Desymmetrization of C2-Symmetric Bis(Boronic Esters) by Zweifel Olefinations

Article abstract of DOI:10.1002/chem.202000599

anti-Configured 1,3-dimethyl deoxypropionate motifs are important sub structures in natural products. Herein, we describe a bidirectional approach for the rapid construction of natural products featuring such motifs by using C₂-symmetrical 1,3-bis(boronic esters). As for its application in convergent syntheses it was important to establish a selective mono-Zweifel olefination we describe the scope and limitations by using different 1,3-bis(boronic esters) and nucleophiles. This protocol takes advantage of the combination of the Hoppe–Matteson–Zweifel chemistry, which was elegantly put into practice by Aggarwal et al. In order to show its applicability the total syntheses of two natural products, serricornin and (+)-invictolide, were performed.

Full text of DOI:10.1002/chem.202000599

A Journal of
Accepted Article
Title: Desymmetrization of C2-symmetric bis(boronic esters) by Zweifel
olefinations
Authors: Markus Kalesse, Yannick Linne, Axel Schönwald, and
Sebastian Weißbach
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Desymmetrization of C2-symmetric bis(boronic esters) by Zweifel
olefinations
Yannick Linne,[a, b] Axel Schönwald,[a] Sebastian Weißbach,[a] and Markus Kalesse*[a, b, c]
Abstract: Anti-configured 1,3-dimethyl deoxypropionate motifs are
important sub-structures in natural products. Herein, we describe a
bidirectional approach for the rapid construction of natural products
featuring such motifs using C2-symmetrical 1,3-bis(boronic esters). As
for its application in convergent syntheses it was important to
establish a selective mono-Zweifel olefination we describe the scope
and limitations using different 1,3-bis(boronic esters) and
nucleophiles. This protocol takes advantage of the combination of the
Hoppe−Matteson−Zweifel chemistry which was elegantly put into
practice by Aggarwal. In order to show its applicability the total
syntheses of two natural products, serricornin and (+)-invictolide,
were performed.
using boronic esters.[3] This method was applied to the total
synthesis of (+)-hydroxyphthioceranic acid, (+)-kalkitoxin (3)[2c]
and many more natural products.[6] One advantage of this strategy,
namely the individual control of each chiral center, is also a
drawback as it requires a linear work flow and an iterative
introduction of every single methyl or ethyl group.[2c, 4] Based on
this state of development and the fact that a large variety of
natural products feature anti-configured 1,3-dimethyl deoxy-
propionate motifs, we envisioned to use C2-symmetrical 1,3-
bis(boronic esters) for the sequential Zweifel olefination with
different nucleophiles. The synthesis of such a precursor was
already described by the group of Aggarwal[5] and recently,
Aggarwal and coworkers subjected this 1,3-bis(boronic ester) to
a double-sided vinylidene homologation.[6] However, our goal was
to use this C2-symmetrical 1,3-bis(boronic esters) in
desymmetrizing mono-Zweifel olefinations (Scheme 1).[7]
In the context of our program to establish synthetic access to
various natural products[1] we focused on those featuring the 1,3-
(poly)deoxypropionate motif. Polyketides and polyketide-peptide
hybrides featuring this motif like sambutoxin (1), borrelidin (2),
(+)-kalkitoxin (3) and (+)-invictolide (4) are challenging to
synthesize without functional group inter-conversions using
established aldol chemistry (Figure 1).[2]
Figure 1. Natural products containing the 1,3-deoxypropionate motif.
Scheme 1. Desymmetrization of C2-symmetric 1,3-bis(boronic esters) by mono-
Zweifel olefination.
On the other hand, an elegant and rapid concept for the
synthesis of those polydeoxypropionates is the so called
assembly line synthesis developed by Aggarwal and coworkers
At the outset of our investigation we started by optimizing the
reaction conditions for the addition of simple vinyl metal species
to 1,3-bis(boronic ester) 5a[5] (Scheme 2). First transformations
with vinyl lithium[8] generated yields around 35%, but during the
upscaling process the yield dropped drastically to 10% or less
(Scheme 2). Subsequently, vinylmagnesium bromide was used
as the nucleophile[9] and the yields were around 25%. We then
further optimized the mono-Zweifel olefination by adding iodine as
a solid[10] and not as a methanolic solution. The subsequent
methanol addition was done with a very slow addition rate of 0.15
mL/min. This variation gave satisfactory and reliable yields
around 48% and 94% based on recovered starting material and
works equally well for small and large scale reactions (for details
see SI). It should be pointed out that separation of the starting
material and the olefination product was easily achievable upon
standard column chromatography.
[a]
[b]
Y. Linne, A. Schönwald, S. Weißbach, Prof. Dr. M. Kalesse
Institute for Organic Chemistry, Schneiderberg 1B, 30167 Hannover
(Germany)
Y. Linne, Prof. Dr. M. Kalesse
Centre of Biomolecular Drug Research (BMWZ)
Gottfried Wilhelm Leibniz Universität Hannover
Schneiderberg 38, 30167 Hannover (Germany)
E-mail: markus.kalesse@oci.uni-hannover.de
Prof. Dr. M. Kalesse
[c]
Helmholtz Centre for Infection Research (HZI)
Inhoffenstrasse 7, 38124 Braunschweig (Germany)
Supporting information for this article is given via a link at the end of
the document.
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Scheme 2. Optimized conditions for the mono-Zweifel olefination. Standard
reaction conditions: 5a (1.0 eq.), vinyl compound (1.1 to 2.0 eq.), THF, 30 min
at −78 °C, then 30 min at rt, I2 (elem., 4.0 eq.), MeOH (0.15 mL/min) 30 min at
−78 °C, then NaOMe (8.0 eq.) in MeOH (0.50 mL/min) at −78 °C, 30 min, then
rt o/n. [b] yield dropped to 10% during upscaling. [c] I2 addition as a methanolic
solution. [d] Ate-complex formation and I2 addition at −95 °C. [e] Ate-complex
formation from −78 °C to 0 °C. a) LDA, Et2O, THF, −78 °C to 0 °C, 1.5 h, 73%.
6a, 7a, 7b and 7c were obtained as single diastereoisomers.
Scheme 3. Substrate scope of the mono-Zweifel olefination. All reactions were
run using the optimized conditions of Scheme 2. Yields refer to isolated yield
after flash column chromatography. The dr was determined by 1H-NMR.
The sterically demanding isobutyl and cyclohexyl residues
were well tolerated by the mono-Zweifel olefination, which is quite
remarkable due to the steric hindrance next to the reaction
center.[7, 8b] Boronic esters containing terminal double bonds (6g-
6h) could also be prepared in good yields. Due to their additional
functionalities they could be valuable building blocks in total
synthesis. Compounds 6i and 6j could also be synthesized using
the mono-Zweifel olefination, but partial epimerization was
observed. In the case of boronic ester 6k we observed traces of
the double olefination product.
After having successfully accomplished the addition of vinyl-
magnesium bromide we turned our attention to other olefins that
could be useful intermediates in total synthesis (Scheme 2). For
this we used but-1-en-2-ylmagnesium bromide[11] and obtained
using the previously optimized conditions the corresponding
product 7a. As we will describe below, this product can be easily
converted to its corresponding ethyl ketone. We also investigated
the addition of different lithiated enolethers[8b, 9, 12], however only
the lithiated vinyl carbamate led to the desired Zweifel olefination
7c. In the beginning we obtained the double-olefination product in
favor of the mono-olefination but we overcame this problem by
reducing the equivalents of the vinyl species to only 1.1 eq. and
obtained the desired mono-Zweifel product 7c in 73% yield.
Beyond that, lithiated vinyl bromide was used as nucleophile
giving us the versatile boronic ester 7b in 46% yield. Interestingly,
all of these nucleophiles were more reactive than vinylmagnesium
bromide, because unreacted starting material could not be
reisolated. To show the utility of these products, mono-Zweifel
product 7c was subjected to an elimination under basic conditions
affording alkyne 8. In our case higher yields were observed when
LDA (73%) was used instead of tBuLi (41%, Scheme 2). In both
cases no epimerization was observed.[12] Alkyne 8 could be a
powerful intermediate in total synthesis after hydrometalation,
cross-coupling or even additional hydroboration.
With a reliable method in hand, we applied building block 6a
to the total synthesis of (+)-invictolide (4) and serricornin (20).[2d-f,
13]
(−)-Invictolide is one of the lactons used for the queen
recognition of the red fire ant Solenopsis invicta (Buren).[13] With
its three methyl groups and the 1,3-anti-deoxypropionate motif
(+)-invictolide (4) matches perfectly our synthetic strategy. In our
retrosynthetic analysis (Scheme 4) we envisioned lactol-
formation and oxidation as the last steps, whereby the double-
bond of Zweifel olefination functions as precursor for the carbonyl
moiety. The required secondary alcohol in linear precursor 9
should be installed by lithiation-borylation chemistry from mono-
Zweifel product 6a and TIB ester 10. The corresponding alcohol
could either be derived from 1-propylboronic acid pinacol
ester (17) by assembly line synthesis or by Myers alkylation using
auxiliary 14. We on purpose performed both routes in order to
compare steps and yields and by doing so obtaining an
assessment of both strategies.
After examination of different nucleophiles we investigated
structural variations of the 1,3-bis(boronic esters, Scheme 3).
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yields are within the same range. However, 1-propylboronic acid
pinacol ester (17) is also commercially available but quite
expensive. However, in the case 17 is purchased three steps
would be needed in both approaches. One drawback of the Myers
route is the use of an auxiliary and the related low atom economy.
Another difference is the number of purification steps, with the
Myers route requiring three and the assembly line approach
requiring two. A practical disadvantage of the assembly line
approach is the required very slow addition rate (20 µL/min) of
nBuLi during the Matteson-homologation.
However, with both fragments in hand, the fragment coupling
using lithiation-borylation chemistry was the next step. The
obtained boronic ester was directly subjected to an oxidation
giving us secondary alcohol 9 in good yield and high
diastereoselectivity.[18] The subsequent two-step sequence
consisting of ozonolysis[19] and Ley-Griffith-oxidation[20] of the
intermediately formed lactol finished the - to the best of our
knowledge - shortest total synthesis of (+)-invictolide (4,
Scheme 7).
Scheme 4. Retrosynthetic analysis of (+)-invictolide (4).
Our synthesis started with gram-scale preparation of C2-
symmetric 1,3-bis(boronic ester) 5a[5] using a slightly modified
protocol of the Aggarwal group (see SI). The described mono-
Zweifel olefination gave us boronic ester 6a in an overall yield of
45%[14] (Scheme 5).
Scheme 5. Synthesis of boronic ester 6a. a) bromoethane, nBu4NHSO4, NaOH,
CHCl3, H2O, rt, o/n, ≥ 95%; b) sBuLi, (+)-sparteine, Et2O, −78 °C, 5 h, then
Me3SnCl, Et2O, −78 °C to rt, 1.5 h, 70%, er 99:1; c) nBuLi, Et2O, −78 °C, 1.5 h,
then pinBCH2Bpin, Et2O, −78 °C to rt, o/n, 82%, dr ≥ 95:5; d) vinylMgBr, THF,
30 min at −78 °C, then 30 min at rt, then I2, MeOH 30 min at −78 °C, then
NaOMe, MeOH, 30 min at −78 °C, then rt o/n, 35%, 81% brsm.
Alcohol 15 was synthesized via two different routes: Myers
alkylation and assembly line synthesis (Scheme 6). The first step
in the Myers alkylation route was the preparation of the
auxiliary 14 from (−)-pseudoephedrine (13).[15] Afterwards,
alkylation using 1-propyliodide (16) and subsequent reduction
finished the synthesis of alcohol 15.[15b, 16] The assembly line route
started with the synthesis of 1-propylboronic acid pinacol
ester (17) from 1-propyliodide (16).[4b] Subsequently, a three-step
Scheme 7. Fragment coupling and endgame in the total synthesis of (+)-invic-
tolide (4). a) sBuLi, (−)-sparteine, Et2O, −78 °C, 5 h, then 6a, THF, −78 °C to
40 °C, o/n; b) 2 M NaOH, H2O2, THF, −20 °C to rt, 2 h, 62% o2s, dr ≥ 95:5; c)
O3, PPh3, CH2Cl2, −78 °C to rt, o/n, 87%; d) TPAP, NMO, 4 Å MS, CH2Cl2, rt,
2.5 h, 80%.
sequence consisting of
lithiation-borylation,
Matteson-
homologation and oxidation led to alcohol 15.[2c] This alcohol was
then subjected to a Mitsunobu reaction[17] using TIBOH (11) to
afford TIB ester 10 in 92% yield.
By using the same strategy we also accomplished the
synthesis of serricornin (20), the female sex pheromone of the
cigarette beetle Lasioderma serricorne.[21] TIB ester 18 was
prepared by using the established phase transfer conditions.[5, 22]
Lithiation-borylation chemistry of boronic ester 7a with TIB
ester 18[5, 22] and subsequent oxidation gave us secondary
alcohol 19 in 70% yield over two steps in a 10:1 diastereomeric
ratio. Finally, ozonolysis[19] provided serricornin (20) as a mixture
of its open chain and hemiketal form (Scheme 8).[11]
Scheme 6. Synthesis of TIB ester 10 via Myers alkylation and assembly line
synthesis. a) LDA, LiCl, THF, −78 °C to rt, 1.5 h, then 1-propyliodide (16), THF,
0 °C to rt, o/n; b) LDA, BH3•NH3, THF, −78 °C to rt, 5 h, 53% o2s, er 99:1;
c) tBuLi, pinBOiPr, Et2O, −105 °C, 10 min, 88%; d) nBuLi, 12, Et2O, −78 °C to
rt, o/n; e) BrCH2Cl, nBuLi, Et2O, −78 °C to rt, o/n; f) 2 M NaOH, H2O2, THF,
−20 °C to rt, 2 h, 45% o3s, er 99:1; g) TIBOH (11), PPh3, DIAD, THF, 0 °C to rt,
o/n, 92%.
Scheme 8. Total synthesis of serricornin (20). a) NaOH, 1-bromopropane,
nBu4NHSO4, H2O, CHCl3, rt, o/n, ≥ 95%; b) sBuLi, (+)-sparteine, Et2O, −78 °C,
5 h, then 7a, THF, −78 °C to 40 °C, o/n; c) 2 M NaOH, H2O2, THF, −20 °C to rt,
2.5 h, 70% o2s, dr 10:1; d) O3, PPh3, CH2Cl2, −78 °C to rt, o/n, 74%, dr 18:1
hemiketal form.
Comparison of the two different routes showed that the
stereoselectivity was excellent (er 99:1) in both approaches. One
step less is needed in the Myers route (three vs. four) while the
In summary, we developed a bidirectional approach to
polyketides or polyketide fragments featuring the 1,3-deoxy-
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in Organoboron Chemistry towards Organic Synthesis, 2019, Ch. 13,
393; b) D. Leonori, V. K. Aggarwal, Acc. Chem. Res. 2014, 47, 3174.
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propionate motif. The key step in this approach is
a
desymmetrization using the mono-Zweifel olefination. We could
show the value of key intermediates 6a and 7a in the syntheses
of (+)-invictolide (4) and serricornin (20). Furthermore, a detailed
substrate scope showed the general applicability of the developed
methodology. Finally, the use of different nucleophiles enables
synthetic access to a variety of valuable building blocks.
[4]
[5]
Synthesis of C2-symmetric 1,3-bis(boronic esters) D. J. Blair, D. Tanini,
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Acknowledgements
For review of Zweifel olefination, see R. Armstrong, V. K. Aggarwal,
Synthesis 2017, 49, 3323.
In terms of preparation of the manuscript we thank Daniel Lücke,
Lucas Millbrodt, Christoph Etling and Alina Eggert. We thank Dr.
J. Fohrer, M. Rettstadt and D. Körtje for detailed 2D-NMR analysis
and A. Schulz and R. Reichel for mass spectra. A generous gift
of B2pin2 from Allychem is also appreciated. Furthermore we
thank Prof. Dr. V. K. Aggarwal and his group for helpful
discussions on Zweifel olefinations.
Common conditions for Zweifel olefination using vinyl lithium a) G.
Dutheuil, M. P. Webster, P. A. Worthington, V. K. Aggarwal, Angew.
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Conditions for Zweifel olefinations using vinylmagnesium bromide a) R.
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132, anie.201914875.
[9]
Keywords: desymmetrization • C2-symmetric 1,3-bis(boronic
esters) • mono-Zweifel olefination • lithiation-borylation chemistry
• natural product synthesis
[10] Personal communication Prof. Dr. V. K. Aggarwal.
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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It’s a crab! By combining the Hoppe-,
Matteson-, Aggarwal- and Zweifel-
chemistry, a bidirectional access to
polyketides featuring the challenging
1,3-deoxypropionate motif is
Yannick Linne, Axel Schönwald,
Sebastian Weißbach, Prof. Dr. Markus
Kalesse*
Page No. – Page No.
presented. The desymmetrization of
various 1,3-bis(boronic esters) led to
synthetically interesting building
blocks. With this reliable method the
pheromones invictolide and
Desymmetrization of C2-symmetric
bis(boronic esters) by Zweifel
olefination
serricornin were synthesized.
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