Reaction of acyclic enaminones with methoxymethylene meldrum's acid: synthetic and structural implications

ARTICLE

Reaction of acyclic enaminones with methoxymethylene meldrum's acid. Synthetic and structural implications^#

Silvio Cunha^I; Viviane C. da Silva^I; Hamilton B. Napolitano^II; Carlito Lariucci^II; Ivo Vencato^II

^IInstituto de Química

IIInstituto de Física, Universidade Federal de Goiás, CP 131, 74001-970 Goiânia, GO, Brazil

^{Address to correspondence}

RESUMO

A reação de enaminonas com o derivado metoximetilênico do ácido de Meldrum forneceu N-adutos e/ou C-adutos das enaminonas, em rendimentos moderados a bons. A regioquímica da reação se revelou dependente do substituinte do nitrogênio da enaminona, e o C-aduto formado é precursor para 2-piridonas. A análise da difração de raios X de dois N-adutos revelou que estes adutos possuem a configuração Z-s-Z.

ABSTRACT

The reaction of acyclic enaminones with methoxymethylene Meldrum's acid afforded N-adduct and/or C-adduct of enaminones in moderate to good yields. The regiochemistry of this reaction depends on the N-amino substituent of the enaminone. The C-adduct is a precursor to 2-pyridones. X-ray analysis of two N-adducts were investigated and the Z-s-Z configuration assigned.

Keywords: enaminones, Meldrum's acid, aza-annulation, 2-pyridone

Introduction

The fascinating chemistry of enaminones and their derivatives has attracted the attention of numerous researchers due to their ambiphilic and ambident properties and their potential in the synthesis of heterocyclic compounds.¹ In this context, the aza-annulation reaction of cyclic and acyclic enaminones has been extensively used in the preparation of a broad spectrum of nitrogen-containing compounds,² mainly in alkaloids³ and conformationally constrained peptide analogues.⁴ Because of these applications several protocols for the synthesis of enaminones have been developed.⁵ Among them, the solid support method developed by Braibante and co-workers⁶ and its systematic use in the synthesis of pyrazoles and isoxazoles derivatives is noteworthy.⁷

While the reaction of methoxymethylene Meldrum's acid (1) with cyclic enaminones has been documented (Scheme 1, reactions 1-4),⁸ much less study has been carried out with 1 and acyclic enaminones. There is only a single paper describing two examples of reaction of 1 with enaminones 11 and 13a (Scheme 1, reactions 5-6).⁸ However, the aza-annulation of derivatives 12 and 14a under pyrolysis conditions (Scheme 1, reaction 7) is not synthetically efficient because mixture of products and poor yields are obtained. In search for a general method of synthesis of derivatives 12 to 14 we undertook a study concerning the reactions of acyclic enaminones and methoxymethylene Meldrum's acid (1). In this paper we report the results of this study with emphasis on synthetic, mechanistic and structural implications.

Results and Discussion

Enaminones may act as an ambident nucleophile by reaction at the nitrogen and at the b-carbon. The reactions of enaminones and methoxymethylene Meldrum's acid depend on the N-amino substituent, Scheme 1. C-Adducts are obtained with NR₂ substituent (R = alkyl) and N-adducts with the NH₂ group. However, when we attempted the reaction of enaminone 13a with 1 under the literature condition⁸ a low yield of the N-adduct 14a was obtained (36%, instead of the reportedly obtained 60% yield). Additionally, a small quantity of the C-adduct 18a was isolated (3.2% yield, Scheme 2), which was not previously reported. The spectral data of compound 14a here obtained were identical with those described.⁸ The ¹H NMR spectra contained a low field N-H (13.93 ppm) which suggests its participation in intramolecular hydrogen bonding. Despite the reportedly E-s-E configuration to 14a we assigned the Z-s-Z configuration to the N-adduct because E-s-E and Z-s-Z configurational isomers of enaminones are well distinguished by typical N-H chemical shifts (E-isomer: 4.1-6.5 ppm; Z-isomer: 9.5-12.0 ppm).⁹ Moreover, the structure of 14a was unambiguously confirmed by X-ray analysis and the Z configuration corroborated, as shown in Figure 1.

In addition, extension of the reported protocol⁸ to other enaminones afforded complex mixtures. Better results were obtained when CH₂Cl₂ was used as solvent instead of CH₃CN (Scheme 2). With this modification N- and C-adducts 14a-b and 18a-b were obtained in a 2:1 ratio, respectively. With enaminone 13c only the C-adduct 18c was formed in good yield.

To our surprise, when we attempted the reaction of 1 with enaminone 13d a complex mixture was obtained, and the 2-pyridone 19d could be isolated in 28% yield (Scheme 3, reaction. 1). Unfortunately, the pyridone 19d was an unstable solid that precluded its complete spectral characterization. However, its structure could be assigned by comparison of its IR and ¹H NMR spectra with analogue 19c (see Experimental). The formation of 19d may be visualized as occurring through the aza-annulation of the initial C-adduct of the reaction of 1 and 13d. To support this mechanistic proposal we decided to perform the thermolysis of the isolated C-adduct 18c. In this way, 18c was refluxed in toluene and the 2-pyridone 19c was obtained in good yield (Scheme 3, reaction 2). The structure of 19c was corroborated by analysis of a long-range heterocorrelation (COLOC) spectrum which showed correlation (³J) of the hydrogen at C-4 with the carbonyl C-2 and with C-6 as well as the other correlations indicated in Scheme 4, which also presents the mechanistic pathway to 19c. Interestingly, in this thermal cyclization the typical CO₂ elimination from the methylene Meldrum's acid moiety was not observed.¹⁰ It should also be pointed out that the relative low temperature required to form the 2-pyridones 19c-d makes this methodology synthetically attractive, contrasting with the literature pyrolysis condition for the N-adduct 14a.⁸

Understanding how enaminones fit together in the solid state is of particular interest to recognize the relationships between structural features and pharmacological properties, e.g. the anticonvulsant activity of enaminones has been associated with the inter- and intramolecular NH...O, CH...O and CH...N hydrogen bonding in the three-dimensional structure.¹¹ To unambiguously assign the structure of the obtained enaminones and to gain insight into intra- and intermolecular interactions the crystal structures of 14a and 14b were determined, and several structural features emerged. As noted in Figure 1, which shows the molecules with labeled atoms, 14a has one strong bifurcated intramolecular hydrogen bonding between the oxygen atoms O1 and O3 and the NH group: N-H1N...O1 [2.699(3) Å] and N-H1N...O3 [2.720(3) Å]) providing two quasi-planar pseudo six-membered rings. The major distance from the least-square plane including all the atoms of these pseudo-rings is 0.187(3) Å for atom O3. In addition, a weak C-H...O intermolecular H-bond was observed: C6-H6...O3ⁱ [1/2-x,-1/2+y, z , 3.379(3)Å]. The Meldrum's acid moiety has an envelope conformation, as can be seen from the Cremer and Pople¹² parameters: O4®C8®...C9 [Q=0.404(2) Å, q = 61.0(3)^o, f = 299.0(4)^o]. The C9 atom is 0.562(3) Å out of the plane defined by the other atoms of the ring. In a similar way, 14b has a weak bifurcated intramolecular hydrogen bonding between the oxygen atoms O3 and O4 and the NH group: N-H2...O3 [2.763(2) Å] and N-H2...O4 [2.753(2) Å], also providing two quasi-planar pseudo six-membered rings. Here again, a weak C-H...O intermolecular H-bond was also noted: C13-H13B...O3' [1-x, y, 1-z, 3.416(3)Å].

In conclusion, the reactions of acyclic enaminones and methoxymethylene Meldrum's acid afford N- and/or C-adducts and 2-pyridones were formed from the latter through an intramolecular aza-annulation. The scope, limitations and the application of the methodology here described in natural products synthesis is under investigation in our lab and will be reported opportunely.

Experimental

Melting points were determined on a Karl Kolb apparatus and are uncorrected. Infrared spectra were recorded as KBr discs on a FT-IR BOMEM MB100 instrument. NMR spectra were obtained for ¹H at 300 MHz and for ¹³C at 75 MHz using a Varian Gemini 300 or a Bruker AC300P spectrometers at Instituto de Química, UNICAMP. Chemical shifts are reported in ppm units downfield from reference (internal TMS). MS spectra were measured on a SHIMADSU CG-MS QP-5050 spectrometer at 70 eV. Elemental analyses were performed on a 2400 CHN Perkin Elmer instrument at Instituto de Química, UNICAMP. Enaminones 13a-b,⁶13c,¹³13d,⁶ Meldrum's acid¹³ and methoxymethylene Meldrum's acid¹³ were prepared according to known procedures. The single crystal X-ray data collections were carried out on a Nonius CAD-4 diffractometer at Departamento de Química, UFSC.

General synthetic procedure

A solution of 2 mmol of Meldrum's acid in 2 mL of trimethyl orthoformate was heated at reflux for 2 h after which time the solvent was evaporated. The solid that formed was dissolved in 5 mL of CH₂Cl₂ and 2 mmol of enaminone was added and the solution was allowed to stand at room temperature for 24 h. The solvent was evaporated and the crude residue was treated as indicated in each case.

2,2-dimethyl-5-[(Z)-1-methyl-3-oxo-1-butenylaminomethylene]-1,3-dioxane-4,6-dione (14a).Purified by silica gel column chromatography (benzene/ethyl acetate 20%), pale orange needles, mp 193-196 ºC. IR (KBr): n_max/cm^-1 3079, 1736, 1690, 1664, 1600, 1580. ¹H NMR (CDCl₃): 1.74 (6H, s); 2.22 (3H, s); 2.26 (3H, s); 5.75 (1H, s); 8.35 (1H, d, J 14.3Hz); 13.93 (1H, br s). ¹³C NMR (CDCl₃): 18.1 (CH₃); 27.3 (CH₃); 30.8 (CH₃); 90.8 (C); 105.0 (C); 109.7 (CH); 148.4 (C); 150.8 (CH); 162.8 (C); 163.7 (C); 199.0 (C). MS, m/z (%): 253 [M⁺, 41%], 195 (100%), 149 (99%); 136 (56%), 108 (37%). Anal. Calcd. for C₁₂H₁₅NO₅: C, 56.90%; H, 5.95%; N, 5.55%. Found: C, 56.97%; H, 6.01%; N, 5.54%.

5-[(Z)-2-acetyl-3-amino-2-butenylidene]-2,2-dimethyl -1,3-dioxane-4,6-dione (18a). Purified by silica gel column chromatography (benzene/ethyl acetate 50%), pale yellow needles, mp 158-161 ºC. IR (KBr): n_max/cm^-1 3420, 3200, 1728, 1690, 1631. ¹H NMR (CDCl₃): 1.75 (6H, s); 2.24 (3H, s); 2.39 (3H, s); 7.97 (1H, br s); 8.64 (1H, s); 10.60 (1H, br s). ¹³C NMR (CDCl₃): 21.5 (CH₃); 27.4 (CH₃); 28.5 (CH₃); 99.7 (C); 103.5 (C); 110.9 (C); 155.1 (CH); 163.0 (C); 164.6 (C); 172.4 (C); 199.4 (C). MS, m/z (%): 253 [M⁺, 70%], 195 (48%), 151 (97%), 149 (37%), 136 (57%), 123 (84%), 108 (100%), 95 (65%), 80 (75%). Anal. Calcd. for C₁₂H₁₅NO₅: C, 56.90%; H, 5.95%; N, 5.55%. Found: C, 56.97%; H, 6.01%; N, 5.54%.

Ethyl (Z)-3-(2,2-dimethyl-4,6-dioxo-1,3-dioxan-5-ylidenmethylamino)-2-butenoate (14b). Purified by silica gel column chromatography (benzene/ethyl acetate 20%), colorless solid, mp 193-194 ºC. IR (KBr): n_max/cm^-1 3136, 1741, 1698, 1651, 1603cm^-1. ¹H NMR (CDCl₃): 1.31 (3H, t, J 7.1Hz); 1.74 (6H, s); 2.22 (3H,s); 4.31 (2H, q, J 7.1Hz); 5.37 (1H, s); 8.32 (1H, d, J 14.3Hz); 13.39 (1H, br s). ¹³C NMR (CDCl₃): 14.2 (CH₃); 18.1 (CH₃); 27.1 (CH₃); 60.7 (CH₂); 90.1 (C); 103.0 (CH); 105.1 (C); 148.8 (C); 150.5 (CH); 163.5 (C); 163.9 (C); 167.1 (C). MS, m/z (%): 283 [M⁺, 22%], 259 (22%), 225 (71%), 196 (100%). Anal. Calcd. for C₁₃H₁₇NO₆: C, 55.12%; H, 6.01%; N, 4.95%. Found: C, 54.83%; H, 5.96%; N, 4.32%.

Ethyl (Z)-3-amino-2-(2,2-dimethyl-4,6-dioxo-1,3-dioxan-5-ylidenmethyl)-2-butenoate (18b). Purified by silica gel column chromatography (benzene/ethyl acetate 30%), yellow needles, mp 172-174 ºC. IR (KBr): n_max/cm^-1 3317, 3109, 1725, 1668. ¹H NMR (CDCl₃): 1.23 (3H, t, J 7.1Hz); 1.66 (6H, s); 2.38 (3H, s); 4.17 (2H, q, J 7.1Hz); 7.04 (1H, br s); 8.40 (1H, s); 9.38 (1H, br s). ¹³C NMR (CDCl₃): 14.4 (CH₃); 21.0 (CH₃); 27.4 (CH₃); 60.7 (CH₂); 101.6 (C); 101.7 (C); 103.8 (C); 153.5 (CH); 163.3 (C); 165.5 (C); 169.2 (C); 170.8 (C). MS, m/z (%): 283 [M⁺, 48%], 225 (36%), 181 (100%), 153 (91%), 136 (31%), 124 (57%). Anal. Calcd. for C₁₃H₁₇NO₆: C, 55.12%; H, 6.01%; N, 4.95%. Found: C, 55.23%; H, 6.07%; N, 4.72%.

Ethyl 2-[(Z)-2-(2,2-dimethyl-4,6-dioxo-1,3-dioxan-5-ylidenmethyl)-1-methyl-3-oxo-1-butenylamino]acetate (18c). Recrystallized from CH₂Cl₂/petroleum ether, yellow solid, mp 139-142 ºC. IR (KBr): n_max/cm^-1 3224, 1736, 1690, 1647. ¹H NMR (CDCl₃): 1.34 (3H, t, J 7.1Hz); 1.74 (6H, s); 2.22 (3H, s); 2.42 (3H, s); 4.26 (2H, d, J 5.1Hz); 4.32 (2H, q, J 7.1Hz); 8.72 (1H, s); 12.64 (1H, br s). ¹³C NMR (CDCl₃): 14.1 (CH₃); 18.6 (CH₃); 27.4 (CH₃); 27.7 (CH₃); 46.1 (CH₂); 62.7 (CH₂); 99.3 (C); 103.3 (C); 111.2 (C); 154.9 (CH); 162.3 (C); 164.5 (C); 166.6 (C); 173.5 (C); 197.5 (C). MS, m/z (%): 339 [M⁺, 34%], 281 (62%), 237 (56%), 135 (100%). Anal. Calcd. for C₁₆H₂₁NO₇ C, 56.64%; H, 6.19%; N, 4.13%. Found: C, 56.47%; H, 6.09%; N, 4.04%.

5-acetyl-1-ethyloxycarbonylmethyl-6-methyl-2-oxo -1,2-dihydro-3-pyridinecarboxylic acid (19c). A solution of 85.1 mg (0.25 mmol) of 18c in 10 mL of toluene was heated at reflux for 24 h, after which time the solvent was evaporated and the residue was recrystallized from CH₂Cl₂/petroleum ether to give 52.9 mg (75% yield) of 19c as colorless needles, mp 121-123 ºC. ¹H NMR (CDCl₃): 1.34 (3H, t, J 7.1Hz), 2.62 (3H, s), 2.74 (3H, s), 4.31 (2H, q, J 7.1Hz), 5.03 (2H, s), 8.90 (1H, s), 13.42 (1H, l). ¹³C NMR (CDCl₃): 14.1 (CH₃), 18.4 (CH₃), 29.6 (CH₃), 46.4 (CH₂), 62.8 (CH₂), 113.9 (C), 119.7 (C), 145.3 (CH), 157.3 (C), 163.9 (C), 164.3 (C), 166.1 (C), 196.4 (C). MS, m/z (%): 281 [M⁺, 43%], 207 (100%). Anal. Calcd. for C₁₃H₁₅NO₆ C, 55.52%; H, 5.34%; N, 4.98%. Found: C, 55.44%; H, 5.19%; N, 5.14%.

1-benzyl-5-ethyloxycarbonyl-6-methyl-2-oxo-1,2-dihydro-3-pyridinecarboxylic acid (19d). Recrystallized from CH₂Cl₂/petroleum ether, colorless solid, mp 135-137 ºC. IR (KBr): n_max/cm^-1 1735, 1720, 1670, 1620. ¹H NMR (CDCl₃): 1.38 (3H, t, J 7.2Hz), 2.86 (3H, s), 4.35 (2H, q, J 7.2Hz), 5.55 (2H, s), 7.11-7.15 (2H, m), 7.32-7.42 (3H, m), 9.07 (1H, s), 13.95 (1H, br s).

Crystal structure of 14a. C₁₂H₁₅NO₅, M_w = 253.25, orthorhombic, space group Pbca [nr. 61], Z = 8, a = 14.321(3), b = 9.261(2), c = 19.064(4) Å, V = 2528.4(9) Å,³ d_c = 1.331 Mg m^-3, l (Mo Ka) = 0.71073 Å, m = 0.104 mm^-1, 2835 measured reflections, 2473 unique (R_int = 0.0) of which 1579 were considered as observed with I ³ 2s(I). The single crystals were obtained by diffusion of petroleum ether into a solution of 14a in CH₂Cl₂ at room temperature.

Crystal structure of 14b. C₁₃H₁₇NO₆, M_w = 283.28, monoclinic, space group C2/m [nr. 12], Z = 4, a = 18.015(4), b = 6.600(1), c = 12.056(2) Å, V = 1430.7(5) Å,³ d_c = 1.315 Mg m^-3, l (Mo Ka) = 0.71073 Å, m = 0.105 mm^-1, 3066 measured reflections, 1535 unique (R_int = 0.016) of which 1225 were considered as observed with I ³ 2s(I). The molecule is placed on the mirror symmetry plane, except the atoms O1, O2, C8 and C11, that are disordered. The single crystals were obtained by diffusion of petroleum ether into a solution of 14b in CH₂Cl₂ at room temperature. The structures were solved with direct methods using SHELXS97¹⁴ and refined anisotropically with full-matrix least-squares on F² using SHELXL97.¹⁵ The hydrogen atoms were placed at calculated position except those involved in H-bonds, found on difference maps and refined. Final indices: R₁(F_o) = 0.043, wR₂ (F²) = 0.131 for 172 refined parameters and R₁(F_o) = 0.048, wR₂ (F²) = 0.128 for 145 refined parameters, for 14a and 14b, respectively.

The crystallographic data (excluding structure factors) for structures in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication numbers CCDC 179109 and CCDC 179108, for 14a and 14b, respectively. Copies of the data can be obtained, free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge, CB21EZ, UK (fax [phone]) or e-mail: [e-mail].)

Acknowledgments

The authors thank the Brazilian Agencies for fellowships to V.C.S. (PIBIC/CNPq), H.B.N. (CAPES), I.V. (CNPq) and financial support from FUNAPE-UFG. The authors also thank Instituto de Química, UNICAMP (NMR and elemental analyses), Departamento de Química, UFSCar (NMR) for measurements and Departamento de Química, UFSC for the X-ray single crystal data collections.

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Received: May 5, 2002

Published on the web: February 12, 2003

^#

Dedicated to Professor Albert James Kascheres, a great mentor and pioneer in ciclopropenone chemistry in Brazil, on the occasion of his 60th birthday.

Publication Dates

• Publication in this collection
  14 Apr 2003
• Date of issue
  Jan 2003

History

• Received
  05 May 2002