Catechols, benzyl ethers

Step 6. The catecholic and phenolic ethers were removed by treatment with hydrobromic acid. Benzyl ethers are frequently removed by reduction (e.g. hydrogenation) but reduction, of course, would not remove the methyl ether. The mechanism of the deprotection is shown below. 

N-Salicylideneamines, to protect amines, 370 Selective cleavage, 411-412 of benzyl carbamates, 339-340 of benzyl ethers, 49-56 of catechol protective groups, 171 of N- vs. 5-dimethylphosphinothioyl groups, 307... 

Diisobutylaluminum hydride (DIBAL-H) hydrogenolyzes simple ortho esters to acetals at room temperature and reduces acetals and ketals to ethers at 70-80 °C (equation 6) benzyl ethers are cleaved at still higher temperatures. This reagent shows good selectivity and considerable versatility. It has been used to reduce acetals of formaldehyde (equation 7), ° which few other reagents can accomplish. With catechol ketals a single reductive cleavage occurs at room temperature (equation 8). ... 

Formation of benzyl ethers from substances related to catechols... 

No chapter on alkylation methodology can hope to cover all facets of the topic, let alone make reference to all papers which bear on the subject. Here, attention has been focused on the most commonly used types of derivatives and procedures which, owing to their reliability, have found wide application. Since esterification with alcohols in the presence of acid catalysts is dealt with in Chapter 2, the method is considered here only with respect to simple glycosidation of sugars and the formation of benzyl ethers from substances related to catechol. Silyl ether derivatives are dealt with in Chapter 4. 

To a solution of chiral sulfide catalyst (9.5 mg, 0.05 mmol) and catechol (1 mg, 1 pmol) in t-BuOH (450 pL) was added the benzaldehyde (26 pL, 0.25 mmol), benzyl bromide (60 pL, 0.5 mmol) and n-Bu4NI (92 mg, 0.25 mmol). The resulting mixture was vigorously stirred at r.t. for 15 min (20-24 °C), and an aqueous solution of NaOH 28.5% (52 pL, 0.5 mmol, giving a 90 10 ratio of -BuOH water) was subsequently added. After 24 h of reaction, water (6 mL) was added, the two phases were separated, and the aqueous layer was extracted with diethyl ether (2x5 mL). The combined organic layers were dried over MgSC>4 and concentrated in vacuo. Purification by silica gel column chromatography afforded the two diastereoisomeric epoxides. 

The protecting group can be removed under mild conditions by treating 372 with one equivalent of catechol boron bromide [118]. This reagent is also effective for cleaving other protecting groups, such as /er/-butyl, allyl, benzyl or MOM ethers, as well as N-Cbz and N-Boc functionalities. 

Preparation.—Ethers. A new method for benzylation of hindered glucidic hydroxy groups e.g. 73) requires reaction of the alkoxide with benzyl bromide and a catalytic quantity of a quaternary ammonium salt, and it is suggested that weakened ion-pairing might account for the increased efficiency. A simple, high yield, methy-lenation of catechols with dihalomethanes, in the absence of the usual strong... 

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