Recently, I came to know that lactols can be converted to lactones but I wasn't able to find the suitable reagent and the reaction mechanism. Can someone elaborate the same?
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$\begingroup$ Can tollen's reagent also be used for this? $\endgroup$– nikolaCommented Jan 17, 2017 at 12:49
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$\begingroup$ Are you oxidizing for analysis or for synthesis? Tollen's reagent is unsuitable for medium or large scale applications due to risk of explosion. $\endgroup$– ZheCommented Jan 17, 2017 at 14:50
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$\begingroup$ I came across this in a question where one of the steps involved using tollens reagent on a lactol, so I was just curious if it could be used or not, is there like a restriction on oxidation by tollens reagent like in case of fehling solution (which does not oxidise aromatic aldehydes). $\endgroup$– nikolaCommented Jan 17, 2017 at 15:12
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2 Answers
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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.
(source: literatumonline.com)
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).
answered Apr 2, 2018 at 17:54
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Lactols are basically aldehydes in ring formation.
What happens is the lactols unwind in solution into chain formation and it is here that they are oxidized to carboxylic acid by something like Tollen's reagent. Then when they reform, they do so in lactone form.
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$\begingroup$ The lactol does not have to open to the hydroxy aldehyde to oxidize. The aldehyde hydrates before oxidation. The lactol is in effect a hydrated aldehyde. PCC oxidations of primary alcohols often give esters as by-products. PCC and Collins oxidation of 1,4-butanediol afford butyrolactone and not succindialdeyde. $\endgroup$ Commented Dec 14, 2017 at 2:19