Oxygen-containing heterocycles such as cyclic ethers and lactones are found in the skeletons of many biologically active natural products. Some contain five-or six-membered rings and mediumring cyclic ethers, such as ciguatoxin, laurencin, and helianane are examples of such compounds. Many pharmaceuticals are derived from the skeletons of biologically active natural products and the activity of oxygen-containing medium rings has attracted attention in drug discovery research. However, compared with that of five-and six-membered rings, the formation of mediummembered rings is more challenging, and the development of a general synthetic method is desired. Furthermore, epoxides and oxetanes, which are three-and four-membered ring ether structures, respectively, are valuable synthetic elements characterized by ring-opening reactions derived from ring distortion. Oxetanes are also known as bioisosteres of gem-dimethyl and carbonyl groups in drug discovery research. Oxygen-containing heterocycles may be synthesized via intramolecular hydroalkoxylation and hydroacyloxylation, in which hydroxy or carboxy groups bond directly to the olefins. We attempted to synthesize oxygen-containing heterocycles by applying the MHAT/RPC mechanism to alkenyl alcohols and carboxylic acids. First, various cyclic ethers 2 were synthesized in high yield using the cobalt complex C1, Nfluoro-2,4,6-trimethylpyridinium trifluoromethanesulfonate (Me3NFPY·OTf), and 1,1,3,3tetramethyldisiloxane [(Me2SiH)2O] in toluene at room temperature (Table ). For example, tetrahydrofurans 2a and 2b were synthesized from alkenyl alcohols containing monosubstituted alkenes and 2c from 1,1-disubstituted olefin in high yields. Starting materials containing acetals, diols, and steroid skeletons could also be used to produce 2d, 2e, and 2f, respectively. When the starting material contained a phenolic hydroxyl group, complex products were formed (2g). Sixmembered products 2h and isochromanes 2i-2k could be synthesized in high yields. Applying the same reaction conditions to alkenyl carboxylic acid 3 resulted in the formation of various five-and six-membered ring lactones 4. In addition to monosubstituted alkenes, 1,1-disubstituted alkenes were used to obtain 4a-4f in high yields.