Sequential Norrish Type II Photoelimination and Intramolecular Aldol Cyclization of 1,2-Diketones in Carbohydrate Systems: Stereoselective Synthesis of Cyclopentitols

摘要

The photochemical behavior of 1,2-diketones differs considerably from that of monoketones and has received a great deal of attention from a theoretical viewpoint over the years. Suitably substituted aliphatic 1,2-diketones exhibit remarkable regioselectivity of intramolecular 1,5-hydrogen-atom transfer, and the 1,4-biradical intermediate yields almost exclusively 2-hydroxycyclobutanones (Norrish-Yang photocyclization). Another important difference between 1,2-diketones and monoketones is that the former compounds do not undergo Norrish type II photoelimination to a large extent. In our opinion, two principal drawbacks have hampered the development of synthetic applications of this 1,5-hydrogen-atom transfer: First, rate constants for the hydrogen abstraction are only about 1% as large as for monoketones. Second, some aliphatic 1,2-diketones are not very stable and are difficult to prepare by standard methods, especially from sensitive substrates.