Preparation of monotetrahydropyranylated short-chain symmetrical diols

Article abstract of DOI:10.1081/SCC-120023450

Efficient and robust methods for the monoprotection of 1,3-propanediol, 1,4-butanediol, and 1,5-pentanediol as THP ethers have been developed. The preparation of 4-(tetrahydro-pyran-2-yloxy)-butan-1-ol from 1,4-butanediol and 5-(tetrahydro-pyran-2-yloxy)-pentan-1-ol from 1,5-pentanediol was accomplished using a two-phase system composed of CH₂Cl_2. 3,4-dihydro-2H-pyran, and aqueous 0.1 N HCL. Preparation of 3-(tetrahydro-pyran-2-yloxy)-propan-1-ol from 1,3-propanediol was accomplished using a water-free biphasic mixture composed of 1,3-propanediol, catalytic pyridinium p-toluenesulfonate, 3,4-dihydro-2H-pyran, and 25% ether in hexane. The compounds are very useful three- to five-carbon homologating agents.

Full text of DOI:10.1081/SCC-120023450

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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 18, pp. 3251–3259, 2003
Preparation of Monotetrahydropyranylated
Short-Chain Symmetrical Diols
*
Richard J. Petroski
USDA,# REE, Agricultural Research Service,
National Center for Agricultural Utilization Research,
Crop Bioprotection Research Unit, Peoria, Illinois, USA
ABSTRACT
Efficient and robust methods for the monoprotection of 1,3-propa-
nediol, 1,4-butanediol, and 1,5-pentanediol as THP ethers have been
developed. The preparation of 4-(tetrahydro-pyran-2-yloxy)-butan-
1-ol from 1,4-butanediol and 5-(tetrahydro-pyran-2-yloxy)-pentan-1-
ol from 1,5-pentanediol was accomplished using a two-phase system
composed of CH2Cl2, 3,4-dihydro-2H-pyran, and aqueous 0.1 N
*Correspondence: Richard J. Petroski, Research Chemist, USDA, REE,
Agricultural Research Service, National Center for Agricultural Utilization
Research, Crop Bioprotection Research Unit, 1815 N. University St., Peoria,
IL 61604, USA; Fax: (309) 681-6693; E-mail: petrosrj@ncaur.usda.gov.
#Names are necessary to report factually on available data; however, the USDA
neither guarantees nor warrants the standard of the product, and the use of the
name by USDA implies no approval of the product to the exclusion of others that
may also be suitable.
3251
DOI: 10.1081/SCC-120023450
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
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3252
Petroski
HCL. Preparation of 3-(tetrahydro-pyran-2-yloxy)-propan-1-ol from
1,3-propanediol was accomplished using a water-free biphasic mix-
ture composed of 1,3-propanediol, catalytic pyridinium
p-toluenesulfonate, 3,4-dihydro-2H-pyran, and 25% ether in hexane.
The compounds are very useful three- to five-carbon homologating
agents.
Key Words: 3-(Tetrahydro-pyran-2-yloxy)-propan-1-ol; 4-(Tetra-
hydro-pyran-2-yloxy)-butan-1-ol; 5-(Tetrahydro-pyran-2-yloxy)-
pentan-1-ol; Monotetrahydropyranylation; Symmetrical diols.
Mono-tetrahydropyranyl (THP) ethers of three- to five-carbon sym-
metrical primary diols are useful as homologating agents because the
THP protective group is stable to base.[1] Although methods have been
reported for the monoprotection of 2- to 12-carbon symmetrical primary
diols as THP ethers,[2–6] the existing methods have serious drawbacks
when used for three- to five-carbon symmetrical primary diols. An article
describing the selective monotetrahydropyranylation of 1,3-propane-
diol[2] recommended the use of a solvent mixture composed 40% 3,4-
dihydro-2H-pyran (DHP) in toluene as the organic phase and either
aqueous NaHSO4 oraqueous HCL as the aqueous phase; however, con-
ducting this reaction in this laboratory resulted in a mixture of products,
including an isomeric monoprotected alcohol. Results from the use of
catalytic ion-exchange resins have been reported to be variable because
the amounts of water contained in the resins tend to decrease in the
course of the reaction.[3] The use of metallic sulfates supported on silica
gel requires close observation to decide when to terminate the reaction
because diethers tend to form rapidly when diols have almost been con-
sumed.[4] Also, the catalyst must be carefully dried under reduced pres-
sure, with stirring, at 230–240ꢀ prior to use.[4] Lithium bromide in
CH2Cl2 has been reported to monotetrahydropyranylate 1,4-butanediol
in 75% yield[5]; however, the amount of diether production was not
reported and the suggested chromatographic workup (silica gel,
EtOAc-hexane, 2:8) has resulted (personal observation) in substantial
cleavage (>20%) of the THP protective group. Attempted solvent-
free monotetrahydropyranylation of 1,4-butanediol with catalytic
.
AlCl3 6H2O[6] resulted in significant (>20%) di-THP ether formation.
In the research presented here, the concept of a biphasic system was
studied, in which significant di-THP ether formation is prevented by
separation of the mono-THP ether from the catalyst. This could occur
by migration of the product into an organic solvent, in which the catalyst

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Preparation of Monotetrahydropyranylated Diols
3253
has very low solubility. This approach has been used previously[2];
however, it was not optimized for short-chain diols. In this article,
methods to prepare mono-THP ethers of these short-chain diols are
reported that minimize the formation of unwanted byproducts and
easily remove unreacted starting materials from the reaction mixture.
RESULTS AND DISCUSSION
The preparation of 4-(tetrahydro-pyran-2-yloxy)-butan-1-ol (1) from
1,4-butanediol and 5-(tetrahydro-pyran-2-yloxy)-pentan-1-ol (2) from
1,5-pentanediol was straightforward using a two-phase system composed
of CH2Cl2, 3,4-dihydro-2H-pyran (DHP), and aqueous HCL (Fig. 1).
Acid is neutralized, prior to workup, using a minimal volume of an
excess of base (saturated aqueous NaHCO3 solution) because aqueous
acid can cleave THP ethers. Both 1 and 2 have enough watersolubility to
recommend using only a minimal volume of aqueous phase in the
workup as well as repeated extraction of the aqueous phase with
CH2Cl2. The starting diols have much greater water solubility so they
can be selectively removed by this extractive workup. Other authors have
Figure 1. Preparation of mono-THP ethers of short-chain primary diols; a: 1,4-
butanediol (88 mmol) or1,5-pentanediol (88 mmol), DHP (80 mmol), 0.1 N HCL
(8 mL), CH2Cl2 (240 mL), b: 1,3-propanediol (120 mmol), DHP (80 mmol), PPTS
(0.16 mmol), 25% diethyl etherin hexane (320 mL). Forconditions and workup,
see Experimental.

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3254
Petroski
used column chromatography on silica gel as part of the workup[3,5,6]
but this process can result in substantial loss of product due to cleavage
of THP ethers (use of either silica gel or Florisil, observations made in
this laboratory).
Isolation of the desired products resulted in 1 and 2 with yields of
75% (97% pure) and 70% (96% pure) respectively. The major impurity,
in both cases, was about 2% of the di-THP ether. The synthetic product
can be used directly in a synthetic pathway, without further purification,
because di-THP ethers of diols are chemically inert to subsequent reac-
tions involving the free hydroxyl function. The diprotected diol impurity
can be removed at the deprotection stage of the synthetic pathway
because 1,4-butanediol and 1,5-pentanediol will be washed away if an
aqueous extractive workup is used.
The above method could not be used to prepare 3-(tetrahydro-pyran-
2-yloxy)-propan-1-ol (3) from 1,3-propanediol due to formation of a
significant amount (ca. 15%) of an isomeric alcohol, 4-[1,3]dioxan-2-yl-
butan-1-ol (4) (Fig. 2). The unwanted isomeric alcohol was tentatively
identified on the basis of mass spectral evidence: MS peaks at m/z 159
(Mþ-1, 8), 142 (Mþ-18, 0.4, loss of water), 113 (0.5), 101 (4) 87 (100),
and 59 (13). The peak at m/z 85 (THP) was notably absent; however,
replaced by a peak at m/z 87, which is consistent with the [1,3]dioxan-2-yl
moiety.[7] Acid-catalyzed isomerization of 3 to 4 has been observed pre-
viously[8,9]; however, no mass spectral data were given. Protection of 1,3-
butanediol as mono- or di-THP ethers resulted in the formation of iso-
meric alcohols containing the 4-methyl-[1,3]dioxan-2-yl moiety.[10]
Compounds 1 and 2 were free of similar impurities. Compound 1 did
not isomerize to an alcohol containing a [1,3]-dioxepane moiety (7-mem-
bered ring) and compound 2 did not isomerize to an alcohol containing a
[1,3]-dioxocane moiety (8-membered ring), possibly because formation of
these larger rings is disfavored.
Isomeric alcohol 4 would interfere with subsequent chemistry
planned for 3, so it would have to be removed. Because of similar
Figure 2. Acid-catalyzed isomerization of 3 to unwanted side product 4.

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Preparation of Monotetrahydropyranylated Diols
3255
structure and a nearly identical GC retention time as the desired product,
a difficult chromatographic purification would be necessary. It was con-
cluded that a new preparation of 3 was needed, and during which the
undesirable side product was not formed.
Preparation of 3 was accomplished using a water-free biphasic mix-
ture composed of 1,3-propanediol, pyridinium p-toluenesulfonate
(PPTS),[11] DHP and 25% etherin hexane. The use of hexane alone as
the upper-phase solvent resulted in as much as 10% of the diprotected
diol. The PPTS catalyst is soluble in 1,3-propanediol (lower phase) but
insoluble in the ether-hexane solvent (upper phase). The mono-THP ether
migrates from the lower phase to the upper phase as it is formed; there-
fore, separation of 3 from the catalyst prevents isomerization to 4. The
reaction can be recycled by reuse of the lower phase. The yield per cycle is
only 20% from DHP; however, 80% of the 1,3-propanediol actually used
in each cycle of the reaction is converted to 3.
Purity was >97% 3. The majorimpuirty was about 2% of the
di-THP ether of 1,3-propanediol. This product can be used directly in
chain-elongation or similar reactions, without further purification,
because the diprotected diol is chemically inert and will be removed at
the deprotection stage workup.
In summary, efficient and robust methods for the monoprotection of
1,3-propanediol, 1,4-butanediol, and 1,5-pentanediol as THP ethers have
been developed. The compounds are very useful three- to five-carbon
homologating agents. The need foronly a minimal wokrup should
be attractive to synthetic organic chemists engaged in the multistep
preparation of complex natural products.
EXPERIMENTAL
General Details
Organic chemicals were purchased from Aldrich (Milwaukee, WI)
unless otherwise stated. Yields were calculated on the basis of the limiting
reactant, 3,4-dihydro-2H-pyran (DHP).
Preparation of 4-(tetrahydro-pyran-2-yloxy)-butan-1-ol (1). To a
500 mL Erlenmeyer flask were added CH2Cl2 (240 mL), 0.1 N aqueous
HCL (8 mL), 1,4-butanediol (7.93 g, 88 mmol) and DHP (6.73 g,
80 mmol). The flask was stoppered with a teflon stopper and the resulting
reaction mixture was magnetically stirred at 25ꢀ forca. 96 h until GC/MS
analysis indicated that the reaction was complete. The reaction was
terminated by the addition of saturated aqueous NaHCO3 solution

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3256
Petroski
(1.0 mL). The aqueous phase tested alkaline by a pH papertest. The
mixture was decanted to a 500 mL separatory funnel. The mixture was
shaken and then the phases were allowed to separate. The lower organic
phase was collected. The remaining aqueous phase was extracted with
CH2Cl2 (2 Â 30 mL) then the organic phases were combined and dried
overanhydrous Na 2SO4. After removal of the drying agent by filtration,
the solvent was evaporated in vacuo to give 1 as a pale yellow oil (10.5 g,
75% yield). Purity was checked by gas chromatography (GC) and found
to be >97% 1. The majorimpurity was about 2% of the di-THP etherof
1,4-butanediol. MS m/z (% base) 173 (Mþ-1, 0.11), 101 (23), 85 (100),
73 (88). 1H NMR ꢀ 1.45–1.85 (10H, m, H-2,3,30,40,50), 2.21 (1H, brs, OH),
3.42 (1H, dt, J4a–4b ¼ 9.8 and J3–4 ¼ 5.7, H-4), 3.49 (1H, ddd,
J6 ax–6 eq ¼ 11.0, J5 ax–6 eq ¼ 6.8 and J5 eq–6 eq ¼ 3.9, H-60eq), 3.65 (2H, t,
J1–2 ¼ 5.9, H-1), 3.79 (1H, dt, J4a–4b ¼ 9.8 and J3–4 ¼ 5.9, H-4), 3.84
0
0
0
0
0
0
(1H, ddd, J6 ax–6 eq ¼ 11.0, J5 ax–6 ax ¼ 7.8 and J5 eq–6 ax ¼ 3.8, H-60ax),
4.59 (1H, brs, H-20). 13C NMR ꢀ 19.4(C-40), 25.2 (C-50), 26.4 (C-3), 30.0
(C-2), 30.5 (C-30), 62.2 (C-60), 62.6 (C-1), 67.4 (C-4), 98.8 (C-20).
Preparation of 5-(tetrahydro-pyran-2-yloxy)-pentan-1-ol (2). A similar
preparation from 1,5-pentanediol (9.17 g, 88 mmol) as the starting diol
gave 2 as a pale yellow oil (10.5 g, 70% yield). Purity was checked by gas
chromatography (GC) and found to be >96% 2. The majorimpurity was
about 2% of the di-THP etherof 1,5-pentanediol. MS m/z (% base) 187
(Mþ-1, 0.11), 101 (23), 87 (39), 85 (100). 1H NMR ꢀ 1.40–1.89 (12H, m, H-
2,3,4,30,40,50), 3.38 (1H, dt, J5a–5b ¼ 9.7 and J4–5 ¼ 6.5, H-5), 3.48 (1H, ddd,
0
0
0
0
0
0
J6 ax–6 eq ¼ 11.4, J5 ax–6 eq ¼ 6.7 and J5 eq–6 eq ¼ 3.9, H-60eq), 3.63 (2H, t,
J1–2 ¼ 6.5, H-1), 3.73 (1H, dt, J5a–5b ¼ 9.6 and J4–5 ¼ 6.7, H-5), 3.84 (1H,
0
0
0
0
0
0
ddd, J6 ax–6 eq ¼ 11.4, J5 ax–6 ax ¼ 7.1 and J5 eq–6 ax ¼ 3.7, H-60ax),
4.55 (1H, brs, H-20). 13C NMR ꢀ 19.5 (C-40), 22.3 (C-3), 25.3 (C-50),
29.3 (C-4), 30.6 (C-30), 32.4 (C-2), 62.2 (C-60), 62.3 (C-1), 67.4 (C-5),
98.8 (C-20).
0
0
0
0
0
0
Preparation of 3-(tetrahydro-pyran-2-yloxy)-propan-1-ol (3). To a
500 mL round-bottomed flask were added hexane (240 mL), diethyl
ether(80 mL), 1,3-porpanediol (9.12 g, 120 mmol), DHP (6.73 g,
80 mmol), and PPTS (40 mg, 0.16 mmol) as the catalyst. The flask was
stoppered with a teflon stopper and the resulting two-phase reaction
mixture was magnetically stirred at 25ꢀ forca. 96 h until GC/MS analysis
indicated that the reaction should be terminated because nearly 2% of the
di-THP ether of 1,3-propanediol had been formed. Extending the reac-
tion beyond this point results in an increased rate of diether production
without significant accumulation of the desired product. The mixture
was allowed to separate into phases. The lower phase consisted of
1,3-propanediol and the catalyst, while the cloudy upper ether-hexane

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Preparation of Monotetrahydropyranylated Diols
3257
phase contained the desired product and some unreacted 1,3-propane-
diol. The upperphase was decanted into a 500 mL Elrenmeyerflask,
stoppered, and placed into the freezer at ꢁ20ꢀ overnight.
The reaction could be recycled by reuse of the lower phase (see below).
When the decanted upper phase was removed from the freezer, an almost-
solid oily residue was observed on the sides and bottom of the cooled flask.
The solution was clear. The product was isolated by decanting the clear
solution, while still cold, into another500 mL Erlenmeyerflask. The oily
residue, containing unreacted 1,3-propanediol, was discarded because it
acquired some water as condensation during the decanting step. The
decanted solution was dried over anhydrous Na2SO4, the drying agent
removed by filtration, and the solvent was evaporated in vacuo to give 3
as a pale yellow oil (2.8 g, 22% from DHP). Purity was checked by gas
chromatography (GC) and found to be >97% 3. The majorimpurity was
about 2% of the di-THP etherof 1,3-propanediol.
To the original 500 mL round-bottomed flask, containing the lower
phase consisting of PPTS catalyst and unreacted 1,3-propanediol, were
added hexane (240 mL), diethyl ether(80 mL), additional 1,3-propanediol
(1.64 g, 21.5 mmol) and 3,4-dihydro-2H-pyran (6.73 g, 80 mmol) to recon-
stitute the reaction mixture to the initial proportions. As before, the flask
was stoppered with a teflon stopper and the resulting reaction mixture was
magnetically stirred at 25ꢀ forca. 96 h until GC/MS analysis indicated that
the reaction should be terminated because nearly 2% of the di-THP ether
of 1,3-propanediol had been formed. Workup afforded an additional 2.5 g
of product (20% yield from DHP). This reaction can be recycled as
described herein to acquire the desired amount of product.
Approximately 80% of the 1,3-propanediol actually used in each cycle
of the reaction (ca. 1.6–1.7 g) was converted to 3. MS m/z (% base) 159
(Mþ-1, 4), 101 (43), 85 (100), 59 (26). 1H NMR ꢀ 1.45–1.90 (8H, m, H-2,
30,40,50), 2.21 (1H, s, OH), 3.51 (1H, dt, J3a–3b ¼ 11.3 and J2–3 ¼ 5.9, H-3),
3.58 (1H, ddd, J6 ax–6 eq ¼ 9.7, J5 ax–6 eq ¼ 7.3 and J5 eq–6 eq ¼ 5.0, H-60eq),
3.77 (2H, t, J1–2 ¼ 6.0, H-1), 3.84 (1H, dt, J3a–3b ¼ 11.3 and J2-3 ¼ 5.8, H-3),
0
0
0
0
0
0
3.92 (1H, ddd, J6 ax–6 eq ¼ 9.7, J5 ax–6 ax ¼ 7.1 and J5 eq–6 ax ¼ 5.3, H-60ax),
4.58 (1H, brs, H-20). 13C NMR ꢀ 19.5 (C-40), 25.2 (C-50), 30.5 (C-30), 31.9
(C-2), 61.5 (C-1), 62.4 (C-60), 66.2 (C-3), 99.0 (C-20).
0
0
0
0
0
0
Analysis of Reaction Products
All reactions were monitored by GC using a Hewlett-Packard (HP)
5890 Series II instrument equipped with flame ionization detector and
interfaced to a HP ChemStation data system. The oven temperature was

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3258
Petroski
programmed from 50 to 250ꢀC at 10ꢀC/min; the programmable cool
on-column inlet temperature was always 3ꢀC hotterthan the oven
temperature and the detector temperature was 250ꢀC. A DB-1 capillary
column (15 m  0.25 mm, 0.25 mm film thickness, J&W Scientific, Folsom,
CA) was used with 1 mL sample injections.
Electron impact mass spectra (70 eV) were obtained with an HP 5973
MSD instrument, equipped with a splitless injector. The oven tempera-
ture was programmed from 50 to 250ꢀC at 10ꢀC/min; the injectortem-
perature was 250ꢀC and the detectortemperature was 250 ꢀC. An EC-1
capillary GC column (30 m  0.25 mm, 0.25 mm film thickness, Alltech,
Deerfield, IL) was used with 0.3 mL sample injections. 1H NMR and
13C NMR spectra (CDCl3) were obtained on a Bruker (Bellerica, MA)
Advance 400 spectrometer at 400 MHz. Chemical shifts are referenced to
tetramethylsilane; coupling constants (J ) are in hertz. Assignments were
1
made with the help of H- and 13C-Predictive software.[12]
ACKNOWLEDGMENTS
I thank Bruce W. Zilkowski and Robert J. Bartelt for acquiring
mass spectral data, David Weisleder for acquiring NMR spectra, and
Robert J. Bartelt for helpful discussions.
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Preparation of Monotetrahydropyranylated Diols
3259
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Received in the USA March 18, 2003

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