New cycloaddition reactions of 1-phenyl-4-vinylpyrazole

Revisiting the Chemistry of Vinylpyrazoles: Properties, Synthesis, and Reactivity

“…Vinylpyrazoles are very reluctant to react as dienes in DA cycloadditions because of the loss of aromaticity of the pyrazole ring on [4 + 2] cycloadduct formation being much less reactive than vinylpyrroles and vinylindoles. Harsh reaction conditions are required, such as sealed vessels and high pressures (8–10 atm) and temperatures (120–140 °C) for long reaction times (several days), to obtain very low or moderate yields [ 50 , 51 ]. So far, the vinylpyrazoles have been used as dienes in Diels–Alder reactions for the preparation of compounds with medicinal interest, being these ones of the most described reactions of vinylpyrazoles in literature.…”

Diels-Alder reactions of 1-amino-1,3-dienes and related systems

“…N-Phenyl-4-vinylpyrazole (426, R = Ph) reacted with DMAD in a sealed vessel at high temperature to give the corresponding indazole derivatives. In the case of using methyl propiolate and NPM at 150 ºC resulted DA adducts 427 and 428, respectively in very poor yields and also other products resulting from an ene-reaction (Scheme 105) [274]. The same group [275] reported cycloaddition reactions of 1-tert-butyl-4-vinylpyrazole (426, R = tBu) with DMAD, methyl propiolate and NPM giving the corresponding products in poor yields (4-15%).…”

Redirecting a Diels–Alder Reaction toward (2 + 2)-Cycloaddition

“…Current approaches for guiding the reaction along the (2 + 2) cyclization path are outlined in Scheme . They consist in (1) stabilization of the s-trans conformation of the intermediate carbocation by conformational constraints , or steric clashes, ,− or alternatively in (2) utilization of non-ene stabilizing groups, such as ethers/sulfides/selenides, − amines, , or aryles. , These stabilizing groups cannot be easily modified, which limits value of this approach in cyclobutane synthesis.…”