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20.5.1.3.1.1.5 Method 5: Using Sodium or Calcium Hypochlorites

DOI: 10.1055/sos-SD-020-00673

Yan, L.; Lin, S.; Liu, P.Science of Synthesis, (200720732.

Inexpensive sodium or calcium hypochlorite, in the presence of acetic acid and an alcohol, can be used to oxidize a variety of aldehydes to the corresponding esters in good yields. Some representative reactions, such as the oxidation of heptanal to afford methyl heptanoate (27), are collected in Scheme 8.[‌23‌‌25‌] In these reactions it is likely that hypochlorous acid, generated in situ, oxidizes a hemiacetal intermediate to form the ester; α-chlorination of ketones is generally not observed (Scheme 8). Using calcium hypochlorite the tert-butyl ester 28 is prepared in 78% yield when tert-butyl alcohol is the co-reagent,[‌24‌] and even the hindered aldehyde 29 can be readily converted into methyl ester 30 using calcium hypochlorite and methanol, followed by O-debenzylation. There are, however, limitations to the use of this methodology; thus, whereas electron-poor aldehydes, such as 3-nitrobenzaldehyde, can be oxidatively esterified to give reasonable yields of the appropriate benzoates, the oxidation of electron-rich aromatic aldehydes, for example 4-methoxybenzaldehyde, often gives low yields due to competitive ring chlorination. Similarly, certain alkenes are susceptible to electrophilic attack by the reagent, so that in some cases benzyl ethers may partially undergo debenzylation under the reaction conditions.[‌25‌] Fortunately, however, the addition of a chlorine scavenger, such as 2-methylbut-2-ene, may suppress such side reactions.[‌26‌,‌27‌] Hypochlorous acid can also be generated by mixing hydrogen peroxide and hydrochloric acid; this reagent is as effective as that prepared from sodium hypochlorite and acetic acid and it converts benzaldehyde into methyl benzoate when methanol is present (Scheme 8).[‌28‌]

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