Simple and efficient method for the protection of hydroxyl groups as 4-methoxybenzyl ethers

Article abstract of DOI:10.1080/00397911.2014.894526

PMB ethers of alcohols are obtained in good yields and under mild conditions using the 4-methoxybenzyl N-allyl thiocarbamate and N-bromosuccinimide (NBS)/TfOH as the catalyst. The present method is very fast, simple, and efficient. Copyright

Full text of DOI:10.1080/00397911.2014.894526

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Simple and Efficient Method for the
Protection of Hydroxyl Groups as 4-
Methoxybenzyl Ethers
Anna Kasprzycka a , Agata Ptaszek-Budniok a & Wiesław Szeja a
a Department of Chemistry , Silesian Technical University , Gliwice ,
Poland
Accepted author version posted online: 05 May 2014.Published
online: 13 Jun 2014.
To cite this article: Anna Kasprzycka , Agata Ptaszek-Budniok & Wiesław Szeja (2014) Simple
and Efficient Method for the Protection of Hydroxyl Groups as 4-Methoxybenzyl Ethers, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
44:15, 2276-2284, DOI: 10.1080/00397911.2014.894526
To link to this article: http://dx.doi.org/10.1080/00397911.2014.894526
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Synthetic Communications1, 44: 2276–2284, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.894526
SIMPLE AND EFFICIENT METHOD FOR THE
PROTECTION OF HYDROXYL GROUPS AS
4-METHOXYBENZYL ETHERS
Anna Kasprzycka, Agata Ptaszek-Budniok, and Wiesław Szeja
Department of Chemistry, Silesian Technical University, Gliwice, Poland
GRAPHICAL ABSTRACT
Abstract PMB ethers of alcohols are obtained in good yields and under mild conditions
using the 4-methoxybenzyl N-allyl thiocarbamate and N-bromosuccinimide (NBS)/TfOH
as the catalyst. The present method is very fast, simple, and efficient.
Keywords 4-Methoxybenzyl alcohol; 4-methoxybenzyl N-allyl thiocarbamate;
4-methoxybenzyl (PMB) ether; 4-methoxybenzyl protection; synthesis of ethers
INTRODUCTION
The synthesis of complex molecules (e.g., natural products) is still a challenge
despite tremendous progress in organic synthesis in recent decades.[1] Most multistep
syntheses still require numerous protection and deprotection procedures. In response
to the increasing complexity of the molecular structures synthesized, a large number
of protecting groups have been developed, as well as methods for their introduction
and their deprotection.[2–4] Nevertheless, new and more selective protecting groups
are still required[5–10] while milder and more selective conditions are actively
pursued.[11–14] A desirable protecting group should satisfy the following main cri-
teria: The formation of the installed protecting group must render inert an otherwise
reactive site during a synthetic sequence aimed at affecting other regions of the
molecular system, the removal (cleavage) of the protecting group must occur easily
and under mild conditions at the appropriate point in the synthetic scheme, and, in
addition, orthogonal reactivity with other common protecting groups is especially
valuable for highly functionalized systems.
The benzyl groups, such as 4-methoxybenzyl (PMB), 3,4-dimethoxybenzyl
(DMB), and others, have been used extensively as protecting groups in carbohydrate
Received December 20, 2013.
Address correspondence to Anna Kasprzycka, Department of Chemistry, Silesian Technical
University, ul. Krzywoustego 4, 44-100 Gliwice, Poland. E-mail: anna.kasprzycka@polsl.pl
2276

4-METHOXYBENZYL ETHERS
2277
chemistry.[15] Benzyl ethers are frequently used as protecting groups indispensable to
the process of obtaining oligosaccharides.[16,17] Benzylated sugars are among the
most popular glycosyl donors. For example, benzyl-protected glycosyl donors are
‘‘armed’’ relative to acylated analogues.[17–19] Deprotection of the corresponding
benzyl ethers may be readily accomplished under mild oxidative conditions with, for
example, ceric ammonium sulfate (CAN) in acetonitrile[20] or with 2,3-dichloro-5,6-
dicyanobenzoquinone (DDQ) in dichloromethane.[21]
Traditionally, preparation of arylmethyl ethers from alcohols is accomplished
by Williamson ether synthesis protocol under basic conditions, which involves
treating the substrate with benzyl halide and a strong base, such as potassium
hydroxide.[22,23] Typical benzylation reactions are thus limited to substrates that
tolerate basic conditions. Benzylation in the synthesis of selectively protected sugars
can be problematic and the selective protection of polyol systems (e.g., carbohy-
drates) can also be complicated by base-catalyzed migration of esters and silyl ethers.
Benzyl trichloroacetamidate, readily obtained from trichloroacetonitrile and benzyl
alcohol,[24–28] has been used to prepare benzylated carbohydrate derivatives. The
reaction is carried out in the presence of Brønsted acids such as para-toluenesulfonic
acid, triflic acid, Lewis acids such as BF3 etherate, TrBF4, TMSOTf, Sn(OTf)2,
TrClO4, and La(OTf)3 in a nonpolar solvent such as tetra- or di-chloromethane
and cyclohexane.[2–4] The disadvantage of the methods described lies in the appli-
cation of heavy-metal salts, which are biologically active and thus prohibited in
the biological and medical experiments.
We report here a new method for the protection of alcohols and sugars as the
corresponding 4-methoxybenzyl ethers by means of the reaction with a new alky-
lation reagent PMB N-allyl thiocarbamate (2).
RESULTS AND DISCUSSION
In the course of our program toward the synthesis of complex glycosides, we
have developed a simple method for activation of anomeric hydroxyl group.[29]
We have already reported the synthesis of O-glycosyl thiocarbamates and evaluated
them in stereoselective glycosylation. Study of the reaction mechanism leads to the
conclusion that the crucial step is activation of thiocarbamate in reaction with an
electrophile and the formation of a carbonium ion following nucleophilic substi-
tution at the anomeric carbon atom. Taking this into account, one can expect that
the electrophilic activation of benzyl thiocarbamates should lead to a relatively
stable benzyl carbonium ion. Our experience with O-glycosyl N-allyl thiocarba-
mates, which are easily activated, efficient glycosylating reagents for various unstable
substrates,[29] encouraged us to try PMB N-allyl thiocarbamate (2) as an alkylating
agent for alcohols. The preparation of this reagent is relatively simple. The synthesis
of 2 is illustrated in Scheme 1.
The PMB alcohol (1) can react with N-allyl isothiocyanate under basic con-
ditions to give the PMB N-allyl thiocarbamate (2). First, several reaction conditions
were examined in the reaction of 4-methoxybenzyl alcohol (1) and N-allyl isothio-
cyanate with various bases. The results are summarized in Table 1.
Potassium carbonate (K2CO3) and N,N-di-isopropylethylamine (DIPEA) appeared
to be too weak as bases for this type of reaction. However, the phosphazene base (BEMP)

2278
A. KASPRZYCKA, A. PTASZEK-BUDNIOK, AND W. SZEJA
Scheme 1. Preparation of PMB N-allyl thiocarbamate.
was too strong and observed in a large number of by-products in these reactions.
The optimal method for receiving methoxybenzyl N-allyl thiocarbamate (2) as donor
protective groups is from the reaction of the corresponding methoxybenzyl alcohol
(1) with N-allyl isothiocyanate in the presence of a strong nonnucleophilic base,
1,8-diazobicyclo[5.4.0]undec-7-en (DBU), in the microwave. Using a microwave
reactor considerably shortened the reaction time, reduced formation of by-products,
and significantly improved performance in comparison with the reaction carried out
under standard conditions. Thus, the starting materials, 4-methoxybenzyl alcohol (1)
and N-allyl isothiocyanate were irradiated at 2 W at 50 ꢀC for 1 h in toluene, in the
˚
presence of a catalytic amount of DBU and 4 A micronized molecular sieves
(Scheme 1). The method produced the desired product (2) at a 90% yield, following
purification using column chromatography. This compound is stable and can be
stored on laboratory bench tops without degradation for several months.
Development and Analysis of the Optimal Benzylation Protocol
Having synthesized thiocarbamate, we turned our attention to the evaluation
of their alkylating properties. At a low temperature (ꢁ25 ꢀC), the PMB thiocar-
bamate (2) is freely soluble in chlorinated solvents (dichloromethane, chloroform,
dichloroethane), in ethereal solvents (THF and ether), and in aromatic hydrocarbons
Table 1. Results from optimization of reaction conditions for preparation of PMB thiocarbamate
Entry
Base
Temp.=conditions
Time
Yieldb (%)
1
2
K2CO3
rt to 50 ꢀC
50 ꢀC, )))
50 ꢀC, mw
rt to 50 ꢀC
50 ꢀC, )))
50 ꢀC, mw
rt to 50 ꢀC
50 ꢀC,)))
8 h
4 h
—
7
3
1 h
8 h
—
6
4
DIPEA
BEMP
DBU
5
4 h
1 h
6
6
7
7
1 h
1 h
45c
50c
55c
65
78
94
8
9
50 ꢀC, mw
rt to 50 ꢀC
50 ꢀC,)))
15 min
6 h
3 h
10
11
12a
50 ꢀC, mw
1 h
aThe method at 50 ꢀC in the reactor microwaves was chosen.
bIsolated yield of pure product.
cIn this method a lot of regrouping product was created.

4-METHOXYBENZYL ETHERS
2279
Scheme 2. Activation pathways for thiocarbamate alkylation.
(toluene). Owing to its ability to dissolve, the initial screening of reaction conditions
was conducted in dichloromethane (DCM).
Considering the multifunctional character of the leaving group, we anticipated
that these could be activated via a number of conceptually different modes. Con-
sidering the multifunctional character of O-alkyl-thiocarbamoyl moiety, we have
developed activation pathways for their use in alkylation reactions (Scheme 2).
In the first approach (a), a ‘‘soft’’ electrophilic promoter such as a bromonium
ion targets the ‘‘soft’’ sulfur of thiocarbamoyl or adds to the double bond (pathway
a). The driving force is the formation of the stable heterocycle. In the second
approach (b), a metal-salt-based activator (AgOTf or CuOTf) can complex to both
sulfur and nitrogen and bring about benzylic activation. Thus, a heavy-metal-salt-
based promoter system would complex sulfur and nitrogen, improving the leaving
group ability by producing a partial positive charge on sulfur. Alternatively, a thio-
philic or electrophilic reagent would act via the a or b pathway. With this consider-
ation in mind, and to evaluate the benzylating properties of PMB thiocarbamate, (2)
was coupled with n-pentanol in the presence of molecular sieves (MS) in nonpolar
solvents (Scheme 3). The results are summarized in Table 2.
Good yields were obtained with thiophilic salts, such as silver or cuprous
triflate. Positive results were also observed using a precursor of the halonium ion.
NBS and NIS in chemical synthesis are known sources of bromonium and iodonium
Scheme 3. General scheme for optimization of preparation of PMB ether of n-pentanol.

2280
A. KASPRZYCKA, A. PTASZEK-BUDNIOK, AND W. SZEJA
Table 2. Results from optimization of reaction conditions for preparation of
PMB ethers of alcohols managed on n-pentanol
Entry
Catalyst
Temp.
–25 ꢀC
Time
Yielda (%)
1
2
3
4
5
Br2=LiClO4=NaHCO3
AgOTf
CuOTf
5 min
1 h
50
62
63
34
83
30 min
15 min
10 min
NIS=TfOH=Et4NTf
NBS=TfOH=Et4NTf
aIsolated yield of pure product.
ions, respectively. Both are frequently used together with different additives to
increase yields and stereoselectivity. Moreover, NBS and NIS are inexpensive and
commercially available reagents. Conditions for the reaction of PMB protection
were established using methylene chloride and a temperature of ꢁ25 ꢀC. PMB
N-allylthiocarbamate (2) was used in double excess to alcohol (3a) to direct the reac-
tion toward the formation of an asymmetrical ether. Trifluoromethanesulfonate acid
(TfOH) served as an additive activating electrophile, whose amount was constantly
adjusted with respect to NBS or NIS, comprising 0.1 of its quantity. An anion of
strong acid (Et4NTf) was added to stabilize the carbonium ion, whereas anhydrous
˚
conditions were maintained by the presence of 4 A micronized molecular sieves. As a
result, while the reaction of n-pentanol with PMB N-allyl thiocarbamate in the
presence of NBS gave 83% of the desired product (4a) within 10 min, the reaction
activated by NIS gave only 34% within 15 min.
Another approach involved activation of the PMB group donor with the aid of
Br2, which is characterized by the ability of exceptionally rapid addition to the dou-
ble bond of the allyl group. The application of bromine necessitates the introduction
of an acceptor (salt) for evolved hydrogen bromide. Thus, reaction was carried out in
CH2Cl2 at a temperature of ꢁ25 ꢀC, in the presence of NaHCO3 as HBr acceptor. As
expected, the reaction was completed within 5 min, but the yield was rather poor,
only 50%. Differences in the promoting activity of NBS, NIS, and Br2 are clearly
shown in Table 2.
In addition to the desired benzyl ether, a by-product was observed in the crude
product mixture: 1,3-thiazolone (5) (Scheme 2). 1,3-Thiazolone was identified as an
expected by-product of the benzylation reactions using 2. It is freely water soluble
and easily removed by aqueous extraction.
The mechanistic course of alkylation reaction using 2 undoubtedly falls along
the continuum between the SN1 and SN2 pathways (Scheme 2). Although we have not
performed detailed kinetic studies, formation of thiazolone 5 appears to be more
consistent with an SN1 mechanism. Addition of a bromonium ion to an allylic double
bond is expected to produce a halonium intermediate followed by alkylation of the
thiocarbonyl group. The formation of a stable heterocycle is the driving force of
alkylation. Activation of 2 provides a highly electrophilic species that is quickly
trapped by the alcohol.
Having established the preferred reaction conditions, PMB protection of
n-pentyl alcohol was performed to demonstrate the versatility of the present
chemical route. Scheme 4 illustrates the benzylation reactions of various alcohols

4-METHOXYBENZYL ETHERS
2281
Scheme 4. PMB protection of representative alcohols.
and sugars containing one unprotected hydroxyl group under our preferred con-
ditions. Primary (3a), secondary (3d), and tertiary (3b) alcohols reacted with PMB
N-allyl thiocarbamate to give corresponding ethers in good yields (Table 3). Simi-
larly, good yields were obtained in the case of allyl alcohol (3c) and epoxy alcohol–
glycidol (3e). Reactions were carried out in DCM as a solvent, with a double excess
of 2 as the PMB donor. In each case examined, the reaction was complete in approxi-
mately 10 min, as monitored by thin-layer chromatography (TLC). The structure of
1
these products was confirmed by H NMR and 13C NMR analysis and compared
with published reports.
The tolerance of this protocol for sensitive functionality will be determined in
due course, but for an initial data point we tested our benzylation reaction in the
presence of a primary silyl ether (3i). The desired benzyl ether was obtained in
excellent yields, and the silyl ether was recovered unchanged. It was observed that
Table 3. Data from PMB protection of representative alcohols
Entry
Substrate
Isolated yield (%)
1
2
3
4
5
6
7
8
9
3a
3b
3c
3d
3e
3f
83
80
70
80
72
78
81
85
84
3g
3h
3i

2282
A. KASPRZYCKA, A. PTASZEK-BUDNIOK, AND W. SZEJA
conventional acetal (3f, 3g) and acyl (3h) protecting groups are stable under the
proposed conditions.
In summary, the reaction of alcohols with 2 in the presence of NBS and
catalytic amounts of triflic acid rapidly affords the corresponding PMB ethers in
good yields. The short reaction time, low catalyst loading, along with ease of
handling and tolerance of protective groups render this an effective method for
the preparation of PMB ethers.
CONCLUSION
Stimulated by an increasing need for selective protection strategies, this syn-
thesis has provided a convenient route to obtaining PMB derivatives of alcohols
and sugars. A mild protection method, preserving the integrity of both alkaline-
and acid-labile protective groups, was developed. In summary, the PMB derivative
of N-allyl thiocarbamate upon treatment with N-bromosuccinimide (NBS), in the
presence of a catalytic amount of trifluoromethanesulfonic acid (TfOH), a
˚
carbocation-stabilizing salt (Et4NTf), and 4 A molecular sieves, transfers the PMB
group to a free hydroxyl of alcohol or sugar.
EXPERIMENTAL
Proton magnetic resonance spectra were recorded with a Varian 300-MHz
spectrometer for solutions in CDCl3. Chemical shifts are reported in (d) ppm relative
to the tetramethylsilane (TMS) peak at d ¼ 0.0 ppm (1H spectra). Mass spectra were
recorded on an electrospray ionization (ESI) mass spectrometer, ABSciex System
4000 QTRAP, at positive and negative modes of ionization. TLC was performed
on precoated plates of silica gel 60F254 (Merck), using hexane=ethyl acetate
(3:1 v=v) or toluene=ethyl acetate (8:1 v=v), and the spots were visualized by spraying
with sulfuric acid and palladium chloride. Chromatographic purification was per-
formed on silica gel 60 (Merck), 0.063–0.2 mm. All solutions were concentrated
under diminished pressure at 40 ꢀC. Organic solutions were dried over anhydrous
MgSO4.
Synthesis 4-Methoxybenzyl N-Allyl Thiocarbamate (2)
4-Methoxybenzyl N-allyl thiocarbamate (2) was prepared according to pro-
cedure of alcohol thiocarbamate preparation in a microwave reactor. A solution
of corresponding alcohol (138 mg, 1 mmol) in toluene was stirred at room tempera-
˚
ture for 20 min in the presence of 4 A micronized molecular sieves (10 mg). Suitable
base (Table 4) and N-allylisothiocyanate (0.39 ml, 4 mmol) were added and subse-
quently reaction mixture was exposed to microware irradiation (2 W) for 1 h at a
temperature of 50 ꢀC. Reaction progress was monitored by TLC (toluene=EtOAc
8=1). Afterward, the reaction mixture was filtered, washed with brine (2 ꢂ 3 ml),
and dried (MgSO4), and the solvent was evaporated. The crude product was purified
by column chromatography on silica gel, eluting with EtOAc=hexane (1=18).
Fractions containing the product were combined, and solvent was removed to afford
corresponding alcohol N-allyl thiocarbamate.

4-METHOXYBENZYL ETHERS
2283
Table 4. Suitable base using to synthesis of alcohol
N-allyl thiocarbamate
Entry
Base
Equivalent (mmol)
1
2
3
4
K2CO3
iPr2NEt
BEMP
DBU
1.5
1
0.01
0.15
4-Methoxybenzyl N-Allyl Thiocarbamate (2)
Yield: 94%; yellow oil; 7.29–7.33 (2H, m, o-Ph), 6.87–6.91 (2H, m, m-Ph), 6.40
0
0
0
(1H, br.s, NH), 5.80–5.91 (1H, 2dt, J 10.2 Hz, J 17.1 Hz, J 5:8 Hz1 CH22 CH3 CH2),
20
30
0
5.45 (2H, ps, CH2Ph), 5.15–5.20 (2H; m;1 CH CH CH2), 4.17–4.21 (2H, tt, J
5.8 Hz, J 3.05 Hz, J 1:52 Hz;1 CH CH CH2), 3.80 (3H, s, PhOMe); dC (100.6 MHz,
2
20
30
0
2
CDCl3) 190.12, 159.60, 132.48, 130.18, 128.66, 117.43, 113.80, 71.82, 55.17, 47.54;
HRMS (ESI-MS): calcd for C12H15NO2S: 260.0721 [M þ Na]þ; found: 260.0709 (as
a supplement material).
FUNDING
Research studies were partly financed by the European Union within the
Structural Funds in Poland (UDA-POKL.04.01.01-00-114=09-01) and the Polish
State Committee for Scientific Research (Grant No. N N209 186336).
SUPPORTING INFORMATION
Supplemental data for this article can be accessed on the publisher’s website.
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