [8] and ketenes. [10] Strained trans-cycloalkenes also serve as ligands for transition metals. [11][12][13][14][15][16] This reactivity profile has made trans-cycloalkanes interesting targets for applications in synthesis and biology.The broadest applications of trans-cycloalkenes are in the field of bioorthogonal chemistry. The inverse-electron demand Diels-Alder reaction between tetrazines and strained alkenes has a rich history in physical organic and synthetic chemistry (Figure 1). [17][18][19] In 2008, three groups described the bioorthogonal reactions of tetrazines with strained alkenes. [20][21][22] The variant introduced by our group that used trans-cyclooctene is marked by exceptionally rapid kinetics, with rate constants that can exceed 10 6 M À 1 s À 1 in the fastest cases. [23][24] With the advances including the development of fluorogenic tetrazines [25] and reactions in live cells [26] and animals, [27] the tetrazine ligation has become a widely used tool for applications that span chemical biology, biomedical imaging, and materials science. [28][29][30][31][32][33][34][35][36][37] This account describes the development of general, flow photochemical methods for the preparative synthesis of transcycloalkene derivatives and enabled applications in synthesis and bioorthogonal chemistry. Selectiv...
