Synthesis of N-Acyl-N,O-acetals from Aldehydes, Amides and Alcohols

Article abstract of DOI:10.1002/ejoc.201500655

A general synthesis of N-acyl-N,O-acetals from aldehydes, amides and alcohols is reported. This new method consists of two single steps: (a) magnesium-mediated addition of an amide or carbamate to an aldehyde and (b) acid-catalyzed conversion of the N-acylhemiaminal to the N-acyl-N,O-acetal. This two-step protocol allows the multigram synthesis of various N-acyl-O-alkyl-N,O-acetals.

Full text of DOI:10.1002/ejoc.201500655

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201500655
Synthesis of N-Acyl-N,O-acetals from Aldehydes, Amides and Alcohols
Juliette Halli,[a] Kamil Hofman,[a] Tamara Beisel,[a] and Georg Manolikakes*[a]
Keywords: N,O-Acetals / Hemiaminals / N-Acylimines / Synthetic methods / Aldehydes
A general synthesis of N-acyl-N,O-acetals from aldehydes,
amides and alcohols is reported. This new method consists
of two single steps: (a) magnesium-mediated addition of an
amide or carbamate to an aldehyde and (b) acid-catalyzed
conversion of the N-acylhemiaminal to the N-acyl-N,O-
acetal. This two-step protocol allows the multigram synthesis
of various N-acyl-O-alkyl-N,O-acetals.
Introduction
The N-acyl-N,O-acetal motif is an important substruc-
ture in organic chemistry. Such N,O-acetals are useful
building blocks for organic synthesis and synthetic equiva-
lents of unstable N-acylimines.[1] On the one hand N-acyl-
N,O-acetals are stable towards air and water and have an
extended shelf-life. On the other hand they can be activated
with suitable Lewis or Brønsted acids to generate reactive
N-acylimines. In addition the N-acyl-N,O-acetal and the
analogous N-acylhemiaminal motifs are found in a number
of natural products with interesting biological activities,
such as zampanolide (1),[2] psymberin (2),[3] or pederin
(3)[4] (Figure 1).
Due to their importance several synthetic routes to N-
acyl-N,O-acetals have been developed. Two of the most
commonly employed syntheses are Katritzky’s benzotri-
azole method[5] and the preparation via amido-sulfones.[6]
In both methods, a suitable precursor, either an N-[α-
(benzotriazol-l-yl)alkyl]amide or an α-amido-sulfone, is
treated with sodium alkoxides. However, for alkyl-aldehyde-
derived substrates, these reactions lead to the formation of
the corresponding enamides.[7] Another approach is based
on the hydrozirconization of nitriles followed by acylation
and trapping of the acylimine with an alcohol.[8] Recently,
the group of Antilla developed an asymmetric synthesis of
N-acyl-N,O-acetals starting from unstable N-acylimines.[9]
Other methods are limited to certain substrate types, such
as chloral,[10] glyoxalates[11] or resin-supported amides[12] or
were developed for the synthesis of specific natural prod-
ucts, e.g. a Curtius rearrangement in the synthesis of zam-
panolide (1), dactylolide or psymberin (2)[13] or by oxidative
decarboxylations from suitable N-acylamino acids.[14] Dur-
Figure 1. Natural compounds containing the N-acyl-N,O-acetal
and the analogous N-acylhemiaminal motifs.
ing our research on N-acylimine-based multicomponent re-
actions[15] we became interested in N-acyl-N,O-acetals as
stable N-acylimine precursors. Therefore, we were looking
for an operational simple, scalable and efficient synthesis of
N-acyl-N,O-acetals. From this point of view the titanium
ethoxide mediated 3-component synthesis reported by Hu-
ang and Wen looked particularly well suited.[16] However,
in our hands this method proved to be unreliable, and we
obtained irreproducible yields between 15 and 50% of the
desired N-acyl-N,O-acetal 8a. Especially for reactions per-
formed on a larger scale (5–50 mmol) the yield dropped
dramatically. We assume, these low yields are associated
with decomposition of the product during the difficult and
slow filtration of TiO2 formed during the aqueous
workup.[17] Herein we describe a new method for the prepa-
ration of N-acyl-N,O-acetals from aldehydes, amides and
alcohols.
[a] Institut für Organische Chemie und Chemische Biologie,
Goethe-Universität Frankfurt
Max-von-Laue-Straße 9, 60438 Frankfurt am Main, Germany
E-mail: g.manolikakes@chemie.uni-frankfurt.de
https://www.uni-frankfurt.de/53469467/Manolikakes
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500655.
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Synthesis of N-Acyl-N,O-acetals from Aldehydes, Amides and Alcohols
that this reaction sequence can be easily performed on a
multigram scale with a slightly reduced yield.[20] We rou-
tinely run this two-step protocol on a 100 mmol scale,
thereby obtaining 12.5 g (52% yield) of the N-acyl-N,O-
acetal 8a within 1 d.
With the reaction conditions in hand we investigated the
scope of our method. Various aryl- and heteroarylaldehydes
6b–6h can be transformed into the desired N-acyl-N,O-acet-
als 8b–8h in 30–76% yield over two steps (Scheme 1). Also
alkylaldehydes proved to be suitable substrates, and the N-
acylimine precursors 8i–8l were obtained in 63–79% yield.
Interestingly, reaction of citronellal with benzamide fol-
lowed by transacetalization furnished the N-acyl-N,O-
acetal 8m in 77% yield. No intramolecular reactions were
observed in this case.
Results and Discussion
Our strategy for the N-acyl-N,O-acetal synthesis was
based on the direct addition of an amide to an aldehyde to
give an N,O-hemiaminal followed by acid-catalyzed reac-
tion with an alcohol. Such direct additions of an amide or
amide anion have been reported as a suitable method for
the preparation of N,O-hemiaminals.[7,18] Indeed, the direct
addition of the magnesium carboxoimidoate 5a, generated
from benzamide 4a and MeMgCl or iPrMgCl·LiCl, to
benzaldehyde 6a furnished the desired hemiaminal 7a in al-
most quantitative yields (Table 1, Entries 1 and 2). Interest-
ingly, the corresponding lithium carboxoimidoate 5b did
not add to benzaldehyde (Entry 3). Stoichiometric amounts
of base were necessary for an efficient addition (Entry 4).
In our hands, hemiaminal 7a proved to be unstable towards
column chromatography or upon prolonged storage. There-
fore, the crude product mixture was directly used for the
second step.[19]
Table 1. Influence of the base on the direct addition.
Entry
Base (equiv.)
Yield [%][a]
1
2
3
4
MeMgCl (1.05)
iPrMgCl·LiCl (1.05)
nBuLi (1.05)
Ͼ 95
Ͼ 95
Ͼ 5
15
MeMgCl (0.20)
[a] Yield determined by NMR spectroscopy.
To our delight, treatment of the crude hemiaminal 7a
with MeOH in the presence of a suitable acidic catalysts
furnished the corresponding N-acyl-N,O-acetal 8a in 52–
76% yield over two steps (Table 2). Best yields were ob-
tained with Sc(OTf)3 and pyridinium p-toluenesulfonate
(PPTS) (Entries 2 and 4). N-Acyl-N,O-acetal 8a proved to
be stable during column chromatography and upon pro-
longed storage at room temperature. It is worth mentioning,
Scheme 1. Reactions with different (hetero)aryl- and alkylalde-
hydes.
Table 2. Influence of the acid on the transacetalization.
As shown in Scheme 2, various aryl- and alkylamides are
suitable substrates for our two-step protocol and furnished
the desired N-acyl-N,O-acetals 9a–f in 34–67% yield. The
reaction of acetamide with different arylaldehydes shows,
that the overall yield is dependent on the exact combination
of amide and aldehyde. Thus, the N-acyl-N,O-acetals 9a–c
were obtained in 34%, 51% and 74% yield, respectively.
Carbamates can be used as amide component in our re-
action, albeit with lower yields. Thus, reaction of benzyl
carbamate or tert-butyl carbamate provided the N-carb-
amoyl-N,O-acetals 9g and 9h in 37 and 32% yield, respec-
Entry
Acid (equiv.)
Yield [%][a]
1
2
3
4
Amberlyst-15 (0.05)
Sc(OTf)3 (0.05)
TsOH·H2O (0.05)
PPTS (0.05)
52
72
66
76, 52[b]
[a] Yield of isolated product; yield over two steps based on 4a as
limiting reagent. [b] Reaction performed on a 100 mmol scale.
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J. Halli, K. Hofman, T. Beisel, G. Manolikakes
SHORT COMMUNICATION
Conclusions
We have developed a new two-step protocol for the syn-
thesis of N-acyl-N,O-acetals from aldehydes, amides and
alcohols. This method is based on the magnesium-mediated
addition of amides to aldehydes followed by an acid-cata-
lyzed transacetalization of the obtained hemiaminal to the
desired N-acyl-N,O-acetals. This method has a very broad
scope, provides an efficient and rapid access to various N-
acyl-N,O-acetals and is amendable to multigram-scale syn-
thesis.
Experimental Section
General Two-Step Procedure for the Synthesis of N,O-Acetals
Step 1. Synthesis of Hemiaminal: A flame-dried and argon-filled
Schlenk flask connected to a Schlenk line was charged with amide
4 (1.0–1.2 equiv., 5.00–6.00 mmol), dry THF (2 mL/mmol amide)
and a suitable stirring bar. The resulting solution was cooled to
–40 to –50 °C, and MeMgCl (1.05 equiv., 2.55 m in THF) was
added slowly through a syringe under vigorous stirring. (Caution!
During the addition a rapid evolution of methane takes place; the
reactions should not be performed in a closed system!) During the
addition a white solid may precipitate, and stirring can become
difficult. The resulting mixture was warmed to room temperature
and stirred for 15 min. Then the reaction mixture was cooled to
0 °C, and aldehyde 6 (1.0 equiv. to 1.1 equiv., 5.0–5.5 mmol) was
added rapidly. The reaction mixture was warmed to room tempera-
ture and stirred until TLC analysis showed complete consumption
(1 h 25 min to 5 h) of the limiting reagent (amide or aldehyde).
After completion, saturated aqueous NaHCO3 (20 mL) was added
to the reaction mixture. The organic layer was separated, and the
aqueous layer was extracted three times with dichloromethane
(20 mL). The combined organic layers were dried with Na2SO4.
(Note! The crude hemiaminals are unstable towards prolonged con-
tact with water; therefore, a rapid workup is recommended!) After
evaporation of the solvents, the crude product is dried at room
temperature under oil-pump vacuum (10–2 mbar) for 2–3 h.
Scheme 2. Selected combinations of aldehydes and amides.
tively. Aromatic amides could be converted into the corre-
sponding N-acyl-N,O-acetals 9i and 9j in satisfactory yields
of 67% and 66%. Only in the case of electron-poor or
heteroaromatic amides were the corresponding N-acylimine
precursors 9k and 9l isolated in low yields.
Our two-step protocol is especially suitable for the syn-
thesis of various N-acyl-O-alkyl-N,O-acetals. Replacement
of MeOH by other alcohols in the transacetalization step
delivers the expected O-alkyl-N,O-acetals in moderate to
good yields. In all cases higher yields were obtained with
Sc(OTf)3 as catalyst (Scheme 3).
Step 2. Transacetalization: The crude hemiaminal 7 was dissolved
in methanol (3 mL/mmol), and 5 mol-% pyridinium p-toluenesulf-
Scheme 3. Reactions with different alcohols.
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Synthesis of N-Acyl-N,O-acetals from Aldehydes, Amides and Alcohols
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[19] Analytically pure samples of the hemiaminal can be obtained
by recrystallization (n-hexane/dichloromethane, 9:1).
[20] The reduced yield is associated with partial decomposition of
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reaction performed on a Ͼ10 mmol scale.
Received: May 20, 2015
Published Online: June 24, 2015
Eur. J. Org. Chem. 2015, 4624–4627
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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