Synthesis of Weinreb amides using diboronic acid anhydride-catalyzed dehydrative amidation of carboxylic acids

Article abstract of DOI:10.1039/d0cc05630h

The first successful example of the direct synthesis of Weinreb amides using catalytic hydroxy-directed dehydrative amidation of carboxylic acids using the diboronic acid anhydride catalyst is described. The methodology is applicable to the concise syntheses of eight α-hydroxyketone natural products, namely, sattabacin, 4-hydroxy sattabacin, kurasoins A and B, soraphinols A and B, and circumcins B and C.

Full text of DOI:10.1039/d0cc05630h

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DOI: 10.1039/D0CC05630H
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Synthesis of Weinreb amides using diboronic acid anhydride-
catalyzed dehydrative amidation of carboxylic acids
Naoyuki Shimada,* Naoya Takahashi, Naoki Ohse, Masayoshi Koshizuka and Kazuishi Makino
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The first successful example of the direct synthesis of Weinreb B active intermediate and found it to be an efficient catalyst in
amides using catalytic hydroxy-directed dehydrative amidation of direct dehydrative amidation or peptide bond formation.14
carboxylic acids using diboronic acid anhydride catalyst is However, there are no examples of the direct synthesis of
described. The methodology is applicable to the concise syntheses Weinreb amides using catalytic dehydrative condensation.
of eight -hydroxyketone natural products, namely, sattabacin, 4-
Hydroxy-directed reaction triggered by the interaction
hydroxy sattabacin, kurasoins A and B, soraphinols A and B, and between hydroxy functional group of substrate and catalyst is
circumcins B and C.
an attractive strategy for molecular transformations.9,15.16 We
developed a newly designed biphenyl-based diboronic acid
anhydride (DBAA) possessing a B–O–B motif, which showed
high catalytic activity for the series of -hydroxy-directed
amidation of carboxylic acids.17 Herein, we report a successful
DBAA-catalyzed dehydrative amidation of -hydroxycarboxylic
acids with N,O-dimethylhydroxylamine, which has never been
used for catalytic dehydrative amidation as an amine
nucleophile because of its poor nucleophilicity. Notably, this is
the first report on the catalytic synthesis of Weinreb amides
using direct dehydrative amidation of carboxylic acids.
Furthermore, this catalysis enabled concise syntheses of
biologically active α-hydroxyketone natural products (Fig 1b).
Initially, we explored the reaction of (S)-2-hydroxy-3-
phenylpropanoic acid (2a) with N,O-dimethylhydroxylamine (3)
N-Methoxy-N-methylamides (i.e., Weinreb amides1) are widely
used as versatile building blocks in organic synthesis for their
conversion to aldehydes or modified ketones.2,3 Recently,
Weinreb amides have also been used as directing groups for
transition metal-catalyzed C–H functionalization.4 Although a
number of methods for the preparation of Weinreb amides
using esters, amides, imides, aldehydes, alcohols, aromatic
halides, and acid chlorides as starting materials have been
reported, the most straightforward approach is the dehydrative
condensation of corresponding carboxylic acids with N,O-
dimethylhydroxylamine.2 However, the common methods for
dehydrative condensation require a stoichiometric amount of
coupling reagents,5 which causes problems associated with
poor atom economy (Fig 1a: conventional method). This
drawback can be overcome using catalytic dehydrative
amidation6 of carboxylic acids (Fig 1a: ideal method). Following
the pioneering work by Yamamoto on dehydrative amidation of
carboxylic acids using electron-deficient group-substituted
aromatic boronic acids as catalysts,7 a wide variety of modified
aromatic boronic acid catalysts has been developed.8 Recently,
other classes of organoboron catalysts have been developed
such as alkylboronic acid,9 boronate ester,10 1,3-dioxa-5-aza-
2,4,6-triborinane (DATB),11 and diboron.12 Most recently, based
on the revised amidation mechanism that the intermediate with
a B–O–B motif by the dimerization of boronic acid is the real
active species reported by Whiting et al.,13 Takemoto has
developed gem-diboronic acid (gem-DBA) composed of a B–C–
Department of Pharmaceutical Sciences, Kitasato University, 5-9-1 Shirokane,
Minatao-ku, Tokyo 108-8641, Japan. E-mail: shimadan@pharm.kitasato-u.ac.jp,
makinok@pharm.kitasato-u.ac.jp
†
Electronic Supplementary Information (ESI) available: Experimental details,
characterization data, copies of 1H- and 13C-NMR spectra for all new compounds.
See DOI: 10.1039/x0xx00000x
Fig 1. Overview of this work
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alkyl side chain at the -position were also applicable, and 4h–
4k were
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DOI: 10.1039/D0CC05630H
Table 1 Optimization of dehydrative amidation conditionsa
Scheme 1. Diboronic acid anhydride-catalyzed synthesis of -
hydroxy Weinreb amidesa
3
Yieldb
Catalyst [mol%]
Entry
[equiv]
[%]
1
2c
3
4
5
6
7
8
9
10
11
12
13
14e
DBAA 1 (2.0)
DBAA 1 (2.0)
DBAA 1 (2.0)
1.0
1.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
78
64
98
DBAA 1 (0.5)
96 [94]d
3,4,5-F3-C6H2B(OH)2 (0.5)
2-(i-Pr)2NCH2-C6H4B(OH)2 (0.5)
2-I-5-MeO-C6H3B(OH)2 (0.5)
2,4-(CF3)2-C6H3B(OH)2 (0.5)
MeB(OH)2 (0.5)
6
3
15
33
4
B2(NMe2)4 (0.5)
B(OCH2CF3)3 (0.5)
DATB (0.5)
2
aThe reactions were carried out in the presence of acid 2 (0.40 mmol,
1.0 equiv), HNMe(OMe) (3) (1.20 mmol, 3.0 equiv), and DBAA 1
(2.00 mol, 0.5 mol%) in DCE (0.2 M) under reflux (bath
temperature, 90°C). The optical purity of amides 4b, 4j, and 4k was
12
25
4
gem-DBA (0.5)
–
1
b
determined by chiral HPLC analysis. Performed with 2.0 mol% of
aThe reactions were carried out in the presence of acid 2a (0.40 mmol,
1.0 equiv), HNMe(OMe) (3), and catalyst in solvent (0.2 M) at 90°C
(bath temp) for 4 h. Determined by H NMR of a crude reaction
mixture of products using 1,1,2,2-tetrachloroethane as an internal
standard. cPerformed in toluene (0.2 M). dIsolated yield. The optical
purity of amides 4a was determined to be >99% by chiral HPLC
analysis. eIn the absence of catalyst.
c
1. Performed using acid 2j with 86% optical purity as a substrate.
dPerformed in DCE (0.1 M).
b
1
obtained in satisfactory yields (83%–98%). Notably, loss of
optical purity was not observed during the synthesis of α-
hydroxy Weinreb amides 4b, 4j, and 4k from chiral carboxylic
acids 2b, 2j, and 2k.
in the presence of DBAA 1 (Table 1). The reaction of an
equimolar mixture of 2a and 3 with 2.0 mol% of 1 in DCE
proceeded at 90°C to afford the corresponding Weinreb amide
4a in 78% yield after 4 h (entry 1). Switching the solvent to
toluene slightly decreased product yield (entry 2), whereas the
use of 3.0 equiv of amine 3 in DCE increased product yield to
98% (entry 3). An excellent isolated yield of 94% was maintained
even when reducing the catalyst loading of DBAA 1 to 0.5 mol%
(entry 4). When other organoboron compounds7-12,14 that are
known as efficient dehydrative amidation catalysts were used
under the same conditions, only low to moderate product yields
were observed (2%–33% yields, entries 5–13).18 A trace amount
of product was obtained in the absence of 1 (entry 14).
For the reaction using a commercially available N,O-
dimethylhydroxylamine hydrochloride salt (3•HCl), the desired
amide 4b was obtained in a 81% yield without racemization by
the reaction of carboxylic acid 2b with amine 3 generated in situ
in the presence of sodium hydrogen carbonate (Scheme 2a).
This protocol is advantageous in terms of practical operation.
Competition experiments using an equimolar mixture of -
hydroxycarboxylic acid 2a and simple carboxylic acid 5 revealed
that DBAA
1
showed high chemoselectivity for -
hydroxycarboxylic acid and afforded -hydroxyamide 4a in
quantitative yield along with simple amide 6 in low yield (6%)
(Scheme 2b). This indicates the characteristic features of
hydroxy-directed reaction.
After identifying the suitability of DBAA 1 as the catalyst for
the Weinreb amide synthesis using dehydrative amidation,
DBAA 1 was also applicable to the catalytic synthesis of -
hydroxy Weinreb amides (Scheme 3). The reaction of -aryl-
substituted carboxylic acids afforded corresponding Weinreb
amides 8a–8c in 81%–94% yields. High yields were also obtained
with an ortho-substituted aryl group or heteroaromatic ring at
the -position to provide 8d and 8e. Carboxylic acids containing
an aliphatic side chain such as phenylethyl or the bulky tert-
butyl group at the -position were also applicable, giving
amides 8f and 8g in high to excellent yields (98 and 86%).
Alkenyl or alkynyl substituents at the -position of carboxylic
acid substrates were tolerated, and the corresponding amides
8h and 8i were obtained in 93% and 82% yields, respectively. A
phenyl substituent at α-position could be also applied, giving8j
next, we investigated the reaction of
a range of -
hydroxycarboxylic acids (Scheme 1). The reaction of chiral (S)-
mandelic acid (2b) with amine 3 in the presence of 0.5 mol% of
catalyst 1 afforded corresponding -hydroxyamide 4b in 92%
yield. We found that -aryl substrates possessing either
electron-donating or electron-withdrawing groups provided
corresponding amides4c–4e in high to excellent yields (84%–
96%). The use of substrates with a meta- or ortho-substituent
afforded amides 4f and 4g in 91% and 80% yield, respectively.
Carboxylic acids containing not only an aromatic ring but also an
2 | J. Name., 2012, 00, 1-3
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in 93% yield, whereas more hindered ,- or , -disubstituted versatile and efficient synthetic method is required.19 Because
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substrates were not suitable for the present catalysis.18
water is the only byproduct in the catalyDsiOs,I:w10e.1d0e39s/igDn0eCdC0a56o3n0eH-
pot
Scheme 2. Diboronic acid anhydride-catalyzed amidation
Scheme 4. Concise syntheses of α-hydroxyketone natural
products
Scheme 3. Diboronic acid anhydride-catalyzed synthesis of -
hydroxy Weinreb amidesa
aThe reactions were carried out in the presence of acid 7
(0.40 mmol, 1.0 equiv), HNMe(OMe) (3) (1.20 mmol,
3.0 equiv), and DBAA 1 (2.00 mol, 0.5 mol%) in DCE (0.2 M)
under reflux (bath temperature, 90°C). bPerformed with
1.0 mmol scale. cPerformed with HCl salt of amine 3 (3.0 equiv)
in the presence of NaHCO3 (3.0 equiv). dPerformed with
2.0 mol% of 1. ePerformed at 60°C.
aThe optical purity was determined by chiral HPLC analysis.
bPerformed using acid 2j with 86% optical purity as a substrate.
sequence of two reactions20 that comprised Grignard addition
after the DBAA-catalyzed dehydrative amidation without the
workup or purification of the Weinreb amide intermediate
(Scheme 4). Thus, the catalytic dehydrative amidation of chiral
carboxylic acid 2a and amine 3 without a workup, followed by
treatment with isopropyl magnesium chloride after switching
the solvent, afforded optically pure sattabacin (9)21 in 92% yield
in two steps. Notably, this sequential protocol with low catalyst
loading (0.5 mol%) of 1 can be easily applied to the gram-scale
synthesis of the natural product, affording 9 in 75% yield
without any racemization. Using analogous two-step
sequences, 4-hydroxy sattabacin (10)21a, and kurasoin A (11)22a,b
were synthesized from chiral carboxylic acid 2j, and kurasoin B
(12)22a,c,e was synthesized from carboxylic acid 2k in good to
high yield (68%–75%). Furthermore, exposure of carboxylic
acids 2k and 2a to amidation conditions, followed by the
treatment with 4-(benzyloxy)benzylmagnesium chloride
resulted in -hydroxyketones 13 and 14, which, upon
deprotection of phenolic benzyl ether, afforded soraphinol A
(17)23a and soraphinol B (18)24, respectively. This is the first
Fig 2. Plausible reaction pathway
Plausible reaction pathway is depicted in Fig 2. Based on
Whiting’s proposed mechanism of boronic acid-catalyzed
amidation,13 our hydroxy-directed reaction might proceed
through
a
bicyclic acyloxydiboronate intermediate via
dehydrative B–O bond formation with the hydroxy substituent
in carboxylic acids.17
-Hydroxyketone structures are widely found in some
potent antiviral agents, in protein farnesylatranferase
inhibitors, or in some antitumor antibiotics, and therefore their
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8
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protocols were also applicable to the synthesis of circumcin B
(19)23a and circumcin C (16).23b
In conclusion, we successfully developed a catalytic method
for the synthesis of Weinreb amides derived from - or -
hydroxycarboxylic acids using diboronic acid anhydride as the
catalyst. This distinctive hydroxy-directed amidation reaction is
the first example of the synthesis of Weinreb amides by the
catalytic dehydrative condensation of carboxylic acids with N,O-
dimethylhydroxylamine. This catalytic reaction proceeded in
high to excellent yields with low catalytic loading without
racemization and any dehydration protocols such as the
addition of molecular sieves or azeotropic reflux using a Dean–
Stark apparatus. Its synthetic utility was demonstrated by the
concise syntheses of eight biologically active -hydroxyketone
natural products. Efforts to expand the utility of DBAA catalysis
are currently underway in our laboratory.
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This research was supported in part by JSPS KAKENHI Grant
Numbers 19K07000 (N.S.) for Scientific Research (C). We thank
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Conflicts of interest
The authors declare no competing financial interest.
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