Improved synthesis of 13C,2H3- and 2H3-salmeterol by Cs2CO3-mediated monoalkylation of a primary amine

Article abstract of DOI:10.1002/jlcr.595

An abbreviated synthesis of isotopically labelled salmeterol has been achieved. The key improvement utilizes a highly selective Cs₂CO₃-mediated one-pot alkylation of benzylamine by 6-bromo-1-(4′-phenylbutoxy)hexane to prepare the limiting reagent, 6-N-benzylamino-1-(4′-phenylbutoxy)hexane without overalkylation. The method was applied to synthesis of the title compounds in >97 at% isotopic purity. Copyright

Full text of DOI:10.1002/jlcr.595

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2002; 45: 755–762.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.595
Research Article
Improved synthesis of C,2H3- and
13
2H3-salmeterol by Cs2CO3-mediated
monoalkylation of a primary amine
Tadeusz F. Molinski*1 and Scott D. Stanley2
1 Department of Chemistry, University of California, Davis, CA 95616, USA
2 K. L. Maddy Equine Analytical Chemistry Laboratory, University of
California, Davis, CA 95616, USA
Summary
An abbreviated synthesis of isotopically labelled salmeterol has been achieved.
The key improvement utilizes a highly selective Cs2CO3-mediated one-pot
alkylation of benzylamine by 6-bromo-1-(40-phenylbutoxy)hexane to prepare
the limiting reagent, 6-N-benzylamino-1-(40-phenylbutoxy)hexane without
overalkylation. The method was applied to synthesis of the title compounds
in >97 at% isotopic purity. Copyright # 2002 John Wiley & Sons, Ltd.
Key Words: phenethylamine; bronchodilator; deuterium; carbon-13; mass
spectrometry
Introduction
Synthesis of pharmacologically active N-alkylated secondary amines by
direct alkylation is often impractical owing to problems of over-
alkylation and separation of intractable mixtures of secondary, tertiary
and quaternary amines. For example, one preparation of the
*Correspondence to: T. F. Molinski, Department of Chemistry, University of California, Davis
1 shields avenue, Davis CA 95616, USA. E-mail: tfmolinski@ucdavis.edu
Contract/grant sponsor: Testing Integrity Program, Inc. Contract/grant number: NSF CHE-980
8183.
Copyright # 2002 John Wiley & Sons, Ltd.
Received 29 January 2002
Revised 14 March 2002
Accepted 22 March 2002

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T. F. MOLINSKI AND S. D. STANLEY
OH
Bn
HN
OH
D
H
N
H
N
13
C
D
HO
HO
CD OH
2
CD OH
2
O
O
O
13
2
2
C, H -salmeterol
3
H -salmeterol
3
2
1
3
Figure 1. Deuterium-labelled salmeterol
bronchodilatory b-adrenergic agonist, salmeterol1 (Figure 1), uses a
work around for the synthesis of the key N-benzyl secondary amine 1.
This is achieved by conversion of the primary alkylating agent to the
corresponding azide by SN2 displacement, followed by reduction then
conversion to an N-benzyl amine by reductive amination with
benzaldehyde in the presence of NaBH4 or NaCNBH3.
An alternative method for selective N-alkylation relies upon
alkylation of an imine, prepared by condensation of the primary amine
with an aldehyde, followed by hydrolysis of the resultant N,N-
dialkyliminium salt to the desired secondary amine.2 Both methods,
and other alternatives (for example, alkylation of N-alkylarenesulfona-
mides followed by hydrolysis),3 add 1–4 steps to the procedure to
achieve the desired outcome-controlled formation of a secondary N-
benzylamine. The key intermediate N-benzylamine 1 (Figure 1),4
employed in the synthesis of salmeterol, is prepared from commercially
available 4-phenylbutanol and 1,6-dibromohexane from operations
requiring five steps.5 We needed a rapid preparation of isotopically
2
13C,2H-labelled salmeterol (13C-2H3-2 and H3-3) for use as internal
standards in analytical LC–MS of equine plasma and urine samples. To
this end, we sought a one-step method for the preparation of 1 by
selective monoalkylation of benzylamine for subsequent conversion to 2
and 3 (Figure 1).
Reports describing chemoselective monoalkylation of primary amines
in the presence of the bases Cs2CO3 and CsOH and DMF have
appeared recently.6 The improved selectivity for monoalkylation
appears to involve initial tight coordination of Cs+ to the primary
amine. Salvatore et al. have explored the scope of the reaction and
propose a mechanism in which the Cs-coordinated primary amine is
deprotonated in the basic medium to give a reactive Cs-amide base.7
The latter is rapidly alkylated, but further alkylation is prevented by the
Copyright # 2002 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2002; 45: 755–762

IMPROVED SYNTHESIS OF LABELLED SALMETEROL
757
steric hindrance to the bulky Cs-coordinated secondary amine product.
We found that in the presence of Cs2CO3 direct alkylation of the
primary bromide 4 gave 1 in one step. This method will find application
in other syntheses of secondary and tertiary amines that are prone to the
complications of multiple alkylation and mixtures of inseparable
products.
Results and discussion
6-Bromo-1-(40-phenylbutoxy)hexane (4, Scheme 1) was prepared by
phase-transfer catalyzed alkylation of 4-phenylbutan-1-ol with
1,6-dibromohexane (2 eq, 10 M NaOH, aq, CH2Cl2, 5 mol% each of
n-Bu4NHSO4 and BnEt3NCl, rt, 8 days, 51%). Purification of crude
mixture was best achieved by short-passage chromatography over TLC-
grade silica gel. Unreacted starting materials and bis-ether byproduct
were eluted with n-hexane followed by 5% MTBE/n-hexane which
provided pure 4. Controlled alkylation of 4 with benzylamine (1.1 eq)
was smoothly carried out using Cs2CO3–DMF (60–808C) to give 1
(38%, pure, unoptimized) uncontaminated by overalkylation products.
The rest of the material was comprised of starting materials, which
could be recovered by chromatography.
The unreported deuterium labelled analog 3 was prepared as follows
(Scheme 1). Methyl a-bromo 4-acetylsalicylate (5), prepared by
NH2
(CH2)4O(CH2)6Br
O
HN
R =
Cs2CO3
DMF, 60-80˚ C, 5 h, 38%
O
4
1
O
O
i-Pr2NEt,
then 1
OH
LiAlD4
Br
N
N
R
R
THF, 2 h
87%
D
HO
HO
THF, 60˚ C
HO
COOMe
5
COOMe
6
7
CD2OH
H2, Pd-C
MeOH
( )-2H3-salmeterol, 3
Scheme 1. Synthesis of deuterium-labelled salmeterols 2 and 3
Copyright # 2002 John Wiley & Sons, Ltd.
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T. F. MOLINSKI AND S. D. STANLEY
(A)
(B)
(C)
Figure 2. Full scan ESI mass spectra recorded in positive-ion mode after direct
sample infusion (see Experimental). Positive-ion mode. (A) 13C,2H3-salmeterol
2
(2); (B) H3-salmeterol (3); and (C) unlabelled salmeterol.
Copyright # 2002 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2002; 45: 755–762

IMPROVED SYNTHESIS OF LABELLED SALMETEROL
759
bromination of methyl 4-acetylsalicylate (Br2, CHCl3), was alkylated
with 1 (i-Pr2EtN, THF, 608C, quantitative) to give 6 which was
2
immediately reduced (LiAlD4) to the corresponding H3-alcohol 7.
Hydrogenolysis of 7 (10% Pd–C, H2) provided pure ( ꢀ )-2H3-
salmeterol (3) as a clear oil after preparative TLC. The isotopic
composition of 3, as measured by ESI MS, was 98% d3, 1.5% d2 and
negligible amounts of lower isotopomers.
Synthesis of the known compound 2 followed the method of
Goodwin et al.8 Methyl 4-(1-13C-a-bromoacetyl)salicylate, prepared
by Friedel–Crafts acylation (AlCl3) of methyl salicylate with 1-13C
acetyl chloride (Aldrich, >99 at%) followed by a-bromination,8 was
alkylated with 1, followed by reduction with LiAlD4 and debenzylation
(Pd–C, H2) to give 2 (>98% d3, ESIMS), identical in every respect to
the reported material.8 Examination of the ESI mass spectra of 2, 3 and
unlabelled salmeterol (1) (Figure 2) confirmed that the compositions for
each compound were >97% isotopic purity.
Conclusion
In summary, samples of deuterated 2 and the new salmeterol derivative
3 were prepared from methyl 4-acetylsalicylate in good yield by
employing a chemoselective Cs+-mediated monoalkylation of benzyla-
mine. The preparation of the limiting reagent 1 from 4-phenylbutanol
was shortened to two steps (cf published five-step procedure)5. This
useful modification can be adapted to recently disclosed syntheses of
enantio-enriched salmeterol,9,5 and preparation of N-alkylated phe-
nethylamines substituted with radioisotope labels in the N-alkyl side
chain.10 In the latter case, the ability to prepare labelled material with
minimum manipulation is most desirable.
Experimental
CH2Cl2 and DMF were distilled from CaH2 and stored under N2. THF
was distilled under N2 from sodium-benzophenone ketyl. Cs2CO3 was
dried overnight under high vacuum at 568C. Compounds were purified
by preparative chromatography using flash chromatography11 or using
vacuum-assisted chromatography on TLC-grade silica. Preparative
TLC was carried out using 20 ꢁ 20 ꢁ 0.2 cm3 plates (E.M. Merck grade
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T. F. MOLINSKI AND S. D. STANLEY
7749). ESI MS spectra were recorded on a ThermoFinnigan LCQ ion
trap. LC–MS analysis was conducted by direct infusion using a mixed
solvent (0.1% HCO2H, 20% CH3CN, 80% H2O) with a flow rate of
1
0.5 ml/min using full scan mode (MS range 150–2000 amu). H NMR
spectra were recorded in Varian Inova 400 or Mercury 300 NMR
spectrometers in CDCl3 and referenced to residual solvent signals,
dH=7.26 ppm and dC=77.00 ppm.
6-bromo-1-(40-phenylbutoxy)hexane ð4 Þ
A solution of 4-phenylbutanol (1.50 g, 10 mmol), 1,6-dibromohexane
(4.88 g,
20.0 mmol)
and
tetra-n-butylammonium
hydrogen
sulphate (169 mg, 0.50 mmol) in CH2Cl2 (10 ml) was treated
with 10 M NaOH aq (1.0 ml) and stirred vigorously for 9 h.
The mixture was treated with benzyltriethylammonium chloride
(5 mol%) and stirred at room temperature for a total of 8 days,
then diluted with CH2Cl2. After washing with H2O, the organic
phase was dried (Na2SO4) and concentrated to give a crude product
that was separated by silica chromatography (TLC grade). Excess
dibromide and bis-ether byproduct were removed by elution with
n-hexane and pure 4 (51%) was recovered after elution with 5% MTBE/
1
n-hexane. The product gave H NMR and MS data identical with
reported values.4
Monoalkylation of 6-bromo-1-(40-phenylbutoxy)hexane with benzylamine
in the presence of Cs2CO3: 6-N-benzylamino-1-(40-phenylbutoxy)hexane
(1)
Anhydrous Cs2CO3 (2.27 g, 7.0 mmol) was added to a stirred solution of
4 (1.82 g, 5.8 mmol) and benzylamine (0.69 g, 6.4 mmol) in dry DMF
(29 ml) and the mixture was stirred at 60–808C for 5 h. The mixture was
poured into ice–NaHCO3 aq (1 M) and extracted with CH2Cl2
(3 ꢁ 100 ml). The combined organic phases were washed with H2O,
dried (Na2SO4) and concentrated. The residue (2.00 g) was dried under
high vacuum to remove DMF and applied to a short, wide column of
TLC-grade silica and eluted with 2–5% MeOH (NH3-saturated)/
CH2Cl2 to obtain product 1 as a clear oil (0.75 g, 38%). This oil
1
displayed a single ninhydrin-positive spot by TLC and a H NMR
spectrum consistent with the reported values.4
Copyright # 2002 John Wiley & Sons, Ltd.
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IMPROVED SYNTHESIS OF LABELLED SALMETEROL
761
(ꢀ)-13C,2H3-salmeterol ð2Þ
This compound was prepared from 1 and the 13C-labelled analog of 5 as
described by Goodwin et al.8 ESI MS m/z [M+H]+ 420 (98% d3), 419
(1.5% d2), 418 (0.5% d). Calcd. for C CH235H3NO4, 420.
13
24
(ꢀ)-2H3-salmeterol ð3Þ
A solution of compound 68 (90 mg, 0.17 mmol) in THF (4 ml) was
2
cooled in an ice-bath and treated with LiAlD4 (98 at % H, 109 mg) in
portions for over 30 min then allowed to warm to room temperature
over 19 h. The mixture was cooled again and cautiously treated with a
few drops of water followed by 1.2 M HCl (10 ml). The mixture was
treated with the theoretical amount of solid NaHCO3 required to
neutralize the acid, adjusted to pH 7, then diluted with H2O (50 ml) and
extracted with CH2Cl2 (3 ꢁ 50 ml). The combined extracts were washed
with water, dried (Na2SO4) and concentrated to give product 7 as a pale
oil (87%). An analytical sample was prepared by preparative TLC
(0.01:1:1 NH4OH aq/n-hexane/EtOAc). 1H NMR (CDCl3) 7.4–7.1
(m, 10 H, ArH), 6.98 (dd, 1 H, J=8.6, 2 Hz), 6.91 (d, 1 H, J= 2 Hz),
6.78 (d, 1 H, J=8.6 Hz), 4.00 (bd, 1 H, CH(OH)CH2N), 3.80 (m, 1 H,
CH(OH)CH2N), 3.39 (t, 2 H, OCH2), 3.35 (t, 2 H, OCH2), 2.7 (m,
NCH2), 2.61 (t, 2 H, J=7.2 Hz, ArCH2), 1.7–1.2 (m, 12 H, 6 ꢁ CH2).
ESI MS m/z [M+H]+ 509 (98% d3), 508 (2% d2), 507 (5 0.1% d1).
Calcd. for C32H41 2H3NO4 509.
A solution of 7 (28 mg) in ethanol (20 ml) was hydrogenated in the
presence of 10% Pd–C (20 mg, 1 atm H2) for 4 h. The catalyst was
removed by filtration through a Celite pad. Removal of the solvent gave
a residue which was purified by preparative TLC (developed with 60:8:1
CH2Cl2/MeOH/NH4OH aq) to provide (ꢀ)-2H3-salmeterol, 3 (17 mg,
77%). ESI MS m/z [M+H]+ 419 (97.9% d3), 418 (1.6% d2), 417 (0.5%
d). Calcd. for C25H35 2H3NO4, 419.
Acknowledgements
We thank Karl Bailey, John MacMillan and Mako Masuno for
1
measurements of H NMR spectra and Wayne S. Skinner for LC–MS
analysis. The 400 MHz NMR spectrometer was partially funded by
NSF CHE-9808183. This work was supported by a grant from the
Testing Integrity Program, Inc.
Copyright # 2002 John Wiley & Sons, Ltd.
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Copyright # 2002 John Wiley & Sons, Ltd.
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