A new synthesis of 2,3-dihydrobenzo[1,4]dioxine and 3,4-dihydro-2H-benzo[1, 4]oxazine derivatives by tandem palladium-catalyzed oxidative aminocarbonylation - Cyclization of 2-prop-2-ynyloxyphenols and 2-prop-2-ynyloxyanilines

Article abstract of DOI:10.1021/jo061229l

(Chemical Equation Presented) 2-[(Dialkylcarbamoyl)methylene]-2,3- dihydrobenzo[1,4]dioxine and 3-[(dialkylcarbamoyl)methylene]-3,4-dihydro-2H- benzo-[1,4]oxazine derivatives (3 and 5, respectively) were synthesized for the first time starting from readil

Full text of DOI:10.1021/jo061229l

SCHEME 1
A New Synthesis of 2,3-Dihydrobenzo[1,4]dioxine
and 3,4-Dihydro-2H-benzo[1,4]oxazine
Derivatives by Tandem Palladium-Catalyzed
Oxidative Aminocarbonylation-Cyclization of
2-Prop-2-ynyloxyphenols and
2-Prop-2-ynyloxyanilines
Bartolo Gabriele,*,† Giuseppe Salerno,‡ Lucia Veltri,‡
Raffaella Mancuso,‡ Zhiyu Li,‡,§ Alessandra Crispini,‡ and
Anna Bellusci‡
Dipartimento di Scienze Farmaceutiche, UniVersita` della
Calabria, 87036 ArcaVacata di Rende (Cosenza), Italy, and
Dipartimento di Chimica, UniVersita` della Calabria,
87036 ArcaVacata di Rende (Cosenza), Italy
SCHEME 2
b.gabriele@unical.it
ReceiVed June 14, 2006
SCHEME 3
2-[(Dialkylcarbamoyl)methylene]-2,3-dihydrobenzo[1,4]dioxine
and 3-[(dialkylcarbamoyl)methylene]-3,4-dihydro-2H-benzo-
[1,4]oxazine derivatives (3 and 5, respectively) were synthe-
sized for the first time starting from readily available 2-prop-
2-ynyloxyphenols 1 and 2-prop-2-ynyloxyanilines 4, respec-
tively, through tandem oxidative aminocarbonylation of the
triple bondsintramolecular conjugate addition. Reactions
were carried out in the presence of catalytic amounts of PdI2
in conjunction with an excess of KI in N,N-dimethylaceta-
mide (DMA) as the solvent at 80-100 °C and under 20 atm
(at 25 °C) of a 4:1 mixture of CO-air. The reaction showed
a significant degree of stereoselectivity, the Z isomers being
formed preferentially or exclusively. The configuration
around the double bond of the major stereoisomers was
unequivocally established by X-ray diffraction analysis.
This reactivity can be exploited for synthesizing functional-
ized heterocycles when applied to alkynes bearing a suitably
placed nucleophilic group. In this context, we have recently
reported the synthesis of 4-dialkylamino-5H-furan-2-ones2 and
4-dialkylamino-1,5-dihydropyrrol-2-ones,3 starting from pro-
pynyl alcohols or amines, respectively, by a sequential oxidative
aminocarbonylation-intermolecular conjugate addition-cy-
clization route (Scheme 2), as well as the synthesis of 2-[(di-
alkylcarbamoyl)methylene]tetrahydrofuran derivatives starting
from 4-yn-1-ols,4 by an oxidative aminocarbonylation-intramo-
lecular conjugate addition sequence (Scheme 3).
We now wish to report a useful extension of this kind of
reactivity, which allows an easy preparation of 2-[(dialkyl-
carbamoyl)methylene]-2,3-dihydrobenzo[1,4]dioxines and 3-
[(dialkylcarbamoyl)methylene]-3,4-dihydro-2H-benzo[1,4]-
oxazines starting from readily available 2-prop-2-ynyloxyphe-
nols or 2-prop-2-ynyloxyanilines, respectively, according to
Scheme 4.5
The PdI2-catalyzed oxidative aminocarbonylation of the triple
bond is an excellent tool for synthesizing 2-ynamides directly
from terminal alkynes (eq 1).1 The reaction occurs through the
formation of an alkynylpalladium complex as the key intermedi-
ate, which then inserts carbon monoxide and undergoes nu-
cleophilic displacement by the secondary amine used as
nucleophile (Scheme 1).
To our knowledge, this protocol represents the first example
of a direct synthesis of 2,3-dihydrobenzo[1,4]dioxine and 3,4-
† Dipartimento di Scienze Farmaceutiche.
‡ Dipartimento di Chimica.
(2) Gabriele, B.; Salerno, G.; Plastina, P.; Costa, M.; Crispini, A. AdV.
Synth. Catal. 2004, 346, 351-358.
§ Present address: Department of Medicinal Chemistry, China Pharmaceutical
University, Nanjing 210009, PRC China.
(3) Gabriele, B.; Plastina, P.; Salerno, G.; Costa, M. Synlett 2005, 935-
938.
(1) Gabriele, B.; Salerno, G.; Veltri, L.; Costa, M. J. Organomet. Chem.
2001, 622, 84-88.
(4) Gabriele, B.; Salerno, G.; Plastina, P. Lett. Org. Chem. 2004, 1, 134-
136.
10.1021/jo061229l CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/08/2006
J. Org. Chem. 2006, 71, 7895-7898
7895

SCHEME 4
employed for the oxidative aminocarbonylation of 4-yn-1-ols
and propargyl alcohols, i.e., in DME as the solvent at 80 °C
under 20 atm (at 25 °C) of a 4:1 mixture of CO-air in the
presence of catalytic amounts of PdI2 in conjunction with KI
and using diethylamine 2a as the amine (PdI2/KI/1a/2a molar
ratio ) 1:10:200:400, 1a concentration ) 0.5 mmol per mL of
DME). After 3 h, we observed the formation of a Z/E mixture
of 2-benzo[1,4]dioxin-2-ylidene-N,N-diethylacetamide 3aa in
43% total yield (Z/E ) 72:28) at 63% conversion of 1a (eq 2)
(Table 1, entry 1, see the Supporting Information).
dihydro-2H-benzo[1,4]oxazine derivatives by direct carbonyl-
ation of acyclic substrates.6-8 The development of new,
selective, and atom-economical synthetic methodologies for the
direct preparation of functionalized 2,3-dihydrobenzo[1,4]-
dioxines and 3,4-dihydro-2H-benzo[1,4]oxazines starting from
simple building blocks through an ordered sequence of steps is
of particular interest, also in view of the wide range of biological
activities shown by many derivatives of these classes of
heterocycles.9,10
The oxidative carbonylation of 2-prop-2-ynyloxyphenol 1a
was initially carried out under conditions similar to those
The stereochemistry around the double bond of the major
stereoisomer was established to be Z by single-crystal X-ray
diffraction analysis (see the Supporting Information for details).
The Z/E ratio reflected the relative stability of the diastereomers
under the reaction conditions, as shown by blank experiments.11
In an attempt at improving this initial result, we carried out
several experiments under different conditions (Table 1, entries
2-9). The use of an equimolar amount of 2a with respect to
1a led to a lower conversion of 1a and to a lower yield of the
final product (30%, entry 2). On the other hand, working with
(5) Formation of the 2-ynamide intermediates shown in Scheme 4 is
strongly suggested on the basis of what we have already demonstrated in
the case of aminocarbonylation of 2-yn-1-ols leading to 4-dialkylamino-
5H-furan-2-ones (Scheme 2)2 and of 4-yn-1-ols leading to 2-[(dialkylcar-
bamoyl)methylene]tetrahydrofurans (Scheme 3).4 In those cases, we were
able to isolate the corresponding 2-ynamide intermediates, which were
shown to convert into the final products under the reaction conditions in
the absence of the metal catalyst (this also proves that the conjugate addition
step is not catalyzed by palladium).2,4 In the present case, unfortunately,
all the attempts to isolate the 2-ynamide intermediates shown in Scheme 4
were unsuccessful. It is, however, interesting to note that the intramolecular
conjugate addition in very similar intermediates, such as 4-(2-hydroxy-
phenoxy)but-2-ynoic acid methyl ester, 4-(2-hydroxyphenoxy)but-2-ynoic
acid methyl ester, 4-(2-hydroxyphenylsulfanyl)but-2-ynoic acid methyl ester,
and 4-(2-mercaptophenoxy)but-2-ynoic acid methyl ester (formed in situ
by the reaction between 1,2-benzenediol, 2-methylaminophenol, or 2-mer-
captophenol with 4-chlorobut-2-ynoic acid methyl ester), has been described
in the literature.6a Moreover, the fact that 2-prop-2-ynyloxyphenols or
2-prop-2-ynyloxyanilines bearing an internal triple bond were unreactive
under our reaction conditions is another indirect proof of the validity of
the mechanism shown in Scheme 4, which clearly can be at work only
starting from substrates bearing a terminal triple bond.
(6) The synthesis of (4H-benzo[1,4]oxazin-3-ylidene)acetic esters and
benzo[1,4]dioxin-2-ylideneacetic acid esters by indirect carbonylation of
2-aminophenols or benzene-1,2-diol with 4-chlorobut-2-ynoic acid methyl
ester has been reported: (a) Cabiddu, S.; Floris, C.; Melis, S.; Sotgiu, F.;
Cerioni, G. J. Heterocycl. Chem. 1986, 23, 1815-1820. The indirect
carbonylation of 2-aminophenol with 4-chloro-3-oxobutyric acid ethyl ester
to give (4H-benzo[1,4]oxazin-3-ylidene)acetic acid ethyl ester has also been
reported: (b) Puebla, P.; Honores, Z.; Medarde, M.; Caballero, E.; Feliciano,
A. S.; Moran, L. J. Heterocycl. Chem. 1999, 36, 1097-1100. For other
syntheses of (4H-benzo[1,4]oxazin-3-ylidene)acetic esters, not involving
cyclization reactions, see: (c) Sabitha, G.; Reddy, M. M.; Srinivas, D.;
Yadov, J. S. Tetrahedron Lett. 1999, 40, 165-166. (d) Pippich, S.; Bartsch,
H.; Holzer, W. Tetrahedron 1997, 53, 8439-8446.
(7) For representative recent examples of synthesis of 2,3-dihydrobenzo-
[1,4]dioxines by cyclization of acyclic substrates, see: (a) Krois, S.; Steglich,
W. Tetrahedron 2004, 60, 4921-4930. (b) Labrosse, J.-R.; Lhoste, P.;
Delbecq, F.; Sinou, D. Eur. J. Org. Chem. 2003, 2813-2822. (c) Labrosse,
J.-R.; Lhoste, P.; Sinou, D. Eur. J. Org. Chem. 2002, 1966-1971. (d)
Kuwabe, S.-I.; Torraca, K. E.; Buchwald, S. L. J. Am. Chem. Soc. 2001,
123, 12202-12206. (e) Wells, G. J.; Tao, M.; Josef, K. A.; Bihovsky, R.
J. Med. Chem. 2001, 44, 3488-3503. (f) Kitaori, K.; Furukawa, Y.;
Yoshimoto, H.; Otera, J. AdV. Synth. Catal. 2001, 343, 95-101. (g) Fukuda,
Y.; Seto, S.; Furuta, H.; Ebisu, H.; Oomori, Y.; Terashima, S. J. Med. Chem.
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D. J. Org. Chem. 2001, 66, 1018-1025. (i) Labrosse, J.-R.; Lhoste, P.;
Sinou, D. J. Org. Chem. 2001, 66, 6634-6642. (j) Fang, Q. K.; Grover,
P.; Han, Z.; McConville, F. X.; Rossi, R. F.; Olsson, D. J.; Kessler, D. W.;
Wald, S. A.; Senanayake, C. H. Tetrahedron: Asymmetry. 2001, 12, 2169-
2174.
(8) For representative recent examples of the synthesis of 3,4-dihydro-
2H-benzo[1,4]oxazines by cyclization of acyclic substrates, see: (a) Omar-
Amrani, R.; Schneider, R.; Fort, Y. Synthesis 2004, 2527-2534. (b) Yang,
S.-C.; Lai, H.-C.; Tsai, Y.-C. Tetrahedron Lett. 2004, 45, 2693-2698. (c)
Banik, B. K.; Banik, I.; Samajdar, S.; Wilson, M. Heterocycles 2004, 63;
283-296. (d) Bunce, R. A.; Herron, D. M.; Hale, L. Y. J. Heterocycl. Chem.
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2003, 46, 1962-1979. (f) Largeron, M.; Neudorffer, A.; Vuilhorgne, M.;
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2894. (h) Stefanic, P.; Turnsek, K.; Kikelj, D. Tetrahedron 2003, 59, 7123-
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(9) For some very recent examples of pharmacologically active 2,3-
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Alam, M.; Rocha, C.; Lin, L.; Bjorner, B.; Nguyen, K.; Grady, C.; Williams,
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7896 J. Org. Chem., Vol. 71, No. 20, 2006

a 2a/1a molar ratio of 5 led to a 46% yield of 3aa at 86%
substrate conversion (entry 3). The selectivity toward 3aa in
this reaction (yield of 3aa/conversion of 1a) was, however, lower
(53%) compared to that obtained in the initial experiment carried
out with a 2a/1a molar ratio of 2 (68%, entry 1). The use of a
KI/PdI2 molar ratio of 100 rather than 10 led to a faster reaction
rate and to a higher yield of 3aa (entry 4). A similar effect was
observed by increasing the concentration of 1a (from 0.5 to
1.0 mmol of product per milliliter of solvent, entry 5) or by
raising the reaction temperature (from 80 to 100 °C, entry 6).
We finally screened the reaction medium (entries 7-9). Of the
solvents tested, N,N-dimethylacetamide (DMA) led to the most
satisfactory results, both in terms of product yield and stereo-
selectivity (entry 9). On the basis of these results, the next
experiment was carried out at 100 °C in DMA as the solvent
with a 2a/1a/KI/PdI2 molar ratio of 400:200:100:1 and a 1a
concentration of 1 mmol per milliliter of DMA. Under these
optimized conditions, the substrate conversion was quantitative
after 2 h, with a 75% GLC yield of 3aa (67% isolated) and a
Z/E ratio of ca. 3:1 (eq 3) (Table 2, entry 10, see the Supporting
Information).
from 100 to 80 °C (entry 14 and eq 4). It should be noted that
the same reaction, carried out for 3 h rather than 2 h, led to a
20% yield of 3ab at total substrate conversion (entry 15).
Substituted substrates 1b,c also led to satisfactory yields in the
corresponding dihydrobenzodioxines 3bb-Z and 3cb (as a 6.3:1
Z:E mixture), after 1-2 h reaction time (entries 16-17 and eq
4).
As we have already observed in the PdI2-catalyzed oxidative
aminocarbonylation of simple terminal alkynes,1 a secondary
nucleophilic amine, such as 2a or 2b, was required for the
reaction to occur. In fact, hindered secondary amines (such as
diisopropylamine) or secondary amines of low basicity (such
as N-ethylaniline) turned out to be unreactive. On the other hand,
primary amines underwent oxidative carbonylation with forma-
tion of ureas and oxamides, according to a reactivity that we
have already described.13
The optimized conditions found for 2-prop-2-ynyloxyphenols
1 were then applied to 2-prop-2-ynyloxyanilines 4. The reaction
of 2-prop-2-ynyloxyphenylamine 4a with diethylamine 2a
afforded (Z)-2-(4H-benzo[1,4]oxazin-3-ylidene)-N,N-diethylac-
etamide 5aa-Z in 59% isolated yield (eq 5) (Table 3, entry 18,
see the Supporting Information), whose structure was confirmed
by single-crystal X-ray analysis (see the Supporting Information
for details).
The reaction was successfully extended to other substrates,
bearing either electron-withdrawing or π-donating groups, as
shown in eq 3 and Table 2, entries 11 and 12.
Interestingly, the use of morpholine 2b instead of diethyl-
amine 2a led to the corresponding 2,3-dihydrobenzo[1,4]dioxine
3ab in only 23% GLC yield (Z/E = 6.7, entry 13) at total
substrate conversion. The formation of unidentified chromato-
graphically immobile materials accounted for the remaining part
of the converted substrate. This low yield was due to the
instability of the product under the reaction conditions, as shown
by blank experiments.12 However, a good yield of 3ab (81%
by GLC, 74% isolated, Z/E ) 6.4, at 83% substrate conversion)
could be obtained by simply decreasing the reaction temperature
(10) For representative recent examples of pharmacologically active 3,4-
dihydro-2H-benzo[1,4]oxazine derivatives, see refs 9h and: (a) Blattes, E.;
Lockhart, B.; Lestage, P.; Schwendimann, L.; Gressens, P.; Fleury, M.-B.;
Largeron, M. J. Med. Chem. 2005, 48, 1282-1286. (b) Wu, Y.-J.; Boissard,
C. G.; Chen, J.; Fitzpatrick, W.; Gao, Q.; Gribkoff, V. K.; Harden, D. G.;
He, H.; Knox, R. J.; Natale, J.; Pieschl, R. L.; Starrett, J. E., Jr.; Sun, L.-
Q.; Thompson, M.; Weaver, D.; Wu, D.; Sworetzky, S. I. Bioorg. Med.
Chem. Lett. 2004, 14, 1991-1996. (c) Torisu, K.; Kobayashi, K.; Iwahashi,
M.; Nakai, Y.; Onoda, T.; Nagase, T.; Sugimoto, I.; Okada, Y.; Matsumoto,
R.; Nanbu, F.; Ohuchida, S.; Nakai, H.; Toda, M. Bioorg. Med. Chem. 2004,
12, 5361-5378. (d) Chen, X.; Kempf, D. J.; Li, L.; Sham, H. L.;
Vasavanonda, S.; Wideburg, N. E.; Saldivar, A.; Marsh, K. C.; MacDonald,
E.; Norbeck, D. W. Bioorg. Med. Chem. Lett. 2003, 13, 3657-3660.
(11) When pure 3aa-Z or 3aa-E was allowed to react in DME (0.5 mmol
per milliliter of DME) at 80 °C for 3 h in the presence of Et2NH (2 equiv),
formation of a Z/E mixture (Z/E ratio ) 2.6) was observed, ensuing from
equilibration of the two diastereomers under these conditions.
No formation of the corresponding E isomer was observed,
while the formation of unidentified chromatographically im-
mobile materials accounted for the substrate conversion (100%).
(12) When pure 3ab-Z or 3ab-E was allowed to react under the same
reaction conditions of entry 13, extensive decomposition occurred, with
formation of chromatographically immobile materials. Only 20-30% of
the starting material remained in the reaction mixture, as shown by GLC
analysis.
(13) (a) Gabriele, B.; Salerno G.; Mancuso, R.; Costa, M. J. Org. Chem.
2004, 69, 4741-4750. (b) Gabriele, B.; Mancuso, R.; Salerno, G.; Costa,
M. Chem. Commun. 2003, 486-487.
J. Org. Chem, Vol. 71, No. 20, 2006 7897

The reason for the exclusive formation of the Z isomer lies in
its much higher stability with respect to the E isomer, owing to
the possibility of an intramolecular hydrogen bond between the
hydrogen of the amino group and the oxygen of the carbonyl.
This was confirmed by the X-ray structure of 5aa-Z at the solid
state, which clearly showed this interaction (see the Supporting
Information).
Other 2-prop-2-ynyloxyphenylamines 4b-d, bearing electron-
donating or electron-withdrawing groups on the aromatic ring,
behaved similarly and afforded the corresponding (Z)-benzox-
azine derivatives in satisfactory isolated yields (54-74%, eq 5
and Table 3, entries 19-21). As expected in view of the higher
nucleophilicity of a secondary cyclic amine compared to that
of an acyclic dialkylamine,14 the aminocarbonylation of 4a-d
was faster using morpholine 2b as the nucleophile rather than
diethylamine 2a and could be successfully carried out at 80 °C
rather than 100 °C for 2-3 h. Also in this case, the reaction
was completely stereoselective, affording the corresponding (Z)-
2,3-dihydrobenzo[1,4]oxazines in good isolated yields (60-
76%, eq 6 and Table 3, entries 22-25).
In conclusion, we have described the one-step synthesis of
2-[(dialkylcarbamoyl)methylene]-2,3-dihydrobenzo[1,4]-
dioxines 3 and (Z)-3-[(dialkylcarbamoyl)methylene]-3,4-dihy-
dro-2H-benzo[1,4]oxazines 5 by tandem PdI2-catalyzed oxida-
tive aminocarbonylation of the triple bond-intramolecular
conjugate addition, starting from readily available 2-prop-2-
ynyloxyphenols and 2-prop-2-ynyloxyanilines. The products
have been obtained in acceptable isolated yields, and the reaction
has showed a high degree of stereoselectivity toward the
formation of the Z isomer, which in several cases has been
obtained exclusively. The structure of some representative
products was confirmed by X-ray diffraction analysis.
Acknowledgment. We thank the Ministero dell’Istruzione,
dell’Universita` e della Ricerca (MIUR) for financial support
and for a grant to Z.L. (Progetto di Ricerca di Interesse
Nazionale PRIN 2003039774).
Supporting Information Available: Tables 1-3, Experimental
Section, X-ray crystallographic data for compounds 3aa-Z and 5aa-
Z, and spectroscopic and elemental analysis data for all compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(14) Katritzky, A. R.; Pozharskii, A. F. Handbook of Heterocyclic
Chemistry, 2nd ed.; Pergamon: Kidlington, Oxford, 2000; p 248.
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