Synthesis of multivalent carbonate esters by divergent growth of branched carbamates

Article abstract of DOI:10.1016/S0040-4039(01)00078-8

Multivalent 4-nitrophenyl (PNP)-carbonate esters were prepared from carbonate ester-containing core groups by a new divergent-growth process. The process doubles the number of PNP-carbonate esters with each round of growth, or branching, which occurs by reacting an N,N-dihydroxylalkyl amine with a PNP-carbonate ester. The resulting N,N-dihydroxyalkyl carbamate contains two hydroxyl groups for each original PNP-carbonate ester. The hydroxyl groups are converted in situ to PNP-carbonate esters, and the process can be repeated to provide successive generations of dendrimer-like branched structures of increased valence.

Full text of DOI:10.1016/S0040-4039(01)00078-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2069–2072
Synthesis of multivalent carbonate esters by divergent growth of
branched carbamates
David S. Jones,* Martina E. Tedder, Christina A. Gamino, Jeffrey R. Hammaker and
Huong-Thu Ton-Nu
La Jolla Pharmaceutical Company, 6455 Nancy Ridge Drive, San Diego, CA 92121, USA
Received 11 December 2000; accepted 12 January 2001
Abstract—Multivalent 4-nitrophenyl (PNP)–carbonate esters were prepared from carbonate ester-containing core groups by a new
divergent-growth process. The process doubles the number of PNP–carbonate esters with each round of growth, or branching,
which occurs by reacting an N,N-dihydroxylalkyl amine with a PNP–carbonate ester. The resulting N,N-dihydroxyalkyl
carbamate contains two hydroxyl groups for each original PNP–carbonate ester. The hydroxyl groups are converted in situ to
PNP–carbonate esters, and the process can be repeated to provide successive generations of dendrimer-like branched structures
of increased valence. © 2001 Published by Elsevier Science Ltd.
A new divergent-growth process has been developed
which provides convenient access to a new class of
multivalent platforms comprised of carbamate link-
ages.† We recognized that readily available
diethanolamine, or other amines containing multiple
hydroxyl groups, could serve as an ABn monomer
which could react with activated carbonate derivatives
to provide branched structures in relatively few syn-
thetic steps. Scheme 1 describes the process, exemplified
with diethanolamine.
ture, represented as 2, by formation of a carbamate.
The hydroxyl groups are converted to reactive carbon-
ate derivatives to provide the first generation branched
structure 3. The process can be repeated to propagate
successive generations of branching. Thus, compound 3
is reacted with diethanolamine to provide structure 4,
which is converted to compound 5. In addition to
having utility in the preparation of multivalent plat-
forms, the chemistry described herein may be applicable
to the preparation of dendrimers.4 Multivalent plat-
forms resemble dendrimers, because they are comprised
of branching groups and linking groups. Dendrimers
are highly branched molecules typically derived from
A reactive carbonate derivative, 1, is reacted with
diethanolamine to provide a hydroxyl terminated struc-
O
OH
O
X
O
O
O
OH
OH
O
O
X
O
N
N
O
N
N
O
O
O
O
O
O
O
O
X
X
OH
OH
RO
X
RO
N
RO
N
RO
N
RO
N
O
O
O
X
O
O
O
O
O
X
OH
2
1
3
4
5
Scheme 1.
* Corresponding author. Fax: 858-626-2845; e-mail: dave.jones@ljpc.com
† Multivalent platforms are branched structures which are important intermediates used to prepare structurally defined multivalent bioconjugates.
Multivalent platforms are key intermediates used to prepare B cell toleragens, therapeutic agents for the treatment of autoimmune diseases. B
cell toleragens are constructed by attaching multiple copies of B cell epitopes to multivalent platforms.1–3 Multivalent platforms are
metabolically-stable biocompatible, non-immunogenic molecules of defined structure with multiple reactive groups to which biologically active
molecules can be attached.
0040-4039/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0040-4039(01)00078-8

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D. S. Jones et al. / Tetrahedron Letters 42 (2001) 2069–2072
Scheme 2. (a) Diethanolamine, pyridine; (b) PNP–chloroformate, pyridine; (c) BocNH(CH2)2NH2, Et3N, CH2Cl2.
OCO2PNP
OCO2PNP
OCO2PNP
PNPO2CO
N
O
N
O
O
a, b
O
O
O
O
O
N
O(CH2CH2O)2
N
8
O
O
N
10 (68%)
N
PNPO2CO
OCO2PNP
OCO2PNP
OCO2PNP
OCO2PNP
OCO2PNP
OCO2PNP
12 (47%)
PNPO2CO
O
N
N
a, b
OCO2PNP
OCO2PNP
PNPO2CO
PNPO2CO
O
O
O
O
O
O
N
N
11
PNPO2CO
O
OCO2PNP
PNPO2CO
PNPO2CO
PNPO2CO
OCO2PNP
OCO2PNP
O
N
O
O
O
N
O
c, b
O
O(CH2CH2O)2
6
13 (62%)
PNPO2CO
Scheme 3. (a) Diethanolamine, pyridine; (b) PNP–chloroformate, pyridine/CH2Cl2; (c) di-(2-(2-hydroxyethoxy)ethyl)amine,
pyridine.
bonate esters, are reactive enough to acylate amines but
not significantly reactive with hydroxyl groups. Reac-
tion of diethanolamine with compound 6, prepared in
78% yield by treating di(ethylene glycol) with PNP–
chloroformate in pyridine, provided the desired tetra-
hydroxy compound, 7, as described in Scheme 2.
ABn monomers where n generally equals 2 or 3. For
more information on dendrimers, the reader is referred
to key review articles.5–9
Divergent growth involving the formation of carba-
mates appeared to be an attractive entry into multiva-
lent platforms, because carbamates are metabolically
stable and otherwise biocompatible as evidenced by
lack of reported toxicity or immunogenicity. Carba-
mates are more resistant to proteases than are
amides,10,11 and tertiary carbamates are more resistant
than secondary carbamates.12 Surprisingly, carbamate
formation has seldom been used to prepare dendrimer-
like compounds. Dendrimeric polymers containing car-
bamates with a reversed orientation were prepared by
polymerization of 3,5-bis-((benzyloxycarbonyl)imino)-
benzyl alcohols.13 These were polydisperse compounds
with secondary carbamate linkages. We are not aware
of any dendrimer-like structures comprised of carba-
mates of the orientation described below.
Isolation and purification of compound 7 was problem-
atic due to its high polarity. It is water-soluble and
tends to remain in the aqueous layer when extractive
workups are attempted. Attempts to purify compound
7 by silica gel chromatography were unsuccessful. Due
to the difficulty of obtaining compound 7 in purified
form, and because an activated carbonate ester such as
compound 8 was ultimately desired, a more convenient
process was developed which avoids the isolation of 7
altogether. Compound 7 was converted in situ to com-
pound 8 by addition of an excess of PNP–chlorofor-
mate to the reaction mixture, thus effecting the one-pot
two-step conversion of 6 to 8 in 83% yield. Compound
8 can be reacted with an amine to provide desired
functionality at the termini as exemplified by the con-
version of 8 to 9 which was accomplished in 80% yield
by reaction with mono-Boc-ethylenediamine.‡
Attempts to react diethanolamine with tri(ethylene gly-
col) bis-chloroformate led to uncharacterizable mix-
tures, presumably due to acylation of both the hydroxyl
groups and the secondary amine. Less activated car-
bonate derivatives, such as 4-nitrophenyl (PNP)–car-
‡ Mono-Boc-ethylenediamine was prepared as described in Ref. 3.

D. S. Jones et al. / Tetrahedron Letters 42 (2001) 2069–2072
2071
Scheme 3 diagrams additional examples of divergent
growth which provide PNP–carbonate esters of
increased valence. Branched structures of higher
valence can be prepared by repetition of the process,
subjecting the purified multivalent PNP–carbonate ester
products to additional rounds of growth. Compound 8,
was converted to an octavalent carbonate ester, com-
pound 10 in 68% yield. Similarly, the tetravalent car-
bonate ester derived from pentaerythritol, compound
11, was converted to the octavalent structure 12 in 47%
yield. As expected, the process is not restricted to the
use of diethanolamine as the ABn monomer. Other
dihydroxy amines can be used as demonstrated by the
conversion of compound 6 to compound 13 in 62%
yield using di-(2-(2-hydroxyethoxy)ethyl)amine.14
3. Jones, D. S.; Coutts, S. M.; Gamino, C. A.; Iverson, G.
M.; Linnik, M. D.; Randow, M. E.; Ton-Nu, H.-T.;
Victoria, E. J. Bioconjugate Chem. 1999, 10, 480–488.
4. Tomalia, D. A.; Baker, H.; Dewald, J. R.; Hall, M.;
Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith., P.
Polym. J. (Tokyo) 1985, 17, 117.
5. Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic
Molecules: Concepts, Synthesis, Perspectives; VCH: Wein-
heim, 1996.
6. Tomalia, D. A.; Naylor, A.; Goddard, W. A. Angew.
Chem., Int. Ed. Engl. 1990, 29, 138–175.
7. Fischer, M.; Vogtle, F. Angew. Chem., Int. Ed. Engl.
1999, 38, 884–905.
8. Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681–
1712.
9. Kim, Y.; Zimmerman, S. C. Curr. Opin. Chem. Biol.
1998, 2, 733–742.
A typical procedure is described as follows for the
preparation of compound 8. A solution of 2.5 g (5.7
mmol) of compound 6 in 17 mL of pyridine was added
10. Cho, C. Y.; Moran, E. J.; Cherry, S. R.; Stephans, J. C.;
Fodor, S. P. A.; Adams, C. L.; Sundaram, A.; Jacobs, J.
W.; Schultz, P. G. Science 1993, 261, 1303–1305.
11. Cho, C. Y.; Liu, C. W.; Wemmer, D. E.; Schultz, P. G.
Bioorg. Med. Chem. 1999, 7, 1171–1179.
12. Hansen, K. T.; Faarup, P.; Bundgaard, H. J. Pharm. Sci.
1991, 80, 793–798.
13. Spindler, R.; Frechet, J. M. J. Macromolecules 1993, 26,
4809–4813.
to
a
0°C solution of 1.8
g
(17.2 mmol) of
diethanolamine in 3 mL of pyridine. The cooling bath
was removed, and the mixture was stirred for 5 h at
room temperature. The mixture was cooled to 0°C, and
40 mL of CH2Cl2 was added. To the resulting mixture
was added a solution of 11.55 g (57.3 mmol) of 4-nitro-
phenylchloroformate in 60 mL of CH2Cl2, and the
mixture was stirred for 20 h at room temperature. The
mixture was cooled to 0°C, acidified with 1N HCl, and
partitioned between 300 mL of 1N HCl and 2×200 mL
of CH2Cl2. The combined organic layers were dried
(MgSO4), filtered, and concentrated to give 13.6 g of
yellow solid. Purification by silica gel chromatography
(CH2Cl2/MeOH and EtOAc/hexanes) provided 4.91 g
(83%) of compound 8 as a sticky amorphous solid. All
new compounds gave satisfactory analytical data.15
14. Bordunov, A. V.; Hellier, P. C.; Bradshaw, J. S.; Dalley,
N. K.; Kou, X.; Zhang, X. X.; Izatt, R. M. J. Org. Chem.
1995, 60, 6097–6102.
15. Compound 6 was isolated as a white powder: mp 110°C;
1H NMR (CDCl3): l 3.89 (t, 4H), 4.50 (t, 4H), 7.40 (d,
4H), 8.26 (d, 4H); 13C NMR (CDCl3): l 68.1, 68.8, 121.9,
125.4, 145.5, 152.6, 155.6; anal. calcd for C18H16N2O11:
C, 49.55; H, 3.70; N, 6.42. Found: C, 49.38; H, 3.76; N,
6.38. Compound 8 was isolated as a sticky amorphous
1
solid: H NMR (CDCl3): l 3.72 (m, 12H), 4.31 (t, 4H),
The multivalent carbonate esters, which we have
described, can be readily reacted with primary and
secondary amine containing compounds. Work is in
progress to use the multivalent active carbonate esters
which have been described to prepare multivalent con-
jugates of biologically active molecules. In addition, we
plan to explore the preparation of dendrimers of higher
valence.
4.48 (m, 8H), 7.40 (m, 8H), 8.29 (m, 8H); 13C NMR
(CDCl3): l 47.1, 47.6, 64.9, 67.1, 67.3, 69.2, 121.8, 121.9,
125.4, 145.5, 152.4, 155.4, 155.5, 156.0; mass spectrum
(ESI): m/z (M+Na)+ 1051; HRMS (MALDI) calculated
for C44H42N4NaO25 (M+Na): 1051.1941. Found:
1051.1632.Compound 9 was isolated as a white crys-
1
talline solid: mp 55–64°C; H NMR (CDCl3): l 1.43 (s,
36H), 3.26 (s, 16H), 3.50 (m, 8H), 3.71 (t, 4H), 4.20 (t,
8H), 4.26 (m, 4H), 5.35 (brd s, 4H), 5.81 (brd s, 2H), 5.98
(brd s, 2H); 13C NMR (CDCl3): l 28.4, 40.4, 41.2, 47.8,
48.3, 62.7, 64.6, 69.3, 79.4, 156.1, 156.8; HRMS
(MALDI) calculated for C46H84NaN10O21 (M+Na):
1135.5705. Found: 1135.5729. Compound 10 was isolated
Acknowledgements
We thank La Jolla Pharmaceutical Company for
support.
1
as a crystalline solid: mp 67–69°C; H NMR (CDCl3): l
3.50–3.80 (m, 28H), 4.22 (m, 12H), 4.43 (m, 16H), 7.40
(m, 16H), 8.30 (m, 16H); 13C NMR (CDCl3): l 46.7, 47.0,
47.4, 47.6, 63.3, 63.9, 64.5, 66.9, 67.3, 69.2, 121.8, 125.3,
145.5, 152.4, 155.3, 155.7, 155.9; mass spectrum (ESI):
m/z (M+Na)+ 2235; anal. calcd for C90H88N14O53: C,
48.83; H, 4.01; N, 8.86. Found: C, 49.07; H, 4.21; N,
8.45; HRMS (ESI) calculated for C90H88NaN14O53 (M+
Na): 2235.4519. Found: 2234.4494. Compound 11 was
References
1. Jones, D. S.; Hachmann, J. P.; Osgood, S. A.; Hayag, M.
S.; Barstad, P. A.; Iverson, G. M.; Coutts, S. M. Biocon-
jugate Chem. 1994, 5, 390–399.
2. Jones, D. S.; Barstad, P. A.; Field, M. J.; Hachmann, J.
P.; Hayag, M. S.; Hill, K. W.; Iverson, G. M.; Liv-
ingston, D. A.; Palanki, M. S.; Tibbetts, A. R.; Yu, L.;
Coutts, S. M. J. Med. Chem. 1995, 38, 2138–2144.
1
isolated as a white crystalline solid: mp 175°C; H NMR
(CDCl3): l 4.61 (s, 8H), 7.40, (m, 8H), 8.30 (m, 8H); 13C
NMR (CDCl3): l 42.4, 66.8, 122.5, 125.4, 145.2, 151.7,
155.1; anal. calcd for C33H24N4O20: C, 49.76; H, 3.04; N,

2072
D. S. Jones et al. / Tetrahedron Letters 42 (2001) 2069–2072
7.03. Found: C, 49.55; H, 3.10; N, 7.06. Compound 12
was isolated as a sticky viscous oil: H NMR (CDCl3): l
Found: 2003.3460. Compound 13 was isolated as a vis-
cous oil: 1H NMR (CDCl3): l 3.55 (m, 8H), 3.64 (m,
12H), 3.73 (m, 8H), 4.22 (t, 4H), 4.40 (t, 8H), 7.40 (d,
8H), 8.30 (d, 8H); 13C NMR (CDCl3): l 47.7, 48.2, 64.3,
68.2, 68.3, 68.4, 69.3, 69.6, 69.9, 121.7, 125.3, 145.4,
152.4, 155.4, 156.0; HRMS (FAB) C50H56CsN6O29 (M+
Cs): 1337.2146. Found: 1337.2079.
1
3.69 (m, 16H), 4.31 (s, 8H), 4.41 (m, 16H), 7.39 (m, 16H),
8.25 (m, 16H);); 13C NMR (CDCl3): l 43.8, 47.3, 48.0,
63.4, 66.9, 121.9, 125.5, 145.8, 152.6, 155.3, 155.4, 155.5;
mass spectrum (ESI): m/z (M+Na)+ 2003; HRMS (ESI)
calculated for C81H72NaN12O48 (M+Na): 2003.3498.
.