The Schenck Ene Reaction: Diastereoselective Oxyfunctionalization with Singlet Oxygen in Synthetic Applications

Oxyfunctionalized molecules are principal building blocks in organic synthesis. In cellular processes highly efficient enzymes serve as selective catalysts for the formation of such synthetic units, for example the oxygenases oxyfunctionalize substrates by activating molecular oxygen. To date no comparable effective chemical oxidation system has been found. A useful photochemical process is the oxyfunctionalization of allylic substrates by sensitized photooxygenation, for which molecular oxygen and light serve as natural sources. This allylic oxidation of olefins by the ene reaction with singlet oxygen (Schenck reaction) figures as a highly versatile synthetic method. While the regioselectivity of this transformation has been studied for decades, only during the last years has attention focused on stereocontrol. Through these recent efforts it has become possible to control high stereoselectivity in the photooxygenation of organic substrates. This breakthrough has enhanced substantially the utility of singlet oxygen in diastereoselective synthesis. The obstinate singlet oxygen can be tamed by a number of factors: steric, stereoelectronic, electronic, and conformational effects of the functional group X play an important role in controlling the direction of the attack of singlet oxygen on alkenes [Eq. (a)]. This unfortunately rarely employed reaction, the Schenck ene reaction, yields diastereoselectively oxyfunctionalized compounds—useful intermediates in organic synthesis. X = hydroxyl, amino, halogen, ether, ester, sulfoxide, sulfone.