Organocatalytic synthesis of an alkyltetrahydropyran

Article abstract of DOI:10.1055/s-0028-1087550

The first synthesis of an alkyltetrahydropyran using a diarylprolinol organocatalyst is described. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1087550

390
LETTER
Organocatalytic Synthesis of an Alkyltetrahydropyran
S
ynthesis of an A
l
.
kyltetrahydro
D
pyran íez,*a M. G. Núñez,a A. Benéitez,a R. F. Moro,a I. S. Marcos,a P. Basabe,a H. B. Broughton,b J. G. Uronesa
a
Departamento de Química Orgánica, Universidad de Salamanca, Plaza de los Caídos 1-5, 37008 Salamanca, Spain
Fax +34(923)294574; E-mail: ddm@usal.es
Lilly S. A., Avda de La Industria 30, 28108 Alcobendas, Madrid, Spain
b
Received 2 September 2008
ran derivatives using trialkylsilyl-protected prolinols as
catalysts. Arvidsson et al.14 have reported a domino reac-
tion involving an oxa-Michael attack of salicylic aldehyde
derivatives onto a,b-unsaturated aldehydes, activated
through iminium ion formation with the organocatalyst,
followed by an intramolecular aldol reaction. The same
strategy has been applied by Córdova et al.15 and Wang et
al.16
Abstract: The first synthesis of an alkyltetrahydropyran using a
diarylprolinol organocatalyst is described.
Keywords: organocatalysis, tetrahydropyrans, diarylprolinol,
cyclization
Tetrahydropyrans are present in many biologically active
marine natural compounds1 such as brevetoxin B2
(Figure 1) and maitotoxin.3 Our group has been interested
for several years in the synthesis of this type of hetero-
cycles, especially the more substituted ones.4
A pioneering application of an organocatalytic intramo-
lecular oxa-Michael reaction was developed by Ishikawa
et al. in the enantioselective synthesis of anti-HIV-1-ac-
tive coumarins such as (+)-calanolide A catalyzed by
quinine.17 Recently, Scheidt et al.18 have developed in-
tramolecular cyclizations for the production of flavanones
and chromanones using a quinine-derived thiourea cata-
lyst to activate a b-keto ester alkylidene substrate and pro-
mote the oxa-Michael addition of the phenol. The group
of Merschaert19 has obtained chiral benzopyrans by in-
tramolecular cyclization of phenols with a,b-unsaturated
esters using cinchona alkaloids as catalysts.
One of the most common procedures for the synthesis of
tetrahydropyrans is the Michael addition of an alcohol to
a,b-unsaturated esters to give cis-2-alkyl-3-oxy-tetrahy-
dropyrans as described by Gung et al.5 and Martín et al.6
This methodology has been applied by Nicolaou et al. in
the synthesis of brevetoxin B.7
The major drawbacks of the oxa-Michael reaction are its
low reactivity, reversibility, and lack of control in stereo-
selectivity.8 In order to solve this problem, several groups
have made use of chiral auxiliaries.9
However, to the best of our knowledge, there have been
no reports on the organocatalytic synthesis of alkyltetra-
hydropyrans, so we decided to investigate an organocata-
lytic intramolecular oxa-Michael addition of alcohols to
a,b-unsaturated aldehydes.
Recently, there has been a growing interest in the organo-
catalytic asymmetric oxa-Michael reaction.10 The first
highly enantioselective organocatalytic b-hydroxylation
of a,b-unsaturated aldehydes was carried out by Jørgensen
using oximes as nucleophiles.11 Maruoka et al. have de-
veloped a biphenyldiamine-based catalyst to perform 1,4-
additions of alcohols to a,b-unsaturated aldehydes.12 In
addition, the group of Falck has reported an enantioselec-
tive intramolecular oxa-Michael addition to g/d-hydroxy-
a,b-enones using a thiourea quinine based organocata-
lyst.13
The first substrate chosen for our study was aldehyde 6
(Scheme 1), which was obtained from commercial d-valero-
lactone (1) in four steps. DIBAL-H reduction20 of the lac-
tone 1 followed by a Wittig olefination21 gave a mixture
of hydroxy esters 3 and 4 in 65% yield and a ratio 1:10.
After separating compound 4 by column chromatography,
it was reduced to the allylic alcohol 5 in 87% yield. When
5 was submitted to MnO2 allylic oxidation,22 a mixture of
the desired aldehyde 6, and the cyclized compound 7 was
obtained (Scheme 1). These compounds could not be sep-
In the last years, several groups have also dedicated much
effort to the organocatalytic synthesis of chiral benzopy-
HO
CHO
O
H
H
O
H
H
O
H
H
O
O
H
O
O
O
H
O
O
H
O
H
H
H
O
H
H
H
Figure 1 Brevetoxin B
SYNLETT 2009, No. 3, pp 0390–0394
x
x.
x
x.
2
0
0
9
Advanced online publication: 21.01.2009
DOI: 10.1055/s-0028-1087550; Art ID: D31408ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of an Alkyltetrahydropyran
391
a
b
+
COOEt
OH
OH
d
O
OH
O
O
O
COOEt
1:10
δ-valerolactone (1)
2
3
4
c
+
OH
OH
CH2OH
CHO
6
CHO
7
5
Scheme 1 Reagents and conditions: a) DIBAL-H, CH2Cl2, –78 °C; b) Ph3P(CH)COOEt, C6H6, r.t., 12 h, 65%; c) DIBAL-H, CH2Cl2, –78 °C,
87%; d) MnO2, CH2Cl2, r.t., 50%.
a
b
c
O
OH
OH
O
OH
TBSO
TBSO
TBSO
TBSO
HO
HO
2
8
9
10
d
g
f
e
HO
CH2OH
CHO
CH2OH
14
(E/Z = 4:1)
13
12
11
COOEt
(E/Z = 4:1)
(E/Z = 4:1)
(E/Z = 4:1)
Scheme 2 Reagents and conditions: a) MeMgBr, THF, 0 °C, 70%; b) TBSCl, imidazole, DMF, 0 °C, 80%; c) TPAP, NMO, 3 Å MS, CH2Cl2,
r.t., 2 h, 94%; d) (EtO)2Ph3P(O)CH2COOEt, NaH, monoglyme, 60 °C, 96%; e) DIBAL-H, CH2Cl2, –78 °C, 93%; f) TBAF, THF, r.t.; g) MnO2,
CH2Cl2, r.t., 70% in 2 steps.
arated by column chromatography, as in the silica acidic Firstly, we performed the reaction at room temperature as
medium aldehyde 6 underwent cyclization to give com- shown in Table 1. The initial experiments showed that
pound 7.
compound 14 did not cyclize when no catalyst was used,
even when it was left reacting for a long period of time
(entry 1). Pyrrolidines C1 and C2 catalyzed the reaction
but aldehyde 15 rapidly oxidized to the carboxylic acid, so
the ee could not be measured (entries 2 and 3, respective-
ly). Catalyst C5 gave no reaction (entry 4) and with C6 the
reaction took place, in the presence of 20 mol% of HCl (2
M), but the dimethyl acetal of 15 was obtained with no ee
(entry 5).
We then decided to test a more hindered starting material
such as aldehyde 14 (Scheme 2), starting again from d-
valerolactone (1).
In this case, lactol 2 was treated with methyl magnesium
bromide,23 obtaining 8 in 70% yield. This diol 8 was se-
lectively monoprotected and oxidized with TPAP24 to the
methyl ketone 10 in 70% yield over two steps (Scheme 2).
This compound was submitted to Horner–Wadsworth–
Emmons reaction conditions25 to obtain an inseparable
mixture (E/Z = 4:1) of a,b-unsaturated esters 11. This
mixture was reduced with DIBAL-H to the allylic alcohol
12 in 93% yield. After deprotection and subsequent allylic
oxidation with MnO2, the required a,b-unsaturated alde-
hyde 14 was obtained in 70% yield over two steps.
In order to avoid the oxidation of the resulting aldehyde
15, its in situ reduction was performed using DIBAL-H or
NaBH4 and a cosolvent when required.29 The results are
shown in Table 2.
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Although compound 14 was obtained as a mixture of dia-
stereoisomers E/Z, it was decided to use it as a substrate
for organocatalytic oxa-Michael cyclizations since there
are literature precedents of E/Z mixtures of olefins that
lead to higher diastereomeric ratio in the final product as
reported by Jørgensen in citral epoxidation26 and of pure
diastereoisomers E and Z that converge to the same enan-
tiomer as in the hydride reduction of a,b-unsaturated alde-
hydes reported by MacMillan.27
N
H
N
N
N
H
H
H
OH
OTMS
OMe
C1
C2
C3
C4
O
O
N
N
N
N
N
H
N
HN
N
N
N
H
H
Ph
Ph
C7
C5
C6
O
O
SO2Ph
N
H
Commercial or easily obtained pyrrolidines C1–C7 and
C9–C10 and pyrrolidine C8 (Figure 2), previously ob-
tained by us,28 were chosen to test as organocatalysts in
the intramolecular oxa-Michael cyclization of substrate
14.
N
N
H
N
H
C8
C9
C10
Figure 2 Organocatalysts tested in the intramolecular oxa-Michael
reaction of substrate 14
Synlett 2009, No. 3, 390–394 © Thieme Stuttgart · New York

392
D. Díez et al.
LETTER
Table 2 Synthesis of Compound 16 with Various Catalysts
Table 1 Synthesis of Compound 15 with Various Catalysts
1. catalyst (20 mol%)
solvent
catalyst
(20 mol%)
CHO
2. reductant
cosolvent
solvent, r.t.
O
HO
O
OH
HO
CHO
15
CHO
16
14
(E/Z = 4:1)
14
E/Z = 4:1
Entry
Catalyst
–
Solvent
Time (h) Yield (%) ee (%)a
Entry Catalyst Solvent
Time Temp Yield ee
Config.b
(h)
(°C)
(%)c (%)a
1
2
3
4
5
toluene
CH2Cl2
toluene
MeOH
MeOH
96
112
2
0
10
75
0
–
1
2
C2
MeOH
toluene
CH2Cl2
toluene
MeOH
toluene
MeOH
toluene
PhCl
7
r.t.
60
74
60
68
30
25
30
43
43
0
7
0
–
–
–
–
–
–
–
–
–
–
–
–
–
–
S
–
R
R
R
R
R
–
R
R
R
R
C1
n.d.
n.d.
–
C2
1
r.t.
C2
3
C2
3
r.t.
0
C5
7
4
C2c
C7
1
r.t.
0
C6b
24
53c
0
a Determined by chiral GC-MS (CHIRASIL DEX-CB).
b HCl (2 M; 20 mol%) was added.
5
2
r.t.
0
c Dimethylacetal of 15 was obtained.
6
C9
3
r.t.
0
7
C10
C10
C10
C10
C10
C10
C10d
C10
C8
0.5
r.t.
5
As is evident from Table 2, catalysts C2, C9, and C10
gave the required alcohol 16 at room temperature, but the
ee30 was zero or very low (entries 1–9). So it was decided
to carry out the reaction at low temperature with catalyst
C10, since it was the one that promoted the fastest cy-
clization. At very low temperatures (–78 °C, –40 °C, en-
tries 10 and 11), the reaction did not take place. However,
raising the temperature to –20 °C, the cyclization took
place in reasonable yield with very low ee.
8
0.5
0.5
3
r.t.
0
9
r.t.
5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
toluene
toluene
toluene
toluene
CH2Cl2
toluene
toluene
toluene
toluene
CH2Cl2
MeOH
THF
–78
–40
–20
–20
–20
–20
–20
–20
–20
–20
–20
–20
r.t.
0
3
0
0
0.5
1
66
64
40
80
70
60
63
70
61
20
62
74
30
50
60
4
5
In order to increase the ee, it was decided to use other cat-
alysts such as C3 and C4, which have been tested before
in Michael additions,10 and catalyst C8, previously syn-
thesized by us.
1
4
2
13
0
C8d
C3
2
As shown in Table 2, catalyst C8 gave compound 16 in
good yield but with low ee (entry 15). Addition of benzoic
acid as co-catalyst did not improve the results (entry 16).
However, the use of catalyst C3 gave better results. When
the reaction was performed in toluene at –20 °C, 60%
yield (in two steps) and 41% ee was obtained (entry 17),
so we next investigated the effect of solvents (entries 19–
21) and found that dichloromethane was the solvent of
choice (entry 19).
2
41
37
44
26
33
0
C3d
C3
2
2
C3
6
C3
6
C3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
toluene
2
When a temperature screening was carried out, it was
found that the ee increased as temperature was lowered,
while the yields remain unaffected (entries 22–25). The
same effect was observed when using catalyst C4, bearing
no methoxy group (entries 26 and 27).
C3
2
0
14
48
57
48
C3
2
–40
–78
–20
C3
12
Determination of the absolute configuration for tetra-
hydropyran 16 was carried out by obtaining the chiral
compound using Sharpless asymmetric epoxidation31 as
follows (Scheme 3).
C4
1
a Determined by chiral GC-MS (CHIRASIL DEX-CB).
b Determined by comparison with the chiral compound 16 obtained as
described in Scheme 3.
c Determined after isolation of pure compound.
d Benzoic acid (20 mol%) was added.
With regard to the mechanism, we assume that the reac-
tion of the pyrrolidine-type catalyst with the a,b-unsatur-
ated aldehyde results in an intermediary iminium ion
which is then attacked by the hydroxyl moiety.10 In order
to explain the results obtained, a molecular modeling
study has been carried out for the reaction of substrate 14
with catalyst C3. A wide variety of possible transition
states for the cyclization process has been explored ac-
cording to the following protocol: RHF/AM1 has been
used to perform geometry optimizations and thermo-
chemical analysis, while for the energies of the optimized
Synlett 2009, No. 3, 390–394 © Thieme Stuttgart · New York

LETTER
Synthesis of an Alkyltetrahydropyran
393
a
c,d
b
O
O
CH2OAc
O
TBSO
TBSO
TBSO
CH2OAc
CH2OH
CH2OH
OH
e
(E/Z = 4:1) 13
16
17
18
f
+
CH2OH
OTs
CH2OH
CH2OAc
O
O
O
2:1
OTs
16
20
19
Scheme 3 Reagents and conditions: a) L-(+)-DET, Ti(i-OPr)4, TBHP, –23 °C, 12 h, 90%; b) Ac2O, pyrdine, r.t., 92%; c) TBAF, THF, r.t.,
80%; d) CSA, CH2Cl2, –40 °C, 60%; e) TsCl, pyridine, 0 °C to r.t., 90%; f) LAH, THF, 0 °C, 50%.
geometries, single-point calculations at the B3LYP/6-
31G* level of theory were performed.32 All these calcula-
tions were carried out with the Gaussian 03 package.33
Our best current estimations of the two preferred transi-
tion states are shown in Figure 3. They are considered on
balance the most likely to be good models for the oxa-
Michael reaction. This choice is based on our analysis of
the single-point B3LYP/6-31G* energy calculations, the
comparison of gas-phase energies vs. solvation models,
the consideration of the qualitative effect of the dipole and
the calculated RHF/AM1 thermochemical parameters.1 In
the solvation model, the energy gap between transition
states is 5.7 kcal/mol in favor of the R-isomer (ee expected
>99.9%), 0.7 kcal/mol smaller than in the gas-phase cal-
culation. Unsurprisingly, this energy difference is not ful-
ly consistent with the observed ee. At this level of theory
we could not be able to make a better prediction, because
the required energetic difference for the observed ee
Figure 3 Chairlike transition states suggest that the methyl group is
equatorial, and attack of the alcohol is from the underside of the inter-
mediate E-iminium salts; favored TS-I leads to the R product
toral fellowships of the Spanish MEC and Universidad de
would be ca. 0.4–0.8 kcal/mol, well within the error of the Salamanca, respectively. Finally, we would like to thank Prof. S. V.
Ley for his kindly gift of catalyst C7.
method. However, the identified transition states – with
minimized steric interactions and chairlike conformations
for the cyclization – are consistent with our expectations,
and we have been able to correctly predict the major iso-
mer generated as R. We continue to work on more compu-
tationally intensive models using higher levels of theory,
References and Notes
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Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
Financial support for this work came from the Spanish MEC
(CTQ2006-08296/BQU) and Junta de Castilla y León (Spain)
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D. Díez et al.
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