Two robust, efficient syntheses of [phenyl ring-U-14C]indole through use of [phenyl ring-U- 14C]aniline

Article abstract of DOI:10.1002/jlcr.1067

Two robust, efficient syntheses of [phenyl ring-U-¹⁴C]indole are presented. In the first synthesis, we developed optimum reaction conditions for the Houben-Hoesch alkylation of chloro acetonitrile with aniline. This was found through the screen

Full text of DOI:10.1002/jlcr.1067

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2006; 49: 615–622.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.1067
Research Article
Two robust, efficient syntheses of [phenyl ring-
U-14C]indole through use of [phenyl ring-U-14C]aniline
Carrie A. B. Wager* and Sandra A. Miller
Chemical Research and Development, Pfizer Global Research and Development, Eastern
Point Road, Groton, CT 06340, USA
Summary
Two robust, efficient syntheses of [phenyl ring-U-14C]indole are presented. In the first
synthesis, we developed optimum reaction conditions for the Houben–Hoesch
alkylation of chloro acetonitrile with aniline. This was found through the screening
of several Lewis acids coupled with the use of Sugasawa conditions for ortho
direction. Following alkylation, the resultant imine was hydrolyzed to the phenone,
and then thermal cyclization followed by reduction led to indole. In the second
synthesis of [phenyl ring-U-14C]indole, we utilized microwave-enhanced conditions for
the key coupling. In this transformation, aniline was alkylated with the acetal of
bromo acetaldehyde. The mono-alkylated aniline was then transformed in a
straightforward manner to indole. Copyright # 2006 John Wiley & Sons, Ltd.
Received 12 October 2005; Revised 5 March 2006; Accepted 6 March 2006
Key Words: indole; Houben–Hoesch; directed acylation; aniline; microwave
Introduction
Heterocycle synthesis is a cornerstone in the synthesis of isotopically labelled
compounds. This is due to the fact that a majority of labelled compounds are
built as radiotracers for adsorption, distribution, metabolism and excretion
(ADME) studies, where it is critical that label placement be located in a
metabolically stable position.1 The indole ring is present in many pharma-
ceutically active compounds,2 and recently, in the course of our work, it
became necessary to synthesize an isotopically labelled indole core and
incorporate it in a drug candidate being pursued for development.
*Correspondence to: Carrie A. B. Wager, Chemical Research and Development, Pfizer Global Research and
Development, Eastern Point Road, Groton, CT 06340, USA. E-mail: carrie.b.wager@pfizer.com
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C. A. B. WAGER AND S. A. MILLER
Labelled [phenyl ring-U-14C]aniline is relatively inexpensive and by nature
the radiolabelled components are encapsulated in a metabolically stable
aromatic ring. These two features are critical when faced with a multi-step
synthesis to the targeted candidate, because more than a few milliCuries of
activity are necessary to carry the synthesis to completion. Therefore, our
research focused on aniline as our primary building block for the synthesis of
unsubstituted indole.
Of the few known options for the synthesis of labelled indole, one synthesis
involves directed lithiation of acylated aniline (Scheme 1).3 This method was
not amenable to the small scale we were working on, due to variable results
based on the need to keep the system rigorously anhydrous. The second
method is an alkylation of the aniline and then cyclization of the resultant
secondary amine to indole (Scheme 2).4 Because the yield of our initial
alkylation of aniline was poor, we set aside this synthetic approach to focus on
the ortho-directed acylation approach (Scheme 3). However, the attractive and
straightforward nature of the alkylation route was worthy of further
consideration.
When an isotope literature search failed to provide a robust and efficient
route, we returned to search for non-labelled options. Most syntheses require
substituted anilines or produce substituted indole products, e.g. the Fisher
indole synthesis. In unsubstituted cases early references for indole synthesis
Scheme 1.
Scheme 2.
Scheme 3.
Copyright # 2006 John Wiley & Sons, Ltd.
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DOI: 10.1002/jlcr

TWO ROBUST, EFFICIENT SYNTHESES OF [PHENYL RING-U-14C]INDOLE
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utilize Friedel–Crafts or Houben–Hoesch chemistry which use either acyl or
nitrile electrophiles, respectively.5 These methods lacked regioselectivity and
therefore the yield of the desired product was moderate and required difficult
separations. Sugasawa overcame the issue of regioselectivity, through the use
of a boron tether to give exclusive alpha substitution.6 This regioselectivity was
a big improvement over existing syntheses, however, yields were moderate
warranting further research. Some investigators extended this method through
the use of boron coordination and additional Lewis acids.7 These served as our
starting point. As the majority of these systems were stacked in favor of
electrophilic aromatic substitution having electron-rich substrates, they were
not ideal for our substrate, aniline, which is only weakly electron rich. We
chose to optimize this reaction chemistry through the use of additional Lewis
acids. We decided that we would take advantage of the Sugasawa method for
ortho direction, and develop optimal conditions for aniline that would boost
reaction rate and yield. We focused our efforts on screening various Lewis
acids to accomplish our goal (increased reaction rate and improved yields.)
The work described herein was performed with cold or stable labelled
reagents.
Discussion
The first pivotal experiments included Sugasawa conditions for ortho direction
and (1) use of an additional amount of boron trichloride as the Lewis acid and
(2) use of aluminum trichloride as an additional Lewis acid. Results with extra
boron trichloride were not promising. We achieved at best only a 3% yield.
The use of an additive (aluminum trichloride) was much more promising. We
were able to achieve the desired acylated product in a moderate 36% yield
(Scheme 4). Although a moderate yield, we believed it to be a promising start
and therefore initiated screening of various Lewis acids using an Argonaut
AS2410 parallel reaction workstation.
Our decision to use aluminum, antimony, titanium, iron and zinc as Lewis
acids came from perusal of the Houben–Hoesch and Friedel–Crafts literature.
Upon screening various Lewis acids, two clearly came out on top of the rest:
Scheme 4.
Copyright # 2006 John Wiley & Sons, Ltd.
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Table 1. Various Lewis Acid Additives with the Houben-Hoesch Acylation of Aniline
titanium tetrachloride and zinc dichloride (Table 1). Both reactions gave
moderate to good yields of the isolated desired product.
Clearly, additional equivalents of the same Lewis acid (i.e. additional boron
trichloride) were not beneficial in our hands. This could be due to several
factors, one being the formation of borate complexes that just do not respond
in these reaction conditions. Interestingly, both antimony and iron gave low
amounts of the desired product and a significant amount of by-products.
Antimony pentachloride was chosen due to its strong Lewis acid character.
It is known that antimony pentachloride can be beneficial even in weakly
electron-rich systems.8 However, it is also known that this Lewis acid can
produce nitrile homo-coupled products5b. Initial results with titanium
tetrachloride looked promising, however, this reaction was slower than we
would like. Historically, both aluminum and zinc have been used in Houben–
Hoesch chemistry: the original Gatterman reaction with cyanic acid used
aluminum trichloride and the initial report of trapping alkyl nitriles by Hoesch
occurred with the aid of zinc dichloride. Even though aluminum trichloride
started to look promising, this reaction stalled after a few hours. Nonetheless,
we were very excited to find the addition of zinc dichloride gave the desired
product in excellent yield (79%). In the case of zinc, both its Lewis acid and
dehydrating properties are mutually beneficial here.
The optimal conditions described above were applied to the synthesis of
both the [phenyl ring-U-14C]indole and then the [phenyl ring-U-14C]-indole-
3-carboxaldehyde, see Scheme 5. A representative ‘Experimental’ section is
presented at the end of this article. The use of inexpensive [phenyl ring-
U-14C]aniline hydrochloride and optimal conditions afforded two isotopically
labelled indole compounds in good yield.
The attractive and straightforward nature of the alkylation route to
unsubstituted indole was revisited with the idea of improving the initial
Copyright # 2006 John Wiley & Sons, Ltd.
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DOI: 10.1002/jlcr

TWO ROBUST, EFFICIENT SYNTHESES OF [PHENYL RING-U-14C]INDOLE
619
Scheme 5.
Scheme 6.
Table 2. Optimization on the Alkylation of Aniline
alkylation step (Scheme 6). This route is promising with the exception of the
first step in the transformation. With extended reaction times and elevated
temperature, we saw this alkylation step as a perfect candidate for microwave
conditions. As seen in Table 2, initial reaction conditions (Reaction 1) show
24% product after 3 days. Without changing the reaction components at all,
however, employing the microwave we were able to achieve a much better
yield, 55%, and drop the reaction time to 0.5 h (Table 2, Reaction 2).
Switching to an inorganic base, sodium bicarbonate under thermal conditions,
we saw improvement on the amount of isolated desired product, although still
requiring a 3 day (Reaction 3). Again, with utilization of microwave
technology, we were able to drop the reaction time from 3 days to half an
Copyright # 2006 John Wiley & Sons, Ltd.
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C. A. B. WAGER AND S. A. MILLER
hour with a further improvement in the isolated yield of the desired product.
Finally, one last variation on base, sodium hexamethyldisilazide, (Reaction 5,
Table 2) resulted in desired product in 69% yield. In summary, there are two
sets of reaction conditions that are optimized: use of sodium bicarbonate as
base with the microwave instrument and use of sodium hexamethyldisilazide
using conventional thermal conditions. Consequently, depending on what
ability is available, then reaction conditions can be chosen accordingly.
Conclusion
Through use of microwave, parallel, and conventional reaction technology, we
have been able to evaluate and determine two robust and efficient systems for
the synthesis of labelled indole. By utilizing a microwave, we have been able to
optimize a route to indole. This route relies on the alkylation of aniline with
the acetal of bromo acetaldehyde. After screening various Lewis acids, we have
found the use of zinc dichloride in addition to boron trichloride allows for the
efficient trapping of chloro acetonitrile by aniline. These reactions are
amenable to small or large scale and are not dependant on an anhydrous or
rigorously inert atmosphere. Both reactions have been optimized to possess a
straightforward reaction processing, by nature of a very clean reaction profile.
We took advantage of an excess of inexpensive reagents, chloro acetonitrile
and the acetal of bromo acetaldehyde. These easily accessible reagents were
coupled with a relatively inexpensive and metabolically stable label position in
aniline. Additional experiments are ongoing in our laboratory to further
elucidate these reaction sequences.
Experimental
[phenyl ring-U-14C]-1-(2-aminophenyl)-2-chloroethanone
[phenyl ring-U-14C]Aniline hydrochloride (200 mCi, 1.6 mmol, source: GE
Biosciences) was suspended in dichloroethane (10 ml) under a nitrogen
atmosphere. Boron trichloride in dichloromethane (5.2 ml, 5.2 mmol) was
added dropwise at room temperature. The mixture was stirred until the
reaction was homogeneous. Chloro acetonitrile (350 ml, 5.5 mmol) was added,
followed by zinc dichloride (870 mg, 6.4 mmol). The now heterogeneous
mixture was stirred at 758C for 22 h. After cooling, 1 N hydrochloric acid
(10 ml, 10 mmol) was added and a yellow precipitate was formed. The mixture
was warmed to 758C for 1 h to ensure ketamine hydrolysis. The reaction was
then cooled to room temperature and the aqueous solution was extracted with
dichloromethane (5 ꢀ 10 ml), the organics were dried (MgSO4), and concen-
trated to give 130 mCi of the crude product. TLC (100% dichloromethane)
showed the desired product to be 92% radiochemically pure ðRf ¼ 0:6Þ(Bios-
can). Silica gel chromatography gave 122 mCi of [phenyl ring-U-14C]-
Copyright # 2006 John Wiley & Sons, Ltd.
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TWO ROBUST, EFFICIENT SYNTHESES OF [PHENYL RING-U-14C]INDOLE
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1-(2-aminophenyl)-2-chloroethanone, 97% radiochemical purity (HPLC),
GCMS gave mass units of 169 m/z (unlabelled) and 183 m/z (six labels).
[phenyl ring-U-14C]-Indole
To a stirred solution of [phenyl ring-U-14C]-1-(2-aminophenyl)-2-chloroetha-
none, (122 mCi, 0.98 mmol) in dioxane (5 ml) containing water (0.5 ml) was
added sodium borohydride (34 mg, 0.90 mmol). The solution was held at reflux
for 17 h. After removal of solvent, the reaction was partitioned between
dichloromethane and water. The aqueous layer was extracted (5 ꢀ 10 ml) with
dichloromethane. The organics were dried (MgSO4) and concentrated to give
110 mCi of crude product, 81% radiochemically pure by HPLC. Silica gel
chromatography (10% ethyl acetate/hexanes) gave 85 mCi of the desired
[phenyl ring-U-14C]-indole, >99% radiochemically pure (HPLC). GCMS gave
masses of 117 m/z (unlabelled) and 131 m/z (six labels).
[phenyl ring-U-14C]-Indole-3-carboxaldehyde
[phenyl ring-U-14C]-indole (85 mCi, 0.68 mmol) was dissolved in dimethylfor-
mamide (200 ml) under a nitrogen atmosphere. In a separate tear-shaped flask
under a nitrogen atmosphere, 340 ml of dimethylformamide was cooled to 08C,
and then phosphorous oxychloride (100 ml, 1.1 mmol) was added. The
presumed Vilsmeier–Haack solution was syringed into the reaction flask
containing the aforementioned [phenyl ring-U-14C]-indole solution. The
resulting reaction mixture was then heated to 358C for 1 h, resulting in a
tan-colored precipitate. Water (500 ml) was added to the mixture to dissolve
the solids, followed by 4.8 N NaOH, (300 ml) dropwise, and then an additional
amount (700 ml) at once. This brown, clear liquid was then heated to reflux for
5 min. After cooling the reaction was extracted with ethyl acetate (5 ꢀ 10 ml)
and the organics dried (MgSO4), filtered, and concentrated to give 75 mCi of
crude product, 87% radiochemically pure (HPLC). Silica gel chromatography
(10% ethyl acetate/hexanes) gave 59 mCi of [phenyl ring-U-14C]-indole-
3-carboxaldehyde >99% radiochemically pure by HPLC. GCMS gave mass
units of 144 m/z (unlabelled) and 158 m/z (six labels).
N-(2,2-Diethoxyethyl)benzenamine
Aniline hydrochloride (125 mg, 0.97 mmol) was suspended in 95% ethanol/
water (3 ml) in a microwave safe 2.5 ml glass vial with magnetic stir bar.
Triethylamine (400 ml, 2.9 mmol) was added, followed by 2-bromo-1,
1-diethoxyethane (180 ml, 1.2 mmol). The vial was sealed and placed in a
Biotage Initiator1 8 microwave instrument. The reaction was stirred for 10 s,
followed by irradiation at high absorption to a temperature of 1808C for
30 min. After cooling, the reaction was concentrated and partitioned between
Copyright # 2006 John Wiley & Sons, Ltd.
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ethyl acetate (10 ml) and water (10 ml). The aqueous was extracted with ethyl
acetate (3 ꢀ 10 ml), the organics were dried (MgSO4), and concentrated to give
the crude oil (316 mg). Silica gel chromatography (10% ethyl acetate/hexanes)
gave N-(2,2-diethoxyethyl)benzenamine (113 mg, 56% yield).
N-(2,2-Diethoxyethyl)benzenamine
Aniline (410 ml, 4.5 mmol) was dissolved in tetrahydrofuran (20 ml). One molar
sodium hexamethydisylazide in tetrahydrofuran (4.5 ml, 4.5 mmol) was added
dropwise. After stirring at room temperature for 2 h, 2-bromo-1,1-diethox-
yethane (810 ml, 5.4 mmol) was added dropwise. The reaction was allowed to
stir for 12 h. The reaction was concentrated and partitioned between ethyl
acetate (20 ml) and water (20 ml). The organic was dried (MgSO4), and
concentrated to give a crude oil (1.42 g). Silica gel chromatography (10% ethyl
acetate/hexanes) gave N-(2,2-diethoxyethyl)benzenamine (651 mg, 69% yield).
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J Label Compd Radiopharm. 2006; 49: 615–622
DOI: 10.1002/jlcr