Stereocontrolled synthesis of 1,5-stereogenic centers through three-carbon homologation of boronic esters

Article abstract of DOI:10.1002/anie.201405700

Allylic pinacol boronic esters are stable toward 1,3-borotropic rearrangement. We developed a Pd^II-mediated isomerization process that gives di- or trisubstituted allylic boronic esters with high E selectivity. The combination of this method with lithiation-borylation enables the synthesis of carbon chains that bear 1,5-stereogenic centers. The utility of this method has been demonstrated in a formal synthesis of (+)-jasplakinolide. Three more: The 3C homologation of chiral pinacol boronic esters gives di- or trisubstituted allylic boronic esters with high yield and E selectivities. The combination of this method with lithiation-borylation enables the synthesis of alkyl chains that bear 1,5-stereogenic centers. The utility of the process was demonstrated in a formal synthesis of (+)-jasplakinolide.

Full text of DOI:10.1002/anie.201405700

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DOI: 10.1002/anie.201405700
Homologation Very Important Paper
Stereocontrolled Synthesis of 1,5-Stereogenic Centers through Three-
Carbon Homologation of Boronic Esters**
Phillip J. Unsworth, Daniele Leonori, and Varinder K. Aggarwal*
Abstract: Allylic pinacol boronic esters are stable toward 1,3-
borotropic rearrangement. We developed a PdII-mediated
isomerization process that gives di- or trisubstituted allylic
boronic esters with high E selectivity. The combination of this
method with lithiation–borylation enables the synthesis of
carbon chains that bear 1,5-stereogenic centers. The utility of
this method has been demonstrated in a formal synthesis of
(+)-jasplakinolide.
P
olyketide natural products are replete with carbon chains
that bear 1,5-stereogenic centers connected by alkyl, di- or tri-
substituted alkenyl groups (Figure 1).[1] Numerous ingenious
Scheme 1. 1) Proposed strategy for the stereocontrolled synthesis of
1,5-stereogenic centers. 2) Previous work: Brown’s three-carbon homo-
logation of propylene glycol boronic esters. 3) This work: three-carbon
homologation of pinacol boronic esters. pin=pinacol, Cb=N,N-
diisopropylcarbamoyl, sp=(À)-sparteine.
ester, this homologation required the use of unstable and
difficult-to-handle propylene glycol boronic esters 3 (Sche-
me 1a).[4] Furthermore, these substrates perform poorly in
lithiation–borylation processes, thus limiting their use in
asymmetric synthesis. In contrast, pinacol boronic esters
perform well in lithiation–borylation processes, and so we
needed to find conditions under which such esters could be
employed in three-carbon homologations.
Figure 1. Examples of natural products that contain 1,5-stereogenic
centers.
strategies have been devised for the synthesis of these natural
products, but control of the double-bond geometry, especially
in the case of tri-substituted alkenyl groups, can sometimes be
challenging.[2] Recently, lithiation–borylation has emerged as
a powerful tool to control the stereochemistry along a carbon
chain and to build up multiple stereogenic centers with high
stereocontrol.[3] In order to use lithiation–borylation to create
compound arrays that bear 1,5-stereogenic centers, a three-
carbon homologation of boronic ester 1 to an allylic boronic
ester intermediate 2 would be required, which would then be
set up for further homologations (Scheme 1a). While there
was one report of a three-carbon homologation of a boronic
In order to achieve our goal, we needed to 1) carry out
a homologation to give allylic boronic ester 4 followed by 2) a
diastereoselective 1,3-borotropic shift to give boronic ester 5
(Scheme 1c). Both steps presented challenges. First of all, we
needed to establish a general and efficient protocol for the
homologation of a broad range of pinacol boronic esters to
allylic boronic esters 4.[5] Secondly, conditions for the key 1,3-
borotropic shift needed to be identified to maximize the
reaction efficiency and more importantly to control the olefin
geometry. It is important to note that while less sterically
hindered allylic boronic esters (and boranes)[6] are known to
undergo a 1,3-borotropic shift upon heating, pinacol allylic
boronic esters have been shown to be thermally stable.[7]
Despite the limited precedence, we initiated a research
program aimed at addressing this challenge, anticipating
that its solution would be highly useful for the synthesis of
many relevant molecules. Herein we describe the first three-
carbon homologation of pinacol boronic esters, introducing
a di- or tri-substituted alkenyl unit with high stereocontrol
over the double-bond geometry. This methodology was
[*] P. J. Unsworth, Dr. D. Leonori, Prof. V. K. Aggarwal
School of Chemistry, University of Bristol
Cantock’s Close, Bristol, BS8 1TS (UK)
E-mail: v.aggarwal@bristol.ac.uk
[**] We thank the EPSRC-funded Bristol Chemical Synthesis Doctoral
Training Centre for a Ph.D. studentship (P.J.U.) and Inochem-
Frontier Scientific for their generous donation of boronic acids and
esters. We thank the EPSRC (EP/I038071/1) and the European
Research Council (FP7/2007–2013, ERC grant no. 246785) for
financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201405700.
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applied to the stereocontrolled synthesis of carbon chains that
bear 1,5-stereogenic centers and to a formal synthesis of the
natural product (+)-jasplakinolide (6).
and allylic alcohols[11] with B2pin2. These reactions are
believed to occur through the reductive elimination of
a Bpin-bound p-allyl–PdII intermediate and generally result
in high E selectivity. We reasoned that access to this
intermediate directly from 8 by using a novel PdII-catalytic
cycle would enable the required 1,3 rearrangement without
the need for further manipulation of the boronic ester (e.g.
oxidation and acylation). We turned this hypothesis into
practice by treating boronic ester 8 with Pd(OAc)2 and B2pin2.
Under these reaction conditions, the rearranged product 11
was formed in an encouraging conversion of 12%, but a poor
E/Z ratio (Table 1, entry 1). During these experiments, we
observed the precipitation of catalytically inactive Pd black
out of the solution. We reasoned that a stoichiometric oxidant
may be required in order for the Pd to turn-over (see below).
We were pleased to find that the addition of CuCl2 gave 11 in
good yield and remarkably high E selectivity (Table 1,
entry 2). Further screening showed that the addition of
basic Na2HPO4 was beneficial, thus leading to 11 essentially
as a single diastereoisomer in high yield. Furthermore, the
catalyst loading could be reduced to 2.5 mol% (Table 1,
entry 3). A control experiment without Pd(OAc)2 resulted in
decomposition of 8, product 11 was not observed (Table 1,
entry 4; see the Supporting Information for full optimization
details).
We began our study by investigating the direct homolo-
gation of boronic ester 7 to allylic boronic ester 8 using 1-
chloroallyllithium. Unfortunately, while this process had
worked well for tertiary pinacol boronic esters,[5a] it worked
poorly for secondary pinacol boronic esters, giving mixtures
of the starting material, the desired product, and over-
homologated products.[8] We then designed a two-step, one-
pot protocol that consists of 1) a Matteson homologation[9]
with dichloromethyl lithium to give 9, followed by 2) the
in situ treatment with vinyl magnesium bromide (Scheme 2).
This procedure routinely gave boronic ester 8 in high yield
and could be easily carried out on multi-gram scale. We also
successfully applied this process to the synthesis of b-methyl-
containing substrates 10 by using isopropenyl magnesium
bromide in the second step. Furthermore, this protocol uses
readily available and nontoxic reagents in contrast to previous
methods.[5b,c]
With this two-step, three-carbon homologation procedure
in hand, we evaluated the scope of the process by testing
a broad range of primary, secondary, and tertiary pinacol
boronic esters (Table 2), all of which gave the desired allylic
boronic esters with very high E selectivity.
Scheme 2. One-pot Matteson homologation/alkylation of boronic ester
7.
Having developed a very efficient method for the syn-
thesis of 8, we next examined the key 1,3 rearrangement
(Table 1). We initially tested thermal and microwave con-
ditions and confirmed that the 1,3-borotropic shift of the
pinacol boronic ester does not occur. An alternative method
was then required in order to achieve our goal. We drew
inspiration from previous syntheses of allylic boronic esters
through the Pd0-catalyzed borylation of allylic carbonates[10]
Table 2: Substrate scope.
Substrate
Yield of Yield of E/Z[b]
Product
A [%][a]
B [%][a]
86
79
>95:5
>95:5
>95:5
Table 1: Optimization of reaction conditions for the 1,3 rearrangement of
boronic ester 8.
77
86
72
84
72
65
50
64
>95:5
>95:5
Entry
Oxidant
([equiv])
Base
Conv.[a]
[%]
Yield[b]
[%]
E/Z
ratio[a]
1
2
–
–
–
12
100
100
–
n.d.
57
94 (79)
–
1:1
CuCl2 (3)
CuCl2 (3)
CuCl2 (3)
>95:5
>95:5
n.d.
86
76
95:5
3[c]
4[d]
Na2HPO4
Na2HPO4
73[c]
79[d]
>95:5
[a] Determined by GC-MS of the crude reaction mixture. [b] Determined
by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as internal
standard (yield of isolated product in parentheses). [c] 2.5 mol%
Pd(OAc)2. [d] No Pd(OAc)2 was used. Entry in bold marks optimized
reaction conditions. n.d.=not determined.
[a] Yields of isolated products after purification by column chromatog-
raphy. [b] Determined by GC-MS of the crude reaction mixture.
[c] Homologation carried out using 1-chloro allyllithium.[5a] [d] Reaction
carried out for 4 days at RT.
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Table 3: Substrate scope for b-methyl substituted allylic boronic esters.
Substrate
Yield of Yield of E/Z[b]
Product
A [%][a]
B [%][a]
77
79
>95:5
>95:5
>95:5
82
88
44[c]
61
Scheme 3. Proposed mechanism of 1,3 rearrangement.
67
59
68
>95:5
>95:5
52[c]
83
65
91:9
96[d]
49[c,e]
>95:5
Scheme 4. Deuterium labeling study.
[a] Yields of isolated products after purification by column chromatog-
raphy. [b] Determined by GC-MS of the crude reaction mixture.
[c] 1.5 equiv B2pin2 and 2.6 equiv Na2HPO4 were used.[13] [d] Homolo-
gation carried out using 1-chloro-methallyl-lithium.[5a] [e] Reaction heated
at 808C.
standard optimized conditions for the 1,3 rearrangement gave
boronic ester [D6]-11 with 90% incorporation of D, thus
showing that the Bpin incorporated in the product originated
from the external B2pin2. Analysis of the crude reaction
mixture by GC-MS showed that the excess of B2pin2 left at the
end of the reaction was a mixture of [D12]-B2pin2 and [D6]-
B2pin2, which is believed to be the source of 10% of the
nondeuterated boronic ester 11 (see the Supporting Informa-
tion for full details as well as further details of mechanistic
studies).
We then illustrated the power of this methodology by the
diastereoselective synthesis of carbon chains that bear 1,5-
stereogenic centers (Scheme 5). First of all we confirmed that
our three-carbon homologation was compatible with enan-
tioenriched substrates by preparing allylic boronic ester (S)-
15 from chiral boronic ester (R)-7[16] with no erosion of
stereochemistry. Homologation of boronic ester (S)-15 with
lithiated carbamate 16a gave syn-boronic ester 17a in good
yield and a d.r. of 94:6. The anti diastereomer 17b was
prepared in a similar yield and selectivity by carrying out the
same homologation, but using the opposite enantiomer of
A highly diastereoselective 1,3 rearrangement could also
be achieved for b-methyl-substituted boronic esters (Table 3).
The optimization of the reaction conditions for these steri-
cally more demanding and challenging substrates showed that
heating to 508C and increasing the amount of reagents was
required to achieve a full conversion. Changing the solvent
from THF to DMF was necessary to maintain a high E/Z
selectivity (see the Supporting Information for further details
of the screening). Again this methodology worked well for
a broad range of primary, secondary, and tertiary pinacol
boronic esters, giving high E/Z ratios throughout. This
methodology enabled the diastereoselective synthesis of
a range of b-methyl-substituted allylic boronic esters, of
which only a few examples have been reported to date.[12]
The proposed mechanism for the 1,3 rearrangement is
shown in Scheme 3. A transmetalation between B2pin2 and
Pd(OAc)2 gives the Bpin-bound PdII intermediate 12.[14]
Coordination of PdII to the alkene of the substrate followed
by base-aided transmetalation gives the key p-allyl-PdII
intermediate 13.[15] Reductive elimination[10a,11c] to give allylic
boronic ester 14 followed by oxidation of Pd0 back to PdII by
CuCl2 completes the cycle. We believe that the origin of the
high E/Z selectivity is the preference of the p-allyl inter-
mediate 13 to adopt an E configuration to minimize A1,3
strain.
To determine whether the Bpin incorporated in the
product originated from the external B2pin2 that was added
or from the starting material, we carried out the 1,3
rearrangement using deuterium-labeled B2pin2 (Scheme 4).
Treatment of boronic ester 8 and [D12]-B2pin2 under the
Scheme 5. Synthesis of boronic esters that contain 1,5-stereogenic
centers.
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lithiated carbamate 16b, which is easily prepared by using
(+)-sparteine[17] in place of (À)-sparteine. As we have access
to both enantiomers of boronic ester 7, this gives us the
potential to easily make all four stereoisomers of boronic
ester 17 at will.
To demonstrate the synthetic utility of our three-carbon
homologation methodology, we carried out a synthesis of the
C1À8 polyketide fragment 18 of (+)-jasplakinolide 6 (Sche-
me 6).[1b] Our synthesis began from commercially available
(S)-(+)-1,3-butane diol by the selective carbamoylation of the
Keywords: 1,5-stereocenters · allylic boronic esters ·
asymmetric synthesis · lithiation–borylation · palladium
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Scheme 6. Synthesis of carboxylic acid 18.
primary alcohol followed by MOM protection of the secon-
dary alcohol to give 19. Lithiation–borylation of 19 with
MeBpin gave 20 with high levels of selectivity. Subsequent
three-carbon homologation worked well, giving allylic bor-
onic ester 21 in good yield and an E/Z ratio of 95:5.
Homologation with lithiated carbamate 16b, followed by
Matteson homologation[18]/oxidation gave alcohol 22 in 71%
yield and excellent anti/syn selectivity in a one-pot double
homologation sequence. Final oxidation with PDC completed
the synthesis of carboxylic acid 18, a known intermediate in
the synthesis of (+)-jasplakinolide,[19] in just seven steps from
(S)-(+)-1,3-butane diol. It is important to note that all of the
stereogenic centers are formed under reagent control, and
thus the same reaction sequence can potentially be applied to
the synthesis of any of the eight stereoisomers of carboxylic
acid 18.
In summary, we have developed the first procedure for
a three-carbon homologation of pinacol boronic esters. This
method enables the highly diastereoselective synthesis of 1,5-
related stereogenic centers along a carbon chain connected by
di- or tri-substituted alkenes, a ubiquitous motif in natural
products. This methodology was applied to a short synthesis
of the C1À8 polyketide fragment of (+)-jasplakinolide.
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Received: May 27, 2014
Published online: July 15, 2014
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