Dibutyltin oxide mediated diastereoselective cyclodehydration/sulfonylation of 1,2,4-triols

Article abstract of DOI:10.1016/j.tetlet.2015.02.083

Dibutyltin oxide (Bu₂SnO) mediated cyclodehydration or sulfonylation of 1,2,4-triols is predictably diastereoselective depending on the steric bulk of the substituents at C4. A larger difference (ΔA-value >1 kcal/mol) leads to the syn-1,2,4-triols favouring cyclodehydration (78-85%) to form 3-hydroxytetrahydrofurans, with the anti-1,2,4-triols favouring monosulfonylation (66-87%). Triols from symmetrical ketones preferentially undergo cyclodehydration in high yield (>75%) due to a gem-disubstituent effect. Thus, the 1,2,4-triols derived from simple cyclic ketones also favour cyclodehydration to form spirocyclic 3-hydroxytetrahydrofurans in 72-79% yields.

Full text of DOI:10.1016/j.tetlet.2015.02.083

Tetrahedron Letters 56 (2015) 1825–1829
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Dibutyltin oxide mediated diastereoselective
cyclodehydration/sulfonylation of 1,2,4-triols
⇑
Makhosazana P. Gamedze, Comfort M. Nkambule
Department of Chemistry, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Dibutyltin oxide (Bu2SnO) mediated cyclodehydration or sulfonylation of 1,2,4-triols is predictably
Received 5 August 2014
Revised 31 January 2015
Accepted 18 February 2015
Available online 24 February 2015
diastereoselective depending on the steric bulk of the substituents at C4. A larger difference (DA-value
>1 kcal/mol) leads to the syn-1,2,4-triols favouring cyclodehydration (78–85%) to form 3-hydroxytetrahy-
drofurans, with the anti-1,2,4-triols favouring monosulfonylation (66–87%). Triols from symmetrical
ketones preferentially undergo cyclodehydration in high yield (>75%) due to a gem-disubstituent effect.
Thus, the 1,2,4-triols derived from simple cyclic ketones also favour cyclodehydration to form spirocyclic
3-hydroxytetrahydrofurans in 72–79% yields.
Keywords:
Hydroxytetrahydrofuran
Cyclodehydration
Sulfonylation
Ó 2015 Elsevier Ltd. All rights reserved.
1,2,4-Triols
gem-Disubstituent
The tetrahydrofuran moiety is present in a wide array of natural
products, most of which exhibit biological activities. Extensive
work has been performed towards the construction of tetrahydro-
furans encompassing a variety of designs, which have been sum-
marized in a comprehensive review by Wolfe and Hay.1 The most
common strategies involve cyclodehydration of 1,4-diols,2
cycloetherification via SN2 displacement (O1?C4 or O4?C1) of
leaving groups,3 epoxide, selenoxide or aziridine ring opening,4
iodo- or silyl-promoted cyclizations of unsaturated alcohols,5 and
nucleophilic addition of alkenes to oxonium ions.6
It is a general observation that in SN2-like (O1?C4 or O4?C1)
displacement reactions, any difference in rates is potentially due
to the varying stabilities of diastereomeric transition states leading
to the THF, due to eclipsing or staggered relationships between the
incoming nucleophile and substituents close to the leaving group.
In the reported cases the diastereomeric ratio is never greater than
4:1 and seems largely independent of the alkyl/aryl group at C4.4d,7
In 1985, Yoshida and co-workers reported the stereoselective
synthesis of THFs from 4-pentene-1,3-diols where, at 0 °C, only
the syn-1,3-diols reacted while the anti-1,3-diols remained unreac-
tive.5c We have previously reported that 3-hydroxytetrahydrofurans
are readily formed during the attempted tin-catalysed, mild
sulfonylation of acyclic 1,2,4-triols.8 Interestingly, we observed
stereo dependency where the syn-1,2,4-triol isomers gave the
anti-THF product exclusively, while the anti-1,2,4-triol isomers
gave a mixture of the anti-2,4-diol monotosylate (major) and
syn-THF (minor) products. We argued that the diastereoselectivity
of the reaction could be explained by a double activation of the tri-
ol moiety by a tin acetal shift from the 1,2-diol to the 2,4-diol. This
would form a six-membered Zimmerman–Traxler-like intermedi-
ate and the reaction should proceed via two energetically different
diastereomeric boat conformer transition states Ia and Ib (Fig. 1). A
similar boat transition state, based on hydrogen bonding between
1,3-diols was proposed by Yoshida and co-workers in the iodocy-
clization of 4-penten-1,3-diols;9 we hypothesized that tin chela-
tion of 1,3-diols should be stronger, and thus more influential
than hydrogen bonding, and therefore would be a significant con-
tributor to the reaction pathway.
In the proposed diastereomeric boat transition states (Fig. 1), it
is clear that tin chelation of the 2,4-diol exacerbates the eclipsing
steric effect in the anti-2,4-diol Ib, and thus would disfavour for-
mation of the syn-THF. Therefore, in the dibutyltin oxide mediated
cyclodehydration of 1,2,4-triols (Scheme 1), we anticipate that the
larger the steric bulk differences (DA-value) between the two
groups at C4 (R1, R2), then the greater would be the observed stereo
dependency for anti-THF formation from syn-1,2,4-triols and the
formation of the anti-2,4-diol tosylate from the anti-1,2,4-triols.
Conversely, as the
DA-value becomes smaller, we predict an
increase in the ratio of the formation of the syn-THF from the
anti-1,2,4-triols.
Overall, we postulate that the THF is the ‘determined or pre-
ferred product’ of the reaction of 1,2,4-triols, but that monosul-
fonylation is an incomplete reaction due to the tin-chelate
⇑
Corresponding author. Tel.: +27 12 3826382; fax: +27 12 3826386.
E-mail address: Nkambulecm@tut.ac.za (C.M. Nkambule).
http://dx.doi.org/10.1016/j.tetlet.2015.02.083
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

1826
M. P. Gamedze, C. M. Nkambule / Tetrahedron Letters 56 (2015) 1825–1829
H
H
Bu
Sn
Bu
O
Bu
Bu
R
O
Cl
Sn
Bu
R
O
O
Sn
Bu
O
OH O Sn Bu
OTs
=
anti-
Bu
O
OTs
OTs
THF
R
R
R
R
OTs
TsO
Ia
R
Bu
Sn
R
Bu
O
Bu
Cl
Bu
O
H
O
Sn
Sn
H
Bu
O
OH O Sn Bu
OTs
Bu
=
O
syn-
THF
O
Bu
OTs
TsO
Ib
Figure 1. Proposed mechanism for the diastereoselective THF synthesis from 1,2,4-triols.
O
O
R2
R2
OH
O
OH
Bu2SnO
TsCl
HO
R1
HO
R1
OH
OH
R2
+
R2
+
OH
OTs
R1
R2
R1
R1
aldehyde/
ketone
1,2,4-triol
anti-THF
syn-THF
anti-2,4-diol tosylate
Scheme 1. General scheme for the cyclodehydration or sulfonylation of 1,2,4-triols.
induced thermodynamic barrier to cyclization for the anti-1,2,4-
triols. This is contrary to the reported observation of cyclization
as an unavoidable nuisance pathway (in quantitative yield) in the
attempted tosylation of anti-1,2,4 triols.10 In this Letter we report
additional evidence for our proposed mechanism on the diastere-
oselectivity of the dibutyltin oxide mediated cyclodehydration or
sulfonylation (CDS) method.
There are a plethora of methods for the preparation of 1,2,4-tri-
ols, including some elegant stereoselective methods for the synthe-
sis and dihydroxylation of homoallylic alcohols and aldol
condensations.11 However, for the purposes of demonstrating the
further reactions of 1,2,4-triols, we have restricted our approach
to the direct Grignard alkylation of the diol aldehyde 1 derived
from (S)-malic acid, and the dihydroxylation of homoallylic alco-
hols derived from the allylation of ketones and aldehydes
(Scheme 2) according to reported methods.12 The details of the
synthesis and characterization of the 1,2,4-triols can be found in
Supplementary material.
Our preliminary objective was to demonstrate that in the
absence of a thermodynamic barrier, the proposed reaction condi-
tions [Bu2SnO (cat.), Et3N (1 equiv), refluxing CH2Cl2] would lead to
the cyclodehydration of the 1,2,4-triols. Hence, the reaction of
1,2,4-butanetriol (2), where the two groups at C4 are both hydro-
gens, was used as a test case (Table 1, entry 1). The exclusive and
quantitative (98%) formation of 3-hydroxytetrahydrofuran con-
firmed our hypothesis and we set out to investigate the diastere-
oselectivity, and indeed, the stereodivergency of the method.
Our postulate was that as the steric size differences between
the two groups at C4 increased (increasing
DA-values), the ratio
of the diastereomeric THFs (c:d) would also increase because the
syn-triol diastereomers would form preferentially the anti-THFs
(c), while the anti-triol diastereomers would form the monosul-
fonyl derivatives (e). Indeed, the results obtained showed that
the anti-THFs (c) were formed in high yields (80–85%) for all the
syn-triols, while the monotosylate (e) was the major product
(66–87%) for all the anti-triols (Table 1, entries 2–11). As predicted,
the diastereomeric ratio (c:d) increased from 7:1 to >99:1 as the
The diastereomers of the 1,2,4-triols synthesized (Fig. 2) were
readily separated by column chromatography; in general the syn-
triols are less polar and elute before the anti-triols. The overall yield
of the triol diastereomers from aldehydes and ketones was very
good and ranged from 39% to 55%. The relative stereochemistry of
D
A value increased from 1.68 to 4.79 kcal/mol (Table 1, entries
2–11). In the case of 5,5-dimethyl-1,2,4-hexanetriols (7a and 7b),
formed from pivaldehyde, where the A-value difference was the
D
largest, the syn-THF (7d) was not observed at all. Our interpreta-
tion of this result is that for 7b the barrier to cyclization is too high
to be overcome (Table 1, entry 11).
the triols was established by comparison (
D
ppm) of the 13C NMR
chemical shifts for the three C–O carbons (C1, C2 and C4) with pre-
viously reported characterizations. With the triols in hand, they
were subjected to the dibutyltin oxide mediated CDS reaction con-
ditions and the results are presented in Tables 1 and 2.13
On the other hand, when the
as was the case for the triols 13a and 13b, the THF diastereomeric
ratio (c:d) also decreased to 2:1 as the yield of the syn-THF increased
D
A value decreased to ꢀ1 kcal/mol,
O
OH
R2
R2
OH
O
HO
R1
HO
R1
O
O
OH
OH
HO
R1
R2
O
H
O
(S)-malic acid
1,2,4-triols
homoallylic alcohols
1
Scheme 2. General scheme for the synthesis of 1,2,4-triols.

M. P. Gamedze, C. M. Nkambule / Tetrahedron Letters 56 (2015) 1825–1829
1827
OH
OH
CH3
HO
H
H
HO
R1
OH
OH
OH
OH
R1
a
a
R2
OH
OH
OH
CH3
HO
R1
HO
R1
HO
R1
OH
b
b
2
3
4
5
6
7
R1 = H
8
9
R1 = R2 = CH3
R1 = R2 = Ph
13 R1 = Ph
R
R
R
R
1 = C3H5 (ω-propenyl)
1 = C11H21 (ω-undecenyl)
1 = C13H25 (ω-tridecenyl)
1 = Ph
10 R1 = R2 = (CH2)4
11 R1 = R2 = (CH2)5
12 R1 = R2 = (CH2)11
R1 = t-Bu
Figure 2. The 1,2,4-triols used in this study.
Table 1
Cyclodehydration or sulfonylation of 1,2,4-triols derived from aldehydes
O
O
OH OH
OH
H
HO
R1
OH
OH
H
H
+
H
+
OH
OTs
R1
R1
R1
anti-2,4 diol tosylate (e)
1,2,4-triol
anti-THF (c)
syn-THF (d)
Entry
Triol
R1
D
A-Value (kcalÁmolÀ1
)
Yield (%)
c
d
e
1
2
H
0
98a
0
2
3
4
5
6
7
8
9
3a
3b
4a
4b
5a
5b
6a
6b
7a
7b
C3H5
C3H5
(
(
x
x
(
(
(
(
-propenyl)
-propenyl)
1.68
1.79
1.79
2.8
82
0
90
0
85
0
84
0
0
12
0
12
0
12
0
9
0
0
0
66
0
74
0
75
0
83
0
87
C
C
C
C
11H21
11H21
13H25
13H25
x
x
x
x
-undecenyl)
-undecenyl)
-tridecenyl)
-tridecenyl)
Ph
Ph
t-Bu
t-Bu
10
11
4.79
80
0
a
Isolated as the benzoate ester.
Table 2
Cyclodehydration or sulfonylation of 1,2,4-triols derived from ketones
O
O
R2
OH OH
OH
R2
R1
HO
R1
OH
OH
R2
+
R2
+
OH
OTs
R1
R1
anti-2,4 diol tosylate (e)
1,2,4-triol
anti-THF (c)
syn-THF (d)
Entry
Triol
R1
R2
D
A-Value (kcalÁmolÀ1
)
Yield (%)
c
d
e
1
2
3
4
5
6
7
8
9
CH3
Ph
CH3
Ph
0
0
0
0
0
78
79
72
79
75
0
0
14
13
0
10
11
12
13a
13b
Cyclopentanone
Cyclohexanone
Cyclododecane
Ph
Ph
CH3
CH3
1.06
1.06
79
0
0
35
0
62
dramatically to 35% (Table 2, entries 6 and 7). The rationale for this
diastereoselectivity is depicted in Figure 3, where the proposed
transition state for cyclodehydration would presumably occur via
the conformational isomer IIb, which clearly shows the improbabil-
ity of the cyclization product (disfavoured), thus the C1 tosylate per-
sists as the major product.
However, the results for compounds 8 and 9 (Table 2, entries 1
and 2), where the groups at C4 are identical (CH3 or Ph, respective-
ly), were surprising in that the THF (only one possible diastereomer
c for the symmetrical ketones 8–12) was formed as the exclusive
product, in very good yields (>75%). Since there is only one possible
conformational transition state, we expected that there would be a

1828
M. P. Gamedze, C. M. Nkambule / Tetrahedron Letters 56 (2015) 1825–1829
R1
R1
Bu
Sn
R1
R2
Bu
O
Bu
R2
O
O
Bu
R2
Sn
O
O
Sn
O
Bu
R2
R1
Bu
O
OTs
OH
OTs
TsO
R1>>R2
ΔA-value > 1 kcal/mol
minor
IIb
R2 OH
R1
OH
OTs
major
Figure 3. A proposed barrier to cyclodehydration for the anti-1,2,4-triols.
barrier to cyclodehydration from the axially positioned group, there-
fore a significant amount of tosylate should form. The exclusive for-
mation of the THF suggests that this cyclization might arise as a
consequence of a gem-disubstituent (Thorpe–Ingold) effect being
the dominant driving force versus the tin chelation effect.14 The
reactions of 1,2,4-triols formed from simple cyclic ketones
(Table 2, entries 3–5) fit this trend favouring the formation of spiro-
cyclic 3-hydroxytetrahydrofurans in good yields (72–79%). However,
the formation of minor monosulfonylated products (<15%) for the
smaller cyclic ketones 10 and 11 (Table 2, entries 3 and 4) suggests
that conformational rigidity may contribute to a slight barrier to
cyclization, thereby increasing the ‘tin chelation effect’.
We were pleasantly surprised to observe that the CDS protocol
proved to be an improvement on the efficiency of previously report-
ed methods for the synthesis of spirocyclic 3-hydroxytetrahydrofu-
rans derived from cyclic ketones.2b,5b,12c,15 For example, via the CDS
method, the 3-hydroxytetrahydrofuran 12c was formed in 79%
yield, compared to the 33% yield (after 4 days) reported by
Vasconcelos et al.15a Given the mild reaction conditions of our
protocol, this method could represent a more favourable option
for the synthesis of spirocyclic 3-hydroxytetrahydrofurans.
In summary, we have described further evidence that the tin-
mediated cyclodehydration or sulfonylation of 1,2,4-triols is pre-
dictably diastereoselective and superior to previous observations
where the observed diastereoselectivity has generally been poor.16
Therefore, this protocol might find application in establishing the
relative configuration of 1,2,4-triols by observing the formation/
non-formation of sulfonylated products. This would be a valuable
addition to the existing circular dichroic (CD) methods.17 Finally,
this method has the potential to be employed as a mild alternative
for the stereoselective synthesis of substituted tetrahydrofurans
from ketones and aldehydes, via 1,2,4-triols.
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.02.083.

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round-bottom flask equipped with
a condenser were added the triol
(0.175 mmol), p-TsCl (0.192 mmol), Bu2SnO (0.0087 mmol) and Et3N
(0.192 mmol), and the mixture was heated to a gentle reflux in CH2Cl2 (2 mL,
0.1 M) for 3.5 h. The reaction was monitored by TLC. On completion, the
reaction was quenched with satd NH4Cl or MeOH (5 mL) and extracted with
EtOAc (50 mL). The organic layer was washed with H2O (15 mL) and brine
(15 mL) before drying over MgSO4. The solvent was evaporated and the crude
residue was purified by column chromatography.