Catalytic Aerobic Oxidation of p-Methoxybenzyl (PMB) Ethers to Aldehydes or Ketones.

An efficient and highly selective aerobic oxidation of p-methoxybenzyl ethers to aldehydes or ketones with Fe(NO3)3 ⋅ 9H2O and TEMPO as catalysts at 25 °C has been developed. This method is compatible with versatile functional groups: adamantyl, ether, alkynyl, and alkenyl, etc. Due to the mild nature of the reaction conditions, even the highly sensitive optically active aldehydes with an easily racemized α-chiral center could be formed from optically active PMB ethers without racemization. Other protecting groups of hydroxyl group remained intact during the current aerobic oxidation. The catalytic protocol has been successfully applied to synthesis of (R)-tert-butyldimethylsilyl hept-1-en-6-yn-4-yl ether, which is the key intermediate for the total synthesis of natural product, (-)-6-O-methylcitreoisocoumarin.