Reactions of enaminones with diazocarbonyl compounds

Article abstract of DOI:10.1002/hlca.201200233

The [Cu(acac)₂]-catalyzed reactions of several tertiary enaminones with three diazocarbonyl compounds, i.e., dimethyl diazomalonate, ethyl diazoacetoacetate, and ethyl diazoacetate, yielded amino- and additionally carbonyl-substituted dihydrofurans, together with further furan derivatives. Due to the conjugation of α-carbonyl/α-Ph groups, reactions proceeded only via 1,5-electrocyclization of corresponding keto-ylides. On the other hand, in the absence of any α-substituent, tertiary enaminone and ethyl diazoacetate, reacted via an accompanying mechanism by a push-pull cyclopropane intermediate, to yield a 2,4-dicarbonyl-substituted furan in one step with moderate yield. Copyright

Full text of DOI:10.1002/hlca.201200233

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Helvetica Chimica Acta – Vol. 96 (2013)
Reactions of Enaminones with Diazocarbonyl Compounds
by Fꢀsun S¸eyma Gꢀngçr*, Neslihan Hancıog˘lu, and Olcay AnaÅ*
Istanbul Technical University, Faculty of Science and Letters, Department of Chemistry, 34469, Maslak,
Istanbul, Turkey (phone: þ 90-212-2856825; fax: þ 90-212-2856386; gungorfus@itu.edu.tr)
The [Cu(acac)2]-catalyzed reactions of several tertiary enaminones with three diazocarbonyl
compounds, i.e., dimethyl diazomalonate, ethyl diazoacetoacetate, and ethyl diazoacetate, yielded
amino- and additionally carbonyl-substituted dihydrofurans, together with further furan derivatives. Due
to the conjugation of a-carbonyl/a-Ph groups, reactions proceeded only via 1,5-electrocyclization of
corresponding keto-ylides. On the other hand, in the absence of any a-substituent, tertiary enaminone
and ethyl diazoacetate, reacted via an accompanying mechanism by a push-pull cyclopropane
intermediate, to yield a 2,4-dicarbonyl-substituted furan in one step with moderate yield.
Introduction. – We have already reported our initial findings on the reactions of
tertiary enaminones 1 with dimethyl diazomalonate 2 in the presence of [Cu(acac)2]
[1]. In contrast to the present literature [2], we determined the formation of significant
amounts of dihydrofuran derivatives 3 along with naphthalenone 4, and formal CaꢀH
insertion products 5. Moreover, we recently performed [Cu(acac)2]-catalyzed reactions
of dimethyl diazomalonate with several more elaborate enaminone systems to extend
this chemistry and to investigate the critical parameters (Scheme 1) [1][3].
The chemoselectivities of the reactions, including those of the new tertiary
enaminones, [1][3] indicated that significant amounts of 2,3-dihydrofuran 3, 2,5-
Scheme 1
ꢀ 2013 Verlag Helvetica Chimica Acta AG, Zꢁrich

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dihydrofuran 6, naphthalenone 4, and quinoline derivatives 7 could be obtained mostly
as main products. When dialkylamino groups were incorporated in the starting
substrates, formal CaꢀH insertion products 5 were also found as minor products.
Obviously, via various possible pathways, this method can be applied in the synthesis of
poly-functional derivatives as valuable building blocks, when the starting enaminones
are well chosen.
Herein, we report our results from the reactions of (E)-enaminones with different
substitution patterns. In addition, the the diazo compound was also varied. Thereby, we
aimed at clarifying the steric and electronic effects of the reactants on the reaction path.
Results and Discussion. – The [Cu(acac)2]-catalyzed (acac ¼ acetylacetonate ¼
pentane-2,4-dionate) reactions of enaminones 8a and 8b, with dimethyl diazomalonate
(2), ethyl diazoacetoacetate (¼ethyl 2-diazo-3-oxobutanoate; 9), and ethyl 2-
diazoacetate (10), respectively, were studied first (Scheme 2). The reasons for starting
with 8a and 8b can be summarized as follows: Compounds 8a and 8b were regarded to
be capable of forming some novel products by electrocyclization of the carbonyl-ylide
intermediates with the Ph ring to yield benzo[c][1,5]oxazocine derivatives. This
pathway is, however, not possible for enaminones of type 8d (Scheme 2).
Rotation around formal C¼C bonds in this kind of conjugated enaminones such as
(E/Z)-8b might be possible [4], and a preference between these two different
conjugated ylides might also be conceivable.
Besides (E/Z)-8b, a-phenyl (E/Z)-enaminone 8c was also used as starting
compound to determine the effect of the a-Ph group on the possible rotation of C¼C
bond of ylide intermediates.
Scheme 2

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Finally, benzoyl enamine 8d, which had been reacted previously with diazo
compound 2 to yield 3 – 5 (Scheme 1) [1][3], was studied once more with respect to its
reaction with ethyl 2-diazoacetate (10) to clarify the effect of diazomethyl moiety in
these enaminone-ylide reactions. Our experimental results are compiled in the Table.
Table. Reactions of Enaminones 8a – 8d with Diazo Compounds 2, 9, and 10a)
Enaminone Diazo compound 11 (Yield [%])b)
12 (Yield [%])b) 13 (Yield [%])b)
8a
8b
8b
8a
2
2
2
10
1 11a2 (60%)
1 11b2 (55%)
1 11a2 (65%)
–
–
–
–
–
–
–
1 cis-11a10 (18%)/
trans-11a10 (27%) 1 : 1.73c)
1 cis-11b10 (45%)/
trans-11b10d) 1 : 1c)
1 cis-11c10/trans-11c10 1 : 1c)
–
1 cis-11a9/trans-11a9 1 : 3.5c)
1 cis-11b9/trans-11b9 1 : 4.53c)
1 cis-11c9/trans-11c9 1 : 1c)
0.2 12a10 (50%)
8b
10
–
–
8c
8d
8a
8b
8c
10
10
9
9
9
–
–
8.7 12d10 (50%)
1 13d10 (40%)
–
–
–
–
–
–
c
a) Normalized by GC values. b) Yield of isolated product. ) The ratio cis-11/trans-11 was determined by
1H-NMR of the reaction mixture. d) Obtained with ethyl 2-(diazoaceto)acetate dimers.
From all reactions of 8a – 8d with diazo compounds 2, 9, and 10, dihydrofurans 11
and furans 12 were obtained by cyclization of the corresponding 1,5-carbonyl-ylide
intermediates.
In the reactions with 9, we could easily detect the dihydrofuran derivatives 11a9 –
1
11c9 by GC/MS and H-NMR of the reaction mixture as the sole formal [1 þ 1]
products (Table). Attempts of purification of 11a9 – 11c9 failed due to decomposition,
however, the decomposition products turned out to be identical with the dihydrofurans
obtained from the reactions of the same enaminones and 10 (cf. the Table). Another
remarkable observation was the formation of cis-11a10 – cis-11c10 from trans-11a9 –
trans-11c9, and the formation of trans-11a10 – trans-11c10 from cis-11a9 – cis-11c9
(Scheme 2). The mechanism of the observed decomposition reaction is not clear.
Moreover, neither analogous derivatives of 5/6 [4] nor novel benzo[c][1,5]oxazo-
cine derivatives were detected in our reactions. Obviously, the presence of neither two
C¼O (i.e., in 8a and 8b) nor C¼O and Ph groups (i.e., in 8c) in the conjugated ene-
carbonyls enhanced the dihydrofuran-producing carbonyl-ylide formation, suppressing
the other pathways by donor-acceptor-substituted cyclopropane intermediates or by
nitrogen-ylide intermediates [3].
The reaction of enaminones 8a and 8d with 10 yielded also 12a10 and 12d10,
respectively, which are formed by elimination of Me2NH from the related parent
compounds, 11a10 and 11d10 (Scheme 3). Compound 12d10 was the main product
(Table). This result is not surprising, considering of the relative stability of the related
carbocations due to the presence of a Ph group for E1 elimination of cis/trans-11d10.

Helvetica Chimica Acta – Vol. 96 (2013)
491
Scheme 3
Additionally, an unexpected derivative, 13d10 was also obtained (Scheme 2). This
reaction probably proceeds by ring opening (due to the adjacent Ph group) of the
donor-acceptor cyclopropane-carboxylate 14. Recyclization of the formed intermediate
15 would lead to a dihydrofuran-carboxylate 16, which, by subsequent Me2NH
elimination, furnishes the 2,4-disubstituted furan 13 (Scheme 4).
Scheme 4
According to literature [5], these kinds of 2-substituted furan-4-carboxylates were
synthesized starting from the appropriate aldehyde in four steps in 30 – 40% yield. Our
method led to the furan derivative 13d10 with the same yield but in only one step. We
performed the same reactions with additional catalysts ([Rh2(OAc)4] and [Cu(OTf)2])
to increase the yield 2,4-disubstituted furan 13, however, without success.
Conclusions. – In summary, [Cu(acac)2]-catalyzed reactions of tertiary enaminones
8a – 8d and diazocarbonyl compounds 2, 9, and 10 led to novel amino- and additionally-
carbonyl-substituted 2,3-dihydrofurans 11 and/or furan derivatives 12. Due to the
additional conjugation of a-carbonyl (in 8a and 8b)/a-Ph (in 8c) groups, the reactions
proceeded only via 1,5-electrocyclization of corresponding keto-ylides. On the other
hand, in the absence of any a-substituent, tertiary enaminone 8d and ethyl 2-
diazoacetate (10) gave a 2,4-substituted furan 13 by a push-pull cyclopropane
intermediate in one step with moderate yield. We also observed that 11a9 – 11c9 easily
decomposed to 11a10 – 11c10 during purification attempts. This degradation mechanism
is not clear.
Experimental Part
General. All of the solvents and reactants are commercially available. The reactions of diazo
compounds and enaminones were carried out under N2. Dimethyl diazomalonate [6], ethyl 2-
diazoacetate [7], and ethyl 2-(diazoaceto)acetate [8] were prepared according to literature procedures.

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1H- and 13C-NMR spectra: in CDCl3, with Bruker AC spectrometers; at 500 and 125, or 250 and 60 MHz,
resp., at r.t.; d in ppm rel. to Me4Si as internal standard, J in Hz. GC/MS: Hewlett-Packard instrument
equipped with a flame ionization detector; in m/z (rel. %). A cross-linked (phenylmethyl)siloxane cap.
column (30 m ꢁ 0.32 mm ꢁ 0.25 mm) was used with He as the carrier gas (25 psi column head pressure);
temp. program: starting with 1008 then 5 min isothermal, ramp 208/min; final 2908, and then 10 min
isothermal, retention times (tR) are given in min.
General Procedure for the Reaction of Enaminones with Diazo Compounds. To a soln. of 8
(2.1 mmol) in benzene (10 ml) was added [Cu(acac)2] (9 ꢁ 10ꢀ3 mmol), and the mixture was heated at
reflux. A soln. of diazo compound (1.4 mmol) in benzene (4 ml) was added to this soln. over 2.5 h under
N2. When the IR spectrum of the reaction mixture indicated total consumption of the diazo compound
(absence of the characteristic diazo band), the mixture was filtered, evaporated, and purified by column
chromatography (CC) or prep. TLC. The crude mixture contained varying amounts of unidentified
compounds (max. 20% by GC).
Dimethyl 4-Acetyl-5-methyl-3-[methyl(phenyl)amino]furan-2,2(3H)-dicarboxylate (11a2): Purifica-
tion by CC (neutral Al2O3; hexane/AcOEt 9 :1). Prep. TLC (Al2O3; hexane/AcOEt 90 :10): 11a2 Yield:
60% (tR 13.86). Dark-yellow oil. 1H-NMR (250 MHz): 7.29 – 7.22 (m, 2 arom. H); 6.95 (d, J ¼ 8.0, 2 arom.
H); 6.79 (distorted t, J ¼ 7.2 – 7.1, 1 arom. H); 6.10 (s, CHN); 3.87 (s, COOMe); 3.48 (s, COOMe), 2.62 (s,
MeN); 2.41 (s, C¼CMe); 1.87 (s, MeCO). 13C-NMR (60 MHz): 194.5; 168.7; 166.8; 164.8; 149.1; 129.2;
118.4; 113.3; 98.7; 91.6; 68.8; 53.8; 52.8; 32.2; 29.2; 14.8. EI-MS (70 eV): 347 (58, Mþ), 241 (30), 197
(100), 167 (48), 151 (30), 107 (91), 77 (32), 59 (20).
4-Ethyl 2,2-Dimethyl 5-Methyl-3-[methyl(phenyl)amino]furan-2,2,4(3H)-tricarboxylate (11b2). Pu-
rification by CC (neutral Al2O3; hexane/AcOEt 92 :8). Prep. TLC (Al2O3; hexane/AcOEt 85 :15). Yield:
55% (tR 14.10). 1H-NMR (250 MHz): 7.20 (t, J ¼ 7.6, 2 H); 6.90 (d, J ¼ 8.0, 2 H); 6.76 (t, J ¼ 7.3, 1 H); 5.94
(br. s, 1 H); 4.00 – 3.94 (m, 2 H); 3.86 (s, 3 H); 3.56 (s, 3 H); 2.56 (s, 3 H); 2.38 (s, 3 H); 0.87 (t, J ¼ 7.0,
3 H). 13C-NMR (125 MHz): 168.7; 167.1; 165.3; 164.6; 150.7; 129.0; 118.5; 114.4; 103.9; 92.1; 70.1; 60.1;
54.0; 53.1; 32.7; 14.3; 14.1. EI-MS (70 eV): 377 (31.5, Mþ), 332 (4), 271 (21), 240 (3), 227 (93), 211 (44),
199 (50), 183 (99), 167 (43), 123 (22), 107 (100), 77 (27), 59 (20).
Dimethyl 5-Methyl-3-[methyl(phenyl)amino]-4-phenylfuran-2,2(3H)-dicarboxylate (11c2). Purifica-
tion by CC (neutral Al2O3; hexane/AcOEt 90 :10). Prep. TLC ( Al2O3; hexane/AcOEt 95 :5). Yield:
65% (tR 15.26). 1H-NMR (250 MHz): 7.33 – 7.09 (m, 2 H); 6.93 (d, J ¼ 7.7, 2 H); 6.76 (distorted t, J ¼ 7.0,
1 H); 6.24 (br. s, 1 H); 3.88 (s, 3 H); 3.46 (s, 3 H); 2.64 (s, 3 H); 2.25 (s, 3 H). 13C-NMR (60 MHz): 168.1;
166.0; 152.4; 149.2; 133.1; 129.0; 128.6; 126.8; 126.4; 117.6; 113.3; 110.1; 89.9; 70.8; 53.6; 52.6; 30.1; 13.8.
EI-MS (70 eV): 381 (29, Mþ), 322 (4), 275 (80), 231 (100), 216 (65), 185 (52), 128 (25), 107 (72), 77 (21),
59 (12).
Ethyl trans-4-Acetyl-5-methyl-3-[methyl(phenyl)amino]-2,3-dihydrofuran-2-carboxylate (trans-
1
11a10). CC (silica gel; hexane/AcOEt 80 :20). Yield: 60% (tR 13.50). Yellow oil. H-NMR (250 MHz):
7.21 (distorted d of distorted t, J ¼ 7.2 – 8.2/2.0 – 1.5, 2 H); 6.96 (d, J ¼ 8.8, 2 H); 6.76 (t, J ¼ 7.3, 1 H); 5.41
(d, J ¼ 1.5, 1 H); 4.75 (d, J ¼ 2.9, 1 H); 4.26 – 4.15 (m, 2 H); 2.70 (s, 3 H); 2.40 (s, 3 H); 2.05 (s, 3 H); 1.24
(distorted t, J ¼ 6.8 – 7.3, 3 H). 13C-NMR (125 MHz): 195.0; 171.2; 169.7; 149.1; 129.6; 119.0; 114.9; 111.5;
82.5; 67.4; 62.3; 31.7; 29.4; 15.2; 14.3. EI-MS (70 eV): 303 (27, Mþ), 230 (9), 196 (13), 181 (18), 153 (16),
125 (48), 107 (100), 77 (19).
Ethyl cis-4-Acetyl-5-methyl-3-[methyl(phenyl)amino]-2,3-dihydrofuran-2-carboxylate (cis-11a10).
CC (neutral Al2O3; hexane/AcOEt 90 :10). Yield: 45% (tR 13.77). Yellow oil. 1H-NMR (250 MHz):
7.23 – 7.20 (m, 2 H); 6.83 (d, J ¼ 8.1, 2 H); 6.76 (t, J ¼ 6.8, 1 H); 5.55 (d, J ¼ 8.7, 1 H); 5.11 (d, J ¼ 8.9, 1 H);
4.03 – 3.91 (m, 2 H); 2.64 (s, 3 H); 2.40 (s, 3 H); 1.89 (s, 3 H); 0.93 (t, J ¼ 7.1, 3 H ) . 13C-NMR (60 MHz):
194.7; 170.5; 167.3; 149.2; 129.1; 118.0; 113.4; 111.7; 83.3; 65.0; 61.6; 32.3; 29.1; 15.0; 13.6.
Diethyl trans-5-Methyl-3-[methyl(phenyl)amino]-2,3-dihydrofuran-2,4-dicarboxylate (trans-11b10).
CC (neutral Al2O3; hexane/AcOEt 95 :5). Prep. TLC (Al2O3; hexane/AcOEt 90 :10). Yield: 26% (tR
13.67). Dark-yellow oil. 1H-NMR (250 MHz): 7.21 (t, J ¼ 7.7, 2 H); 6.90 (d, J ¼ 8.1, 2 H); 6.76 (t, J ¼ 7.0,
1 H); 5.42 (br. s, 1 H); 4.84 (d, J ¼ 2.7, 1 H); 4.25 – 4.19 (m, 2 H); 4.02 – 3.97 (m, 2 H); 2.67 (s, 3 H); 2.37
(s, 3 H); 1.30 (distorted t, J ¼ 6.8, 3 H); 0.97 (t, J ¼ 7.1, 3 H ) . 13C-NMR (125 MHz): 169.6; 168.7; 163.8;
148.6; 127.9; 117.1; 113.5; 102.2; 81.6; 66.9; 60.9; 58.7; 30.5; 13.2. EI-MS (70 eV): 333 (20, Mþ), 288 (5),
226 (31), 197 (29), 181 (38), 170 (23), 153 (46), 127 (24), 107 (100), 77 (20), 51 (18).

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Diethyl cis-5-Methyl-3-[methyl(phenyl)amino]-2,3-dihydrofuran-2,4-dicarboxylate (cis-11b10). CC
(neutral Al2O3; hexane/AcOEt 95 :5). Prep. TLC (Al2O3; hexane/AcOEt 90 :10). Yield: 45% (tR 13.89).
Yellow oil. 1H-NMR (250 MHz): 7.18 (t, J ¼ 7.8, 2 H); 6.80 – 6.70 (m, 3 H); 5.41 (d, J ¼ 8.7, 1 H); 5.10 (d,
J ¼ 9.1, 1 H); 4.15 – 3.83 (m, 4 H); 2.59 (s, 3 H); 2.37 (s, 3 H); 1.29 (t, J ¼ 7.1, 3 H); 1.00 (t, J ¼ 7.2, 3 H).
13C-NMR (125 MHz): 169.2; 166.5; 163.9; 149.4; 127.8; 116.9; 113.3; 102.5; 82.4; 65.0; 60.5; 60.2; 30.5;
13.1; 13.0.
Diethyl trans-2-Acetyl-5-methyl-3-[methyl(phenyl)amino]-2,3-dihydrofuran-2,4-dicarboxylate
1
(trans-11b9). CC: tR 13.67. H-NMR (500 MHz): 7.19 (t, J ¼ 8.1, 2 H); 6.90 (d, J ¼ 7.8, 2 H); 6.73 (t, J ¼
7.1, 1 H); 5.95 (br. s, 1 H); 4.30 – 4.22 (m, 2 H); 3.93 – 3.88 (m, 1 H); 3.83 – 3.79 (m, 1 H); 2.60 (s, 3 H);
2.35 (s, 3 H); 2.15 (s, 3 H); 1.31 – 1.26 (m, 3 H); 0.77 (t, J ¼ 6.8, 3 H). 13C-NMR (125 MHz): 196.4; 167.8;
166.6; 163.4; 128.0; 117.5; 112.8; 102.5; 95.2; 68.7; 62.2; 59.7; 31.3; 25.8; 24.8; 13.0; 12.7. EI-MS (70 eV):
375 (33, Mþ), 288 (8), 227 (99), 199 (88), 181 (20), 153 (13), 107 (100), 77 (13).
Ethyl 2-Acetyl-5-methyl-3-[methyl(phenyl)amino]-4-phenyl-2,3-dihydrofuran-2-carboxylate (11c9).
Prep. TLC (neutral Al2O3; hexane/AcOEt 80 :20): 11c9 (3 :1 isomer mixture (1H-NMR). tR 15.26. Light-
yellow oil. 1H-NMR (250 MHz): 7.29 – 7.05 (m, 7 H); 6.95 (d, J ¼ 7.9, 2 H); 6.78 (t, J ¼ 7.1, 1 H); 6.21 (br. s,
1 H); 4.36 – 4.31 (m, 2 H); 2.58 (s, 3 H); 2.26 (s, 3 H); 2.23 (s, 3 H); 1.33 (t, J ¼ 7.1, 3 H ) . 13C-NMR
(125 MHz): 199.8; 168.9; 152.7; 149.5; 133.4; 129.5; 128.8; 126.6; 126.5; 118.2; 113.4; 110.5; 94.7; 71.4;
63.0; 32.4; 27.3; 14.3; 14.1. EI-MS (70 eV): 379 (10, Mþ), 231 (100), 203 (63), 186 (56), 158 (23), 128 (30),
107 (74), 77 (21).
Ethyl 4-Acetyl-5-methylfuran-2-carboxylate (12a10). Yield: 55%. tR 10.04. White solid. M.p. 98 – 1008.
1H-NMR (250 MHz): 8.01 (s, C¼CH); 4.37 – 4.29 (m, COOCH2Me); 2.52 (s, Me); 2.41 (s, COMe); 1.39 –
1.32 (m, COOCH2Me). 13C-NMR (60 MHz): 192.8; 159.2; 149.9; 142.2; 130.1; 128.0; 61.0; 28.7; 14.3; 10.4.
EI-MS (70 eV): 196 (88, Mþ), 162 (90), 153 (100), 137 (8), 109 (20).
Ethyl 5-Phenylfuran-2-carboxylate (12d10): Yield 50%. tR 10.8. Yellow oil. 1H-NMR (250 MHz): 7.78
(d, J ¼ 7.0, 2 H); 7.41 (distorted t, J ¼ 7.2, 2 H); 7.35 – 7.33 (m, 1 H); 6.72 (d, J ¼ 3.5, 1 H); 4.37 (q, J ¼ 7.1,
2 H); 1.38 (t, J ¼ 7.1, 3 H). EI-MS (70 eV): 217 (15, [M þ 1]þ), 216 (100), 188 (72), 171 (60), 144 (70), 115
(95).
Ethyl 5-Phenylfuran-3-carboxylate (13d10): Yield: 40%. tR 10.69. 1H-NMR (250 MHz): 8.01 (s,
C¼CH); 7.66 (d, J ¼ 7.3, 2 arom. H); 7.39 (t, J ¼ 7.4, 2 arom. H); 7.30 (d, J ¼ 7.1, 1 arom. H) 6.95 (s, C¼CH),
4.31 (q, J ¼ 7.1, COOCH2Me); 1.35 (t, J ¼ 7.1, COOCH2Me). 13C-NMR (125 MHz): 162.1; 154.1; 127.8;
127.1; 123.0; 120.3; 103.5; 59.5; 13.3.
This work was supported by a grant from Istanbul Technical University Research Fund (Project
Nos. 33711 and 33340).
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Received May 7, 2012