Synthesis of azaspirocycles and their evaluation in drug discovery

Article abstract of DOI:10.1002/anie.200907108

(Figure Presented). Make It spirol Readily synthesized heteroatom- substituted spiro[3.3]heptanes generally show higher aqueous solubility than their cyclohexane analogues, and show a trend towards higher metabolic stability. The novel framework can be mo

Full text of DOI:10.1002/anie.200907108

Communications
DOI: 10.1002/anie.200907108
Drug Design
Synthesis of Azaspirocycles and their Evaluation in Drug Discovery**
Johannes A. Burkhard, Bjꢀrn Wagner, Holger Fischer, Franz Schuler, Klaus Mꢁller,* and
Erick M. Carreira*
In modern drug discovery, high-throughput screening may
generate promising lead compounds that are less than ideal
with respect to important parameters such as absorption,
distribution, metabolism, and excretion (ADME). In the
process of lead optimization, various structural features are
fine-tuned and physicochemical properties such as the
basicity (pKa[1]), lipophilicity (logD), solubility (Solint), and
clearance rates (CLint) are improved.[2] Herein, we introduce
heteroatom-substituted spiro[3.3]heptanes as viable and
promising surrogates for piperazines, piperidines, morpho-
lines, and thiomorpholines—all of which are common build-
ing blocks in medicinal chemistry (Figure 1).
compound.[4] Furthermore, these structures allow the explo-
ration of novel chemical space in biology and medicine.[5]
We have suggested a number of useful ways of perceiving
oxetanes as structural analogues. For example, oxetanes may
be considered as structural mimics of carbonyl groups;
consequently 2-oxa-6-azaspiro[3.3]heptane has a close struc-
tural resemblance to azetidin-3-one. Interestingly, this same
oxetanyl spiroazetidine also exhibits similarities to morpho-
line.[6] In view of the use of saturated heterocycles in
medicinal chemistry,[7] we turned our attention to other
members of the spiro[3.3]heptane family. For a survey of
fundamental physicochemical and biochemical properties, we
chose molecules of topological C2 symmetry with a common
piperonyl-tagged[6] azetidine moiety as model compounds.
2,6-Diazaspiro[3.3]heptanes 2–7 and azetidine-thietane
spirocycles 9–11 were prepared from dibromide 1[8]
(Scheme 1). After removal of the N-tosyl group of common
Figure 1. Heteroatom-substituted spiro[3.3]heptanes as novel building
blocks in drug discovery.
The vast majority of compounds generated in pharma-
ceutical and chemical research contain saturated and unsatu-
rated six- and five-membered rings that may be fused or
linked.[3] In striking contrast, four-membered rings are found
comparatively rarely. As previously described with oxetanes,
the incorporation of four-membered heterocycles into drug-
like scaffolds provides an opportunity to uniquely tune the
physicochemical and biochemical properties of the parent
[*] J. A. Burkhard, Prof. Dr. E. M. Carreira
Laboratorium fꢀr Organische Chemie, ETH Zꢀrich, HCI H335
8093 Zꢀrich (Switzerland)
Fax: (+41)44-632-1328
E-mail: carreira@org.chem.ethz.ch
Homepage: http://www.carreira.ethz.ch
Scheme 1. R=Piperonyl. Reagents and conditions: a) Piperonylamine,
90%; b) Mg, MeOH, ultrasound; c) CH2O, NaBH(OAc)3, 71%
(2 steps); d) Ac2O, Et3N, 71% (2 steps); e) PhCHO, NaBH(OAc)3,
62% (2 steps); f) Boc2O, Et3N, 66% (2 steps); g) 1-Br-3,5-F2-C6H3,
(ꢀ)-binap, [Pd2(dba)3], KOtBu, 69% (2 steps); h) Na2S, 86%; i) see
(b), then C2H2O4; j) piperonal, NaBH(OAc)3, 70% (3 steps);
k) K2OsO4·2H2O, NMO, 99%; l) H2O2, AcOH, 68%; m) see (a), 69%;
n) Swern oxidation, 91%; o) (R)-tBu-S(O)NH2, Ti(OEt)4, 79%; p) MeLi,
52:48 d.r.; PhLi, 73:27 d.r.; q) KOtBu, 40% (2 steps; for 15), 73%
(2 steps; for 16); r) see (b) and (j); s) HCl, then (c), 42% (4 steps;
15!17), 46% (4 steps; 16!18). Ts=p-toluenesulfonyl, Bn=benzyl,
Boc=tert-butoxycarbonyl, binap=2,2’-bis(diphenylphosphino)-1,1’-
binaphthalene, dba=trans,trans-dibenzylideneacetone, NMO=
N-methylmorpholine N-oxide.
B. Wagner, Dr. H. Fischer, Dr. F. Schuler, Prof. Dr. K. Mꢀller
F. Hoffmann–La Roche AG, Pharmaceuticals Division
4070 Basel (Switzerland)
Fax: (+41)61-688-6965
E-mail: klaus.mueller@roche.com
[**] We thank F. Hoffmann–La Roche AG for support of this research.
J.A.B. is grateful to Novartis and the Roche Research Foundation for
graduate fellowship support. This research was supported by a
grant from ETH-Z (0-20449-07). Dr. Sven Hobbie (University of
Zurich) is acknowledged for measuring the MIC values.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200907108.
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Chemie
intermediate 2, functionalization of the amine furnished the
desired diazaspiro[3.3]heptanes 3–7 in good yields.[9] Treat-
ment of 1 with Na2S delivered thietane 8, which was
elaborated to 9–11. Clean oxidation to sulfone 10 was
observed using K2OsO4·2H2O/NMO. Homospiropiperidine
13 was readily synthesized from 12.[10,11]
The general route described above was successfully
adapted to introduce substituents a to the nitrogen atom in
a stereoselective manner. Swern oxidation of bromoalcohol
14 and subsequent condensation with Ellmanꢀs auxiliary[12]
afforded the sulfinylimine. Whereas reagents such as
MeMgBr failed to add to the sterically hindered electro-
phile,[13] more reactive MeLi as well as PhLi added at ꢁ788C.
Ring closure was then performed at 08C using KOtBu,[14] and
subsequent elaboration led to enantiomerically pure
diazaspiro[3.3]heptanes 17 and 18.
amine of the spirocycle is generally more basic. If piperidine
34 and homospiropiperidine 13, which have identical basicity,
are taken as a common reference, the decrease in the
pKa values in the homospiro series as a consequence of Y is
typically a factor of 0.5–0.6 lower than that observed for the
corresponding members in the six-membered monocyclic
series (Figure 2). This holds true for the whole range of
The spirocycles and their six-membered-ring counterparts
were analyzed with respect to their lipophilicity, aqueous
solubility, metabolic stability, and amine basicity (Table 1). A
survey of the pKa values reveals a general trend that the
Figure 2. Correlation of pKa decrements in the homospirocyclic series,
DpKa(hsP), against the six-membered monocyclic series, DpKa(P),
relative to the reference compounds 13 and 34, respectively, containing
no heterofunctionality (Y=CH2, see Table 1). The black, orange, and
blue dots refer to Y=NR, S(O)x (x=0–2), and O functionalities,
respectively.
Table 1: Physicochemical and biochemical properties.
Compound[a]
logD[b] (logP)[c]
Solint
CLint (h/m)[e] pKa[f]
[d]
pKa values, and is consistent with the fact that the Y
functionality (see Table 1) exerts its influence in the six-
membered monocyclic series through two paths of three
s bonds,[1] but through four s bonds in the homospirocyclic
series. In this correlation we exclude the dibasic compounds,
as their pKa values cannot be assigned unambiguously to
specific protonation sites (for further discussions, see the
Supporting Information). The pKa data in Table 1 are con-
sistent with previously discussed pKa decrements,[1] and nicely
complement hitherto unavailable data for basicity modula-
tion by heterofunctionalities Y at the b or g positions to an
amine unit.
The spiro[3.3]heptanes generally exhibit lower lipophilic-
ities than their monocyclic counterparts (average DlogD =
ꢁ0.75). Interestingly, their neutral bases are also more polar,
as can be seen from their logP values (average DlogP =
ꢁ0.21). One notable exception is sulfone 9, which has
higher lipophilicity than its thiomorpholine counterpart 32,
but it is still more soluble in aqueous phosphate buffer. Most
of the spirocyclic compounds have a higher intrinsic solubility,
even for substances that have very similar logP values (for
example, 8 and 31, or 13 and 34), or when DlogP > 0 (9 and
32). The differences are in some cases quite remarkable; for
example, tert-butyl carbamates 29 and 6 show a solubility
difference of roughly an order of magnitude.
A, Y=NTs
B, Y=NTs
A, Y=NMe
B, Y=NMe
A, Y=NAc
B, Y=NAc
A, Y=NBn
B, Y=NBn
A, Y=NBoc
B, Y=NBoc
A, Y=NAr[h]
B, Y=NAr[h]
A, Y=S
25
2
26
3
27
4
28
5
29
6
30
7
31
8
32
9
3.4(3.4)
2.5(2.8)
13
23
310/586
25/114
18/21
12/4
13/39
6/10
26/156
6/41
32/1150
2/35
184/495
29/241
28/2300
18/330
7/100
21/30
0/6
9/0
8/18
9/26
9/8
3/7
0/23
9/39
16/91
16/67
6.0
7.4
7.9
9.5
6.4
7.7
7.6
8.4
6.7
7.8
6.9
8.1
7.3
8.1
4.0
6.7
5.5
7.2
9.6
9.6
7.0
8.0
7.9
9.3
7.1
8.4
0.5(1.1) >59800
26800
ꢁ0.5(1.6)
0.9(1.0) >14900
0.0(0.5) >10600
2.8(3.2)
1.6(2.7)
3.1(3.1)
n.d.[g]
n.d.[g]
287
2620
12
2.2(2.8)
>3.7(>3.8)
>3.0(>3.8)
2.2(2.4)
12
1740
4560
1440
4930
B, Y=S
1.6(2.3)
0.1(0.1)
0.5(0.6)
0.5(0.5) >33000
0.1(0.3) >32000
0.9(3.1)
1.0(3.2)
1.5(1.6)
A, Y=SO2
B, Y=SO2
A, Y=SO
B, Y=SO
A, Y=CH2
B, Y=CH2
A, Y=O
33
10
34
13
35
36
2060
3280
36300
B, Y=O[6]
0.5(1.2) 100000
A, Y=NMe;a-Me 37
B, Y=NMe;a-Me 17
A, Y=NMe;a-Ph 38
B, Y=NMe;a-Ph 18
n.d.[g]
n.d.[g]
n.d.[g]
796
n.d.[g]
2.9(3.1)
2.0(3.1)
2490
[a] R=piperonyl. [b] Log n-octanol/water distribution coefficient at
pH 7.4. [c] Intrinsic lipophilicity of neutral base according to
log P ¼ log D þ log10ð1 þ 10ðpK ꢁpHÞÞ. [d] Intrinsic molar solubility
a
Although metabolic susceptibility is dependent on the
structural context, and thus cannot generally be attributed on
a given structural subunit of interest, we found that most
spirocyclic compounds were oxidatively degraded at lower
rates than the six-membered monocyclic analogues in both
human (h) and mouse (m) liver microsomes. Striking differ-
ences are observed for the more lipophilic pairs; in particular,
[mmolLꢁ1] of the neutral base obtained from the experimental thermo-
dynamic solubility in phosphate buffer (50 mm) at pH 9.9 and (22.5ꢀ
1)8C, and corrected for pKa. [e] Intrinsic clearance rates in minꢁ1
/
(mg(protein)/mL) measured in human (h) and mouse (m) liver micro-
somes. [f] Amine basicity in H2O measured spectrophotometrically at
248C; for details, see the Supporting Information. [g] n.d.=not deter-
mined. [h] Ar=3,5-difluorophenyl.
Angew. Chem. Int. Ed. 2010, 49, 3524 –3527
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Communications
carbamate 6, sulfonamide 2, and aniline 7 display pronounced
human microsomal assays (Table 2), whereas ciprofloxacin
trifluoroacetate shows slight metabolic degradation. Like-
wise, 21 was as stable as ciprofloxacin in a human hepatocyte
assay, whereas the homospiromorpholine analogue 22
remained essentially unaffected under the same whole-cell
assay conditions.
In summary, we have developed an efficient and scalable
synthesis of various heteroatom-substituted spiro-
[3.3]heptanes with topological C2 symmetry, and have shown
that important pharmacokinetic properties such as lipophi-
licity and metabolic stability may be advantageously altered
when compared with their traditional piperidine, piperazine,
or thiomorpholine counterparts. Moreover, the spirocyclic
building blocks can be easily grafted onto frameworks of
druglike structures, such as the fluoroquinolones, to give
compounds that retain notable activity and populate chemical
space otherwise not previously accessed. In this regard, we
expect the spirocyclic systems to find wide applications in
medicinal chemistry and beyond.
greater metabolic stability than their monocyclic analogues.
This finding suggests that the higher aqueous solubility may
result in the more polar compounds having fewer interactions
with cytochromes P450 and other membrane-bound oxidizing
enzymes, thereby avoiding metabolic degradation. Only in a
few cases, particularly where the six-membered ring deriva-
tives exhibit low intrinsic clearance rates, the spirocyclic
analogues show similar or even slightly higher rates. In
general, the data in Table 1 suggest that the concept of
replacing a six-membered monocyclic unit in a drug candidate
by a corresponding spiro[3.3]heptane analogue is worth
implementing, as it may significantly improve relevant aspects
of the pharmacokinetic profile by increasing the aqueous
solubility and reducing both lipophilicity and metabolic
degradation.
To demonstrate the usefulness of spirocyclic building
blocks 23[8] and 24[6] in applied medicinal chemistry, the
synthesis of analogues of the antibacterial compound cipro-
floxacin (19) was undertaken (Table 2).[15] The commercially
Received: December 17, 2009
Published online: April 1, 2010
Table 2: Synthesis, activity (MIC) against S. aureus, and metabolic
stability of ciprofloxacin (19) and analogues 21 and 22.[a]
Keywords: amines · drug design · heterocycles · metabolism ·
.
spiro compounds
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MIC
CLint (human)
S. aureus
microsomal
hepatocytes
[mgmLꢁ1
]
[minꢁ1 (mgprotein/mL)ꢁ1
]
[minꢁ1 (106 cells/mL)ꢁ1
]
19
21
22
0.125
0.5–1.0
0.125
5
0
0
7
5
1
[a] Reagents and conditions: a) 23, KOtBu, DMSO, 62%; b) TFA, 99%;
c) 24, see (a), 68%. TFA=trifluoroacetic acid.
[5] C. Lipinski, A. Hopkins, Nature 2004, 432, 855 – 861.
[6] G. Wuitschik, M. Rogers-Evans, A. Buckl, M. Bernasconi, M.
Marki, T. Godel, H. Fischer, B. Wagner, I. Parrilla, F. Schuler, J.
Schneider, A. Alker, W. B. Schweizer, K. Mꢂller, E. M. Carreira,
Angew. Chem. 2008, 120, 4588 – 4591; Angew. Chem. Int. Ed.
2008, 47, 4512 – 4515.
[7] A search of the Prous Science Integrity database (August 2009)
revealed launched drugs containing heterocycles not substituted
at carbon atoms: piperazine (90), morpholine (20), piperidine
(21), thiomorpholines (0; lead compounds in a preclinical phase:
84).
[8] J. Burkhard, E. M. Carreira, Org. Lett. 2008, 10, 3525 – 3526.
[9] For other syntheses of differentiated 2,6-diazaspiro[3.3]heptanes
and their possible use in medicinal chemistry, see a) D. Hamza,
M. J. Stocks, A. Decor, G. Pairaudeau, J. P. Stonehouse, Synlett
2007, 2584 – 2586; b) M. J. Stocks, D. Hamza, G. Pairaudeau, J. P.
Stonehouse, P. V. Thorne, Synlett 2007, 2587 – 2589; c) M. J.
Stocks, G. R. H. Wilden, G. Pairaudeau, M. W. D. Perry, J.
Steele, J. P. Stonehouse, ChemMedChem 2009, 4, 800 – 808;
d) M. C. Hillier, C.-Y. Chen, J. Org. Chem. 2006, 71, 7885 – 7887;
available aryl chloride 20 was treated with 23 or 24 (KOtBu/
DMSO) at 1308C to give 21 or 22, respectively, in good yields.
These were then tested against clinical isolates of S. aureus.
Ciprofloxacin trifluoroacetate was measured for the purpose
of comparison. Azetidine analogue 21 displayed 4–8-fold
weaker inhibition than ciprofloxacin. By contrast, 22 dis-
played comparable activity. The fact that the analogues 21 and
22 show comparable activities in an otherwise non-optimized
scaffold suggests that spirocyclic amines can be considered in
lead optimization. Furthermore, inhibitory properties may be
retained with refinement of pharmacokinetic and -dynamic
properties and the overall efficacy may even be improved. For
example, metabolic studies reveal azetidine 21 and oxetane 22
to have high stabilities, with no observable metabolism in
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e) W. Engel, W. Eberlein, G. Trummlitz, G. Mihm, H. Doods, N.
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0–417631, 1991.
[12] For a review, see J. A. Ellman, T. D. Owens, T. P. Tang, Acc.
Chem. Res. 2002, 35, 984 – 995.
[13] For a similar steric environment, see Z. X. Han, D. Krishna-
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Lett. 2002, 4, 4025 – 4028.
[10] J. Foos, F. Steel, S. Q. A. Rizvi, G. Fraenkel, J. Org. Chem. 1979,
44, 2522 – 2529.
[14] The azetidines can also be isolated after warming the reaction
mixture of the addition reaction to 08C, but cleaner conversions
and higher yields were obtained in the two-step procedure.
[15] For an overview of ciprofloxacin and other fluoroquinolones, see
a) P. C. Appelbaum, P. A. Hunter, Int. J. Antimicrob. Agents
2000, 16, 5 – 15; b) D. T. W. Chu, P. B. Fernandes, Antimicrob.
Agents Chemother. 1989, 33, 131 – 135.
[11] For other uses of 2-azaspiro[3.3]heptanes, see a) T. A. Blizzard
et al., see the Supporting Information; b) M. Altman et al., see
the Supporting Information. The synthesis of a related building
block was reported recently: M. J. Meyers, I. Muizebelt, J. van
Wiltenburg, D. L. Brown, A. Thorarensen, Org. Lett. 2009, 11,
3523 – 3525.
Angew. Chem. Int. Ed. 2010, 49, 3524 –3527
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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