Boron Tribromide‐Assisted Chiral Phosphoric Acid Catalysts for Enantioselective [2+2] Cycloaddition

Abstract

BBr -assisted chiral phosphoric acid catalysts for enantioselective [2+2] cycloaddition were developed. The reactions of phenyl vinyl sulfide with α-substituted acroleins proceeded, and the corresponding [2+2] cycloadducts were obtained with high enantioselectivity. In particular, the [2+2] cycloadduct obtained from methacrolein is a synthetically useful optically active cyclobutane, which could be transformed to a key intermediate for (+)-frontalin, a pheromone of Asian elephants.

Tris(pentafluorophenyl)borane‐Assisted Chiral Phosphoric Acid Catalysts for Enantioselective Inverse‐Electron‐Demand Hetero‐Diels‐Alder Reaction of α,β‐Substituted Acroleins

“…In our previous report, we achieved a highly enantioselective [2+2] cycloaddition of methacrolein 4 a with phenyl vinyl sulfide 3 a in the presence of BBr 3 ‐( R )‐ 2 a (Ar=4‐(2‐naphthyl)C 6 H 4 ), as an in situ‐ prepared chiral supramolecular Brønsted acid catalyst, in dichloromethane at −78 °C, and 5 aa was obtained in 93% yield with 95% ee (Scheme , eq. 1).…”

“…2). However, at that time, 6 ba was generated not only directly but also via the enantiomerically‐uncontrolled isomerization from [2+2] cycloadduct 5 ba . Therefore, to achieve both high reaction selectivity and high asymmetric induction of the inverse‐electron‐demand hetero‐Diels‐Alder reaction, a more careful combination of substrates and the corresponding catalysts might be necessary.…”

Intramolecular [2+2]‐Cycloaddition in the Synthesis of Polycyclic Tetrahydrothiopyran Derivatives Bearing a Cyclobutane Scaffold

“…Cycloaddition reactions where cyclic products are formed starting from acyclic precursors constitute a convenient and frequently employed synthetic strategy for the construction of carbo-and heterocyclic frameworks. [1][2][3][4][5][6] Among a myriad of available cycloadditions, [4 + 2]-variant including Diels-Alder reaction [2] and its inverse-electron-demand modification [3] are frequently employed. Interestingly, a related [2 + 2]-cycloaddition has been less explored (Scheme 1, top).…”

“…Interestingly, a related [2 + 2]-cycloaddition has been less explored (Scheme 1, top). [4][5][6] This important transformation leads to the formation of highly sterically congested cyclobutane derivatives. According to Woodward-Hoffman rules these reactions are photochemically induced.…”

“…According to Woodward-Hoffman rules these reactions are photochemically induced. However, organocatalytic [5] and Lewis-acidpromoted [6] formal [2 + 2]-cycloadditions proceeding via step-wise mechanism are known in the literature.…”