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Dihydrobenzofuran Neolignans

Uncategorized

May 132016

Figure 1 Structures of dihydrobenzofuran neolignans 2a and 2b.

Scheme 1 (i) Ag₂O, (CH₃)₂CO:C₆H₆ 3:5, r.t., 20 h (2a: 36% yield; 2b: 43% yield).

Figure 3 Main nuclear Overhauser effect (NOE) correlations observed in the nuclear Overhauser effect spectroscopy (NOESY) spectra of compounds 2a and 2b.

Table 1 ¹H and ¹³C NMR data assignments for compound 2a (400 MHz, CDCl₃)

	δ_Ca	δ_H (integral, multiplicityb), J/ Hz
1	132.0 (C)	−
2=6	127.5 (CH)	7.27 (2H, ddd, J_2,5 = J_6,3 0.3, J_2,7 = J_6,7 0.6, J_2,3 = J_6,5 8.3)
3=5	115.7 (CH)	6.84 (2H, dd, J_3,6 = J_5,2 0.3, J_3,2 = J_5,6 8.3)
4	156.1 (C)	−
7	87.7 (CH)	6.09 (1H, dt, J_7,2 = J_7,6 0.6, J_7,8 7.2)
8	55.1 (CH)	4.27 (1H, dd, J_8,6′ 1.4, J_8,7 7.2)
9	170.9 (C)	−
10	52.9 (CH₃)	3.83 (3H, s)
1′	127.8 (C)	−
2′	130.8 (CH)	7.43 (1H, ddd, J_2′,7‘ 1.1, J_2′,6′ 2.0, J_2′,3′ 8.3)
3′	110.3 (CH)	6.89 (1H, dd, J_3′,6′ 0.4, J_3′,2′8.3)
4′	161.2 (C)	−
5′	125.1 (C)	−
6′	124.9 (CH)	7.55 (1H, dddd, J_6′,3′ 0.4, J_6′,7′ 0.7, J_6′,8 1.4, J_6′,2′2.0)
7′	144.7 (CH)	7.66 (1H, ddd, J_7′,6′ 0.7, J_7′,2′ 1.1, J_7′,8′ 15.9)
8′	115.2 (CH)	6.32 (1H, d, J_8′,7′ 15.9)
9′	167.9 (C)	−
10′	51.7 (CH₃)	3.81 (3H, s)

^aMultiplicities assigned on the basis of distortionless enhancement by polarization transfer (DEPT) 135 experiments;

^bmultiplicities and coupling constant values measured within ¹H NMR and J-resolved spectra with the help from¹H-¹H correlation spectroscopy (COSY) results.

Table 2 2D NMR data for compound 2a (400 MHz, CDCl₃)

C	H	gCOSYa	gHMBCb	gHMQCc	NOESYd
1	–	–	H₃ =H₅, H₇, H₈	–	–
2=6	2=6	H₃, H₅, H₇	H₃ =H₅, H₇	H₂=H₆	H₇, H₈
3=5	3=5	H₂, H₆	H₅	H₃=H₅	–
4	–	–	H₃ =H₅, H₂ =H₆	–	–
7	7	H₂=H₆, H₈	H₆, H₈	H₇	H₂=H₆*
8	8	H₇, H_6′	H₇, H_6′	H₈	H₂=H₆, H_6′*
9	–	–	H₇, H₈, H₁₀	–	–
10	10	–	−	H₁₀	–
1′	–	–	H_3′, H_8′	–	–
2′	2′	H_3′, H_6′, H_7′	H_6′, H_7′	H_2′	H_7′, H_8′*
3′	3′	H_2′, H_6′	−	H_3′	–
4′	–	–	H_2′, H_3′, H_6′, H₇, H₈	–	–
5′	–	–	H₈, H_3′	–	–
6′	6′	H_2′, H_3′, H_7′, H₈	H_2′, H_7′, H₈	H_6′	H_8′, H_7′, H₈*
7′	7′	H_2′, H_8′, H_6′	H_2′, H_6′, H_8′	H_7′	H_6′*, H_2′
8′	8′	_H7′	H_7′	H_8′	H_6′*, H_2′
9′	–	–	H_7′, H_8′, H_10′	–	–
10′	10′	–	–	H_10′	–

^bgradient-selected heteronuclear multiple bond coherence;
^cgradient-selected heteronuclear multiple quantum coherence;
^dnuclear Overhauser effect spectroscopy.
^*mean weak correlation.

^aGradient-selected correlation spectroscopy;

Table 3 ¹H and ¹³C NMR data assignments for compound 2b (400 MHz, acetone-d₆)

	δ_Ca	δ_H (integral, multiplicityb); J/ Hz
1	132.5 (C)	–
2	111.2 (CH)	7.10 (1H, ddd, J_2,5 0.3, J_2,7 0.8, J_2,6 2.1)
3	149.1 (C)	–
4	148.5 (C)	–
5	116.3 (CH)	6.84 (1H, dd, J_5,2 0.3, J_5,6 8.3)
6	120.7 (CH)	6.92 (1H, ddd, J_6,7 0.6, J_6,2 2.1, J_6,5 8.3)
7	88.8 (CH)	6.04 (1H, ddd, J_7,6 0.6, J_7,2 0.8, J_7,8 7.3)
8	57.0 (CH)	4.47 (1H, dd, J_8,6′ 1.4, J_8,7 7.3)
9	172.1 (C=O)	–
10	53.5 (CH₃)	3.81 (3H, s)
11	56.4 (CH₃)	3.84 (3H, s)
1′	129.9 (C)	–
2′	113.9 (CH)	7.33 (1H, dd, J_2′,7′ 0.4, J_2’6′ 2.6)
3′	146.3 (C)	–
4′	151.5 (C)	–
5′	127.8 (C)	–
6′	119.5 (CH)	7.29 (1H, ddd, J_6′,7′ 0.8, J_6′,8 1.4, J_6′,2′ 2.6)
7′	145.9 (CH)	7.63 (1H, ddd, J_7′,2′ 0.4, J_7′,6′ 0.8, J_7′,8′ 15.8)
8′	116.8 (CH)	6.44 (1H, d, J_8′,7′ 15.8)
9′	168.2 (C)	–
10′	52.1 (CH₃)	3.73 (3H, s)
11′	56.8 (CH₃)	3.92 (3H, s)

^aMultiplicities assigned on the basis of distortionless enhancement by polarization transfer (DEPT) 135 experiments;
^bmultiplicities and coupling constant values measured within ¹H-NMR and J-resolved spectra with the help from¹H-¹H correlation spectroscopy (COSY) results.

Table 4 2D NMR data for compound 2b (400 MHz, acetone-d₆)

C	H	gCOSYa	gHMBCb	gHMQCc	NOESYd
1	–	–	H₂, H₆, H₇, H₈	–	–
2	2	H₅, H₆, H₇	H₅, H₆, H₇	H₂	H₇, H₈, H₁₁
3	–	–	H₂, H₅, H₁₁	H₃	–
4	–	–	H₂, H₅, H₆	–	–
5	5	H₂, H₆	H₆	H₅	–
6	6	H₂, H₅, H₇	H₂, H₅	H₆	H₇, H₈
7	7	H₂, H₆, H₈	H₂, H₆, H₈	H₇	H₆*, H₂
8	8	H_6′, H₇	H₂, H_6′	H₈	H_6′*, H₂, H₆
9	–	–	H₇, H₈, H₁₀	H₉	–
10	10	–	−	H₁₀	–
11	11	–	−	H₁₁	H₂
1′	–	–	H_7′, H_8′	H_1′	–
2′	2′	H_6′, H_7′	H_6′, H_7′	H_2′	H_7′, H_8′*, H_11′
3′	–	–	H_11′	–	–
4′	–	–	H_2′, H_6′, H₇, H₈	–	–
5′	–	–	H₇, H_8′	–	–
6′	6′	H_7′, H_2′, H₈	H_2′, H_7′, H₈	H_6′	H_8′, H_7′, H₈*
7′	7′	H_2′, H_6′, H_8′	H_2′, H_6′, H_8′	H_7′	H_6′, H_2′,
8′	8′	H_7′	H_7′	H_8′	H_6′, H_2′
9′	–	–	H_8′, H_10′	–	–
10′	10′	–	–	H_10′	–
11′	11′	–	–	H_11′	H_2′

^aGradient-selected correlation spectroscopy;
^bgradient-selected heteronuclear multiple bond coherence;
^cgradient-selected heteronuclear multiple quantum coherence;
^dnuclear Overhauser effect spectroscopy.
^*mean weak correlation.

¹H and ¹³C NMR data previously reported for compound2a and 2b were obtained in CDCl₃ or acetone-d₆. Most of the signals in the ¹H NMR spectrum were between δ_H 6.0 and δ_H 8.0, but the hydrogen signal multiplicities are ambiguous. In this work, we found that for compound2a in acetone-d₆, the signals at δ_H 7.6-7.7 are referred to four hydrogen atoms and their overlapping precluded their correct assignment (Figure 2). Therefore, CDCl₃ provided much clearer spectra for 2a, but not for 2b, due to the solvent influence on chemical shifts. For compound 2b, three hydrogen atoms resonate at δ_H 6.91 in the ¹H HMR spectrum in CDCl₃. On the other hand, the ¹H NMR signals of 2b were resolved by using acetone-d₆ as solvent, which allowed verification of the multiplicities, observation of the chemical shifts and measurement of the coupling constants.

Figure 2 Expansions of the ¹H NMR spectrum of compounds 2a and 2b obtained in CDCl₃ and acetone-d₆.

Journal of the Brazilian Chemical Society

On-line version ISSN 1678-4790

J. Braz. Chem. Soc. vol.27 no.1 São Paulo Jan. 2016

http://dx.doi.org/10.5935/0103-5053.20150262

ARTICLES

Detailed ¹H and ¹³C NMR Spectral Data Assignment for Two Dihydrobenzofuran Neolignans

Talita C. T. Medeiros^a^#, Herbert J. Dias^a^#, Eliane O. Silva^a, Murilo J. Fukui^b, Ana Carolina F. Soares^b, Tapas Kar^c, Vladimir C. G. Heleno^b, Paulo M. Donate^a, Renato L. T. Parreira^b, Antônio E. M. Crotti^a^*

^{^a}Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, 14040-901 Ribeirão Preto-SP, Brazil

^{^b}Núcleo de Pesquisas em Ciências Exatas e Tecnológicas, Universidade de Franca, 14404-600 Franca-SP, Brazil

^{^c}Department of Chemistry and Biochemistry, Utah State University, 84322-0300 Logan-UT, United States

ABSTRACT

In this work we present a complete proton (1H) and carbon 13 (¹³C) nuclear magnetic resonance (NMR) spectral analysis of two synthetic dihydrofuran neolignans (±)-trans-dehydrodicoumarate dimethyl ester and (±)-trans-dehydrodiferulate dimethyl ester. Unequivocal assignments were achieved by ¹H NMR, proton decoupled ¹³C (¹³C{¹H}) NMR spectra, gradient-selected correlation spectroscopy (gCOSY), J-resolved, gradient-selected heteronuclear multiple quantum coherence (gHMQC), gradient-selected heteronuclear multiple bond coherence (gHMBC) and nuclear Overhauser effect spectroscopy (NOESY) experiments. All hydrogen coupling constants were measured, clarifying all the hydrogen signals multiplicities. Computational methods were also used to simulate the ¹H and ¹³C chemical shifts and showed good agreement with the transconfiguration of the substituents at C₇ and C₈.