Structure-activity relationships for ketamine esters as short-acting anaesthetics

Article abstract of DOI:10.1016/j.bmc.2013.06.047

A series of aliphatic esters of the non-opioid anaesthetic/analgesic ketamine were prepared and their properties as shorter-acting analogues of ketamine itself were explored in an infused rat model, measuring the time after infusion to recover from both the anaesthetic (righting reflex) and analgesic (response to stimulus) effects. The potency of the esters as sedatives was not significantly related to chain length, but Me, Et and i-Pr esters were the more dose potent (up to twofold less than ketamine), whereas n-Pr esters were less potent (from 2- to 6-fold less than ketamine). For the Me, Et and i-Pr esters recovery from anaesthesia was 10-15-fold faster than from ketamine itself, and for the n-Pr esters it was 20-25-fold faster than from ketamine. A new dimethylamino ketamine derivative (homoketamine) had ketamine-like sedative effects but was slightly less potent than, but ester analogues of homoketamine had very weak sedative effects.

Full text of DOI:10.1016/j.bmc.2013.06.047

Bioorganic & Medicinal Chemistry 21 (2013) 5098–5106
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Structure–activity relationships for ketamine esters as short-acting
anaesthetics
Jiney Jose a, Swarna A. Gamage a, Martyn G. Harvey b, Logan J. Voss b, James W. Sleigh b, William A. Denny a,
⇑
a Auckland Cancer Society Research Centre, School of Medical Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand
b Waikato Clinical School, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of aliphatic esters of the non-opioid anaesthetic/analgesic ketamine were prepared and their
properties as shorter-acting analogues of ketamine itself were explored in an infused rat model, measur-
ing the time after infusion to recover from both the anaesthetic (righting reflex) and analgesic (response
to stimulus) effects. The potency of the esters as sedatives was not significantly related to chain length,
but Me, Et and i-Pr esters were the more dose potent (up to twofold less than ketamine), whereas n-Pr
esters were less potent (from 2- to 6-fold less than ketamine). For the Me, Et and i-Pr esters recovery from
anaesthesia was 10–15-fold faster than from ketamine itself, and for the n-Pr esters it was 20–25-fold
faster than from ketamine. A new dimethylamino ketamine derivative (homoketamine) had ketamine-
like sedative effects but was slightly less potent than, but ester analogues of homoketamine had very
weak sedative effects.
Received 22 April 2013
Revised 13 June 2013
Accepted 19 June 2013
Available online 27 June 2013
Keywords:
Ketamine
Esters
Anaesthesia
Short-acting
Structure–activity relationship
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
The most clinically significant adverse effect of 1 is its halluci-
nogenic properties which, together with its relatively long half-life
Racemic (2-(2-chlorophenyl)-2-(methylamino)cyclohexanone
(ketamine; 1; Fig. 1) is an effective and widely-used non-opioid
anaesthetic/analgesic.1,2 With respect to its analgesic activity it is
(2–3 h) means that it is normally administered together with sed-
ative or hypnotic drugs like midazolam and/or propofol to control
the prolonged period of post-anesthesia hallucinations.6,7 While
the (S)-enantiomer (1S) has somewhat faster elimination than
the racemic material,6 there is still a need for analogues with much
shorter half-lives to avoid the concomitant use of sedatives/
hypnotics.
thought to act primarily at N-methyl-D-aspartate (NMDA) receptors
as a non-competitive antagonist of the calcium channel pore, but
also has effects on a wide variety of other muscarinic and monoam-
inergic receptors.3 Ketamine’s major advantages over opioids are a
lack of respiratory depression or hyperalgesic effects (it also has a
primary role in pain management as an ‘antihyperalgesic’ or ‘toler-
ance-protective’ compound4), and an absence of longer-term effects
such as increased tolerance and immune suppression. Ketamine is
normally used as the (cheaper) racemate, but more recently the
more active (S)-enantiomer (1S) has begun to be employed. (S)-Ket-
amine has similar pharmacological, analgesic and anaesthetic prop-
erties to the racemate, but is about twice as potent.5
The limited structure–activity relationships of analogues of 1
have shown that its anaesthetic effects are related closely to its
physicochemical properties, with its (more polar) secondary
6-hydroxy metabolite (2) having no anaesthetic properties.8 This
suggested to us that an ester with similar lipophicity to 1 might
retain desirable anaesthetic properties, but would be rapidly
hydrolysed by serum esterases to the corresponding very polar
and thus non-anaesthetic ionised acid. This concept has been ap-
plied successfully to the sedative-hypnotic drug etomidate (3)
which, as a concomitant very potent inhibitor of 11b-hydroxylase,
has the side-effect of prolonged adrenocortical suppression.
Development of the shorter-acting ester derivative methoxycar-
bonyl-etomidate (MOC-etomidate; 4)9 which is readily hydrolysed
to the inactive acid 5 (Fig. 1) resulted in faster recovery from both
hypnosis and adrenal suppression. A similar strategy has also been
followed in the development of the fast-acting analgesic opioid
remifentanil, where a ester moiety is rapidly hydrolysed to an inac-
tive carboxylic acid metabolite.10
OH
O
O
O
O
Cl
Cl
O
Cl
Me
R
N
NHMe
NHMe
(ketamine)
NHMe
N
1
1S
2
3: etomidate: R = Et
4: MOC-etomidate: R = (CH2)2CO2Me
5: MOC-etomidateacid: R = (CH2)2CO2H
Abbreviations: DCM, dichloromethane; LRR, loss of righting reflex; NMDA, N-
methyl-D-aspartate; PWR, pedal withdrawal reflex score.
⇑
Corresponding author. Tel.: +64 9 923 6144; fax: +64 9 3737 502.
E-mail address: b.denny@auckland.ac.nz (W.A. Denny).
0968-0896/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmc.2013.06.047

J. Jose et al. / Bioorg. Med. Chem. 21 (2013) 5098–5106
5099
A
B
1.0
0.5
0.0
1.0
0.5
0.0
0
10
20
30
40
0
10
20
30
40
time (min)
time (min)
Figure 1. Time-course for anaesthesia (loss and recovery of righting reflex) for compounds 10 (A) and 14 (B). The grey panel shows the duration of drug infusion
(measurement taken every minute). dddd: test compound. jjjj: Ketamine.
Table 1
2. Results and discussion
Physiciochemical properties of ketamine esters
O
2.1. Chemistry
Cl
N
X
The compounds of interest were prepared from norketamine
(25), which was in turn prepared from commercially available
R
c
No.
X
R
Puritya (%)
clogPb
pKa
(2-chlorophenyl)(cyclopentyl)methanone (22) following
a re-
ported procedure11 (Scheme 1). Bromination of 22 with CuBr2 gave
the bromide 23, which was converted to the imine 24 with NH4OH/
NH3, and thermal rearrangement of the hydrochloride salt of 24 in
Dowtherm A gave racemic norketamine (25). Resolution of this via
the L-(R,R)-(+)-tartaric acid salts gave (25S). Acetates 7 and 7S of
Table 1 were prepared by reaction of amines 25 or 25S, respec-
tively, with 3-bromopropyl acetate, and NaOH hydrolysis of 7S
gave alcohol 6S. Similar reaction of 25 or 25S with the appropriate
alkyl halides Br(CH2)nCO2(CH2)R and conversion of the products to
the hydrochloride salts with HCl gas (Scheme 1) gave compounds
8–17 of Table 1. The ‘homoketamines’ 18 and 18S were obtained
by treating a methanolic solution of norketamines 25 and 25S suc-
cessively with sodium cyanoborohydride and then formaldehyde
for 24 h (Scheme 2). Finally, compounds 19–21 were obtained by
reaction of ketamine ester 1 with sodium cyanoborohydride and
then formaldehyde for 24 h (Scheme 2).
The structures and physicochemical properties of the ketamine
analogues prepared are given in Table 1. Their lipophilicities
(clogP) were calculated using ChemBioDraw v12.02 (Cambridge-
Soft, UK) and pKa values were calculated using ACD/PhysChem
Suite v12; ACD/Labs, Toronto, Canada). Ketamine (1) has a mea-
sured12 aqueous pKa of 7.49 and a calculated clogP of 2.22. The
closest match to this were the acetates (7, 7S), which were evalu-
ated since acetate hydrolysis to the more polar alcohols is known
1
6S
7
7S
8
8S
9
9S
9R
10
11
12
13
14
14S
15
16
17
18
18S
19
20
21
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
(CH2)3OH
3.20d
2.85
3.76
3.76
3.93
3.93
4.24
4.24
4.24
4.46
4.29
4.60
4.82
3.74
3.74
4.27
4.58
4.80
3.49
3.49
4.40
4.29
4.04
7.49
6.20
6.20
6.20
4.35
4.35
4.35
4.35
4.35
4.35
5.86
5.86
5.85
6.29
6.29
6.29
6.29
6.29
5.86
5.86
4.77
5.51
5.74
97.0
97.2
95.8
97.2
99.1
99.0
99.5
99.5
99.0
95.3
98.4
97.2
99.1
97.0
94.4
97.6
95.4
98.5
99.7
93.0
94.0
94.8
(CH2)3OAc
(CH2)3OAc
(CH2)2CO2Et
(CH2)2CO2Et
(CH2)2CO2iPr
CH2)2CO2iPr
CH2)2CO2iPr
(CH2)2CO2nPr
(CH2)3CO2Et
(CH2)3CO2iPr
(CH2)3CO2nPr
(CH2)4CO2Me
(CH2)4CO2Me
(CH2)4CO2Et
(CH2)4CO2iPr
(CH2)4CO2nPr
Me
H
H
Me
Me
Me
Me
Me
Me
(CH2)2CO2Et
(CH2)3CO2Et
(CH2)4CO2Me
a
b
c
Purity by reverse-phase HPLC.
clog p calculated using ChemBioDraw Ultra v12.02.
pKa calculated using ACD/PhysChem Suite v12.
pKa Measured value from Ref. 9.
d
O
O
Cl
Cl
Cl
O
Cl
O
i
Br
compounds 8-
17, 8S, 9S, 9R,
14S of Table 1
i
NH2
NMe2
22
23
18
25
25S
vi
18S
ii
vi
O
O
Cl
Cl NH
OH
Cl
O
O
Cl
Cl
iii
v
i
NH2
NH2
HCl
24
NHR
N
25S
25
R
Me
19: R=(CH2)2CO2Et
20
vii
O
1
Cl
:
R=(CH2)3CO2Et
7S
: R=OAc
6S: R=OH
21: R=(CH2)4CO2Me
viii
OR
HN
Scheme 2. Reagents and conditions: (i) NaCNBH3, HCHO, AcOH, MeOH, 25 °C, 24 h.
Scheme 1. Reagents and conditions: (i) CuBr2, EtOAc, reflux, 3 h; (ii) (a) NH3/
NH4OH, 25 °C, 5 days; (b) HClg, isopropanol/diethyl ether, 0 °C, 3 h; (iii) dowtherm
A, 200 °C, 12 min; (v) L-(R,R)-(+)-tartaric acid, Me2CO, 3x crystallisation; (vi)
Br(CH2)nCO2R (R = Me, Et, iPr, nPr), KI, K2CO3, MeCN; (vii) Br(CH3)2OAc, KI, K2CO3,
MeCN; (viii) 0.2 N NaOH, MeCN, 25 °C, 2 h.
to be reasonably rapid in blood.13 Next closest in physicochemical
properties were the C4 methyl esters (14, 14S). The esters overall

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J. Jose et al. / Bioorg. Med. Chem. 21 (2013) 5098–5106
showed a range of both pKa values (from 4.35 to 6.29) and lipophi-
licities (from 2.77 to 3.92), so that any dependency of activity on
these parameters could be evaluated.
maintain loss of righting reflex (LRR) for 10 min. The PWR is a
standardised response to a stimulus, and is a measure of analgesic
effect. LRR is a measure of anaesthetic hypnotic effect. Following an
initial single-animal exploratory study, three rats were used in a
confirmatory study, with each group of rats also acting as their
own ketamine control. Data were collected on time (seconds),
and total dose of drug (mg/kg), to achieve LRR and a PWR = 1. Also
recorded were the times (in seconds, from cessation of the infu-
sion) to recovery of righting reflex (RRR) (recovery from the hyp-
notic anaesthesia effect), and time to normalisation of the PWR
(loss of analgesic effect). Full definitions of LRR and PWR are given
2.2. Biology
The compounds were evaluated for their ability to anaesthetise
rats when administered by continuous intravenous infusion, with
the results given in Tables 2 and 3. Compounds were administered
so as to initially deliver 20 mg/kg/min (weight-adjusted flow) to
uniform pedal withdrawal reflex score (PWR = 1), then titrated to
Table 2
Anaesthetic effects of ketamine esters in a rat infusion study, compared head-to-head each time to racemic ketamine (1) as a positive control
No.
LRRa
PWR = 1b
RRRc
Walkd
Timee (sec)
Dosef (mg/kg)
Timee (sec)
Dosef (mg/kg)
Timee (sec)
Timee (sec)
1
6S
69
59
7
9
23
21
2
3
84
102 16
4
28
34
2
4
874 81
341 60
1384 374
588 108
1
7S
59
81 10
5
20
26
2
3
93
104
8
7
31
34
2
1
863 153
566 30
1918 518
983 93
1
8
77 10
135 61
27
4
95 13
154 68
38 10
55 25
1602 549
177 51
2536 250
253 68
48 23
20
59 19
1
8S
53
1
1
73
3
26
1
1315 215
2163 722
171 54
185 55
62 20
62
8
80
8
1
9
62
103 17
7
22
33
2
6
71
127 13
9
24
37
3
6
760 144
83 19
1100 144
153 33
1
9S
65 10
222 18
30
74
4
9
77 12
247 13
34
84
4
4
900 60
15 15
1200 180
224 90
1
9R
53
170
24
71
69
190
26
82
1170
0
1629
900
1
10
51
404 196
3
18
131 62
1
63
420 180
3
21
136 67
1
1060 221
0
1500
5
0
1
11
59
70
6
6
17
24
2
2
76
137 14
7
24
44
4
5
1523 131
95 12
2122 131
170 17
1
12
69
125 21
7
23
42
2
9
84
192 47
4
28
2
874 81
37 22
1384 374
110 45
66 17
28
209 20
22
44 10
25
44 34
23
1
13
69
329 106
7
23
137 48
2
84
558 42
4
2
874 81
10 5.5
1384 374
65 33
1
14
70 28
92 14
20
34
5
7
81 23
124 21
4
1104 95
99 16
1462 113
126 32
1
14S
55
107
2
9
20
36
1
3
72
130 10
2
1
1286 127
95 18
1740 282
900 30
1
15
56
97
6
8
17
34
2
4
76
172 36
7
4
1523 131
134 22
2122 107
112 18
57 12
1
16
54
122
19
38
66
122
22
38
1281
180
1499
320
1
17
65 10
271 97
23
82 34
3
77 12
277 97
26
83 34
4
900 60
45 15
1200 180
90 30
1
18
56
125
19
43
60
130
21
43
480
1500
900
1740
1
18S
53
94
20
32
69
103
21
35
1170
880
1620
1215
1
19
60
280
21
95
69
290
25
99
1230
0
1475
0
1
20
75
600
25
196
86
600
29
196
1500
0
1920
0
1
21
57
420
20
145
82
435
28
148
2040
0
2340
201
a
LRR: (loss of righting reflex) assesses anaesthetic hypnotic effect. Righting reflex is considered absent when the animal fails to right from a position of dorsal recumbency
to a position of sternal recumbency on three attempts performed in rapid succession.
b
PWR (pedal withdrawal reflex) assesses analgesic effect, and is conducted by a 1-second application of firm constant pressure (for rats, firm digital pressure) over the
forepaw of the animal. A PWR = 1 (a flicker of response) indicates a satisfactory level of analgesia (nociception).
c
RRR (recovery of righting reflex; ability to right from dorsal recumbency).
Walk (ability to sustain independent locomotion).
Time: the time from onset of the infusion of drug to achieve LRR or PWR = 1, or the time from the end of the infusion of drug to achieve RRR or walk.
Dose: the total drug administered to achieve LRR or PWR = 1. Where no errors are given (compounds 9R, 18, 18S, 19 and 21) the results are from a single animal.
d
e
f

J. Jose et al. / Bioorg. Med. Chem. 21 (2013) 5098–5106
5101
Table 3
compound (14). Loss of righting are shown in Figure 1a and b,
respectively, and pedal withdrawal scoring shown in Figure 2a
and b. Figure 3 depicts a scatter-plot of effective potency (dose
[mg/kg] to loss of righting reflex) versus duration (time to return
of righting reflex) for all compounds.
Head-to-head ratios of the anaesthetic properties of ketamine esters to ketamine
No.
LRRa
PWR = 1b
Timee
RRRc
Walkd
Timee
Timee
Dosef
Dosef
Timee
6S
7S
8
8S
9
0.86
1.37
1.75
3.22
1.66
3.41
3.20
7.92
1.19
3.23
1.81
4.77
1.31
1.94
1.73
2.26
4.17
2.23
1.77
4.67
8.00
7.37
0.91
1.30
1.78
2.95
1.50
2.46
2.96
7.28
1.41
2.95
1.83
5.95
1.70
1.80
2.00
2.00
3.56
2.26
1.60
4.50
7.84
7.25
1.21
1.12
1.62
2.53
1.79
3.14
2.75
6.67
1.80
2.53
2.29
6.64
1.53
1.81
2.26
1.85
3.60
2.17
1.49
4.20
6.98
5.30
1.21
1.10
1.45
2.38
1.54
2.47
3.15
6.48
1.83
2.38
2.36
7.46
2.00
1.76
2.48
1.73
3.19
2.05
1.67
3.96
6.76
5.29
0.39
0.65
0.11
0.05
0.11
0.02
NAg
0.42
0.51
0.10
0.04
0.14
0.19
0.06
NCh
2.3. Structure–activity relationships
The acetate 7S was the most potent of the compounds studied
(about as potent as ketamine), but showed only moderately faster
recoveries (1.5- to 2-fold) than ketamine itself. This is most likely
not due to slow acetate hydrolysis, but to the fact that the alcohol
product 6S is itself a potent hypnotic/analgesic (the most potent of
the compounds evaluated here), even though it was no more polar
(clogP 2.10) than 7S itself.
However, the norketamine esters proved to be a more useful
class. The more potent compounds (up to twofold less dose-potent
than ketamine itself) 8, 9, 11, 12, 14, 15 and 16 comprised a series
of esters with chain lengths across the whole range studied (thus
the full range of pKas) and a variety of Me, Et and iPr esters. In con-
trast, compounds 10, 13 and 17 (from 2- to 6-fold less potent than
ketamine) also contained a wide range of different chain lengths,
but were all n-Pr esters, and at the higher end of the lipophilicity
range. Thus the potency of the esters was more dependent on
the nature of the ester functionality than its separation from the
amine moiety. Since dose-potency and rapidity of recovery from
both LRR and PWR are broadly reciprocal, it is not surprising that
the norketamine esters resulting in fastest recovery (20–25-fold
faster than ketamine) are in each case the same n-Pr esters 10,
13 and 17.
The bulk of the norketamine esters explored were racemic, but
in three cases the S-enantiomers were also evaluated (8S, 9S, and
14S), since (S)-ketamine (1S) is known to be as active but about
twice as potent as its racemate [5]. However two of the S-enantio-
mer esters (8S, 9S) while they were active, were only half as potent
as the corresponding racemates and showed faster recoveries, sug-
gesting more rapid hydrolysis of the S-enantiomer esters. The 9R
enantiomer was also prepared and had similar potencies and kinet-
ics of recovery to 9S. The third pair (14/14S) had broadly equivalent
properties.
9S
9R
10
11
11S
12
13
14
14S
15
16
17
18
18S
19
20
21
NCh
0.062
0.05
0.042
0.012
0.090
0.07
0.09
1.78
0.05
3.12
0.75
NCh
0.08
0.04
0.08
0.05
0.086
0.51
0.05
1.17
0.08
1.88
0.75
NCh
NCh
NCh
0.09
NCh
a-fAs for Table 2.
g
NA: not active.
NC: not calculable.
h
in the ⁄⁄Experimental Section. For the ketamine standard, the
average values of the parameters measured over the different
experiments are given in Table 4. Given the complexity of the
experimental protocol, the pre-sedation data (time and total dose
for LRR; Table 2) are very consistent, with ranges of only 1.5-fold.
The consistency of the post-sedation recovery times are expectedly
lower, with ranges of about 2.5-fold.
Representative plots of compound performance—each com-
pared with their ketamine control—are presented in Figures 1
and 2. As illustrations we have chosen an ultra-rapidly metabo-
lised, relatively non-potent compound (10), and more potent
Table 4
We also prepared the tertiary amine 18 (‘homoketamine’),
which has not been previously reported. The racemic compound
18 exhibited ketamine-like sedative effects but was slightly less
potent than ketamine, with a duration of action somewhat longer
than the average value for ketamine. The (S)-enantiomer 18S was
somewhat more dose-potent than racemic 18 but had broadly sim-
ilar sedative properties. These thus seemed good targets for ester
formation, and the corresponding esters 19–21 were prepared
and exploratory studies were done (mostly by single-animal
Average parameters determined for the ketamine standard
Property
Average
Range
Time to achieve LRR (sec)
Total dose to LRR (mg/kg)
Time to PWR = 1 (sec)
Total dose to PWR = 1 (mg/kg)
Time to RRR (sec)
61
21
76
26
8
51–75
17–27
60–95
21–34
863–2040
1100–2340
7
9
4
1212 318
1709 400
Time to walking (sec)
A
B
4
2
0
4
2
0
0
10
20
30
40
0
10
20
30
40
time (min)
time (min)
Figure 2. Time-course for analgesia (pedal withdrawal reflex score) with representative compounds. Loss and recovery of pedal withdrawal reflex for compounds 10 (A) and
14 (B). The grey panel shows the duration of drug infusion (measurement taken every minute). Error bars are SEM. dddd: Test compound. jjjj: Ketamine.

5102
J. Jose et al. / Bioorg. Med. Chem. 21 (2013) 5098–5106
10000
1000
100
10
18
Ketamine
18S
7S
6S
16
8
15
14
11
14S
9
8S
17
12
9S
13
9R
19
1021
20
1
10
100
1000
log Dose to LRR (mg/kg)
Figure 3. Effective potency versus duration for all compounds. Plot (log10) of effective potency (dose [mg/kg] to LRR) versus duration (time to RRR) for studied compounds.
The alkyl chain length of compounds is denoted by symbol: w = C1; s = C2; d = C3; j = C4; N = ketamine.
evaluations). The racemic C2 ethyl ester 19 was about as potent as
the n-Pr norketamine esters above, but had a very weak sedative
effect, with very rapid recovery. Longer chain lengths were ex-
plored with the C3 ethyl and C4 methyl esters 20 and 21, but these
were even less potent, with very poor sedative activity. There was
no clear effect of pKa on anaesthetic activity, although the weakest
bases (compounds 8–10, 19) were among the least potent of the
norketamine esters.
determined on a Bruker micrOTOFQ II mass spectrometer. Final
products were analysed by reverse-phase HPLC (Alltima C18
5
lm column, 150 mm  3.2 mm; Alltech Associated, Inc., Deer-
field, IL) using an Agilent HP1100 equipped with a diode array
detector. The mobile phase was 80% MeCN/20% H2O (v/v) in
45 mM HCO2NH4 at pH 3.5 and 0.5 mL/min. The purity was deter-
mined by monitoring at 272 nm and was P95% for final products
unless otherwise stated. Enantiomeric purity was analysed by chi-
ral HPLC (Chiralcel OJ-H column, 0.46 cm  45 cm). The mobile
phase was 85% hexanes/15% EtOH with a flow rate of 0.6 mL/
min. The purity was determined by monitoring at 254 and
280 nm and was P95% unless otherwise stated. The final product
purity was also assessed by combustion analysis carried out in
the Campbell Micro analytical Laboratory, University of Otago
(Dunedin, New Zealand). Melting points were determined on an
Electrothermal 2300 Melting Point Apparatus and are uncorrected.
DCM refers to dichloromethane, DMF refers to N,N-dimethylform-
amide, EtOAc refers to ethyl acetate, EtOH refers to ethanol.
2.4. Conclusions
The above results show that short-chain aliphatic ester ana-
logues of ketamine broadly retain its desirable anaesthetic and
analgesic activities, yet are metabolised to the more polar and inac-
tive acids sufficiently rapidly to minimise the drawbacks of keta-
mine itself in this capacity. While the structure activity
relationships for the ester were not straightforward, the results
suggest that the n-propyl esters, and the S-enantiomers tend to
be less potent; likely due to their more rapid hydrolysis.
3.1.1. 3-((1-(2-Chlorophenyl)-2-oxocyclohexyl)amino)propyl
acetate hydrochloride (7) (Scheme 1)
3. Experimental
3.1. Chemistry
(2-Chlorophenyl)(cyclopentyl)methanone
(22)
(10 g,
48.0 mmol) was dissolved in EtOAc (100 mL) followed by addition
of Cu(II)Br2 (27 g, 120.9 mmol). The solution was refluxed for 3 h
and cooled to 25 °C. The solid was filtered and the filtrate was
evaporated under reduced pressure. Some solid began to form
while evaporating solvent under reduced pressure. DCM (100 mL)
was added to the solid formed and solution cooled to 0 °C in an
ice bath. After standing for 10 min. the solution was filtered and
the filtrate concentrated under reduced pressure to obtain
(1-bromocyclopentyl)(2-chlorophenyl)methanone14 (23) as a yel-
low oil (12.3 g, 89%). 1H NMR (CDCl3) d 7.70 (dd, J = 7.4, 1.8 Hz,
1H), 7.43 (dd, J = 7.9, 1.3 Hz, 1H), 7.37 (td, J = 7.4, 1.8 Hz, 1H),
7.30 (td, J = 7.4, 1.3 Hz, 1H), 2.45–2.27 (m, 4H), 2.09–2.01 (m,
2H), 1.89–1.82 (m, 2H); 13C NMR (CDCl3) d 199.47, 138.87,
130.83, 130.55, 130.16, 128.33, 126.49, 74.29, 40.42, 23.26. MS
m/z 289.3 (M2H+, 24 %) 207.4 (MÀBrÀ, 100%).
All reagents and solvents were obtained from commercial sup-
pliers and used without further purification unless otherwise sta-
ted. Reactions requiring anhydrous conditions were performed
under nitrogen atmospheres. Reactions were monitored by thin
layer chromatography (TLC) on preloaded silica gel F254 plates
(Sigma–Aldrich) with a UV indicator. Column chromatography
was performed with Merck 230–400 mesh silica gel. 1H and 13C
NMR spectra were obtained with a Bruker Avance 400 spectrome-
ter at 400 MHz for 1H and 101 MHz for 13C spectra. Spectra were
obtained in CDCl3 or (CD3)2SO. The chemical shifts are reported
in parts per million (d) downfield using tetramethylsilane (SiMe4)
as internal standard. Spin multiplicities are given as s (singlet), d
(doublet), dd (double doublet), br (broad), m (multiplet), and q
(quartet). Coupling constants (J values) were measured in hertz
(Hz). All LC/MS data were gathered by direct injection of methano-
lic solutions into a Surveyor MSQ mass spectrometer using an
atmospheric pressure chemical ionisation (APCI) with a corona
voltage of 50 V and a source temperature of 400 °C. High-resolu-
tion electrospray ionisation (HRESIMS) mass spectra were
Ammonium hydroxide (200 mL) was cooled to 0 °C in an ice
bath and was saturated with NH3 gas for 5 min. The solution was
added to a flask containing 23 (12.74 g, 44.5 mmol) and stirred vig-
orously at 25 °C for 5 days. The brown clumps formed were sepa-
rated from the solvent and resuspended in hexanes (150 mL).
After stirring in hexanes for 4 h, the precipitate formed was filtered

J. Jose et al. / Bioorg. Med. Chem. 21 (2013) 5098–5106
5103
and dried to obtain 24 (8.15 g, 81%) as a pale yellow solid. This was
suspended in 8 mL of 2-propanol and cooled to 0 °C in an ice bath.
HCl gas was bubbled through the solution for 2 min. and Et2O
(16 mL) was added. Upon standing at 0 °C for 3 h a pale yellow pre-
cipitate was formed which was filtered, dried under vacuum to ob-
tain 1-((2-chlorophenyl)(imino)methyl)cyclopentanol15 (24) as the
HCl salt (7.21 g). 1H NMR (CDCl3) d 14.05 (br, 1H), 12.28 (br, 1H),
7.61–7.32 (m, 4H), 2.23 (br, 2H), 1.98 (m, 4H), 1.69 (br, 2H); 13C
NMR (CDCl3) d 195.76, 132.74, 131.42, 130.57, 128.99, 128.85,
126.82, 85.56, 38.78, 23.85 (two Peaks overlapping). MS m/z
224.4 (MH+).
To Dowtherm A (142 mL) heated to 200 °C was added in por-
tions 24 (18 g, 69.2 mmol). The heating was continued for
12 min. and cooled to 0 °C in an ice bath. The reaction mixture
along with precipitate formed was poured into diethyl ether
(500 mL) and allowed to stand overnight. The white precipitate
formed was filtered and washed with Et2O (100 mL). The precipi-
tate was dissolved in water (200 mL) and neutralized with 2 N
NaOH. The water layer was extracted with DCM (3 Â 100 mL),
dried over Na2SO4 and solvent evaporated. The residue obtained
was purified by passing through a short silica gel column eluting
with DCM (100%) to 10% MeOH/DCM to give racemic 2-amino-2-
(2-chlorophenyl)cyclohexanone.15 (norketamine) (25) (9.2 g,
59%). 1H NMR (CDCl3) d 7.69 (dd, J = 7.8, 1.7 Hz, 1H), 7.39–7.33
(m, 2H), 7.26 (td, J = 7.6, 1.6 Hz, 1H), 2.79–2.72 (m, 1H), 2.63–
2.56 (m, 1H), 2.51–2.43 (m, 1H), 2.08–2.0 (m, 1H), 1.88 (br, 1H),
1.81–1.75 (m, 2H), 1.72–1.63 (m, 1H).
A solution of racemic 25 (200 mg, 0.89 mmol), 3-bromopropyl
acetate16 (194 mg, 1.07 mmol), KI (45 mg, 0.27 mmol), K2CO3
(371 mg, 2.7 mmol) was dissolved in MeCN (5 mL). The reaction
mixture was heated to reflux for 24 h. After completion of reaction
the reaction mixture was cooled to room temperature and solvent
evaporated under reduced pressure. The residue was purified by
column chromatography on silica gel eluting with hexanes,
EtOAc/hexanes (40%). The solvent was evaporated under reduced
pressure to obtain the desired product as yellow oil (173 mg,
59%). The yellow oil was dissolved in Et2O (5 mL) and was cooled
to 0 °C in an ice bath. Dry HCl gas was bubbled through the solu-
tion at 0 °C for 2 min. The solvent was evaporated under reduced
pressure to obtain a yellow solid. The yellow solid was taken up
in EtOAc (1 mL) and sonicated at 25 °C for 2 min. The white precip-
itate formed was diluted with EtOAc (5 mL) and filtered, washed
with EtOAc and dried under vacuum to give racemic 7 as the HCl
salt (107 mg, 33%), mp 180–183 °C.1H NMR (CDCl3) d 11.71 (br,
1H), 8.19 (d, J = 7.6 Hz, 1H), 8.09 (br, 1H), 7.58 (m, 1H), 7.46 (d,
J = 4.0 Hz, 2H), 4.15 (m, 1H), 4.08 (m, 1H), 3.81 (dm, J = 12.0 Hz,
1H), 3.19 (br, 1H), 2.74 (d, J = 12.0 Hz, 1H), 2.68–2.60 (m, 2H),
2.47 (br, 1H), 2.28 (t, J = 14 Hz, 1H), 2.12 (br, 2H), 2.09 (s, 3H),
1.84 (br, 2H), 1.54 (br, 1H); 13C NMR (CDCl3) d 206.20, 171.04,
135.14, 132.56, 132.34, 131.79, 129.16, 128.92, 77.49, 62.22,
41.87, 40.63, 40.01, 29.91, 25.84, 21.82, 21.02. MS m/z 324.2
(MH+). Anal. Calcd for C17H23Cl2NO3: C, 56.67; H, 6.43; N, 3.89;
Cl, 19.68. Found: C, 56.49; H, 6.61; N, 3.69.
and recrystallized two additional times in acetone (1750 and
800 mL, respectively) to obtain (S)-2-amino-2-(2-chloro-
phenyl)cyclohexanone (25S) as the tartrate salt, mp: 190–191 °C.
1H NMR (DMSO-d6) d 7.85 (d, J = 7.8 Hz, 1H), 7.39 (t, J = 7.4 Hz,
2H), 7.34 (d, J = 7.5 Hz, 1H), 4.21 (s, 2H), 2.78–2.70 (m, 1H), 2.32
(dt, J = 15.1, 4.4 Hz, 1H), 1.96–1.81 (m, 3H), 1.73–1.60 (m, 2H),
one proton submerged with DMSO-d6 peak; 13C NMR (DMSO-d6)
d 208.6, 173.320, 131.96, 130.28, 129.1 (2), 128.93, 127.09, 71.93,
64.84, 38.31, 37.5, 25.79, 20.84. MS m/z 224.2 (MH+).
The (S)-norketamine tartrate salt was dissolved in water
(200 mL), neutralized with 2 N NaOH. Extraction with DCM (3
 100 mL) gave the free base of 25S (4.96 g) as a pale yellow vis-
cous oil. 1H NMR (CDCl3) d 7.70 (dd, J = 7.8, 1.7 Hz, 1H), 7.38–
7.31 (m, 2H), 7.28–7.23 (m, 1H), 2.79–2.71 (m, 1H), 2.63–2.56
(m, 1H), 2.51–2.43 (m, 1H), 2.08–2.02 (m, 1H), 1.89–1.74 (m,
3H), 1.71–1.63 (m, 1H); 13C NMR (CDCl3) d 212.75, 140.49,
133.02, 131.0, 128.97, 128.32, 127.20, 66.42, 41.25, 38.98, 28.32,
22.16. MS m/z 224.2 (MH+).
A solution of 25S (1 g, 4.47 mmol) was treated with 3-bromo-
propyl acetate, KI and K2CO3 in MeCN as above to give a crude
product that was purified by column chromatography on silica
gel eluting with hexanes, EtOAc/hexanes (40%) to give a yellow
oil (695 mg, 48%). This dissolved in Et2O (20 mL), cooled to 0 °C
in an ice bath and treated with dry HCl gas at 0 °C for 2 min. The
precipitate formed was filtered, resuspended in EtOAc (20 mL),
stirred for 10 min at room temperature and filtered to give 7S as
the HCl salt (512 mg, 29%), mp 169–172 °C. 1H NMR (CDCl3) d
11.87 (br, 1H), 8.20 (d, J = 8.2 Hz, 1H), 7.62–7.54 (m, 1H), 7.48 (d,
J = 3.8 Hz, 2H), 7.39 (br, 1H), 4.12 (q, J = 7.1 Hz, 2H), 3.83 (dm,
J = 14.4 Hz, 1H), 3.11–3.02 (m, 1H), 2.75 (d, J = 12.5 Hz, 1H), 2.70–
2.61 (m, 1H), 2.51 (br, 1H), 2.32 (t, J = 7.3 Hz, 2H), 2.24 (t,
J = 11.1 Hz, 1H), 2.05 (br, 1H), 1.99–1.88 (m, 2H), 1.83 (d,
J = 14.5 Hz, 2H), 1.76–1.61 (m, 3H), 1.24 (t, J = 7.1 Hz, 3H); 13C
NMR (CDCl3) d 205.41, 173.07, 135.21, 132.58, 132.06, 131.67,
129.34, 128.99, 73.20, 60.57, 43.59, 40.82, 39.62, 33.65, 29.68,
26.07, 22.27, 21.77, 14.31. MS m/z 352.2 (MH+). Anal. Calcd for C19-
H27Cl2NO3: C, 58.77; H, 7.01; N, 3.61; Cl, 18.26. Found: C, 58.81; H,
7.1, N, 3.51; Cl, 18.31.
3.1.3. (S)-2-(2-Chlorophenyl)-2-(3-hydroxypropylamino)-
cyclohexanone hydrochloride (6S)
(Scheme 1). A solution of the HCl salt of 7S (360 mg, 1.11 mmol)
in 0.2 N NaOH (20 mL)/MeCN (2 mL) was stirred at 25 °C for 2 h,
then the reaction mixture was made acidic (pH 2–3) by addition
of 1.25 N HCl in MeOH. The solvent was evaporated under reduced
pressure and the white solid obtained was suspended in MeCN
(20 mL) and stirred for 2 h at 25 °C, then filtered and the filtrate
dried over Na2SO4. Evaporation of the solvent under reduced pres-
sure afforded a colourless viscous oil that was suspended in EtOAc
(20 mL) and sonicated for 2 min. The white solid formed was fil-
tered to obtain (S)-2-(2-chlorophenyl)-2-(3-hydroxypropylami-
no)cyclohexanone hydrochloride (6S) (263 mg, 74%), mp 187–
190 °C. 1H NMR (CDCl3) d 10.82 (br, 1H), 8.72 (br, 1H), 8.13 (d,
J = 7.9 Hz, 1H), 7.59–7.54 (m, 1H), 7.49–7.44 (m, 2H), 3.97–3.93
(m, 1H), 3.79 (t, J = 9.3 Hz, 1H), 3.66 (d, J = 12.1 Hz, 1H), 3.21 (br,
1H), 2.85 (br, 1H), 2.70 (d, J = 12.8 Hz, 1H), 2.60 (td, J = 13.1,
6.0 Hz, 1H), 2.39–2.22 (m, 2H), 2.09–2.0 (m, 2H), 1.92–1.79 (m,
3H), 1.58 (q, J = 13.8 Hz, 1H); MS m/z 282.5 (MH+). Anal. Calcd for
3.1.2. (S)-3-((1-(2-Chlorophenyl)-2-oxocyclohexyl)amino)propyl
acetate hydrochloride (7S) (Scheme 1)
Resolution of norketamine was achieved by following a pub-
lished procedure.16 A solution of racemic 25 (13.2 g, 59.1 mmol)
in MeOH (33 mL) was treated with L-(R,R)-(+)-tartaric acid (8.9 g,
59.1 mmol) in MeOH (118 mL). The reaction mixture was stirred
overnight at 25 °C and filtered to remove any solid impurities.
The filtrate was evaporated and the white solid obtained washed
with 2-butanone (264 mL). The solid was suspended in acetone
(1750 mL) and heated to reflux until most of the solid was
dissolved. The solution was cooled to room temperature and
allowed to stand for 2 days. The crystals formed were filtered
C15H21Cl2NO2 : C, 56.61; H, 6.65; N, 4.40. Found: C, 55.21; H,
6.70; N, 4.37.
3.1.4. General procedure for synthesis of N-alkylated
norketamine esters (Scheme 1)
A solution of racemic 25 or 25S (1 equiv), the appropriate alkyl
halide (1.2 equiv or 6 equiv in case of ethyl-3-bromo propionate),
KI (0.3 equiv) and K2CO3 (3 equiv) was dissolved in MeCN

5104
J. Jose et al. / Bioorg. Med. Chem. 21 (2013) 5098–5106
(4.5 mL/mmol). The solution was heated to 80 °C in a sealed tube
for 24 h (72 h in case of ethyl-3-bromo propionate). The reaction
mixture was cooled to room temperature and solvent evaporated.
The residue was purified by column chromatography on silica gel
eluting with hexanes, EtOAc/hexanes (20–35%). The solvent was
evaporated and the HCl salts of the amines were prepared as above
with dry HCl gas, followed by evaporation under reduced pressure
to dryness. The crude products were taken up in EtOAc (2 mL), son-
icated at 25 °C for 2 min, then diluted with EtOAc (10 mL), filtered,
washed with EtOAc and dried under vacuum to obtain the pure HCl
salts. The following compounds were prepared according to this
general procedure:
3.1.9. (R)-Isopropyl 3-((1-(2-chlorophenyl)-2-
oxocyclohexyl)amino)propanoate hydrochloride (r-C2iPr) (9R)
From 25R and isopropyl 3-bromopropanoate (29%), mp 216–
219 °C. 1H NMR (CDCl3) d 12.20 (br, 1H), 8.14 (dbr, J = 8.1 Hz,
2H), 7.60–7.56 (m, 1H), 7.49 (br, 2H), 5.14–5.04 (m, 1H), 3.80
(dm, J = 13.6 Hz, 1H), 3.51–3.44 (m, 1H), 3.26 (br, 1H), 2.73 (br,
2H), 2.64–2.56 (m, 2H), 2.21 (t, J = 13.2 Hz, 1H), 2.06 (br, 1H),
1.89–1.79 (m, 2H), 1.64 (br, 1H), 1.26 (app. dd, J = 4.81, 1.40 Hz,
6H); 13C NMR (CDCl3) d 206.71, 172.36, 135.23, 132.67, 132.51,
131.85, 129.11, 128.39, 73.5, 69.94, 40.24, 39.95, 30.26, 30.13,
21.95, 21.89, 21.85. MS m/z (MH+). HRMS calculated for C18H25-
ClNO3 (MH+) 338.1517, found 338.1521.
3.1.10. n-Propyl 3-((1-(2-chlorophenyl)-2-oxocyclohexyl)-
amino)propanoate hydrochloride (10)
3.1.5. Ethyl 3-((1-(2-chlorophenyl)-2-oxocyclohexyl)-
amino)propanoate hydrochloride (8)
From racemic 25 and propyl 3-bromopropanoate (44%) mp
163–165 °C. 1H NMR (CDCl3) d 11.68 (br, 1H), 8.69 (br, 1H), 8.14
(d, J = 8.0 Hz, 1H), 7.61–7.54 (m, 1H), 7.49 (br, 2H), 4.07 (t,
J = 6.8 Hz, 2H), 3.74 (dm, J = 14.3 Hz, 1H), 3.53–3.43 (m, 1H), 3.38
(br, 1H), 2.81–2.71 (m, 3H), 2.64–2.57 (m, 1H), 2.35 (td, J = 13.8,
3.2 Hz, 1H), 2.07 (br, 1H), 1.92–1.80 (m, 2H), 1.68–1.54 (m, 3H),
0.92 (t, J = 7.4 Hz, 3H); 13C NMR (CDCl3) d 206.03, 172.0, 135.28,
132.48, 132.37, 131.84, 128.99, 128.6, 73.4, 67.22, 40.31, 39.57,
30.28, 29.9, 21.9, 21.81, 10.42. MS m/z 338.2 (MH+). HRMS calcu-
lated for C18H25ClNO3 338.1517, found 338.1526.
From racemic 25 and ethyl 3-bromopropionate (33% yield), mp
199–202 °C. 1H NMR (CDCl3) d 12.24 (br, 1H), 8.13 (d, J = 8.0 Hz,
2H), 7.61–7.54 (m, 1H), 7.49 (d, J = 3.8 Hz, 2H,), 4.23 (q, J = 7.2 Hz,
2H), 3.78 (dm, J = 14.3 Hz, 1H), 3.59–3.45 (m, 1H), 3.25 (q,
J = 5.4 Hz, 1H), 2.73 (br, 2H), 2.68–2.54 (m, 2H), 2.23 (td, J = 13.7,
2.5 Hz, 1H), 2.14–2.02 (m, 1H), 1.89–1.82 (m, 2H), 1.65–1.59 (m,
1H), 1.28 (t, J = 7.2 Hz); 13C NMR (CDCl3)
d 206.10, 171.94,
135.27, 132.57, 132.40, 131.85, 129.05, 128. 57, 73.42, 61.67,
40.30, 39.71, 39.63, 30.38, 29.93, 21.86, 14.18; MS m/z 324.2
(MH+). Anal. Calcd for C17H23Cl2NO3: C, 56.67; H, 6.43; N, 3.89;
Cl, 19.68. Found: C, 56.65; H, 6.57; N, 3.89; Cl, 19.90.
3.1.11. Ethyl 4-((1-(2-chlorophenyl)-2-
oxocyclohexyl)amino)butanoate hydrochloride (11)
3.1.6. (S)-Ethyl 3-((1-(2-chlorophenyl)-2-oxocyclohexyl) amino)
propanoate hydrochloride (s-C2Et) (8S)
From racemic 25 and ethyl 4-bromobutanoate (37% yield), mp
186–189 °C. 1H NMR (CDCl3) d 11.14 (br, 1H), 9.14 (br, 1H), 8.26
(d, J = 8.1 Hz, 1H), 7.62–7.49 (m, 1H), 7.45 (d, J = 3.8 Hz, 2H), 4.11
(q, J = 7.1 Hz, 2H), 3.75 (dm, J = 14.4 Hz, 1H), 3.38–3.26 (m, 1H),
2.75–2.64 (m, 2H), 2.64–2.58 (m, 1H), 2.43–2.26 (m, 2H), 2.45–
2.28 (m, 2H), 2.14–2.07 (m, 1H), 1.98(br, 2H), 1.91–1.79 (m, 1H),
1.58–1.44 (m, 1H), 1.23 (t, J = 7.1 Hz, 3H); 13C NMR (CDCl3) d
206.03, 173.16, 135.11, 132.76, 132.08, 131.61, 129.19, 129.02,
73.17, 61.04, 43.54, 40.86, 40.00, 32.24, 29.72, 21.64, 14.27. MS
m/z 338.2 (MH+). Anal. Calcd for C18H25Cl2NO3: C, 57.76; H, 6.73;
N, 3.74; Cl, 18.94. Found: C, 57.55; H, 6.92; N, 3.64; Cl, 18.73.
From 25S and ethyl 3-bromopropanoate (54%), mp 208–210 °C.
1H NMR (CDCl3) d 12.08 (br, 1H), 8.25 (br, 1H), 8.13 (d, J = 8.0 Hz,
1H), 7.61–7.56 (m, 1H), 7.49 (br, 2H), 4.21 (q, J = 7.2 Hz, 2H), 3.76
(dm, J = 14.3, 3.2 Hz, 1H), 3.55–3.46 (m, 1H), 3.28 (q, J = 9.97 Hz,
1H), 2.75–2.56 (m, 4H), 2.26 (td, J = 14.14 Hz, 1H), 2.08 (br, 1H),
1.90–1.78 (m, 2H), 1.61 (br, 1H), 1.28 (t, J = 7.2 Hz, 3H); 13C NMR
(CDCl3) d 206.35, 172.38, 135.08, 132.39, 132.33, 131.71, 128.91,
128.27, 73.32, 61.75, 40.08, 39.57, 29.91, 29.89, 21.69, 14.02 (1C
overlapping). MS m/z 324.2 (MH+). HRMS calculated for C17H23-
ClNO3 (MH+) 324.1361, found 324.1370.
3.1.12. Isopropyl 4-((1-(2-chlorophenyl)-2-oxocyclohexyl)-
amino)butanoate hydrochloride (12)
3.1.7. Iso-Propyl 3-((1-(2-chlorophenyl)-2-oxocyclohexyl)-
amino)propanoate hydrochloride (9)
From racemic 25 and isopropyl 4-bromobutanoate17 (24%
yield), mp 167–169 °C. 1H NMR (CDCl3) d 11.41 (br, 1H), 8.94 (br,
1H), 8.26 (d, J = 8.1 Hz, 1H), 7.68–7.50 (m, 1H), 7.45 (d, J = 3.9 Hz,
2H), 4.99 (m, 1H), 3.79 (dm, J = 14.4 Hz, 1H), 3.31–3.21 (m, 1H),
2.75–2.68 (m, 1H), 2.66–2.59 (m, 2H), 2.58–2.49 (m, 1H), 2.42–
2.48 (m, 1H), 2.34 (t, J = 10.8 Hz, 2H), 2.11–1.98 (m, 2H), 1.84 (d,
J = 10.2 Hz, 2H), 1.58–1.46 (m, 1H), 1.22 (dd, J = 6.28, 2.2 Hz, 6H)
From racemic 25 and isopropyl 3-bromopropanoate (48%), mp
203–205 °C. 1H NMR (CDCl3) d 12.0 (br, 1H), 8.27 (br, 1H), 8.14
(d, J = 8.0 Hz, 1H), 7.61–7.55 (m, 1H), 7.49 (br, 2H), 5.13–5.04 (m,
1H), 3.79 (dm, J = 14.3 Hz, 1H), 3.52–3.44 (m, 1H), 3.28 (br, 1H),
2.74 (br, 2H), 2.65–2.56 (m, 2H), 2.24 (td, J = 13.8 Hz, 3.2 Hz, 1H),
2.07 (br, 1H), 1.89–1.78 (m, 2H), 1.65–1.62 (m, 1H), 1.26 (d,
J = 5.01 Hz, 6H); 13C NMR (CDCl3) d 206.57, 172.2, 135.23, 132.63,
132.49, 131.84, 129.07, 129.03, 73.51, 69.86, 40.23, 39.85, 30.27,
30.08, 21.93, 21.84. MS m/z 338.2 (MH+). Calculated for C18H25-
ClNO3 (MH+) 338.1517, found 338.1529.
;
13C NMR (CDCl3) d 206.29, 173.46, 135.04, 132.84, 132.13,
131.62, 129.08, 129.06, 73.11, 68.81, 43.70, 40.84, 40.21, 32.79,
29.90, 21.91, 21.68. MS m/z 352.2 (MH+). Anal. Calcd for C19H27Cl2-
NO3: C, 58.77; H, 7.01; N, 3.61. Found: C, 58.57; H, 7.2; N, 3.54.
3.1.8. (S)-Isopropyl 3-((1-(2-chlorophenyl)-2-oxocyclohexyl)-
amino)propanoate hydrochloride (s-C2iPr) (9S)
3.1.13. n-Propyl 4-((1-(2-chlorophenyl)-2-oxocyclohexyl)-
amino)butanoate hydrochloride (13)
From 25S and isopropyl 3-bromopropanoate (29%), mp 208–
211 °C. 1H NMR (CDCl3) d 12.22 (br, 1H), 8.14 (dbr, J = 8.1 Hz,
2H), 7.61–7.55 (m, 1H), 7.49 (br, 2H), 5.12–5.06 (m, 1H), 3.79
(dm, J = 14.4 Hz, 1H), 3.52–3.43 (m, 1H), 3.26 (q, J = 11.9 Hz, 1H),
2.71 (br, 2H), 2.67–2.55 (m, 2H), 2.21 (td, J = 14.1, 3.3 Hz, 1H),
2.07 (br, 1H), 1.89–1.78 (m, 2H), 1.63 (br, 1H), 1.27 (app. dd,
J = 4.93, 1.25 Hz, 6H); 13C NMR (CDCl3) d 206.62, 172.23, 135.24,
132.62, 132.48, 131.84, 129.08, 128.42, 73.48, 69.86, 40.24, 39.87,
30.28, 30.09, 21.93, 21.87, 21.84. MS m/z 338.2 (MH+). HRMS calcu-
lated for C18H25ClNO3 (MH+) 338.1517, found 338.1524.
From racemic 25 and n-propyl 4-bromobutanoate (19%), mp
160–161 °C. 1H NMR (CDCl3) d 11.10 (br, 1H), 9.23 (br, 1H), 8.27
(d, J = 8.1 Hz, 1H), 7.59–7.52 (m, 1H), 7.44 (d, J = 5.1 Hz, 2H), 4.01
(t, J = 6.8 Hz, 2H), 3.76 (dm, J = 14.4 Hz,1H), 3.39–3.28 (m, 1H),
2.70 (t, J = 7.8 Hz, 1H), 2.66 (t, J = 6.9 Hz, 1H), 2.62–2.54 (m, 1H),
2.51 (td, J = 7.0, 2.8 Hz, 2H), 2.47–2.41 (m, 1H), 2.39–2.30 (m,
1H), 2.15–2.06 (m, 1H), 2.0 (br, 1H), 1.85 (td, J = 8.0, 3.9 Hz, 2H),
1.57–1.66 (m, 2H), 1.50 (q, J = 14.4 Hz, 1H), 0.91 (t, J = 7.4 Hz,
3H); 13C NMR (CDCl3) d 205.92, 173.47, 135.12, 132.78, 132.05,
131.59, 129.23, 129.0, 73.16, 66.62, 43.54, 40.86, 39.99, 32.19,

J. Jose et al. / Bioorg. Med. Chem. 21 (2013) 5098–5106
5105
29.77, 22.0, 21.78, 21.70, 10.48. MS m/z 352.2 (MH+). Anal. Calcd
for C19H27Cl2NO3: C, 58.77; H, 7.01; N, 3.61; Cl, 18.26. Found: C,
58.88; H, 7.15; N, 3.51; Cl, 18.28.
2.04–1.95 (m, 1H), 1.92–1.89 (m, 1H), 1.87–1.83 (m, 1H), 1.79
(dbr, J = 15.3 Hz, 1H), 1.73–1.57 (m, 5H), 1.54–1.47 (m, 1H), 0.91
(t, J = 7.4 Hz, 3H); 13C NMR (CDCl3) d 205.98, 173. 29, 135.14,
132. 6, 132.19, 131.74, 129.17, 129.08, 73.28, 66.31, 43.68, 40.77,
39.88, 33.58, 29.85, 26.16, 22.16, 22.06, 21.81, 10.5. MS m/z
366.2 (MH+). HRMS calculated for C20H29ClNO3 (MH+) 366.1830,
found 366.1839.
3.1.14. rac-Methyl 4-((1-(2-chlorophenyl)-2-oxocyclohexyl)-
amino)pentanoate hydrochloride (14)
From racemic 25 and ethyl 5-bromopentanoate, followed by
purification by preparative HPLC (41%). 1H NMR (CDCl3) d 7.53
(dd, J = 7.8, 1.6 Hz, 1H), 7.36 (dd, J = 7.8, 1.4 Hz, 1H), 7.31 (dt,
J = 7.8, 7.6, 1.5 Hz, 1H), 7.25–7.21 (m, 1H), 3.64 (s, 3H), 2.77–2.69
(m, 1H), 2.55–2.42 (m, 2H), 2.36–2.30 (m. 1H), 2.26 (t, J = 7.4,
2H), 2.09–1.73 (m, 7H), 1.66–1.58 (m, 2H), 1.55–1.40 (m, 2H);
13C NMR (CDCl3) d 205.35, 173.0, 134.5, 131.86, 131.45, 131.09,
128.38, 72.56, 59.91, 51.21, 42.96, 40.13, 39.18, 32.75, 29.14,
25.58, 21.54, 13.71. MS m/z 338.2 (MH+).
3.1.19. 2-(2-Chlorophenyl)-2-(dimethylamino)cyclohexanone
hydrochloride(18) (Scheme 2)
Racemic 25 (200 mg, 0.9 mmol) was dissolved in MeOH (20 mL)
and cooled to 0 °C in an ice bath. Acetic acid (0.2 mL, 3.6 mmol) and
NaCNBH3 (112 mg, 1.8 mmol) was added to the above solution and
stirred at 0 °C for 5 min. Formaldehyde (37% in H2O, 2.2 mmol) was
added at 0 °C and reaction mixture allowed to stir at 25 °C for 24 h.
The reaction mixture was quenched with NaHCO3 and diluted with
water. The aqueous layer was extracted with DCM (3 Â 20 mL),
washed with brine and dried over Na2SO4. The solvent was evapo-
rated under reduced pressure to obtain the product as yellow oil.
The yellow oil was dissolved in Et2O (5 mL), cooled to 0 °C in an
ice bath and treated with HCl gas for 1 min. Solvent was evapo-
rated and the residue was resuspended in EtOAc (2 mL) and soni-
cated. The precipitate formed was diluted with EtOAc (10 mL)
and filtered, dried to give 18 as the HCl salt (155 mg, 60%), mp
192–194 °C. 1H NMR (CDCl3) d 12.38 (br, 1H), 8.11 (d, J = 8.0 Hz,
1H), 7.62–7.58 (m, 1H), 7.52 (br, 2H), 3.39 (d, J = 12.4 Hz, 1H),
3.07 (s, 3H), 2.78 (s, 3H), 2.72–2.59 (m, 2H), 2.44 (td, J = 13.5,
3.2 Hz, 1H), 2.05–1.97 (m, 1H), 1.90–1.81 (m, 2H), 1.46 (t,
J = 9.4 Hz, 1H); 13C NMR (CDCl3) d 205.07, 135.99, 132.76, 132.62,
132.35, 128.89, 128.71, 42.11, 40.37, 39.36, 37.3, 29.25, 22.18.
MS m/z 252.1 (MH+).
3.1.15. (S)-Methyl 4-((1-(2-chlorophenyl)-2-oxocyclohexyl)-
amino)pentanoate hydrochloride (14S)
From 25S and ethyl 5-bromopentanoate (42%), mp (MeOH/
EtOAc) 188–191 °C, 1H NMR (CDCl3) d 7.52 (dd, J = 7.82, 1.68 Hz,
1H), 7.36 (dd, J = 7.8, 1.4 Hz, 1H), 7.31 (dt, J = 7.8, 7.60, 1.45 Hz,
1H), 7.23 (dt, J = 8.0; 1.7 Hz, 1H), 3.65 (s, 3H), 2.76–2.68 (m, 1H),
2.55–2.42 (m, 2H), 2.36–2.30 (m, 1H), 2.26 (t, J = 7.4 Hz, 2H),
2.08–1.74 (m, 7H), 1.66–1.58 (M, 2H), 1.57–1.37 (m, 3H); 13C
NMR (CDCl3) d 206.14, 173.65, 135.12, 132.56, 132. 2, 131.75
(2C), 129.1, 73.27, 60.54, 51.87, 43.66, 40.75, 33.34, 29.87, 26.14,
22.1, 14.35. MS m/z 338.2 (MH+). Analysis calculated for C18H25Cl2-
NO3: C, 57.8; H, 6.7, Cl, 18.9, N, 3.7; found C, 57.7, H, 6.8 Cl, 18.9 N,
3.7.
3.1.16. rac-Ethyl 5-((1-(2-chlorophenyl)-2-oxocyclohexyl)-
amino)pentanoate hydrochloride (15)
From racemic 25 and ethyl 5-bromopentanoate (29%), mp 169–
172 °C. 1H NMR (CDCl3) d 11.87 (br, 1H), 8.20 (d, J = 8.2 Hz, 1H),
7.62–7.54 (m, 1H), 7.48 (d, J = 3.8 Hz, 2H), 7.39 (br, 1H), 4.12 (q,
J = 7.1 Hz, 2H), 3.83 (dm, J = 14.4 Hz, 1H), 3.11–3.02 (m, 1H), 2.75
(d, J = 12.5 Hz, 1H), 2.70–2.61 (m, 1H), 2.51 (br, 1H), 2.32 (t,
J = 7.3 Hz, 2H), 2.24 (t, J = 11.1 Hz, 1H), 2.05 (br, 1H), 1.99–1.88
(m, 2H), 1.83 (d, J = 14.5 Hz, 2H), 1.76–1.61 (m, 3H), 1.24 (t,
J = 7.1 Hz, 3H); 13C NMR (CDCl3) d 205.41, 173.07, 135.21, 132.58,
132.06, 131.67, 129.34, 128.99, 73.20, 60.57, 43.59, 40.82, 39.62,
33.65, 29.68, 26.07, 22.27, 21.77, 14.31. MS m/z 352.2 (MH+). Anal.
Calcd for C19H27Cl2NO3: C, 58.77; H, 7.01; N, 3.61; Cl, 18.26. Found:
C, 58.81; H, 7.1, N, 3.51; Cl, 18.31.
3.1.20. (S)-2-(2-Chlorophenyl)-2-
(dimethylamino)cyclohexanone hydrochloride(18S)
From 25S following the procedure for 18, (42%). 1H NMR (CDCl3)
d 12.45 (br, 1H), 8.09 (d, J = 7.9 Hz, 1H), 7.62–7.58 (m, 1H), 7.51 (br,
2H), 3.39 (d, J = 12.8 Hz, 1H), 3.08 (s, 3H), 2.78 (s, 3H), 2.72–2.59
(m, 2H), 2.44 (t, J = 13.5 Hz, 1H), 2.02–1.97 (m, 1H), 1.90–1.81
(m, 2H), 1.53 (m, 1H); 13C NMR (CDCl3) d 205.05, 135.06, 133.93,
131.7, 130.09, 129.04, 126.67, 42.0, 39.45, 38.53, 29.42, 22.48. MS
m/z 252.1 (MH+). HRMS calculated for C14H19ClNO (MH+)
252.1150, found 252.1156.
3.1.21. Ethyl 3-((1-(2-chlorophenyl)-2-
oxocyclohexyl)(methyl)amino)propanoate (19)
3.1.17. Isopropyl 5-((1-(2-chlorophenyl)-2-
oxocyclohexyl)amino)pentanoate hydrochloride (16)
From reductive methylation of 8, (97 %). 1H NMR (CDCl3) d 7.38
(dd, J = 7.8, 1.4 Hz, 1H), 7.36–7.33 (m, 1H), 7.30 (td, J = 7.9, 1.4 Hz,
1H), 7.25–7.21 (m, 1H), 4.10 (q, J = 7.2 Hz, 2H), 3.11–3.04 (m, 1H),
2.97–2.90 (m, 1H), 2.80–2.73 (m, 1H), 2.58 (t, J = 6.7 Hz, 2H), 2.49–
2.45 (m, 2H), 2.43 (s, 3H), 2.05–1.91 (m, 2H), 1.89–1.72 (m, 2H),
1.65–1.56 (m, 1H), 1.24 (t, J = 7.2 Hz, 3H); 13C NMR (CDCl3) d
208.1, 172.7, 138.0, 134.07, 131.68, 129.86, 18.64, 126.65, 74.58,
60.37, 47.76, 41.19, 36.97, 36.14, 34.73, 27.28, 22.33, 14.27. MS
m/z 338.5 (MH+). HRMS calculated for C18H25ClNO3 (MH+)
338.1517, found 338.1514.
From racemic 25 and isopropyl 5-bromovalerate (40%), mp
161–163 °C. 1H NMR (CDCl3) d 11.04 (br, 1H), 8.78 (br, 1H), 8.24
(d, J = 8.0 Hz, 1H), 7.59–7.53 (m, 1H), 7.47 (br, 2H), 5.01–4.92 (m,
1H), 3.74 (dm, J = 14.4 Hz, 1H), 3.29–3.21 (m, 1H), 2.73 (d,
J = 12.2 Hz, 1H), 2.64 (td, J = 13.3 Hz, 6.3, 1H), 2.54–2.42 (m, 2H),
2.26–2.21 (m, 2H), 2.10–1.99 (m, 2H), 1.94–1.83 (m, 2H), 1.78 (d,
J = 17.6 Hz, 1H), 1.71–1.47 (m, 3H), 1.20 (dd, J = 6.3, 1.2 Hz, 6H);
13C NMR (CDCl3) d 205.7, 172.68, 135.18, 132.61, 132.13, 131.72,
129.26, 129.04, 73.25, 67.95, 43.66, 40.80, 39.76, 33.95, 29.78,
26.12, 22.25, 21.97, 21.8. MS m/z 366.2 (MH HRMS calculated for
3.1.22. Ethyl 4-((1-(2-chlorophenyl)-2-
C
20H29ClNO3 (MH+) 366.1830, found 366.1842.
oxocyclohexyl)(methyl)amino)butanoate hydrochloride (20)
From reductive methylation of 11 (97%). Mixture of rotamers.
3.1.18. n-Propyl 5-((1-(2-chlorophenyl)-2-
1H NMR (CDCl3)
d 11.97 (br, 1H), 11.79 (br, 1H), 8.46 (d,
oxocyclohexyl)amino)pentanoate hydrochloride (17)
J = 8.0 Hz, 1H), 7.93 (d, J = 7.6 Hz, 1H), 7.64–7.46 (m, 6H), 4.12–
4.04 (m, 4H), 3.96 (t, J = 9.3 Hz, 1H), 3.69 (d, J = 14.8 Hz, 1H), 3.47
(1H), 3.25 (d, J = 14.5 Hz, 1H), 3.16 (s, 3H), 2.78 (br, 6H), 2.69–
2.55 (m, 5H), 2.48–2.34 (m, 3H), 2.14 (br, 3H), 1.97 (br, 3H), 1.84
(br, 5), 1.48–1.39 (m, 2 H), 1.31–1.19 (m, 6H); 13C NMR (CDCl3) d
205.26, 204.28, 172, 136.05, 135.91, 133.57, 132.87, 132. 77,
From racemic 25 and propyl 5-bromopentanoate (45 %), 1H
NMR (CDCl3) d 11.23 (br, 1H), 8.49 (br, 1H), 8.23 (d, J = 8.0 Hz,
1H), 7.59–7.52 (m, 1H), 7.46 (br, 2H), 4.0 (t, J = 6.8 Hz, 2H), 3.76
(dm, J = 14.3 Hz, 1H), 3.24–3.18 (m, 1H), 2.74 (br, 1H), 2.69–2.61
(m, 1H), 2.46 (t, J = 14.0 Hz, 2H), 2.29 (td, J = 7.5, 2.9 Hz, 2H),

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J. Jose et al. / Bioorg. Med. Chem. 21 (2013) 5098–5106
132.65, 132.55, 132.11, 129.2, 128.53, 127.69, 60.85, 53.32, 52.16,
42.65, 41.99, 37.37, 37.17, 36.59, 35.39, 31.49, 29.16, 22.22,
22.13, 20.64, 20.55, 14.3 (some C not seen for both rotamers). MS
m/z 352.2 (MH+). HRMS calculated for C19H27ClNO3 (MH+)
352.1674, found 352.1687.
3.2.2. Pedal withdrawal reflex (PWR) scoring
Nociceptive testing in animals was conducted via 1 sec applica-
tion of constant pressure (firm digital pressure) over the forepaw of
the animal. Pedal withdrawal reflex testing is primarily used to as-
sess analgesic effect, and responses are graded accordingly: 0, ab-
sent; 1, flicker; 2, moderate withdrawal; 3, fast withdrawal; 4,
Fast withdrawal with cry/preceding apnoea (modified from Ref. 18).
3.1.23. Methyl 5-((1-(2-chlorophenyl)-2-
oxocyclohexyl)(methyl)amino)pentanoate (21)
From reductive methylation of 14, (97 %). 1H NMR (CDCl3) d
7.41 (d, J = 7.5 Hz, 2H), 7.38–7.29 (m, 2H), 3.66 (s, 3H), 2.83 (br,
2H), 2.61–2.57 (m, 3H), 2.49 (br, 3H), 2.30 (t, J = 7.1 Hz, 2H),
2.10–1.94 (m, 3H), 1.88–1.77 (m, 3H), 1.62–1.59 (m, 3H); 13C
NMR (CDCl3) d 206.75, 173.94, 134.77, 133.19, 132.01, 131.14,
130.12, 127.44, 52.02, 51.62, 41. 63, 37.0, 35.86, 33.66, 28.04,
27.07, 22.58, 22.27 (1C overlapping). MS m/z 352.2 (MH+). HRMS
calculated for C19H27ClNO3 (MH+) 352.1674, found 352.1683.
3.2.3. Loss of righting reflex (LRR)
This is primarily used to assess anaesthetic hypnotic effect.
Righting reflex is judged absent when the rat fails to right from a
position of dorsal recumbency to a position of sternal recumbency
on three attempts performed in rapid succession. Dose to LRR is
termed effective potency.
Acknowledgement
3.2. Biological activity
The authors thank the Biopharma Thematic Research Initiative,
University of Auckland, for funding.
3.2.1. General
All animal experiments were conducted at the Ruakura
Research Centre, Hamilton, New Zealand, using experimental pro-
tocols reviewed and approved by the Ruakura Animal Ethics Com-
mittee (ethics ref 12604). Following acquisition of baseline
physiologic parameters (heart rate, respiratory rate, PWR, and
righting reflex (RR)) adult female Sprague-Dawley rats of approxi-
mately 350–450 g were put under non-traumatic restraint and the
marginal vein of the tail was cannulated. Ketamine or an experi-
mental compound at 10 mg/ml was administered via a minibore
extension tube adequately secured to the tail. Infusions were com-
menced at a rate (weight-adjusted) to deliver 20 mg/kg/min
initially (continued until the pedal withdrawal reflex score
PWR = 1), then were reduced to a rate of 6.7 mg/kg/min. Infusion
rate was then titrated in an up-and-down fashion to maintain dor-
sal recumbency and a PWR = 1 to 10 min before cessation. Three
rats were used in each study, with each group of rats also acting
as their own ketamine control. The order of study drug administra-
tion was determined by prior odds/evens randomisation with a
recovery interval of at least one hour afforded between experi-
ments. PWR and RR were recorded at 1 min intervals throughout.
The times from cessation of infusion to return of righting reflex
(RRR), and from cessation of infusion to the animals displaying
independent locomotion (walk) were recorded.
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