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DOI: 10.1039/C5RA09986B (Paper) RSC Adv., 2015, 5, 59114-59119

SeO2 in water: a mild and efficient promoter for deprotection of acetyl, methoxymethyl and tetrahydropyranyl ethers and sequel oxidation of methyl/methylene carbons of alpha carbonyl carbon†

Gulab Khushalrao Pathe and Naseem Ahmed*
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247 667, India. E-mail: nasemfcy@iitr.ac.in; Fax: +91 1332 285745

Received 27th May 2015 , Accepted 1st July 2015

First published on 2nd July 2015

Abstract

SeO2 in water provided an efficient and one-pot deprotection of acetyl, tetrahydropyranyl and methoxymethyl ethers in alcohols and phenols and sequel oxidation of methyl/methylene carbons of alpha carbonyl carbon to dicarbonyl functional groups at 80 °C in 30–60 min. Using a substrate, SeO2 in a 1[thin space (1/6-em)]:[thin space (1/6-em)]3 ratio, the reaction gave excellent yields (85–95%) for acetyl and tetrahydropyranyl deprotections and a moderate yield (35–40%) for methoxymethyl deprotection without affecting other functional groups.

Introduction

Protection and deprotection of functional groups is one of the most frequently used strategies in multi-step organic syntheses. In particular, the protection and the deprotection of hydroxyl and phenolic groups is extremely important because of their presence in a number of compounds of natural products, biological and synthetic compounds such as carbohydrates, macrolides, peptides, steroids, nucleotides and polyethers.1 The protection of hydroxyl groups with 3,4-dihydro-2H-pyran (DHP) is the most common method because of the stability of the product, 2-tetrahydropyranyl ethers (THPEs) in strong basic conditions such as Grignard reagents, organolithium, metal hydrides, catalytic hydrogenation, alkylating and acylating conditions.2 Similarly, methoxymethyl chloride (MOMCl) and acetyl chloride/acetic anhydride (CH3COCl/Ac2O) reagents are used for the hydroxyl and phenolic group protection.3 Deprotection of these groups (acetyl, THP and MOM ethers) therefore required efficient methods to avoid product decomposition and/or loss of other functional groups in the molecules. Several catalytic methods have been explored for the selective deacetylation of alcohols and phenols under acidic and basic conditions. For example, the deprotection of acetate includes NaOMe,4a micelles,4b Zn–MeOH,4c cyclodextrin,4d enzymes,4e mettallo–enzyme,4f metal complexes,4g and antibodies,4h montmorillonite k-10,4i I2,4j NaBO3,4k and HCOONH4–SiO2 (ref. 4l) and of detetrahydropyranylation includes protic acids,5a–d Lewis acids like BF3–etherate,5e LiBr,5f LiBF4,5g LiOTf,5h LiClO4,5i Sc(OTf)3,5j In(OTf)3,5k I2,5l InCl3,5m ZrCl4,5n CuCl2,5o NH4Cl,5p graphite,5q clay materials,5r silica-supported sulfuric acid,5s electrogenerated acids,5t bis(trimethylsilyl)sulphate,5u Distannoxane,5v triphenyl phosphine dibromide,5w DDQ,5x and heteropoly acids.5y Similarly, for demethoxymethylation includes HCl,6a BBr3,6b pyridinium p-toluene sulphonate under strong acidic condition,6c ZnBr2 and TiCl4 in aprotic solvents,6d Me2BBr6e and (i-PrS)2BBr.6f Most of these methods have one or other drawbacks such as long reaction time, low yields, reflux at high temperature and tedious workup procedures.7 Hence, there is still scope to develop mild and efficient methods in the deacetylation, detetrahydropyranylation and demethoxymethylation of hydroxyl groups.

In the alpha carbonyl carbon (active methyl and methylene) oxidation to dicarbonyl group, various reagents and reaction conditions have been explored that include ammonium chlorochromate,8a I2,8b CrO3–NH4Cl,8c HBr,8d MeSSMe–CuCl2–CuO,8e Cu(OAc)2·H2O,8f P–Me–sulphonic acid,8g SeO2–DMSO,8h KI + O2-t-BuNH2.8i However, SeO2 was not reported as reagent for one-pot deprotection of acetyl, THP and MOM ethers and sequel alpha carbonyl carbons oxidation in eco-friendly environment. In continuation of our interest.9 to develop new methods for the organic synthesis, herein, we report an efficient one-pot protocol for the deprotection of acetyl, tetrahydropyranyl and methoxymethyl ethers and sequel oxidation of methyl/methylene carbons of alpha carbonyl carbon to dicarbonyl functional group using SeO2 in water as a novel reagent.

Results and discussion

We screened different oxidizing agents in water for the deprotection and sequel oxidation as CrO3, DDQ and PCC in H2O, however failed to give the products (Table 1, entries 1, 3 & 6), OsO4 and MnO2 gave a poor yield (5–10%) at 80 °C after 6 h (Table 1, entry 2 & 7), while, SeO2 gave the optimal yield (85–95%) using substrate[thin space (1/6-em)]:[thin space (1/6-em)]SeO2 (1[thin space (1/6-em)]:[thin space (1/6-em)]3 ratio) in 1 ml H2O at 80 °C within 1 h (Table 1, entry 5). Under similar conditions, demethoxymethylation and sequel oxidation gave moderate yields (40%) in 1–3 h. Therefore, substrate[thin space (1/6-em)]:[thin space (1/6-em)]SeO2 (1[thin space (1/6-em)]:[thin space (1/6-em)]3 ratio) in 1 ml H2O was selected as an optimized conditions. When, we applied substrate[thin space (1/6-em)]:[thin space (1/6-em)]SeO2 (1[thin space (1/6-em)]:[thin space (1/6-em)]1 and 1[thin space (1/6-em)]:[thin space (1/6-em)]2 ratios) in 1 ml H2O, got only deprotection product in 95% yields for Ac and THP and demethoxymethylation gave moderate yields (35–40%) at 80 °C in 1–3 h (Table 1, entry 4 and Table 3). In case of indanone only deprotection was observed no oxidation of alpha hydroxyl to dicarbonyl (Table 3, entry 11). The products were confirmed on the basis of their spectral data (ESI†). For example, product 7, the 1H-NMR spectra showed the characteristic broad peak at δ 5.45 ppm for hydroxyl group. IR value at 3425 cm−1 for –OH groups indicates only deprotection without oxidation of methyl/methylene carbons of alpha carbonyl carbon to dicarbonyl group.
Table 1 Optimization conditions in deprotection of acetate, tetrahydropyranyl and methoxymethyl ethers and sequel oxidation of methyl/methylene carbons of alpha carbonyl carbon

image file: c5ra09986b-u1.tif
Entry Substrate[thin space (1/6-em)]:[thin space (1/6-em)]reagent (equiv.) Time (h) Yield (%) (deacetylation) Yield (%) (detetrahydro-pyranylation)
1 Sub.[thin space (1/6-em)]:[thin space (1/6-em)]CrO3(1[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 h No reaction No reaction
2 Sub.[thin space (1/6-em)]:[thin space (1/6-em)]OsO4(1[thin space (1/6-em)]:[thin space (1/6-em)]1) 6 h 8 5
3 Sub.[thin space (1/6-em)]:[thin space (1/6-em)]DDQ(1[thin space (1/6-em)]:[thin space (1/6-em)]1) 24 h No reaction No reaction
4 Sub.[thin space (1/6-em)]:[thin space (1/6-em)]SeO2(1[thin space (1/6-em)]:[thin space (1/6-em)]1) 1 h–3 h Only deprotection Only deprotection
5 Sub.[thin space (1/6-em)]:[thin space (1/6-em)]SeO2(1[thin space (1/6-em)]:[thin space (1/6-em)]3) 1 h 95 94
6 Sub.[thin space (1/6-em)]:[thin space (1/6-em)]PCC(1[thin space (1/6-em)]:[thin space (1/6-em)]1) 6 h No reaction No reaction
7 Sub.[thin space (1/6-em)]:[thin space (1/6-em)]MnO2(1[thin space (1/6-em)]:[thin space (1/6-em)]1) 6 h 10 10

Table 2 Solvents effects in deprotection of acetate, tetrahydropyranyl and methoxymethyl ethers and sequel oxidation of methyl/methylene carbons of alpha carbonyl carbon

image file: c5ra09986b-u2.tif
Entry H2O (1 ml)[thin space (1/6-em)]:[thin space (1/6-em)]Solvents (3–4 drops) Yielda (%) Yieldb (%)
a Yields of deacetylation.b Yields of detetrahydropyranylation.c DMSO, Dioxane, THF, DMF, etc.
1 H2O 30 35
2 Organic solventsc 0 0
3 H2O[thin space (1/6-em)]:[thin space (1/6-em)]THF 65 60
4 H2O[thin space (1/6-em)]:[thin space (1/6-em)]Dioxane 95 94
5 H2O[thin space (1/6-em)]:[thin space (1/6-em)]DMF 50 40
6 H2O[thin space (1/6-em)]:[thin space (1/6-em)]DEE 25 20
7 H2O[thin space (1/6-em)]:[thin space (1/6-em)]Ethanol 0 0
8 H2O[thin space (1/6-em)]:[thin space (1/6-em)]methanol 0 0
9 H2O[thin space (1/6-em)]:[thin space (1/6-em)]CHCl3 0 0
10 H2O[thin space (1/6-em)]:[thin space (1/6-em)]DMSO 50 40

Table 3 Examples of deprotection (only) of acetate, tetrahydropyranyl and methoxymethyl ethers in alcohols and phenols

image file: c5ra09986b-u3.tif
Entry ROAc/THP/MOM ROH Time (min) Yielda (%) Yieldb (%) Yieldc (%)
a Yields of deacetylation.b Yields of detetrahydropyranylation.c Yields of demethoxymethylation.
1 image file: c5ra09986b-u4.tif image file: c5ra09986b-u5.tif 30 92 89 40
2 image file: c5ra09986b-u6.tif image file: c5ra09986b-u7.tif 30 92 88 40
3 image file: c5ra09986b-u8.tif image file: c5ra09986b-u9.tif 30 95 92 35
4 image file: c5ra09986b-u10.tif image file: c5ra09986b-u11.tif 30 93 95 40
5 image file: c5ra09986b-u12.tif image file: c5ra09986b-u13.tif 30 95 95 32
6 image file: c5ra09986b-u14.tif image file: c5ra09986b-u15.tif 30 96 95 35
7 image file: c5ra09986b-u16.tif image file: c5ra09986b-u17.tif 30 90 87 35
8 image file: c5ra09986b-u18.tif image file: c5ra09986b-u19.tif 30 96 94 40
9 image file: c5ra09986b-u20.tif image file: c5ra09986b-u21.tif 30 95 90 35
10 image file: c5ra09986b-u22.tif image file: c5ra09986b-u23.tif 30 90 96 35
11 image file: c5ra09986b-u24.tif image file: c5ra09986b-u25.tif 30 92 94 30

In the case of low soluble or insoluble compounds under optimized conditions, the reaction gave moderate yield (35%). Therefore, 3–4 drops of organic solvents (THF, dioxane, DMF, DEE, ethanol, methanol, CHCl3 and DMSO) were used to improve the solubility, in which THF and dioxane gave the maximum yields (60% and 94%) respectively (Table 2, entries 1–10). However, SeO2 in any organic solvent failed to give the product without H2O (Table 2, entry 2). In the MOM ethers deprotection and sequel oxidation of alpha carbon gave only 40% yields even under above conditions.

Under optimized reaction conditions using substrate[thin space (1/6-em)]:[thin space (1/6-em)]SeO2,1[thin space (1/6-em)]:[thin space (1/6-em)]3 ratio, the reaction gave the deprotection and sequel oxidation of methyl/methylene carbons of alpha carbonyl carbon to di-carbonyl group in excellent yield (85–95%) for the deacetylation, detetrahydropyranylation and moderate yield 30–40% for demethoxymethylation (Table 4, entries 3–10) within 30–60 min at 80 °C. Interestingly, acetyl and THP removal and sequel oxidation of methyl/methylene carbons of alpha carbonyl carbon to di-carbonyl group were observed in the protected hydroxyl acetophenone and dihydrochalcones to give the corresponding hydroxy dicarbonyl derivatives 14–21 in excellent yield (85–95%). We further used excess of substrate[thin space (1/6-em)]:[thin space (1/6-em)]SeO2 in 1[thin space (1/6-em)]:[thin space (1/6-em)]4, 1[thin space (1/6-em)]:[thin space (1/6-em)]5 & 1[thin space (1/6-em)]:[thin space (1/6-em)]6 ratio and prolonging the reaction time (>120 minutes) for the oxidation of another alpha carbon, but failed to give oxidation product (Table 4, entry 11). Similarly, the di-carbonyl compounds were failed to undergo alpha carbon oxidation under optimized condition even prolonging the reaction time (Table 4, entry 12).

Table 4 Deprotection of Ac, THP and MOM groups and sequel oxidation of methyl/methylene carbons of alpha carbonyl carbon

image file: c5ra09986b-u26.tif
Entry Compounds with alpha carbonyl carbon Dicarbonyl Time (min) Yielda (%) Yieldb (%) Yieldc (%)
a Yields of deacetylation.b Yields of detetrahydropyranylation.c Yields of demethoxymethylation.d Used substrate[thin space (1/6-em)]:[thin space (1/6-em)]SeO2 ratio: 1[thin space (1/6-em)]:[thin space (1/6-em)]4, 1[thin space (1/6-em)]:[thin space (1/6-em)]5 & 1[thin space (1/6-em)]:[thin space (1/6-em)]6, with prolonging reaction time but no further oxidation was observed.e Under optimized condition, di-carbonyl compound failed to give alpha carbon oxidation product.
1 image file: c5ra09986b-u27.tif image file: c5ra09986b-u28.tif 60 92 95 32
2 image file: c5ra09986b-u29.tif image file: c5ra09986b-u30.tif 60 90 88 32
3 image file: c5ra09986b-u31.tif image file: c5ra09986b-u32.tif 60 93 95 40
4 image file: c5ra09986b-u33.tif image file: c5ra09986b-u34.tif 30 94 94 35
5 image file: c5ra09986b-u35.tif image file: c5ra09986b-u36.tif 60 91 85 40
6 image file: c5ra09986b-u37.tif image file: c5ra09986b-u38.tif 60 93 88 40
7 image file: c5ra09986b-u39.tif image file: c5ra09986b-u40.tif 60 90 87 30
8 image file: c5ra09986b-u41.tif image file: c5ra09986b-u42.tif 60 95 85 35
9 image file: c5ra09986b-u43.tif image file: c5ra09986b-u44.tif 60 94 82 35
10 image file: c5ra09986b-u45.tif image file: c5ra09986b-u46.tif 60 95 80 30
11d image file: c5ra09986b-u47.tif image file: c5ra09986b-u48.tif >120 — — —
12e image file: c5ra09986b-u49.tif image file: c5ra09986b-u50.tif >60 — — —

The reagent is also useful for the deprotection of methoxymethyl ether (MOM) of phenolic compounds like, chalcone and chalcone epoxides but the yield of the product was moderate 30–40% (Table 4, entries 3–12). In the case of demethoxymethylation, we performed reaction for longer reaction time (1–3 h), but no further conversion was observed. Therefore, the remaining starting material was recovered by column chromatography.

The products were characterized on the basis of their spectral analysis 1H- and 13C-NMR, GC-MS (ESI†). For example, product 16, the 1H-NMR spectra showed the characteristic singlet peak at δ 3.99 ppm for –CH2 and disappear the characteristic two triplet peak at δ 2.80 and 2.72 ppm (J = 5.5–6.5 Hz) of –CH2–CH2– and broad peak at δ 5.19 ppm for hydroxyl group, indicates oxidation of alpha carbonyl carbon. In 13C-NMR spectra, the characteristic peak at δ 197.12 and 191.10 ppm for two carbonyl (–CO–CO–) groups and peak at δ 50.89 ppm for –CH2 confirms the oxidation of alpha carbonyl carbon, this confirmation also support by the disappearance of peak at 46.35 and 30.51 ppm of –CH2–CH2– groups in dihydrochalcone. IR value at 3415 cm−1 for –OH groups, 1705 and 1715 cm−1 for dicarbonyl indicates the deprotection and sequel oxidation of alpha carbonyl carbon. The structures of all other compounds were further confirmed by GC-MS (ESI†).

A plausible mechanism was proposed (Scheme 1). First, selenium dioxide and water reacts to form the selenous acid in situ which reacted with tetrahydropyranyl dihydrochalcone I to make complex II. Selenous acid ligated with oxygen of tetrahydropyranyl dihydochalcone II, followed by removal of DHP via intramolecular abstraction of proton by selanolate ion to give compound III.


image file: c5ra09986b-s1.tif
Scheme 1 Propose mechanism for the deprotection of tetrahydropyranyl ethers and sequel oxidation of methyl/methylene carbons of alpha carbonyl carbon to di-carbonyl group in SeO2/water.

Further, complex III undergoes keto-enol tautomerization to get compound IV. The key step is the beta-ketoseleninic acid V formation by the dihydrochalcone followed by Pummerer-like10a,b reaction to obtain compound VI. Then, the hydrolysis gave the deprotected alpha-diketone VII of THP dihydochalcone.10c

Conclusions

In conclusion, we have developed an efficient one-pot protocol for the deprotection of acetyl, tetrahydropyranyl and methoxymethyl ethers and sequel oxidation sequel oxidation of methyl/methylene carbons of alpha carbonyl carbon to di-carbonyl functional group using substrate[thin space (1/6-em)]:[thin space (1/6-em)]SeO2 (1[thin space (1/6-em)]:[thin space (1/6-em)]3 ratio) in H2O. The reaction gave excellent yield (85–95%) for acetyl and THP ethers and moderate yield (30–40%) at 80 °C in 30–60 min. However, a selective deprotection of acetyl, tetrahydropyranyl and methoxymethyl was afforded using substrate[thin space (1/6-em)]:[thin space (1/6-em)]SeO2 (1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio) in H2O. This methodology has advantages such as short reaction time, high yields, and easy workup procedures.

Experimental section

General methods

Organic solvents were dried by standard methods; the reagents (chemicals) were purchased from commercial sources, and used without further purification. All reactions were monitored by TLC using precoated silica gel aluminum plates. Visualization of TLC plates was accomplished with an UV lamp. Column chromatography was performed using silica gel 60–120 mesh size (RANKEM Limited) with EtOAc–hexanes as eluent. Melting points were recorded on Perfit apparatus and are uncorrected. All products were characterized by NMR, IR and MS spectra. 1H and 13C NMR spectra were recorded in deuterated chloroform (CDCl3) on a 500 MHz and 125 MHz spectrometer (Bruker), respectively. Chemical shifts were reported in parts per million (ppm, δ) downfield from tetramethylsilane. Proton coupling patterns are described as singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m), and broad (br).
General procedure for deprotection of acetyl, tetrahydropyranyl and methoxymethyl ethers of alcohol and phenol. SeO2 (1 mmol) was added to a stirred solution of Esters and ethers (1 mmol) in a water (1 ml) and 3–4 drops of dioxane, suspension obtained, applied heating to 80 °C. After TLC monitoring, the resulting reaction mixture was poured in cold water and extracted with EtOAc. The organic layer was washed with brine, dried with anhyd. Na2SO4, and concentrated in vacuo to give the corresponding product which was purified by silica gel column chromatography with hexane–EtOAc eluent to obtain the products 1 to 11 (Table 3) in excellent yield 85–95% and 30–40% for deacetylation, detetrahydropyranylation and demethoxymethylation respectively. Similarly, 1-(4-bromophenyl)-3-(4-hydroxyphenyl)propan-1-one (7): 1H NMR (CDCl3, 500 MHz) δ ppm 8.02 (d, J = 7.5 Hz, 2H), 7.74 (d, J = 7.5 Hz, 1H), 7.57 (d, J = 8.5 Hz, 2H), 7.51 (d, J = 8.0 Hz, 2H), 7.39 (d, J = 8.5 Hz, 2H), 5.45 (s, 1H), 2.80 (t, J = 6.5 Hz, 2H), 2.73 (t, J = 6.0 Hz, 2H); 13C NMR (CDCl3, 125 MHz) δ ppm 199.12, 157.13, 136.27, 133.63, 131.54, 130.78, 129.62, 129.30, 115.19, 45.81, 30.17; IR νmax (KBr, cm−1): 3425 (OH str), 2928, 2885 (aromatic C–H str), 1705 (C[double bond, length as m-dash]O str), 1599 (aromatic, C[double bond, length as m-dash]C str), 1265, 1079, 862, 725; GC-MS (m/z): 304, 306 [M+˙, C15H13BrO2].
General procedure for deprotection of acetyl, tetrahydropyranyl and methoxymethyl ethers in alcohols and phenols and sequel oxidation of methylene carbon alpha carbonyl carbon to carbonyl functional groups. SeO2 (3 mmol) was added to a stirred solution of esters and ethers (1 mmol) in a water (1 ml) and 3 to 4 drops of dioxane, suspension obtained, applied heating to 80 °C, gave products 12 to 21 (Table 4) in excellent yield 85–95% and 30–40% for deacetylation, detetrahydropyranylation and demethoxymethylation followed by sequel oxidation of methylene carbon alpha carbonyl carbon to carbonyl groups respectively. 3-(4-fluorophenyl)-1-(4-hydroxyphenyl)propane-1,2-dione (16): 1H NMR (CDCl3, 500 MHz) δ ppm 7.73 (d, J = 9.0 Hz, 1H), 7.57 (d, J = 7.5 Hz, 2H), 7.53 (d, J = 9.0 Hz, 1H), 7.38 (d, J = 8.5 Hz, 2H), 6.98 (d, J = 8.5 Hz, 2H), 5.19 (s, 1H), 3.99 (s, 2H); 13C NMR (CDCl3, 125 MHz) δ ppm 197.12, 191.10, 166.42, 163.17, 131.54, 130.71, 129.62, 129.30, 122.38, 117.19, 116.11, 50.89; IR νmax (KBr, cm−1): 3415 (OH str), 2935, 2879 (aromatic C–H str), 1705, 1715 (C[double bond, length as m-dash]O str), 1593 (aromatic, C[double bond, length as m-dash]C str), 1268, 1087, 865, 731; GC-MS (m/z): 258 [M+˙, C15H11FO3].

Acknowledgements

Financial support and award of junior research fellowship by the Department of science and Technology (DST) New Delhi is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedure, characterization data and 1H, 13C NMR spectra associated with this article are available. See DOI: 10.1039/c5ra09986b
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