The stereochemistry of the 1,3-dipolar cycloadditions of diazomethane to pseudoguaianolides

Tetrahedron: Asymmetry 28 (2017) 367–373

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The stereochemistry of the 1,3-dipolar cycloadditions of diazomethane to pseudoguaianolides Adriana Ortiz-León a, J. Martín Torres-Valencia a,⇑, J. Jesús Manríquez-Torres a, José G. Alvarado-Rodríguez a, Carlos M. Cerda-García-Rojas b, Pedro Joseph-Nathan b a b

Área Académica de Química, Universidad Autónoma del Estado de Hidalgo, km 4.5 Carretera Pachuca-Tulancingo, Mineral de la Reforma, Hidalgo 42184, Mexico Departamento de Química, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Apartado 14-740, Mexico City 07000, Mexico

a r t i c l e

i n f o

Article history: Received 24 December 2016 Accepted 13 January 2017 Available online 2 February 2017

a b s t r a c t The 1,3-dipolar cycloaddition of diazomethane to the sesquiterpene lactones, parthenin, coronopilin, and psilostachyin, gave their respective spiropyrazolines with complete chemoselectivity, while the diastereoselectivity in favour of the (11S)-stereoisomer was 86–98%. Similarly, mexicanin I acetate, helenalin, and helenalin acetate provided the (11R)-diastereoisomer. When helenalin and its acetate were treated with a large excess of diazomethane, they afforded their respective dipyrazolines with 98% diastereoselectivity in favour of the (2R,3S,11R)-diastereoisomer. All compounds were characterized by their physical and spectroscopic properties and their absolute configuration was established by X-ray diffraction analysis calculating the Flack and Hooft parameters. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction During the structural elucidation of sesquiterpene lactones over half a century ago, the 1,3-dipolar cycloaddition of diazomethane to provide pyrazolines, followed by elemental analysis, was used as a chemical test for the presence of an exocyclic methylene a, b-unsaturated-c-lactone functionality. However, the stereochemistry of this reaction has not been scrutinized, nor has it been established as to whether the addition of diazomethane occurs with chemoselectivity, since tools for these purposes were not available at that time. With the advent of 60 MHz 1H NMR spectrographic methodologies in the 1960 s, the pyrazoline derivatives were no longer routinely prepared. In recent times we reported the first case of a stereochemical study of this reaction using the guaianolide zaluzanina A, which possesses an a,b-unsaturated-dlactone in which an 86% of diastereoselectivity towards the (11S)-stereoisomer was observed.1 In order to know if the chemoselectivity and the diastereoselectivity of the addition of diazomethane to sesquiterpene lactones proceeded systematically, we carried out the reaction using the pseudoguaianolides parthenin,2 coronopilin,3 psilostachyin,4 mexicanin I acetate,5 helenalin acetate,6 and helenalin.7 Parthenin, mexicanin I acetate, helenalin and its acetate possess, in addition to the exocyclic a,b-unsaturated methylene c-lactone, an a,b-unsaturated ketone ⇑ Corresponding author. Tel.: +52 771 717 2000x2205. E-mail address: [email protected] (J.M. Torres-Valencia). http://dx.doi.org/10.1016/j.tetasy.2017.01.009 0957-4166/Ó 2017 Elsevier Ltd. All rights reserved.

system in a five-membered ring, also susceptible to the formation of pyrazolines. All these compounds constitute an important class of sesquiterpene lactones, which can be found in traditional medicinal plants from the Asteraceae family and possess relevant biological properties that include antibacterial, antifungal, antiprotozoal, insect-feeding deterrent, cytotoxic,8,9 and anti-inflammatory activities,10 as well as inhibition of Myb-dependent gene expression.11 Helenalin also inhibits the hepatitis C virus replication by its capability to potentiate the interferon a-exerted anti-hepatitis C virus effect,12 and showed potent inhibition of human telomerase by means of alkylation of the CYS445 residue.13,14 As a result of the 1,3-dipolar cycloaddition of diazomethane to the aforementioned psedoguaianolides, pyrazolines 1–8 (Fig. 1) were obtained and characterized by their physical and spectroscopy properties, which were compared with those of the starting sesquiterpene lactones.15–18 Their absolute configuration was established by chemical correlation with the starting material and by X-ray diffraction analysis including the calculation of Flack19 and Hooft20 parameters. 2. Results and discussion 2.1. Preparation of pyrazolines The 1,3-dipolar cycloaddition of diazomethane to the pseudoguaianolides parthenin, coronopilin, psilostachyin, and mexicanin I acetate in ether solutions afforded the monospiro

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A. Ortiz-León et al. / Tetrahedron: Asymmetry 28 (2017) 367–373 14

2

HO 10 1

3

H

5 4 15

O

H

8

6

O

HO

9

13

7

16

11 12

O

O

N N

O O

1

O

4

3

O

2

2

H

1

HO

H

H

5

O

O

N N

O

N

O

N 4

H H

O

AcO

3

O

N N

N N

O

RO

N

O

N 5:R=H 6 : R = Ac

17

H H

O

O N

RO 16

O

N

7:R=H 8 : R = Ac

missing, as were the signals of the vinyl hydrogen atoms at C-2 and C-3 in compounds 7 and 8. Instead, two double double doublets or multiplet signals were displayed at dH 2.18–1.57 (H-13pro-R) and 2.33–1.78 (H-13pro-S) for the methylene hydrogen atoms at C-13, and two double triplets or double double doublets were observed at dH 5.03–4.55 (H-16pro-R) and 4.85–4.66 (H-16pro-S) due to the methylene hydrogen atoms at C-16. Furthermore, the 1H NMR spectrum of 7 displayed signals at dH 5.04 (dd, J = 19.2 and 10.0 Hz, H-17pro-R) and at 4.70 (ddd, J = 19.2, 5.9 and 3.0 Hz, H-17pro-S) for the methylene hydrogen atoms at C-17, while H-3 and H-2 were observed at dH 5.42 (dd, J = 10.0 and 3.0 Hz) and dH 2.84 (tdd, J = 10.0, 8.0 and 5.9 Hz), respectively. Similarly, the 1H NMR spectrum of 8 showed the presence of the bicyclopyrazoline consisting of two double double doublets at dH 5.06 (J = 19.0, 10.8 and 0.8 Hz, H-17pro-R) and 4.70 (J = 19.0, 6.0 and 3.6 Hz, H-17pro-S) for the methylene hydrogen atoms bearing the diazo group, a double double doublets at dH 5.41 (J = 10.0, 3.6 and 0.8 Hz) for H-3, and a double double double doublet at dH 2.85 (J = 10.8, 10.0, 8.0 and 6.0 Hz) for H-2. The multiplicity of the H-3 and H-17 signals of the bicyclopyrazoline residue is explicated on bases of their azohomoallylic interactions.21 The 13C NMR spectra of 1–8 (Table 2) showed signals for the spiropyrazoline moiety at dC 102.0–96.4 (C-11), 25.3–21.4 (C-13), and 79.6–78.3 (C-16). Moreover, 7 and 8 displayed signals due to the pyrazoline at C-2–C-3 around dC 32.0 (C-2), 100.0 (C-3), and 82.2 (C-17). In all cases, the assignment of their 1H and 13C NMR spectra were corroborated by gCOSY, gHSQC and gHMBC experiments. It is noteworthy that in pyrazoline 3, the 1H NMR multiplicity of the signals was not easy to define, and therefore most signals were described as multiplets.

Figure 1. Chemical formulae of pseudoguaianolide pyrazolines 1–8.

2.3. Stereochemical and conformational analysis of pyrazolines derivatives parthenin pyrazoline 1, coronopilin pyrazoline 2, psilostachyin pyrazoline 3, and mexicanin I acetate pyrazoline 4, respectively (Fig. 1). When helenalin and helenalin acetate were reacted with an excess of diazomethane, helenalin dipyrazoline 7, and helenalin acetate dipyrazoline 8 were obtained (Fig. 1). However, when helenalin and helenalin acetate were dissolved in a dichloromethane-ethyl ether mixture, cooled to 4 °C, and treated dropwise with a yellow ether diazomethane solution with stirring until persistence of color, the respective monopyrazolines 5 and 6 were obtained in almost quantitative yields and P98% of ds towards the (11R)-stereoisomer. The addition of diazomethane to mexicanin I was not studied because of the marked insolubility of this substance in contrast to its acetyl derivative. Careful 1H NMR analysis of crude reaction outcomes, and integration of their signals, allowed us to determine the chemo- and diastereoselectivity of the cycloadditions. Spiropyrazolines 1–3 proceeded with complete chemoselectivity and with 81, 94, and 98% diastereoselectivity (ds), respectively, towards the (11S)-stereoisomer, while 4–6 also progressed with complete chemoselectivity but with 98% of ds towards the (11R)-stereoisomer. In the case of dipyrazolines 7 and 8, a 98% ds was observed towards the (11R)stereoisomer and exclusively towards the (2R,3S)-pyrazoline. All compounds were obtained as white or slightly yellow powders whose purification by recrystallization or column chromatography led to obtain in all cases crystalline pyrazolines. These were characterized by X-ray diffraction analysis as well as by 1D and 2D NMR spectroscopy whose data were compared with those of the starting sesquiterpene lactones.15–18 Pyrazolines 12 and 23 are known, while derivatives 3–8 are described for the first time herein. 2.2. NMR analysis of pyrazolines In the 1H NMR spectra of 1–8 (Table 1), the signals due to the exocyclic methylene group of the a,b-unsaturated-c-lactone were

In all of the 1,3-dipolar cycloadditions, it was observed that the diazomethane methylene carbon attacked the C-13 atom of the exocyclic a,b-unsaturated-c-lactones from the a-face of the molecules 1a, 1b, 5a, and 5b while the attack at C-2 of the a,b-unsaturated ketone occurred from the b-face in 6a and 6b as illustrated in Fig. 2. In order to explain the diastereoselectivity of the addition of diazomethane to the sesquiterpene lactones, parthenin (lactone closed at C-6), helenalin and its acetate (lactones closed at C-8) were selected to carry out a conformational analysis. The molecular models of these pseudoguaianolides were built and subjected to a conformational search with the Monte Carlo method using Molecular Mechanics calculations, followed by single-point energy calculations at the DFT B3LYP/6-31G(d) level of theory. Further geometry optimization of all resulting structures at the B3LYP/DGDZVP level of theory, using a described protocol,22 allowed us to obtain the two main conformers for each compound (Fig. 2), which represented 98–100% of the conformational population. Examination of the optimized structures 1a and 1b shows the steric hindrance of the re-face for the exocyclic methylene (C-11@C-13). In these structures, the C-8 methylene group adopts a pseudo-axial position with respect to the lactone ring. Additionally, in 1b, the double bond reface remains inside the cavity formed by the seven-membered ring and C-15. Therefore, addition of diazomethane takes place on the siface to generate the 11S-spiropyrazoline 1. The same analysis is valid for the generation of 2 and 3. The main conformer of helenalin 5a (Fig. 2), identical to that deduced from its 1H NMR spectrum 45 years ago,16 and that of its acetate 6a are quite similar and the same is true for the second minimum structures 5b and 6b. In structures 5a and 6a, the si-face of the exocyclic methylene has a large steric hindrance because it is positioned in the cavity of the seven-membered ring, while the C-15 methyl group is relatively close to this face. Also, in the minor conformers 5b and 6b, the steric hindrance generated by the C-9

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A. Ortiz-León et al. / Tetrahedron: Asymmetry 28 (2017) 367–373 Table 1 H NMR data for 1–8 (400 MHz, CDCl3)

1

Proton

dH, mult. (J in Hz) 1

2

3

4

5

6

7

8

1

–

–

–

7.53, d (5.9)

2.58, m

2pro-R

–

3.10, ddd (10.8, 3.0, 2.0) 7.72, dd (6.0, 2.0) –

2.77, dd (11.0, 8.0) 2.84, tdd (10.0, 8.0, 5.9) –

3

6.27, d (5.9)

3pro-S 6

– 5.68, d (5.9)

2.43, ddd (19.0, 10.0, 1.7) 2.51, dt (19.0, 10.0) 2.65, dt (13.6, 10.0) 1.66, m 5.60, d (5.8)

3.14, ddd (11.2, 3.0, 1.7) 7.73, dd (6.0, 1.7) –

2.59, dd (11.3, 8.0)

2

2.89, ddd (10.5, 2.8, 2.0) 7.59, dd (6.1, 2.0) – 6.10, dd (6.1, 2.8) – 5.60, d (4.9)

dd (4.1,

6.09, dd (6.0, 3.0) – 5.20, br s

5.42, dd (10.0, 3.0) – 4.08, br s

5.41, ddd (10.0, 3.6, 0.8) – 5.07, br s

7

3.13, ddd (13.0, 5.9, 1.6) 1.40, dddd (13.8, 6.0, 1.9, 1.6) 1.99, tdd (13.8, 13.0, 1.5) 2.18, tdd (13.8, 6.4, 1.9) 1.74, ddt (13.8, 6.0, 1.5) 2.38, qdd (7.8, 6.4, 1.5) 1.58, ddd (13.0, 9.6, 8.6) 2.08, ddd (13.0, 8.6, 3.5) 1.15, d (7.8) 1.42, s 4.60, dt (17.7, 8.6)

2.90, dd (12.2, 5.8) 1.34, dd (14.7, 6.6) 1.76, brdd (14.7, 12.2) 2.09, m

3.08, br m

6.08, 3.0) – 4.14, 2.1) 3.12, 2.1) 5.68, 2.3) –

dd (7.0,

2.83, d (6.2)

2.67, br d (6.5)

td (7.0,

5.70, td (6.2, 2.0) –

2.93, dd (6.9, 1.1) 5.66, td (6.9, 1.2) –

5.61, td (6.5, 1.3)

1.93, ddd (15.4, 11.0, 1.2) 2.51, ddd (15.4, 6.9, 1.9) 2.20, tqd (11.0, 6.7, 1.9) 2.07, ddd (13.0, 8.9, 3.5 1.78, ddd (13.0, 9.6, 8.2) 1.16, d (6.7) 0.52, s 4.94, ddd (18.0, 9.6, 3.5) 4.67, ddd (18.0, 8.9, 8.2) 5.04, dd (19.2, 10.0) 4.70, ddd (19.2, 5.9, 3.0) –

1.80, 11.3, 2.59, 2.0) 2.25, 2.0) 2.06, 4.2) 2.15, 7.7) 1.17, 0.58, 4.97, 4.2) 4.69, 7.7) 5.06, 10.8, 4.70, 3.6) 1.98,

8a 8b 9a 9b

2.55, m 2.07, m 2.69, m 5.50, d (7.2)

3.82, dd (9.4, 4.9) –

1.64, m 2.04, m

5.00, ddd (11.8, 9.4, 3.2) 1.67, dt (13.2, 11.8) 2.64, ddd (13.2, 4.6, 3.2) 2.23, m

1.26, m

1.92, ddd (15.2, 9.3, 2.3) 2.50, ddd (15.2, 7.0, 3.1) 2.22, m

1.82, ddd (15.5, 11.1, 2.0) 2.64, ddd (15.5, 6.2, 2.2) 2.29, m

2.14, ddd (13.0, 8.6, 3.7) 1.88, ddd (13.0, 9.5, 8.1) 1.28, d (6.7) 1.04, s 4.94, ddd (17.7, 9.5, 3.7) 4.69, ddd (17.7, 8.6, 8.1) –

2.18, ddd (13.0, 8.5, 4.0) 2.33, ddd (13.0, 9.4, 7.8) 1.30, d (6.7) 1.05, s 5.03, ddd (17.7, 9.4, 4.0) 4.74, ddd (17.7, 8.5, 7.8)

1.64, m

1.63, m

2.14, m

2.42, m 1.63, m

17pro-R

4.84, ddd (17.7, 9.6, 3.5) –

1.57, ddd (13.0, 9.6, 8.5) 2.03, ddd (13.0, 8.5, 3.4) 1.18, d (7.4) 1.21, s 4.55, dt (17.7, 8.5) 4.80, ddd (17.7, 9.6, 3.4) –

0.99, d (7.1) 1.57, s 4.66, ddd (17.8, 8.8, 7.9) 4.85, ddd (17.8, 9.5, 4.0) –

1.90, ddd (13.0, 8.4, 4.1) 1.84, ddd (13.0, 9.0, 8.4) 1.29, d (6.6) 1.20, s 4.86, ddd (17.7, 9.0, 4.1) 4.66, dt (17.7, 8.4) –

17pro-S

–

–

–

–

–

COCH3

–

–

–

2.11, s

–

10 13pro-R 13pro-S 14 15 16pro-R 16pro-S

2.06, m

dd (6.0,

1.95, s

2.85, dddd (10.8, 10.0, 8.0, 6.0) –

– ddd (15.5, 1.3) ddd (15.5, 6.5, tqd (11.3, 6.7, ddd (13.0, 8.7, ddd (13.0, 9.3, d (6.7) s ddd (17.9, 9.3, ddd (17.9, 8.7, ddd (19.0, 0.8) ddd (19.0, 6.0, s

Table 2 C NMR data for 1–8 (100 MHz, CDCl3)

13

Carbon

dC 1

2

3

4

5

6

7

8

COCH3

84.8 163.2 132.3 210.3 59.2 80.2 48.4 21.3 31.3 40.2 101.5 172.6 21.4 18.0 20.0 78.5 – –

85.2 31.7 33.2 219.2 58.7 81.9 49.4 20.5 31.8 41.9 102.0 172.9 21.5 17.8 16.2 78.3 – –

94.6 28.1 30.1 176.7 78.4 86.1 43.1 24.7 26.9 41.4 100.3 172.5 25.3 16.6 21.8 78.5 – –

54.1 160.3 130.4 209.5 56.0 66.7 51.6 77.5 44.5 27.3 96.4 172.1 23.8 19.6 21.6 79.2 – 21.1

52.0 163.7 129.7 212.3 57.3 72.0 53.7 80.6 39.7 25.9 101.4 173.0 23.1 19.6 19.0 79.3 – –

53.7 162.2 129.4 208.9 54.7 74.3 51.9 80.7 40.7 25.9 101.8 173.0 22.2 19.6 17.7 79.6 – 20.9

43.5 31.8 100.1 208.1 55.6 72.4 52.9 80.9 38.1 25.2 101.7 173.1 22.6 20.7 15.1 79.5 82.2 –

45.5 31.9 99.5 205.1 53.8 74.6 50.6 80.1 38.4 25.4 101.5 172.8 22.0 20.7 14.9 79.6 82.1 20.9

COCH3

–

–

–

168.9

–

169.7

–

169.3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

methylene and the C-15 methyl group is also present. Therefore, diazomethane is added from the re-face of the C-11@C-13 exocyclic methylene to afford the (11R)-spiropyrazolines. This also occurs in compounds 4, 7 and 8. With regards to the addition to the a,b-unsaturated ketone, structures 5a–6b show that H-1a

exerts a steric hindrance for the approximation of the diazomethane molecule from the 2si,3re-face23 of the C-2@C-3 endocyclic double bond. Therefore, the addition of diazomethane proceeds on the opposite side to form the (2R,3S)-pyrazoline in compounds 7 and 8.

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i

i

1a (89.4%)

1b (8.4%)

ii

ii

i

i

5a (71.4%)

5b (28.6%)

ii

ii

i

i 6a (94.7%)

6b (5.3%)

Figure 2. The lowest DFT B3LYP/DGDZVP minimum energy conformers of parthenins 1a and 1b, helenalins 5a and 5b, and helenalin acetates 6a and 6b, and the preferred addition of diazomethane to (i) the exocyclic methylene a,b-unsaturated-c-lactone, and subsequently to (ii) the cyclopentenone functionalities. The green arrows indicate the preferred face of diazomethane attack and the orange arcs represent the steric hindrance.

2.4. X-ray diffraction analysis of pyrazolines A useful property of pyrazoline derivatives, particularly explored in sesquiterpene lactones, is their high tendency to generate crystalline products. Herein, all pyrazoline derivatives yielded high quality crystals that allowed us to undertake X-ray diffraction analyses. This advantage might be useful in the case of new compounds that contain an a,b-unsaturated lactone or a ketone moieties to increase the probability of obtaining crystalline derivatives suitable for X-ray studies, including the calculation of the Flack19 and Hooft20 parameters to determine their absolute configuration. The X-ray data of 1–8 were measured by mounting a single crystal of each compound on an X-ray diffractometer equipped with graphite monochromated Cu Ka radiation. The molecular structures for 1–8 are depicted in Figures 3 and 4, while the crystal data and collection details are summarized in Tables 3 and 4. In the solid state, the preferred conformation for the sevenmembered ring of compounds 1 and 2 is quite comparable, while the conformation of this ring in 3 undergoes a drastic variation, which alters the orientation of the C-14 and C-15 methyl groups, and also modifies the conformation of the pyrazoline ring (Fig. 3). In lactones closed at C8, the seven-membered ring adopts an analogous conformation in 4 (Fig. 3) and in 6–8 (Fig. 4). Pyrazoline 5 (Fig. 4) crystallized in a different conformation which corre-

sponds to the second minimum energy structure of helenalin, labelled as 5b in Figure 2. It was also observed that the ring closure (trans in 4 and cis in 6–8) slightly modifies the conformation of the seven-membered ring but significantly alters the conformation of the pyrazoline moiety. Compounds 1–8 were subjected to Flack and Hooft parameter calculations, listed in Tables 3 and 4, in order to test their absolute configuration, as that depicted in Figure 1. Additionally, the X-ray analysis permitted the unambiguous determination of the absolute configuration of newly created stereogenic centers in these substances (C-2 and C-3 in 7 and 8, and C-11 in 1–8). 3. Conclusion The treatment of the pseudoguaianolides parthenin, coronopilin, psilostachyin, and mexicanin I acetate with ether solutions of diazomethane afforded their corresponding pyrazolines 1–4 in good yields, while helenalin and helenalin acetate showed the ability to afford monopyrazolines 5 and 6 or dipyrazolines 7 and 8, respectively, depending on the amount of added diazomethane and the reaction conditions. The chemoselectivity towards the spiropyrazoline formation at C-11–C-13 over the pyrazoline at C-2–C-3 in helenalin and its acetate can be explained on the basis of the high reactivity of the exocyclic methylene a,b-unsaturated-c-lactone

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1

5

2

6

3

7

4

8

Figure 3. X-ray diffraction structures of pyrazolines 1–4.

system. The diastereoselectivity of the cycloaddition at C-11–C-13 can be understood on the basis of the steric effects generated by the seven-membered ring together with the angular Me-15, which always remains b-oriented. The diastereoselectivity for the diazomethane addition to the a,b-unsaturated ketone to form dipyrazolines 7 and 8 seems to be determined by the steric hindrance of the axial H-1 atom, located in the adjacent position (C-2@C-3) of the 1,3-dipolar cycloaddition. Finally, the success in obtaining good quality crystals in all of the studied compounds reveals that the formation of pyrazoline derivatives can be very useful for the determination of the stereochemistry of new compounds that contain a, b-unsaturated lactone or ketone moieties to increase the probability of obtaining crystalline derivatives suitable for X-ray studies.

Figure 4. X-ray diffraction structures of pyrazolines 5–8.

4. Experimental 4.1. General Samples of parthenin, coronopilin and psilostachyin came from the group of the late Professor Jesús Romo (Instituto de Química, UNAM, Mexico City), while mexicanin I acetate, helenalin, and helenalin acetate were available from previous studies.24,25 Their purity was verified by 1H and 13C NMR spectra. 1H (400 MHz)

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Table 3 Crystal data for pyrazolines 1–4

Empirical formula Formula weight Crystal size (mm) Crystal system Space group Unit cell dimensions (Å) a b c b (deg) Volume (Å3) Z, q calculated (mg/mm3) Absorption coefficient (mm-1) F(000) h range for data collection (deg) Limiting indices

Collected reflections Unique reflections Completeness to h (%) Data/restraints/parameters Goodness of fit on F2 Final R indices [I > 2r(I)] (%) Largest diff. peak and hole (e Å3) Flack and Hooft parameters Inverted Flack and Hooft parameters CCDC deposition No.

1

2

3

4

C16H20N2O4 304.34 0.30  0.10  0.10 Orthorhombic P212121

C16H22N2O4 306.36 0.25  0.20  0.10 Monoclinic P21

C16H22N2O5 322.36 0.40  0.25  0.10 Orthorhombic P212121

C18H22N2O5 346.38 0.40  0.20  0.10 Monoclinic P21

6.9341(3) 12.5202(4) 18.0576(7) 90 1567.7(1) 4, 1.289 0.769 648 4.30–77.79 8 6 h 6 8, 0 6 k 6 15, 0 6 l 6 22 19872 [R(int) = 0.0001] 3305 99.2 2825/0/226 1.040 R1 = 3.6, wR2 = 9.7 0.109 and 0.094 0.00(3) and 0.03(11) 0.90(3) and 1.03(11) 1462805

9.648(2) 11.199(2) 10.256(2) 115.63(3) 999.1(3) 2, 1.018 0.604 328 4.78–77.31 0 6 h 6 12, 13 6 k 6 12, 12 6 l 6 11 19832 [R(int) = 0.0001] 2958 76.4 2958/1/210 1.077 R1 = 5.3, wR2 = 14.7 0.152 and 0.146 0.04(15) and 0.07(11) 1.20(15) and 1.10(11) 1462806

6.3749(1) 12.0900(3) 20.4655(5) 90 1577.33(6) 4, 1.357 0.842 688 4.32–77.53 7 6 h 6 7, 15 6 k 6 15, 25 6 l 6 25 26678 [R(int) = 0.0353] 3290 98.4 3065/0/226 1.013 R1 = 4.1, wR2 = 11.2 0.224 and 0.217 0.10(2) and 0.06(7) 0.90(2) and 0.94(7) 1462807

9.426(2) 7.477(2) 12.352(3) 97.90(3) 862.3(3) 2, 1.334 0.811 368 3.61–77.86 11 6 h 6 11, 8 6 k 6 9, 15 6 l 6 15 26980 [R(int) = 0.0518] 3538 98.8 3143/1/258 1.075 R1 = 3.6, wR2 = 9.0 0.147 and 0.132 0.08(19) and 0.06(9) 0.88(19) and 0.94(9) 1462808

Table 4 Crystal data for pyrazolines 5–8

Empirical formula Formula weight Crystal size (mm) Crystal system Space group Unit cell dimensions (Å) a b c b (°) Volume (Å3) Z, q calculated (mg/mm3) Absorption coefficient (mm 1) F(000) h range for data collection (°) Limiting indices

Collected reflections Unique reflections Completeness to h (%) Data/restraints/parameters Goodness of fit on F2 Final R indices [I > 2r(I)] (%) Largest diff. peak and hole (e Å3) Flack and Hooft parameters Inverted Flack and Hooft parameters CCDC deposition No.

5

6

7

8

C16H20N2O4 304.34 0.26  0.18  0.10 Monoclinic P21

C18H22N2O5 346.38 0.20  0.20  0.10 Orthorhombic P212121

C17H22N4O4 346.39 0.40  0.40  0.10 Orthorhombic P212121

C19H24N4O5 388.42 0.60  0.50  0.45 Monoclinic P21

9.3569(3) 6.2001(5) 12.559(4) 93.930(4) 726.9(2) 2, 1.390 0.829 324 4.74–77.64 11 6 h 6 11, 7 6 k 6 7, 15 6 l 6 15 6165 [R(int) = 0.0001] 2984 97.0 2851/0/230 1.037 R1 = 5.3, wR2 = 14.8 0.386 and 0.284 0.1(3) and 0.2(11) 0.9(3) and 1.02(11) 1516983

7.1418(3) 9.3728(4) 26.122(1) 90 1748.6(1) 4, 1.316 0.800 736 5.01–77.52 8 6 h 6 9, 12 6 k 6 11, 31 6 l 6 32 22386 [R(int) = 0.0001] 2787 77.5 2787/0/258 1.070 R1 = 4.2, wR2 = 11.0 0.161 and 0.182 0.00(3) and 0.02(15) 0.90(3) and 1.03(15) 1462809

9.196(2) 10.223(2) 17.478(4) 90 1643.2(6) 4, 1.400 0.840 736 5.01–77.52 11 6 h 6 11, 12 6 k 6 12, 22 6 l 6 21 21062 [R(int) = 0.0395] 3101 92.7 2873/0/260 1.022 R1 = 2.9, wR2 = 7.4 0.185 and 0.120 0.10(19) and 0.13(10) 1.09(19) and 1.13(10) 1462812

9.1226(2) 8.4012(2) 12.5997(3) 99.841(3) 951.45(4) 2, 1.356 0.827 412 3.56–77.33 11 6 h 6 11, 10 6 k 6 10, 15 6 l 6 15 12948 [R(int) = 0.0001] 3947 98.1 3896/1/284 1.030 R1 = 2.8, wR2 = 7.8 0.160 and 0.139 0.06(13) and 0.07(3) 0.93(13) and 0.93(3) 1512966

and 13C (100 MHz) NMR measurements, including gCOSY, gHSQC, and gHMBC experiments, were performed on a Varian VNMRS or a Bruker Ascend 400 spectrometer from CDCl3 solutions using TMS as the internal standard. Optical rotations were determined in EtOH or CHCl3 on a Perkin-Elmer 341 polarimeter. Column chromatography was carried out on Merck silica gel 60 (Aldrich, 230– 400 mesh ASTM).

4.2. Treatment of pseudoguaianolides with diazometane Samples of the pseudoguaianolides parthenin, coronopilin, psilostachyin, and mexicanin I acetate (100 mg) in MeOH (2 mL) were cooled to 4 °C, mixed with 2 mL of an ether solution of diazomethane (prepared with 3 g of N-nitrosomethylurea), allowed to stand at 4 °C overnight and evaporated to dryness under an N2

A. Ortiz-León et al. / Tetrahedron: Asymmetry 28 (2017) 367–373

stream at room temperature to give pyrazolines 1–4, respectively, in almost quantitative yields. Likewise, helenalin and helenalin acetate (100 mg each) in MeOH (2 mL) were treated with 15 mL of the same ethereal solution of diazomethane to provide dipyrazolines 7 and 8, correspondingly, in almost quantitative yields. Crystallization of the residues or column chromatography (CC) purification provided pure pyrazolines. In addition, cold (4 °C) solutions of helenalin and helenalin acetate (50 mg) in CH2Cl2–Et2O (1:1, 2 mL) were treated with an ethereal solution of diazomethane (ca. 1 mL) which was added dropwise under stirring until persistence of the yellow color to afford monopyrazolines 5 and 6, respectively, in almost quantitative yields and P98% of ds. Crystallization of the products from CH2Cl2–MeOH yielded pure 5 and 6. 4.3. (1S,5S,6R,7S,10S,11S)-Parthenin pyrazoline 1 The reaction product was column chromatographed using hexanes-EtOAc (7:3) collecting fractions of 10 mL. Fraction 5 gave 1 as colorless prisms, mp 145–147 °C (dec.) [lit.2 mp 146–148 °C (dec.)]; [a]20 107.2 (c 0.5, EtOH). 1H and 13C NMR, see Tables D = 1 and 2. 4.4. (1R,5S,6R,7S,10S,11S)-Coronopilin pyrazoline 2 The reaction product was purified by column chromatography employing hexanes-EtOAc (9:1 and 2:1). The fractions eluted with hexanes-EtOAc 2:1 provided pyrazoline 2, which was obtained as colorless prisms after crystallization from CHCl3-MeOH, mp 136– 138 °C (dec.) [lit.3 mp 145 °C (dec.)]; [a]20 79.6 (c 0.9, EtOH). D = 1 H and 13C NMR, see Tables 1 and 2. 4.5. (1R,5S,6R,7S,10S,11S)-Psilostachyin pyrazoline 3 The reaction product was subjected to crystallization from CH2Cl2-MeOH to yield 3 as colorless prisms, mp 143–146 °C (dec.); [a]20 93.1 (c 0.5, EtOH). 1H and 13C NMR, see Tables 1 D = and 2. 4.6. (1R,5R,6R,7R,8S,10R,11R)-Mexicanin I acetate pyrazoline 4 The reaction product was column chromatographed using hexanes-EtOAc (1:4), and collecting 10 mL fractions to give pyrazoline 4, which was obtained as colorless prisms after crystallization from CHCl3-MeOH, showing mp 151–154 °C (dec.); [a]20 D = +36.9 (c 0.8, EtOH). 1H and 13C NMR, see Tables 1 and 2. 4.7. (1R,5R,6S,7R,8R,10R,11R)-Helenalin pyrazoline 5 The reaction product was subjected to crystallization from CH2Cl2-MeOH to yield 5, which was obtained as colorless prisms 1 mp 154–157 °C (dec.); [a]20 H and 13C D = +97.0 (c 0.1, EtOH). NMR, see Tables 1 and 2. 4.8. (1R,5R,6S,7R,8R,10R,11R)-Helenalin acetate pyrazoline 6 The reaction product was crystallized using CH2Cl2-MeOH to yield 6 as colorless prisms, mp 111–113 °C (dec.); [a]20 31.2 (c D = 0.8, EtOH). 1H and 13C NMR, see Tables 1 and 2. 4.9. (1R,2R,3S,5R,6S,7S,8R,10R,11R)-Helenalin dipyrazoline 7 The reaction product was chromatographed using hexanes– EtOAc (3:7 and 1:4), collecting 10 mL fractions. Dipyrazoline 7 was isolated from fractions 3 and 4 as colorless prisms, mp 135– 137 °C (dec.); [a]20 124.3 (c 0.4, CHCl3). 1H and 13C NMR, see D = Tables 1 and 2.

373

4.10. (1R,2R,3S,5R,6S,7S,8R,10R,11R)-Helenalin acetate dipyrazoline 8 The reaction product was crystallized from CH2Cl2-MeOH to yield dipyrazoline 8 as colorless prisms, mp 186–189 °C (dec.); 1 13 [a]20 C NMR, see Tables 1 and 2. D = +51.4 (c 1.08, EtOH). H and 4.11. Single crystal X-ray studies The data were collected on an Oxford Diffraction Gemini CCD instrument using graphite-monochromated CuKa (k = 1.54184 Å) radiation at 294 K in the x scan mode. Crystal data for compounds 1–8 are summarized in Tables 1 and 2. The structures were solved by direct methods using the SHELXS-97 program included in the WinGX v1.70.01 crystallographic software package. For the structural refinement, the non-hydrogen atoms were treated anisotropically, and the hydrogen atoms, included in the structure factor calculation, were refined isotropically. Crystallographic data (excluding structure factors) have been deposited at the Cambridge Crystallographic Data Centre. Copies of the data can be obtained free of charge on application to the CCDC, 12 Union Road, Cambridge CB2 IEZ, UK. Fax: +44-(0)1223-336033 or e-mail: [email protected]. Acknowledgments One of us (PJN) thanks to the late Professor Jesús Romo for samples of parthenin, coronopilin, and psilostachyin. Partial financial support from CONACYT-Mexico (Grant No. 238206) and from PRODEP-Mexico (Grant Síntesis Química y Supramolecular 2016-2017) is acknowledged. AOL thanks CONACYT, Mexico for fellowship 389269. References 1. Ortiz-León, A.; Torres-Valencia, J. M.; Manriquez-Torres, J. J.; AlvaradoRodriguez, J. G.; Hernández-Balderas, U.; Cerda-García-Rojas, C. M.; JosephNathan, P. Nat. Prod. Commun. 2014, 9, 753–756. 2. Herz, W.; Watanabe, H.; Miyazaki, M.; Kishida, Y. J. Am. Chem. Soc. 1962, 84, 2601–2610. 3. Herz, W.; Högenauer, G. J. Org. Chem. 1961, 26, 5011–5013. 4. Mabry, T. J.; Miller, H. E.; Kagan, H. B.; Renold, W. Tetrahedron 1966, 22, 1139– 1146. 5. Domínguez, E.; Romo, J. Tetrahedron 1963, 19, 1415–1421. 6. Clark, E. P. J. Am. Chem. Soc. 1936, 58, 1982–1983. 7. Adams, R.; Herz, W. J. Am. Chem. Soc. 1949, 71, 2146–2551. 8. Picman, A. K. Biochem. Syst. Ecol. 1986, 14, 255–281. 9. Scotti, M. T.; Fernandes, M. B.; Ferreira, M. J. P.; Emerenciano, V. P. Bioorg. Med. Chem. 2007, 15, 2927–2934. 10. Dirsch, V. M.; Stuppner, H.; Ellmerer-Müller, E. P.; Vollmar, A. M. Bioorg. Med. Chem. 2000, 8, 2747–2753. 11. Schomburg, C.; Schuehly, W.; Da Costa, F. D.; Klempnauer, K.-H.; Schmidt, T. J. Eur. J. Med. Chem. 2013, 63, 313–320. 12. Hwang, D.-R.; Wu, Y.-S.; Chang, C.-W.; Lien, T.-W.; Chen, W.-C.; Tan, U.-K.; Hsu, J. T. A.; Hsied, H.-P. Bioorg. Med. Chem. 2006, 14, 83–91. 13. Huang, P.-R.; Ming, Y.; Wang, T.-C. V. Cancer Lett. 2005, 227, 169–174. 14. Zhang, Z.; Xu, L.; Cheung, H.-Y. J. Mol. Graph. Model. 2014, 51, 97–103. 15. Díaz, E.; Joseph-Nathan, P.; Romo de Vivar, A.; Romo, J. Bol. Inst. Quím. Univ. Nal. Autón. Méx. 1965, 17, 122–138. 16. Joseph-Nathan, P.; Díaz, E. Rev. Latinoamer. Quím. 1971, 2, 54–57. 17. Joseph-Nathan, P. Rev. Soc. Quím. Méx. 1976, 20, 255–259. 18. Joseph-Nathan, P. Resonancia Magnética Nuclear de Hidrógeno-1 y de Carbono13; The General Secretariat of the Organization of the American States: Washington, DC, USA, 1982. pp. 136 139. 19. Flack, H. D.; Bernardinelli, G. Chirality 2008, 20, 681–690. 20. Hooft, R. W. W.; Straver, L. H.; Spek, A. L. J. Appl. Crystallogr. 2008, 41, 96–103. 21. Joseph-Nathan, P.; García, G. E.; Díaz, E. J. Magn. Reson. 1973, 9, 378–382. 22. Velázquez-Jiménez, R.; Torres-Valencia, J. M.; Valdez-Calderón, A.; AlvaradoRodríguez, J. G.; Hernández-Hernández, J. D.; Román-Marín, L. U.; CerdaGarcía-Rojas, C. M.; Joseph-Nathan, P. Tetrahedron: Asymmetry 2016, 27, 193– 200. 23. McMurry, J. E. Organic Chemistry, 8th ed.; Brooks/Cole: Belmont, CA, 2010. p. 167. 24. Romo, J.; Joseph-Nathan, P.; Díaz, A. Chem. Ind. 1963, 1839. 25. Romo, J.; Joseph-Nathan, P.; Díaz, A. F. Tetrahedron 1964, 20, 79–85.