Studies on electrophilic reaction of tertiary 2,3-allenols with NBS in H2O or aqueous MeCN: An efficient selective synthesis of 2-bromoallylic ketones, 1,2-allenyl ketones, or 3-bromo-2,5-dihydrofurans

Article abstract of DOI:10.1021/jo9018717

(Figure Presented) Previously, we observed that the electrophilic reaction of 2,3-allenols with X⁺ (X=Br, I) affords 3-halo-2-alkenals or 2-halo-2-alkenyl ketones in aqueous MeCN (MeCN/H₂O=15:1). However, the reaction of tertiary 2,3

Full text of DOI:10.1021/jo9018717

pubs.acs.org/joc
Studies on Electrophilic Reaction of Tertiary 2,3-Allenols with
NBS in H2O or Aqueous MeCN: An Efficient Selective Synthesis
of 2-Bromoallylic Ketones, 1,2-Allenyl Ketones,
or 3-Bromo-2,5-dihydrofurans
Jing Li,† Wangqing Kong,† Yihua Yu,‡ Chunling Fu,*,† and Shengming Ma*,†
†Laboratory of Molecular Recognition and Synthesis, Department of Chemistry, Zhejiang University,
Hangzhou 310027, Zhejiang, People’s Republic of China, and ‡Shanghai Key Laboratory of Functional
Magnetic Resonance Imaging, Department of Physics, East China Normal University,
Shanghai 200062, People’s Republic of China
fucl@zju.edu.cn; masm@mail.sioc.ac.cn
Received August 31, 2009
Previously, we observed that the electrophilic reaction of 2,3-allenols with Xþ (X = Br, I) affords 3-halo-
2-alkenals or 2-halo-2-alkenyl ketones in aqueous MeCN (MeCN/H2O = 15:1). However, the reaction
of tertiary 2,3-allenols with NBS under these reaction conditions affords a mixture of rearrangement
products (aldehydes or ketones) together with 1-bromovinyl epoxides in a low selectivity. Due to the
synthetic potential of 2-haloallylic ketones, we decided to explore this reaction further. After screening, we
observed that the electrophilic reaction of terminal tertiary 2,3-allenols with NBS in water affords
2-bromoallylic ketones highly selectively in up to 97% yields via a sequential electrophilic interaction of
Brþ with the allene moiety to form a possible three-membered bromonium intermediate with the more
substituted CdC bond, which would be followed by 1,2-aryl or 1,2-alkyl shift to open the strained three-
membered ring and proton elimination to form the carbonyl functionality. When both R1 and R2 (the
substituents on the carbon atom connected to the hydroxyl groups) are alkyl groups, one of these two
groups is migrated; with 1,2-propadienyl cycloalkanols, a ring expansion reaction was observed; with
R1 being an aryl group, R1 would be transferred exclusively to form the 2-bromoallylic ketones.
Interestingly, these 2-bromo-1-aryl-2-propenyl ketones may easily be converted into 1,2-allenyl ketones
after column chromatography on silica gel pre-eluented with 10 drops of Et3N; when there is at least an
alkyl substituent on the 4-position of the tertiary 2,3-allenols, their electrophilic reaction with NBS
in CH3CN/H2O = 15/1 or H2O, under the same reaction conditions as above, affords 3-bromo-
2,5-dihydrofurans in 61-84% yields, indicating that the electronic effect and the steric effect of the two
CdC bonds determine the reaction pathway, i.e., the Brþ interacts with the CdC bond at the 3-position.
Introduction
functionalities may be introduced within one synthetic
operation.1,2 In 2005, we reported that the electrophilic
reaction of 2,3-allenols with Xþ affords 3-halo-3-alkenals
or 2-halo-2-alkenyl ketones in 37% to 93% yields with good
Electrophilic addition reactions of allenes have been be-
coming more and more useful in organic synthesis since two
DOI: 10.1021/jo9018717
r
Published on Web 10/29/2009
J. Org. Chem. 2009, 74, 8733–8738 8733
2009 American Chemical Society

Li et al.
JOCArticle
TABLE 1. Electrophilic Reaction of 1-(Propadienyl)cyclohexanol 1a with NBSa
entry
solvent
T (°C)
t (h)
yield of 2a (%)
yield of 3a (%)
recovery of 1a (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14b
15c
16d
17e
18b
19b
MeCN/H2O = 15/1
MeCN/H2O = 15/1
THF
MeCN
MeNO2
CH2Cl2
CCl4
MeOH
1,4-dioxane
MeCN/H2O = 5/1
MeCN/H2O = 1/1
MeCN/H2O = 1/10
H2O
H2O
H2O
H2O
H2O
H2O
H2O
rt
0
1
2
3
10
2
3
3
2
2
10
10
2
2
2
2
2
2
2
50
54
29
22
6
19
25
13
21
17
4
0
0
6
33
29
62
60
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
60
80
5
0
complicated
0
10
50
26
12
10
8
7
7
4
6
7
34
48
55
52
0
0
0
0
0
0
0
0
0
0
58
60 (58)
51
50
20
51
49
2
8
aNMR yields determined by using CH2Br2 as the internal standard. The isolated yield is given in parentheses. b1.0 equiv of NBS was used. c1.2 equiv of
NBS were used. d1.5 equiv of NBS were used. e2.0 equiv of NBS were used.
regioselectivity in aqueous MeCN (MeCN/H2O = 15/1), in
which a C-C bond was cleaved due to the rearrangement of
a cationic intermediate.3 However, when we tried to extend
this chemistry to tertiary 2,3-allenols, it was observed that
the formation of the epoxide is a major side reaction. In 2008,
Okamoto et al. reported the reaction of 2-substituted tertiary
2,3-allenols with NBS, affording cyclic 2-haloallylic ketones
in MeCN/H2O = 15/1 in moderate yields (38-79%) with the
formation of epoxide being reported only in one case.4 In this
paper, we wish to report our detailed study on the electro-
philic reaction of tertiary 2,3-allenols with NBS, especially
the solvent effect on the selectivity of carbonyl products vs.
epoxides as well as the electronic and steric effects on the
regioselectivity of the reaction of Brþ with the different CdC
bonds of the allene moiety.
entry 1 for 12 h did not yield any 3a. These two facts indicate
that epoxide 2a is not the kinetic product and did not serve as
an intermediate for the formation of ketone 3a. To increase
the selectivity of ketone/epoxide, screening of different reac-
tion conditions was conducted: the reaction at 0 o C led to a
slight increase of the yield (Table 1, entry 2). Then we
screened the most commonly used solvents, such as THF,
MeCN, MeNO2, CH2Cl2, CCl4, MeOH, and 1,4-dioxane;
however, all the results were poor with 1a being recovered to
some extent (Table 1, entries 3-9). The effect of the ratio of
the mixed solvent MeCN/H2O on the reaction was also
screened (Table 1, entries 10-12). To our surprise, the
reaction in pure water afforded the R-(2-bromoallyl)cyclo-
heptanone 3a in 58% yield together with epoxide 2a in 10%
yield (Table 1, entry 13). Best results were achieved by
running the reaction in water, using 1.0 equiv of NBS to
afford 3a in 60% yield (58% isolated yield) and epoxide 2a in
8% yield (Table 1, entry 14). Increasing the amount of NBS
led to reduced yields (Table 1, entries 15-17). The reaction at
a higher temperature afforded the products in slightly lower
yields (Table 1, entries 18 and 19). Thus, the reaction
conditions presented in entry 14 of Table 1 were defined as
the standard conditions for further study.
The electrophilic reaction of tertiary 2,3-allenols 1 with
different types of R1 and R2 at the 1-position with NBS was
then studied by applying the standard reaction conditions:
The reaction of cyclic 2,3-allenols 1b-d with NBS afforded
ring-expanded products R-(1-bromovinyl)cycloketones
3b-d in moderate yields via a 1,2-alkyl shift process
(Table 2, entries 1-4). The reaction of 1-(propadienyl)cyclo-
pentanol 1b in H2O afforded the corresponding rearrange-
ment product 3b in moderate NMR yield (59%) together
with another unidentified product (Table 2, entry 1), while
the reaction in MeCN/H2O = 15/1 afforded the compound
3b as the only product (Table 2, entry 2). It is interesting to
Results and Discussion
Electrophilic Reaction of Tertiary 2,3-Allenols. First, we
used 1-(propadienyl)cyclohexanol 1a as the model substrate.
The reaction in MeCN/H2O = 15/13 afforded a mixture of
2-bromovinyl-substituted epoxide 2a5 and the ring-ex-
panded 2-bromovinyl seven-membered cyclic ketone 3a in
69% combined yield with a selectivity of 2.6:1 (Table 1, entry
1). It should be noted that the reaction afforded 2a and 3a in
essentially the same ratio even after 4 h. Furthermore,
treatment of pure epoxide 2a under the conditions listed in
(1) For a review, see: Ma, S. Ionic Addition to Allenes. In Modern Allene
Chemistry; Krause, N., Hashmi, A. S. K., Eds.; Wiley-VCH: Weinheim,
Germany, 2004.
(2) (a) Ma, S. Pure Appl. Chem. 2007, 79, 261. (b) Ma, S. Acc. Chem. Res.
DOI: 10.1021/ar900153r. Published Online: July 15, 2009.
(3) (a) Fu, C.; Li, J.; Ma, S. Chem. Commun. 2005, 4119. (b) Li, J.; Fu, C.;
Chen, G.; Chai, G.; Ma, S. Adv. Synth. Catal. 2008, 350, 1376.
(4) He, J.; Shibata, D.; Ohno, C.; Okamoto, S. Tetrahedron Lett. 2008, 49,
6724.
(5) Friesen, R. W.; Blouin, M. J. Org. Chem. 1993, 58, 1653.
8734 J. Org. Chem. Vol. 74, No. 22, 2009

Li et al.
JOCArticle
TABLE 2. Electrophilic Reaction of Tertiary 2,3-Allenols 1 with NBS in Water
entry
R1
R2
isolated yield of 3 (%)
NMR yield of 2 (%)
1
2b
3
4
5
6
7
8
9
(CH2)4 (1b)
(CH2)4 (1b)
(CH2)6 (1c)
(CH2)7 (1d)
-a (3b)
53 (3b)
62 (3c)
58 (3d)
60 (3e)
60 (3f)
97 (3g)
93 (3h)
82 (3i)
79 (3j)
79 (3k)
0
0
12
8
4
4
0
0
0
0
n-C3H7
n-C4H9
Ph
n-C3H7 (1e)
n-C4H9 (1f)
CH3 (1g)
C2H5 (1h)
CH3 (1i)
Ph
m-CH3C6H4
p-CH3C6H4
Ph
10
11
CH3 (1j)
Ph (1k)
0
a59% NMR yield together with another unidentified product. bMeCN/H2O = 15/1 was used as solvent.
SCHEME 1
observe that even the not readily available eight- or nine-
membered rings6 may be formed easily (Table 2, entries 3
and 4). The yields of epoxides 2c,d were at the level of
8-12% for 1c,d (Table 2, entries 3 and 4) while the forma-
tion of epoxide 2b was not observed in the reaction of 1b
(Table 2, entry 2). Furthermore, we were happy to see that
even the reaction of the substrates with R1 and R2 being
separate alkyl groups, i.e., 1e and 1f, afforded 1,2-alkyl shift
products 3e and 3f in moderate yields with 4% of epoxide
products 2e and 2f (Table 2, entries 5 and 6). The Brþ-
induced 1,2-shift reaction of the 2,3-allenols 1 with R1 being
an aryl group showed very good selectivity, but only the
1,2-aryl (R1) shift products 3 were formed in good to
excellent yields (79-97%) (Table 2, entries 7-11). The
reaction of the substrate 1k with R1 and R2 both being
phenyl also afforded the 1,2-aryl shift product 3k in 79%
yield (Table 2, entry 11).
The reaction may also be extended to 1-(propadienyl)-
1,2,3,4-tetrahydronaphth-1-ol 1l, affording the 1,2-aryl shift
product 5-(10-bromovinyl)-6,7,8,9-tetrahydro-5H-benzo[7]-
annulen-6-one 3l in only 76% yield in 2 h with a 0.3 mmol
scale reaction; when 3.0 mmol of 1l was used, 3l was afforded
in a much higher yield (92%) in 5 h with the complete
consumption of the starting material. The structure of the
product 3l was further confirmed by its X-ray diffrac-
tion study (Figure 1 in the Supporting Information).7 The
reaction of 1-(propadienyl)cyclobutanol 1m also afforded
2-(10-bromovinyl)cyclopentanone 3m in a higher yield (71%)
when 10.0 mmol of 1m was used as compared to a 0.3 mmol
scale reaction (64%) (Scheme 1).
ketones may be partially converted to 1-aryl-1,2-propadienyl
ketones8 upon purification via chromatography on silica gel.
Further study led us to note that 1-aryl-1,2-propadienyl
ketones 3g-k and 3n may be converted to 1,2-allenyl ketones
4g-k and 4n in moderate to good yields cleanly and exclu-
sively upon column chromatography on silica gel pre-
eluented with 10 drops of Et3N. Both 1-aryl-1,2-propdienyl
aryl and alkyl ketones may be prepared (Table 3). The
structure of the product 4i was further confirmed by its
X-ray diffraction study (Figure 2 in the Supporting
Information).9 When tertiary 2,3-allenol 1l was treated with
NBS followed by column chromatography on silica gel pre-
eluented with 10 drops of Et3N, 1,2-allenyl ketone 4l was
One-Pot Synthesis of 1-Aryl-1,2-allenyl Ketones. It is
interesting for us to observe that 1-aryl-2-halo-2-propenyl
(8) (a) For a report on the formation of 1,2-propadienyl methyl ketone via
the elimination of HBr from the Z/E mixture of 3-bromobut-3-en-2-one, see:
Buono, G. Synthesis 1981, 872. (b) For the formation of N-phenyl-2,3-
butadienamide from the elimination of N-phenyl-3-bromo-3-butenamide,
see: Zhao, Q.; Li, C. Org. Lett. 2008, 10, 4037.
(6) For a review of limitations in the synthesis of medium-sized ring
carbocycles, see: Majhi, T. P.; Achari, B.; Chattopadhyay, P. Heterocycles
2007, 71, 1011.
(7) Crystal data for 3l: C13H13OBr, MW = 265.14, monoclinic, space
group P2(1)/n, final R indices [I > 2σ(I)], R1 = 0.0204, wR2 = 0.0516, R
(9) Crystal data of 4i: C12H12O, MW = 172.22, monoclinic, space group
P2(1)/n, final R indices [I > 2σ(I)], R1 = 0.0373, wR2 = 0.0997, R indices (all
˚
indices (all data) R1 = 0.0216, wR2 = 0.0524, a = 12.7860(4) A, b =
˚ ˚
data) R1 = 0.0411, wR2 = 0.1044, a = 7.5594(3) A, b = 13.6697(5) A, c =
o 3
o
˚
˚
˚
˚
6.0794(2) A, c = 14.2698(4) A, R = 90°, β = 99.934(2) , γ = 90°, V =
1092.58(6) A , T = 173(2) K, Z = 4, reflections collected/unique 9937/1922
9.4364(3) A, R = 90°, β = 93.7300(10) , γ = 90°, V = 973.04(6) A , T =
173(2) K, Z = 4, reflections collected/unique 11007/1704 (Rint = 0.0170),
number of observations [>2σ(I)] 1535, parameters 126. File CCDC 736404
can be obtained free of charge from the CCDC via www.ccdc.cam.ak/conts/
retrieving.html.
3
˚
(Rint = 0.0389), number of observations [>2σ(I)] 1831, parameters 145. File
CCDC 713084 can be obtained free of charge from the CCDC via www.ccdc.
cam.ak/conts/retrieving.html.
J. Org. Chem. Vol. 74, No. 22, 2009 8735

Li et al.
JOCArticle
SCHEME 2
TABLE 3. Synthesis of 1-Aryl-1,2-allenyl Ketones 4 from Tertiary 2,3-
Allenols 1g-n
TABLE 4. Electrophilic Cyclization of 4-Substituted Tertiary
2,3-Allenols 1o-g with NBS
entry
R1
R2
R3
Me
Me
Me
Me
n-C5H11
n-C5H11
R4
isolated yield (%)
entry
R1
R2
isolated yield (%)
1
(CH2)4
(CH2)4
Ph
Ph
(CH2)4
(CH2)4
Me (1o)
Me (1o)
Me (1p)
Me (1p)
H (1q)
71 (5o)
67 (5o)
84 (5p)
69 (5p)
73 (5q)
61 (5q)
2a
3
1
2
3
4
5
6
Ph
Ph
CH3 (1g)
C2H5 (1h)
CH3 (1i)
CH3 (1j)
Ph (1k)
74 (4g)
77 (4h)
88 (4i)
89 (4j)
74 (4k)
47 (4n)
Me
Me
4a
5
m-CH3C6H4
p-CH3C6H4
Ph
6a,b
H (1q)
p-NO2C6H4
CH3 (1n)
aWith H2O as solvent. b1.2 equiv of NBS were used.
formed via the intermediacy of 3l (Scheme 1). However, this
allenyl ketone is unstable, thus it was further reduced with
LiAlH4 to form cyclic 2,3-allenol 1r in 51% combined yield
for the three-step procedure (Scheme 2).
bromonium A, which undergoes a 1,2-shift of the aryl
or alkyl group12 to open the strained three-membered
ring forming the cationic homoallylic alcohol interme-
diate B. Upon release of Hþ the reaction would form
2-bromoallylic ketones 3, which can easily be converted
to 1,2-allenyl ketones 4 via elimination of HBr due to the
presence of the electron-withdrawing carbonyl group and
the aryl group in 3 (path a).9 When R1 and R2 are both
alkyl groups, the 1,2-alkyl shift may be slower, and the
competitive reaction occurs to form the epoxide product
2 as a byproduct via the intramolecular attack of the
hydroxyl oxygen atom (path b). When R3 or R4 ¼ H, the
CdC bond at the 3-position of 2,3-allenols becomes more
electron rich and less sterically hindered (as compared to
the CdC bond at the 2-position) for 4-monosubstituted
ones and very electron rich for 4,4-disubstituted ones,
thus its reaction with “Xþ” would favorably form posi-
tively charged three-membered bromonium C, which
would be attacked by the intramolecular hydroxyl group
affording 3-bromo-2,5-dihydrofuran 5 as the only product
(Scheme 3, path c).10 Regarding the effect of H2O, it may
stabilize the allenyl bromonium species and its hydrogen
bonding with the hydroxyl group may slow down the
epoxide formation, while allowing the slower aryl/alkyl
Meerwein-Wagner 1,2-shift to compete.
Synthesis of 3-Bromo-2,5-dihydrofurans from Substituted
Tertiary 2,3-Allenols. However, this reaction could not be
extended to substrates with substituent(s) at the 4-position
of the starting 2,3-allenols: the reaction of 4-substituted
tertiary 2,3-allenols 1o-q with NBS failed to afford either
3-type ketones or 2-type epoxides. Instead, 3-bromo-
2,5-dihydrofurans 5o-q were formed as the only product
in high yields either in H2O (entries 2, 4, and 6, Table 4) or in
MeCN:H2O (15:1) (entries 1, 3, and 5, Table 4), indicating
that Brþ interacts with the CdC bond at the 3-position of
2,3-allenols exclusively (Table 4), which has been observed
by Marshall et al.10,11
Mechanistic Discussion. On the basis of these observa-
tions, it is concluded that the regioselectivity for the CdC
bond in the allene interacting with Brþ depends largely on
the electronic and steric effects of these two CdC bonds:3
When R3 = R4 = H, the interaction of “Xþ” with the
relatively electron-rich internal CdC bond of the allene
moiety would form the positively charged three-membered
(10) (a) Marshall, J. A.; Wang, X.-J. J. Org. Chem. 1990, 55, 2995. (b)
Marshall, J. A.; Wang, X.-J. J. Org. Chem. 1991, 56, 4913.
(11) For recent reports on Agþ or Auþ-catalyzed cycloisomerization
reactions of 2,3-allenols, see: (a) Mitasev, B.; Brummond, K. M. Synlett
€
2006, 3100. (b) Hoffmann-Roder, A.; Krause, N. Org. Biomol. Chem. 2005, 3,
387. (c) Wei, L.-L.; Xiong, H.; Hsung, R. P. Acc. Chem. Res. 2003, 36, 773.
(12) (a) Nakamura, K.; Osamura, Y. J. Am. Chem. Soc. 1993, 115, 9112.
(b) Hu, X.; Fan, C.; Zhang, F.; Tu, Y. Angew. Chem., Int. Ed. 2004, 43, 1702.
8736 J. Org. Chem. Vol. 74, No. 22, 2009

Li et al.
JOCArticle
SCHEME 3. Proposed Mechanism for the Formation of Allylic
Ketones 3, Epoxides 2, and 3-Bromo-2,5-dihydrofurans 5
ether (3 ꢀ 10 mL), washed with brine (10 mL), and dried over
anhydrous Na2SO4. Filtration and evaporation afforded the
crude product, which showed 60% of 3a and 8% of 2a with
CH2Br2 as the internal standard according to the 1H NMR
analysis. Column chromatography on silica gel pre-eluted with
pure petroleum ether then10 drops of AcOH (eluent: petroleum
ether/ethyl acetate = 30/1) afforded 3a (37.8 mg, 58%): oil; 1H
NMR (300 MHz, CDCl3) δ 5.70 (d, J = 1.8 Hz, 1 H), 5.58 (d,
J = 1.8 Hz, 1 H), 3.45 (dd, J1 = 10.8 Hz, J2 = 3.3 Hz, 1 H),
2.71-2.50 (m, 2 H), 2.09-1.73 (m, 5 H), 1.62-1.28 (m, 3 H); 13
C
NMR (75 MHz, CDCl3) δ 210.9, 131.6, 118.7, 62.2, 43.4, 30.8,
29.6, 28.3, 24.6; IR (neat) ν (cm-1) 2931, 2856, 1709, 1625, 1454,
1322, 1231, 1146; MS (70 eV, EI) m/z (%) 218 (Mþ(81Br), 26.3),
216 (Mþ(79Br), 26.6), 67 (100); HRMS calcd for C9H13O79Br
(Mþ) 216.0150, found 216.0150.
(2) 2-(10-Bromovinyl)cyclohexanone (3b). Typical Procedure 2.
To a reaction tube were added MeCN (4.0 mL), H2O (0.27 mL),
1-propadienylcyclopentanol 1b (37.8 mg, 0.30 mmol), and
NBS (54.2 mg, 0.30 mmol). After 2 h the starting material 1b
was completely consumed as indicated by TLC, and the
resulting mixture was quenched with a saturated aqueous
solution of Na2S2O3 (1 mL), extracted with ether (3 ꢀ 10
mL), washed with brine (10 mL), and dried over anhydrous
Na2SO4. Filtration and evaporation afforded the crude pro-
duct, which showed 55% of 3b with CH2Br2 as the internal
Conclusion
1
standard according to the H NMR analysis. Column chro-
In summary, it is concluded that the regioselectivity
referring to which CdC bond of the allene moiety in
tertiary 2,3-allenols would interact with NBS depends
on the steric and electronic effects: with terminal tertiary
2,3-allenols, the relatively electron-rich CdC bond at
the 2-position would react with NBS to form either
2-bromo-2-propenyl ketones or 1-bromovinyl epoxides
via 1,2-shift/Hþ-elimination or intramolecular attack of
the hydroxyl group, respectively. The formation of epoxi-
des may be reduced dramatically by conducting the reac-
tion in water. With 4-substituted tertiary allenols, the
substituent(s) at the 4-position increases the electron
richness of the CdC bond at the 3-position, thus it reacts
exclusively with NBS followed by intramolecular attack of
the hydroxyl group to form 3-bromo-2,5-dihydrofurans.
This electrophilic interaction of tertiary 2,3-allenols with
NBS is useful for the selective synthesis of 2-bromoallylic
ketones, 1,2-allenyl ketones, or 3-bromo-2,5-dihydro-
furans. Due to the synthetic potential of these products,
this methodology may be useful in organic synthesis.
Further investigation in this area is being intensively
carried out in our laboratory.
matography on silica gel pre-eluented with 10 drops of AcOH
(eluent: petroleum ether/ether = 10/1) afforded 3b (33.1 mg,
53%): oil; 1H NMR (300 MHz, CDCl3) δ 5.69-5.64 (m, 2 H),
3.34 (dd, J1 = 11.7 Hz, J2 = 5.4 Hz, 1 H), 2.54-2.43 (m, 1 H),
2.40-2.27 (m, 1 H), 2.25-2.13 (m, 1 H), 2.11-1.88 (m, 3 H),
1.82-1.62 (m, 2 H); 13C NMR (75 MHz, CDCl3) δ 207.3,
130.5, 119.4, 60.5, 41.9, 33.2, 27.2, 24.3; IR (neat) ν (cm-1
)
2940, 2865, 1716, 1630, 1449, 1425, 1297, 1204, 1129, 1069; MS
(70 eV, EI) m/z (%) 204 (Mþ(81Br), 30.4), 202 (Mþ(79Br), 31.3),
95 (100); HRMS calcd for C8H11O81Br (Mþ) 203.9973, found
203.9973.
Synthesis of 1,2-Allenyl Ketones 4g-n: 3-Phenylpenta-3,4-
dien-2-one (4g). Typical Procedure. The reaction of 2-phenyl-
penta-3,4-dien-2-ol 1g (46.2 mg, 0.29 mmol) and NBS (53.0 mg,
0.30 mmol) in H2O (4.0 mL) afforded 4g (33.6 mg, 74%)
(column chromatography on silica gel pre-eluted with pure
petroleum ether then 10 drops of Et3N (eluent: petroleum
ether/ethyl acetate = 20/1)): oil; 1H NMR (300 MHz, CDCl3)
δ 7.43-7.26 (m, 5 H), 5.45 (s, 2 H), 2.46 (s, 3 H); 13C NMR
(75 MHz, CDCl3) δ 218.1, 197.1, 131.6, 128.9, 128.2, 127.7,
110.7, 80.3, 28.0; IR (neat) ν (cm-1) 3057, 2980, 2927, 1948,
1922, 1684, 1599, 1494, 1447, 1415, 1357, 1277, 1232, 1133,
1074, 1019, 1002; MS (70 eV, EI) m/z (%) 159 (Mþ þ 1, 11.4),
158 (Mþ, 100); HRMS calcd for C11H10O (Mþ) 158.0732, found
158.0734.
Experimental Section
Synthesis of 3-Bromo-2,5-dihydrofurans 5o-q: 3-Bromo-2,2-
dimethyl-1-oxaspiro[4.4]non-3-ene (5o). Following typical pro-
cedure 2, the reaction of 1-(3-methylbuta-1,2-dienyl)cyclo-
pentanol 1o (45.8 mg, 0.30 mmol) and NBS (53.8 mg,
0.30 mmol) in a mixed solvent of MeCN (4.0 mL) and H2O
(0.27 mL) afforded 5o (49.5 mg, 71%) (eluent: petroleum ether/
ethyl ether = 50/1): oil; 1H NMR (300 MHz, CDCl3) δ 5.78 (s,
1 H), 1.85-1.50 (m, 8 H), 1.34 (s, 6 H); 13C NMR (75 MHz,
CDCl3) δ 131.5, 124.2, 95.7, 87.6, 39.9, 27.7, 24.2; IR (neat) ν
(cm-1) 2962, 2925, 2871, 1629, 1454, 1376, 1360, 1330, 1290,
1260, 1150, 1015; MS (70 eV, EI) m/z (%) 232 (Mþ(81Br), 7.04),
230 (Mþ(79Br), 7.04), 122 (100); HRMS calcd for C10H15O79Br
(Mþ) 230.0306, found 230.0309.
Synthesis of Starting Materials. Tertiary 2,3-allenols were
ꢀ
prepared from the modified Crabbe reaction of corresponding
propargyl alcohols,13 or the LiAlH4 reduction of corresponding
DHP-protected propargyl alcohols.14
Synthesis of r-Bromoallylic Ketones 3a-m: (1) 2-(10-
Bromovinyl)cycloheptanone (3a). Typical Procedure 1. To a
reaction vessel were added 1-propadienylcyclohexanol 1a (41.3
mg, 0.30 mmol), H2O (4.0 mL), and NBS (54.8 mg, 0.31 mmol).
After 2 h the starting material 1a was completely consumed as
indicated by TLC, and the resulting mixture was quenched with
a saturated aqueous solution of Na2S2O3 (1 mL), extracted with
ꢀ
ꢀ
(13) (a) Crabbe, P.; Fillion, H.; Andre, D.; Luche, J.-L. J. Chem. Soc.,
Chem. Commun. 1979, 859. (b) Kuan, J.; Ma, S. J. Org. Chem. 2009, 74, 1763.
(14) Cowie, J. S.; Landor, P. D.; Landor, S. R. J. Chem. Soc., Perkin
Trans. 1 1973, 720.
Acknowledgment. We gratefully acknowledge the State
Key Basic Research and Development Program of China
J. Org. Chem. Vol. 74, No. 22, 2009 8737

Li et al.
JOCArticle
(No. 2009CB825300). Shengming Ma is a Qiu Shi Adjunct
Professor at Zhejiang University. We thank Mr. Wangqing
Kong in our group for reproducing the results presented in
entry 14 in Table 1 and entries 5 and 11 in Table 2.
Supporting Information Available: Complete set of experi-
mental procedures and copies of 1H and 13C NMR spectra of all
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
8738 J. Org. Chem. Vol. 74, No. 22, 2009