Stereoselective synthesis of 2,6-disubstituted-4-aryltetrahydropyrans using sakurai-hosomi-prins-friedel-crafts reaction

Article abstract of DOI:10.1002/ejoc.200900006

The reaction of aldehydes with allyltrimethylsilane in arene solvents gives symmetrical 2,6-disubsLituLed-4-ary]tetrahydropyrans in good yields. The reaction is highly stereoselective. Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/ejoc.200900006

FULL PAPER
DOI: 10.1002/ejoc.200900006
Stereoselective Synthesis of 2,6-Disubstituted-4-Aryltetrahydropyrans Using
Sakurai–Hosomi–Prins–Friedel–Crafts Reaction
Udagandla C. Reddy,[a] Somasekhar Bondalapati,[a] and Anil K. Saikia*[a]
Keywords: Multicomponent reactions / Diastereoselectivity / Cyclization / Oxygen heterocycles
The reaction of aldehydes with allyltrimethylsilane in arene
solvents gives symmetrical 2,6-disubstituted-4-aryltetrahy-
dropyrans in good yields. The reaction is highly stereoselec-
tive.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
ence of BF3·Et2O (1.2 mmol) at 0 °C, and the reaction was
warmed to room temperature. 2,4,6-Triphenyltetrahydro-
pyran was obtained in 74% yield. To prove the general appli-
cability of this reaction, a variety of alkyl and aryl alde-
hydes were investigated as shown in Scheme 1. The results
are summarized in Table 1.
Introduction
Multicomponent reactions are important in organic syn-
thesis because of their ability to construct multiple bonds
in a single step, which is crucial for pharmaceutically active
and natural product synthesis.[1] The synthesis of the tetra-
hydropyran unit is important because of its presence in
many natural products.[2] The all-cis 2,6-disubstituted-4-ar-
yltetrahydropyrans have olfactory properties[3] and shows
nonredox 5-lipoxygenase inhibiting properties.[4] These
tetrahydropyrans are prepared by hetero-Diels–Alder meth-
ods,[5] manipulation of carbohydrates,[6] Prins cyclization,[7]
and intramolecular Michael reactions.[8] Although the syn-
thesis of 4-halo-,[9] 4-thio-,[10] 4-azido-,[11] 4-amino-,[12] and
4-hydroxytetrahydropyrans[7e,7f,9d,13] have been reported in
the literature, the synthesis of 2,6-disubstituted-4-aryl-
tetrahydropyrans is limited.[14,2h]
Scheme 1. Synthesis of symmetrical 2,6-disubstituted-4-aryltetra-
hydropyran (R = alkyl, aryl).
In all the cases studied, 4-aryltetrahydropyrans 1b–16b
(Table 1) could be obtained in high purity without any side
products. Both aliphatic and aromatic aldehydes gave good
yields with high diastereoselectivity, as determined from the
1H and 13C NMR spectra of the crude products. The sub-
stituents on the aromatic ring play an important role in this
reaction: electron-withdrawing substituents and simple al-
dehydes gave better yields than those obtained when elec-
tron-donating groups were on the ring. Aliphatic aldehydes
were found to be better substrates for this reaction. The
substituents at the 2-, 4-, and 6-positions of the tetra-
hydropyran ring are in a cis relationship and are equatorial.
This is revealed from the coupling constants of the 2,6-H
(J = 11.2 and 2.0 Hz) and the 4-H (J = 11.2 and 2.0 Hz)
hydrogen atoms of compound 2b (Figure 1). This was also
confirmed by an NOE experiment and single-crystal X-ray
analysis.[15]
We have shown in our previous investigation that 2,6-
disubstituted-4-amidotetrahydropyrans can be synthesized
in high yields with excellent stereoselectivity through
BF3·Et2O-mediated cyclization of aldehydes and allyltri-
methylsilane in acetonitrile. In this protocol, a Sakurai–
Hosomi–Prins–Ritter reaction sequence was used.[12e]
We disclose here a stereoselective one-pot, three-compo-
nent synthesis of 2,6-disubstituted-4-aryltetrahydropyrans
from aldehydes, allyltrimethylsilane, and arenes by using a
Sakurai–Hosomi–Prins–Friedel–Crafts reaction.
Results and Discussion
To start, benzaldehyde (1.0 mmol) was treated with allyl-
trimethylsilane (0.6 mmol) in benzene (5.0 mL) in the pres-
[a] Department of Chemistry, Indian Institute of Technology
Guwahati,
To explore further utility of the method, other arenes
were also studied as nucleophiles as shown in Table 2. Thus,
the reaction of m-nitrobenzaldehyde (3a) in toluene gave
product 17 as an inseparable mixture of two regioisomers
with a ratio of 4.7:1 and 97% overall yield. o-Xylene gave
Guwahati 781039, India
Fax: +91-361-2690762
E-mail: asaikia@iitg.ernet.in
Supporting information for this article is available on the
WWW under http://www.eurjoc.org/ or from the author.
Eur. J. Org. Chem. 2009, 1625–1629
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U. C. Reddy, S. Bondalapati, A. K. Saikia
FULL PAPER
Table 1. Synthesis of 2,6-disubstituted-4-phenyltetrahydropyran.
product 18 as an inseparable mixture of two regioisomers
with a ratio of 4:1 and 100% overall yield. In contrast, p-
xylene gave single product 19 with 100% yield.
Table 2. Reaction of m-nitrobenzaldehyde with other aromatic nu-
cleophiles.
[a] Yields refer to isolated products. Compounds were charac-
1
terized by H NMR, 13C NMR, and IR spectroscopy. [b] Insepa-
1
rable mixture of regioisomers. Ratio was determined by H NMR
spectroscopy. [c] o/p-Isomers were separated and their ratio was 2:1.
[d] Reaction with 5.0 equiv. of anisole in CH2Cl2.
[a] Yields refer to isolated products. Compounds were charac-
1
terized by H NMR, 13C NMR, and IR spectroscopy.
Similarly, m-xylene gave product 20 as an inseparable
mixture of two regioisomers with a ratio of 1:2 and 100%
overall yield. Anisole is a good nucleophile, as it gave 21
with 95% yield within 30 min. (21p/21o, 1:2). Even at a low
concentration of anisole in dichloromethane a good yield
was obtained (90%, Table 2). Electron-deficient aromatic
compounds such as halobenzenes and nitrobenzene do not
act as nucleophiles in the Friedel–Crafts reaction. Aromatic
compounds having electron-donating groups, such as
Figure 1. Coupling constants and NOE of compound 2b.
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Eur. J. Org. Chem. 2009, 1625–1629

Stereoselective Synthesis of 2,6-Disubstituted-4-Aryltetrahydropyrans
methyl and methoxy groups, behave as strong nucleophiles
The mechanism of the reaction can be explained as fol-
in comparison to benzene, which is evident from their reac- lows: In the presence of Lewis acid allyltrimethylsilane (1)
tion times and yields (Tables 1 and 2). Other aromatic com- reacts with the aldehyde to afford intermediate
2
pounds like naphthalene, 2-methoxynaphthalene, and other (Scheme 2). Intermediate 2 reacts with another molecule of
fused-ring aromatic compounds were inactive under these the aldehyde to give tetrahydropyranyl cation 3, which in
reaction conditions. This might be due to steric hindrance.
the presence of an aryl nucleophile, gives intermediate 4.
The reaction was also carried out with two different alde- Species 4 after deprotonation gives 2,6-disubstituted-4-aryl-
hydes. The reaction was performed by allowing aldehyde A tetrahydropyran 5.
to react with allyltrimethylsilane for 1 h, followed by the
addition of aldehyde B. The reaction was not selective and
Conclusions
gave three different products: symmetric products C and D
and cross product E (Table 3). The yield of the cross prod-
uct was always less than that of the symmetric products.
Allowing the first aldehyde to react with allyltrimethylsilane
for a longer time period resulted in lower amounts of cross
products. This indicates that the rate of formation of homo-
allylic alcohol (Sakurai–Hosomi reaction) is slower than the
Prins cyclization reaction. That is, all the molecules of alde-
hyde A do not form the homoallylic alcohol intermediate
so that aldehyde B can take part in Prins cyclization to give
only the cross product. The possibility of allyl transfer from
the initially formed homoallyloxy intermediate (species 2,
Scheme 2) to the second aldehyde cannot be ruled out.
In summary, an efficient, highly diastereoselective, one-
pot method for the synthesis of 2,6-disubstituted-4-aryl-
tetrahydropyrans in good yields has been developed. The
scope and synthetic applications of this novel reaction is
under investigation in our laboratory.
Experimental Section
General Procedure for the Synthesis of 2,6-Disubstituted-4-aryl-
tetrahydropyrans: To a mixture of aldehyde (1.0 mmol), BF3·Et2O
(1.2 mmol), and aryl compound (3.0 mL) was added a mixture of
allyltrimethylsilane (0.6 mmol) and aryl compound (2.0 mL) drop
by drop at 0 °C; the temperature was slowly increased to room
temperature over 1 h. The reaction mixture was stirred at room
temperature for the specified time. The progress of the reaction was
monitored by TLC (ethyl acetate/hexane). After completion of the
reaction, the product was extracted with ethyl acetate and washed
with brine and water. The organic layer was dried (Na2SO4) and
evaporated to leave the crude product, which was purified by short-
column chromatography over silica gel to give the title compounds.
Table 3. Reaction with mixed aldehydes.
2,4,6-Triphenyltetrahydropyran (1b): To a mixture of benzaldehyde
(1a; 0.10 mL, 1.0 mmol), benzene (3.0 mL), and BF3·Et2O
(0.15 mL, 1.2 mmol) was added allyltrimethylsilane (0.10 mL,
0.6 mmol.) in benzene (2.0 mL) drop by drop at 0 °C; the tempera-
ture was slowly increased to room temperature over 1 h. The reac-
tion mixture was stirred at room temperature for 14 h. The progress
of the reaction was monitored by TLC (EtOAc/hexane, 1:9). After
completion of the reaction, the product was extracted with ethyl
acetate and washed with brine and water. The organic layer was
dried (Na2SO4) and evaporated to leave the crude product, which
was purified by short-column chromatography over silica gel to
give 1b (116 mg, 74%) as a gum. 1H NMR (400 MHz, CDCl3,
25 °C): δ = 1.76–1.85 (m, 2 H, 3ax,5ax-CH), 2.16–2.20 (m, 2 H,
3eq,5eq-CH), 3.17 [tt, J4ax,(3,5)ax = 12.0 Hz, J4ax,(3,5)eq = 3.6 Hz, 1
H, 4ax-CH], 4.75 (dd, J2ax,3ax = 11.2 Hz, J2ax,3eq = 2.0 Hz, 2 H,
2ax,6ax-CH), 7.19–7.37 (m, 12 H, ArH), 7.47–7.49 (m, 3 H, ArH)
ppm. 13C NMR (100 MHz, CDCl3): δ = 41.4, 42.6, 80.0, 126.0,
[a] Yields refer to isolated products. Compounds were charac-
1
terized by H NMR, 13C NMR, and IR spectroscopy.
126.6, 126.9, 127.5, 128.5, 128.7, 143.1, 145.3 ppm. IR: ν = 3063,
˜
2915, 2874, 1598, 1573, 1476, 1443, 1208, 1133, 1079, 1051, 1035,
753 cm–1. C23H22O (314.42): calcd. C 87.86, H 7.05; found C 87.68,
H 7.21.
4-(4-Methoxyphenyl)-2,6-bis(3-nitrophenyl)tetrahydro-2H-pyran (21p)
and 4-(2-Methoxyphenyl)-2,6-bis(3-nitrophenyl)tetrahydro-2H-pyran
(21o): To a solution of 3a (302 mg, 2.0 mmol) and BF3·Et2O
(0.30 mL, 2.4 mmol) in dichloromethane (2.0 mL) was added a
solution of allyltrimethylsilane (0.22 mL, 1.4 mmol.) and anisole
(1.10 mL, 10.0 mmol) drop by drop at 0 °C; the temperature was
slowly increased to room temperature over 1 h. The reaction mix-
ture was stirred at room temperature for 24 h. The progress of the
Scheme 2. Mechanism of the reaction.
Eur. J. Org. Chem. 2009, 1625–1629
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1627

U. C. Reddy, S. Bondalapati, A. K. Saikia
FULL PAPER
reaction was monitored by TLC (EtOAc/hexane; 1:9). After com-
pletion of the reaction, the product was extracted with ethyl acetate
and washed with brine and water. The organic layer was dried
(Na2SO4) and evaporated to leave the crude product, which was
purified by thin-layer chromatography over silica gel to give 21o
and 21p in a 2:1 ratio (390 mg, 90% of overall yield) as a solid.
1 H), 2.15–2.18 (m, 2 H), 3.17 (tt, J = 12.0, 3.6 Hz, 1 H), 4.73 (dd,
J = 11.2, 2.0 Hz, 1 H), 4.80 (dd, J = 11.2, 2.0 Hz, 1 H), 7.18–7.39
(m, 8 H), 7.46 (d, J = 8.8 Hz, 2 H), 7.59 (d, J = 8.4 Hz, 2 H), 8.16
(d, J = 8.8 Hz, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 41.0,
41.2, 42.3, 78.9, 80.1, 123.7, 125.9, 126.3, 126.6, 126.9, 127.7, 128.6,
128.8, 142.5, 144.6, 147.2, 150.3 ppm. IR: ν = 3062, 3029, 2941,
˜
Data for 21p: Solid. M.p. 164–168 °C. 1H NMR (400 MHz, 2849, 1602, 1516, 1495, 1345, 1287, 1102, 1078, 1030, 987, 851,
CDCl3): δ = 1.74–1.84 (m, 2 H, 3ax,5ax-CH), 2.21–2.25 (m, 2 H,
3eq,5eq-CH), 3.20 [tt, J4ax,(3,5)ax = 12.4 Hz, J4ax,(3,5)eq = 3.6 Hz, 1
748 cm–1. C23H21NO3 (359.42): calcd. C 76.86, H 5.89, N 3.90;
found C 76.92, H 5.78, N 3.88.
H, 4ax-CH], 3.78 (s, 3 H, -OCH3), 4.88 (dd, J2ax,3ax = J6ax,5ax
=
Supporting Information (see footnote on the first page of this arti-
10 Hz, J2ax,3eq = J6a,5eq = 1.2 Hz, 2 H, 2ax,6ax-CH), 6.86 (d, J =
8.4 Hz, 2 H, ArH),7.18 (d, J = 8.4 Hz, 2 H, ArH), 7.56 (m, 2 H,
ArH), 7.83 (d, J = 7.6 Hz, 2 H, ArH), 8.15 (d, J = 8.0 Hz, 2 H,
ArH), 8.34 (s, 2 H, ArH) ppm. 13C NMR (100 MHz, CDCl3): δ =
41.2, 41.4, 55.5, 79.2, 114.3, 121.1, 122.8, 127.8, 129.7, 132.1, 136.3,
1
1
cle): H and 13C NMR spectra of 1b–8b, 11b–16b, and 17–21; H,
13C, and 19F NMR spectra of compounds 9b and 10b; crude 1H
NMR spectra of compound 11b; NOESY spectrum of 2b; X-ray
structure and crystal parameters of compound 8b.
144.5, 148.5, 158.6 ppm. IR: ν = 2933, 2853, 1528, 1350, 1250,
˜
1103, 1072, 810, 737 cm–1. C24H22N2O6 (434.45): calcd. C 66.35, H
Acknowledgments
5.10, N 6.45; found C 66.50, H 5.27, N 6.57. Data for 21o: Solid.
1
M.p. 124–128 °C. H NMR (400 MHz, CDCl3): δ = 1.73–1.85 (m,
A. K. S. and U. C. R. are grateful to the Council of Scientific &
Industrial Research (CSIR), New Delhi for financial support
[Grant No. 01(1809)/02/EMR II] and a fellowship, respectively. The
authors gratefully acknowledge the X-ray facility provided by the
department of Science and Technology (DST) under the FIST pro-
gram.
2 H, 3ax,5ax-CH), 2.20–2.24 (m, 2 H, 3ex,5ex-CH), 3.69 [tt,
J4ax,(3,5)ax = 12.4 Hz, J4ax,(3,5)eq = 3.6 Hz, 1 H, 4ax-CH], 3.90 (s, 3
H, -OCH3), 4.92 (dd, J2ax,3ax = J6ax,5ax = 10 Hz, J2ax,3eq = J6a,5eq
=
1.2 Hz, 2 H, 2ax,6ax-CH), 6.88–6.95 (m, 2 H, ArH), 7.16–7.24 (m,
2 H, ArH), 7.56 (m, 2 H, ArH), 7.84 (d, J = 7.6 Hz, 2 H, ArH),
8.14–8.17 (m, 2 H, ArH), 8.33 (s, 2 H, ArH) ppm. 13C NMR
(100 MHz, CDCl3): δ = 35.1, 39.6, 55.6, 79.4, 110.7, 121.0, 121.1,
122.7, 126.6, 127.8, 129.6, 132.2 (2 C), 144.7, 148.5, 156.9 ppm.
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˜
C24H22N2O6 (434.44): calcd. C 66.35, H 5.10, N 6.45; found C
66.52, H 5.23, N 6.49.
General Procedure for Crossed 2,6-Disubstituted-4-phenyltetra-
hydropyrans: To a mixture of aldehyde (1.0 mmol) and BF3·Et2O
(1.2 mmol) in benzene (2.0 mL) was added allyltrimethylsilane
(0.6 mmol) in benzene (1.0 mL) drop by drop at 0 °C. The reaction
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Received: January 5, 2008
Published Online: February 18, 2009
Eur. J. Org. Chem. 2009, 1625–1629
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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