Synthesis of potassium and tetra n-butylammonium 2-substituted-1,3-dithianotrifluoroborate salts and addition to chiral cyclic N-acyliminium ions

Article abstract of DOI:10.1016/j.tet.2008.05.085

The synthesis of potassium 2-substituted-1,3-dithianotrifluoroborate salts and tetra-n-butyl ammonium derivatives is described. The reaction proceeds under mild reaction conditions and the corresponding products were obtained in moderate to good yields. T

Full text of DOI:10.1016/j.tet.2008.05.085

Tetrahedron 64 (2008) 7234–7241
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of potassium and tetra n-butylammonium 2-substituted-1,3-
dithianotrifluoroborate salts and addition to chiral cyclic N-acyliminium ions
Adriano S. Vieira a, Pedro F. Fiorante a, Julio Zukerman-Schpector b, Diego Alves a,
a
a,c,
*
´
Giancarlo V. Botteselle , Helio A. Stefani
a
´
ˆ
ˆ
˜
˜
Departamento de Farmacia, Faculdade de Ciencias Farmaceuticas, Universidade de Sao Paulo, Sao Paulo, SP, Brazil
b Departamento de Qu´ımica, Universidade Federal de S˜ao Carlos, S˜ao Carlos, SP, Brazil
c Departamento de Biof´ısica, Universidade Federal de S˜ao Paulo, S˜ao Paulo, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of potassium 2-substituted-1,3-dithianotrifluoroborate salts and tetra-n-butyl ammonium
derivatives is described. The reaction proceeds under mild reaction conditions and the corresponding
products were obtained in moderate to good yields. The reactivity of these compounds in reactions with
chiral cyclic N-acyliminium ions was evaluated.
Received 31 March 2008
Received in revised form 15 May 2008
Accepted 16 May 2008
Available online 21 May 2008
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
for over 40 years has become a mainstay for the union of both
simple and complex fragments.4
Carbon–carbon bond formation is a major focus of research in
organic synthesis. Tools for making such bonds are indispensable for
the construction of complex molecules and intense efforts are con-
tinuously directed toward developing novel and selective bond
forming reactions. In the recent years, organoboron compounds
have become one of the most popular organometallic reagents in the
carbon–carbon bond formation chemistry.1 The great applicability
(in place of other organometallic reagents) is due to some features of
the organoboron compounds; e.g., (1) compatibility with many
functional groups; (2) availability of reagents via hydroboration and
transmetallation; (3) low toxicity; (4) their ultimate degradation
into the environmentally friendly boric acid and in addition (5) the
handling and removal of boron-containing byproducts is easier
when compared to other organometallic reagents.
Among the most important features of these compounds is their
stability under both acidic and basic conditions5 and their potential
utility in organic synthesis as acyl carbanion equivalents in carbon–
carbon bond forming reactions.3,6 A wide range of electrophiles,
including alkyl halides, aldehydes, and epoxides, react smoothly
with the derived acylanion equivalents.7
To the best of our knowledge only three papers in the literature
describe the reaction of 2-lithio-1,3-dithianes with trialkylboranes8
(R3B) and trimethyl borates ((RO)3B).9
N-Acyliminium ions10 are very important in organic synthesis
since they are reactive intermediates involved in the synthesis of
many compounds with interesting biological properties. Nucleo-
philic additions to N-acyliminium ions constitute an important
method to provide a-functionalized amino compounds and for the
The organoboron compounds more used are boronic acids and
boronate esters, but these compounds have some drawbacks,
among them we can mention the low stability, very high price of
some reagents, and high sensitivity to air and moisture. To solve the
problems those organoboron have been replaced by organo-
trifluoroborate salts.2 These latter reagents are very stable to air and
moisture, crystalline solids, easily prepared from inexpensive ma-
terials, and these compounds show a greater nucleophilicity com-
pared with their boronic acids or boronate esters analogs.2
preparation of alkaloids and many other biologically active nitrogen
heterocycles.11
2. Results and discussion
In connection with our research interest on the preparation and
reactivity of potassium organotrifluoroborate salts, and their use as
intermediates in organic synthesis,9,11m,12 we wish to report here
a general procedure to access various air-stable and storable po-
tassium 2-organo-1,3-dithianotrifluoroborate salts readily available
from the corresponding 1,3-dithiane reagents (Scheme 1).
We initially focused our attention on the optimization of the
reaction conditions for the synthesis of the potassium 2-organo-
1,3-dithianotrifluoroborate salts starting from the 2-methyl-1,3-
dithiane 1a (Table 1).
In 1965, Corey and Seebach introduced the use of 1,3-dithianes
as important umpolung linchpins in organic synthesis.3 This tactic
* Corresponding author. Tel.: þ55 11 3091 3654; fax: þ55 11 3815 4418.
E-mail address: hstefani@usp.br (H.A. Stefani).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.05.085

A.S. Vieira et al. / Tetrahedron 64 (2008) 7234–7241
7235
1) n-BuLi, THF,
This problem can be solved by a simple counterion exchange
reaction.13 Therefore, the potassium 2-organo-1,3-dithianotri-
fluoroborates 2a–k were submitted to a counterion exchange
protocol with tetra-n-butylammonium hydroxide (TBAOH) afford-
ing the TBA derivatives 3a–k (Table 2). As shown in Table 2, the
products 3a–k were obtained in very high yields (90–97%). In all
cases further purification was unnecessary.
S
S
S
S
R1
-78 °C
R1
S
R1
B(OMe)3
Li
KHF2, H2O
S
2) B(OCH3)3
H
BF3K
1
2
ate complex
R1= alkyl, aryl, alkenyl, alkynyl, TMS
In order to test the potential use of the 2-organo-1,3-dithiano-
trifluoroborate salts as transfer of acyl group in the stereoselective
synthesis of functionalized N-heterocycles via N-acyliminium ions,
we performed the reaction of the 5-acetoxy-2-pyrrolidinone with
various 2-organo-1,3-dithianotrifluoroborate salts, following the
methodology recently developed by us.11m For this purpose, the N-
Scheme 1.
In a first test, the deprotonation of the corresponding 1,3-
dithiane 1a with 1.0 equiv of n-BuLi at ꢀ78 ꢁC in THF was followed
by reaction of the lithium salt intermediate with 1.2 equiv of
B(OCH3)3. The transmetallation reaction was performed at ꢀ30 ꢁC
for 1 h and then 1 h at room temperature. The resulting ‘ate com-
benzyl-3,4,5-triacetoxy-2-pyrrolidinone
4
was treated with
BF3$Et2O (4.0 equiv) in CH2Cl2 at ꢀ78 ꢁC for 1 h to ensure in situ
formation of the corresponding N-acyliminium ion. Next, potas-
sium 1,3-dithianotrifluoroborate (1.2 equiv) was added and the
reaction was allowed to warm to room temperature and stirred for
6 h and these results were summarized in Table 4.
plex’ (Scheme 1), which could be detected as a singlet at d 5.49 ppm
in the 11B NMR spectra, was reacted with saturated aqueous solu-
tion of KHF2 at ꢀ20 ꢁC for 1 h and then at room temperature for 1 h
to afford the desired potassium 2-methyl-1,3-dithianotri-
fluoroborate salt 2a in only 21% isolated yield (Table 1, entry 1).
A variety of reaction conditions were tested as shown in Table 1.
Among the tested solvent systems, the use of THF and 1.0 equiv of
HMPA as additive provided the best result and the desired product
was yielded in 79% (Table 1, entry 5). The use of t-BuLi instead of n-
BuLi did not increase the yield, and when the reaction was carried
out in hexane in presence of TMEDA (1.0 equiv) no product was
obtained (Table 1, entry 8).
The reaction proceeded with moderate to good yields and in
good levels of diastereoselection in most cases. However, to our
surprise no product was obtained when 2-aryl-1,3-dithianes were
employed. We believe that the low reactivity of the compounds
containing aryl groups was attributed to poor nucleophilicity of the
carbon bonded to boron.
In order to demonstrate the efficiency of our methodology we
carried out the reaction of 2-lithium-1,3-dithiane with N-acylimi-
nium ion mediated by BF3$OEt2. For this purpose, the 2-pyrrolidi-
none 4 was treated with BF3$Et2O (4.0 equiv) in CH2Cl2 at ꢀ78 ꢁC
for 1 h11m followed by addition of 2-lithium-1,3-dithiane3b and
stirring at room temperature for 6 h. For our surprise, the com-
pound 5b was obtained in only 23% isolated yield in an 80:20 di-
astereoisomeric ratio in favor to the anti-isomer.
These optimized conditions were subsequently applied to the
different 1,3-dithianes 1b–k as outlined in Table 2. The reaction
proceeded with satisfactory to good yields in most cases for alkyl,
alkenyl, alkynyl, and silyl 1,3-dithianes (Table 2, entries 1–7). De-
spite the use of a fluorine source, 2-trimethylsilyl group remains
intact (Table 2, entry 4).
However, much to our surprise, under these conditions the
reaction proceeded in low yields when we used 2-aryl or 2-hetero-
aryl-1,3-dithianes, probably due to the less nucleophilic 2-lithium-
2-aryl-1,3-dithiane intermediate in comparison to his aliphatic
analog.
The stereochemistry of the newly created stereogenic center of
5b was determined by 1H NMR analysis of the crude mixture. The
cis relative stereochemistry of the major product was established
after analysis of the multiplicity and vicinal coupling constant of
the hydrogen attached to the a-nitrogen (H-5) and then was ex-
To improve the yields, different boron sources were examined
(Table 3). The use of BH3$S(CH3)2 and BF3$OEt2 provided the best
results (Table 3, entries 4 and 5). Between these two options, we
decided to use BF3$OEt2, because it is less expensive and most
environmentally friendly. Using this protocol, the corresponding
products were obtained in moderated yields (Table 2, entries 8–11).
Although Kþ salts have been synthetically useful, they are in-
soluble in some organic media, and require polar solvents such as
acetonitrile, acetone, methanol or water at elevated temperatures
for dissolution.
tended to the other related compounds.
For the analogous 2-pyrrolidinones the vicinal coupling con-
stant 3J(H5–H4) for the syn-isomer shows smaller value than the anti-
isomer. These chemical correlations are in agreement with the
major isomer obtained by us and the relative stereochemistry of the
major isomer was assigned as being syn.11m,14
The compound 5b (in a 70:30 mixture of syn/anti diastereomers)
was submitted to thioketal hydrolysis using Dess–Martin period-
inane (DMP) in CH3CN/CH2Cl2/H2O according to methodology
recently described by Panek and co-workers15 (Scheme 2). By
use of this protocol, the respective aldehyde was obtained in 78%
yield.
Table 1
Search for the best reactions conditions
The crystal structure of potassium (1,3-dithian-2-yl)-2-n-butyl
trifluoroborate salt is a representative for this class of compounds
(Fig. 1). The compound 2e was carefully recrystallized from dry
methanol to afford the desired crystal.
1) RLi (1.0 equiv),
S
S
additive, solvent, -78 °C
2) B(OCH3)3 (1.2 equiv)
3) KHF2 (4.0 equiv), H2O
S
CH3
BF3K
2a
S
CH3
H
The compound crystallizes as discrete Kþ and C8H15BF3S2ꢀ moi-
eties. The closest distance between a cation and an anion is K F1
2.638(2) Å, which is between K and F1. Each Kþ is surrounded by
four anions making close contacts with four F atoms with distances
ranging between 2.638(2) and 2.793(2) Å. There is a C–H/F in-
termolecular short contact: C6/F1i¼3.580(4), H6B/F1i¼2.67 Å,
C6–H6B/F1i¼157ꢁ (i¼1ꢀx, 1ꢀy, 1ꢀz).
1a
Entry
R–Li
Solvent
Additive
Yielda (%)
1
2
3
4
5
6
7
8
9
n-BuLi
n-BuLi
n-BuLi
n-BuLi
n-BuLi
n-BuLi
n-BuLi
n-BuLi
t-BuLi
THF
THF
THF
THF
d
21
25
28
37
79
7
10
n.r.
19
TMEDA
12-Crown-4
DMPU
HMPA
d
THF
Et2O
DME
Hexane
THF
3. Conclusion
d
TMEDA
d
In summary, we have described the first synthesis of potassium-
and tetra-n-butylammonium-2-organo-1,3-dithianotrifluoroborate
a
Isolated yield after recrystallization from diethyl ether.

7236
A.S. Vieira et al. / Tetrahedron 64 (2008) 7234–7241
Table 2
Synthesis of potassium 2-substituted-1,3-dithianotrifluoroborate salts 2a–k and their tetra-n-butyl ammonium derivatives 3a–k
1) n-BuLi, HMPA
S
S
S
S
R1
n-Bu4NOH
THF, -78 °C
S
R1
S
R1
CH3OH, H2O,
0 °C - r.t., 30 min
2) B(OCH3)3
3) KHF2, H2O
H
BF3K
BF3(n-Bu4N)
1a-k
2a-k
3a-k
Entry
1
R1
Product (2a–k)
Yield (%)
79a
Product (3a–k)
Yieldc (%)
92
S
S
S
S
CH3
BF3(n-Bu4N)
3a
CH3
BF3K
2a
CH3
S
S
n-C5H11
n-C5H11
S
S
2
3
4
5
6
7
n-C5H11C^C
63a
83a
75a
58a
67a
41a
90
91
97
91
95
91
BF3(n-Bu4N)
BF3K
2b
3b
S
S
S
H
S
H
H
BF3(n-Bu4N)
3c
BF3K
2c
S
S
S
S
Si(CH3)3
BF3(n-Bu4N)
3d
S
Si(CH3)3
S
S
S
Si(CH3)3
BF3K
2d
S
n-C4H9
BF3(n-Bu4N)
3e
S
n-C4H9
n-C4H9
BF3K
2e
S
S
H2C]CHCH2
(E)-CH3(H)C]CH
BF3(n-Bu4N)
BF3K
2f
3f
S
S
S
S
S
CH3
CH3
BF3K
2g
BF3(n-Bu4N)
3g
S
S
13a
45b
S
8
C6H5
95
97
BF3(n-Bu4N)
3h
BF3K
2h
S
S
S
S
8a
O
O
9
2-furyl
33b
BF3K
BF3(n-Bu4N)
3i
2i
O
O
O
S
S
11a
S
S
O
10
3,4-(CH2)O2C6H3
91
93
48b
BF3K
2j
BF3(n-Bu4N)
3j
9a
Cl
Cl
S
S
S
S
11
4-Cl–C6H4
37b
BF3K
BF3(n-Bu4N)
3k
2k
a
Condition A: (i) n-BuLi (1.0 equiv), HMPA (1.0 equiv), THF, ꢀ78 ꢁC,1 h; (ii) B(OMe)3 (1.2 equiv), ꢀ30 ꢁC for 1 h, then 1 h at rt; (iii) KHF2 (4.0 equiv), H2O, ꢀ20 ꢁC,1 h then 1 h at rt.
Condition B: (i) n-BuLi (1.0 equiv), HMPA (1.0 equiv), THF, ꢀ78 ꢁC, 1 h. (ii) BF3$OEt2 (1.2 equiv), ꢀ30 ꢁC, 1 h, then 1 h at rt; (iii) KHF2 (4.0 equiv), H2O, ꢀ20 ꢁC for 1 h and then
b
1 h at rt.
c
Isolated yield after extraction.
salts and this work represents an expansion in the chemistry
of organotrifluoroborate salts. These compounds were obtained
in moderate to good yields under very mild reaction conditions.
We demonstrated that the obtained potassium 1,3-
dithianotrifluoroborates salts have interesting synthetic properties
as transfer of acyl group to N-acyliminium ions. Studies of the re-
activity of these compounds in other kind of reactions are ongoing
in our lab and will be reported in due course.

A.S. Vieira et al. / Tetrahedron 64 (2008) 7234–7241
7237
Table 3
OAc
AcO
AcO
O
OAc
AcO
O
OAc
H
OAc
Improved reaction conditions for aromatic derivatives
I
O
S
1) n-BuLi (1.0 equiv),
HMPA (1.0 equiv),
THF, -78 °C
S
O
S
O
N
N
S
S
H
CH3CN, CH2Cl2,
Bn
Bn
S
H2O, r.t.
2) Boron source
5b
6
BF3K
H
1h
(1.2 equiv)
2h
Scheme 2.
3) KHF2 (4.0 equiv),H2O
Entry
Boron source
Yielda (%)
4.2. General procedure for the synthesis of potassium
2-substituted-1,3-dithianotrifluoroborate salts
1
2
3
4
5
B(OCH3)3
B(O-i-Pr)3
B(n-Bu)4
BH3$S(CH3)2
BF3$Et2O
13
15
n.r.
44
45
A solution of 2-methyl-1,3-dithiane (1.34 g, 10 mmol, 1.0 equiv)
in 20 mL of dry THF was cooled to ꢀ78 ꢁC under nitrogen. n-BuLi
(7.33 mL, 1.5 M in hexane, 10 mmol, 1.0 equiv) and HMPA (1.8 mL,
10 mmol, 1.0 equiv) were added dropwise, and the solution was
stirred for 1 h at this temperature. B(OCH3)3 (12 mmol, 1.2 equiv)
was then added dropwise at ꢀ78 ꢁC. The solution was stirred at
ꢀ30 ꢁC for 1 h and at room temperature for another 1 h after which
a saturated aqueous solution of potassium hydrogen difluoride
(3.12 g, 40 mmol, 4.0 equiv) was added to the vigorously stirred
solution. The resulting mixture was allowed to stir for 1 h at ꢀ20 ꢁC,
after which it was allowed to warm to room temperature for 1 h.
a
Isolated yield after recrystallization from diethyl ether.
4. Experimental
4.1. General
All air-sensitive and/or water-sensitive reactions were carried
out under nitrogen atmosphere with dry solvents under anhydrous
conditions. Standard syringe techniques were applied for transfer
of dry solvents and some air-sensitive reagents and were in-
troduced into reaction vessels through a rubber septum. The re-
actions were monitored by TLC carried out on Merck silica gel (60
C3
C2
S1
F3
K
F
254) by using UV light, 5% vanillin in 10% H2SO4 and heat as
developing agents. Merck silica gel (particle size 0.040–0.063 mm)
was used for flash chromatography. THF was distilled from sodium–
benzophenone before use. CH2Cl2 was distilled from P2O5 and
stored over MS 4 Å under atmosphere of dry nitrogen. NMR spectra
were recorded with Bruker DPX 300 (300 MHz) instrument using
DMSO-d6, CD3OD or CDCl3 as solvent and calibrated using tetra-
methylsilane as internal standard. Chemical shifts are reported in
C1
F1
C4
B
S2
F2
d
ppm relative to (CH3)4Si for 1H and DMSO-d6, CD3OD or CDCl3 for
13C NMR. The 19F NMR chemical shifts were referenced to external
CFCl3 (0.0 ppm). The 11B NMR spectra were obtained on a spec-
trometer equipped with the appropriate decoupling accessories. All
11B chemical shifts were referenced to external BF3$OEt2 (0.0 ppm),
with a negative sign indicating an upfield shift. Coupling constants
(J) are reported in hertz. Ratios of mixtures of diastereoisomers
were determined by peak integration in the 1H NMR spectra of the
crude product. Infrared (IR) spectra were obtained from CH3OH or
CHCl3 solutions, using a Varian 3100 FT-IR spectrophotometer and
wavelengths are reported in cmꢀ1. Mass spectra (MS) were mea-
sured on a Shimadzu GCMS-QP5050A mass spectrometer. The
HRMS spectra were measured on a Bruker Daltonics Micro TOF
(direct inlet probe).
C5
C6
C7
C8
Figure 1. ORTEP depiction of the crystal structure of 2e; C8H15$BF3KS2, Mw¼378.31,
P21/a, a¼10.218(1), b¼10.2843(7), c¼12.628(1) Å;
b
¼108.835(4)ꢁ, Robs¼0.0553 for 2818
reflections and 136 refined parameters. C.C.D.C. number 686771.
Table 4
5-Substituted 2-pyrrolidinones 5a–e
AcO
O
OAc
OAc
AcO
AcO
O
OAc
OAc
1) BF3.OEt2
CH2Cl2, -78 °C, 1h
S
S
S
S
+
O
N
N
N
S
2)
R1
S
R1
R1
Bn
Bn
5 anti
Bn
5 syn
2
BF3K
4
-78 °C- r.t., 6h
Entry
R1
Product
Yield (%)
syn/anti Ratioa
1
2
3
4
5
CH3
H
n-C4H9
Me3Si
H2C]CHCH2
5a
5b
5c
5d
5e
68
54
42
63
51
80:20
70:30
90:10
70:30
90:10
a
Ratios determined by 1H NMR (300 MHz) integration of the hydrogen attached to the
a-nitrogen carbon on the crude reaction mixture.

7238
A.S. Vieira et al. / Tetrahedron 64 (2008) 7234–7241
After this time, the solvent was removed under reduced pressure,
and the resulting white solid was dried under high vacuum for 3 h
to remove all water. The solid was then washed with hot acetone.
The resulting organic solution was filtered, and the solvent was
removed to afford a fluffy white solid. This solid was then dissolved
in hot acetone and precipitated with diethyl ether, after which the
solution was cooled to ꢀ20 ꢁC to complete precipitation of the
solid. The product 2a was collected as a white crystalline solid
(1.72 g, 79%).
(282 MHz, DMSO-d6)
DMSO-d6)
d
ꢀ152.59 (q, J¼50.7 Hz); 11B NMR (96 MHz,
d
4.20 (q, J¼56.2 Hz). Anal. Calcd for C7H11BF3KS2: C,
31.58; H, 4.17. Found: C, 31.47; H, 4.03. LRMS m/z: 305 (MþK); IR
(CH3OH solution, cmꢀ1): 2986, 2911, 1618, 1418, 1238, 1044, 944.
4.2.7. Potassium (1,3-dithian-2-yl)-(E)-2-(prop-1-enyl)
trifluoroborate (2g)
Mp¼259 ꢁC (decomp.); 1H NMR (300 MHz, DMSO-d6)
d 5.90 (d,
J¼10.8 Hz, 1H), 2.56–2.83 (m, 4H), 1.90–2.12 (m, 2H), 1.51–1.60 (m,
1H), 0.76 (d, J¼7.0 Hz, 3H); 13C NMR (75 MHz, DMSO-d6)
d
149.0,
ꢀ152.41
4.31 (q, J¼56.8 Hz).
4.2.1. Potassium (1,3-dithian-2-yl)-2-methyl trifluoroborate (2a)
115.1, 31.0, 30.1, 26.3, 16.7; 19F NMR (282 MHz, DMSO-d6)
d
Mp¼241 ꢁC (decomp.); 1H NMR (300 MHz, DMSO-d6)
d
2.91–
(q, J¼52.5 Hz); 11B NMR (96 MHz, DMSO-d6)
d
3.00 (m, 2H), 2.56–2.63 (m, 2H), 1.83–2.01 (m, 2H), 1.64 (s, 3H); 13
C
Anal. Calcd for C7H11BF3KS2: C, 31.58; H, 4.17. Found: C, 31.29; H,
3.85. LRMS m/z: 305 (MþK); IR (CH3OH solution, cmꢀ1): 2981, 2915,
1639, 1420, 1239, 1047, 931.
NMR (75 MHz, DMSO-d6)
(282 MHz, DMSO-d6)
DMSO-d6)
25.01; H, 3.78. Found: C, 25.05; H, 3.66. LRMS m/z: 279 (MþK); IR
d
30.7, 27.7, 25.1 (2C), 24.6; 19F NMR
d
ꢀ152.79 (q, J¼50.5 Hz); 11B NMR (96 MHz,
d
4.67 (q, J¼50.3 Hz). Anal. Calcd for C5H9BF3KS2: C,
4.2.8. Potassium (1,3-dithian-2-yl)-2-phenyl trifluoroborate (2h)
(CH3OH solution, cmꢀ1): 2978, 2907, 1411, 1241, 1049, 946.
Mp¼277 ꢁC (decomp.); 1H NMR (300 MHz, DMSO-d6)
d 6.92–
7.18 (m, 5H), 2.38–2.52 (m, 2H), 2.03–2.25 (m, 2H), 1.54 (t, J¼6.7 Hz,
4.2.2. Potassium (1,3-dithian-2-yl)-2-(hept-1-ynyl) trifluoroborate
2H); 13C NMR (75 MHz, DMSO-d6)
d 146.3, 129.9 (2C), 127.3 (2C),
(2b)
123.5, 27.8, 26.4, 25.3 (2C); 19F NMR (282 MHz, DMSO-d6)
Mp¼261 ꢁC (decomp.); 1H NMR (300 MHz, DMSO-d6)
d
3.00–
d
ꢀ149.59 (q, J¼47.9 Hz); 11B NMR (96 MHz, DMSO-d6)
d 3.07 (q,
3.07 (m, 1H), 2.78 (t, J¼5.4 Hz, 2H), 2.35 (t, J¼8.0 Hz, 1H), 1.79–1.88
J¼47.6 Hz). Anal. Calcd for C10H11BF3KS2: C, 39.74; H, 3.67. Found: C,
39.55; H, 3.52. LRMS m/z: 341 (MþK); IR (CH3OH solution, cmꢀ1):
2982, 2910, 2347, 1415, 1046, 944.
(m, 2H), 1.45–1.67 (m, 1H), 1.09–1.32 (m, 5H), 0.91 (t, J¼8.0 Hz, 2H),
0.82 (t, J¼8.0 Hz, 3H); 13C NMR (75 MHz, DMSO-d6)
d 55.8, 44.6,
31.4, 27.9, 26.6, 25.9, 25.8 (2C), 24.8, 14.8, 14.5; 19F NMR (282 MHz,
DMSO-d6)
d
ꢀ151.55 (q, J¼53.4 Hz); 11B NMR (96 MHz, DMSO-d6)
4.2.9. Potassium (1,3-dithian-2-yl)-2-(2-furanyl) trifluoroborate
d
2.58 (q, J¼51.0 Hz). Anal. Calcd for C11H17BF3KS2: C, 41.25; H, 5.35.
(2i)
Found: C, 40.94; H, 5.06. LRMS m/z: 359 (MþK); IR (CH3OH solution,
Mp¼271 ꢁC (decomp.); 1H NMR (300 MHz, DMSO-d6)
d 7.43 (s,
cmꢀ1): 2989, 2907, 2119, 1411, 1243, 1041, 935.
1H), 6.31 (s, 1H), 6.14 (s, 1H), 2.67–2.77 (m, 2H), 2.31–2.38 (m, 2H),
1.82–1.93 (m, 2H); 13C NMR (75 MHz, DMSO-d6)
d 160.6, 140.4,
4.2.3. Potassium (1,3-dithian-2-yl) trifluoroborate (2c)
110.4, 106.7, 26.4, 26.0 (2C); 19F NMR (282 MHz, DMSO-d6)
Mp¼237 ꢁC (decomp.); 1H NMR (300 MHz, DMSO-d6)
d
3.26 (t,
d
ꢀ150.50 (q, J¼49.3 Hz); 11B NMR (96 MHz, DMSO-d6)
d 3.51 (q,
J¼3.0 Hz, 1H), 2.72–2.82 (m, 2H), 2.60 (dt, J¼12.0, 3.0 Hz, 2H), 1.99–
J¼48.7 Hz). Anal. Calcd for C8H9BF3KOS2: C, 32.88; H, 3.10. Found: C,
32.73; H, 2.89. LRMS m/z: 331 (MþK); IR (CH3OH solution, cmꢀ1):
2975, 2907, 1647, 1373, 1069, 904.
2.08 (m, 1H), 1.76–1.89 (m, 1H); 13C NMR (75 MHz, DMSO-d6)
d
31.8,
ꢀ148.31 (q,
ꢀ1.18 (q, J¼50.8 Hz).
31.6 (2C), 28.2; 19F NMR (282 MHz, DMSO-d6)
d
J¼50.7 Hz); 11B NMR (96 MHz, DMSO-d6)
d
Anal. Calcd for C4H7BF3KS2: C, 21.25; H, 3.12. Found: C, 21.28; H,
2.83. LRMS m/z: 265 (MþK); IR (CH3OH solution, cmꢀ1): 2925, 2911,
1417, 1043, 909.
4.2.10. Potassium (1,3-dithian-2-yl)-2-(3,4-methylenedioxy-
phenyl) trifluoroborate (2j)
Mp¼268 ꢁC (decomp.); 1H NMR (300 MHz, DMSO-d6)
d 7.24 (s,
1H), 7.19 (d, J¼8.2 Hz, 1H), 6.63 (d, J¼8.2 Hz, 1H), 5.82 (s, 2H), 2.34–
4.2.4. Potassium (1,3-dithian-2-yl)-2-(trimethylsilyl)
2.44 (m, 2H), 2.15–2.20 (m, 2H), 1.62–1.72 (m, 2H); 13C NMR
trifluoroborate (2d)
(75 MHz, DMSO-d6) d 146.9, 143.8, 140.5, 122.8, 110.6, 107.0, 100.7,
Mp¼283 ꢁC (decomp.); 1H NMR (300 MHz, DMSO-d6)
d
3.11–
26.4, 25.3 (2C); 19F NMR (282 MHz, DMSO-d6)
d
ꢀ148.90 (q,
3.23 (m, 2H), 2.21–2.78 (m, 2H), 1.70–1.89 (m, 2H), 0.20 (s, 9H); 13
C
J¼49.7 Hz); 11B NMR (96 MHz, DMSO-d6)
d
3.25 (q, J¼53.4 Hz).
NMR (75 MHz, DMSO-d6)
d
27.4, 27.1 (2C), ꢀ1.4; 19F NMR (282 MHz,
Anal. Calcd for C11H11BF3KO2S2: C, 38.16; H, 3.20. Found: C, 38.09; H,
3.11. LRMS m/z: 385 (MþK); IR (CH3OH solution, cmꢀ1): 2989, 2923,
1419, 1230, 1045, 946.
DMSO-d6)
d
ꢀ136.78 (q, J¼50.8 Hz); 11B NMR (96 MHz, DMSO-d6)
d
5.92 (q, J¼49.3 Hz). Anal. Calcd for C7H15BF3KS2Si: C, 28.18; H,
5.07. Found: C, 28.09; H, 4.98. LRMS m/z: 337 (MþK); IR (CH3OH
solution, cmꢀ1): 2986, 2911, 1417, 1239, 1044, 944.
4.2.11. Potassium (1,3-dithian-2-yl)-2-(4-chloro-phenyl)
trifluoroborate (2k)
4.2.5. Potassium (1,3-dithian-2-yl)-2-n-butyl trifluoroborate (2e)
Mp¼287 ꢁC (decomp.); 1H NMR (300 MHz, DMSO-d6)
d 7.81 (d,
Mp¼252 ꢁC (decomp.); 1H NMR (300 MHz, DMSO-d6)
d
2.66–2.71
J¼8.4 Hz, 2H), 7.24 (d, J¼8.4 Hz, 2H), 2.27–2.52 (m, 4H), 1.69–1.81
(m, 2H), 2.31–2.38 (m, 2H),1.79–1.84 (m, 3H), 1.59–1.72 (m, 1H), 1.31–
(m, 2H); 13C NMR (75 MHz, DMSO-d6)
d
145.4, 131.3 (2C), 127.8,
ꢀ149.64
3.39 (q, J¼52.5 Hz).
1.42 (m, 2H), 1.19 (sext, J¼7.5 Hz, 2H), 0.83 (t, J¼7.5 Hz, 3H); 13C NMR
126.6 (2C), 25.8, 24.8 (2C); 19F NMR (282 MHz, DMSO-d6)
d
(75 MHz, DMSO-d6)
d
35.9, 28.6, 26.3, 23.3 (2C), 23.1, 14.3; 19F NMR
d
(q, J¼54.8 Hz); 11B NMR (96 MHz, DMSO-d6)
d
(282 MHz, DMSO-d6)
DMSO-d6)
ꢀ153.13 (q, J¼53.5 Hz); 11B NMR (96 MHz,
Anal. Calcd for C10H10BClF3KS2: C, 35.67; H, 2.99. Found: C, 35.59; H,
2.79. LRMS m/z: 375 (MþK); IR (CH3OH solution, cmꢀ1): 2974, 2903,
1657, 1393, 1067, 880.
d
4.10 (q, J¼57.6 Hz). Anal. Calcd for C8H15BF3KS2: C, 34.04;
H, 5.36. Found: C, 33.92; H, 5.16. LRMS m/z: 321 (MþK); IR (CH3OH
solution, cmꢀ1): 2987, 2912, 1419, 1238, 1043, 945.
4.3. General procedure for preparation of tetra-n-butyl-
ammonium 2-substituted-1,3-dithianotrifluoroborates
salts (3)
4.2.6. Potassium (1,3-dithian-2-yl)-2-allyl trifluoroborate (2f)
Mp¼225 ꢁC (decomp.); 1H NMR (300 MHz, DMSO-d6)
d 5.89–
6.03 (m, 1H), 4.88 (d, J¼17.3 Hz, 1H), 4.76 (d, J¼8.6 Hz, 1H), 2.65–2.78
(m, 4H), 2.32–2.40 (m, 2H), 1.82–1.86 (m, 1H); 1.72–1.74 (m, 1H); 13
NMR (75 MHz, DMSO-d6)
139.0,112.7, 40.7, 26.0, 23.2 (2C); 19F NMR
C
A
solution of potassium (1,3-dithian-2-yl)-2-methyl tri-
d
fluoroborate 2a (0.24 g, 1 mmol) in MeOH (3 mL) and water (3 mL)

A.S. Vieira et al. / Tetrahedron 64 (2008) 7234–7241
7239
was cooled to 0 ꢁC and a solution of n-Bu4NOH (0.74 mL, 1.50 M,
1.1 mmol) was slowly added over 5 min. The reaction was warmed
to room temperature and stirred for 30 min. The reaction mixture
was diluted with CH2Cl2 (10 mL), the layers separated, and the
aqueous phase further extracted with CH2Cl2 (3ꢂ10 mL). The
combined organic layers were dried over MgSO4, filtered, and
concentrated in vacuo to afford 0.40 g of a colorless oil (92%, 3a).
C, 59.36; H, 10.59; N, 2.88. Found: C, 59.11; H, 10.28; N, 2.67. LRMS
m/z: 243 (MꢀTBAþ); IR (CHCl3 solution, cmꢀ1): 2980, 2911, 1429,
1046, 935.
4.3.6. Tetra-n-butylammonium (1,3-dithian-2-yl)-2-allyl
trifluoroborate (3f)
Colorless oil. 1H NMR (300 MHz, CDCl3)
d 6.15–6.28 (m, 1H), 5.02
(d, J¼16.8 Hz, 1H), 4.93 (d, J¼10.0 Hz, 1H), 3.26 (t, J¼8.0 Hz, 8H),
2.86–2.94 (m, 3H), 2.63 (dt, J¼13.6, 3.5 Hz, 3H), 1.93–1.96 (m, 2H),
1.58–1.69 (m, 8H), 1.39–1.51 (m, 8H), 1.01 (t, J¼8.0 Hz, 12H); 13C
4.3.1. Tetra-n-butylammonium (1,3-dithian-2-yl)-2-methyl
trifluoroborate (3a)
Colorless oil. 1H NMR (300 MHz, CDCl3)
d
3.22 (t, J¼8.0 Hz, 8H),
NMR (75 MHz, CDCl3) d 139.0, 112.7, 58.5 (4C), 40.7, 28.9, 26.6, 24.3,
2.89–2.98 (m, 2H), 2.63 (dt, J¼13.6, 3.5 Hz, 2H), 1.86–1.95 (m, 2H),
23.7 (4C), 19.6 (4C), 13.6 (4C); 19F NMR (282 MHz, CDCl3)
d
ꢀ158.50
1.69 (s, 3H), 1.48–1.60 (m, 8H), 1.40 (sext, J¼8.0 Hz, 8H), 0.97 (t,
(q, J¼62.0 Hz); 11B NMR (96 MHz, CDCl3)
d
4.13 (q, J¼55.5 Hz). Anal.
J¼8.0 Hz, 12H); 13C NMR (75 MHz, CDCl3)
d
58.4 (4C), 29.6, 26.8,
Calcd for C23H47BF3NS2: C, 58.93; H, 10.09; N, 2.98. Found: C, 58.64;
H, 9.80; N, 2.79. LRMS m/z: 227 (MꢀTBAþ); IR (CHCl3 solution,
cmꢀ1): 2981, 2918, 1428, 1047, 936.
24.5, 23.9 (4C), 19.6 (C), 13.6 (4C); 19F NMR (282 MHz, CDCl3)
d
ꢀ154.13 (q, J¼53.5 Hz); 11B NMR (96 MHz, CDCl3)
d 4.17 (q,
J¼51.9 Hz). Anal. Calcd for C21H45BF3NS2: C, 56.87; H, 10.23; N, 3.16.
Found: C, 56.63; H, 9.94; N, 2.98. LRMS m/z: 201 (MꢀTBAþ); IR
(CHCl3 solution, cmꢀ1): 2979, 2902, 1428, 1043, 930.
4.3.7. Tetra-n-butylammonium (1,3-dithian-2-yl)-(E)-2-
(prop-1-enyl) trifluoroborate (3g)
Colorless oil. 1H NMR (300 MHz, CDCl3)
d
5.90 (d, J¼10.8 Hz, 1H),
4.3.2. Tetra-n-butylammonium (1,3-dithian-2-yl)-2-(hept-1-ynyl)
trifluoroborate (3b)
3.26 (t, J¼8.0 Hz, 8H), 2.63–2.83 (m, 4H), 1.90–2.12 (m, 2H), 1.58–
1.69 (m, 8H), 1.39–1.51 (m, 8H), 1.01 (t, J¼8.0 Hz, 15H); 13C NMR
Colorless oil. 1H NMR (300 MHz, CDCl3)
d
3.26 (t, J¼8.0 Hz, 8H),
(75 MHz, CDCl3) d 149.1, 115.2, 58.5 (4C), 31.1, 30.2, 26.6, 23.7 (4C),
2.79–2.83 (m, 2H), 2.41–2.54 (m, 4H), 1.84 (t, J¼8.0 Hz, 2H), 1.55–
1.65 (m, 8H), 1.44 (sext, J¼8.0 Hz, 8H), 1.21–1.31 (m, 6H), 0.97 (t,
J¼8.0 Hz, 12H), 0.85 (t, J¼8.0 Hz, 3H); 13C NMR (75 MHz, CDCl3)
19.6 (4C), 16.8, 13.6 (4C); 19F NMR (282 MHz, CDCl3)
d
ꢀ156.30 (q,
J¼62.5 Hz); 11B NMR (96 MHz, CDCl3)
d
4.15 (q, J¼55.6 Hz). Anal.
Calcd for C23H47BF3NS2: C, 58.93; H, 10.09; N, 2.98. Found: C, 58.67;
H, 9.86; N, 2.83. LRMS m/z: 227 (MꢀTBAþ); IR (CHCl3 solution,
cmꢀ1): 2982, 2911, 1428, 1047, 936.
d
58.6 (4C), 56.4, 45.3, 32.1, 28.6, 27.2, 26.5, 26.3 (2C), 25.4,14.8,14.6,
23.7 (4C), 19.6 (4C), 13.7 (4C); 19F NMR (282 MHz, CDCl3)
d
ꢀ155.19
(q, J¼61.5 Hz); 11B NMR (96 MHz, CDCl3)
d
4.20 (q, J¼55.7 Hz). Anal.
Calcd for C27H53BF3NS2: C, 61.93; H, 10.20; N, 2.67. Found: C, 61.64;
H, 10.01; N, 2.46. LRMS m/z: 281 (MꢀTBAþ); IR (CHCl3 solution,
cmꢀ1): 2961, 2912, 1428, 1047, 934.
4.3.8. Tetra-n-butylammonium (1,3-dithian-2-yl)-2-phenyl
trifluoroborate (3h)
Mp¼109 ꢁC; 1H NMR (300 MHz, CDCl3)
d
8.05 (d, J¼8.0 Hz, 2H),
7.30 (t, J¼8.0 Hz, 2H), 7.05 (t, J¼8.0 Hz, 1H), 3.11 (t, J¼8.0 Hz, 8H),
2.68–2.76 (m, 2H), 2.36 (dt, J¼13.6, 3.5 Hz, 2H), 1.81–2.06 (m, 2H),
1.50–1.61 (m, 8H), 1.37–1.46 (m, 8H), 0.99 (t, J¼8.0 Hz, 12H); 13C
4.3.3. Tetra-n-butylammonium (1,3-dithian-2-yl) trifluoroborate
(3c)
Colorless oil. 1H NMR (300 MHz, CDCl3)
d
3.47 (s, 1H), 3.16 (t,
NMR (75 MHz, CDCl3) d 146.4, 129.9 (2C), 127.5 (2C), 123.6, 58.4
J¼8.0 Hz, 8H), 2.66–2.76 (m, 2H), 2.56 (dt, J¼13.6, 3.5 Hz, 2H), 1.82–
(4C), 29.8, 27.1, 26.5, 23.8 (4C), 19.5 (4C), 13.5 (4C), 19F NMR
2.01 (m, 2H), 1.49–1.59 (m, 8H), 1.37 (sext, J¼8.0 Hz, 8H), 0.92 (t,
(282 MHz, CDCl3)
CDCl3)
H, 9.37; N, 2.77. Found: C, 61.50; H, 9.11; N, 2.64. LRMS m/z: 263
(MꢀTBAþ), IR (CHCl3 solution, cmꢀ1): 2983, 2927, 1425, 1056, 939.
d
ꢀ154.70 (q, J¼60.5 Hz); 11B NMR (96 MHz,
J¼8.0 Hz, 12H); 13C NMR (75 MHz, CDCl3)
d
58.3 (4C), 31.0 (2C), 27.2,
ꢀ151.18
4.50 (q, J¼55.6 Hz). Anal.
d
4.09 (q, J¼55.8 Hz). Anal. Calcd for C26H47BF3NS2: C, 61.76;
23.9 (4C), 19.6 (4C), 13.6 (4C); 19F NMR (282 MHz, CDCl3)
d
(q, J¼54.5 Hz); 11B NMR (96 MHz, CDCl3)
d
Calcd for C20H43BF3NS2: C, 55.93; H, 10.09; N, 3.26. Found: C, 55.73;
H, 9.82; N, 3.05. LRMS m/z: 187 (MꢀTBAþ); IR (CHCl3 solution,
cmꢀ1): 2978, 2900, 1429, 1041, 920.
4.3.9. Tetra-n-butylammonium (1,3-dithian-2-yl)-2-(2-furanyl)
trifluoroborate (3i)
Yellow oil. 1H NMR (300 MHz, CDCl3)
d 7.37 (s, 1H), 6.40 (s, 1H),
4.3.4. Tetra-n-butylammonium (1,3-dithian-2-yl)-2-(trimethylsilyl)
trifluoroborate (3d)
6.32 (s, 1H), 3.20 (t, J¼8.0 Hz, 8H), 2.92–3.01 (m, 2H), 2.44 (dt,
J¼13.6, 3.5 Hz, 2H), 1.97–2.05 (m, 2H), 1.55–1.65 (m, 8H), 1.37–1.49
Colorless oil. 1H NMR (300 MHz, CDCl3)
d
3.24 (t, J¼8.0 Hz, 8H),
(m, 8H), 1.00 (t, J¼8.0 Hz, 12H); 13C NMR (75 MHz, CDCl3)
d 160.7,
2.74–2.86 (m, 2H), 2.33 (dt, J¼13.6, 3.5 Hz, 2H), 1.80–2.05 (m, 2H),
140.5, 110.5, 106.8, 58.4 (4C), 30.6, 27.2, 26.5, 23.7 (4C), 19.6 (4C),
1.56–1.66 (m, 8H), 1.42 (sext, J¼8.0 Hz, 8H), 0.95 (t, J¼8.0 Hz, 12H),
13.4 (4C); 19F NMR (282 MHz, CDCl3)
NMR (96 MHz, CDCl3)
d
ꢀ154.50 (q, J¼60.5 Hz); 11
B
0.20 (s, 9H); 13C NMR (75 MHz, CDCl3)
d
58.3 (4C), 30.8, 27.1, 26.4,
d
4.23 (q, J¼55.8 Hz). Anal. Calcd for
23.8 (4C),19.5 (4C),13.5 (4C), ꢀ1.75 (3C); 19F NMR (282 MHz, CDCl3)
C24H45BF3NOS2: C, 58.17; H, 9.15; N, 2.83. Found: C, 57.90; H, 8.86;
d
ꢀ155.30 (q, J¼61.5 Hz); 11B NMR (96 MHz, CDCl3)
d
4.12 (q,
N, 2.67. LRMS m/z: 253 (MꢀTBAþ); IR (CHCl3 solution, cmꢀ1): 2989,
J¼54.8 Hz). Anal. Calcd for C23H51BF3NS2Si: C, 55.06; H, 10.25; N,
2.79. Found: C, 54.77; H, 10.03; N, 2.65. LRMS m/z: 259 (MꢀTBAþ);
IR (CHCl3 solution, cmꢀ1): 2982, 2909, 1427, 1045, 936.
2929, 1426, 1059, 947.
4.3.10. Tetra-n-butylammonium (1,3-dithian-2-yl)-2-
(3,4-methylenedioxy-phenyl) trifluoroborate (3j)
4.3.5. Tetra-n-butylammonium (1,3-dithian-2-yl)-2-(n-butyl)
Yellow oil. 1H NMR (300 MHz, CDCl3)
d 7.29 (s, 1H), 7.21 (d,
trifluoroborate (3e)
J¼8.2 Hz, 1H), 6.71 (d, J¼8.2 Hz, 1H), 5.89 (s, 2H), 3.25 (t, J¼8.0 Hz,
Colorless oil. 1H NMR (300 MHz, CDCl3)
d
3.25 (t, J¼8.0 Hz, 8H),
8H), 2.95–3.04 (m, 2H), 2.51 (dt, J¼13.6, 3.5 Hz, 2H), 1.99–2.07 (m,
2.85–2.94 (m, 2H), 2.62 (dt, J¼13.6, 3.5 Hz, 2H), 1.92–2.06 (m, 4H),
1.54–1.68 (m, 10H), 1.45 (sext, J¼8.0 Hz, 8H), 1.27–1.36 (m, 2H), 1.01
(t, J¼8.0 Hz, 12H), 0.95 (t, J¼8.0 Hz, 3H); 13C NMR (75 MHz, CDCl3)
2H), 1.57–1.69 (m, 8H), 1.38–1.50 (m, 8H), 1.02 (t, J¼8.0 Hz, 12H); 13
C
NMR (75 MHz, CDCl3) d 148.9, 143.9, 140.6, 122.9, 110.7, 107.1, 100.7,
58.4 (4C), 30.7, 27.3, 26.6, 23.8 (4C), 19.7 (4C), 13.5 (4C); 19F NMR
d
58.5 (4C), 36.6, 28.9, 26.6, 24.3, 23.9, 23.7 (4C), 19.6 (4C), 14.3, 13.6
(282 MHz, CDCl3)
CDCl3)
d
ꢀ153.60 (q, J¼60.8 Hz); 11B NMR (96 MHz,
(4C); 19F NMR (282 MHz, CDCl3)
(96 MHz, CDCl3)
d
ꢀ157.59 (q, J¼62.5 Hz); 11B NMR
d
4.21 (q, J¼54.8 Hz). Anal. Calcd for C27H47BF3NO2S2: C,
d
4.03 (q, J¼54.5 Hz). Anal. Calcd for C24H51BF3NS2:
59.00; H, 8.62; N, 2.55. Found: C, 58.72; H, 8.33; N, 2.43. LRMS

7240
A.S. Vieira et al. / Tetrahedron 64 (2008) 7234–7241
m/z: 307 (MꢀTBAþ); IR (CHCl3 solution, cmꢀ1): 2945, 2926, 1423,
cmꢀ1): 3020, 2933, 1759, 1725, 1457. GC/MS m/z (%): 409 (1) [Mþ],
318 (15), 290 (35), 119 (12), 106 (21), 91 (100), 43 (78). HRMS (ESI,
positive) m/z calcd for C19H23NO5S2: 410.1096 ([MþH]þ); found:
410.1141 ([MþH]þ). Minor isomer: anti 5b: 1H NMR (300 MHz,
1059, 949.
4.3.11. Tetra-n-butylammonium (1,3-dithian-2-yl)-2-
(4-chloro-phenyl) trifluoroborate (3k)
CDCl3)
d
7.32–7.54 (m, 5H), 5.58 (d, J¼8.2 Hz, 1H), 5.32 (dd, J¼8.2,
Mp¼127 ꢁC; 1H NMR (300 MHz, CDCl3)
d
7.97 (d, J¼8.5 Hz, 2H),
8.2 Hz, 1H), 5.05 (d, J¼15.0 Hz, 1H), 4.55 (dd, J¼8.2, 4.1 Hz, 1H), 4.35
7.18 (d, J¼8.5 Hz, 2H), 3.07 (t, J¼8.0 Hz, 8H), 2.62–2.70 (m, 2H), 2.34
(dt, J¼13.6, 3.5 Hz, 2H), 1.75–2.02 (m, 2H), 1.48–1.56 (m, 8H), 1.34–
1.44 (m, 8H), 0.97 (t, J¼8.0 Hz, 12H); 13C NMR (75 MHz, CDCl3)
(d, J¼15.0 Hz, 1H), 4.27 (d, J¼4.1 Hz, 1H), 2.73–3.12 (m, 2H), 2.56–
2.69 (m, 2H), 1.85–1.98 (m, 2H); 13C NMR (75 MHz, CDCl3)
d 169.8,
168.3, 166.9, 134.6, 128.6, 128.5, 128.3, 75.3, 73.5, 64.1, 54.2, 43.5,
31.4, 29.6, 27.0, 20.5, 20.3.
d
145.4, 131.4 (2C), 127.9, 126.7 (2C), 58.4 (4C), 26.0, 25.8, 24.8 (2C),
23.8 (4C), 19.5 (4C), 13.5 (4C); 19F NMR (282 MHz, CDCl3)
d
ꢀ155.70
(q, J¼61.5 Hz); 11B NMR (96 MHz, CDCl3)
d
4.02 (q, J¼54.8 Hz). Anal.
4.4.3. (3R,4R)-3,4-Diacetoxy-5-(2-n-butyl-1,3-dithianyl)-1-benzyl-
2-pyrrolidinone (5c)
The product 5c was prepared as described in the general pro-
cedure and was obtained in a 90:10 syn/anti mixture as a colorless
oil in 42% yield (195 mg). Major isomer: syn 5c: 1H NMR (300 MHz,
Calcd for C26H46BF3NClS2: C, 57.83; H, 8.59; N, 2.59. Found: C, 57.54;
H, 8.25; N, 2.41. LRMS m/z: 297 (MꢀTBAþ); IR (CHCl3 solution,
cmꢀ1): 2985, 2923, 1429, 1049, 944.
4.4. General procedure for the synthesis of 5-(2-organo-1,3-
dithianyl)-2-pyrrolidinones (5a–e)
CDCl3)
d
7.29–7.46 (m, 5H), 5.41 (d, J¼5.0 Hz, 1H), 5.33 (dd, J¼5.0,
5.0 Hz, 1H), 5.08 (d, J¼14.5 Hz, 1H), 4.16 (d, J¼14.5 Hz, 1H), 4.06 (d,
J¼5.0 Hz, 1H), 2.88–2.96 (m, 2H), 2.50–2.65 (m, 2H), 2.17 (s, 3H),
2.12 (s, 3H), 1.96 (t, J¼7.1 Hz, 2H), 1.80–1.89 (m, 2H), 1.29–1.38 (m,
4.4.1. (3R,4R)-3,4-Diacetoxy-5-(2-methyl-1,3-dithianyl)-1-benzyl-
2-pyrrolidinone (5a)
4H), 0.91 (t, J¼7.0 Hz, 3H); 13C NMR (75 MHz, CDCl3)
d 170.7, 170.3,
To a solution of the 5-acetoxy-2-pyrrolidinone 4 (349 mg,
1.0 mmol) in CH2Cl2 (4.0 mL) at ꢀ78 ꢁC under nitrogen atmosphere
was added dropwise the BF3$Et2O (0.50 mL, 4.0 mmol, 4.0 equiv).
The reaction mixture was kept 1 h at ꢀ78 ꢁC when potassium (2-
methyl-1,3-dithianyl) trifluoroborate (2a) (288 mg, 1.2 mmol,
1.2 equiv) was added in one portion. After 1 h at ꢀ78 ꢁC, the re-
action mixture was stirred 6 h at room temperature when it was
quenched with saturated NaHCO3 (10 mL). The mixture was diluted
with CH2Cl2 (20 mL), and the organic phase was washed with 10%
HCl (10 mL), saturated NaHCO3 (10 mL), and dried over MgSO4.
Evaporation under reduced pressure, followed by column chro-
matography on silica gel (20% ethyl acetate in hexanes) afforded 5a
in an 80:20 syn/anti mixture as a yellow oil, in 68% yield (287 mg).
Both isomers have nearly the same Rf values and they could not be
separated by column chromatography. The syn/anti ratio was de-
termined to be 80:20 by the 1H NMR (300 MHz) analysis of the
crude product. Major isomer: syn 5a: 1H NMR (300 MHz, CDCl3)
168.5,135.2,128.8,128.5,128.3, 75.6, 73.7, 64.7, 54.5, 43.7, 31.5, 29.3,
27.9, 26.6, 24.2, 23.1, 20.8, 20.6, 14.6; IR (CHCl3 solution, cmꢀ1):
3025, 2935, 1751, 1720, 1229. GC/MS m/z (%): 465 (2) [Mþ], 375 (12),
332 (23), 290 (17), 175 (10), 106 (19), 91 (100), 43 (71). HRMS (ESI,
positive) m/z calcd for C23H31NO5S2: 466.1722 ([MþH]þ); found:
466.1751 ([MþH]þ). Minor isomer: anti 5c: 1H NMR (300 MHz,
CDCl3)
d
7.29–7.46 (m, 5H), 5.51 (d, J¼8.2 Hz, 1H), 5.30 (dd, J¼8.2,
8.2 Hz, 1H), 5.03 (d, J¼14.8 Hz, 1H), 4.10 (d, J¼14.8 Hz, 1H), 4.01 (d,
J¼8.2 Hz, 1H), 2.88–2.96 (m, 2H), 2.50–2.65 (m, 2H), 2.16 (s, 3H),
2.10 (s, 3H), 1.96 (t, J¼7.1 Hz, 2H), 1.80–1.89 (m, 2H), 1.29–1.38 (m,
4H), 0.91 (t, J¼7.0 Hz, 3H); 13C NMR (75 MHz, CDCl3)
d 170.5, 170.1,
168.4, 135.1, 128.8, 128.5, 128.3, 75.3, 73.5, 64.4, 54.2, 43.5, 31.4,
29.3, 27.9, 26.6, 24.2, 23.1, 20.8, 20.6, 14.6.
4.4.4. (3R,4R)-3,4-Diacetoxy-5-(2-trimethylsilyl-1,3-dithianyl)-
1-benzyl-2-pyrrolidinone (5d)
The product 5d was prepared as described in the general pro-
cedure and was obtained in a 70:30 syn/anti mixture as a colorless
oil in 63% yield (303 mg). Major isomer: syn 5d: 1H NMR (300 MHz,
d
7.27–7.34 (m, 5H), 5.39 (d, J¼4.8 Hz, 1H), 5.35 (dd, J¼4.8, 4.8 Hz,
1H), 5.11 (d, J¼15.0 Hz,1H), 4.20 (d, J¼15.0 Hz,1H), 4.15 (d, J¼4.8 Hz,
1H), 2.87–2.95 (m, 2H), 2.52–2.69 (m, 2H), 2.16 (s, 3H), 2.05 (s, 3H),
CDCl3)
d
7.26–7.35 (m, 5H), 5.38 (d, J¼4.7 Hz, 1H), 5.34 (dd, J¼4.7,
1.95 (s, 3H), 1.82–1.91 (m, 2H); 13C NMR (75 MHz, CDCl3)
d
170.4,
4.7 Hz, 1H), 5.10 (d, J¼15.2 Hz, 1H), 4.18 (d, J¼15.2 Hz, 1H), 4.14 (d,
J¼4.7 Hz, 1H), 2.88–2.97 (m, 2H), 2.53–2.70 (m, 2H), 2.15 (s, 3H),
2.07 (s, 3H), 1.83–1.93 (m, 2H), 0.13 (s, 9H); 13C NMR (75 MHz,
169.9, 167.0, 134.9, 128.9, 128.4, 128.2, 75.5, 73.8, 64.8, 54.6, 43.9,
31.6, 29.2, 27.8, 24.2, 20.8, 20.6; IR (CHCl3 solution, cmꢀ1): 3021,
2937, 1755, 1721, 1451. GC/MS m/z (%): 423 (2) [Mþ], 309 (10), 290
(24), 230 (28), 188 (63), 160 (13), 133 (17), 106 (15), 91 (100), 65 (11),
43 (45). HRMS (ESI, positive) m/z calcd for C20H25NO5S2: 424.1253
([MþH]þ); found: 424.1241 ([MþH]þ). Minor isomer: anti 5a: 1H
CDCl3) d 170.0, 169.5, 167.3, 134.7, 128.6, 128.5, 128.1, 75.1, 73.3, 62.6,
51.8, 43.7, 31.5, 29.3, 27.5, 20.9, 20.5, 1.3; IR (CHCl3 solution, cmꢀ1):
3021, 2937, 1750, 1724. GC/MS m/z (%): 481 (1) [Mþ], 390 (21), 347
(15), 290 (24), 106 (13), 91 (100), 43 (75). HRMS (ESI, positive) m/z
calcd for C22H31NO5S2Si: 482.1491 ([MþH]þ); found: 482.1536
NMR (300 MHz, CDCl3)
d
7.27–7.34 (m, 5H), 5.56 (d, J¼8.0 Hz, 1H),
5.30 (dd, J¼8.0, 8.0 Hz, 1H), 5.08 (d, J¼15.0 Hz, 1H), 4.72 (d,
J¼8.0 Hz, 1H), 4.05 (d, J¼15.0 Hz, 1H), 2.87–2.95 (m, 2H), 2.52–2.69
([MþH]þ). Minor isomer: anti 5d: 1H NMR (300 MHz, CDCl3)
d 7.26–
7.35 (m, 5H), 5.55 (d, J¼8.3 Hz, 1H), 5.29 (dd, J¼8.3, 8.3 Hz, 1H), 5.07
(d, J¼14.7 Hz, 1H), 4.14 (d, J¼14.7 Hz, 1H), 4.03 (d, J¼8.3 Hz, 1H),
2.86–2.93 (m, 2H), 2.51–2.68 (m, 2H), 2.14 (s, 3H), 2.06 (s, 3H), 1.83–
(m, 2H), 2.18 (s, 3H), 2.14 (s, 3H), 1.94 (s, 3H), 1.82–1.91 (m, 2H); 13
C
NMR (75 MHz, CDCl3) d 170.2,169.7, 165.8, 134.4, 128.8, 128.3, 128.0,
75.4, 72.7, 64.0, 53.8, 43.7, 31.4, 29.0, 27.6, 24.0, 20.7, 20.5.
1.93 (m, 2H), 0.12 (s, 9H); 13C NMR (75 MHz, CDCl3)
d 170.1, 169.4,
167.2, 134.6, 128.6,128.5,128.0, 74.8, 73.2, 62.5, 51.6, 43.6, 31.4, 29.3,
27.5, 20.8, 20.4, 1.3.
4.4.2. (3R,4R)-3,4-Diacetoxy-5-(1,3-dithianyl)-1-benzyl-2-
pyrrolidinone (5b)
The product 5b was prepared as described in the general pro-
cedure and was obtained in a 70:30 syn/anti mixture as a colorless
oil in 54% yield (220 mg). Major isomer: syn 5b: 1H NMR (300 MHz,
4.4.5. (3R,4R)-3,4-Diacetoxy-5-(2-allyl-1,3-dithianyl)-1-benzyl-
2-pyrrolidinone (5e)
The product 5e was prepared as described in the general pro-
cedure and was obtained in a 90:10 syn/anti mixture as a colorless
oil in 51% yield (229 mg). Major isomer: syn 5e: 1H NMR (300 MHz,
CDCl3)
d
7.32–7.54 (m, 5H), 5.42 (d, J¼4.9 Hz, 1H), 5.38 (dd, J¼4.9,
4.9 Hz, 1H), 5.14 (d, J¼15.0 Hz, 1H), 4.52 (dd, J¼4.9, 4.3 Hz, 1H), 4.32
(d, J¼15.0 Hz, 1H), 4.23 (d, J¼4.3 Hz, 1H), 2.73–3.12 (m, 2H), 2.56–
2.69 (m, 2H), 2.19 (s, 3H), 2.10 (s, 3H), 1.85–1.98 (m, 2H); 13C NMR
CDCl3)
d
7.28–7.35 (m, 5H), 5.73–5.91 (m, 1H), 5.40 (d, J¼4.9 Hz,1H),
5.30 (dd, J¼17.3, 1.5 Hz, 1H), 5.26 (dd, J¼4.9, 4.9 Hz, 1H), 5.22 (dd,
J¼10.0, 1.5 Hz), 5.10 (d, J¼15.1 Hz, 1H), 4.23 (d, J¼15.1 Hz, 1H),
4.12 (d, J¼4.9 Hz, 1H), 2.87–2.95 (m, 2H), 2.75 (d, J¼5.7 Hz, 2H),
(75 MHz, CDCl3)
d 170.1, 168.5, 167.1, 134.7, 128.7, 128.5, 128.2, 75.4,
73.6, 64.3, 54.3, 43.7, 31.7, 29.7, 27.1, 20.6, 20.4; IR (CHCl3 solution,

A.S. Vieira et al. / Tetrahedron 64 (2008) 7234–7241
7241
2.51–2.68 (m, 2H), 2.15 (s, 3H), 2.09 (s, 3H), 1.83–1.92 (m, 2H); 13C
NMR (75 MHz, CDCl3) 170.1, 169.8, 167.3, 135.3, 134.8, 128.8, 128.3,
2. For reviews of organotrifluoroborate salts, see: (a) Darses, S.; Geneˆt, J. P. Eur. J.
Org. Chem. 2003, 4313; (b) Molander, G. A.; Figueroa, R. Aldrichimica Acta 2005,
38, 49; (c) Molander, G. A.; Ellis, N. Acc. Chem. Res. 2007, 40, 275; (d) Stefani,
d
128.2, 118.1, 75.3, 73.4, 63.8, 54.1, 43.9, 34.7, 29.2, 27.8, 24.2, 20.8,
20.6; IR (CHCl3 solution, cmꢀ1): 3036, 2939, 1750, 1707. GC/MS m/z
(%): 449 (2) [Mþ], 408 (24), 358 (15), 290 (25), 159 (12), 91 (100), 43
(67), 41 (35). HRMS (ESI, positive) m/z calcd for C22H27NO5S2:
450.1409 ([MþH]þ); found: 450.1396 ([MþH]þ). Minor isomer: anti
ˆ
H. A.; Cella, R.; Vieira, A. S. Tetrahedron 2007, 63, 3623; (e) Darses, S.; Genet, J. P.
Chem. Rev. 2008, 108, 288.
3. (a) Corey, E. J.; Seebach, D. Angew. Chem., Int. Ed. Engl. 1965, 4, 1075; (b) Seebach,
D.; Corey, E. J. J. Org. Chem. 1975, 40, 231.
4. For reviews see: (a) Seebach, D. Synthesis 1969, 17; (b) Grobel, B. T.; Seebach, D.
Synthesis 1977, 357; (c) Page, P. C. B.; Van Niel, M. B.; Prodger, J. C. Tetrahedron
1989, 45, 7643; (d) Yus, M.; Najera, C.; Foubelo, F. Tetrahedron 2003, 59,
6147.
5. (a) Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic Synthesis, 2nd ed.;
Wiley: New York, NY, 1991; pp 178–207; (b) Corey, E. J.; Seebach, D. J. Org. Chem.
1966, 31, 4097; (c) Eliel, E. L.; Morris-Natschke, S. J. Am. Chem. Soc. 1984, 106,
2937.
6. (a) Garlaschelli, L.; Vidari, G. Tetrahedron Lett. 1990, 31, 5815; (b) Hauptmann,
H.; Campos, M. M. J. Am. Chem. Soc. 1950, 72, 5815; (c) Seebach, D.; Kolb, M.
Chem. Ind. (London) 1974, 687; (d) Ong, B. S. Tetrahedron Lett. 1980, 21, 4225; (e)
Kumar, V.; Dev, S. Tetrahedron Lett. 1983, 24, 1289; (f) Ku, B.; Oh, D. Y. Synth.
Commun. 1989, 19, 433; (g) Ong, B. S.; Chan, T. H. Synth. Commun. 1977, 7, 283;
(h) Corey, E. J.; Simoji, K. Tetrahedron Lett. 1983, 24, 169; (i) Komatsu, N.; Uda,
M.; Suzuki, H. Synlett 1995, 984; (j) Choudray, B. M.; Sudha, Y. Synth. Commun.
1996, 26, 2993.
7. (a) Smith, A. B., III; Condon, S. M.; McCauley, J. A. Acc. Chem. Res. 1998, 31, 35 and
references therein; (b) Smith, A. B., III; Lodise, S. A. Org. Lett. 1999, 1, 1249; (c)
Smith, A. B., III; Doughty, V. A.; Lin, Q.; Zhuang, L.; McBriar, M. D.; Boldi, A. M.;
Moser, W. H.; Murase, N.; Nakayama, K.; Sobukawa, M. Angew. Chem., Int. Ed.
2001, 40, 191; (d) Smith, A. B., III; Lin, Q.; Doughty, V. A.; Zhuang, L.; McBriar,
M. D.; Kerns, J. K.; Brook, C. S.; Murase, N.; Nakayama, K. Angew. Chem., Int. Ed.
2001, 40, 196; (e) Smith, A. B., III; Adams, C. M.; Lodise Barbosa, S. A.; Degnan,
A. P. J. Am. Chem. Soc. 2003, 125, 350.
8. Hughes, R. J.; Ncube, S.; Pelter, A.; Smith, K. J. Chem. Soc., Perkin Trans. 1 1977,
1172.
9. (a) Stefani, H. A.; Cella, R.; Zukerman-Schpector, J.; Caracelli, I. Z. Kristallogr. NCS
2006, 221, 167; (b) Caracelli, I.; Stefani, H. A.; Vieira, A. S.; Machado, M. M. P.;
Zukerman-Schpector, J. Z. Kristallogr. NCS 2007, 222, 345.
10. For reviews of N-acyliminium ion chemistry, see: (a) de Koning, H.; Speck-
amp, W. N. Houben-Weyl, Stereoselective Synthesis; Helmchen, G., Hoffmann,
R. W., Mulzer, J., Schaumann, E., Eds.; 1995; Vol. E21, p 1953; (b) Speckamp,
W. N.; Moolenaar, M. J. Tetrahedron 2000, 56, 3817; (c) Maryanoff, B. E.;
Zhang, H.-C.; Cohen, J. H.; Turchi, I. J.; Maryanoff, C. A. Chem. Rev. 2004, 104,
1431.
11. See, for example: (a) Pilli, R. A.; Dias, L. C.; Maldaner, A. O. J. Org. Chem. 1995, 60,
717; (b) Pilli, R. A.; Russowsky, D. J. Org. Chem. 1996, 61, 3187; (c) Dhimane, H.;
Vanucci, C.; Lhommet, G. Tetrahedron Lett. 1997, 38, 1415; (d) Batey, R. A.;
Mackay, D. B. Tetrahedron Lett. 2000, 41, 9935; (e) El-Nezhawy, A. O. H.; El-Di-
wani, H. I.; Schmidt, R. R. Eur. J. Org. Chem. 2002, 4137; (f) Bennet, D. J.; Blake,
A. J.; Cooke, P. A.; Godfrey, C. R. A.; Pickering, P. L.; Simpkins, N. S.; Walker, M. D.;
Wilson, C. Tetrahedron 2004, 60, 4491; (g) Osante, I.; Lete, E.; Sotomayor, N.
Tetrahedron Lett. 2004, 45, 1253; (h) Huang, P.-Q.; Lu, L.-X.; Wei, B.-G.; Ruan, Y.-P.
Org. Lett. 2003, 5, 1927; (i) Meng, W.-H.; Wu, T.-J.; Zhang, H.-K.; Huang, P.-Q.
Tetrahedron: Asymmetry 2004, 15, 3899; (j) Chen, B.-F.; Tasi, M.-R.; Yang, C.-Y.;
Chang, J.-K.; Chang, N.-C. Tetrahedron 2004, 60, 10223; (k) Huang, P.-Q. Synlett
2006,1133; (l) Washburn, D. G.; Heidebrecht, R. W.; Martin, S. F. Org. Lett. 2003, 5,
3523; (m) Vieira, A. S.; Ferreira, F. P.; Fiorante, P. F.; Guadagnin, R. C.; Stefani, H. A.
Tetrahedron 2008, 64, 3306; (n) For leading references, see: The Alkaloids,
Chemistry and Biology; Cordell, G. A., Ed.; Academic: San Diego, CA, 1998; Vol.
50.
5e: 1H NMR (300 MHz, CDCl3)
d 7.28–7.35 (m, 5H), 5.73–5.91 (m,
1H), 5.51 (d, J¼8.1 Hz), 5.30 (dd, J¼17.3, 1.5 Hz, 1H), 5.15 (dd, J¼8.1,
8.1 Hz, 1H), 5.22 (dd, J¼10.0, 1.5 Hz), 5.05 (d, J¼15.1 Hz, 1H), 4.19 (d,
J¼15.1 Hz, 1H), 4.08 (d, J¼8.1 Hz, 1H), 2.87–2.95 (m, 2H), 2.75 (d,
J¼5.7 Hz, 2H), 2.51–2.68 (m, 2H), 2.13 (s, 3H), 2.07 (s, 3H), 1.83–1.92
(m, 2H); 13C NMR (75 MHz, CDCl3)
d 170.0, 169.6, 167.1, 135.2, 134.5,
128.8, 128.2, 128.0, 118.1, 75.1, 73.3, 63.6, 54.0, 43.7, 34.5, 29.1, 27.8,
24.2, 20.7, 20.4.
4.5. Procedure for the thioketal hydrolysis of
5-(1,3-dithianyl)-2-pyrrolidinone 5b15
4.5.1. (3R,4R)-3,4-Diacetoxy-5-formyl-1-benzyl-2-pyrrolidinone (6)
To a solution of the compound 5b (205 mg, 0.5 mmol) in 2.5 mL
of 8:1:1 CH3CN/CH2Cl2/H2O (0.2 M) was added 424 mg of Dess–
Martin periodinane (1.0 mmol) in one portion. The reaction mix-
ture was stirred at room temperature, exposed to air, for 2 h until
complete consumption of starting material as observed by TLC. The
reaction was quenched with 20 mL of 50% aqueous NaHCO3. The
layers were separated and the aqueous layer was extracted 3ꢂ into
50 mL of CH2Cl2. The combined organic layers were washed with
brine, dried over MgSO4, filtered, and concentrated. Purification by
flash chromatography (silica, 10% EtOAc/hexanes) affords the de-
sired aldehyde 6 as a colorless oil in 78% yield (124 mg). Major
isomer: syn 6: 1H NMR (300 MHz, CDCl3)
d
10.16 (d, J¼3.2 Hz, 1H),
7.21–7.43 (m, 5H), 5.48 (d, J¼5.1 Hz, 1H), 5.21 (dd, J¼5.1, 5.1 Hz, 1H),
5.10 (d, J¼14.8 Hz, 1H), 4.72 (dd, J¼5.1, 3.2 Hz, 1H), 4.42 (d,
J¼14.8 Hz, 1H), 2.15 (s, 3H), 2.11 (s, 3H); 13C NMR (75 MHz, CDCl3)
d
200.3, 170.3, 169.6, 167.5, 134.9, 128.8, 128.6, 128.3, 76.2, 74.1, 67.5,
45.2, 21.3, 21.1; IR (CHCl3 solution, cmꢀ1): 3029, 2935, 2861, 2765,
1741, 1725, 1703. GC/MS m/z (%): 319 (2) [Mþ], 276 (31), 228 (23), 91,
(100), 43 (72). HRMS (ESI, positive) m/z calcd for C16H17NO6:
320.1134 ([MþH]þ); found: 320.1201 ([MþH]þ).
Acknowledgements
The authors would like to acknowledge Conselho Nacional de
´
´
˜
Desenvolvimento Cientıfico e Tecnologico (CNPq) and Fundaçao de
12. (a) Cella, R.; Cunha, R. L. O. R.; Reis, A. E. S.; Pimenta, D. C.; Klitzke, C. F.; Stefani,
H. A. J. Org. Chem. 2006, 71, 224; (b) Cella, R.; Stefani, H. A. Tetrahedron 2006, 62,
5656; (c) Stefani, H. A.; Cella, R.; Do¨rr, F. A.; Pereira, C. M. P.; Zeni, G.; Gomes, M.,
Jr. Tetrahedron Lett. 2005, 46, 563; (d) Cella, R.; Orfa˜o, A. T. G.; Stefani, H. A.
Tetrahedron Lett. 2006, 47, 5075; (e) Cella, R.; Venturoso, R. C.; Stefani, H. A.
Tetrahedron Lett. 2008, 49, 16.
`
˜
Amparo a Pesquisa do Estado de Sao Paulo (FAPESP 06/50190-7 and
05/59141-6) for financial support.
References and notes
13. Batey, R. A.; Quach, T. D. Tetrahedron Lett. 2001, 42, 9099.
14. Metten, B.; Kostermans, M.; Van Baelen, G.; Smet, M.; Dehaen. Tetrahedron
2006, 62, 6018.
15. Langille, N. F.; Dakin, L. A.; Panek, J. S. Org. Lett. 2003, 5, 575.
1. (a) Matteson, D. S. Stereodirected Synthesis with Organoboranes; Springer: Berlin,
1995; (b) Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents; Academic: London,
1988; (c) Ling, R.; Yoshida, M.; Mariano, P. S. J. Org. Chem. 1996, 61, 4439.