moiety in the presence of propargylic carbonates affording
allenyl cyclic products, in which the CO2 was eliminated
and released.6 Based on these previous works and retro-
synthetic analysis, we envisioned that allenic oxazolidin-2-
ones 3 may be efficiently constructed by using 2,3-allenyl
amines7 1 and propargylic carbonate 2 in an atom-
economic manner provided that the to-be-released CO2 can
be recycled8 intermolecularly with high efficiency (3 and 6 vs
4 and 5, Scheme 1) and the issue of regioselectivity can be
addressed (3 vs 6, Scheme 1). Herein, we report an efficient
synthesis of 5-(1,3,4-alkatrien-2-yl)oxazolidin-2-one 3 via
palladium-catalyzed cyclization reactions of 2,3-allenyl
amines in the presence of propargylic carbonates with
the in situ generated CO2 recycled under mild conditions.
obtained in 94% NMR yield and 88% isolated yield by
using 5 mol % Pd(PPh3)4, 1.5 equiv of 2a, and 2 equiv of
K2CO3 in DMSO at 70 °C for 4 h (entry 1, Table 1). Also,
we were pleasedtofind that the formation of 4- and 5-types
of non-CO2-incorporated cyclization products9,10 or the
6-type CO2-incorportaed regioisomer was not observed.
Encouraged by these results, further optimization of the
reaction conditions was carried out subsequently. Chang-
ing the palladium catalyst (entries 2 and 3, Table 1) or the
solvent (entries 4ꢀ7, Table 1) led to a decrease in the yield
of 3aa. Other bases such as Cs2CO3 or NEt3 were less
effective (entries 8 and 9, Table 1). Notably, 66% and 75%
NMR yields of 3aa were observed even in the absence of
K2CO3, indicating that the CO2 trapped in the oxazolidin-
2-one indeed comes from propargylic carbonates (entries 9
and 10, Table 1). Lowering or elevating the temperature
proved to be deleterious (entries 11 and 12, Table 1)
whereas lowering the amount of 2a to 1.2 equiv improved
the yield of 3aa to 99% by NMR (entry 13, Table 1). In
contrast, increasing the amount of 2a to 2 equiv resulted in
a lower yield of 3aa (entry 14, Table 1). Thus, 5 mol %
Pd(PPh3)4, 1.2 equiv of 2a, and 2 equiv of K2CO3 in
DMSO at 70 °C for 4 h were established as the standard
conditions for further study.
Scheme 1. Concepts and Selectivity Issue for the Synthesis of
Allenic Oxazolidin-2-ones 3
Table 1. Optimization of Reaction Conditions for the
Pd-Catalyzed Cyclization of 1a with 2aa
Initially, terminal 2,3-allenyl amine 1a and propargylic
carbonate 2a were chosen to test the feasibility of our
hypothesis. Gratifyingly, the expected product 3aa was
(6) (a) Ma, S.; Gu, Z.; Deng, Y. Chem. Commun. 2006, 94. (b) Shu,
W.; Jia, G.; Ma, S. Org. Lett. 2009, 11, 117. (c) Shu, W.; Ma, S.
Tetrahedron 2010, 66, 2869. (d) Chen, G.; Zhang, Y.; Fu, C.; Ma, S.
Tetrahedron 2011, 67, 2332.
yield of
3aa (%)b
entry
[Pd]
solvent
base
(7) For a report on the synthesis of oxazolidin-2-ones by Pd-cata-
lyzed cyclization of 2,3-allenyl amines with pressurized CO2 in 2ꢀ65%
NMR yields, see: Kayaki, Y.; Mori, N.; Ikariya, T. Tetrahedron Lett.
2009, 50, 6491.
(8) For reviews on synthesis of cyclic carbonates via intramolecular
“CO2 recycling” from allylic or propargylic carbonates, see: (a) Yoshida,
M.; Ihara, M. Chem.;Eur. J. 2004, 10, 2886. (b) Yoshida, M. J. Synth.
Org. Chem. Jpn. 2010, 68, 160and references cited therein. For reports
on the synthesis of oxazolidin-2-ones, see: (c) Yoshida, M.; Ohsawa, Y.;
Sugimoto, K.; Tokuyama, H.; Ihara, M. Tetrahedron Lett. 2007, 48,
8678. (d) Yoshida, M.; Komatsuzaki, Y.; Ihara, M. Org. Lett. 2008, 10,
2083.
(9) For reviews on Pd-catalyzed cyclization of allenes with a nucleo-
philic moiety, see: (a) Bates, R. W.; Satcharoen, V. Chem. Soc. Rev.
2002, 31, 12. (b) Ma, S. Acc. Chem. Res. 2003, 36, 701. (c) Ma, S. In
Topics in Organometallic Chemistry; Tsuji, J., Ed.; Springer-Verlag:
Heidelberg, 2005; pp 183ꢀ210. (d) Alcaide, B.; Almendros, P.; del Campo,
T. M. Chem.;Eur. J. 2010, 16, 5836.
1
Pd(PPh3)4
Pd(dba)2/TFPd
Pd(OAc)2/TFPe
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
DMSO
DMSO
DMSO
CH3CN
DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
NEt3
94(88)c
52
2
3
67
4
89
5
82
6
THF
39
7
DCE
77
8
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
22
9
66
f
10
11g
12h
13i
14j
ꢀ
75
K2CO3
K2CO3
K2CO3
K2CO3
72
36
99
83
(10) For selected examples on Pd-catalyzed cyclization reactions of
2,3-allenyl amines with organic halides, see: (a) Ohno, H.; Toda, A.;
Miwa, Y.; Taga, T.; Osawa, E.; Yamaoka, Y.; Fujii, N.; Tanaka, T.;
Ibuka, T. J. Org. Chem. 1999, 64, 2992. (b) Ohno, H.; Anzai, M.; Toda,
A.; Ohishi, S.; Fujii, N.; Tanaka, T.; Takemoto, Y.; Ibuka, T. J. Org.
Chem. 2001, 66, 4904 and references cited therein. (c) Dieter, R. K.; Yu,
H. Org. Lett. 2001, 2, 3855. (d) Shibata, T.; Kadowaki, S.; Takagi, K.
Heterocycles 2002, 57, 2261. (e) Ma, S.; Yu, F.; Gao, W. J. Org. Chem.
2003, 68, 5943. (f) Shu, W.; Yu, Q.; Jia, G.; Ma, S. Chem.;Eur. J. 2011,
17, 4720.
a The reaction was carried out on a 0.1 mmol scale of 1a in 1 mL of the
indicated solvent. b NMR yield of 3aa determined by 1H NMR analysis
of the crude reaction mixture using 1,3,5-trimethylbenzene as the
internal standard. c The value in parentheses is the isolated yield of
3aa. d Pd(dba)2 (5 mol %) and TFP (10 mol %) were used; TFP = tri-(20-
furyl)phosphine. e Pd(OAc)2 (5 mol %) and TFP (10 mol %) were used.
f No base was added. g Run at 50 °C. h Run at 90 °C. i 2a (1.2 equiv) was
used. j 2a (2 equiv) was used.
Org. Lett., Vol. 14, No. 9, 2012
2313