Molecules 2014, 19, 1976-1989; doi:10.3390/molecules19021976 molecules ISSN 1420-3049 www.mdpi.com/journal/molecules Article Pyrrolidine-Mediated Direct Preparation of (E)-Monoarylidene Derivatives of Homo- and Heterocyclic Ketones with Various Aldehydes Xin Gu 1,† , Xiaoyan Wang 2,† , Fengtian Wang 1 , Hongbao Sun 1 , Jie Liu 1, *, Yongmei Xie 1, * and Mingli Xiang 1 1 State Key Laboratory of Biotherapy, West China Hospital, West China Medical School, Sichuan University, Chengdu 610041, China; E-Mails: [email protected] (X.G.); [email protected] (F.W.); [email protected] (H.S.); [email protected] (M.X.) 2 Analytical & Testing Center, Sichuan University, Chengdu 610064, China; E-Mail: [email protected]† These authors contributed equally to this work. * Authors to whom correspondence should be addressed; E-Mails: [email protected] (J.L.); [email protected] (Y.X.); Tel/Fax.: +86-28-8550-3817 (J.L.). Received: 18 December 2013; in revised form: 21 January 2014 / Accepted: 26 January 2014 / Published: 12 February 2014 Abstract: An efficient method for the facile synthesis of (E)-monoarylidene derivatives of homo- and heterocyclic ketones with various aldehydes in the presence of a pyrrolidine organocatalyst has been achieved. A range of α,β-unsaturated ketones were obtained in moderate to high yields (up to 99%). Unlike the Claisen-Schmidt condensation process, the formation of undesired bisarylidene byproducts is not observed. The possible reaction mechanism suggests that the reaction proceeds via a Mannich-elimination sequence. Keywords: α,β-unsaturated ketones; pyrrolidine; 1-methyl-4-piperidone; Mannich-elimination sequence; Claisen-Schmidt condensation 1. Introduction α,β-Unsaturated ketones represent an important class of compounds, as they possess a broad spectrum of biological activity such as anticancer, cytotoxic, anti-inflammatory, analgesic, and OPEN ACCESS
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Pyrrolidine-Mediated Direct Preparation of (E ... · project in our laboratory, we required mono-2-arylidene derivatives of ketones, particularly of piperidone. The ideal choice in
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Although the reaction solvents influenced the rate of the reaction, they did not affect the formation of
(E)-4a as single product during the course of reaction, regardless of the protic or aprotic nature of the
solvent. Aldol products and bisarylmethylidenes of piperidone were not observed. The temperature
also influenced the rate of the reaction. Elevating the reaction temperature resulted in a high reactivity
(Table 1, entries 17, 18), while conducting the reaction at 40 °C provided the best results. Through
extensive screening, the optimized reaction conditions were found to be 2a/3a/1a = 1/1/1.2 and 1.0 mL
of CH2Cl2 as solvent at 40 °C.
The application scope of the catalytic system was then examined under the optimal conditions. As
shown in Table 2, a variety of aromatic aldehydes bearing various substituents were investigated, and
the corresponding products were obtained in moderate to high yields (up to 99%, Table 2, entries 1–18).
The electronic properties and steric hindrance of the substituents at the aromatic ring affected the
yields strongly (Table 2, entries 1–13). Aromatic aldehydes with electron-withdrawing groups gave
higher yields than those with electron-donating groups (Table 2, entries 2–6, 10, 12, 13 vs. 7, 8, 11).
ortho-Substituted aromatic aldehydes gave higher yields than para- and meta-substituted aromatic
aldehydes (Table 2, entries entries 4, 5, 8 vs. 10–13). Naphthyl and heterocyclic aromatic aldehydes
also participated in this reaction in moderate yields (Table 2, entries 14–18). Moreover, an aliphatic
aldehyde was investigated and it was transformed with a yield of 51% (Table 2, entry 19).
To further extend the application of our procedure, the reactions of other ketones, such as
cyclohexanone, cyclopentanone and 4-oxotetrahydropyran with several representative aldehydes
were also examined (Table 2, entries 20–24). Interestingly, ketones with different structures worked
well under the optimized conditions and these reactions gave the corresponding products in good
yields. Similarly, the electronic nature of the substrate influenced the reactivity. Aromatic aldehydes
with electron-withdrawing groups gave higher yields (Table 2, entry 21 vs. 22).
Based on the results and previous reports [29,34–38], two possible reaction mechanisms for the
formation of (E)-monoarylidene derivatives of homo- and heterocyclic ketones with various aldehydes
have been proposed. As depicted in Scheme 2, one route is the generation through an aldol reaction of
product a, which then undergoes a dehydration process (Scheme 2, mechanism 1). The reaction
proceeded through a course of enamine activation. Another route is the generation of product a
through a Mannich-elimination sequence (Scheme 2, mechanism 2). The 1-methyl-4-piperidone
attacks the iminium complex formed from pyrrolidine and benzaldehyde to give intermidate c, which
then undergoes a elimination process to afford product a. The iminium species formation is an
important mode of activation and facilitates this reaction.
Molecules 2014, 19 1980
Table 2. Synthesis of monoarylmethylidenes of various homo- and heterocyclic ketones.
Entry 2 3 4 Yield (%) b
1 X = N-CH3, n = 2 R1 = Ph 4a 94 2 X = N-CH3, n = 2 R1 = 4-NO2-Ph 4b 90 3 X = N-CH3, n = 2 R1 = 4-CN-Ph 4c 99 4 X = N-CH3, n = 2 R1 = 4-F-Ph 4d 75 5 X = N-CH3, n = 2 R1 = 4-Br-Ph 4e 73 6 X = N-CH3, n = 2 R1 = 3,4-diCl-Ph 4f 80 7 X = N-CH3, n = 2 R1 = 4-CH3-Ph 4g 54 8 X = N-CH3, n = 2 R1 = 4-CH3O-Ph 4h 50 9 X = N-CH3, n = 2 R1 = 3-Cl-Ph 4i 81 10 X = N-CH3, n = 2 R1 = 3-Br-Ph 4j 86 11 X = N-CH3, n = 2 R1 = 3-CH3O-Ph 4k 65 12 X = N-CH3, n = 2 R1 = 2-F-Ph 4l 84 13 X = N-CH3, n = 2 R1 = 2-Br-Ph 4m 94 14 X = N-CH3, n = 2 R1 = 2-naphthyl 4n 60 15 X = N-CH3, n = 2 R1 = 1-naphthyl 4o 93 16 X = N-CH3, n = 2 R1 = 2-pyridinyl 4p 78 17 X = N-CH3, n = 2 R1 = 4-pyridinyl 4q 59 18 X = N-CH3, n = 2 R1 = 2-thienyl 4r 46 19 X = N-CH3, n = 2 R1 = CH3CH2CH2 4s 53 20 X = N-Boc, n = 2 R1 = Ph 4t 92 21 X = O, n = 2 R1 = Ph 4u 53 22 X = O, n = 2 R1 = 4-NO2-Ph 4v 64 23 X = C, n = 2 R1 = 4-NO2-Ph 4w 84c 24 X = C, n = 1 R1 = 4-NO2-Ph 4x 95 a Unless indicated otherwise, the reaction was carried out in 0.1 mmol scale in CH2Cl2 (1.0 mL) at 40 °C for 4 h,
and the ratio of 1a/2/3 is 1.2/1/1; b Isolated yield based on ketones; c Reaction time is 20 h.
Scheme 2. Two proposed mechanisms for the formation of α, β-unsaturated ketones.
To obtain a better view of the nature of the catalytic species at work in this reaction, first careful
monitoring of the course of the reaction of 1-methyl-4-piperidone with benzaldehyde in the presence
of 1.2 eq pyrrolidine in CH2Cl2 has been performed by TLC. During the course of the reaction, the
N N
N H
OO
N
N O
H
O
H
N N
N
O OH
N
O
N
O N
N
O
aldol reaction
dehydration
Mannich reaction
deamination
NH
NH
Mechanism 1:
Mechanism 2:
b a
O
ca
enamine activation
iminium activation
Molecules 2014, 19 1981
aldol product was never detected. Therefore, we hypothesized the generation of product a may go
through a Mannich-elimination sequence (Scheme 2, mechanism 2). We next validated the possible
reaction mechanism for the formation of product a using some spectroscopic studies. NMR spectra of
three raw materials are presented in Figure 2a. The 1H-NMR spectra were then recorded over time
(Figure 2b). After 15 min, the appearance of multiple peaks in the δ 7.2–7.4, 2.1–2.5, 1.5–1.8 ppm
region and a doublet at δ 4.62 ppm evidenced formation of the intermediate. From HSQC spectra, the
carbon at 64.7 ppm was a typical signal which linked with the hydrogen at 4.62 ppm. As the reaction
proceeded, the peaks of the signal became more apparent. After 5 h, the amount of intermediate did not
increase. It is hard to distinguish whether the intermediate was b or c from the 1H-NMR spectra, but
the 13C spectra provided some additional information. Three peaks at δ 209.5, 208.4 and 192.2 ppm
were assigned to the carbonyl carbons of the two unreacted raw materials and the intermediate. A
chemical shift in the 180–220 ppm range for the iminium carbon of intermediate b is not reasonable,
therefore the intermediate should be c. The quantitative 13C-NMR results showed that the integration
of the carbon at 209.5 ppm and the integration of the one at 64.7 ppm were approximately equal,
suggesting the two carbons were those of intermediate c (Figure 2c). Moreover, the structure of
intermediate c was also verified by A NOE experiment (Figure 2d). The obvious cross-peak between the
pyrrole and the phenyl ring illustrated that the intermediate should be c instead of b. This, based on 1H-, 13C-, quantitative 13C-NMR, and NOESY data of the mixture, the structure of the intermediate
was identified as c (Figure S3). However, we do not see evidence for product formation under these
reaction conditions. It should be noted that although no products are evident in the solution during the
NMR reactions, the intermediate turns into product during subsequent purification on silicagel.
Figure 2. (a) 1H spectra of the starting material; (b) 1H spectra recorded at different time;
(c) selected 1H and 13C chemical shift of c; and (d) The NOESY spectra recorded after 5 h.
(a)
Molecules 2014, 19 1982
Figure 2. Cont.
(b)
(c)
(d)
Molecules 2014, 19 1983
3. Experimental
3.1. General
All chemicals were obtained from commercial sources and used without further purification.
Column chromatography was carried out on silica gel (300–400 mesh, Qingdao Marine Chemical Ltd.,
Qingdao, China). Thin layer chromatography (TLC) was performed on TLC silica gel 60 F254 plates. 1H-NMR spectra were recorded on Bruker AVII-400 or 600 MHz instruments. The chemical shifts
were recorded in ppm relative to tetramethylsilane and with the solvent (CDCl3) resonance as the
internal standard. Data were reported as follows: chemical shift, multiplicity (s = singlet, d = doublet,
t = triplet, q = quartlet, m = multiplet), coupling constants (Hz), integration. 13C-NMR data were
collected at 100 or 150 MHz with complete proton decoupling. Chemical shifts were reported in ppm
from tetramethylsilane with the solvent (CDCl3) resonance as internal standard. MS spectra were
obtained on a Waters Quattro Premier XETM triple quadrupole mass spectrometer and methanol was
used to dissolve the sample. Melting points were recorded on a SGW X-4 melting point instrument