NaHSO 4 .H 2 O promoted oxidative deprotection of trimethylsilyl, tetrahydropyranyl and methoxymethyl ethers with HIO 3

Different types of trimethylsilyl, tetrahydropyranyl and methoxymethyl ethers are efficiently converted to their corresponding aldehydes and ketones with HIO3 in the presence of NaHSO4.H2O. All reactions were performed in moistured CH3CN in good to high yields.


Introduction
Aldehydes and ketones represent an important class of products and intermediates in the fine chemicals and specialities. 13][4][5][6][7][8][9] Some of these methods involve the use of expensive reagents, long reaction times, low yields of the products and tedious work-up.Therefore, introduction of new methods and reagents for such functional group transformation is still in demand.
Application of solid acids in organic transformations have a very important role, because solid acids have many advantages such as simplicity in handling, decrease reactor and plant corrosion problems, and more environmentally safe disposal.Also wastes and by-products can be minimized or avoided by developing cleaner synthesis routes. 10During the last decade, the use of metal hydrogen sulfates have attracted the attention of many organic chemists.The stability and cheapness of most of these compounds, high yields of the products and relatively short reaction times are important advantages of these reagents.
To the best of our knowledge, there are only three reports in the literature related to the preparation of carbonyl compounds via the oxidation of alcohols by using HIO 3 , [11][12][13] but there is not any report about the oxidation of trimethylsilyl and tetrahydropyranyl ethers using this reagent.
In continuation of our studies on the application of metal hydrogen sulfates in the promotion of organic reactions [14][15][16][17][18] and also using HIO 3 in organic transformations , 12,13,[19][20][21] we have found that HIO 3 in the presence of NaHSO 4 .H 2 O is able to oxidize TMS and THP ethers to their corresponding carbonyl compounds under mild reaction conditions.Herein we wish to report an efficient, simple and cheap method for the oxidation of the above mentioned substrates to aldehydes and ketones using HIO 3 / NaHSO 4 .H 2 O reagent system in moistured CH 3 CN.All reactions were performed under mild conditions in good to high yields (Scheme 1).Yields and reaction times are shown in Tables 1 and 2. Using this method benzylic TMS and THP ethers, including electron donating and withdrawing groups are converted to their corresponding carbonyl compounds in good to high yields.Aliphatic TMS and THP ethers are also efficiently oxidized to their corresponding aldehydes and ketones under the same reaction conditions.In all cases, no over-oxidation products were observed.
The methoxymethylation is also used for the protection of alcoholic hydroxyl groups.Although a number of methods are available for deprotection of methoxymethyl ethers to the parent alcohols, 22,23 but, to the best of our knowledge and in spite of TMS and THP ethers, there is only one report for the direct oxidation of these type of ethers to their corresponding carbonyl compounds. 24Our investigation clarified that HIO 3 in the presence of NaHSO 4 .H 2 O can also be used for the oxidation of various methoxymethyl ethers to the aldehydes and ketones mostly in quantitative yields as well as TMS and THP ethers (Scheme 1, Table 3).
It is important to note that the progress of the reaction is extremely depends on the presence of NaHSO 4 .H 2 O in the reaction mixture, so that in the absence of this reagent, the starting material remains intact during the course of the reaction.This observation suggests the important role of sodium hydrogen sulfate monohydrate in the promotion of the oxidation reactions.On the other hand when the reaction is carried out in dry CH 3 CN the reaction time became very long.This result clarifies that the presence of water is essential for such processes.A plausible mechanism for the oxidation reactions is shown in Scheme 2.

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a Products were characterized by their physical constants, comparison with authentic samples and IR and NMR spectroscopy.b Isolated yields.
In conclusion, in this study we have developed a mild, efficient and simple method for the oxidative deprotection of trimethylsilyl, tetrahydropyranyl and methoxymethyl ethers.In addition, the low cost and availability of the reagents, easy and clean work-up, and high yields are other advantages of the present method.

Experimental Section
General Procedures.[27] All oxidation products are known compounds; they are identified by comparison of their physical data, IR and NMR spectra with those of authentic samples.Yields refer to isolated products or their 2,4dinitrophenylhydrazones.

General Procedure
To a solution of the substrate (1 mmol) in CH 3 CN (3 mL), which is moistured witha drop of water, were added HIO 3 (1.5 mmol, 0.264 g) and NaHSO 4 .H 2 O (0.25 mmol, 0.035 g) and refluxed for the appropriate time (Tables 1 and 2).The progress of the reaction was monitored by TLC.After completion of the reaction, the mixture was filtered and the solid residue was washed with CH 3 CN (5 mL).The solvent was evaporated and the residue was triturated with H 2 O (10 mL).The product was extracted with diethyl ether (3×5 mL) the combined organic solution was dried over MgSO 4 and filtered.Evaporation of the solvent followed by column chromatography on silica gel gave the corresponding carbonyl compounds in good to high yields. 28 Scheme 1

Table 1 .
Oxidative deprotection of trimethylsilyl ethers a a Products were characterized by their physical constants, comparison with authentic samples and IR and NMR spectroscopy.b Isolated yields.

Table 2 .
Oxidative deprotection of tetrahydropyranyl ethers a

Table 3 .
Oxidative deprotection of methoxymethyl ethers (MOM-ethers).a a Products were characterized by their physical constants, comparison with authentic samples and IR and NMR spectroscopy.b Isolated yields.