A 2-pyrone cycloaddition route to functionalised aromatic boronic esters

Article abstract of DOI:10.1016/j.tet.2007.08.118

A [4+2] cycloaddition/retro-cycloaddition route to functionalised aromatic boronic esters is outlined. A range of electron deficient dienes (2-pyrones) and dienophiles (alkynylboronates) were found to participate in the reaction. Furthermore, high levels

Full text of DOI:10.1016/j.tet.2007.08.118

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Tetrahedron 64 (2008) 866e873
www.elsevier.com/locate/tet
A 2-pyrone cycloaddition route to functionalised
aromatic boronic esters
Patrick M. Delaney a, Duncan L. Browne a, Harry Adams a,
Andrew Plant b, Joseph P.A. Harrity a,
*
a Department of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK
b Research Chemistry, Syngenta, Jealott’s Hill International Research Centre, Bracknell, Berkshire, RG42 6EY, UK
Received 3 July 2007; accepted 24 August 2007
Available online 24 October 2007
Abstract
A [4þ2] cycloaddition/retro-cycloaddition route to functionalised aromatic boronic esters is outlined. A range of electron deficient dienes
(2-pyrones) and dienophiles (alkynylboronates) were found to participate in the reaction. Furthermore, high levels of regiocontrol could be
obtained in this process and a consistent mode of alkyne insertion has been uncovered.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
appropriate substrate bearing a boronate moiety, whereby
ring formation and instalment of the boronate were accom-
plished in a single operation. This concept has been put into
practise via cycloaddition reactions of alkynylboronates.5 To
date, metal-mediated6 and metal-catalysed reactions7 have
been developed, as well as techniques based on dipolar cyclo-
addition8 and [4þ2] cycloaddition9 processes.
Organoboron reagents are among the most attractive syn-
thetic intermediates in modern organic chemistry.1 These com-
pounds show good stability and their versatility provides the
opportunity to construct new compounds through various
carbonecarbon bond forming processes and functional group
interconversion reactions. Aromatic boronic acids and esters
have emerged as an important sub-class of these reagents.
The rich chemistry of organoboron compounds allows the in-
corporation of these aromatic motifs into a diverse range of
carbon-skeleta. Classically, these compounds have been pre-
pared from main group organometallics (usually Grignard or
organolithium reagents),2 or more recently via milder Pd-
catalysed CeB bond forming processes.3 A powerful alterna-
tive strategy exploits metal-catalysed CeH bond activation
processes.4 Whilst both of these routes deliver a diverse range
of benzenoid and heterocyclic boronates, they require an ap-
propriately functionalised aromatic precursor. A complemen-
tary strategy, therefore, would be the benzannulation of an
Our preliminary work in this area showed that hydroquinone
and quinone boronic esters could be prepared in good yield and
with excellent regiocontrol through the employment of a Dotz
¨
benzannulation reaction.6a,b Whilst this process allows highly
functionalised aromatic derivatives to be generated in a single
operation, it requires one equivalent of a chromium Fischer car-
bene complex. Accordingly, in an effort to avoid the need for
stoichiometric quantities of transition metal complex, we subse-
quently investigated a ‘metal free’strategy that involved a [4þ2]
cycloaddition of cyclopentadienones with alkynylboronates fol-
lowed by chelotropic extrusion of CO.9a Whilst this process also
delivered functionalised boronic esters in good overall yield, the
products were limited to heavily substituted polyaromatic com-
pounds. Therefore, our more recent efforts have focused on the
development of a more general protocol. In this context, we con-
sidered an alternative approach via the [4þ2] cycloaddition/
retro-cycloaddition of 2-pyrones as outlined in Figure 1.
* Corresponding author. Tel.: þ44 114 222 9496; fax: þ44 114 222 9346.
E-mail address: j.harrity@sheffield.ac.uk (J.P.A. Harrity).
0040-4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.08.118

P.M. Delaney et al. / Tetrahedron 64 (2008) 866e873
867
Table 1
Cycloaddition of 2-pyrones 1 and 2
O
R2
O
[4 + 2]
R2
O
R1
+
R1
O
O
MeO2C
MeO2C
O
B
R
B
B(OR)2
R
B(OR)2
O
O
5a-b
1 or 2
+
R2
O
170 °C, 15 h
R
B
-CO2
R1
O
6a,7a
6b,7b
B(OR)2
Entrya
2-Pyrone
R
Solvent
Yield (%) (a:b)
Figure 1. 2-Pyrone cycloaddition of alkynylboronates.
1
2
3
4
1
1
2
2
Me3Si; 5a
H; 5b
Neat
Ph2O
Neat
Ph2O
6; 19 (1:1)
7; 21 (1:5)
6; 0
We report herein the development of this concept into an effi-
cient method for generating functionalised aromatic boronic
esters.10,11
Me3Si; 5a
H; 5b
7; 0
a
Reactions run with 1e3 equiv of alkyne.
2. Results and discussion
reaction mixture to such high temperatures was resulting in
compound decomposition. Therefore, we screened a number
of solvents and found o-dichlorobenzene (DCB) to be optimal.
To our delight, the reaction of 3 and 5a proceeded in quantita-
tive yield when carried out at a similar temperature, albeit
again with no regiocontrol (entry 2). Cycloaddition of phenyl-
acetylene boronic ester 5c proceeded in high yield and with
excellent levels of regiocontrol (entries 3 and 4), whilst alkyne
5d showed modest levels of regioselectivity, albeit with an
analogous insertion pattern. Protected propargyl alcohol de-
rived alkynylboronate 5e underwent cycloaddition with poor
regiocontrol (entries 7 and 8) whereas terminal alkyne 5b
provided 12 in high yield and with good selectivity for
regioisomer 12b.13
The final class of ester substituted 2-pyrone 4 was exam-
ined next, our results are summarised in Table 3. Cycloaddi-
tion of alkyne 5a was found to proceed efficiently with
2-pyrone 4 to provide aromatic product 8 in high yield when
the reaction was heated as a neat mixture or in DCB. Moreover,
and in contrast to the cycloaddition with diene 3, we were
pleased to find that this cycloaddition exhibited some regiocon-
trol, providing modest selectivity for 8b (entries 1 and 2).
Our initial goal was to establish some preliminary data on
the reaction scope and regioselectivity, and so we opted to in-
vestigate the cycloaddition of a series of alkynylboronates
with isomeric 2-pyrones 1e4. We were particularly drawn to
these substrates because they were readily available,12 and
they provided a straightforward method for establishing the ef-
fect that an electron withdrawing ester group had on reaction
efficiency and regiocontrol, given that 1/2 and 3/4 provided
identical pairs of regioisomeric cycloadducts (Fig. 2).
Our investigations began with the cycloaddition reactions of
2-pyrones 1 and 2, and our results are summarised in Table 1.
Disappointingly, diene 1 was found to react sluggishly with
alkynes 5a,b providing the corresponding benzene boronic es-
ters in poor yield. Furthermore, the isomeric 2-pyrone 2 failed
to undergo cycloaddition with these compounds and starting
material was recovered, even after prolonged reaction times.
We next turned our attention to the cycloaddition of methyl
coumalate 3 and, as outlined in Table 2, were pleased to find
that this pyrone performed significantly better in these cyclo-
addition reactions. Heating a neat mixture of 3 and 5a at high
temperature afforded the desired aromatic product in good
yield but with no regiocontrol (entry 1). Examination of the
crude reaction mixtures suggested that exposing the neat
Table 2
Cycloaddition of methyl coumalate 3
O
CO2Me
O
B
R
MeO2C
+
R
B
O
O
O
MeO2C
O
MeO2C
O
O
5a-e
O
3
B
O
O
O
O
O
R
8a-12a
MeO2C
8b-12b
CO2Me
1
2
3
4
Entry
R
Conditionsa
Yield (%) (a:b)
1
2
3
4
5
6
7
8
9
a
Me3Si; 5a
Me3Si; 5a
Ph; 5c
A
B
A
B
A
B
A
B
8; 85 (1:1)
8; 100 (1:1)
9; 57 (14:1)
9; 75 (14:1)
10; 59 (3:1)
10; 80 (3:1)
11; 31 (1:1)
11; 60 (1:1)
12; 77 (1:5)b
CO2Me
B(OR)2
R2
B(OR)2
Ph; 5c
MeO2C
MeO2C
R2
n-Bu; 5d
n-Bu; 5d
BnOCH2; 5e
BnOCH2; 5e
H; 5b
B(OR)2
R2
B(OR)2
R2
Ph2O, 170 ꢀC
CO2Me
A: neat, 170 ꢀC, 15 h. B: o-dichlorobenzene, 180 ꢀC, 18 h.
Regioisomer a is identical to 7b.
b
Figure 2. Cycloadditions of isomeric 2-pyrones.

868
P.M. Delaney et al. / Tetrahedron 64 (2008) 866e873
Table 3
Cycloaddition of 2-pyrone 4
Table 4
Cycloaddition of 2-pyrone 13
O
O
O
Br
O
B
R
R
B
MeO2C
+
R
B
O
MeO2C
5a,c,d
O
O
5a-e
O
4
B
O
DCB, 180 °C, 18 h
O
MeO2C
R
8a-12a
8b-12b
13
Br
R
Br
O
B
Entry
R
Conditionsa
Yield (%) (a:b)
O
1
2
3
4
5
6
7
a
Me3Si; 5a
Me3Si; 5a
Ph; 5c
A
B
A
B
A
B
B
8; 70 (1:3)
8; 97 (1:3)
O
B
R
MeO2C
MeO2C
9; 42 (1:1)
9; 67 (1:1)
O
14a-16a
14b-16b
Ph; 5c
n-Bu; 5d
n-Bu; 5d
H; 5b
10; 24 (1:10)
10; 54 (1:10)
12; 72 (2:3)b,c
Entry
R
Yield (%) (a:b)
1
2
3
Me3Si; 5a
Ph; 5c
n-Bu; 5d
14; 77 (3:2)
15; 77 (>95:<5)
16; 82 (10:1)
A: neat, 170 ꢀC, 15 h. B: o-dichlorobenzene, 180 ꢀC, 18 h.
Regioisomer a is identical to 7b.
For this cycloaddition, 3.5 equiv of alkyne is used.
b
c
Alkyne 5c produced an equal mixture of boronic esters 9a and
9b in good yield when the reaction was conducted in DCB
(entries 3 and 4), whereas 1-hexyne derived alkynylboronate
5d was highly selective for the formation of 10b (entries 5
and 6). Finally, the terminal alkyne 5b provided the corre-
sponding cycloadduct in good yield but with poor regiocontrol
in this case (entry 7).
These preliminary studies suggested that 2-pyrones with
substituents in the 3- and 6-positions would be sluggish
towards cycloaddition (cf. dienes 1, 2 vs 3, 4), presumably
because of steric hinderance. Moreover, conjugation of the
electron withdrawing group with the enol ester moiety of the
2-pyrone appeared to provide enhanced reactivity (cf. 1 vs 2,
3 vs 4). In the context of regiochemistry, these investigations
also highlighted an interesting trend whereby those cycloaddi-
tions that were selective, followed a consistent pattern of al-
kyne insertion irrespective of the position of the ester group.
Specifically, substituted alkynylboronates underwent cycload-
dition with the regiochemistry highlighted in Figure 3. Finally,
the observation that 2-pyrone 3 generally favoured formation
of regioisomer a, whereas pyrone 4 was more generally selec-
tive for b demonstrates that this strategy offers access to differ-
ent product regioisomers by judicious choice of diene
component.14
silylalkyne 5a underwent smooth cycloaddition to provide
14, unfortunately however, with poor levels of regiocontrol
(entry 1). More promising was the reaction of 5c with 13,
which provided the corresponding boronic ester 15 as a single
regioisomer in 77% yield (entry 2). Finally, butyl-substituted
boronate 5d was also found to undergo efficient and selective
cycloaddition, moreover, with an identical regiochemical in-
sertion pattern to that of the Ph-substituted alkyne predominat-
ing (entry 3).16
Having explored the cycloaddition efficiency with regard to
the diene component, we decided to briefly explore the effects
of changing the boronic ester moiety of the dienophile. As
shown in Figure 4, we envisaged that the electronic nature
of the boronate unit would be affected by the ability of the
flanking O-atoms to donate electron density to the vacant p-
orbital on boron. Specifically, the angle strain imposed by
having contiguous OeBeO sp2-centres may be better accom-
modated in a six-membered ring than in a five-membered ring.
Given that recent studies in our labs have suggested that alky-
nylboronates act as electron rich dieneophiles in [4þ2] cyclo-
additions,17 the donor/acceptor ability of the boronate could
have an effect on reactivity as well as regioselectivity.
In order to gather evidence for this effect, we prepared al-
kynes 17e18 (Fig. 5). Compounds 5c and 17 were found to be
crystalline solids and so we undertook X-ray crystallographic
The apparent increased reactivity of 2-pyrones bearing an
ester group in the 5-position suggested that smaller substitu-
ents might be incorporated at C-3 or C-6 without significantly
affecting the cycloaddition efficiency. In an effort to test this
idea, we prepared pyrone 1315 and examined its reactivity
with our alkynylboronate substrates. As outlined in Table 4,
O
()n
C
C
B
O
Figure 4. Boronate donor ability.
R
O
MeO2C
O
B
O
O
O
O
O
O
Ph
B
Ph
B
R=TMS, Bu, Ph
17
18
Figure 5. Six-membered ring boronates.
Figure 3. Cycloaddition regiochemistry.

P.M. Delaney et al. / Tetrahedron 64 (2008) 866e873
869
results for the cycloaddition of 5c are included for direct com-
parison). In general, we found the six-membered boronic
esters to be quite prone to protodeboronation at high temper-
ature; moreover, they were relatively sluggish towards cyclo-
addition. Nonethless, the corresponding aromatic products
19 and 20 could be obtained in acceptable yields. Interest-
ingly, however, an increase in product regioselectivity was
obtained in these reactions by comparison to the cycloaddition
of compound 5c. Whilst we have insufficient data to make
general conclusions at this stage, it would appear that the
cycloaddition of alkynylboronates bearing various boronic
ester moieties may provide a further means by which the
regioselectivity of these reactions can be optimised. Studies
towards this end are currently underway and will be reported
in due course.
ꢀ
˚
Figure 6. ORTEP diagram of 5c. Selected bond distances [A] and angles [ ]:
C(1)eC(2) 1.201(3), C(1)eB(1) 1.535(3), B(1)eO(1) 1.358(2), B(1)eO(2)
1.362(2); O(1)eB(1)eO(2) 114.84(17).
3. Conclusion
The [4þ2] cycloaddition of 2-pyrones with alkynylboro-
nates provides an efficient route to functionalised boronic es-
ters with good to excellent levels of regiocontrol in many
cases. Moreover, reaction regioselectivity was found to vary
according to the position of electron withdrawing groups on
the diene substrate. This observation suggests therefore that
cycloaddition regiochemistry can, to some extent, be con-
trolled by judicious positioning of electron deficient substitu-
ents on the 2-pyrone substrate. Finally, alternative boronic
ester moieties were also found to participate in the cycloaddi-
tion process and may provide a further means to controlling
reaction regioselectivity.
ꢀ
˚
Figure 7. ORTEP diagram of 17. Selected bond distances [A] and angles [ ]:
C(7)eC(8) 1.205(5), C(8)eB(1) 1.541(6), B(1)eO(1) 1.339(5), B(1)eO(2)
1.364(5); O(1)eB(1)eO(2) 124.3(4).
analysis of these alkynes to compare the relative bond lengths
and angles around the boronate unit in each case, selected data
are provided in Figures 6 and 7.18 A striking difference in
these structures was that 17 showed two quite different BeO
bond lengths whereas those of 5c are equal (within experi-
mental error). The generally shorter BeO bonds in 17 appear
to suggest greater donation from O/B with increased Oe
BeO bond angle and increased alkyne CeB bond length
relative to 5c.
4. Experimental
4.1. General
In the context of cycloadditions, the reaction of alkynes 17
and 18 with methyl coumalate was performed in the absence
of solvent and our results are highlighted in Table 5 (the
Alkynylboronates 5 were prepared from the corresponding
terminal alkynes by the method of Brown.19 Alkynes 1720 and
18 were prepared from the corresponding alkynyltrifluorobo-
rate salts by the method of Vaultier21 and Yamamoto.7g Pyra-
nones 1,12 412 and 1315 were prepared according to literature
procedures.
Table 5
Cycloaddition of methyl coumalate 3
Ph
B(OR)2
Ph
MeO2C
MeO2C
Ph
B(OR)2
+
3
4.2. General procedure for the [4þ2] cycloaddition
B(OR)2
neat, 180 °C, 18 h
reaction of pyranone with alkynylboronates
9a, 19a, 20a
9b, 19b, 20b
Entry
1
B(OR)2
Yield (%) (a:b)
19; 58 (20:1)
4.2.1. Methyl 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-3-(trimethylsilyl)benzoate (6a) and methyl 3-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2-
O
O
17
B
(trimethylsilyl)benzoate (6b)
A mixture of pyranone (1) (0.173 g, 1.12 mmol) and
4,4,5,5-tetramethyl-2-trimethylsilanylethynyl-1,3,2-dioxaboro-
lane (5a) (0.755 g, 3.37 mmol) was heated at 170 ꢀC and
stirred for 15 h under N2. The resulting brown solution was
cooled to room temperature and purified by flash column chro-
matography (eluting solvent petroleum ether/ethyl acetate
50:1 ratio) to give the title compounds (6a) and (6b) (1:1 ratio)
O
18
5c
2
B
B
20; 64 (20:1)
9; 57 (14:1)a
O
O
O
3
a
Reaction runs at 170 ꢀC, 15 h.

870
P.M. Delaney et al. / Tetrahedron 64 (2008) 866e873
as a colourless solid, 0.071 g, 19%. 1H NMR (250 MHz,
CDCl3): d 0.25 (9H, s, Si(CH3)3), 0.39 (9H, s, Si(CH3)3),
1.33 (12H, s, 4ꢁCH3), 1.49 (12H, s, 4ꢁCH3), 3.85 (3H, s,
CO2CH3), 3.89 (3H, s, CO2CH3), 7.31e7.35 (2H, m, Ar-H ),
7.67e7.71 (2H, m, Ar-H ), 7.81 (1H, dd, J¼7.5, 1.0 Hz,
Ar-H ), 7.91 (1H, dd, J¼7.5, 1.5 Hz, Ar-H ); 13C NMR
(62.9 MHz, CDCl3) d 0.4, 1.7, 25.2, 26.3, 52.1, 52.2,
84.2 (ꢁ2), 127.3 (ꢁ2), 127.5 (ꢁ2), 129.0 (ꢁ2), 130.1
(ꢁ2), 137.0, 137.8, 167.4, 167.5. FTIR: nmax/CHCl3,
(w) cmꢂ1. HRMS (EIþ) calcd for C17H27O4BSi: 335.1850.
Found: 335.1857.
4.2.4. Methyl 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)biphenyl-4-carboxylate (9a) and methyl 6-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)biphenyl-3-carb-
oxylate (9b)
A mixture of (3) (0.1 g, 0.649 mmol) and (5c) (0.296 g,
1.298 mmol) in o-dichlorobenzene (0.649 ml) was heated at
180 ꢀC for 18 h under N2. The product was purified by flash
column chromatography (eluting solvent petroleum ether/ethyl
acetate 25:1 ratio) to give compounds (9a) and (9b) (14:1 ra-
tio) as an inseparable mixture of regioisomers as a colourless
oil, 0.164 g, 75%. Compound (9a): 1H NMR (250 MHz,
CDCl3): d 1.21 (12H, s, 4ꢁCH3), 3.92 (3H, s, CO2CH3),
7.35e7.48 (5H, m, Ar-H ), 7.56 (1H, d, J¼8.0 Hz, Ar-H ),
7.98 (1H, dd, J¼8.0, 1.5 Hz, Ar-H ), 8.04 (1H, dd, J¼1.0,
0.5 Hz, Ar-H ); 13C NMR (62.9 MHz, CDCl3) d 24.7, 52.2,
84.1, 127.0, 127.2, 127.5, 128.0, 129.0, 129.1, 135.7, 142.0,
147.9, 167.0. FTIR: nmax/CHCl3, 2991 (w), 2979 (w), 2940
(w), 1723 (s), 1600 (w) cmꢂ1. HRMS (EIþ) calcd for
C20H23O4B: 338.1689. Found: 338.1687. Compound (9b):
1H NMR (250 MHz, CDCl3): d 1.23 (12H, s, 4ꢁCH3), 3.95
(3H, s, CO2CH3), 7.35e7.48 (6H, m, Ar-H ), 8.12 (1H,
dd, J¼8.0, 2.0 Hz, Ar-H ), 8.38 (1H, dd, J¼2.0, 0.5 Hz,
Ar-H ); 13C NMR (62.9 MHz, CDCl3) d 24.6, 52.1, 84.0,
127.0, 127.5, 127.9, 128.9, 129.0, 130.0, 135.6, 142.1,
151.9, 167.0.
2977 (m), 2953 (m), 2900 (w), 1577 (w), 1558 (w) cmꢂ1
.
HRMS (ESþ) calcd for C17H27O4BSiNa: 357.1669. Found:
357.1657.
4.2.2. Methyl 3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)benzoate (7b)
A solution of pyranone (1) (0.100 g, 0.65 mmol) and 2-
ethynyl-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane (5b) (0.09 g,
0.65 mmol) in diphenylether (1 ml) was heated at 170 ꢀC
and stirred for 15 h under N2. The resulting brown solution
was cooled to room temperature and purified by flash column
chromatography (eluting solvent petroleum ether/ethyl ace-
tate 50:1 ratio) to give the title compounds (7a) and (7b)
(1:5 ratio) as a colourless solid, 0.036 g, 21%. Compound
1
(7b): H NMR (250 MHz, CDCl3): d 1.28 (12H, s, 4ꢁCH3),
3.84 (3H, s, CO2CH3), 7.38 (1H, dd, J¼8.0, 8.0 Hz, Ar-H ),
7.90 (1H, ddd, J¼8.0, 1.0, 1.0 Hz, Ar-H ), 8.00 (1H, ddd,
J¼8.0, 1.0, 1.0 Hz, Ar-H ), 8.05 (1H, br, Ar-H ); 13C NMR
(62.9 MHz, CDCl3) d 24.9, 52.0, 84.1, 127.8, 129.6, 132.3,
135.8, 139.2, 167.2. FTIR: nmax/CHCl3, 2976 (w), 1718
(s) cmꢂ1. HRMS (EIþ) calcd for C14H19BO4: 262.1376.
Found: 262.1384.
4.2.5. Methyl 4-butyl-3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)benzoate (10a) and methyl 3-butyl-4-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
benzoate (10b)
4.2.3. Methyl 3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-4-(trimethylsilyl)benzoate (8a) and methyl 4-
A mixture of (3) (0.1 g, 0.649 mmol) and (5d) (0.270 g,
1.298 mmol) in o-dichlorobenzene (0.65 ml) was heated at
180 ꢀC for 18 h under N2. The product was purified by flash
column chromatography (eluting solvent petroleum ether/ethyl
acetate 25:1 ratio) to give compounds (10a) and (10b) (3:1 ra-
tio) as an inseparable mixture of regioisomers as a colourless
oil, 0.172 g, 80%. Compound (10a): 1H NMR (250 MHz,
CDCl3): d 0.85 (3H, t, J¼7.5 Hz, CH2CH3), 1.28 (12H, s,
4ꢁCH3), 1.22e1.37 (2H, m, CH2CH3), 1.38e1.56 (2H, m,
CH2CH2CH3), 2.85 (2H, app t, J¼8.0 Hz, C]CCH2), 3.89
(3H, s, CO2CH3), 7.79 (2H, s, Ar-H ), 7.81 (1H, s, Ar-H );
13C NMR (62.9 MHz, CDCl3) d 13.9, 22.7, 24.8, 35.2, 35.4,
52.0, 83.8, 125.6, 129.9, 131.8, 135.8, 150.2, 167.3. FTIR:
nmax/CHCl3, 2977 (m), 2956 (m), 2931 (m), 2870 (m), 1723
(s) cmꢂ1. HRMS (EIþ) calcd for C18H27O4B: 318.2002.
Found: 318.2012. Compound (10b): 1H NMR (250 MHz,
CDCl3): d 0.85 (3H, t, J¼7.5 Hz, CH2CH3), 1.28 (12H, s,
4ꢁCH3), 1.22e1.37 (2H, m, CH2CH3), 1.38e1.56 (2H, m,
CH2CH2CH3), 2.85 (2H, app t, J¼8.0 Hz, C]CCH2), 3.88
(3H, s, CO2CH3), 7.16 (1H, d, J¼8.0 Hz, Ar-H ), 7.92 (1H,
dd, J¼8.0, 2.0 Hz, Ar-H ), 8.34 (1H, d, J¼2.0 Hz, Ar-H );
13C NMR (62.9 MHz, CDCl3) d 13.9, 22.7, 24.8, 35.3, 35.6,
51.9, 83.7, 126.8, 129.3, 131.8, 137.2, 155.5, 167.3.
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-3-
(trimethylsilyl)benzoate (8b)
A mixture of (3) (0.1 g, 0.65 mmol) and (5a) (0.291 g,
1.3 mmol) in o-dichlorobenzene (0.65 ml) was heated at
180 ꢀC for 18 h under N2. The product was purified by flash
column chromatography (eluting solvent petroleum ether/ethyl
acetate 25:1 ratio) to give the separated title compounds (8a)
as a colourless oil, 0.108 g, 50% and (8b) as a colourless
oil, 0.109 g, 50%. Compound (8a): 1H NMR (250 MHz,
CDCl3): d 0.36 (9H, s, SiMe3), 1.35 (12H, s, CH3), 3.91
(3H, s, CO2CH3), 7.96 (2H, m, Ar-H ), 8.24 (1H, d,
J¼1.5 Hz, Ar-H ); 13C NMR (62.9 MHz, CDCl3) d 0.4, 25.0,
52.1, 84.2, 128.5, 130.5, 134.8, 135.9, 147.3, 167.5. FTIR:
nmax/CHCl3, 2979 (m), 2952 (w), 1726 (s), 1595 (w), 1548
(w) cmꢂ1. HRMS (EIþ) calcd for C17H27O4BSi: 335.1850.
Found: 335.1839. Compound (8b): 1H NMR (250 MHz,
CDCl3): d 0.29 (9H, s, SiMe3), 1.29 (12H, s, CH3), 3.86
(3H, s, CO2CH3), 7.62 (1H, d, J¼8.0 Hz, Ar-H ), 7.93 (1H,
dd, J¼8.0, 1.5 Hz, Ar-H ), 8.43 (1H, d, J¼1.5 Hz, Ar-H );
13C NMR (62.9 MHz, CDCl3) d 0.8, 25.0, 52.0, 84.1, 129.7,
130.2, 134.4, 136.4, 153.3, 167.4. FTIR: nmax/CHCl3, 2979
(m), 2951 (w), 2901 (w), 1726 (s), 1593 (w), 1551

P.M. Delaney et al. / Tetrahedron 64 (2008) 866e873
871
4.2.6. Methyl 4-(benzyloxymethyl)-3-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)benzoate (11a) and methyl 3-
(benzyloxymethyl)-4-(4,4,5,5-tetramethyl-1,3,2-
4.2.10. Methyl 4-butyl-3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)benzoate (10a) and methyl 3-butyl-4-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzoate (10b)
dioxaborolan-2-yl)benzoate (11b)
A mixture of (4) (0.050 g, 0.325 mmol) and (5c) (0.135 g,
0.649 mmol) in o-dichlorobenzene (0.65 ml) was heated at
180 ꢀC for 36 h under N2. The product was purified by flash
column chromatography (eluting solvent petroleum ether/ethyl
acetate 25:1 ratio) to give compounds (8a) and (8b) (1:10
ratio) as a clear oil, 0.056 g, 54%.
A mixture of (3) (0.1 g, 0.649 mmol) and (5e) (0.353 g,
1.298 mmol) in o-dichlorobenzene (0.65 ml) was heated at
180 ꢀC for 18 h under N2. The product was purified by flash col-
umn chromatography (eluting solvent petroleum ether/ethyl
acetate 25:1 ratio) to give compounds (11a) and (11b) (1:1 ratio)
as a clear oil, 0.149 g, 60%. 1H NMR (250 MHz, CDCl3): d 1.25
(24H, s, CH3), 3.93 (6H, s, CO2CH3), 4.61 (4H, m, ArCH2O),
4.84 (2H, s, ArCH2O), 4.89 (2H, s, ArCH2O), 7.29e7.43
(10H, m, Ar-H ), 7.64 (1H, d, J¼8.0 Hz, Ar-H ), 7.85 (1H, d,
J¼8.0 Hz, Ar-H ), 7.90 (1H, dd, J¼8.0, 2.0 Hz, Ar-H ), 8.11
(1H, dd, J¼8.0, 1.5 Hz), 8.46e8.47 (1H, m, Ar-H ), 8.46 (1H,
d, J¼2.0 Hz, Ar-H ); 13C NMR (62.9 MHz, CDCl3) d 24.8ꢁ2,
51.9, 52.1, 71.3ꢁ2, 72.6ꢁ2, 83.9, 84.0, 127.2ꢁ2, 127.5,
127.6ꢁ2, 127.8, 127.9ꢁ2, 128.3, 128.4ꢁ2, 128.6, 132.0,
4.2.11. Methyl 3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)benzoate (7b) and methyl 4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)benzoate (12b)3a
A mixture of (4) (0.050 g, 0.325 mmol) and (5b) (0.172 g,
1.136 mmol) in o-dichlorobenzene (0.325 ml) was heated at
180 ꢀC for 18 h under N2. The product was purified by flash
column chromatography (eluting solvent petroleum ether/ethyl
acetate 25:1 ratio) to give compounds (7b) and (12b)3a (2:3
ratio) as a colourless crystalline solid, 0.061 g, 72%.
132.1, 135.5, 136.7, 144.7, 144.9, 167.1, 167.2. FTIR: nmax
/
CHCl3, 2993 (w), 2972 (w), 1725 (s), 1640 (w) cmꢂ1. HRMS
(EIþ) calcd for C22H27BO5: 382.1952. Found: 382.1949.
4.2.12. Methyl 3-bromo-5-(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)-4-trimethylsilyl benzoate (14 major) and methyl
3-bromo-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-5-(tri-
methylsilyl)benzoate (14 minor)
4.2.7. Methyl 3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)benzoate (7b) and methyl 4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)benzoate (12b)3a
A mixture of (13) (0.05 g, 0.215 mmol) and (5a) (0.096 g,
0.429 mmol) in o-dichlorobenzene (0.215 ml) was heated at
180 ꢀC for 18 h under N2. The product was purified by flash
column chromatography containing a plug of 10% AgNO3 im-
pregnated silica (eluting solvent petroleum ether/ethyl acetate
25:1 ratio) to give compounds (14a) and (14b) (3:2 ratio) as
a colourless crystalline solid, 0.068 g, 77%. 1H NMR
(250 MHz, CDCl3): d 0.36 (5.4H, s, SiCH3), 0.47 (3.6H, s,
SiCH3), 1.37 (4.8H, s, CCH3), 1.46 (7.2H, s, CCH3), 3.90
(3H, s, COCH3), 8.08e8.16 (2H, m, Ar-H ); 13C NMR
(62.9 MHz, CDCl3) d 0.0, 1.7, 25.3, 25.9, 25.3ꢁ2, 84.5,
85.1, 127.8, 130.8, 131.0, 131.5, 132.5, 132.7, 132.8, 134.6,
147.5, 149.8, 166.1ꢁ2. FTIR: nmax/CHCl3, 2952 (w), 1729
(s), 1338 (m) cmꢂ1. HRMS (ESþ) calcd for C17H27BBrO4Si:
413.0955. Found: 413.0974.
A mixture of (3) (0.1 g, 0.649 mmol), (5b) (0.09 g,
0.649 mmol) and diphenylether (1 ml) was heated at 170 ꢀC
with stirring, for 15 h under a nitrogen atmosphere. The product
was purified by flash column chromatography (eluting solvent
petroleum ether/EtOAc 100:1) to give compounds (12b) and
(7b) (5:1 ratio) as a colourless crystalline solid, 0.130 g, 77%.
Compound (12b):3a 1H NMR (250 MHz, CDCl3): d 1.28
(12H, s, CH3), 3.84 (3H, s, CH3OCO), 7.79 (2H, d, J¼8.5 Hz,
Ar-H ), 7.96 (2H, d, J¼8.5 Hz, Ar-H ); 13C NMR (62.9 MHz,
CDCl3) d 24.8, 52.1, 84.1, 127.8, 135.8, 134.6, 167.1.
4.2.8. Methyl 3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-4-(trimethylsilyl)benzoate (8a) and methyl 4-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-3-
(trimethylsilyl)benzoate (8b)
A mixture of (4) (0.05 g, 0.325 mmol) and (5a) (0.145 g,
0.649 mmol) in o-dichlorobenzene (0.325 ml) was heated at
180 ꢀC for 15 h under N2. The product was purified by flash
column chromatography (eluting solvent petroleum ether/ethyl
acetate 25:1 ratio) to give (6a) and (6b) (1:3 ratio) as colour-
less oils, 0.107 g, 97%.
4.2.13. Methyl 2-bromo-6-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)biphenyl-4-carboxylate (15)
A mixture of (13) (0.05 g, 0.215 mmol) and (5c) (0.098 g,
0.429 mmol) in o-dichlorobenzene (0.215 ml) was heated at
180 ꢀC for 18 h under N2. The product was purified by flash
column chromatography containing a plug of 10% AgNO3 im-
pregnated silica (eluting solvent petroleum ether/ethyl acetate
25:1 ratio) to give compound (15) as a colourless crystalline
4.2.9. Methyl 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)biphenyl-4-carboxylate (9a) and methyl 6-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)biphenyl-3-carboxylate
(9b)
A mixture of (4) (0.05 g, 0.324 mmol) and (2a) (0.148 g,
0.649 mmol) in o-dichlorobenzene (0.325 ml) was heated at
180 ꢀC for 36 h under N2. The product was purified by flash
column chromatography (eluting solvent petroleum ether/ethyl
acetate 25:1 ratio) to give (9a) and (9b) (1:1 ratio) as a clear
oil, 0.074 g, 67%.
1
solid, 0.069 g, 77%. Mp 109e111 ꢀC. H NMR (250 MHz,
CDCl3): d 1.01 (12H, s, CH3), 3.85 (3H, s, CH3OCO), 7.30
(2H, m, Ar-H ), 7.71 (3H, m, Ar-H ), 8.17 (1H, d, J¼2.0 Hz,
Ar-H ), 8.28 (1H, d, J¼2.0 Hz, Ar-H ); 13C NMR (62.9 MHz,
CDCl3) d 24.9, 52.6, 82.1, 122.9, 128.5, 128.8, 129.5, 130.9,
131.6, 134.7, 140.6, 147.4, 166.1. FTIR: nmax/CHCl3, 2978
(w), 1727 (s) cmꢂ1. HRMS (EIþ) calcd for C20H22BBrO4:
417.0873. Found: 417.0886.

872
P.M. Delaney et al. / Tetrahedron 64 (2008) 866e873
4.2.14. Methyl 3-bromo-5-(4,4,5,5-tetramethyl-1,3,2-
References and notes
dioxaborolan-2-yl)-4-butyl benzoate (16)
1. Boronic Acids; Hall, D. G., Ed.; Wiley-VCH: Weinheim, 2005.
2. (a) Gilman, H.; Moore, L. O. J. Am. Chem. Soc. 1958, 80, 3609; (b) Wong,
K. T.; Chien, Y. Y.; Liao, Y. L.; Lin, C. C.; Chou, M. Y.; Leung, M. K.
J. Org. Chem. 2002, 67, 1041.
A mixture of (13) (0.05 g, 0.215 mmol) and (5d) (0.089 g,
0.429 mmol) in o-dichlorobenzene (0.215 ml) was heated at
180 ꢀC for 18 h under N2. The product was purified by flash
column chromatography containing a plug of 10% AgNO3 im-
pregnated silica (eluting solvent petroleum ether/ethyl acetate
25:1 ratio) to give compound (16) (10:1 ratio) as a clear oil,
0.070 g, 82%. 1H NMR (250 MHz, CDCl3): d 0.90e0.95
(3H, t, J¼8.0 Hz, CH2CH3), 1.23 (12H, s, CCH3), 1.25e
1.38 (4H, m, CH2CH2CH3), 3.04 (2H, t, J¼8.0 Hz,
ArCH2CH2), 3.88 (3H, s, CO2CH3), 8.18 (1H, d, J¼2 Hz,
Ar-H ), 8.28 (1H, d, J¼2 Hz, Ar-H ); 13C NMR (62.9 MHz,
CDCl3) d 13.9, 23.0, 24.8, 33.2, 35.1, 52.2, 84.0, 125.4,
128.4, 136.0, 136.2, 153.7, 166.0. FTIR: nmax/CHCl3, 2956
(w), 1728 (s), 1596 (m), 1338 (m) cmꢂ1. HRMS (ESþ) calcd
for C18H27BBrO4: 397.1186. Found: 397.1178.
3. (a) Ishiyama, T.; Murata, M.; Miyaura, N. J. Org. Chem. 1995, 60, 7508;
(b) Ishiyama, T.; Ishida, K.; Miyaura, N. Tetrahedron 2001, 57, 9813.
4. (a) Boller, T. M.; Murphy, J. M.; Hapke, M.; Ishiyama, T.; Miyaura, N.;
Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 14263; (b) Coventry, D. N.;
Bastanov, A. S.; Goeta, A. E.; Howard, J. A. K.; Marder, T. B.; Perutz,
R. N. Chem. Commun. 2005, 2172; (c) Cho, J. Y.; Iverson, C. N.; Smith,
M. R., III. J. Am. Chem. Soc. 2000, 122, 12868.
5. For recent overviews, see: (a) Hilt, G.; Bolze, P. Synthesis 2005, 2091; (b)
Gandon, V.; Aubert, C.; Malacria, M. Chem. Commun. 2006, 2209.
6. (a) Davies, M. W.; Johnson, C. N.; Harrity, J. P. A. Chem. Commun. 1999,
2107; (b) Davies, M. W.; Johnson, C. N.; Harrity, J. P. A. J. Org. Chem.
2001, 66, 3525; (c) Gandon, V.; Leca, D.; Aechtner, T.; Vollhardt, K. P. C.;
Malacria, M.; Aubert, C. Org. Lett. 2004, 6, 3405; (d) Gandon, V.;
Leboeuf, D.; Amslinger, S.; Vollhardt, K. P. C.; Malacria, M.; Aubert,
C. Angew. Chem., Int. Ed. 2005, 44, 7114; (e) Geny, A.; Leboeuf, D.;
´
Rouquie, G.; Vollhardt, K. P. C.; Malacria, M.; Gandon, V.; Aubert, C.
Chem.dEur. J. 2007, 13, 5408.
4.2.15. 2-(4,4,6-Trimethyl-1,3,2-dioxaborinan-2-yl)-
biphenyl-4-carboxylic acid methyl ester (19)
7. (a) Ester, C.; Maderna, A.; Pritzkow, H.; Siebert, W. Eur. J. Inorg. Chem.
2000, 1177; (b) Hilt, G.; Smolko, K. I. Angew. Chem., Int. Ed. 2003, 42,
2795; (c) Hilt, G.; Luers, S.; Smolko, K. I. Org. Lett. 2005, 7, 251; (d)
Hilt, G.; Hess, W.; Schmidt, F. Eur. J. Org. Chem. 2005, 2526; (e) Yama-
moto, Y.; Ishii, J.-I.; Nishiyama, H.; Itoh, K. J. Am. Chem. Soc. 2004, 126,
3712; (f) Yamamoto, Y.; Ishii, J.-I.; Nishiyama, H.; Itoh, K. Tetrahedron
2005, 61, 11501; (g) Yamamoto, Y.; Hattori, K.; Ishii, J.-I.; Nishiyama, H.
Tetrahedron 2006, 62, 4294; (h) Yamamoto, Y.; Hattori, K.; Ishii, J.-I.;
Nishiyama, H.; Itoh, K. Chem. Commun. 2005, 4438.
A mixture of 3 (0.15 g, 0.974 mmol) and 17 (0.444 g,
1.948 mmol) was heated at 180 ꢀC for 18 h. The resulting brown
solution was cooled to room temperature and purification by
flash column chromatography (eluting solvent petroleum
ether/ethyl acetate 25:1 ratio) gave (19a) and (19b) (20:1 ratio)
1
as a yellow oil, 0.191 g, 58%. H NMR (250 MHz, CDCl3):
d 0.82e0.95 (9H, m, CH3), 1.10e1.2 (2H, m, CCH2CH), 3.86
(3H, s, CO2CH3), 4.20e4.35 (1H, m, CH2CHCH3), 7.30e7.42
(6H, m, Ar-H ), 8.04 (1H, dd, J¼8.0, 2.0 Hz, Ar-H ), 8.30 (1H,
d, J¼2.0 Hz, Ar-H ); 13C NMR (62.9 MHz, CDCl3) d 22.8,
27.8, 30.7, 45.7, 52.0, 65.3, 71.5, 127.1, 127.9, 128.7, 128.9,
130.2, 134.8, 143.2, 151.1, 167.4. FTIR: nmax/CHCl3, 2974
(m), 1721 (s), 1598 (m), 1384 (m) cmꢂ1. HRMS (EIþ) calcd
for C20H23BO4: 338.2052. Found: 338.2054.
8. (a) Bianchi, G.; Cogoli, A.; Grunanger, P. J. Organomet. Chem. 1966, 6,
¨
598; (b) Matteson, D. S. J. Org. Chem. 1962, 27, 4293; (c) Davies, M. W.;
Wybrow, R. A. J.; Johnson, C. N.; Harrity, J. P. A. Chem. Commun. 2001,
1558; (d) Moore, J. E.; Goodenough, K. M.; Spinks, D.; Harrity, J. P. A.
Synlett 2002, 2071; (e) Moore, J. E.; Davies, M. W.; Goodenough, K. M.;
Wybrow, R. A. J.; York, M.; Johnson, C. N.; Harrity, J. P. A. Tetrahedron
2005, 61, 6707.
9. (a) Moore, J. E.; York, M.; Harrity, J. P. A. Synlett 2005, 860; (b) Sato, S.;
Isobe, H.; Tanaka, T.; Ushijima, T.; Nakamura, E. Tetrahedron 2005, 61,
11449; (c) Helm, M. D.; Moore, J. E.; Plant, A.; Harrity, J. P. A. Angew.
Chem., Int. Ed. 2005, 44, 3889.
4.2.16. 2-(5,5-Dimethyl-1,3,2-dioxaborinan-2-yl)-
biphenyl-4-carboxylic acid methyl ester (20a)
10. For our preliminary account of this work, see: Delaney, P. M.; Moore,
J. E.; Harrity, J. P. A. Chem. Commun. 2006, 3323.
A mixture of 3 (0.2 g, 1.298 mmol) and 18 (566 mg,
2.6 mmol) was heated at 180 ꢀC for 18 h. The resulting brown
solution was cooled to room temperature and purification by
flash column chromatography (eluting solvent petroleum
ether/ethyl acetate 25:1 ratio) to give compounds (20a) and
11. For the cycloaddition of alkynylstannanes with 2-pyrones, see: (a) Evnin,
A. B.; Seyferth, D. J. Am. Chem. Soc. 1967, 89, 952; (b) Seyferth, D.;
White, D. J. Organomet. Chem. 1972, 34, 119.
12. Pyrones 2 and 3 were purchased from commercial suppliers, compounds 1
and 4 were prepared according to literature procedures. Compound 1: Dun-
kelblum, A. Helv. Chim. Acta 1970, 53, 2159; Compound 4: Haneda, A.;
Uenakai, H.; Imagawa, T.; Kawanisi, M. Synth. Commun. 1976, 6, 141.
13. The regiochemistry of compounds 8e10 was established by NOE stud-
ies,10 whilst the regiochemistry of compound 12 was made on the basis
1
(20b) (20:1 ratio) as a brown oil, 0.269 g, 64%. H NMR
(250 MHz, CDCl3): d 0.96 (6H, s, 2ꢁCH3), 3.57 (4H, s,
CH2), 3.92 (3H, s, CH3), 7.30e7.43 (5H, m, Ar-H ), 8.05
(1H, d, J¼2.0 Hz, Ar-H ), 8.10 (1H, d, J¼2.0 Hz, Ar-H ),
8.37 (1H, d, J¼1.0 Hz, Ar-H ); 13C NMR (62.9 MHz,
CDCl3) d 26.5, 32.2, 52.1, 70.6, 127.0, 127.3, 128.1, 128.9,
129.5, 130.1, 131.0, 149.6, 153.7, 167.6. FTIR: nmax/CHCl3,
2900 (w), 1720 (m), 1439 (w), 1290 (m) cmꢂ1. HRMS (EIþ)
calcd for C19H21BO4: 324.1786. Found: 324.1789.
1
of He1H coupling patterns. Compound 11 was not prepared with regio-
control and therefore regioisomer elucidation was not carried out in this
case.
14. For studies on the effects of halogen substituents on pyrone cycloaddition
reactivity and regioselectivity with alkenes, see: Afarinkia, K.; Bearpark,
M. J.; Ndibwami, A. J. Org. Chem. 2005, 70, 1122.
15. Ashworth, I. W.; Bowden, M. C.; Dembofsky, B.; Levin, D.; Moss, W.;
Robinson, E.; Szczur, N.; Virica, J. Org. Process Res. Dev. 2003, 7, 74.
16. The regiochemistry of compound 15 was confirmed by X-ray crystallo-
graphy.10 The regiochemistry of compound 16 was made on the basis
of 1H NOE spectroscopy.
Acknowledgements
We thank the EPSRC (GR/S91161/01) and Syngenta for
studentship funding and the University of Sheffield for finan-
cial support.
17. Gomez-Bengoa, E.; Helm, M. D.; Plant, A.; Harrity, J. P. A. J. Am. Chem.
Soc. 2007, 129, 2691.

P.M. Delaney et al. / Tetrahedron 64 (2008) 866e873
873
18. CCDC 650935 and CCDC 650936 contain the supplementary crystallo-
graphic data for compounds 5c and 17, respectively. This data can beobtained
free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge,
CB2 1EZ, UK; fax: (þ44) 1223 336 033; or request@ccdc.cam.ac.uk).
19. Brown, H. C.; Bhat, N. G.; Srebnik, M. Tetrahedron Lett. 1988, 29, 2631.
20. To a solution of 2-methyl-2,4-bis-trimethylsilanyloxy-pentane (13.126 g,
50 mmol) and potassium(phenylethynyl)trifluoroborate (10.401 g,
50 mmol) in acetone (50 ml) was added chlorotrimethyl silane
(10.864 g, 100 mmol). The reaction mixture was allowed to stir at room
temperature for 18 h. The reaction mixture was filtered and washed
with acetone and then the solvent was removed in vacuo to give compound
17 as a colourless crystalline solid, 11.064 g, 97%. Mp 63e65 ꢀC. 1H
NMR (250 MHz, CDCl3): d 1.31 (3H, d, J¼6.0 Hz, C(CH3)2CH2CH3),
1.35 (6H, s, C(CH3)2CH2CH3), 1.50e1.60 (1H, m, C(CH3)2CH2CH3),
1.83 (1H, dd, J¼14.0, 3.0 Hz, C(CH3)2CH2CH3), 4.25e4.34 (1H, m,
CHCH3), 7.26e7.33 (3H, m, Ar-H ), 7.50e7.54 (2H, m, Ar-H ); 13C
NMR (62.9 MHz, CDCl3) d 22.9, 28.2, 31.0, 46.0, 66.8, 85.9, 90.7,
122.2, 128.0, 129.0, 132.4. FTIR 2975 (m), 2192 (m), 1490 (m), 1397
(s), 1294 (s), 1199 (s) cmꢂ1. HRMS (EIþ) calcd for C14H17BO2:
228.1322. Found: 228.1328.
21. Blanchard, C.; Framery, E.; Vaultier, M. Synthesis 1996, 45.