Molecules 2019, 24, 3193
7 of 10
1H-NMR (400 MHz, DMSO-d6):
δ
2.12 (s, 3H, C=C-CH3), 5.17 (s, 1H, C=CH), 5.92 (s, 1H, CO-CH),
11.58 (s, 1H, OH). 13C-NMR (100 MHz, DMSO-d6): δ 35.9, 93.3, 105.4, 168.5, 169.1, 175.7.
3.9. (E)-2-methyl-7-(prop-1-en-1-yl)-2,3-dihydropyrane[4,3-b]pyran-4,5-dione ((±)-3-deoxyradicinin; (±)-2)
Under a nitrogen atmosphere, TiCl4 (0.234 mL, 213 mmol, 1.8 eq) and crotonoyl chloride (0.170 mL,
1.77 mmol, 1.5 eq) were dropwise added in sequence to a solution of pyranodienone 7a (180 mg,
1.18 mmol) in 2 mL of CHCl2CHCl2. The mixture was first stirred for 15 min at room temperature
and then heated to 100 ◦C for 3 h. After cooling to room temperature, the mixture was poured into
water/ice (10 mL) and extracted with ethyl acetate. The organic layer was washed with brine and
concentrated under reduced pressure. The crude product was purified by column chromatography on
silica gel (n-hexane:acetone 1:1), to afford (±)-2 [24] (130 mg) in 50% yield.
1H-NMR (400 MHz, CDCl3):
2.59–2.70 (m, 2H, COCH2), 4.73–4.80 (m, 1H, OCHCH3), 5.83 (s, 1H, C=CH), 6.02 (d, J = 15.6 Hz, 1H,
δ 1.54 (d, J = 6.4 Hz, 3H, CHCH3), 1.95 (d, J = 6.8 Hz, 3H, CH=CHCH3),
CH=CHCH3), 6.89–6.97 (m, 1H, CH=CHCH3). 13C-NMR (100 MHz, CDCl3):
δ 18.7, 20.3, 43.7, 53.4,
98.1, 100.2, 122.7, 140.0, 157.2, 163.4, 175.9, 186.4. MS(EI): m/z 220 (44, M+), 205 (100), 179 (24), 111 (17),
69 (77). HRESIMS (+) m/z 221.0810 [M + H]+ (calcd for C12H13O4 221.0814).
3.10. 2,7-Dimethyl-2,3-dihydropyrane[4,3-b]pyran-4,5-dione ((±)-8)
Following the same procedure as above, the compound analogue (
45% yield.
±
1H-NMR (400 MHz, CDCl3):
δ 1.54 (d, J = 6.8 Hz, 3H, CH-CH3), 2.27 (s, 3H, CH=C-CH3), 2.59–2.68
(m, 2H, CH-CH2), 4.74–4.79 (m, 1H, CH-CH2), 5.90 (s, 1H, C=CH). 13C-NMR (100 MHz, CDCl3):
δ 20.2,
20.7, 29.2, 43.7, 95.1, 99.6, 157.8, 168.7, 175.9, 186.4. MS(EI): m/z 194 (28, M+), 179 (100), 153 (16), 69 (61),
41 (16). HRESIMS (+) m/z 195.0651 [M + H]+ (calcd for C10H11O4 195.0657).
3.11. Oxidation of (±)-3-deoxyradicinin (2) with Pb(OAc)4
A mixture of compound 2 (40 mg, 0.18 mmol), Pb(OAc)4 (120 mg, 1.5 eq) and acetic acid (3 mL)
was stirred and heated to 100 ◦C for 2h. The mixture was then cooled to room temperature, poured
into water/ice and extracted with dichloromethane. The organic layer was washed with a 5% NaHCO3
solution and brine and concentrated under reduced pressure. By column chromatography on silica gel
(n-hexane: acetone 1:1) of the residue, two fractions were eluted, corresponding to a 6:4 mixture of
trans:cis (±)-radicinin acetate 9 [21] (9.5 mg, 19%) and the pyranopyrandione 10 [26] (29 mg, 74%).
(E)-2-methyl-4,5-dioxo-7-(prop-1-en-1-yl)-2,3,4,5-tetrahydropyrano[4,3-b]pyran-3-yl acetate (9a
+ 9b
1
mixture). H-NMR (400 MHz, CDCl3):
δ
1.45 (d, J = 6.4 Hz, 3H, CHCH3, 9b), 1.55 (d, J = 6.4 Hz, 3H,
9b), 2.17 (s, 3H, COCH3, 9b), 2.21 (s, 3H, COCH3,
9a), 4.69–4.75 (m, 1H, CHCH3, 9a), 4.85–4.92 (m, 1H, CHCH3, 9b), 5.25 (d, J = 10.8 Hz, 1H,CHOAc, 9a),
CHCH3, 9a), 1.97 (d, J = 7.2 Hz, 3H, CH=CHCH3, 9a
+
5.49 (d, J = 3.6 Hz, 1H, CHOAc, 9b), 5.85 (s, 1H, C=CH, 9a
+
9b), 6.04 (dd, J = 15.2 Hz, 1.6 Hz, 1H,
CH=CHCH3, 9a + 9b), 6.93–7.00 (m, 1H, CH=CHCH3, 9a + 9b).
1
(E)-2-methyl-7-(prop-1-en-1-yl)pyrano[4,3-b]pyran-4,5-dione (10). H-NMR (400 MHz, CDCl3):
δ 1.97
(d, J = 7.2 Hz, 3H, CH=CHCH3), 2.30 (s, 3H, CH=CCH3), 6.06 (s, 1H, CH=CCH3), 6.07 (d, J = 15.6 Hz,
1H, CH=CHCH3), 6.17 (s, 1H, C=CH), 6.90–6.99 (m, 1H, C=CH-CH3). 13C-NMR (100 MHz, CDCl3):
δ 18.7, 19.6, 96.7, 105.7, 115.7, 122.3, 139.8, 156.9, 162.2, 163.6, 169.3, 174.3. HRESIMS (+) m/z 219.0653
[M + H]+ (calcd for C12H11O4 219.0657).
3.12. Leaf Puncture Bioassays
The synthetic derivatives (
buffelgrass (C. ciliaris) following a procedure previously reported [22
with that of natural 18]. Accordingly, compounds were first dissolved in MeOH (final concentration
±
)-
2
and 10 were assayed at 2.5
×
10−3 M for phytotoxicity on leaves of
25] and their activity compared
,
1
[