Molecular iodine-catalyzed, one-pot, diastereoselective synthesis of 4-amido tetrahydropyrans

Article abstract of DOI:10.1080/00397910903422534

A simple and efficient one-pot synthesis of symmetrical 4-amido tetrahydropyrans via the Sakurai-Prins-Ritter reaction sequence using molecular iodine as catalyst from an aldehyde and allyltrimethylsilane in acetonitrile is described. This new method has

Full text of DOI:10.1080/00397910903422534

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Molecular Iodine–Catalyzed, One-Pot,
Diastereoselective Synthesis of 4-Amido
Tetrahydropyrans
Gowravaram Sabitha a , M. Bhikshapathi a , Sambit Nayak a & J. S.
Yadav a
a Organic Division I, Indian Institute of Chemical Technology,
Hyderabad, India
Version of record first published: 14 Dec 2010.
To cite this article: Gowravaram Sabitha, M. Bhikshapathi, Sambit Nayak & J. S. Yadav (2010):
Molecular Iodine–Catalyzed, One-Pot, Diastereoselective Synthesis of 4-Amido Tetrahydropyrans,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 41:1, 8-15
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Synthetic Communications1, 41: 8–15, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903422534
MOLECULAR IODINE–CATALYZED, ONE-POT,
DIASTEREOSELECTIVE SYNTHESIS OF 4-AMIDO
TETRAHYDROPYRANS
Gowravaram Sabitha, M. Bhikshapathi, Sambit Nayak, and
J. S. Yadav
Organic Division I, Indian Institute of Chemical Technology, Hyderabad,
India
A simple and efficient one-pot synthesis of symmetrical 4-amido tetrahydropyrans via the
Sakurai–Prins–Ritter reaction sequence using molecular iodine as catalyst from an aldehyde
and allyltrimethylsilane in acetonitrile is described. This new method has the advantage of
good to excellent yields (60–98%) and short reaction times (20–150 min) at ambient
temperature with high diastereoselectivity.
Keywords: Aldehydes;
tetrahydropyrans
allyltrimethylsilane;
diastereoselectivity;
iodine;
Sakurai–Prins–Ritter;
INTRODUCTION
Substituted tetrahydropyrans are common structural fragments of many
natural products and biologically active compounds, and many strategies for their
synthesis have been reported.[1–3] Among them, the amino tetrahydropyran core
unit[4–8] constitutes a prominant structural feature of many natural products repre-
sented by ambruticins, oligomers of glucoamino acids, sialic acid, and desyherbaine.
These compounds are also used in photographic films[9] and host–guest chemistry.[10]
However, methods for the synthesis of 4-amino tetrahydropyrans are limited by
means of the Sakurai–Prins–Ritter reaction.[11] Recently, BF3.(OEt2)-catalyzed syn-
thesis of symmetrically substituted 4-amido tetrahydropyrans was developed, but it
requires stoichiometric amounts of catalyst and longer reaction time (8–36 h).[12]
Because of the versatility of these compounds, still there is still a need for the devel-
opment of a new catalytic system with high efficiency for a Sakurai–Prins–Ritter
reaction sequence.
Multicomponent reactions (MCRs) have attracted great attention from
research groups working on medicinal chemistry, drug discovery, and material
science because of their productivity and simple procedures. Over the past years,
molecular iodine[13–16] has attracted attention as an inexpensive, nontoxic, readily
Received August 4, 2009.
Address correspondence to Gowravaram Sabitha, Organic Division I, Indian Institute of Chemical
Technology, Hyderabad 500 007, India. E-mail: gowravaramsr@yahoo.com
8

SYNTHESIS OF 4-AMIDO TETRAHYDROPYRANS
9
available catalyst for various organic transformations. Now, we describe a single-
step MCR for the synthesis of symmetric 2,6-disubstituted-4-amido tetrahydropyr-
ans promoted by a catalytic amount of iodine under ambient conditions with good
to excellent yields. Significant features of the present iodine-catalyzed synthesis of
4-amidotetrahydropyrans are its efficiency, generality, mild, cost-effectiveness and
convenience from a practical point of view.
RESULTS AND DISCUSSION
Initially, we studied the reaction of 1.0 eq of n-butyraldehyde 1 with 0.6 eq of
allyltrimethylsilane 2 in acetonitrile by means of a Sakurai–Prins–Ritter reaction
sequence in the presence of 10 mol% I2. The reaction proceeded rapidly and com-
pleted in about 20 min, and the product, 2,6-dipropyl-4-acetamido tetrahydropyran
3, was obtained in 98% yield. The reaction is highly diastereoselective as determined
from the NMR spectra of the crude products (Scheme 1).
This procedure was applied to a wide range of aldehydes (Table 1, entries
1b–k), with allyltrimethylsilane in acetonitrile; in all cases, the reaction works well,
leading to the desired 4-acetamidotetrahydropyrans (Table 1, 3b–k) in good yields.
The structure of the 2,6-disubstituted-4-acetamido tetrahydropyran was estab-
lished by Nuclear Overhauser effect (NOE) experiments. From NOE studies, it was
confirmed that all three substituents occupy equatorial positions on the tetrahydropyr-
anyl ring (Fig. 1). For 3i (Table 1), 1H NMR experiments were performed at 500 MHz
in CDCl3 solution. Assignments were carried out by using double-quantum filtered
correlation spectroscopy (DQFCOSY). The presence of a plane of symmetry in the
1
molecule is evident from the H NMR spectrum. We noticed that dH1 ¼ dH10,
dH2a ¼ dH2a0, and dH2b ¼ dH2b0. For 3i, NOE cross peak CH1=CH3 (CH10=CH3)
reveals that H1(H10) and H3 are on the same side of the ring and take a diaxial orien-
tation. The proposed structure of 3i is further supported by the coupling constants
3
3
3
3JCH1=CH2a ¼ 1.8, JCH1=CH2b ¼ 11.7, JCH2a=CH3 ¼ 4.3, and JCH2b=CH3 ¼ 11.7 Hz.
Thus CH2b(CH2b0) is antiperiplanar to both CH1(CH10) and C3H. Interestingly, we
also observed a x coupling of ꢀ2 Hz between H2a and H2a0, further supporting their
equatorial orientation. All the findings confirm that the pyran ring takes a chair con-
formation, where substituents at C1, C10, and C3 are present in equatorial positions.
The reaction was also successful with other nitriles such as benzonitrile,
4-methyl benzonitrile, and isobutyronitrile to produce the corresponding
2,6-disubstituted-4-amido tetrahydropyrans (Table 2) in good yields. In the absence
of I2, the reaction did not proceed, even after a long time. All the reactions were
clean at room temperature with high diastereoselectivity and shorter reaction times.
Scheme 1. Synthesis of 4-amidotetrahydropyrans.

10
G. SABITHA ET AL.
Table 1. Iodine-catalyzed synthesis of 4-amidotetrahydropyransa,b via Sakurai–Prins–Ritter sequence
Entry
a
Aldehde
Product
Time (h)
0.60
Yield (%)
85
b
1.25
70
c
d
e
f
0.75
0.75
0.75
1.00
88
86
84
80
g
3.0
60
h
i
0.4
0.4
0.4
98
96
94
j
k
0.4
95
aAll products were characterized by spectral data and confirmed by NOE studies.
bIsolated pure products.

SYNTHESIS OF 4-AMIDO TETRAHYDROPYRANS
11
Figure 1. Schematic NOE diagram of 3i.
It should be noted that aliphatic aldehydes serve as best substrates when compared
to aromatic aldehydes in terms of reaction times and yields.
When compared with other catalysts (Table 3) I2 gives better results in terms of
catalytic loading and reaction time required for completion of the reaction.
In conclusion, we have successfully demonstrated a simple and efficient meth-
odology by means of a Sakurai–Prins–Ritter reaction sequence to prepare a wide
variety 4-amidotetrahydropyrans using iodine in catalytic amounts. The powerful
Table 2. Iodine-catalyzed synthesis of 4-amidotetrahydropyransa,b
Entry
a
Aldehyde
Nitrile
Product
Time (h)
Yield (%)
84
0.5
b
c
1.25
0.5
80
82
d
e
f
1.0
1.0
1.5
76
84
60
aAll products were characterized by spectral data and confirmed by NOE studies.
bIsolated pure products.

12
G. SABITHA ET AL.
Table 3. Comparison of various catalysts
No.
Catalyst
Time (h)
Yield (%)
1
2
3
4
5
I2 (10 mol%)
BF3.OET2 (100 mol%)
PMA-SiO2 (50 mol%)
Silica Sulphuric (50 mol%)
Amberlite-120
0.4
12.0
6.0
96
98
80
84
3.0
12.0
Trace
catalytic activity of iodine for these transformations can be substantiated by the
shorter reaction times as well as better product yields. The diastereoselective version
of this method adds significant value.
EXPERIMENTAL
Reactions were conducted under nitrogen atmosphere. All reactions were
monitored by thin-layer chromatography (TLC) using silica-coated plates. Light
petroleum of the distillation range 60–80 ꢁC was used. Evaporation of solvents was
1
performed at reduced pressure using a Buchi rotary evaporator. H NMR spectra
were recorded on Varian FT 200-MHz (Gemini) and Bruker UXNMR FT
300-MHz (Avance) instruments in CDCl3. Chemical shift values were reported in
parts per million (d) relative to tetramethylsilane (d 0.0) as an internal standard. Mass
spectra were recorded under electron impact at 70 eV on a liquid chromatography–
mass spectrometer (Agilent Technologies). Column chromatography was performed
on silica gel (60–120 mesh) supplied by Acme Chemical Co., India. TLC was
performed on Merck 60 F-254 silica-gel plates.
Typical Experimental Procedure for Compound 3i
Allyltrimethylsilane 2 (0.6 mmol) was added to a mixture of n-butyraldehyde
(1.0 mmol) and iodine (10 mol%) in acetonitrile (5 mL), and the reaction mixture
was stirred at room temperature for 20 min. After completion of the reaction (as indi-
cated by TLC), the reaction mixture was quenched with aqueous sodium thiosulfate,
extracted with ethyl acetate (2 ꢂ 5 mL), and washed with brine and water. The
Scheme 2. Mechanism of the reaction.

SYNTHESIS OF 4-AMIDO TETRAHYDROPYRANS
13
combined organic layer was dried (anhyd. NaSO4) and concentrated to leave the
crude compound, which was subjected to flash-column chromatography to get a pure
compound 3i in 96% yield.
Selected Data
N1-(2,6-Diphenyltetrahydro-2H-4-pyranyl)acetamide 3a (Table 1). White
1
solid; mp: 220–222 ꢁC; IR (KBr): cmꢃ1 3279, 3069, 2927, 2842, 1643, 1552, 747; H
NMR (CDCl3, 500 MHz): d 7.41 (m, 4H), 7.34 (m, 4H), 7.26 (m, 2H), 5.43 (d,
J ¼ 8.4 Hz, 1H), 4.66 (dd, J ¼ 1.8, 11.7 Hz, 2H), 4.39 (ttd, J ¼ 4.3, 11.7, 8.4 Hz, 1H),
2.29 (dddd, J ¼ 1.8, 2.0, 4.3, 13.0 Hz, 2H), 1.96 (s, 3H), 1.42 (td, J ¼ 11.7, 13.0 Hz,
2H); 13C NMR (CDCl3, 100 MHz): d 168.3, 142.2, 128.8, 128.2, 126.2, 78.7, 46.0,
41.9, 23.9; HRMS (m=z): calc.: 318.1469; found: 318.1458 (MþþNa).
N1-[2,6-Di(4-fluorophenyl)tetrahydro-2H-4-pyranyl]acetamide3c(Table1).
Brown solid, mp: 176–178 ꢁC; (KBr): cmꢃ1 3275, 3082, 2947, 2925, 2840, 1643, 1603,
1
835; H NMR (CDCl3, 300 MHz): d 7.65 (d, J ¼ 7.5 Hz, 2H), 7.34–7.25 (m, 4H),
7.23–7.16 (m, 2H), 5.21 (d, J ¼ 8.3 Hz, 1H), 5.02 (d, J ¼ 9.8 Hz, 2H), 4.48–4.34 (m,
1H), 2.51–2.44 (m, 2H), 1.93 (s, 3H), 1.32–1.18 (m, 2H); 13C NMR (CDCl3,
75 MHz): d 167.0, 161.3, 136.6, 125.8, 113.2, 112.9, 75.3, 44.1, 20.9; HRMS (m=z)
calc.: 354.1281; found: 354.1270 (MþþNa).
N1-[2,6-di(2,6-Dimethoxyphenyl)tetrahydro-2H-4-pyranyl]acetamide 3g
(Table 1). Brown solid, mp: 226–228ꢁC; (KBr): cmꢃ1 3282, 3078, 2956, 2914, 2872,
1
1636, 1608; H NMR (CDCl3, 300 MHz): d 7.16 (d, J ¼ 2.2 Hz, 2H), 6.72–6.70 (m,
4H), 5.22 (d, J ¼ 8.3 Hz, 1H), 4.96 (d, J ¼ 10.5 Hz, 2H), 4.45–4.34 (m, 1H), 3.81 (s,
12H), 2.43–2.33 (m, 2H), 1.95 (s, 3H), 1.26–1.13 (m, 2H); HRMS (m=z) calc.:
438.1892; found: 438.1824 (MþþNa).
N1-(2,6-Dipropyltetrahydro-2H-4-pyranyl)acetamide3i(Table1). Semisolid;
IR (KBr): cmꢃ1 3287, 2929, 2869, 1647, 1561, 1146; 1H NMR (CDCl3, 500 MHz): d
5.32 (d, J ¼ 8.4, 1H), 4.00 (ttd, J ¼ 4.3, 11.7, 8.4 Hz, 1H), 3.32 (m, 2H), 1.94 (s, 3H),
1.91 (dddd, J ¼ 1.8, 2.0, 4.3, 13.0 Hz, 2H), 0.97 (td, J ¼ 11.7, 13.0 Hz, 2H), 1.50 (m,
2H), 1.45 (m, 2H), 1.36 (m, 2H), 1.35 (m, 2H), 0.90 (m, 6H, CH3); 13C NMR (CDCl3,
100 MHz): d 168.5, 76.2, 47.1, 38.8, 37.1, 22.8, 15.2, 13.7; HRMS (m=z) calc.:
250.1782; found: 250.1782 (Mþ þ Na).
N1-(2,6-Dipentyltetrahydro-2H-4-pyranyl)acetamide 3j (Table 1). White
1
solid, mp: 80–82ꢁC; (KBr): cmꢃ1 3287, 3089, 2928, 2855, 1643, 1556; H NMR
(CDCl3, 300 MHz): d 5.60 (d, J ¼ 7.5 Hz, 1H), 4.02–3.88 (m, 1H), 3.33–3.24 (m,
2H), 1.93 (s, 3H), 1.92–1.85 (m, 2H), 1.53–1.24 (m, 16H), 1.01–0.86 (m, 8H); 13C
NMR (CDCl3, 75 MHz): d 169.2, 76.0, 46.4, 38.6, 36.1, 31.7, 25.2, 23.5, 22.6, 14.0;
HRMS (m=z) calc.: 306.2408; found: 306.2421 (MþþNa).
N1-(2,6-Diisopropyltetrahydro-2H-4-pyranyl)acetamide 3k (Table 1).
Semisolid; (KBr): cmꢃ1 3283, 3091, 2943, 2879, 1644, 1556; 1H NMR (CDCl3,
300 MHz): d 5.62 (d, J ¼ 7.5 Hz, 1H), 3.98–3.85 (m, 1H), 3.03–2.95 (m, 2H), 1.94
(s, 3H), 1.93–1.87 (m, 2H), 1.67–1.59 (m, 2H), 0.95–0.86 (m, 14H); HRMS (m=z)
calc.: 250.1782; found: 250.1787 (MþþNa).

14
G. SABITHA ET AL.
N1-(2,6-Dipropyltetrahydrꢃo1-2H-4-pyranyl)benzamide 3a (Table 2). White
1
solid, mp: 126–128ꢁC; (KBr): cm 3281, 3065, 2956, 2865, 1629, 1543; H NMR
(CDCl3, 300 MHz): d 7.71 (dd, J ¼ 8.0, 1.4 Hz, 2H), 7.43 (m, 3H), 5.84 (d, J ¼ 8.8 Hz,
1H), 4.21–4.15 (m, 1H), 3.42–3.35 (m, 2H), 2.09–1.97 (m, 2H), 1.60–1.30 (m, 8H),
1.17–0.97 (m, 2H), 0.93 (t, J ¼ 7.3 Hz, 6H); HRMS (m=z) calc: 312.1939; found:
312.1950 (MþþNa).
N1-(2,6-Diphenyltetrahydro-2H-4-pyranyl)benzamide 3b (Table 2).
1
White solid, mp: 202–204ꢁC; (KBr): cmꢃ1 3282, 3081, 2923, 2843, 1634, 1546; H
NMR (CDCl3, 300 MHz): d 7.65 (d, J ¼ 7.5 Hz, 2H), 7.48–7.42 (m, 6H), 7.35 (t,
J ¼ 8.3 Hz, 4H), 7.30–7.26 (m, 3H) 5.95 (d, J ¼ 8.3 Hz, 1H), 4.76 (d, J ¼ 10.5 Hz,
2H), 4.67–4.53 (m, 1H), 2.50–2.42 (m, 2H), 1.61–1.48 (m, 2H); 13C NMR (CDCl3,
75 MHz): d 163.9, 140.6, 132.7, 129.1, 126.3, 126.2, 123.7, 75.8, 44.7; HRMS (m=z)
calc: 380.1626; found: 380.1632 (MþþNa).
N1-(2,6-Diethyltetrahydro-2H-4-pyranyl)-4-methylbenzamide
3c
(Table 2). White solid, mp: 172–174ꢁC; (KBr): cmꢃ1 3290, 3037, 2959, 2920,
1
2875, 1632, 1541; H NMR (CDCl3, 300 MHz): d 7.60 (d, J ¼ 8.3, Hz, 2H), 7.19
(d, J ¼ 8.3, Hz, 2H), 5.81 (d, J ¼ 8.3 Hz, 1H), 4.24–4.09 (m, 1H), 3.36–3.26
(m, 2H), 2.41 (s, 3H), 2.09–2.01 (m, 2H), 1.62–1.42 (m, 4H), 1.12–0.99 (m, 2H),
0.93 (t, J ¼ 7.5 Hz, 6H); HRMS (m=z) calc.: 298.1782; found: 298.1795 (MþþNa).
N1-[2,6-Di(4-fluorophenyl)tetrahydro-2H-4-pyranyl]-4-methylbenzamide
3d (Table 2). White solid, mp: 212–214ꢁC; (KBr): cmꢃ1 3294, 3044, 2925, 2850,
1
1630, 1534; H NMR (CDCl3, 300 MHz): d 7.62 (d, J ¼ 7.5 Hz, 2H), 7.43–7.37 (m,
4H), 7.22 (d, J ¼ 8.3 Hz, 2H), 7.08–7.01 (m, 4H), 5.88 (d, J ¼ 7.5 Hz, 1H), 4.72 (d,
J ¼ 11.3 Hz, 2H), 4.63–4.50 (m, 1H), 2.47–2.39 (m, 5H), 1.57–1.43 (m, 2H); HRMS
(m=z) calc.: 430.1594; found: 430.1594 (MþþNa).
N1-(2,6-Diheptyltetrahydro-2H-4-pyranyl)-2-methylpropanamide
3e
(Table 2). White solid, mp: 103–105ꢁC; (KBr): cmꢃ1 3283, 2924, 2853, 1646,
1
1551; H NMR (CDCl3, 200 MHz): d 5.10 (d, J ¼ 8.0 Hz, 1H), 4.03–3.89 (m, 1H),
3.37–3.24 (m, 2H), 2.33–2.14 (m, 1H), 1.96–1.84 (m, 2H), 1.50–1.24 (m, 24H), 1.13
(d, J ¼ 6.5 Hz, 6H), 0.98–0.85 (m, 8H); HRMS (m=z) calc.: 390.3347; found:
390.3353 (MþþNa).
N1-[2,6-Di(4-methylphenyl)tetrahydro-2H-4-pyranyl]ꢃ-21-methylpropana-
mide 3f (Table 2). White solid, mp: 254–256ꢁC; (KBr): cm 3284, 3089, 2932,
1
28640, 1648, 1556; H NMR (CDCl3, 300 MHz): d 7.29 (d, J ¼ 8.3 Hz, 4H), 7.13
(d, J ¼ 8.3 Hz, 4H), 5.22 (d, J ¼ 7.5 Hz, 1H), 4.64 (d, J ¼ 10.5 Hz, 2H), 4.41–4.28
(m, 1H), 2.36 (s, 6H), 2.32–2.25 (m, 3H) 1.44–1.31 (m, 2H), 1.15 (d, J ¼ 6.8 Hz,
6H); HRMS (m=z) calc.: 374.2095; found: 374.2099 (MþþNa).
ACKNOWLEDGMENT
M.B. and S.N. thank the Council of Scientific and Industrial Research, New
Delhi, for the award of fellowships.

SYNTHESIS OF 4-AMIDO TETRAHYDROPYRANS
15
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