Protective groups are commonly used in the synthesis of pharmaceuticals and natural products; however, more steps are required for protection and deprotection processes. However, protecting groups improve solubility, reduce polarity, or temporarily stabilize starting materials. Shigehisa found that cyclized products could be obtained directly from protected alkenyl alcohols and alkenyl carboxylic acids after deprotection under MHAT/RPC conditions. For example, when protected alkenyl alcohols 9a-9d were subjected to MHAT/RPC conditions, cyclic ether 10 was obtained in a high yield (Table ). The substrate scope was investigated for three typical protective groups (TBS, MOM, and Bn), and the substrates comprising the MOM group generally afforded the product in the highest yields. Substrates with phenolic hydroxyl groups showed a significant improvement in product yield when protective groups were used. 12 was obtained from 11 in high yield, and the MOM group, which was not involved in the cyclization, remained unchanged. Deprotective cyclization also proceeded from various alkenyl esters 13a-13d, directly yielding lactone 14 in a high yield. Deprotection occurred after formation of an oxonium intermediate. Structural determination of the coproducts revealed that the MOM, Bn, and Me groups, which protected the hydroxyl and carboxyl groups, were eventually trapped by collidine.