Allylic Oxidations in Natural Product Synthesis

Although C–H oxidation of hydrocarbons is generally difficult, allylic C–H oxidation is relatively simple and predictable, even on a preparative scale, because active species generated at the allylic position are stabilized by the double bond. Therefore, allylic oxidation has been employed in natural product synthesis, and a variety of reagents and conditions for allylic oxidation have been reported. However, reagents and conditions suitable for natural product synthesis are limited in terms of efficiency and chemo-, regio-, and stereoselectivity, owing to the structural and characteristic diversity of natural products. This review addresses allylic oxidations, highlighting reagents and conditions that meet the requirements for natural product synthesis. 1 Introduction 2 Selenium Reagents 2.1 Selenium Dioxide 2.2 Diphenyldiselenide–Iodoxybenzene 3 Chromium(VI) Reagents 3.1 Chromic Acid and Chromate Ester 3.2 Chromium Trioxide–3,5-Dimethylpyrazole (CrO 3 ·3,5-DMP) 3.3 PCC and PDC 4 Transition-Metal Reagents 5 Others 6 Conclusion