Asymmetric synthesis of 4-amino-4H-chromenes by organocatalytic oxa-Michael/aza-Baylis-Hillman tandem reactions

Article abstract of DOI:10.1002/chem.201001293

Quick off the mark! In this work we present the first highly enatioselective oxa- Michael/aza-Baylis-Hillman tandem reaction between 2-alkynals and salicyl Ntosylimine leading to optically active 4-amino-4H-chromenes (see scheme). This reaction takes plac

Full text of DOI:10.1002/chem.201001293

COMMUNICATION
DOI: 10.1002/chem.201001293
Asymmetric Synthesis of 4-Amino-4H-Chromenes by Organocatalytic Oxa-
Michael/Aza-Baylis–Hillman Tandem Reactions
Josꢀ Alemꢁn,* Alberto NfflÇez, Leyre Marzo, Vanesa Marcos,
Cuauhtꢀmoc Alvarado, and Josꢀ Luis García Ruano[a]
4-Aminochromanes (Scheme 1) are a class of structures
that are integrated in hundreds of natural and bioactive
compounds. Their importance is reflected in the existence of
dozens of patents related to 4-aminochromanes bearing dif-
ferent aromatic rings at the NH2 group.[1] Among these com-
as well as the synthesis of related products have been re-
ported.[5]
The most direct organocatalytic asymmetric method for
obtaining these skeletons would involve the oxa-Michael re-
action of a,b-unsaturated aldehydes to salicylaldehyde[6] or
salicylimines,[7] followed by al-
dolic (or Mannich, Z=NR3) re-
action of the resulting inter-
mediate A (Scheme 1). Howev-
er, when R2 is hydrogen, the in-
termediate B cannot be isolat-
ed, because H2Z (Z=O, NR) is
quickly eliminated, affording
2H-chromenes C,[7a–d] due to
the large acidity of such hydro-
gen. When R2 is not hydrogen,
no reaction takes place, because
Scheme 1. Approach to the synthesis of 4-aminochromenes.
the reactivity of A is strongly
decreased.[8] At this point we
reasoned that reactions of N-ac-
pounds, those with a CH2OH group at C-3 (3-hydroxymeth-
yl-4-aminochromanes)[2] exhibit interesting biological prop-
erties (e.g., antiobiotic),[2d] and take part in the tetrahydro-
chromanoquinolines core,[3] whereas 1,4a-5,10b-tetrahydro-
4H-chromen[4,3b]pyridines are considered as aza analogs of
D1-transtetrahydrocannabinols.[3hÀi] Moreover, some attrac-
tive alkaloids (e.g., martinelli[4]) with the 4-aminochromane
structure have been described. Finally, the 4-aminochroma-
nol moiety is also important and dozens of medicinal studies
tivated 2-hydroxy benzaldimines with alkynals would afford
4-amino-4H-chromenes E (Scheme 2c), which could retain
the stereochemical information associated to the nitrogen
function and could be used as precursors of B by reduc-
tion.[9] To the best of our knowledge, this transformation in-
volving an oxa-Michael/aza-Baylis–Hillman (aza-BH)[10]
tandem process[11] with alkynals has never been reported
and prompted us to study it.
The classic aza-BH reaction[10a] consists of the reaction of
a nucleophile (usually a catalyst) with a deactivated double
bond and further addition of the resulting a-stabilized carb-
anion to a C=N bond, being finally the catalyst recovered
after elimination (Scheme 2a). The use of deactivated triple
bonds as electrophiles in aza-BH determines that the final
elimination cannot take place (Scheme 2b). These reactions
have hardly been explored,[12] which increased our interest
for studying them. Since natural 4-aminochromanes are op-
tically pure, we decided to study the oxa-Michael/aza-BH
tandem reaction by using activation with silyl prolinol ethers
[a] Dr. J. Alemꢀn, A. NfflÇez, L. Marzo, V. Marcos, Dr. C. Alvarado,
Dr. J. L. García Ruano
Departamento de Química Orgꢀnica (C-I)
Universidad Autónoma de Madrid, Cantoblanco
28049-Madrid (Spain)
Fax : (+34)91497466
E-mail: jose.aleman@uam.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001293.
Chem. Eur. J. 2010, 16, 9453 – 9456
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9453

not enough to activate the alky-
nal and no reaction was ob-
served (entry 13).
In order to check the scope
of the reaction, we explored re-
actions of different aryl (1a–g),
alkyl (1h) and alkenyl (1i) alky-
nals with 2 under the previously
optimized
conditions
(en-
tries 10–12, Table 1). Results
are summarized in Table 2.
Most reactions were completed
in less than 2 h (8 h when the
catalytic loading was 5 mol%,
entries 1 and 2). All reactions
were performed on a 0.2 mmol
scale in 0.2 mL of toluene,
Scheme 2. Different approaches for the aza-BH reaction.
in an asymmetric version,[13] taking advantage of the effi-
cient activation of alkynals[14] in their reactions with nitroal-
kenes has been recently reported.[14b]
Table 1. Optimization of the reaction of aldehyde (1a) with salicyl N-to-
sylimine 2.[a]
In this work we present the first highly enantioselective
organocatalytic[15] oxa-Michael/aza-BH tandem reaction be-
tween 2-alkynals and salicyl N-tosylimine, leading to optical-
ly active 4-amino-4H-chromenes by iminium activation
(Scheme 2c).
The screening of salicyl N-tosylimine 2 with alkynal 1a
was used as the model reaction. Different secondary amines
were used as catalyst and all reactions were stopped after
2 h (see Table 1). With proline 5a and prolinol 5c, none or
very low conversion was observed (entries 1 and 3). In the
rest of the cases, the reaction provided mixtures of two com-
pounds, the expected 4-amino-4H-chromene 4a and the 4’a.
With prolinamide 5b full conversion was achieved in less
than 2 h, but a 1:1 mixture of both compounds (4a and 4’a)
was obtained with rather low stereoselectivity (33% ee for
4a, entry 2).[16] Better enantiomeric excesses were obtained
with protected silyl prolinol ethers 5d (95% conversion)
and 5e (77% conversion), but a significant amount of 4’a
was also formed (entries 4 and 5). The amount of the by-
product 4’a could not be reduced by dilution in toluene or
CH2Cl2 (entries 6 and 7), but it was increased when the ratio
1a/2 become smaller (entry 8). To our delight, the use of a
1.5:1 ratio of 1a/2 (entry 9), afforded a 80:20 mixture of 4a
and 4’a and the result was even better by using an 2:1 ratio
of 1a/2 (entry 10). Under these last conditions, we observed
full conversion after two hours into a 95:5 mixture of 4a
and 4’a, isolating 4a with 80% yield. Both compounds 4a
and 4’a were obtained with identical ee (entry 10). All men-
tioned reactions were performed by using 20 mol% of the
catalysts. We studied the influence of the catalytic loading.
Thus, good results were also obtained with 10 mol%
(99% ee and 58% yield, entry 11) with only traces of 4’a
and also with 5 mol% of the catalyst (97% ee, 60% yield,
entry 12); however, under these conditions the reaction time
was slightly longer (8 h). The use of only 1 mol% of 5d was
Catalyst
(mol%)
Solvent
1a/2
Conver-
4a/4’a
ee
G
sion [%][c]
[%][d]
1
2
3
4
5
6
7
8
9
10
11
12
13
5a (20)
5b (20)
5c (20)
5d (20)
5e (20)
5d (20)
5d (20)
5d (20)
5d (20)
5d (20)
5d (10)
5d (5)
toluene
toluene
toluene
toluene
toluene
toluene[d]
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1.5
1.5:1
2:1
nr
>98
nr
95
77
85
60
>98
–
–
33
–
96
98
nd
96
94
50:50
–
55:45
41:59
40:60
70:30
50:50
80:20
95:5
[d]
CH2Cl2
toluene
toluene
toluene
toluene
toluene[g]
toluene[g]
>98
96
>98 (80)[e]
>98 (58)[e]
>98 (60)[e]
nr
98 (98)[f]
2:1
2:1
2:1
>98:2
93:7
99
97
–
5d (1)
–
[a] All reactions were performed on a 0.2 mmol scale in 0.2 mL of solvent
and stopped after 2 h. [b] Conversion was determined by 1H NMR spec-
troscopy. [c] Enantiomeric ratio was determined by chiral HPLC; nr=no
reaction. [d] Diluted up to 0.1m. [e] Isolated yield after flash chromatog-
raphy. [f] Enantiomeric ratio of the byproduct 4’a. [g] These reactions
were stopped after 8 h.
except for entry 3 that was carried out in 2.0 mmol scale.
Upon scaling up the reaction, 4a was obtained in 76% iso-
lated yield without decreasing the optical purity (98% ee).
The incorporation of electron-donating groups (p-Me, o-
MeO and p-MeO) at the alkynalꢂs aromatic ring did not
affect the stereoselectivity with eeꢂs ranging between 94 to
9454
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Chem. Eur. J. 2010, 16, 9453 – 9456

Asymmetric Synthesis
COMMUNICATION
Table 2. Scope results for the reaction of salicyl N-tosylimine (2) with al-
dehydes 1a–j.[a]
R
Mol%
Product
Yield [%]
ee [%][b]
catalyst
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Ph (1a)
Ph (1a)
Ph (1a)
p-Me-C6H4 (1b)
o-MeO-C6H4 (1c)
o-MeO-C6H4 (1c)
p-MeO-C6H4 (1d)
3-5-CF3-C6H4 (1e)
tBu-C6H4 (1 f)
tBu-C6H4 (1 f)
20
5
5
4a
4a
4a
4b
4c
4c
4d
4e
4 f
4 f
4g
4g
4h
4i
80
60
98
98
99
96
96
98
94
99
99
98
98
96
98
99
99
–
76[c]
97
20
20
5
20
40
20
10
20
5
80
82
94
55[d]
70
67
p-
p-
U
68
84[d]
79
n-pent (1h)
20
20
5
Scheme 3. Proposed mechanism for the oxa-Michael/Baylis–Hillman
tandem reaction.
1-cyclohexenyl (1i)
1-cyclohexenyl (1i)
TMS (1j)
77
4i
4j
78
20
nr[e]
[a] All reactions were performed with salicyl N-tosylimine (0.2 mmol),
alkyne (0.4 mmol), and the indicated amount of catalyst in solvent
(0.2 mL) and stopped after 2 h. [b] Determined by chiral HPLC (see Sup-
porting Information). [c] This reaction was carried out in a 2.0 mmol
scale. [d] This reaction was stopped after 20 h. [e] No reaction.
less than 2 h with high yields and excellent enantioselectivi-
ties. The catalytic loading could be reduced to 5 mol% with
slight increase in reaction times.
Acknowledgements
98% (Table 2, entries 4–7). No erosion of yield or stereose-
lectivity was observed by decreasing the catalytic loading to
5 mol% (entry 6). Interestingly, compound 1e, bearing an
electron-poor aromatic ring, also evolved with good enantio-
selectivity. However, the reactivity was substantially de-
creased and was necessary 40 mol% of the catalyst and 20 h
for the consumption of the starting material (entry 8). Other
alkyl groups at para-position, such as n-Pent and tBu also
produced excellent eeꢂs with both 20 mol% and 5 mol% of
catalyst (entries 9–12). Finally, reactions of alkynals bearing
alkyl or alkenyl chains, instead of aryl ones, produced good
stereoselectivity and isolated yields (entries 13–15). Interest-
ingly the reaction with the alkynal 1j did not work at stan-
dard conditions (entry 16). The structure of 4a and absolute
configuration of compound 4’a were unequivocally estab-
lished by X-ray analysis (see the Supporting Information for
more details).[17]
A plausible catalytic cycle for explaining the course of
these reactions is depicted at Scheme 3. First, the catalyst 5
activates the alkynal 1, forming an iminium intermediate I,
that undergoes the oxa-Michael addition with the salicyl N-
tosylimine 2. Resulting alenamine intermediate II[18] reacts
with the imine in an intramolecular fashion leading com-
pounds 4. The catalyst 5 is recovered in this last step.
Financial support of this work by the Spanish Government (CTQ-2009-
12168) and CAM (“programa AVANCAT CS2009/PPQ-1634”) is grate-
fully acknowledged. A. N. and V. M. thank the Universidad Autónoma
de Madrid and “Ministerio de Ciencia e Innovación”, respectively, for
predoctoral fellowships. J. A. thanks the “Ministerio de Ciencia e Innova-
ción” for a “Juan de la Cierva contract”. C. A. thanks the “Consejo Na-
cional de Ciencia y Tecnología de MØxico” for a postdoctoral fellowship.
Keywords: asymmetric synthesis
· aza-Baylis–Hillman ·
diarylprolinol ethers · domino reactions · organocatalysis
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after 2 h. The product 4’a was treated with different acid solutions
(HCl 10%, H2SO4 5%) in order to obtain product 4a, however we
only detected starting material.
[17] CCDC-766985 (4’a) and 766986 (4a) contains the supplementary
crystallographic data (see the Supporting Information for more de-
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[18] The allenamine intermediate II has been previously proposed by
Wang et al, see reference [14b].
Received: May 12, 2010
Published online: July 26, 2010
9456
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Chem. Eur. J. 2010, 16, 9453 – 9456