New methodology for the N-demethylation of opiate alkaloids

Article abstract of DOI:10.1021/jo071171q

(Chemical Equation Presented) N-Demethylation is a key step in the preparation of a number of semisynthetic opiate pharmaceuticals. Herein we report a high-yielding, catalytic procedure for the N-demethylation of opiates which has a number of advantages over existing methods. For example, tetrasodium 5,10,15,20-tetra(4-sulfophenyl)-porphyrinatoiron(II) (0.3 molar equiv) effected the transformation of codeine methyl ether to the corresponding N-nor analogue in 91% yield. The catalyst was readily removed and recycled.

Full text of DOI:10.1021/jo071171q

New Methodology for the N-Demethylation of Opiate Alkaloids
Zemin Dong and Peter J. Scammells*
Department of Medicinal Chemistry, Victorian College of Pharmacy, Monash UniVersity, 381 Royal
Parade, ParkVille, Victoria 3052, Australia
peter.scammells@Vcp.monash.edu.au
ReceiVed June 5, 2007
N-Demethylation is a key step in the preparation of a number of semisynthetic opiate pharmaceuticals.
Herein we report a high-yielding, catalytic procedure for the N-demethylation of opiates which has a
number of advantages over existing methods. For example, tetrasodium 5,10,15,20-tetra(4-sulfophenyl)-
porphyrinatoiron(II) (0.3 molar equiv) effected the transformation of codeine methyl ether to the
corresponding N-nor analogue in 91% yield. The catalyst was readily removed and recycled.
CHART 1. Nalorphine (1), Nalbuphine (2), Buprenorphine
(3), Naloxone (4), Naltrexone (5), and Nalmefene (6)
Introduction
The N-methyl group is a characteristic group associated with
many naturally occurring alkaloids of biological significance,
including the opiate alkaloids. In a number of semisynthetic
pharmaceutical opiates this N-methyl group has been replaced
by other alkyl substituents such as N-allyl, N-cyclopropylmethyl,
and N-cyclobutylmethyl. A number of examples are depicted
in Chart 1. Since these compounds are typically prepared from
naturally occurring opiate precursors, a key step in their
synthesis is the N-demethylation of these substrates.
N-Methyl alkaloids can be N-demethylated via a variety of
procedures.1 The von Braun reaction, in which the N-methyl
amine is reacted with cyanogen bromide prior to cleavage of
the resultant cyanamide, was developed in the early 1900s.2
More recently chloroformates have been the reagents of choice
for the N-demethylation of tertiary N-methyl amines. A range
of different chloroformates have been developed which give
rise to carbamates that can be cleaved under a variety of different
reaction conditions.3 Vinyl chloroformate has been one of the
most effective members of this family for the N-demethylation
of opiates.4 An example of this is the conversion of oxymor-
phone to noroxymorphone in 98% yield. Diethyl azodicarboxy-
* Address correspondence to this author. Phone: +61 3 9903 9542. Fax:
+61 3 9903 9582.
(1) Thavaneswaran, S.; McCamley, K.; Scammells, P. J. Nat. Prod.
Commun. 2006, 1, 885-895.
(2) von Braun, J. Chem. Ber. 1900, 33, 1438-1452.
(3) Cooley, J. H.; Evian, E. J. Synthesis 1989, 1-7.
10.1021/jo071171q CCC: $37.00 © 2007 American Chemical Society
Published on Web 11/17/2007
J. Org. Chem. 2007, 72, 9881-9885
9881

Dong and Scammells
TABLE 1. N-Demethylation of CME N-Oxide with Various Fe(III)
SCHEME 1. N-Demethylation of Codeine Methyl Ether
with Use of a Nonclassical Polonovski Reaction
Catalystsa
reaction
time (h)
isolated
yield (%)
entry
catalyst
equiv
additive
1
2
3
4
Fe(III)(TPP)Cl
Fe(III)(TPPS)Cl
Fe(III)(TPPS)Cl
Fe2(SO4)3.9H2O
1.0
1.0
0.3
0.3
tetrazole
tetrazole
48
120
168
168
24
0b
0b
0b
a Reactions were performed in methanol at room temperature. b No
1
product was apparent from TLC and H NMR analysis.
SCHEME 2. N-Demethylation of OPC-31260
by treatment with 5,10,15,20-tetraphenylporphyrinatoiron(III)
chloride [Fe(III)(TPP)Cl]. The axial ligand of the iron complex
proved to be an influential factor in this reaction, with tetrazole
giving the best results.
Herein we report a new procedure for the N-demethylation
of opiate alkaloids that initially involves conversion to the
corresponding N-oxide and then employs 5,10,15,20-tetra(4-
sulfophenyl)porphyrinatoiron(II) [Fe(II)TPPS] as the catalyst of
choice.
Results and Discussion
late has also been reported to be an effective reagent for the
N-demethylation of thebaine to northebaine, though it has not
seen widespread use for the N-demethylation of tertiary amines.5
More recently the “nonclassical” or iron salt mediated
Polonovski reaction has been applied to the N-demethylation
of opiate alkaloids.6-9 In this procedure, the tertiary N-methyl
amine was converted to the corresponding N-oxide prior to
treatment with iron sulfate (2 molar equiv). A range of opiate
and tropane alkaloids were successfully N-demethylated in this
fashion in moderate to high yield. A high-yielding example of
this procedure is the preparation of N-norcodeine methyl ether
in 87% yield over two steps (Scheme 1).6 The presence of excess
iron sulfate presents purification problems, particularly with
larger scale syntheses. In some cases, this problem could be
overcome by the use of iron chelating agents such as EDTA
and tetraphenylporphine (TPP). A range of other chemical,
photochemical, and microbial procedures that have been used
for the N-demethylation of alkaloids have been summarized in
a recent review on this topic.1
Cytochrome P-450 enzymes effect a range of metabolic
oxidation reactions including N-demethylation. A range of
biomimetic oxidations modeled on cytochrome P-450’s action
have also been studied.10 A relevant example of this was
described by Otsubo and co-workers, who employed a metal-
loporphyrin-catalyzed demethylation reaction to prepare a
metabolite (10) of the vasopressin V2 receptor antagonist, OPC-
31260 (Scheme 2).11 In this case the relevant N,N-dimethyla-
lkylamine N-oxide was selectively demethylated in 86% yield
We initially evaluated the use of Fe(III)(TPP)Cl for the
N-demethylation of opiate alkaloids. When codeine methyl ether
N-oxide was treated with Fe(III)(TPP)Cl under the same
conditions used by Otsubo and co-workers,11 N-norcodeine
methyl ether was isolated in 24% yield (entry 1, Table 1). The
use of a stoichiometric amount of Fe(III)(TPPS)Cl and tetrazole
failed to yield any of the desired N-demethylated product.
Likewise, no N-norcodeine methyl ether was detected when
substoichiometric amounts of either Fe(III)(TPPS)Cl or iron-
(III) sulfate were employed.
We also chose to investigate the use of Fe(II) porphyrins in
N-demethylation reactions. This first necessitated the preparation
of the desired porphyrins (Scheme 3), which are not com-
mercially available. Tetrasodium 5,10,15,20-tetra(4-sulfonatophe-
nyl)porphyrin (TPPS) was prepared in two steps via literature
methodology.13 The use of microwave irradiation (rather than
conventional heating) greatly reduced the reaction time and
slightly improved the yield of the initial reaction between pyrrole
and benzaldehyde to form 5,10,15,20-tetraphenylporphyrin
(TPP). Iron was incorporated by treating TPPS with iron(II)
sulfate in a 1 M acetate buffer to form 5,10,15,20-tetra(4-
sulfonatophenyl)porphyrinatoiron(II) as a tetrasodium salt.
Reaction of Fe(II)TPPS (0.3 molar equiv) with codeine
methyl ether N-oxide hydrochloride proceeded very effectively,
delivering a 91% yield of the N-norcodeine methyl ether after
isolation and purification (entry 1, Table 2). By comparison,
the corresponding reaction with 0.3 equiv of iron(II) sulfate
heptahydrate produced only 52% of the N-nor product and 12%
of the parent CME after the same reaction time (entry 2, Table
2). Fe(II)TPPS also proved to be an effective catalyst for the
N-demethylation of dextromethorphan N-oxide, affording the
N-nor product in 93% yield (entry 3, Table 2). Tetraphenylpor-
phineiron(II) [Fe(II)TPP] also catalyzed the N-demethylation
of dextromethorphan N-oxide, albeit in a slightly lower yield
and longer reaction time than the sulfonated catalyst (entry 4,
Table 2). In contrast with most of the Fe(III) catalysts evaluated
(4) Olofson, R. A.; Schnur, R. C.; Bunes, L.; Pepe, J. P. Tetrahedron
Lett. 1977, 18, 1567-1571.
(5) Scwab, L. S. J. Med. Chem. 1980, 23, 698-702.
(6) McCamley, K.; Ripper, J. A.; Singer, R. D.; Scammells, P. J. J. Org.
Chem. 2003, 68, 9847-9850.
(7) Scammells, P. J.; Gathergood, N.; Ripper, J. A. PCT Int. Appl. WO
2002/016367, 2002.
(8) Smith, C.; Purcell, S.; Waddell, L.; Hayes, N.; Ritchie, J. PCT Int.
Appl. WO 2005/028483, 2002.
(9) Thavaneswaran, S.; Scammells, P. J. Bioorg. Med. Chem. Lett. 2006,
16, 2868-2871.
(10) For a review see: Bernadou, J.; Meunier, B. AdV. Synth. Catal.
2004, 346, 171-184.
(12) Caldwell, G. W.; Gauthier, A. D.; Mills, J. E. Magn. Reson. Chem.
1996, 34, 505-511.
(11) Kawano, Y.; Otsubo, K.; Matsubara, J.; Kitano, K.; Ohtani, T.;
Morita, S.; Uchida, M. Heterocycles 1999, 50, 17-20.
(13) Rothemund, P.; Menotti, A. R. J. Am. Chem. Soc. 1941, 63, 267-
270.
9882 J. Org. Chem., Vol. 72, No. 26, 2007

N-Demethylation of Opiate Alkaloids
SCHEME 3. Synthesis of Fe(II)TPPS
TABLE 2. N-Demethylation Reactions with Alternative Iron
Catalysts
TABLE 3. N-Demethylation with Use of Different Catalyst
Loading
reaction yield
equiv time (h) (%)a
Fe(II)TPPS
(molar equiv)
reaction
time (h)a
yield
(%)c
entry
substrate
codeine methyl ether Fe(II)TPPS
codeine methyl ether FeSO4‚7H2O 0.3
catalyst
entry
1
2
3
4
5
6
0.3
72
72
72
144
192
168
91
1
2
3
4
1.0
0.3
0.2
0.1
44
72
84
85
80
nd
52b
93
dextromethorphan
dextromethorphan
dextromethorphan
dextromethorphan
Fe(II)TPPS
Fe(II)TPP
Hemin
0.3
0.3
0.3
120
264d
84
23
a Reactions were monitored by TLC and worked up when CME
N-oxide‚HCl was consumed completely. b Isolated yield over two steps after
column chromatography. d Some CME N-oxide‚HCl still remained after 264
h of stirring.
hemin/dithio 0.3/1.0
erythritol
47c
7
thebaine
Fe(II)TPPS
0.3
240
68
a Isolated yield over two steps after column chromatography. b CME
(12%) was also isolated from this reaction. c Dextromethorphan (37%) was
also isolated from this reaction.
TABLE 4. N-Demethylation of CME and Dextromethorphan with
Fe(II)TPPS in Aqueous Alcohol
solvent
(H2O/methanol)
reaction
time (h)
yield
(%)a
previously, hemin produced a modest amount of N-nordex-
tromethorphan (23%) though this may have resulted from the
presence of trace amounts of Fe(II) in the reaction mixture.
When this reaction was repeated with added dithioerythritol to
increase the Fe(II) content, a higher yield on the N-demethylated
product was isolated (47%, entry 6, Table 2). Thebaine N-oxide
gave a lower yield of the desired N-demethylated product than
the other alkaloids (entry 7, Table 2). The thebaine N-oxide
isomers were present in a ratio of 65:35 by 1H NMR. Caldwell
and co-workers have reported that the major isomer (where the
N-CH3 group is axial) completely decomposed upon standing
in solution for 5 days while the minor isomer (where the N-CH3
group is equatorial) remained stable over this period.12 Thus,
the lower yield of the N-northebaine may have resulted from
competing degradation processes over the course of the reaction
(10 days).
reactant
CME
dextromethorphan
1:3
7:3
72
72
84
87
a Isolated yield after column chromatography.
We have also been able to demonstrate that the Fe(II)TPPS
catalyst can be recovered and reused. The same batch of catalyst
was used for the N-demethylation of dextromethorphan N-oxide
over four cycles. After the first cycle, Fe(II)TPPS was precipi-
tated from the reaction mixture by the addition of diethyl ether.
The catalyst was collected by suction filtration, washed with
CHCl3/Et2O, dried in vacuo and then reused without further
purification. Pure N-nordextromethorphan was isolated in yields
of 87%, 94%, 97%, and 85%, respectively, following column
chromatography.
Fe(II)TPPS clearly produced the most promising results of
the iron catalysts that were evaluated in the initial study. The
Fe(II)TPPS catalyst loading was further explored and the results
of this investigation are described in Table 3. The use of a
stoichiometric amount of Fe(II)TPPS shortened the reaction time
to 44 h and gave a comparable yield of the corresponding
N-demethylated product (entry 1, Table 3). Conversely the use
of less than 0.3 molar equiv of the catalyst resulted in a
significant increase in the time taken for the reaction to go to
completion. In the reaction where Fe(II)TPPS (0.2 equiv) was
used, the reaction time increased to 120 h, while the reaction
with Fe(II)TPPS (0.1) failed to proceed to completion in 264 h
(entries 3 and 4, Table 3). Comparable yields were obtained
when 0.2, 0.3, and 1.0 equiv of the catalyst were used.
We also wished to explore the possibility of performing
N-demethylation reactions in aqueous systems. The results of
this work are summarized in Table 4. The limiting factor in
these experiments proved to be the water solubility of the
substrate. However, once in solution the reactions with codeine
methyl ether N-oxide and dextromethorphan N-oxide proceeded
in good yield (84% and 87%, respectively).
Conclusions
In conclusion, Fe(II)TPPS is a promising reagent for the
N-demethylation of N-methyl alkaloids, with 0.3 equiv affording
excellent yields of the desired N-nor products. Furthermore, the
high water solubility of the iron porphyrin catalyst facilitates
J. Org. Chem, Vol. 72, No. 26, 2007 9883

Dong and Scammells
(71 mg, 0.41 mmol) was added and the reaction was refluxed for
10 min. The reaction was then cooled to room temperature and put
into an ice bath for 15 min. The reaction mixture was diluted with
deionized water (40 mL) and a precipitate formed. The solid was
collected by suction filtration, washed well with deionized water,
and dried by high vacuum to afford Fe(II)TPP16 (182 mg, 84%
yield). ESI-MS (70V): m/z 668.2 (M + H+, 100%).
its removal at the completion of the reaction via precipitation
and filtration following the addition of diethyl ether. As the
N-demethylation of alkaloids has potential industrial applica-
tions, the principles of green chemistry have been given due
consideration throughout the course of this research. The use
of 0.3 equiv of Fe(II)TPPS rather than the 2.0 equiv of FeSO4
commonly employed in the Polonovski approach, the capacity
to recycle and reuse the catalyst, and the use of partially aqueous
reaction media represent progress in this area.
General Procedure for N-Demethylation. Codeine methyl ether
(730 mg, 2.33 mmol) was dissolved in methanol (35 mL) and
hydrogen peroxide (7.9 g, 30% w/v, 70 mmol) was added dropwise
at 0 °C. The reaction mixture was stirred at room temperature for
2 days. The excess H2O2 was deactivated with MnO2 at 0 °C for 1
h and the solution was filtered through a Celite pad. The Celite
pad was washed with additional methanol and the filtrate was
evaporated in vacuo to give CME N-oxide. The CME N-oxide was
dissolved in brine (10 mL), cooled on ice, and acidified to pH 1-2
with 6 M HCl. The resulting solution was extracted with CHCl3 (4
× 20 mL). The CHCl3 extracts were combined, dried (MgSO4),
and then evaporated in vacuo to give CME N-oxide‚HCl salt as an
off-white solid. The crude CME N-oxide‚HCl salt was dissolved
in methanol (120 mL) and Fe(II)TPPS (690 mg, 0.64 mmol) was
added. The reaction mixture was stirred at room temperature and
monitored by TLC with use of CHCl3/MeOH/NH4OH (85:15:1)
as an eluent. After stirring at room temperature for 72 h, TLC
showed complete consumption of the N-oxide. Methanol was
removed in vacuo and the residue was taken up in a mixture of
water (25 mL) and CHCl3 (50 mL). The aqueous layer was then
extracted with CHCl3 (4 × 30 mL). The CHCl3 extracts were
combined, washed with brine (25 mL), dried (MgSO4), and
evaporated to afford a crude solid. After column chromatography
on silica gel with a CHCl3/MeOH/NH4OH gradient (100:0:1 to 95:
5:1), pure norcodeine methyl ether17 was obtained as a pale yellow
solid (631 mg) in 91% yield.
N-Norcodeine methyl ether: mp 104-106 °C (lit.17 mp 98-
100 °C); 1H NMR δ 6.69 (1H, d, J ) 8.1 Hz), 6.56 (1H, d, J ) 8.1
Hz), 5.79 (1H, d, J ) 9.9 Hz), 5.34 (1H, dt, J ) 9.9, 2.7 Hz), 5.01
(1H, dd, J ) 5.7, 1.2 Hz), 3.88 (3H, s), 3.86-3.82 (1H, m), 3.72-
3.69 (1H, m), 3.58 (3H, s), 3.09-3.02 (1H, m), 2.99-2.87 (3H,
m), 2.63 (1H, m), 1.98-1.88 (2H, m); 13C NMR δ 147.6, 142.1,
130.8, 130.6, 128.6, 127.1, 118.7, 113.4, 89.8, 75.9, 57.0, 56.4,
51.9, 44.3, 41.6, 38.5, 36.8, 31.5; ESI-MS (20 and 70 V) m/z 300
(M + H+, 100%). HRMS C18H21NO3 calcd for [M + H]+ 300.1594,
found 300.1595.
N-Nordextromethorphan. The target compound was prepared
from dextromethorphan (1.49 g, 5.2 mmol) with the general
procedure described above. N-Nordextromethorphan18 was purified
by column chromatography on silica gel with a CHCl3/MeOH/NH4-
OH gradient (100:0:1 to 95:5:1) to afford a pale yellow oil (1.31
g, 93%). 1H NMR δ 7.05 (1H, d, J ) 8.4 Hz), 6.80 (1H, d, J ) 2.4
Hz), 6.72 (1H, dd, J ) 8.4, 2.4 Hz), 4.43 (1H, br s), 3.78 (3H, s),
3.27 (1H, br s), 3.13 (1H, dd, J ) 18.0, 6.0 Hz), 2.91 (1H, d, J )
18.0 Hz), 2.87-2.81 (1H, m), 2.64 (1H, t, J ) 12.1 Hz), 2.32 (1H,
d, J ) 12.6 Hz), 1.74 (1H, dd, J ) 12.6, 4.8 Hz), 1.68-1.62 (1H,
m), 1.54-1.51 (1H, m), 1.43-1.25 (5H, m), 1.07 (1H, t, J ) 12.6
Hz); 13C NMR δ 158.3, 141.4, 129.7, 128.7, 111.3, 110.9, 55.2,
51.2, 45.5, 42.3, 39.0, 38.2, 36.9, 33.0, 26.8, 26.7, 22.1; ESI-MS
(20V) m/z 300 (M + H+, 85%), 102 (100%). HRMS C17H23NO
calcd for [M + H]+ 258.1852, found 258.1850.
Experimental Section
Synthesis of 5,10,15,20-Tetraphenylporphyrin (TPP). Pyrrole
(0.7 mL, 10 mmol) and benzaldehyde (1.0 mL, 10 mmol) were
dissolved in propionic acid (18.3 mL) in a 20 mL microwave vial.
The vial was capped and irradiated in a microwave reactor (Biotage
Initiator 2.0) at 200 °C for 15 min. The reaction mixture was cooled
to 50 °C, filtered, and washed with methanol until the methanol
washings were colorless. The resultant purple solid was dried in
1
high vacuum to give pure TPP13 (0.39 g) in 25% yield. H NMR
δ 8.91 (8H, s), 8.30-8.27 (8H, m), 7.87-7.78 (12H, m), -2.70
(2H, s); ESI-MS (70V) m/z 615.4 (M + H+, 100%).
Synthesis of Tetrasodium 5,10,15,20-Tetra(4-sulfophenyl)-
porphyrin (TPPS).14 Tetraphenylporphyrin (TPP) (2.0 g, 3.25
mmol) and concentrated sulfuric acid (10 mL) were ground into a
homogeneous paste with a mortar and pestle. The paste was
transferred to a 100-mL round-bottomed flask and additional sulfuric
acid (30 mL) was added. The mixture was heated in an oil bath
(100-110 °C) for 4 h and then allowed to stand at room temperature
for 18 h. The solution was filtered through a sintered frit and the
filtrate was diluted carefully to 400 mL by addition of distilled
water. The solution was stirred with CaO until a purple color
persisted. Calcium sulfate was filtered off and washed with a
minimum quantity of hot water. Crushed dry ice was added to the
combined filtrate, which was filtered again. The filtrate was
concentrated (to ∼150 mL) and the pH of the final solution was
regulated at 8-10 by adding 1 M NaOH solution or 1 M HCl
solution. The solution was again filtered to remove inorganic salt.
The solution was cooled with liquid nitrogen until frozen then dried
on a freeze dryer to give TPPS14 (3.32 g) in quantitative yield. 1H
NMR (DMSO-d6) δ 8.86 (8H, s), 8.18 (8H, d, J ) 8.1 Hz), 8.06
(8H, d, J ) 8.1 Hz), -2.90 (2H, s, NH); ESI-MS (20V) m/z 935.0
(M + H+ - 4Na+, 100%), 957.3 (M + H+ + Na, 56%), 979.2 (M
+ H+ + 2Na, 45%), 1001.1 (M + H+ + 3Na, 28%), 1024.3 (M +
H++ 4Na, 13%).
Synthesis of Tetrasodium 5,10,15,20-Tetra(4-sulfophenyl)-
porphyrinatoiron(II) [Fe(II)TPPS]. Tetrasodium tetra(4-sulfophe-
nyl)porphyrin (TPPS) (1.75 g, 1.71 mmol) and FeSO4‚7H2O (0.43
g, 1.63 mmol) were dissolved in 1 M acetate buffer (120 mL, pH
4). The resulting mixture was refluxed in an oil bath for 3.5 h and
the reaction mixture was allowed to cool to room temperature. The
reaction mixture was then stored at 4 °C overnight and filtered.
The filtrate was poured into acetone (960 mL) and a precipitate
formed. The precipitate was separated by centrifugation and then
it was mixed with methanol (60 mL). Acetone (480 mL) was added
and a second precipitate formed. The above procedure was repeated.
The precipitate was finally filtered off and dried under high vacuum
for 4 h to give Fe(II)TPPS15 as a green solid (1.69 g, 92%). ESI-
MS (70V): m/z 988.2 (M + H+, 100%).
N-Northebaine. The target compound was prepared from
thebaine (47 mg, 0.15 mmol) with the general procedure described
above. N-Northebaine19 was purified by column chromatography
Synthesis of 5,10,15,20-Tetraphenylporphyrinatoiron(II) [Fe-
(II)TPP]. TPP (200 mg, 0.33 mmol) and N,N-dimethylformamide
(20 mL) were warmed in an oil bath under an atmosphere of
nitrogen. The TPP was dissolved at 60-80 °C then ferrous acetate
(16) Coleman, J. P.; Hoard, J. L.; Kim, N.; Lang, G.; Reed, C. A. J.
Am. Chem. Soc. 1975, 97, 2676-2681.
(17) Dhar, N. C.; Maehr, R. B.; Masterson, L. A.; Midgley, J. M.;
Stenlake, J. B.; Wastila, W. B. J. Med. Chem. 1996, 39, 556-561.
(18) Sawa, Y.; Prefecture, H.; Tsuki, N.; Tada, H.; Prefecture, O. US
3,230,224, 1966.
(14) Srivastava, T. S.; Tsutsui, M. J. Org. Chem. 1973, 38, 2103-2103.
(15) Miyoshi, S.; Kamei, T.; Ota, Y.; Masunaga, C.; Izuhara, Y.;
Tomochika, K.; Shinoda, S.; Yamamoto FEBS Microbiol. Lett. 2002, 208,
77-81.
(19) Bartels-Keith, J. R. J. Chem. Soc. 1966, 617-624.
9884 J. Org. Chem., Vol. 72, No. 26, 2007

N-Demethylation of Opiate Alkaloids
on silica gel with a CHCl3/MeOH/NH4OH gradient (100:0:1 to 95:
5:1) to afford the title compounds as a brown solid (31 mg, 69%).
mp 148-153 °C (lit.19 mp 157-158 °C); 1H NMR δ 6.73 (1H, d,
J ) 8.1 Hz), 6.66 (1H, d, J ) 8.1 Hz), 5.56 (1H, d, J ) 6.3 Hz),
5.32 (1H, s), 5.09 (1H, d, J ) 6.3 Hz), 3.98 (1H, d, J ) 4.2 Hz),
3.91 (3H, s), 3.66 (3H, s), 3.25 (1H, dt, J ) 13.5, 3.3 Hz), 3.17
(2H, br s), 2.99 (1H, dd, J ) 13.5, 4.5 Hz), 2.23 (1H, br s), 2.14
(1H, dt, J ) 12.6, 5.1 Hz), 1.89 (1H, dd, J ) 12.6, 2.4 Hz); 13C
NMR δ 152.8, 145.0, 143.0, 133.3, 127.7, 119.4, 113.2, 110.8, 95.9,
89.2, 56.5, 55.1, 55.0, 53.9, 46.6, 40.3, 38.4, 37.5; ESI-MS (20V)
m/z 298 (M + H+, 100%), 152 (100%). HRMS C18H19NO3 calcd
for [M + H]+ 298.1438, found 298.1443.
General Procedure for Catalyst Recovery and Reuse. Dex-
tromethorphan (228 mg, 0.84 mmol) was dissolved in methanol
(25 mL), magnesium bis(monoperoxyphthalate) hexahydrate (MMPP)
(0.46 g, 0.93 mmol) was added, and the reaction mixture was stirred
for 0.5 h at room temperature. Methanol (30 mL) was added and
the solution was filtered through a plug of Celite. After washing
with an additional 30 mL of methanol, the solution was evaporated
in vacuo. The crude dextromethorphan N-oxide was redissolved in
methanol (90 mL), Fe(II)TPPS (250 mg, 0.25 mmol) was added,
and the reaction mixture was stirred at room temperature for 72 h.
TLC (with CHCl3/MeOH/NH4OH, 85:15:1 as an eluent) showed
complete consumption of the dextromethorphan N-oxide. The
reaction mixture was poured into diethyl ether (270 mL) and the
catalyst precipitated. The catalyst was collected by suction filtration
with a Millipore filtration apparatus (0.45 µm filter paper), washed
with CHCl3/diethyl ether (4:1, 2 × 30 mL), and dried under vacuum
prior to reuse. The filtrate was evaporated in vacuo and the residue
partitioned between brine (15 mL) and CHCl3 (30 mL). After further
extraction of the aqueous layer with CHCl3 (2 × 30 mL), the
combined CHCl3 extracts were evaporated to afford the crude N-nor
product. Pure N-nordextromethorphan (188 mg, 87% yield) was
obtaining following column chromatography on silica gel with a
CHCl3/MeOH/NH4OH gradient (100:0:1 to 95:5:1).
H2O2 was deactivated by stirring with MnO2 for 1 h at 0 °C. The
solution was then filtered through a Celite plug, which was further
washed with methanol (10 mL). Evaporation of the solvent afforded
CME N-oxide, which was dissolved in brine (5 mL), cooled on
ice, and acidified to pH 1-2 with 6 M HCl. The resulting solution
was extracted with CHCl3 (4 × 10 mL). The CHCl3 extracts were
combined, dried (MgSO4), and then evaporated in vacuo to give
CME N-oxide‚HCl as an off-white solid. The crude CME N-oxide
hydrochloride was dissolved in aqueous alcohol (1 mL of water
and 3 mL of methanol) and Fe(II)TPPS (35 mg, 0.04 mmol) was
added. The reaction mixture was stirred at room temperature and
monitored by TLC (eluted with CHCl3/MeOH/NH4OH, 85:15:1).
After 72 h, TLC showed complete consumption of the CME
N-oxide. After evaporation of the solvent under reduced pressure,
the residue was partitioned between chloroform (15 mL) and water
(5 mL). The aqueous layer was extracted with CHCl3 (4 × 15 mL)
and the combined CHCl3 extracts were washed with brine (15 mL),
dried over anhydrous MgSO4, and evaporated to afford a crude
product. After column chromatography on silica gel with a CHCl3/
MeOH/NH4OH gradient (100:0:1 to 95:5:1), pure norcodeine
methyl ether (30.5 mg) was obtained in 84% yield.
N-Nordextromethorphan. The target compound was prepared
from dextromethorphan (38 mg, 0.14 mmol) with the general
procedure described above. Crude dextromethorphan N-oxide‚HCl
was dissolved in aqueous alcohol (3 mL of water, 7 mL of
methanol) and stirred with Fe(II)-TPPS (40 mg, 0.04 mmol).
N-Nordextromethorphan was purified by column chromatography
with a CHCl3/MeOH/NH4OH gradient (100:0:1 to 95:5:1) to afford
the title compound (31.5 mg) in 87% yield.
Acknowledgment. The authors wish to thank the ARC
Centre for Free Radical Chemistry and Biotechnology for
financial support and GlaxoSmithKline for donation of opiate
starting materials. The efforts of Dr. Simon Egan and Mr. Trent
Ashton in obtaining the MS and HR-MS data are also appreci-
ated.
This procedure was repeated with use of the recovered catalyst
to afford N-nordextromethorphan: 203 mg (94%), 209 mg (97%),
and 184 mg (85%), respectively.
General Procedure for N-Demethylation in Aqueous Alcohol.
N-Norcodeine Methyl Ether. Codeine methyl ether (38 mg, 0.12
mmol) was dissolved in methanol (5 mL) and hydrogen peroxide
(400 mg, 30% w/v, 3.6 mmol) was added dropwise at 0 °C. The
reaction mixture was stirred at room temperature for 2 days. Excess
Supporting Information Available: UV spectra of Fe(II)TPP
1
and Fe(II)TPPS, and H and 13C NMR spectra of N-norcodeine
methyl ether, N-northebaine, and N-nordextromethorphan. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
JO071171Q
J. Org. Chem, Vol. 72, No. 26, 2007 9885