A palladium-catalyzed carbonylative-deacetylative sequence to 1,3-keto amides

Article abstract of DOI:10.1002/adsc.201400545

An efficient three-component reaction involving carbon monoxide with a range of aryl bromides and N-substituted acetoacetamides is reported for the synthesis of b-keto amides. This transformation is promoted by Pd-catalysis followed by an acid-mediated deacetylation upon work-up, enabling a large number of b-keto amides to be isolated. Finally, d2-13C-dyclonine could be synthesized in three steps utilizing the developed catalytic system as the key step.

Full text of DOI:10.1002/adsc.201400545

COMMUNICATIONS
DOI: 10.1002/adsc.201400545
A Palladium-Catalyzed Carbonylative–Deacetylative Sequence to
1,3-Keto Amides
Dennis U. Nielsen,a Signe Korsager,a Anders T. Lindhardt,b,*
and Troels Skrydstrupa,*
a
Center for Insoluble Protein Structures (inSPIN), Interdisciplinary Nanoscience Center (iNANO), Department of
Chemistry, Aarhus University, Gustav Wieds Vej 14, 8000 Aarhus C, Denmark
Fax : (+45)-8619-6199; phone: (+45)-8715-6757; e-mail: ts@inano.au.dk
Interdisciplinary Nanoscience Center (iNANO), Biological and Chemical Engineering, Departmant of Engineering,
b
Aarhus University, Finlandsgade 22, 8200 Aarhus N, Denmark
E-mail: lindhardt@eng.au.dk
Received: June 3, 2014; Revised: July 25, 2014; Published online: November 5, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400545.
Abstract: An efficient three-component reaction in-
volving carbon monoxide with a range of aryl bro-
mides and N-substituted acetoacetamides is report-
ed for the synthesis of b-keto amides. This transfor-
mation is promoted by Pd-catalysis followed by an
acid-mediated deacetylation upon work-up, ena-
bling a large number of b-keto amides to be isolat-
ed. Finally, d2-13C-dyclonine could be synthesized in
three steps utilizing the developed catalytic system
as the key step.
we envisioned a carbonylative version of the a-aryla-
tion of tertiary amides to be an elegant and direct
route to 1,3-keto amides.[7] Although few desired 1,3-
keto amides could be obtained in acceptable yields
(results not shown) employing this approach, we
struggled with a lack of reproducibility and a limited
substrate scope, which forced us to re-think the initial
strategy. Recently, we reported a Pd-catalyzed car-
bonylative a-arylation–decarboxylative sequence in
a one-pot method for the synthesis of 1,3-keto esters
from malonate derivatives (Scheme 1a).[8]
Keywords: carbonylation; catalysis; isotope label-
ling; 1,3-keto amides; palladium
Applying the same catalytic system, we were able
to develop a method for the formation of 1,3-dike-
tones employing acetylacetone, which readily under-
goes
(Scheme 1b).[9]
In this communication, we wish to report on a mild
deacetylation
during
acidic
work-up
1,3-Keto amides are a desirable group of compounds and effective route to 1,3-keto amides via a Pd-cata-
owing to their utility as building blocks for accessing lyzed carbonylative–deacetylative pathway applying
pharmaceutically relevant heterocycles.[1] The need aryl bromides and a-acetylated amides (Scheme 1c).
for these precursors calls for continued improvement This method allows a variety of aryl bromides carry-
of synthetic routes that allows the isolation of such ing different functional groups to be transformed into
structures in a selective and efficient manner. Known the corresponding b-aryl-b-keto amides. An aryl tri-
procedures exploit b-ketocarboxylic acids either pre- flate and benzyl chloride could be activated as well.
activated as the corresponding acid chlorides or acti- Furthermore, different a-acetylated amides were con-
vated in situ by employing standard coupling re- verted into the desired 1,3-keto amides. Finally, a stoi-
agents.[2] Alternatively, 1,3-keto amides can be ac- chiometric amount of ex situ generated CO is em-
cessed by the oxidation of the aldol product from the ployed and the advantage gained by this is illustrated
reaction of an acetamide with an aldehyde.[3] Howev- in the synthesis of the d2-13C-labelled local anesthetic
er, a one-pot catalytic approach to 1,3-keto amides dyclonine in just three steps.[10,11]
would circumvent the need for applying stoichiomet-
In the aforementioned synthesis of 1,3-diketones
ric coupling reagents or oxidants as are needed in the and b-keto esters, both were formed from either de-
aforementioned approaches. Previously, our group has acetylation or decarboxylation reaction, respectively,
published carbonylative versions of the Pd-catalyzed via a tricarbonyl species formed in the initial carbony-
a-arylation of ketones both for aryl iodides and aryl lation step employing a catalyst derived from Pd(0)
bromides.[4–6] Inspired by these useful transformations, and Xantphos (Scheme 1). Applying the same reac-
Adv. Synth. Catal. 2014, 356, 3519 – 3524
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3519

COMMUNICATIONS
Dennis U. Nielsen et al.
diethylacetoacetamide and the desired 1,3-ketoamides
9–14 could be isolated in excellent yields. Further-
more, 1 could be synthesized in an 81% yield from 4-
chlorobenzonitrile by raising the temperature to
1208C. Switching the focus to the more challenging
aryl bromides carrying ortho-substituents gave a drop
in the isolated yield of 15–17, which is consistent with
previous observations.[14] Heterocyclic bromides such
as pyridine, thiophene, benzohydrofuran and indole
were all converted to the corresponding 1,3-keto
amides 18–22 in good yields. Benzyl chloride could be
utilized as an electrophile providing the correspond-
ing 1,3-keto amide 23. However, secondary benzyl
chlorides carrying a b-hydrogen only provided styrene
products, whereas blocking the presence of a b-hydro-
gen as for chlorodiphenylmethane did not result in
Scheme 1. Pd-catalyzed carbonylative a-arylations forming
1,3-keto esters, 1,3-diketones and 1,3-keto amides.
tion conditions as before utilizing this time an aceto- the formation of the desired 1,3-ketoamide (results
acetamide as the nucleophile, should form a similar not shown). Finally, a vinyl bromide could be success-
tricarbonyl compound, which can be deacetylated fully employed as an electrophile exemplified by 24.
under acidic conditions. As illustrated with 4-cyano- Next, we examined variations in the nucleophile
ACHTUNGTRENNUNG
phenyl bromide, this approach led to the isolation of (Scheme 3), with 4-bromobenzonitrile as the electro-
the b-keto amide 1 in an 87% yield after column phile for this study. Decreasing the steric bulk of the
chromatography (Scheme 2).
amide by employing N,N-dimethyl-3-oxobutanamide
With these conditions in hand, we set out to ex- resulted in a lower isolated yield of the aryl 1,3-keto
plore the scope of this transformation. N,N-Diethyl- amide 25 than expected. This could possibly be ex-
ACHTUNGTRENNUNGacetoacetamide was chosen as the amide precursor plained by loss of selectivity during hydrolysis. Never-
and the reaction conditions and the following scope theless, using the piperidine equivalent raised the iso-
are illustrated in Table 1. Aryl bromides carrying lated yield to 84% (compound 26). To our delight,
either a branched or a terminal olefin could be trans- when employing N-methoxy-N-methyl-3-oxobutana-
formed to amides 2 and 3 in 94% and 65% yield, re- mide, the desired product 27 could be isolated in
spectively. The presence of a phenyl group in the a good 73% yield.[15] Furthermore, the unfunctional-
para-position led to the isolation of the corresponding ized acetoacetamide could also be coupled, but pro-
1,3-keto amide starting with either an aryl bromide or vided compound 28 in a 50% yield. Formation of
an aryl triflate (compound 4). Employing more elec- a 1,3-diketone in a 30% yield was also obtained from
tron-rich aryl bromides did not interfere with the high this example after hydrolysis of the intermediary tri-
selectivity for the desired 1,3-keto amide (compounds carbonyl (result not shown). Nevertheless, employing
5–7). Notably, the reaction occurred chemoselectively the corresponding benzylated or methylated secon-
in the presence of a tosylate in the para-position, pro- dary 1,3-keto amide restored the efficiency of the re-
viding the keto amide 8.[13] Aryl bromides substituted action as illustrated with compounds 29 and 30. The
with electron-withdrawing groups coupled with N,N- formation of 1,3-diketones was also observed when
Scheme 2. Three-component formation of b-keto amides 1.
3520
asc.wiley-vch.de
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2014, 356, 3519 – 3524

COMMUNICATIONS
A Palladium-Catalyzed Carbonylative–Deacetylative Sequence
Table 1. Pd-catalyzed carbonylative a-arylation with aryl halides forming aryl b-keto amides.[a]
[a]
Reaction conditions: Chamber A: Aryl halide (0.50 mmol), Pd(cod)Cl2 (0.025 mmol), Xantphos (0.025 mmol), N,N-dieth-
yl-3-oxobutanamide (0.55 mmol), MgCl2 (0.60 mmol), Et3N (2.0 mmol) and dioxane (4 mL) at 808C. Chamber B: COgen
(0.75 mmol), Pd(cod)Cl2 (0.038 mmol), HBF4P(t-Bu)3 (0.038 mmol), Cy2NMe (1.5 mmol) and dioxane (3 mL) at 808C for
18 h. DEA=N,N-diethylacetamide. Reactions were quenched with 2M HCl for 1 h at 808C.
Aryl triflate used as electrophile.
Reaction conducted at 1208C with Cy2NMe as the base, butyronitrile as the solvent and 4-chlorobenzonitrile as the elec-
[b]
[c]
trophile.
[d]
Reaction conducted at 1008C with Cy2NMe as the base.
employing b-keto amides with nitrogen bound aro- odology could provide the same product. Indeed, this
matic substituents, such as a phenyl, 4-methoxyphenyl turned out to be the case as depicted in Scheme 4.
and 4-trifluoromethylphenyl group (results not The aryl b-ketoethyl ester 32 could be secured from
shown). Finally, the a-fluoroacetoacetamide provided ethyl acetoacetate in an excellent 83% isolated
the fluorinated keto amide 31. However, with yield.[16] Interestingly, the corresponding b-keto thio-
a benzyl group at the a-position none of the desired ester 33 was isolated in a good yield employing S-
1,3-keto amide was obtained (result not shown).
ethyl 3-oxobutanethioate as the nucleophile. Even the
Having applied the carbonylative–deacetylative ap- a-acylated menthol derivative 34 could be synthe-
proach for the synthesis of b-keto amides and dike- sized. It is well established that b-keto esters undergo
tones, we speculated whether other functional groups hydrolysis and subsequent decarboxylation in the
could be introduced in a similar manner. In our previ- presence of acid to generate acetophenones.[17] As
ous work, we prepared b-keto esters via a carbonyla- shown in Scheme 4, the choice of acid and work-up
tion–decarboxylation sequence (Scheme 1a). Howev- conditions can lead either to the 1,3-keto ester prod-
er, this required a monoester potassium malonate for uct 35 or the corresponding thioacetophenone 36.
the subsequent acid-mediated decarboxylation. We
Quenching with formic acid at 808C for 1 h yielded
therefore examined whether the deacetylative meth- the b-keto ester, whereas a quench with 6M HCl at
Adv. Synth. Catal. 2014, 356, 3519 – 3524
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3521

COMMUNICATIONS
Dennis U. Nielsen et al.
Scheme 4. Access to b-keto esters, b-keto thioesters and
thiophenones. Reaction conditions: as in Table 1.
AHCTUNGTRENNUNG
Scheme 3. Pd-Catalyzed carbonylative a-arylation of differ-
ent a-acetylated amides using 4-bromobenzonitrile. Reaction
conditions: as in Table 1.
was achieved by a Pd-catalyzed carbonylation of ace-
toacetamides followed by a deacetylation. A variety
of different aryl bromides could be utilized and the
808C for 6 h formed the corresponding thioacetophe- corresponding 1,3-keto amide could be isolated in
none both in very good yields. good to excellent yields. A number of acetoaceta-
Applying ex situ generated CO enables the delivery mides could be employed as well, demonstrating the
of stoichiometric amounts of 13C-labelled CO, render- robustness of this reaction. Moreover, the same strat-
ing this technique ideal for isotope labelling of phar- egy could be applied to generate b-keto esters, a b-
maceutically relevant molecules. This is exemplified keto thioester and an acetophenone. Finally, a triple
in Scheme 5 with the synthesis of d2-13C-dyclonine via isotopically labelled drug, dyclonine, could be formed
coupling of 1-bromo-4-butoxybenzene with 13CO and in three steps, which underlines the use of the two-
1-(piperidin-1-yl)butane-1,3-dione to form the corre- chamber technique for late-stage installation of
sponding 1,3-keto amide 37 in a 92% isolated yield carbon isotopes.
after acidic hydrolysis. Notably, this reaction could be
scaled up to 3.0 mmol. Employing an excess of
LiAlD4 reduced 37 to the corresponding d3-a-
Experimental Section
hydroxyphenpropylamine, which could be oxidized to
ACHTUNGTRENNUNG
ketone 38 by employing the Dess–Martin reagent.
Additional incorporation of two deuterium atoms in-
creases the mass unit of dyclonine by 3 units totally.
This difference is crucial in isotope mass dilution
spectrometry (ID-MS), representing one of the lead-
ing techniques in the field of trace analysis.[18]
Typical Procedure for the Syntheses of 1–30:
Synthesis of 1
Chamber A: In an argon-filled glovebox, to chamber A of
the two-chamber system (20 mL total volume) was added 4-
bromobenzonitrile (91 mg, 0.5 mmol), Pd(cod)Cl2 (7.1 mg,
0.025 mmol), Xantphos (14.5 mg, 0.025 mmol), N,N-diethyl-
3-oxobutanamide (87 mg, 0.55 mmol), MgCl2 (57 mg,
In conclusion, a new and useful methodology for
synthesizing b-keto amides has been presented. This 0.6 mmol), Et3N (280 mL, 2.0 mmol) and dioxane (4.0 mL) in
3522
asc.wiley-vch.de
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2014, 356, 3519 – 3524

COMMUNICATIONS
A Palladium-Catalyzed Carbonylative–Deacetylative Sequence
References
[1] a) K. Komori, T. Taniguchi, S. Mizutani, K. Monde, K.
Kuramochi, K. Tsubaki, Org. Lett. 2014, 16, 1386–1389;
b) Y. Wei, S. Lin, H. Xue, F. Liang, B. Zhao, Org. Lett.
2012, 14, 712–715; c) U. S. Sørensen, E. Falch, P. Krogs-
gaard-Larsen, J. Org. Chem. 2000, 65, 1003–1007.
[2] For 1,3-ketoacid chlorides, see: a) S. Peukert, Y. Sun,
R. Zhang, B. Hurley, M. Sabio, X. Shen, C. Gray, J.
Dzink-Fox, J. Tao, R. Cebula, S. Wattanasin, Bioorg.
Med. Chem. Lett. 2008, 18, 1840–1844; b) J. Wenner-
berg, A. Bjçrk, T. Fristedt, B. Granquist, K. Jansson, I.
Thuvesson, Org. Process Res. Dev. 2007, 11, 674–680;
c) C.-W. Lee, R. Lira, J. Dutra, K. Ogilvie, B. T.
OꢂNeill, M. Brodney, C. Helal, J. Young, E. Lachapelle,
S. Sakya, J. C. Murray, J. Org. Chem. 2013, 78, 2661.
For 1,3-keto acids see: d) R. T. Ruck, M. A. Huffman,
M. K. Kim, M. Shevlin, W. V. Kandur, I. W. Davies,
Angew. Chem. 2008, 120, 4789–4792; Angew. Chem.
Int. Ed. 2008, 47, 4711–4714; e) O. Renꢃ, D. Lapointe,
K. Fagnou, Org. Lett. 2009, 11, 4560–4563; f) J. Wang,
Y. Yuan, R. Xiong, D. Zhang-Negrerie, Y. Du, K.
Zhao, Org. Lett. 2012, 14, 2210–2213; g) S. Bhunia, S.
Ghosh, D. Dey, A. Bisai, Org. Lett. 2013, 15, 2426–
2429.
[3] a) M. Nakada, S. Kobayashi, S. Iwasaki, M. Ohno, Tet-
rahedron Lett. 1993, 34, 1035–1038; b) M. Pꢃrez, C. del
Pozo, F. Reyes, A. Rodrꢄguez, A. Francesch, A. M.
Echavarren, C. Cuevas, Angew. Chem. 2004, 116, 1756–
1759; Angew. Chem. Int. Ed. 2004, 43, 1724–1727;
c) L. K. Ransborg, L. Albrecht, C. F. Weise, J. R. Bak,
K. A. Jørgensen, Org. Lett. 2012, 14, 724–727.
[4] a) T. M. Gøgsig, R. H. Taaning, A. T. Lindhardt, T.
Skrydstrup, Angew. Chem. 2012, 124, 822–825; Angew.
Chem. Int. Ed. 2012, 51, 798–801; b) D. U. Nielsen, C.
Lescot, T. M. Gøgsig, A. T. Lindhardt, T. Skrydstrup,
Chem. Eur. J. 2013, 19, 17926–17938.
[5] For recent reviews on a-arylation of carbonyl-contain-
ing compounds, see: a) C. C. C. Johansson, T. J. Cola-
cot, Angew. Chem. 2010, 122, 686–718; Angew. Chem.
Int. Ed. 2010, 49, 676–707; b) F. Bellina, R. Rossi,
Chem. Rev. 2010, 110, 1082–1146.
[6] For recent reviews on Pd-catalyzed carbonylations, see:
a) X.-F. Wu, H. Neumann, M. Beller, ChemSusChem
2013, 6, 229–241; b) X.-F. Wu, H. Neumann, M. Beller,
Chem. Rev. 2013, 113, 1–35; c) X.-F. Wu, H. Neumann,
M. Beller, Chem. Soc. Rev. 2011, 40, 4986–5009.
[7] For examples on a-arylation of amides, see: a) K. H.
Shaughnessy, B. C. Hamann, J. F. Hartwig, J. Org.
Chem. 1998, 63, 6546–6553; b) S. Lee, J. F. Hartwig, J.
Org. Chem. 2001, 66, 3402–3415; c) D. A. Culkin, J. F.
Hartwig, J. Am. Chem. Soc. 2001, 123, 5816–5817; d) T.
Hama, X. Liu, D. A. Culkin, J. F. Hartwig, J. Am.
Chem. Soc. 2003, 125, 11176–11177; e) J. Cossy, A. de
Filippis, D. G. Pardo, Org. Lett. 2003, 17, 3037–3039;
f) T. Hama, D. A. Culkin, J. F. Hartwig, J. Am. Chem.
Soc. 2006, 128, 4976–4985; g) A. M. Taylor, R. A.
Altman, S. L. Buchwald, J. Am. Chem. Soc. 2009, 131,
9900–9901; h) P. Li, S. L. Buchwald, Angew. Chem.
2011, 123, 6520–6524; Angew. Chem. Int. Ed. 2011, 50,
6396–6400.
Scheme 5. Synthesis of d2-13C-dyclonine.
that order. The chamber was sealed with a screwcap fitted
with a Teflonꢁ seal.
Chamber B (1.5 equiv. CO): In an argon-filled glovebox,
to chamber B of the two-chamber system was added
HBF4P(t-Bu)3 (2.2 mg, 0.0075 mmol), Pd(cod)Cl2 (2.1 mg,
0.0075 mmol),
9-methylfluorene-9-carbonyl
chloride
(COgen) (182 mg, 0.75 mmol), dioxane (3.0 mL) and
Cy2NMe (320 mL, 1.5 mmol) in that order. The chamber was
sealed with a screwcap fitted with a Teflonꢁ seal.
The loaded two-chamber system was removed from the
glovebox and heated to 808C for 18 h. The reaction was
quenched with 2M HCl (1 mL) and stirred for 1 hour at
808C. The two phases were separated and the water phase
was extracted three times with CH2Cl2. The combined or-
ganic phases were dried over anhydrous Na2SO4, filtered
and concentrated under vacuum. The crude residue was pu-
rified by flash chromatography using pentane/EtOAc (3:1)
as eluent which resulted in 1 obtained as mixture between
the keto- and the enol form as a colorless oil; yield:106.0 mg
1
(87%). H NMR (400 MHz, CDCl3): d=8.07 (d, J=8.4 Hz,
2H, keto), 7.84 (d, J=8.4 Hz, 2H, enol), 7.76 (d, J=8.8 Hz,
2H, keto), 7.67 (d, J=8.4 Hz, 2H, enol), 5.77 (s, 1H, enol),
4.15 (s, 2H, keto), 3.51–3.30 (m, 8H, keto+enol), 1.33–1.08
(m, 12H, keto+enol). 13C NMR (100 MHz, CDCl3) d (ppm)
193.3, 171.0, 169.1, 165.4, 139.6, 139.5, 132.6 (2C), 132.3
(2C), 129.4 (2C), 126.6 (2C), 118.6, 118.0, 116.7, 113.8, 86.9,
45.8, 42.9, 42.5, 40.8, 40.5, 14.5, 14.4, 13.4, 13.0; HR-MS:
m/z=245.1288, calculated for C14H16N2O2 [M+H+]:
245.1285.
Acknowledgements
We thank the Danish National Research Foundation (grant
no. DNRF59), the Villum Foundation, the Danish Council
for Independant Research: Technology and Production Sci-
ences, the Lundbeck Foundation, the Carlsberg Foundation,
and Aarhus University for generous financial support of this
work.
Adv. Synth. Catal. 2014, 356, 3519 – 3524
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3523

COMMUNICATIONS
Dennis U. Nielsen et al.
[8] S. Korsager, D. U. Nielsen, R. H. Taaning, T. Skrydstr-
up, Angew. Chem. Int. Ed. 2013, 52, 9763–9766.
[9] S. Korsager, D. U. Nielsen, R. H. Taaning, A. T. Lind-
hardt, T. Skrydstrup, Chem. Eur. J. 2013, 19, 17687–
17691.
[10] For previous work using ex situ generated CO in Pd
catalysis to form amides, see: a) P. Hermange, A. T.
Lindhardt, R. H. Taaning, K. Bjerglund, D. Lupp, T.
Skrydstrup, J. Am. Chem. Soc. 2011, 133, 6061–6071;
b) D. U. Nielsen, R. H. Taaning, A. T. Lindhardt, T. M.
Gøgsig, T. Skrydstrup, Org. Lett. 2011, 13, 4454–4457;
c) D. U. Nielsen, K. Neumann, R. H. Taaning, A. T.
Lindhardt, A. Modvig, T. Skrydstrup, J. Org. Chem.
2012, 77, 6155–6165.
[11] For literature on dyclonine see: a) L. C. Harris Jr, J. C.
Parry, F. E. Greifenstein, Anesthesiology 1956, 5, 648–
652; b) H. Groeben, T. Growendt, M.-T. Silvanus, G.
Pavlakovic, J. Peters, Anesthesiology 2001, 3, 423–428.
[12] CO was generated ex situ from COgen in a two-cham-
ber reactor (COware). COgen and COware are both
commercially available at SyTracks.com or Sigmaal-
drich.com.
[13] For examples of aryl tosylates as electrophiles in cross-
coupling reactions, see: a) A. H. Roy, J. F. Hartwig, J.
Am. Chem. Soc. 2003, 125, 8704–8705; b) R. H.
Munday, J. R. Martinelli, S. L. Buchwald, J. Am. Chem.
Soc. 2008, 130, 2754–2755; c) L. J. Goossen, N. Rodrꢄ-
guez, P. L. Lange, C. Linder, Angew. Chem. 2010, 122,
1129–1132; Angew. Chem. Int. Ed. 2010, 49, 1111–1114.
[14] The detrimental effect of ortho-substituents can be
overcome by reducing the sterical bulk on the ligand:
C. A. Fleckenstein, H. Plenio, Chem. Soc. Rev. 2010,
39, 694–711.
[15] For a recent review on Weinreb amides see: S. Balasu-
bramaniam, I. S. Aidhen, Synthesis 2008, 23, 3707–3738.
[16] It should be noted that the acidic work-up was done by
employing formic acid instead of an aqueous acid due
to the increased reactivity of esters towards hydrolysis
compared to amides.
[17] a) D. F. Taber, J. C. Amedio Jr, F. Gulino, J. Org.
Chem. 1989, 54, 3474–3475; b) D. D. Dhavale, N. N.
Saha, V. N. Desai, J. Org. Chem. 1997, 62, 7482–7484.
[18] M. Rychlik, S. Asam, Anal. Bioanal. Chem. 2008, 390,
617–628.
3524
asc.wiley-vch.de
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2014, 356, 3519 – 3524