What is the mechanism for the oxidation of a lactol to a lactone?
Methyl(trifluoromethyl)dioxirane (TFDO) in 1,1,1‐trifluoro‐2‐propanone (TFP)may be able to do your sought oxidation (see image [1] below): In there, cyclic ethers (shown example is tetrahydropyran) yield lactones by subsequent oxidation of a stable cyclic hemiacetal intermediate such as lactol in your case.



It is considered as selective and reactive oxidizing agent, capable of epoxidation of unreactive alkenes and arenes, oxyfunctionalization of alkanes, and oxidation of alcohols and ethers in following reference: Encyclopedia of Reagents for Organic Synthesis (1; March 2016); 11 pages. (https://doi.org/10.1002/047084289X.rm267.pub3)

You may also read: Oxidations by methyl(trifluoromethyl)dioxirane. 5. Conversion of alcohols into carbonyl compounds: J. Am. Chem. Soc., 1991, 113(6), 2205–2208 (DOI: 10.1021/ja00006a044).