A tosyliminium ion-based total synthesis of (±)-schefferine

Article abstract of DOI:10.1139/v00-120

The synthesis of the phenolic tetrahydroprotoberberine alkaloid (±)-schefferine is reported, featuring as key steps a tosyliminium ion-mediated Friedel-Crafts alkylation of eugenol dimethyl ether and an intramolecular Mitsunobu-type amination.

Full text of DOI:10.1139/v00-120

1165
A tosyliminium ion-based total synthesis of
(±)-schefferine
Darío A. Bianchi and Teodoro S. Kaufman
Abstract: The synthesis of the phenolic tetrahydroprotoberberine alkaloid (±)-schefferine is reported, featuring as key
steps a tosyliminium ion-mediated Friedel–Crafts alkylation of eugenol dimethyl ether and an intramolecular
Mitsunobu-type amination.
Key words: total synthesis, (±)-schefferine, natural product, tosyliminium ion, Mitsunobu amination.
Résumé : On a réalisé la synthèse de la (±)-schefférine, un alcaloïde phénolique dérivé de la tétrahyroprotoberbérine.
Les étapes clés de cette synthèse sont une alkylation de Friedel–Crafts l’éther diméthylique de l’eugénol assistée par
l’ion tosyliminium et un amination intramoléculaire du type Mitsunobu.
Mots clés : synthèse totale, (±)-schefférine, produit naturel, ion tosyliminium, amination de Mitsunobu.
[Traduit par la Rédaction] Bianchi and Kaufman 1169
In 1972, Gellert and Rudzats (1) reported the isolation of
schefferine from the bark of Schefferomitra subaequalis
Diels (Anonaceae), a New Guinea liana found as a climber
on rain forest trees. The structure of this phenolic tetrahy-
droprotoberberine alkaloid, also known as tetrahydropalma-
1
trubine (2), was formulated as 1 on the basis of its H NMR
and mass spectral data.
Several syntheses of (±)-schefferine have been previously
reported, including reduction of protoberberines as well as
Mannich cyclization and photolysis of 1-substituted
tetrahydroisoquinolines. Späth and Burger (3) prepared (±)-1
long before its isolation from a natural source by reduction
of palmatrubine, a monodemethylation product of the widely
known alkaloid palmatine. A similar approach, involving the
synthesis and reduction of a palmatrubine derivative, was
followed by Dutta and Bradsher (2b).
More recently, Kametani and co-workers informed that
the Mannich cyclization of a conveniently substituted 1-ben-
zyltetrahydroisoquinoline under careful pH control, fur-
nished moderate yields of (±)-schefferine (4a). The same
group also reported the synthesis of this natural product em-
ploying the Mannich reaction of a brominated 1-benzyl
tetrahydroisoquinoline, produced by a Bischler–Napieralski
reaction (4b). Noteworthy, they were unable to obtain (±)–1
by phenolic cyclization of a related substrate (4c).
In addition, (±)-schefferine was isolated as a minor prod-
uct resulting from the Mannich reaction of a 1-benzyltetra-
hydroisoquinoline with formaldehyde generated in situ
during the photolysis of the starting heterocycle (5). Interest-
ingly, better yields of (±)–1 were realized when photolysis
of bromoenamides was employed as synthetic strategy (6).
Tracer experiments on Stephania glabra Miers (Menis-
permaceae) evidenced high incorporation of (±)-[aryl-3H]-
schefferine into corydalmine but not into stepholidine, indi-
cating that 1 might be a possible intermediate of the bio-
synthesis of corydalmine from reticuline (7a). Likewise,
schefferine has been postulated as an intermediate in the
biosynthesis of sinactine from reticuline (2a) and as a pre-
cursor of tetrahydropalmatine (7b).
We have reported a convenient entry to 3-substituted tetra-
hydroisoquinolines featuring the reaction of silicon-based
nucleophiles with tosyliminium ions, generated upon addi-
tion of Lewis acids to tosylamidals (8). By capturing the
tosyliminium ions with electron rich aromatics, such as phe-
nols and their ethers, we recently extended the scope of this
transformation to the elaboration of the very useful 3-aryl
tetrahydroisoquinolines (9).
Received May 1, 2000. Published on the NRC Research Press
website on August 18, 2000.
D.A. Bianchi and T.S. Kaufman.1 Instituto de Química
Orgánica de Síntesis (CONICET-UNR) and Facultad de
Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional
de Rosario, Suipacha 531, (2000) Rosario, Argentina.
1Author to whom correspondence should be addressed.
Telephone: 54-341-4804600. Fax: 54-341-4370477.
e-mail: tkaufman@agatha.unr.edu.ar
Herein, we disclose the details of a new and efficient total
synthesis of (±)-schefferine via a new approach, consisting
in the tosyliminium ion mediated preparation of a 3-aryl
Can. J. Chem. 78: 1165–1169 (2000)
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Can. J. Chem. Vol. 78, 2000
Scheme 1. Reagents and conditions: (i) 4-allyl-1,2-dimethoxybenzene (3), BF3·Et2O, CH2Cl2, –78°C, 15 min, –30°C, 30 min; (ii) OsO4
(cat.), NaIO4, THF–H2O (3:1); (iii) OsO4 (cat.), NMO, Me2CO–H2O–tBuOH (4:2:1); (iv) NaIO4, THF–H2O (3:1); (v) NaBH4, MeOH–
Et2O (4:1); (vi) (1) Na, NH3, –33°C; (2) NH4Cl, –33°C→ rt; and (vii) PPh3, DEAD, HBF4, THF, reflux.
tetrahydroisoquinoline intermediate and a Mitsunobu-type
intramolecular amination (10).
tion of undesirable over-reduced side-products and furnish
optimum yields of tetrahydroisoquinolines (13), reaction of
7 with sodium in liquid ammonia was found to cleanly effect
simultaneous cleavage of both protective groups (14), pro-
viding amino alcohol 8 in 83% yield.
Although syntheses of alkaloids have made extensive use
of acyliminium ion chemistry (11), the reactivity of the re-
lated tosyliminium ions has been comparatively less explored.
The known amidal 2 (8) was treated at –78°C with the
commercially available 4-allyl-1,2-dimethoxybenzene (3) and
BF3·Et2O to give the 3-aryl tetrahydroisoquinoline derivative
4 in 88% yield. β-Phenethyl alcohol 7 was then synthesized
from 4 in two steps. First, sodium periodate-assisted oxida-
tive olefin cleavage with osmium tetraoxide as catalyst in
THF–water (12) gave aldehyde 5 in 77% yield; then, reduc-
tion of aldehyde 5 with sodium borohydride in dry methanol
furnished alcohol 7 in 92% yield.
Finally, intramolecular Mitsunobu amination of 8 with the
DEAD–PPh3 couple in dry THF containing 1 equiv of HBF4
gave 82% of (±)-schefferine [(±)–1], exhibiting a melting
1
point and H NMR spectrum identical with reported data
(4b, 5b). The use of HBF4 as additive was key for the suc-
cess of this transformation to cyclized product in good
yields. Protonation of the DEAD-derived hydrazide anion in-
termediate, is presumed to avoid its involvement as a com-
peting nucleophile in the amination process (10b).
Because phenylacetaldehyde 5 tended to overoxidize,
probably through its enol form, the dihydroxylation and
oxidative fission steps were carried out separately, employ-
ing NMO as cooxidant during the dihydroxylation step,
which resulted in an increased overall yield of 89%.
Debenzylation, desulfonylation, and formation of ring B
were required to complete the synthesis. Our previous work
suggested converting 7 into 8 by a stepwise process, consist-
ing of catalytic debenzylation to yield phenol 9, followed by
sodium-mediated cleavage of the sulfonamide moiety. Al-
though this approach allowed a better control of the delivery
of the alkaline metal to the reaction medium to avoid forma-
Dean and Rapoport (15) reported the syntheses of 8- and
13-methylberbines 10 and 11 by intramolecular cyclizations
involving iminium ions formed by α-aminoacid decarbonyl-
ation. More recently, Padwa and Waterson (16) described a
protoberberine synthesis which furnished 12, employing a
thionium–N-acyliminium ion cyclization of amido sulfoxides
for the assembly of rings B and C. Unlike the above pre-
sented sequence leading to (±)-schefferine, these iminium
ion-based synthetic strategies involved construction of ring
B by formation of the C6—N bond prior to the elaboration
of the C14—C14a bond between rings A and C. The scope
and limitations of this alternative protocol are currently
© 2000 NRC Canada

Bianchi and Kaufman
1167
1600, 1500, 1440, 1350, 1280, 1160, 1070, 910, 820, 730,
and 660. 1H NMR δ: 2.41 (s, 3H), 2.79 (dd, J = 9.0,
11.8 Hz, 1H), 3.36 (brd, 2H), 3.60 (s, 3H), 3.68 (dd, J = 5.4,
11.8 Hz, 1H), 3.85 (s, 3H), 3.87 (s, 3H), 3.89 (d, J =
15.9 Hz, 1H), 4.38 (dd, J = 5.4, 9.0 Hz, 1H), 4.55 (d, J =
15.9 Hz, 1H), 4.93–5.10 (m, 2H), 5.03 (d, J = 11.1 Hz, 1H),
5.12 (d, J = 11.1 Hz, 1H), 5.85–6.06 (m, 1H), 6.23 (s, 1H),
6.48 (d, J = 8.7 Hz, 1H), 6.68 (s, 1H), 6.71 (d, J = 8.7 Hz,
1H), 7.25 (d, J = 8.2 Hz, 2H), 7.30–7.50 (m, 5H) and 7.57
(d, J = 8.2 Hz, 2H). 13C NMR δ: 21.37, 36.91, 39.77, 44.31,
49.89, 55.69,* 55.81, 74.22, 111.20, 112.42, 112.71, 115.79,
124.39, 126.77, 127.49,* 127.97,# 128.25,* 129.47,* 130.11,
130.22, 132.34, 133.57, 137.32, 137.42, 143.28,* 147.24,
147.62, and 150.25. HREIMS calcd. for C35H37NO6S:
599.2342; found: 599.2347.
being evaluated for the synthesis of protoberberine-type nat-
ural products.
Experimental section
Melting points were measured on an Ernst Leitz hot-stage
microscope apparatus and are uncorrected. IR spectra were
taken on a Beckman Acculab 8 spectrophotometer with solid
1
samples as KBr pellets and liquid samples as films. H and
13C NMR spectra were recorded in CDCl3, on a Bruker AC-
200E instrument at 200.13 and 50.33 MHz respectively, with
TMS as the internal standard. 13C NMR resonances corre-
sponding to two carbon atoms are designated with an aster-
isk while those assigned to three carbons are marked with
“#.” High resolution mass spectra were obtained from
UMYMFOR (FCEN, Buenos Aires).
2-{2-[8-Benzyloxy-7-methoxy-2-(toluene-4-sulfonyl)-
1,2,3,4-tetrahydroisoquinolin-3-yl]-4,5-
4-Allyl-1,2-dimethoxybenzene was purchased from
Aldrich Chemical Co. and used without further purification.
All reactions were carried out in a dry oxygen-free nitrogen
atmosphere, monitored by thin layer chromatography on
Merck’s precoated silica gel 60 F254 TLC plates developed
in hexane–EtOAc (7:3) or CHCl3–EtOH (8:2) and detected
by examination under UV light followed by spraying with
2% p-anisaldehyde–sulfuric acid reagent in EtOH or 0.2%
ninhydrin in EtOH. Careful heating improved the sensitivity
of the detection. All new compounds gave a single spot by
TLC. Flash chromatography was carried out on Merck
Kieselgel 60 (0.04–0.063 mm), packed in hexane; elution
was with mixtures of hexane–EtOAc, using gradient tech-
niques. Compounds were preadsorbed from Et2O or CH2Cl2
solutions onto the adsorbent before column chromatography.
dimethoxyphenyl}-ethanol (7)
A 2% w/v solution of OsO4 in tert-butanol (0.051 mL)
was added to a mixture of allyl tetrahydroisoquinoline 4
(125 mg, 0.21 mmol) and potassium periodate (144 mg,
0.63 mmol) in 3:1 THF–water (4 mL). The mixture was
stirred overnight at room temperature and quenched with
10% sodium hydrogensulfite (5 mL); Celite (1 g) was added
and stirring continued for an additional 1 h, then the suspen-
sion was filtered under reduced pressure through Celite con-
tained in a Büchner funnel and the crude reaction product
was exhaustively extracted with EtOAc (5 × 20 mL). The or-
ganic portion was dried (Na2SO4), concentrated, and
chromatographed to furnish aldehyde 5 (96 mg, 77%) as an
oil. IR (neat) (cm–1): 2960, 2870, 1750, 1630, 1510, 1480,
1380, 1300, 1190, 1090, 970, 840, 760, and 680. 1H NMR δ:
2.42 (s, 3H), 2.83 (dd, J = 8.5, 11.9 Hz, 1H), 3.55–375 (m,
3H), 3.60 (s, 3H), 3.85 (s, 3H), 3.86 (s, 3H), 3.93 (d, J =
16.5 Hz, 1H), 4.22 (dd, J = 5.0, 8.5 Hz, 1H), 4.47 (d, J =
16.5 Hz, 1H), 5.03 (d, J = 11.2 Hz, 1H), 5.13 (d, J =
11.2 Hz, 1H), 6.34 (s, 1H), 6.52 (d, J = 8.6 Hz, 1H), 6.63 (s,
1H), 6.72 (d, J = 8.6 Hz, 1H), 7.25 (d, J = 8.3 Hz, 2H),
7.35–7.52 (m, 5H), 7.58 (d, J = 8.3 Hz, 2H) and 9,70 (s,
1H). 13C NMR δ: 21.32, 40.46, 44.22, 47.64, 49.72, 55.62,
55.72,* 74.16, 111.28, 112.80, 113.41, 122.17, 124.36,
126.61, 127.38,* 127.96, 128.24 (4 carbons), 129.33,
129.49,* 133.41, 133.56, 137.28, 143.18, 143.39, 147.86,
148.19, 150.39, and 198.74. HREIMS calcd. for
C34H35NO7S: 601.2134; found: 601.2132.
3-(2-Allyl-4,5-dimethoxyphenyl)-8-benzyloxy-7-methoxy-
2-(toluene-4-sulfonyl)-1,2,3,4-tetrahydroisoquinoline (4)
A
CH2Cl2 solution of 4-allyl-1,2-dimethoxybenzene
(0.88 mL, 0.49 mmol) was added to amidal 2 (110 mg,
0.243 mmol) in anhydrous CH2Cl2 (3 mL) and the resulting
solution was cooled to –78°C. Then, a solution of BF3·Et2O
in CH2Cl2 (0.51 mL, 0.29 mmol) was added dropwise.
Stirring continued for 15 min at –78°C and 30 min at –30°C.
Then the reaction mixture was rapidly poured over brine
(10 mL) and the products were extracted with EtOAc (4 ×
25 mL). Drying (Na2SO4), concentration, and chromatogra-
phy of the combined ethereal extracts afforded the 3-aryl-
tetrahydroisoquinoline derivative 4 (128 mg, 88%) as a
colourless oil which crystallized on standing, mp 120–
121.5°C (hexane–EtOAc). IR (neat) (cm–1): 2920, 2850,
Without further purification, aldehyde
0.11 mmol) was dissolved in a 4:1 MeOH–Et2O mixture
(10 mL) kept at 0°C in an ice water bath. Sodium
5 (66 mg,
© 2000 NRC Canada

1168
Can. J. Chem. Vol. 78, 2000
borohydride (10 mg, 0.26 mmol) was added, stirring contin-
ued during 15 min and the reaction was quenched with 10%
w/v citric acid solution (10 mL). The organic solvent was
evaporated under reduced pressure and the reaction product
was extracted with EtOAc (4 × 15 mL). The organic extract
was dried (Na2SO4), concentrated under reduced pressure,
and the glassy residue was chromatographed, yielding alco-
hol 7 (61 mg, 92%), as a solid, mp 148–149.5°C. IR (KBr)
(cm–1): 3520, 2960, 2860, 1620, 1530, 1480, 1370, 1310,
was slowly evaporated and the reaction products were mixed
with silica gel. The solvent was removed under reduced
pressure and the adsorbed reaction products were
chromatographed (CH2Cl2–EtOH), furnishing 8 (25 mg,
83%) as an oil. IR (neat) (cm–1): 3550–2500, 2960, 2880,
1
1630, 1500, 1470, 1280, 1060, 920, and 740. H NMR δ:
2.91 (dd, J = 8.8, 12.3 Hz, 1H), 2.94 (brd, 2H), 3.31 (dd, J =
5.2, 12.3 Hz, 1H), 3.64 (s, 3H), 3.75–3.92 (m, 1H), 3.80 (s,
3H), 3.86 (s, 3H), 4.00–4.30 (m, 6H), 4.35 (dd, J = 5.2,
8.8 Hz, 1H), 6.25 (d, J = 8.5 Hz, 1H), 6.34 (s, 1H), 6.59 (d,
J = 8.5 Hz, 1H) and 6.72 (s, 1H). 13C NMR δ: 35.99, 39.31,
43.09, 50.76, 55.72, 55.77, 55.85, 63.62, 106.92, 112.18,
112.84, 119.97, 121.75, 129.17, 131.31, 134.07, 141.37,
144.17, 147.31, and 147.42. HREIMS calcd. for C20H25NO5:
359.1733; found: 359.1736.
1
1190, 1090, 980, 830, 760, and 680. H NMR δ: 1.90 (brs,
w1/2 = 13 Hz,1H), 2.40 (s, 3H), 2.84 (dd, J = 7.4, 10.2 Hz,
1H), 2.88–3.02 (m, 2H), 3.59 (s, 3H), 3.69 (dd, J = 4.5,
10.2 Hz, 1H), 3.76–3.90 (m, 2H), 3.84 (s, 3H), 3.87 (s, 3H),
3.91 (d, J = 15.9 Hz, 1H), 4.43 (dd, J = 4.5, 7.4 Hz, 1H),
4.55 (d, J = 15.9 Hz, 1H), 5.03 (d, J = 11.2 Hz, 1H), 5.12 (d,
J = 11.2 Hz, 1H), 6.22 (s, 1H), 6.48 (d, J = 8.6 Hz, 1H),
6.70 (d, J = 8.6 Hz, 1H), 6.74 (s, 1H), 7.25 (d, J = 8.2 Hz,
2H), 7.30–7.51 (m, 5H) and 7.58 (d, J = 8.2 Hz, 2H). 13C
NMR δ: 21.34, 35.69, 39.60, 44.32, 50.14, 55.64,* 55.77,
63.60, 74.18, 111.16, 112.23, 112.76, 124.35, 126.73,
127.39,* 127.95, 128.20,* 128.25,* 129.04, 129.48,* 130.11,
132.61, 133.54, 137.37, 143.24, 143.33, 147.38, 147.59, and
150.25. HREIMS calcd. for C34H37NO7S: 603.2291; found:
603.2287.
(±)-2,3,10-Trimethoxy-9-hydroxy-5,6,13,13a-tetrahydro-
8H-dibenzo[a,g]quinolizine ((±)-schefferine, (±)-1)
Diethyl azodicarboxylate (0.022 mL, 0.122 mmol) was
added all at once to a stirred solution of 8 (22 mg,
0.061 mmol), triphenylphosphine (32 mg, 0.122 mmol), and
ethereal HBF4 (0.008 mL, 0.067 mmol) in THF (2 mL). The
reaction was stirred under reflux until complete conversion
of the aminoalcohol was assessed by TLC; then, the
volatiles were removed under reduced pressure and the re-
maining oil was chromatographed, yielding (±)-1 (17 mg,
82%) as a solid, mp 146.5–148°C (lit. (5b) 144–145°C and
(4b) 147–148°C). IR (KBr) (cm–1): 3430, 2930, 2850, 2800–
2720, 1630, 1590, 1500, 1450, 1340, 1290, 1200, 1140,
2-{2-[8-Benzyloxy-7-methoxy-2-(toluene-4-sulfonyl)-
1,2,3,4-tetrahydroisoquinolin-3-yl]-4,5-
dimethoxyphenyl}-acetaldehyde (5)
A 2% w/v solution of OsO4 in tert-butanol (0.100 mL)
was added to a mixture of allyl tetrahydroisoquinoline 4
(250 mg, 0.42 mmol) and NMO (103 mg, 0.88 mmol) in
4:2:1 acetone–water–tert-butanol (8 mL). The mixture was
stirred overnight at room temperature and then quenched
with 10% sodium hydrogensulfite (5 mL). Celite (1 g) was
added and stirring continued for an additional 1 h, when the
suspension was filtered under reduced pressure through
Celite contained in a Büchner funnel and the crude reaction
product was exhaustively extracted with EtOAc (5 × 20 mL).
The organic portion was dried (Na2SO4) and concentrated to
yield 6, as an oily 1:1 epimeric mixture (251 mg, 95%). IR
(film) (cm–1): 3450, 2950, 2860, 1600, 1500, 1290, 1190,
1070, 920, and 740. Without further purification, the mixture
of diols (125 mg) was dissolved in 3:1 THF–water (4 mL)
and treated with sodium periodate (101 mg, 0.44 mmol).
The reaction was stirred 2 h at room temperature, then it was
diluted with brine (5 mL) and extracted with EtOAc (5 ×
20 mL). The combined organic extracts were dried
(Na2SO4), concentrated in vacuum, and chromatographed,
providing aldehyde 5 (111 mg, 94%) as an oil.
1
1070, 990, and 740. H NMR δ: 2.18 (dd, J = 3.8, 13.3 Hz,
1H), 3.03 (dd, J = 2.3, 13.3 Hz, 1H), 3.18–3.56 (m, 5H),
3.68 (brs, 1H), 3.80 (s, 3H), 3.84 (s, 3H), 3.93 (s, 3H), 4.16
(d, J = 17.8 Hz, 1H), 4.33 (d, J = 17.8 Hz, 1H), 6.49 (s, 1H),
6.54 (d, J = 8.4 Hz, 1H), 6.67 (d, J = 8.4 Hz, 1H) and 6.82
(s, 1H). 13C NMR δ: 34.15, 43.39, 48.42, 52.19, 55.76,
55.90, 56.06, 56.13, 108.89, 112.96, 114.36, 120.69, 121.95,
129.25, 133.09, 137.89, 140.49, 143.93, 146.45, and 146.51.
HREIMS calcd. for C20H23NO4: 341.1627; found: 341.1622.
Acknowledgements
The authors gratefully acknowledge CONICET, SECyT-
UNR, and Fundación Antorchas for financial support and a
fellowship (D.A.B).
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© 2000 NRC Canada