Total Synthesis of (+)-Mesembrine Applying Asymmetric Gold Catalysis

Article abstract of DOI:10.1021/acs.joc.6b00985

The total synthesis of enantiomerically pure (+)-mesembrine is described. The central pyrrolidine moiety incorporating a quaternary, all-carbon-substituted stereocenter was constructed employing an asymmetric gold-catalyzed cycloisomerization of a 1,4-diynamide.

Full text of DOI:10.1021/acs.joc.6b00985

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Note
Total Synthesis of (+)-Mesembrine Applying Asymmetric Gold Catalysis
Michael Spittler, Kiril Lutsenko, and Constantin Czekelius
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b00985 • Publication Date (Web): 15 Jun 2016
Downloaded from http://pubs.acs.org on June 20, 2016
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Total Synthesis of (+)-Mesembrine Applying Asymmetric Gold
Catalysis
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Michael Spittler, Kiril Lutsenko, and Constantin Czekelius*
Institute for Organic Chemistry and Macromolecular Chemistry, Universität Düsseldorf,
Universitätsstr. 1, D-40225 Düsseldorf, Germany
Fax: +49 211 81 11501
E-mail: constantin.czekelius@hhu.de
ABSTRACT: The total synthesis of enantiomerically pure (+)-mesembrine is described. The central pyrrolidine moiety
incorporating a quaternary, all-carbon-substituted stereocenter was constructed employing an asymmetric gold-
catalyzed cycloisomerization of a 1,4-diynamide.
(−)-Mesembrine is a naturally occurring alkaloid including a pyrrolidine substructure which was originally isolated
from the succulent plant Sceletium tortuosum. This plant is also known as kanna and has been used as a stimulant by
South African natives.1 In 1957, Bodendorf and Krieger1a published the first accurate structural elucidation of
mesembrine and three years later Popelak et al.2 determined its absolute configuration (Figure 1).
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Figure 1 (−)-Mesembrine, a Naturally Occurring Alkaloid.
Shamma and Rodriguez were the first to report a total synthesis of racemic mesembrine in 1965.3 In 1971, Yamada and
coworkers chose a chiral auxiliary-controlled approach to perform the first synthesis of enantiomerically enriched, non-
natural (+)-mesembrine.4 Ten years later, the group of Takano published the ex-chiral-pool total synthesis of the
naturally occurring (−)-mesembrine starting from
(D
)-mannitol.5 The first total synthesis of enantiopure
(+)-mesembrine was performed by the Meyers group, following a reagent-controlled strategy.6 Since then, over 40
syntheses have been published, rendering mesembrine one of the calibration standards for the application of new
stereoselective methodology in the context of total synthesis.7 In particular, the quaternary, all-carbon substituted
stereocenter poses a challenge since methods for its preparation are limited in number.8
Within the last two decades, gold-catalyzed reactions have evolved as a remarkable field of research and today various
transformations of alkenes, alkynes and allenes via carbophilic Lewis activation are well-established.9 Employing chiral
catalysts enantioselective protocols have been developed which also found application in total synthesis.10 Recently, we
have reported the desymmetrization of 1,4-diynols and 1,4-diynamides by gold-catalyzed cycloisomerization leading to
heterocyclic products such as dihydro-dioxepines, tetrahydro-oxazepines, methylene tetrahydrofuranes and methylene
pyrrolidines.11 While catalysts with chiral phosphine or carbene ligands gave no satisfying enantioselectivities, optically
active phosphate counter-anions derived from 3,3'-disubstituted BINOL12 were successfully employed. The
cycloisomerization of diynamides 1 led to methylene pyrrolidines 3 in enantioselectivities up to 92% ee (Equation 1).
(1)
Given the new access to pyrrolidines with a quaternary stereocenter in 3-position we became interested in
exemplifying our gold-catalyzed asymmetric desymmetrization for heterocyclic natural product synthesis and chose
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mesembrine as a target molecule. We reasoned that not only mesembrine but also other alkaloids such as
spirooxindoles or aspidospermines are accessible via the new route. Herein, we report the implementation of the
enantioselective, gold-catalyzed diyne cycloisomerization as a key step for the total synthesis of (+)-mesembrine or
related alkaloids including a pyrrolidine substructure.
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Starting from commercially available 4-bromoveratrole (4) an Ullmann-type cross-coupling reaction with
acetylacetone was performed (Scheme 1).11b, 13 Copper(I) iodide was used as the catalyst and the coupling product was
obtained in 43% yield. The resulting diketone was C-selectively allylated and then transformed into ene-1,4-diyne 5 via
the corresponding enol phosphate ester using a modified protocol by Negishi.14 The alkene moiety was chemoselectively
dihydroxylated in an Upjohn reaction and the resulting glycol was cleaved using sodium periodate on silica gel to give
aldehyde 6.15
Scheme 1 Preparation of Diynal 6: (a) MeCOCH2COMe, CuI, L-proline, K2CO3, DMSO, 90 °C, 43%; (b) allyl iodide,
t
NaH, DMF, 74%; (c) LDA, ClPO(OEt)2, −78 °C, 77%; (d) K2OsO4, NMO, BuOH/H2O, 83%; (e) NaIO4, silica gel,
97%.
The transformation of aldehyde 6 to diynamide 8 was surprisingly challenging (Scheme 2). Following the protocol
developed earlier we first investigated the formation of a N-tosylimine followed by a reduction.11c, 16 However,
condensation with p-tosylamide and precipitation of the p-toluenesulfinate adduct, which was found successful for
related compounds, gave no satisfying results. The reductive amination of aldehyde 6 with different ammonium salts in
the presence of NaBH3CN or via a reduction of the corresponding oxime gave the desired diynamide 8 in a maximum
yield of only 30%. Therefore, we considered the nucleophilic substitution of alcohol 7 as an alternative approach. For
this purpose, aldehyde 6 was reduced with sodium borohydride (Scheme 2). The nucleophilic displacement of the
tosylate derived from alcohol 7 with p-tosylamide was not productive, however. Furthermore, the employment of other
N-nucleophiles such as ammonia or sodium azide at elevated temperatures resulted in a non-selective ring-closure. We
were pleased to find that the desired product 8 could be efficiently prepared by a Mitsunobu-type reaction with Boc-
p-tosylamide17 followed by acid-mediated deprotection of the Boc group. This two-step procedure provided diynamide 8
in 87% yield.
Scheme 2 Preparation of 1,4-Diynamide 8: (a) NaBH4, MeOH, 73%; (b) p-TsNHBoc, PPh3, DEAD, THF, 94%; (c)
TFA, CH2Cl2, 93%.
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At this point the stage was set for the gold-catalyzed, enantioselective desymmetrization which was carried out under
the previously optimized conditions.11 In former studies it was observed that both the solvent and the reaction
temperature are critical parameters for the selectivity of the reaction. The enantioselectivity was found to be highest in
non-coordinating chlorinated solvents such as chloroform or dichloromethane and severely decreased in a coordinating
solvent like THF. Furthermore, running the reaction at low temperatures significantly improved the enantiomeric
excess. Presumably, one carbon-carbontriple bond is activated by coordination of the cationic gold complex, which
forms a contact ion pair with the chiral phosphate.19 Upon this activation the amide nucleophile can attack the alkyne
moiety to form methylene pyrrolidine 9. The enantioselective cycloisomerization gave methylene pyrrolidine 9 in 76%
yield and 70% ee after stirring for 19 h at −55 °C (Equation 2).
Ts
N
NHTs
(tBu3P)Au[(R)-TriP] (2)
CHCl3, 55 °C
MeO
MeO
76%, 70% ee
OMe
OMe
8
9
(2)
It was shown earlier that the enantiomeric excess of related pyrrolidine compounds could be improved by
recrystallization (e.g. from n-hexane). However, in the case of product 9 recrystallization attempts from various solvents
did not significantly increase the enantiomeric excess. Therefore, we decided to proceed with the synthetic sequence
since recrystallization of
a
later intermediate was known to result in enantiomerically pure material.7a
Methylene pyrrolidine 9 was transformed into the corresponding unsaturated ester by treatment of the acetylide with
methyl chloroformate (Scheme 3). The subsequent conjugate reduction was performed by catalytic hydrogenation using
palladium on carbon at 85 bar. Ester 10 was formed in 98% yield without concomitant reduction of the enamide moiety
or the aromatic substituent. In contrast, reduction using Stryker's reagent gave mixtures of the desired ester 10 and the
partially reduced alkenoic ester.
n
Scheme 3 Total Synthesis of (+)-Mesembrine: (a) ClCO2Et, BuLi, 98%; (b) 85 bar H2, Pd/C, 98%; (c) NaC10H8,
THF, −78 °C, 87%; (d) Boc2O, DMAP, Et3N, 91%; (e) recrystallization from n-hexane/Et2O; (f) 1. (CF3)2CHOH, MW,
120 °C, 40 min; 2. NaH, MeI, THF, 97%; (g) Li/NH3 (liq.), tBuOH, −78 °C, 77%.
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Gratifyingly, deprotection of the p-tosyl protecting group with sodium naphthalenide directly resulted in the
formation of the bicyclic hexahydro-6H-indol-6-one 11 in a yield of 87%.20 Vinylogous amide 11 was converted into the
N-Boc-protected derivative, recrystallized from n-hexane/Et2O to increase the enantiomeric excess to >99% and then
transformed into the N-methylation product 12.7a For the conjugate reduction of 12, lithium in liquid ammonia was used
and (+)-mesembrine was obtained in 77% yield.7a, 21 1H-NMR, 13C-NMR data7a and the optical rotation
(c = 0.37, MeOH) are in line with literature values.6, 7t
α
= +56
ꢀ ꢁꢃꢄ
ꢂ
CONCLUSION
In summary, we have demonstrated the applicability of asymmetric gold catalysis for the total synthesis of
(+)-mesembrine. We have shown that pyrrolidine natural products incorporating a quaternary stereocenter, a
challenging but also common structural motif, are accessible by an asymmetric 1,4-diynamide cycloisomerization. Using
this general method not only mesembrine, but also various related natural products like spirooxindole or
aspidospermine alkaloids could be synthesized.
EXPERIMENTAL SECTION
General
All reagents used were purchased or purified to reagent grade. Solvents were dried by common laboratory techniques.
Reactions were monitored with thin-layer chromatography. For reactions requiring an inert atmosphere the glassware
was dried at 120 °C and standard Schlenk techniques were employed. For column chromatography silica gel 60 or
aluminum oxide were used. Silica gel was deactivated by conditioning with the respective eluent and 1% (v/v)
triethylamine. For reactions under microwave irradiation the CEM Discover SP-D microwave was used with 100 W. The
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enantiomeric excess was determined using a HPLCsystem equipped with a DAICEL CHIRALPAK® IA™ or DAICEL
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CHIRALPAK® IC™ column. H- and 13C-NMR-spectra were recorded at 600 and 300 or 151 and 75 MHz, respectively.
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IR-spectra were measured as thin films on a NaCl single crystal. High resolution mass spectra (HRMS) were measured
with a Q-TOF spectrometer.
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3-Ethynyl-3-(3,4-dimethoxyphenyl)hex-1-ene-5-yne (5): Under N2 atmosphere, dry diisopropylamine (5.5 g, 54 mmol,
2.4 eq.) was dissolved in dry THF (135 mL, 0.4
added dropwise. The solution was stirred at –78 °C for 30 min, warmed up to rt and stirred for 30 min. A solution of
allylated product 14 (6.2 g, 23 mmol, 1 eq.) in THF (45 mL, 0.5 ), which was cooled in an ice bath, was transferred into
M). At –78 °C n-butyllithium (1.6 M in hexane, 31 mL, 49 mmol, 2.2 eq.) was
M
the LDA-solution via cannula. After stirring for 1 h at −78 °C, diethyl chlorophosphate (7.3 g, 42 mmol, 2.1 eq.) was added
via a nitrogen-flushed syringe. The solution was stirred at −78 °C for 2.5 h and then allowed to warm to rt. It was cooled
to −78 °C again and in another flask dry diisopropylamine (9.7 g, 96 mmol, 4.8 eq.) was dissolved in dry THF (430 mL,
0.25 M). At −78 °C n-butyllithium (1.6 M in hexane, 55 mL, 88 mmol, 4.4 eq.) was added dropwise. The solution was
stirred at –78 °C for 30 min, warmed up to rt and stirred for 30 min. It was cooled again to −78 °C and to this solution the
enol phosphate ester solution was added via cannula. The solution was stirred in the acetone/ dry ice bath and allowed
to warm to rt overnight. The reaction was quenched with a mixture of dist. water and brine (2:1) after 20 h reaction time.
The aqueous phase was extracted with diethyl ether and the organic phase was washed with brine. The organic phase
was dried over Na2SO4 and concentrated under reduced pressure. Purification by flash chromatography (eluent =
1
n-hexane:EtOAc = 5:1) gave a viscous oil (4.2 g, 17 mmol, 77%). Rf = 0.44 (n-hexane:EtOAc = 3:1). H-NMR (300 MHz,
CDCl3): δ [ppm] = 7.24 - 7.17 (m, 2H), 6.84 (d, J = 8.3 Hz, 1H), 5.86 (ddt, J = 17.4, 10.3, 7.1 Hz, 1H), 5.19 - 4.95 (m, 2H), 3.90
(s, 3H), 3.87 (s, 3H), 2.69 (dt, J = 7.1, 1.1 Hz, 2H), 2.53 (s, 2H). 13C-NMR (75 MHz, CDCl3): δ [ppm] = 148.8, 148.5, 133.1, 132.9,
119.1, 118.7, 110.9, 110.0, 84.3, 72.2, 56.0, 50.2, 39.8. IR (film), ꢅꢆ [cm-1]: 3289, 3078, 3003, 2955, 2936, 2912, 2836, 1516, 1264,
1236, 1144, 1028, 649. HRMS (ESI): m/z calculated for C16H17O2 [M+H]+: 241.1223; found: 241.1223.
3-(3,4-Dimethoxyphenyl)-3-ethynyl-pent-4-ynal (6): To a vigorously stirred solution of diol 15 (2.79 g, 10.2 mmol, 1 eq.) in
dry dichloromethane (100 mL, 0.1
) NaIO4 adsorbed on silica gel (21.6 g; 0.613 mmol⋅g-1, 13.2 mmol, 1.30 eq.) was added.
M
After complete conversion the silica gel was filtered off and the residue was washed with dichloromethane. The filtrate
was evaporated and the residue dried in vacuo. A yellowish, viscous product was obtained (2.39 g, 9.85 mmol, 97 %). No
1
further purification was necessary. Rf = 0.87 (n-hexane:EtOAc = 1:1). mp = 61-64 °C. H-NMR (300 MHz, CDCl3): δ [ppm]
= 9.85 (t, J = 2.6 Hz, 1H), 7.22 (dd, J = 8.3, 2.3 Hz, 1H), 7.18 (d, J = 2.3 Hz, 1H), 6.85 (d, J = 8.3 Hz, 1H), 3.89 (s, 3H), 3.87 (s,
3H), 2.89 (d, J = 2.7 Hz, 2H), 2.65 (s, 2H). 13C-NMR (75 MHz, CDCl3): δ [ppm] = 199.6, 149.2, 148.9, 131.7, 118.4, 111.2, 109.5,
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82.8, 73.7, 56.7, 56.1, 35.7. IR (film), ꢅꢆ [cm-1]: 3284, 3004, 2961, 2937, 2912, 2838, 2744, 1726, 1517, 1262, 1146, 1026, 666.
HRMS (ESI): m/z calculated for C15H15O3 [M+H]+: 243.1016; found: 243.1011.
6
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3-(3,4-Dimethoxyphenyl)-3-ethynyl-pent-4-yn-1-ol (7): Under nitrogen atmosphere aldehyde 6 (0.205 g, 0.845 mmol, 1 eq.)
8
9
was dissolved in dry methanol (8.5 mL, 0.1
M
) and cooled to 0 °C. NaBH4 (64.0 mg, 1.69 mmol, 2.00 eq.) was added. After
) was added
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complete conversion within 15 min the reaction was quenched with dist. water and hydrochloric acid (1
M
until pH = 2 was reached. The solution was stirred for 30 min and then neutralised with sodium hydroxide solution.
Brine was added and the aqueous phase was extracted with EtOAc. The organic phase was washed with brine and dried
over Na2SO4. The product was purified by column chromatography over silica gel (eluent = n-hexane:EtOAc:CH2Cl2 =
62:28:10). The alcohol 7 was obtained as a white solid (0.150 g, 0.614 mmol, 73%). Rf = 0.45 (n-hexane:EtOAc = 1:1).
1
mp = 78-79 °C. H-NMR (300 MHz, CDCl3): δ [ppm] = 7.29 – 7.16 (m, 2H), 6.86 (d, J = 8.4 Hz, 1H), 3.97 – 3.85 (m, 8H),
2.57 (s, 2H), 2.26 (t, J = 6.4 Hz, 2H), 1.77 (s, 1H). 13C-NMR (75 MHz, CDCl3): δ [ppm] = 149.0, 148.6, 133.2, 118.4, 111.0, 109.6,
84.3, 77.2, 72.5, 60.4, 56.0, 48.0, 37.8. IR (film), ꢅꢆ [cm-1]: 3499, 3393, 3287, 3005, 2960, 2935, 2837, 1516, 1261, 1145, 1027, 651.
HRMS (ESI): m/z calculated for C15H17O3 [M+H]+: 245.1172; found: 245.1171.
N-(4-Toluenesulfonyl)-(3-(3,4-dimethoxyphenyl)-3-ethynyl-pent-4-yn-1-amine (8): To
a
solution of protected
sulfonamide 17 (0.134 g, 0.270 mmol, 1 eq.) in dichloromethane (3 mL, 0.1 ) was added trifluoroacetic acid (0.56 mL,
M
7.3 mmol, 27 eq.). After stirring for 7 h at rt, a saturated NaHCO3 solution was added dropwise until the gas evolution
ceased and pH = 8 was reached. The aqueous phase was extracted with dichloromethane and the organic phase was
dried over Na2SO4. The drying agent was filtered off, the solution was concentrated and the residue dried in vacuo giving
a brownish solid (0.998 g, 0.251 mmol, 93%). No further purification was necessary. Rf = 0.54 (n-hexane:EtOAc = 1:1).
1
mp = 117-120 °C. H-NMR (300 MHz, CDCl3): δ [ppm] = 7.73 – 7.66 (m, 2H), 7.33 – 7.27 (m, 2H), 7.13 (dd, J = 8.3, 2.3 Hz,
1H), 7.09 (d, J = 2.2 Hz, 1H), 6.81 (d, J = 8.4 Hz, 1H), 4.69 (s, 1H), 3.87 (s, 6H), 3.23 (ddd, J = 8.1, 7.1, 6.2 Hz, 2H2.53 (s, 2H),
2.43 (s, 3H), 2.18 – 2.09 (m, 2H). 13C-NMR (75 MHz, CDCl3): δ [ppm] = 149.0, 148.7, 143.5, 137.0, 132.5, 129.8, 127.2, 118.4,
111.0, 109.5, 83.6, 72.9, 56.1, 56.0, 44.9, 40.4, 38.1, 21.6. IR (film), ꢅꢆ [cm-1]: 3285, 3003, 2960, 2937, 2838, 1597, 1515, 1260, 1160,
661. HRMS (ESI): m/z calculated for C22H24NO4S [M+H]+: 398.1421; found: 398.1419.
(S)-3-(3,4-Dimethoxyphenyl)-3-ethynyl-2-methylene-1-(4-toluene-sulfonyl)-pyrrolidine (9):
diynamide 8 (0.435 g, 1.10 mmol, 1 eq.) was dissolved in dry CHCl3 (5.5 mL, 0.2 ). At –55 °C, freshly prepared
(R)-TriPAu(PtBu3) (2) (56 mg, 0.049 mmol, 4.4 mol%) in dry CHCl3 (5.5 mL, 0.01
) was added and the solution stirred
Under
N2
atmosphere,
M
M
for 19 h at −55 °C. The reaction was quenched by adding triethylamine (50 µL, 0.36 mmol) and the solvent was
evaporated. Purification by flash chromatography on deactivated silica gel (n-hexane:EtOAc:CH2Cl2 = 65:11:24) gave a
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colorless solid (0.334 g, 0.840 mmol, 76%, 70% ee). The product was combined with material from earlier experiments
and recrystallized from n-hexane/CH2Cl2 (3:1) giving pyrrolidine 9 with an enantiomeric excess of 68% ee.
6
7
-1
-1
ꢀ ꢁ20
α
= +87 ml⋅g ⋅dm (c = 0.58; 68% ee, CHCl3). Enantiomeric excess was determined by chiral HPLC analysis
D
8
9
(DAICEL CHIRALPAK® IC™, 4.6 x 250 mm, eluent = n-hexane:iso-propanol = 55:45, 0.63 mL∙min-1, λ = 254 nm)
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1
Rt (major) = 25.8 min, Rt (minor) = 33.0 min). Rf = 0.50 (n-hexane:EtOAc = 6:4). mp = 143-145 °C. H-NMR (300 MHz,
CDCl3): δ [ppm] = 7.80 – 7.72 (m, 2H), 7.32 – 7.26 (m, 2H), 6.89 (d, J = 2.2 Hz, 1H), 6.82 (dd, J = 8.4, 2.2 Hz, 1H), 6.67 (d,
J = 8.4 Hz, 1H), 5.34 (d, J = 1.7 Hz, 1H), 4.32 (d, J = 1.7 Hz, 1H), 3.85 (s, 3H), 3.81 (s, 3H), 3.77 (dd, J = 7.1, 5.8 Hz, 2H), 2.43
(s, 3H), 2.27 (tt, J = 12.2, 5.8 Hz, 2H), 2.10 (s, 1H). 13C-NMR (151 MHz, CDCl3): δ [ppm] = 148.8, 148.8, 148.5, 144.1, 134.0,
132.1, 129.4, 127.9, 119.5, 110.7, 110.6, 94.0, 84.4, 77.2, 73.4, 56.0, 50.8, 48.2, 38.9, 21.7. IR (film), ꢅꢆ [cm-1]: 3283, 2956, 2935,
2837, 1646, 1518, 1259, 1167, 659. HRMS (ESI): m/z calculated for C22H24NO4S [M+H]+: 398.1421; found: 398.1423.
(R)-Methyl
3-(3-(3,4-dimethoxyphenyl)-2-methylene-1-(4-toluenesulfo-nyl)-pyrrolidin-3-yl)propanoate (10):
Ester 18
(0.121 g, 0.266 mmol) was dissolved in a mixture of THF:MeOH (1:1, 2.8 mL, 0.1
M
) and Pd/C (12.1 mg, 10 w%) was added
under inert conditions. The reaction was carried out in an autoclave at 85 bar hydrogen pressure for 18.5 h. Pd/C was
filtered off by filtration through a celite pad and the solvent was removed to obtain a viscous white product (0.12 g,
-1
-1
ꢀ ꢁ20
0.26 mmol, 98%). No further purification was necessary. Rf = 0.35 (n-hexane:EtOAc = 6:4).
α
= +17 ml⋅g ⋅dm
D
1
(c = 0.42; 67.9% ee, CHCl3). H-NMR (300 MHz, C6D6): δ = 7.52 – 7.46 (m, 2H), 6.62 – 6.55 (m, 2H), 6.37 (d, J = 2.1 Hz,
1H), 6.29 – 6.16 (m, 2H), 5.71 (d, J = 1.4 Hz, 1H), 4.36 (d, J = 1.4 Hz, 1H), 3.61 (ddd, J = 9.7, 7.6, 2.3 Hz, 1H), 3.37 (s, 3H), 3.25
(d, J = 13.4 Hz, 7H), 2.10 – 1.98 (m, 1H), 1.96 (s, 3H), 1.95 – 1.82 (m, 3H), 1.76 (ddd, J = 12.6, 5.8, 2.0 Hz, 1H), 1.27 (ddd,
J = 12.7, 10.1, 7.7 Hz, 1H). 13C-NMR (75 MHz, C6D6): δ [ppm] = 173.0, 150.3, 149.7, 148.9, 143.5, 135.0, 132.5, 127.6, 118.5, 111.2,
111.1, 92.1, 55.4, 55.1, 51.1, 48.0, 34.5, 32.0, 30.0, 21.2. IR (film), ꢅꢆ [cm-1]: 2953, 2926, 2854, 1735, 1519, 1342, 1166, 661.
HRMS (ESI): m/z calculated for C24H30NO6S [M+H]+: 460.1788; found: 460.1789.
(R)-3a-(3,4-Dimethoxyphenyl)-1,2,3,3a,4,5-hexahydro-6H-indol-6-one (11):7g Under inert conditions pyrrolidine 10 (0.167 g,
0.363 mmol, 1 eq.) was dissolved in dry THF (2 mL, 0.2
M). In another flask napthalene (0.335 g, 2.61 mmol, 7.19 eq.) was
dissolved in THF (3 mL, 0.9 ) and sodium (50 mg, 2.2 mmol, 6.1 eq.) was added. The educt solution was cooled to
M
−78 °C and the NaC10H8 solution was added dropwise until a slight green colouring persisted (1.6 mL NaC10H8 solution,
≈3 eq.). The reaction was quenched with sat. NaHCO3 solution and the aqueous phase was extracted with EtOAc. The
combined organic phases were washed with brine and the solvent was removed. Purification by
column chromatography (eluent = CH2Cl2:MeOH = 97:3) on neutral aluminum oxide (Brockmann III) yielded the
-1
-1
ꢀ ꢁ20
product as a white solid (86.4 mg, 0.316 mmol, 87%). Rf = 0.10 (CH2Cl2:MeOH = 20:1). α = +155 ml⋅g ⋅dm (c = 0.505;
D
1
67.9% ee, CHCl3). H-NMR (300 MHz, CDCl3): δ [ppm] = 7.07 (s, 1H, -NH-), 6.90 – 6.81 (m, 2H), 6.75 (d, J = 8.4 Hz, 1H),
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5.38 (s, 1H, =CH-CO-), 3.83 (s, 3H), 3.82 (s, 3H), 3.40 (ddd, J = 10.4, 8.1, 2.1 Hz, 1H, -NH-CH2-CH2-), 3.15 (tdd, J = 10.8, 5.9,
3.5 Hz, 1H, -NH-CH2-CH2-), 2.47 – 2.19 (m, 2H, -CO-CH2-CH2-), 2.19 – 1.79 (m, 4H, -NH-CH2-CH2- + -CO-CH2-CH2-).
13C-NMR (75 MHz, CDCl3): δ [ppm] = 197.1, 173.5, 148.9, 148.1, 133.3, 119.6, 111.0, 110.2, 94.8, 56.1, 55.9, 51.5, 45.1, 40.2, 36.0,
33.5. IR (film), ꢅꢆ [cm-1]: 3183, 2939, 2872, 1575, 1514, 1264, 1219, 1026, 753. HRMS (ESI): m/z calculated for C16H20NO3
[M+H]+: 274.1438; found: 274.1439.
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(R)-3a-(3,4-Dimethoxyphenyl)-1-methyl-1,2,3,3a,4,5-hexahydro-6H-indol-6-one (12):7a
Boc-protected
pyrrolidine 19
(43.4 mg, 0.116 mmol, 1 eq.) was dissolved in (CF3)2CHOH (1 mL) and heated to 115 °C for 40 min by microwave
irradiation. The solvent was removed on a rotary evaporator and the crude product was dissolved in dry THF (1.3 mL,
0.09 M). It was added at 0 °C to a suspension of sodium hydride (60% in mineral oil, 5.6 mg, 0.14 mmol, 1.2 eq.) in THF
(1 mL). After 15 min the cooling bath was removed and methyl iodide (9.0 µL, 0.14 mmol, 1.2 eq.) was added via syringe.
Due to an incomplete conversion methyl iodide (8.o µL, 0.13 mmol, 1.1 eq.) and sodium hydride (60w% in mineral oil,
5.6 mg, 0.14 mmol, 1.2 eq.) were added. After quenching by addition of a mixture of dist. water and brine, the aqueous
phase was extracted with EtOAc. Combined organic phases were washed with brine and dried over Na2SO4. Purification
by column chromatography on deactivated silica gel (eluent = CH2Cl2:MeOH = 95:5) yielded the product as a slightly
1
yellowish material (32.3 mg, 0.112 mmol, 97%). Rf = 0.46 (CH2Cl2:MeOH = 10:1). H-NMR (300 MHz, CDCl3): δ [ppm] =
6.81 – 6.72 (m, 3H), 5.19 (s, 1H), 3.85 (d, J = 1.1 Hz, 6H), 3.33 – 3.23 (m, 2H), 2.96 (s, 3H), 2.44 – 2.34 (m, 1H), 2.31 – 2.22 (m,
1H), 2.20 – 2.01 (m, 3H), 2.01 – 1.82 (m, 1H).
3-(3,4-Dimethoxyphenyl)pentane-2,4-dione (13):11b,13 Under N2 atmosphere, copper(I) iodide (2.00 g, 10.5 mmol, 0.100 eq.),
D/L-proline (2.42 g, 21.0 mmol, 0.200 eq.) and potassium carbonate (58.0 g, 420 mmol, 4.00 eq.) were suspended in dry
DMSO (420 mL, 0.25 m). After addition of 4-bromo-1,2-dimethoxybenzene (22.8 g, 105 mmol, 1 eq.) and acetyl acetone
(31.5 g, 200 mmol, 3.00 eq.), the mixture was stirred for 41 h at 90 °C. The dark green solution was cooled to 0 °C and
slowly poured into HCl (1 M) while vigorously stirring. The layers were separated and the aqueous layer extracted with
EtOAc. The combined organic phases were washed with brine, dried over MgSO4 and concentrated in vacuo. The
residue was purified by flash chromatography (eluent = n-hexane:EtOAc = 5:1). Recrystallization from n-hexane yielded
white crystals (10.8 g, 45.8 mmol, 43%). Rf = 0.27 (n-hexane:EtOAc = 5:1). 1H-NMR (300 MHz, CDCl3): δ [ppm] = 6.86 (d,
J = 8.1 Hz, 1H), 6.70 (dd, J = 8.1 Hz, 2.0 Hz, 1H), 6.66 (d, J = 2.0 Hz, 1H), 3.89 (s, 3H), 3.86 (s, 3H), 1.89 (s, 6H). 13C-NMR
(151 MHz, CDCl3): δ [ppm] = 191.2, 149.1, 148.5, 129.5, 123.5, 115.0, 114.1, 111.4, 77.2, 77.2, 56.0, 55.9, 24.2. IR (film), ꢅꢆ [cm-1]:
3006, 2952, 2929, 2831, 1587, 1521, 1454, 1252, 1137, 1021. HRMS (ESI): m/z calculated for C13H17O4 [M+H]+: 237.1121; found:
237.1120.
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3-Allyl-3-(3,4-dimethoxyphenyl)pentane-2,4-dione (14): Under N2 atmosphere, dione 13 (13.0 g, 55.0 mmol, 1 eq.) was
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dissolved in dry DMF (220 mL, 0.25 M). The solution was cooled to 0 °C and sodium hydride (60% in mineral oil, 2.3 g,
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58 mmol, 1.1 eq.) was added in portions. After stirring for 1 h at 0 °C and 1 h at rt the solution was cooled to 0 °C.
Allyl iodide (5.5 mL, 61 mmol, 1.1 eq.) was added via syringe, the solution was allowed to warm to rt and it was stirred for
3 h. Cold water was added and the aqueous phase was extracted with EtOAc. The combined organic phases were washed
with brine, dried over Na2SO4 and concentrated in vacuo. Purification by flash chromatography (eluent =
n-hexane:EtOAc = 4:1) gave a white solid (11.3 g, 40.7 mmol, 74%). Rf = 0.24 (n-hexane:EtOAc = 4:1). mp = 95-96 °C.
1H-NMR (300 MHz, CDCl3): δ [ppm] = 6.86 (d, J = 8.4 Hz, 1H), 6.79 (dd, J = 8.4, 2.2 Hz, 1H), 6.72 (d, J = 2.2 Hz, 1H), 5.69
(ddt, J = 17.2, 10.2, 7.0 Hz, 1H), 5.16-5.02 (m, 2H), 3.87 (s, 3H), 3.84 (s, 3H), 3.02 (dt, J = 7.0, 1.4 Hz, 2H), 2.10 (s, 6H).
13C-NMR (75 MHz, CDCl3): δ [ppm] = 206.1, 149.2, 148.9, 133.7, 129.2, 120.7, 118.6, 111.4, 111.2, 74.2, 56.1, 56.0, 37.7, 28.0.
IR (film), ꢅꢆ [cm-1]: 3379, 3082, 3006, 2968, 2937, 2838, 1705, 1519, 1150, 1024, 928, 766. HRMS (ESI): m/z calculated for
C16H21O4 [M+H]+: 277.1435; found: 277.1434.
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3-Ethynyl-3-(3,4-dimethoxyphenyl)-hex-5-yn-1,2-diol (15): Diyne 5 (2.95 g, 12.3 mmol, 1 eq.) was dissolved in a mixture of
tert-butanol and water (24.5 mL, 1:1, 0.5
potassium osmate dihydrate (67.8 mg, 0.184 mmol, 1.49 mol%) were added in portions, resulting in a brown solution.
After 66 h tert-butanol was evaporated under reduced pressure and hydrochloric acid (1 ) was added until pH = 1 was
M). N-Methylmorpholine N-oxide (3.02 g, 25.8 mmol, 2.10 eq.) as well as
M
reached. The aqueous phase was extracted with EtOAc. The organic phase was washed with brine and dried over
Na2SO4. The crude product was purified via column chromatography over silica (eluent = n-hexane:EtOAc = 1:3),
1
yielding a yellowish substance (2.79 g, 10.2 mmol, 83%). Rf = 0.12 (n-hexane:EtOAc = 1:1). H-NMR (300 MHz, CDCl3):
δ [ppm] = 7.42 (dd, J = 8.7, 2.6 Hz, 1H), 7.19 (d, J = 2.3 Hz, 1H), 6.85 (d, J = 8.4 Hz, 1H), 4.17 (s, 1H), 3.90 (s, 3H), 3.87 (s,
3H), 3.66 – 3.41 (m, 2H), 2.75 (d, J = 2.9 Hz, 1H), 2.63 (s, 1H), (s, 1H), 2.21 (dd, J = 14.2, 7.9 Hz, 1H), 2.04 (dd, J = 14.2,
2.8 Hz, 1H). 13C-NMR (75 MHz, CDCl3): δ [ppm] = 149.1, 148.8, 133.1, 118.4, 111.1, 109.6, 84.7, 84.0, 73.1, 73.0, 70.2, 66.7, 56.1,
49.4, 38.1. IR (film), ꢅꢆ [cm-1]: 3398, 3286, 3003, 2935, 2838, 1593, 1516, 1261, 1145, 1026, 648. HRMS (ESI): m/z calculated for
C16H19O4 [M+H]+: 275.1278; found: 275.1278.
tert-Butyl-4-toluenesulfonyl-carbamate (16):22 Under nitrogen atmosphere 4-methylbenzenesulfonamide (15.8 g,
92.2 mmol, 1 eq.) and N,N-dimethyl-4-aminopyridine (0.563 g, 4.61 mmol, 5.00 mol%) were dissolved in dry
dichloromethane (190 mL, 0.5
(21.7 mL, 101 mmol, 1.10 eq.) were added. After 30 min the solution was concentrated and the crude product was
dissolved in EtOAc. It was washed with hydrochlorid acid (1 ), dist. water as well as brine. The organic phase was dried
M). Dry triethylamine (14.1 mL, 101 mmol, 1.10 eq.) as well as di-tert-butyl dicarbonate
M
over Na2SO4 and concentrated under reduced pressure to give a white powder. The product was recrystallized from a
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mixture of diethyl ether and n-hexane (1:1). The crystals were washed with ice cold n-hexane and vacuum dried to obtain
white crystals (21.6 g, 79.6 mmol, 86%). mp = 119 °C (lit.22 mp = 117-119 °C).1H-NMR (300 MHz, CDCl3): δ [ppm] = 7.93 –
7.86 (m, 2H), 7.45 (s, 1H), 7.38 – 7.31 (m, 2H), 2.44 (s, 3H), 1.38 (s, 9H).
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tert-Butyl (3-(3,4-dimethoxyphenyl)-3-ethynyl-pent-4-yn-1-yl) (4-toluene-sulfonyl)carbamate (17): Under N2 atmosphere,
alcohol 7 (1.51 g, 6.17 mmol, 1 eq.), triphenylphosphine (2.43 g, 9.26 mmol, 1.50 eq.) and tert-butyl tosylcarbamate (16)
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(1.84 g, 6.79 mmol, 1.10 eq.) were dissolved in dry THF (62 mL, 0.1 M). To this solution, di-ethyl azodicarboxylate (4.2 mL,
9.3 mmol, 1.5 eq., 40 w% in toluene) was added dropwise. The solution was cooled with a water bath (15 °C). After
stirring for 3 h, the solution was concentrated in vacuo. Purification by flash column chromatography over silica gel
(n-hexane:EtOAc = 7:2) gave a white solid (2.87 g, 5.77 mmol, 94%). Rf = 0.77 (n-hexane:EtOAc = 1:1). mp = 129-132 °C.
1H-NMR (300 MHz, CDCl3): δ [ppm] = 7.78 – 7.72 (m, 2H), 7.33 – 7.23 (m, 3H), 7.24 (d, J = 2.2 Hz, 1H), 6.87 (d, J = 8.3 Hz,
1H), 4.12 – 4.02 (m, 2H), 3.93 (s, 3H), 3.88 (s, 3H), 2.59 (s, 2H), 2.47 – 2.38 (m, 5H), 1.33 (s, 9H). 13C-NMR (75 MHz, CDCl3):
δ [ppm] = 150.8, 149.0, 148.7, 144.3, 137.3, 132.7, 129.4, 127.9, 118.6, 111.0, 109.7, 84.4, 83.4, 72.6, 56.1, 44.5, 44.4, 37.8, 28.0,
21.7. IR (film), ꢅꢆ [cm-1]: 3286, 2979, 2936, 2837, 2256, 1731, 1516, 1354, 1153, 734. HRMS (ESI): m/z calculated for
C27H35N2O6S [M+NH4]+: 515.2210; found: 515.2211, C27H31NNaO6S [M+Na]+: 520.1764; found: 520.1763.
(R)-Methyl 3-(3-(3,4-dimethoxyphenyl)-2-methylene-1-(4-toluene-sulfonyl)-pyrrolidin-3-yl)propiolate (18): Under N2
atmosphere, alkyne 9 (0.228 g, 0.574 mmol, 1 eq.) was dissolved in dry THF (5.7 mL). At –80 °C, n-butyllithium solution
(1.6
M in hexane, 0.39 mL, 0.63 mmol, 1.1 eq.) was added dropwise and stirred for 20 min. Next, methyl chloroformate
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(88 µL, 1.2 mmol, 2.1 eq.) was added and the solution was allowed to warm to rt. After 30 min, a mixture of water, sat.
NaHCO3 and brine (4:1:2, 28 mL) was added, EtOAc was added and the layers were separated. The aqueous phase was
extracted with EtOAc. The combined organic phases were washed with brine (15 mL) and dried over Na2SO4.
Purification by flash chromatography on deactivated silica gel (n-hexane:EtOAc:CH2Cl2 = 65:10:25) gave a white solid
-1
-1
ꢀ ꢁ20
(0.255 g, 0.560 mmol, 98%). Rf = 0.29 (n-hexane:EtOAc: CH2Cl2 = 6:1:3). mp = 115-120 °C. α = +54 ml⋅g ⋅dm (c = 0.49;
D
1
67.9% ee, CHCl3). H-NMR (300 MHz, C6D6): δ [ppm] = 7.81 – 7.73 (m, 2H), 6.83 – 6.75 (m, 4H), 6.34 (d, J = 9.0 Hz, 1H),
5.75 (d, J = 1.6 Hz, 1H), 4.52 (d, J = 1.6 Hz, 1H), 3.53 (ddd, J = 6.8, 5.9, 2.1 Hz, 2H), 3.34 (s, 3H), 3.34 (s, 3H), 3.21 (s, 3H), 1.95
(s, 3H), 1.92 – 1.74 (m, 2H). 13C-NMR (75 MHz, C6D6): δ [ppm] = 153.5, 150.1, 149.9, 148.0, 144.0, 134.6, 131.0, 129.6, 127.9,
119.8, 111.7, 111.4, 94.7, 88.0, 77.3, 55.6, 55.5, 52.1, 51.1, 48.3, 37.9, 21.3. IR (film), ꢅꢆ [cm-1]: 3004, 2955, 2838, 2235, 1715, 1518,
1266, 1167, 751, 658. HRMS (ESI): m/z calculated for C24H26NO6S [M+H]+: 456.1475; found: 456.1479.
(R)-tert-Butyl
3a-(3,4-dimethoxyphenyl)-6-oxo-2,3,3a,4,5,6-hexahydro-1H-indole-1-carboxylate (19):7a
Pyrrolidine 11
(0.135 g, 0.494 mmol, 1 eq., 68% ee) was dissolved in dry CH2Cl2 (6 mL, 0.1
M
). N,N-Dimethylaminopyridine (8.4 mg,
0.069 mmol, 14 mol%), triethylamine (95 µL, 0.68 mmol, 1.4 eq.) and di-tert-butyl dicarbonate (0.149 g, 146 µL,
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0.684 mmol, 1.38 eq.) were added. After 20 min a mixture of dist. water and brine was added and the aqueous phase was
extracted with CH2Cl2. The organic phase was washed with a mixture of dist. water and brine (1:3) and the solvent was
removed. Purification by flash column chromatography (eluent = n-hexane:EtOAc:CH2Cl2 = 43:42:15) on deactivated
silica gel yielded the product as a white substance (0.168 g, 0.449 mmol, 91%). The product was recrystallized from
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n-hexane and a mixture of n-hexane:Et2O (2:1) in two batches, giving crystals (78.9 mg) with an ee >99%. H-NMR
(300 MHz, CDCl3): δ [ppm] = 6.84 – 6.76 (m, 3H), 6.55 (s, 1H), 3.86 (s, 3H), 3.84 (s, 3H), 3.78 (dd, J = 11.0, 8.1 Hz, 1H), 3.24
(td, J = 11.4, 5.5 Hz, 1H), 2.46 – 2.35 (m, 1H), 2.31 (dd, J = 12.0, 5.4 Hz, 1H), 2.26 – 2.14 (m, 2H), 2.13 – 1.92 (m, 2H), 1.54 (s,
9H). The enantiomeric excess was determined by chiral HPLC analysis (DAICEL CHIRALPAK® IA™, 4.6 x 250 mm,
eluent = n-hexane:iso-propanol = 82:18, 0.82 mL∙min-1, λ = 254 nm) Rt (major) = 13.8 min, Rt (minor) = 10.8 min.
(+)-Mesembrine:7a, 21 Enone 12 (38.6 mg, 0.134 mmol, 1 eq.) and dry tert-butanol (24 mg, 0.32 mmol, 2.8 eq.) were
dissolved in dry THF (2 mL, 0.06 M). The solution was cooled to −78 °C and liquid ammonia (≈40 mL) was condensed
into the flask. A piece of lithium (1.9 mg, 0.27 mmol, 2.0 eq.) was added and the solution was stirred for 45 min.
Ammonia was evaporated and the solution was diluted with a mixture of dist. water and brine. The aqueous phase was
extracted with EtOAc and the combined organic phases were washed with brine and dried over Na2SO4. Purification by
column chromatography (eluent = CH2Cl2:acetone = 8:2) on neutral aluminum oxide (Brockmann III) yielded the
ꢀ ꢁꢃꢄ
product as a slightly yellowish substance (30.0 mg, 0.104 mmol, 77%). Rf = 0.25 (CH2Cl2:acetone = 6:2.5). α = +56
D
6
1
(c = 0.37, MeOH); (lit.7t
α
D
ꢀ ꢁꢃꢄ
ꢀ ꢁ
= +50.0, c = 0.53, MeOH; lit. α = +58.5, c = 0.04, MeOH). H-NMR (300 MHz, CDCl3):
δ [ppm] = 6.93 (dd, J = 8.3, 2.3 Hz, 1H), 6.89 (d, J = 2.2 Hz, 1H), 6.84 (d, J = 8.3 Hz, 1H), 3.90 (s, 3H), 3.88 (s, 3H), 3.14 (ddd,
J = 9.3, 7.5, 3.1 Hz, 1H), 2.94 (t, J = 3.6 Hz, 1H), 2.60 (d, J = 3.7 Hz, 2H), 2.47 – 2.03 (m, 10H). 13C-NMR (75 MHz, CDCl3):
δ [ppm] = 211.5, 149.1, 147.6, 140.3, 118.0, 111.1, 110.1, 77.2, 70.5, 56.1, 56.0, 55.0, 47.6, 40.7, 40.2, 39.0, 36.4, 35.4.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
Experimental details, spectral data for all new compounds and HPLC traces for compounds 9 and 19 (PDF).
AUTHOR INFORMATION
Corresponding Author
*E-Mail: constantin.czekelius@hhu.de.
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENT
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Financial support of the German Research Council (DFG) by an Emmy-Noether grant (CZ 183/1) is most gratefully
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acknowledged. We thank Maria Baumann for initial experiments addressing the preparation of diynesulfonamide 8.
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