Thermally induced substrate release via intramolecular cyclizations of Amino esters and Amino carbonates

Article abstract of DOI:10.1016/j.tet.2014.03.092

The relative cleavage of an alcohol from a panel of amino esters and amino carbonates via intramolecular cyclization was examined as a mechanism for substrate release. Thermal stability at 37 °C was observed only for the seven-membered ring progenitors. Applicability of the approach was illustrated by δ-lactam formation within a poly(dimethylsiloxane) microchannel for release of a captured fluorescent probe.

Full text of DOI:10.1016/j.tet.2014.03.092

Tetrahedron 70 (2014) 3422e3429
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Thermally induced substrate release via intramolecular cyclizations
of Amino esters and Amino carbonates
,
Ralph J. Knipp a, Rosendo Estrada b, Palaniappan Sethu b, Michael H. Nantz a
*
a Department of Chemistry, University of Louisville, Louisville, KY 40292, USA
b Department of Bioengineering, University of Louisville, Louisville, KY 40292, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The relative cleavage of an alcohol from a panel of amino esters and amino carbonates via intramolecular
cyclization was examined as a mechanism for substrate release. Thermal stability at 37 ꢀC was observed
only for the seven-membered ring progenitors. Applicability of the approach was illustrated by d-lactam
formation within a poly(dimethylsiloxane) microchannel for release of a captured fluorescent probe.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 20 January 2014
Received in revised form 17 March 2014
Accepted 25 March 2014
Available online 1 April 2014
Keywords:
Lactam
Oxazolidinone
Drug delivery
Poly(dimethylsiloxane) microchannel
1. Introduction
We disclose here a brief study on the propensity of amino ester
and amino carbonate substrates of the type indicated in Fig. 1 to
undergo intramolecular cyclization for purposes of releasing sub-
strate HO-R0. Similar intramolecular cyclization modes of substrate
release previously have been exploited as mechanisms for drug
delivery.1e4 In these cases, the released substrate is either a phenol
(R0¼Ar) or an aniline derivative so that the intramolecular cycli-
zation occurs rapidly at physiological temperature,5 generally with
a half-life from minutes to 1 h.6 Intramolecular cyclizations of this
type also have been used to unmask aromatic hydroxyl,7 amine8 or
thiol9 moieties as mechanisms to initiate electron cascade reactions
or for release of polymer-bound drugs.10 Again the focus was to use
the intramolecular cyclization for rapid drug release at physiolog-
ical temperature, and this required R0 to be aromatic. Few examples
have used this approach to release non-aromatic alcohols,11,12 the
most common being for the delivery of 5-halo deoxyuridine
analogs.13e15
Fig. 1. Heat-induced cyclization via (a) lactamization or (b) carbamate formation.
determine factors that would minimize the rate of cyclization at
37 ꢀC while allowing cyclization to proceed at higher temperature
(Fig. 2).
Our interest in control over heat-induced intramolecular cycli-
zation as a mechanism for substrate release led us to investigate
linkers that would expel HOeR0 at temperatures above 37 ꢀC, such
as in response to an externally triggered local hyperthermia. Con-
sequently, we prepared16 a panel of amino carbonyl substrates to
2. Results and discussion
2.1. Cyclization panel (Fig. 2)
All substrates were prepared to contain an N-allyl moiety for
convenient functionalization, such as hydrosilylation or thiol in-
corporation, for eventual covalent attachment to various supports.
* Corresponding author. Tel.: þ1 502 852 8069; fax: þ1 502 852 7214; e-mail
address: michael.nantz@louisville.edu (M.H. Nantz).
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.03.092

R.J. Knipp et al. / Tetrahedron 70 (2014) 3422e3429
3423
Scheme 2. Synthesis of amino carbonates 2. Conditions: (a) TBSCl, Et3N, imidazole,
CH2Cl2, 0 ꢀC to rt, 95e98%; (b) allyl bromide, NaH, THF, 0 ꢀC to rt, 77e84%; (c) TBAF,
THF, 0 ꢀC to rt, 90e95%; (d). (imid)2C]O, (i-Pr)2NEt, CH2Cl2, 0 ꢀC to rt, 93e95%; (e) 2-
(9-anthracenyl)ethanol, KOH, toluene, 60 ꢀC, 34e45%; (f) TFA, CH2Cl2,
96e100%.
0
ꢀC, 1 h,
Fig. 2. Cyclization precursors (R¼2-(9-anthracenyl)ethyl). Ring size after intra-
molecular cyclization given in parentheses.
To quantify release of the ‘payload’ (e.g., HO-R0 in 1e3 and 5,6, H2N-
R0 in 4) on heating, R0 was selected as a 9-substituted anthracene for
ease of UV and fluorescent measurements. In addition to the cy-
clization ring size (italics, Fig. 2), we varied the carbonyl function-
ality to include ester, carbonate, carbamate, and amide examples.
The influence of gem-dimethylation, as in seven-membered ring
precursors ester 5 and carbonate 6, also was examined as a means
to enhance intramolecular cyclization via the ThorpeeIngold
effect.17
Scheme 3. Synthesis of amino carbamate 3. Conditions: (a) phthalic anhydride, tolu-
ene, reflux, 77%; (b) allyl bromide, NaH, THF, 0 ꢀC to rt, 58%; (c) hydrazine mono-
hydrate, 2:1 CH2Cl2/EtOH, 0 ꢀC to rt, 92%; (d) ArCH2CH2OC(O)Cl, Et3N, CH2Cl2, 0 ꢀC to rt,
12 h, 41%; (e) TFA, CH2Cl2, 0 ꢀC, 1 h, 88%.
2.2. Syntheses (Schemes 1e5)
N-Allylation18 of commercial Boc-protected amino acids 7 fol-
lowed by carbodiimide-mediated esterification with 2-(9-
anthracenyl)ethanol19 and Boc-deprotection furnished amino
esters 1.1e3 (Scheme 1). Condensation of amino acid 8.1 with 2-(9-
anthracenyl)ethanamine20 gave the amino amide substrate 4. In
similar fashion, N-allylation of Boc-protected alcohols 10
(Scheme 2), after necessary silylation and desilylation steps, gave
alcohols 11. The carbonate moiety was installed by first forming the
corresponding acyl imidazole intermediates 12. Reaction with
anthracenyl ethanol under basic conditions21 then gave amino
carbonates 2.1 and 2.2.
The gem-dimethyl amino ester 5 was prepared by applying the
above-described three-step sequence of N-allylation, esterification,
and Boc-deprotection to commercially available amino acid 15
(Scheme 4). Synthesis of gem-dimethyl amino carbonate 6 followed
from bis-a
-methylation10 of the ester derived from amino acid 8.1
(Scheme 5). Subsequent chemoselective ester reduction22 and
carbonate formation using the established method gave desired
substrate 6.
Scheme 4. Conditions: (a) allyl bromide, NaH, THF,
0
ꢀC to rt, 68%; (b) 2-(9-
anthracenyl)ethanol, DIC, cat. DMAP, CH2Cl2, 12 h, 87%; (c) TFA, CH2Cl2, 0 ꢀC, 1 h, 85%.
2.3. Cyclization study
Scheme 1. Synthesis of amino esters 1 and amide 4. Conditions: (a) allyl bromide,
NaH, THF, 0 ꢀC to rt, 81e98%; (b) 2-(9-anthracenyl)ethanol (or 2-(9-anthracenyl)
ethanamine), DIC, cat. DMAP, CH2Cl2, 12 h, 59e97%; (c) TFA, CH2Cl2, 0 ꢀC, 1 h, 72e100%.
To determine the release rates of the 9-substituted anthracenes
(e.g., 20, Scheme 6) from the panel of amino carbonyl substrates,
dilute methanol solutions of each compound were heated at 55 ꢀC.
Aliquots taken at various times were analyzed by normal phase
HPLC for the appearance of 20 or 21. Amino carbamate 3 and amino
amide 4 were unreactive under the conditions and did not release
Chloroformate acylation of N-allyl-N-Boc-ethanediamine pro-
vided a convenient route for synthesis of the amino carbamate
substrate 3 (Scheme 3).

3424
R.J. Knipp et al. / Tetrahedron 70 (2014) 3422e3429
that lead to seven-membered rings. In regards to potential heat-
triggered release applications, the high thermal responsiveness of
esters 1.1 and 1.2 and carbonates 2.1 and 2.2 make these constructs
ideal for applications involving thermally sensitive substrates, such
as in cell studies involving triggered release at or below 37 ꢀC. In
contrast, the resistance to cyclization of the 7-ring progenitor ester
1.3 is promising for higher temperature applications (Fig. 3), al-
though the overall release of 20 from 1.3 on heating at 55 ꢀC was
low. Comparison of amino esters 1.3 and 5 shows that gem-dime-
thylation more than doubled the heat-induced release of anthra-
cene 20 after heating 24 h. The gem-dimethyl amino carbonate 6
provided an even higher thermal response than gem-dimethyl ester
5 with a nearly 40% release of payload at 55 ꢀC over 24 h. Fur-
thermore, the gem-dimethyl seven-ring carbonate motif resists
cyclization at lower temperatures. Incubation of 6 at 37 ꢀC for 24 h
resulted in only 5.7ꢁ0.7% release of 20.
Scheme 5. Conditions: (a) MeOH, DIC, cat. DMAP, rt, 80%; (b) LiHMDS, MeI, THF,
e78 ꢀC to rt, 67%; (c) LiBH4, THF, 0 ꢀC to rt, 73%; (d) (imid)2C]O, (i-Pr)2NEt, CH2Cl2,
0 ꢀC to rt, 92%; (e) 2-(9-anthracenyl)ethanol, NaH, THF, ꢂ5 ꢀC to rt, 48%; (f) TFA, CH2Cl2,
2.4. Microchannel application
0
ꢀC, 1 h, 92%.
With these cyclization trends established, we set out to apply
the approach toward mild heat-induced release of a substrate from
a poly(dimethylsiloxane) (PDMS) microchannel using the ester 1.2
motif as a representative example (Scheme 7). Microchannels
fabricated using PDMS have shown great utility for molecular and
cellular separations due to their extremely high surface to volume
ratio.23 Further, small PDMS channel heights and incorporation of
features, such as microfabricated grooves, enhance fluid mixing
within the channel to increase interactions of functionalized sur-
faces with target molecules flowing through the channel.24
The N-allyl moiety of ammonium aminooxy ester 22 (Scheme 7)
was hydrosilylated25 to attach a terminal triethoxysilane group. The
Scheme 6. Heat-induced release of anthracene probes.
detectable 20 or 21, respectively (Supplementary data, Table 1). As
could be expected, anthracene alcohol 20 was released from the
amino ester and amino carbonate progenitors of the five- and six-
membered lactams (19, Y¼CH2, n¼0, 1) and oxazolidinones (19,
Y¼O, n¼0, 1) at rates faster than from the progenitors of corre-
sponding seven-membered rings (Fig. 3). Control experiments in-
dicated that ester or carbonate methanolysis did not occur under
the conditions to yield 20. Whereas dilute conditions were used to
reduce the incidence of intermolecular reactions, these cannot be
ruled out as a possible source of 20 particularly with the substrates
Fig. 3. Percent release of 20 from indicated substrates in MeOH at 55 ꢀC. Shown are
the standard deviations from the mean (n¼3).
Scheme 7. Poly(dimethylsiloxane) microchannel functionalization and heat-induced
release study (FL¼fluorescein, Xꢂ ¼ CF3CO2ꢂ).

R.J. Knipp et al. / Tetrahedron 70 (2014) 3422e3429
3425
ammonium aminooxy substructure of ester 22 follows from our
3. Conclusion
previously published work on ammonium aminooxy reagents for
capture of aldehyde and ketone metabolites from aqueous cell
extracts.26,27 Subsequent condensation28,29 with the PDMS micro-
channel (Fig. 4a) covalently attached 23 to afford an aminooxy-
functionalized microchannel specific for reaction with aldehydes
and ketones. Injection of an acetonitrile solution containing
a fluorescent aldehyde probe, FITC (fluorescein isothiocyanate)
reacted with a 4-aminobutanal equivalent,30 anchored the fluo-
rophore to the surfaces of the microchannel via oxime ether for-
mation. The quaternary ammonium salt of 23 ensures availability of
the aminooxy moiety for reaction with aldehydes since charged
polar groups are repelled by the hydrophobic PDMS matrix. The
capture and covalent attachment of the FITC-derived fluorophore to
the microchannel was verified using fluorescent microscopy
(Fig. 4b). Boc-deprotection using trifluoroacetic acid afforded tri-
fluoroacetate salt 25. Concerned that loss of captured aldehyde
probe may occur at this stage, we verified that no fluorescent probe
was released prior to neutralization and thermal triggering. Indeed,
no fluorescence was observed in the microchannel effluent after
heating 25 at 60 ꢀC for 30 min. Injection of a dilute solution of
triethanolamine into the microchannel neutralized the linker. After
subsequent rinsing (CH3CN) of the microchannel, we were gratified
to observe that mild heating (60 ꢀC, 30 min) promoted the intra-
molecular cyclization to release alcohol 27. As can be seen in Fig. 4c,
nearly complete release of the bound fluorophore was achieved.
Further utilization of this release mechanism in microchannel
studies of cell metabolites requiring hydroxide-free cleavage con-
ditions are ongoing.
In summary, a brief study on the cyclization of amino esters and
amino carbonates as a mode for substrate release revealed that
both ring size and carbonyl functionality significantly influence the
release rate. The process is facile at mild temperatures in cases
where five- and six-membered rings are formed, even for non-
phenolic alcohols as illustrated in a PDMS microchannel applica-
tion. In contrast, an amino carbonate motif leading to a seven-
membered ring appears well suited for applications requiring
thermal stability at 37 ꢀC while still releasing the alcohol substrate
at slightly elevated temperatures. This particular secondary
amineecarbonate structural combination may be a promising
linker for drug delivery vehicles relying on the generation of local
hyperthermia.
4. Experimentals
4.1. General
Reagent grade solvents were used for extraction and flash
chromatography. Tetrahydrofuran was dried by distillation from
LiAlH4. Acetonitrile was dried by distillation from CaH2. All other
commercial reagents were used as received without additional
purification. The progress of reactions was checked by thin-layer
ꢀ
chromatography (TLC, silica gel 60 A F-254 plates). The plates
were visualized first with UV illumination followed by staining
using iodine, p-anisaldehyde, phosphomolybdic acid hydrate, or
ninhydrin. Column chromatography was performed using silica gel
(230e400 mesh). NMR spectra were obtained using a Varian/Agi-
lent 400-MR NMR spectrometer equipped with a 5 mm z-axis
gradient AutoX probe operating at the nominal 1H frequency of
399.66 MHz and 13C frequency of 100.49 MHz. All spectra are re-
ported in parts per million (ppm) relative to the residual solvent
peak in 1H NMR and the deuterated solvent peak in 13C NMR. High-
resolution mass spectra were obtained using a Finnigan LTQ-FT
spectrometer (Thermo Electron Corp). Release results were
quantitated using a Waters Delta 600 HPLC fitted with a Waters
ꢀ
Nova-Pak HR Silica 6 mm 60 A 3.9ꢃ300 mm Prep Column and
a Waters 2487 detector set at 254 nm.
4.2. Representative amino ester synthesis
4.2.1. 5-(Allyl(tert-butoxycarbonyl)amino)pentanoic acid (8.2). Boc-
protected amine 7.2 (4.65 g, 21.4 mmol) was added to a slurry of
60% NaH (4.28 g, 107 mmol) in dry THF (140 mL) at 0 ꢀC. After 1 h of
stirring, allyl bromide (5.56 mL, 64.2 mmol) was added dropwise.
After 24 h, the reaction mixture was cooled to 0 ꢀC and quenched
with water until the reaction mixture became transparent. The
reaction mixture was acidified to pH w3 by addition of 1 M HCl and
the layers were separated. The aqueous phase was extracted with
EtOAc (2ꢃ30 mL) and the combined organic phase was washed
with brine (50 mL), dried over Na2SO4, filtered, and concentrated in
vacuo. The crude material was purified by column chromatography
(SiO2, 1:1 EtOAc/hexanes) to give 8.2 as an oil (4.77 g, 87%); Rf 0.36
(1:1 EtOAc/hexanes); 1H NMR (400 MHz, CDCl3)
d 1.43 (s, 9H),
1.51e1.64 (m, 4H), 2.35 (t, J¼7.0 Hz, 2H), 3.17 (br s, 2H), 3.78 (br s,
2H), 5.09 (d, J¼11.6 Hz, 2H), 5.70e5.78 (m, 1H); 13C NMR (100 MHz,
CDCl3)
d 22.1, 27.8, 28.6, 33.9, 46.2, 49.7, 79.9, 116.5, 134.4, 155.8,
179.6; FT-ICR-MS calcd for C13H22NOꢂ4 [MꢂH]ꢂ m/z 256.1554,
found 256.1555.
4.2.2. 2-(Anthracen-9-yl)ethyl 5-(allyl(tert-butoxycarbonyl)amino)
pentanoate (9.2). To amino acid 8.2 (255 mg, 0.99 mmol) and al-
cohol 20 (197 mg, 0.88 mmol) in dry CH2Cl2 (8 mL) were added DIC
Fig. 4. (a) Schematic of poly(dimethylsiloxane) microchannel; (b) fluorescence mi-
croscopy image of FITC-derived substrate 12 within the microchannel and (c) same
field of view showing microchannel fluorescence after heating neutralized 13.
(211
mL, 1.35 mmol) and 4-(N,N-dimethylamino)pyridine (DMAP,

3426
R.J. Knipp et al. / Tetrahedron 70 (2014) 3422e3429
pinch). After 3 h, the white solids were filtered and the filter cake
was washed with CH2Cl2. The combined filtrate was condensed in
vacuo and the crude material was purified by column chromatog-
raphy (SiO2, 0:100 to 1:19 EtOAc/CH2Cl2 gradient) to give 9.2
(242 mg, 59%) as an oil; Rf 0.46 (1:19 EtOAc/CH2Cl2); FTIR 3058,
NMR (100 MHz, CDCl3) d 27.6, 28.0, 28.6, 43.9, 49.8, 66.0, 67.3, 79.9,
116.8, 124.1, 125.2, 126.4, 127.2, 128.3, 129.5, 130.5, 131.7, 134.3,
155.5, 155.6; FT-ICR-MS calcd for C28H33NNaOþ5 [MþNa]þ m/z
486.2251, found 486.2251.
Trifluoroacetic acid (0.50 mL, 6.53 mmol) was added to a solu-
tion of Boc-protected 2.2 (12 mg, 0.026 mmol) in dry CH2Cl2
(0.50 mL) at 0 ꢀC. After stirring for 1 h, the volatiles were removed
in vacuo to afford the TFA salt of 2.2 (12 mg, 96%) as an oil; 1H NMR
2981, 1729, 1685 cmꢂ1 1H NMR (400 MHz, CDCl3)
; d 1.46 (s, 9H),
1.50e1.61 (m, 4H), 2.33 (t, J¼7.4 Hz, 2H), 3.16 (br s, 2H), 3.79 (br s,
2H), 3.97 (t, J¼7.8 Hz, 2H), 4.48 (t, J¼7.8 Hz, 2H), 5.11 (d, J¼11.6 Hz,
2H), 5.72e5.82 (m, 1H), 7.45e7.49 (m, 2H), 7.51e7.59 (m, 2H), 8.01
(d, J¼8.4 Hz, 2H), 8.34 (d, J¼9.2 Hz, 2H), 8.39 (s, 1H); 13C NMR
(400 MHz, CDCl3)
d
2.10 (t, J¼6.8 Hz, 2H), 3.00 (br s, 2H), 3.53 (br s,
2H), 4.02 (t, J¼8.4 Hz, 2H), 4.23 (t, J¼6.0 Hz, 2H), 4.50 (t, J¼8.4 Hz,
2H), 5.12e5.48 (m, 2H), 5.83e5.96 (m, 1H), 7.45e7.56 (m, 4H), 8.00
(d, J¼8.4 Hz, 2H), 8.30 (d, J¼8.8 Hz, 2H), 8.39 (s, 1H), 9.64 (br s, 2H);
(100 MHz, CDCl3) d 22.3, 27.5, 27.9, 28.6, 34.2, 46.3, 49.9, 64.3, 79.6,
116.2,124.3,125.1,126.2,127.0,129.2,129.4,130.5,131.7,134.5,155.7,
173.8; FT-ICR-MS calcd for C29H35NNaOþ4 [MþNa]þ m/z 484.2458,
found 484.2459.
13C NMR (100 MHz, CDCl3)
d 25.7, 27.4, 43.9, 50.1, 64.6, 67.7, 124.1,
124.3, 125.2, 126.4, 127.2, 127.6, 128.3, 129.5, 130.5, 131.7, 155.3; FT-
ICR-MS calcd for C23H26NOþ3 [MþH]þ m/z 364.1907, found 364.1911.
4.2.3. 2-(Anthracen-9-yl)ethyl 5-(allylamino)pentanoate (1.2). Tri-
fluoroacetic acid (0.74 mL, 9.60 mmol) was added to a solution of
9.2 (68 mg, 0.15 mmol) in dry CH2Cl2 (0.74 mL) at 0 ꢀC. After stirring
for 1 h, the volatiles were removed in vacuo and the remaining
residue was diluted with Et2O (10 mL) and washed with NaHCO3
(3ꢃ5 mL). The organic phase was washed with brine (5 mL), dried
over Na2SO4, filtered, and concentrated in vacuo to give 1.2 (53 mg,
100% yield) as an oil; Rf 0.20 (1:9 MeOH/CH2Cl2); 1H NMR
4.4. Synthesis of carbamate 3 (Scheme 3)
4.4.1. tert-Butyl
allyl(2-(1,3-dioxoisoindolin-2-yl)ethyl)carbamate
(14). tert-Butyl
N-(2-(1,3-dioxo-2,3-dihydro-1H-isoindol-2-yl)
ethyl)carbamate phthalimide (5.76 g, 19.8 mmol) was added to
a slurry of NaH (1.59 g of 60% in mineral oil, 39.7 mmol) in dry THF
(83 mL) at 0 ꢀC. After stirring for 1 h, allyl bromide (2.23 mL,
25.8 mmol) was added dropwise. The slurry was stirred for 3 days
and then quenched by addition into water (50 mL). The layers were
separated and the aqueous phase was extracted with Et2O
(2ꢃ35 mL). The combined organic phase was washed with brine
(50 mL), dried over Na2SO4, filtered, and concentrated in vacuo. The
crude material was purified by column chromatography (SiO2, 1:19
EtOAc/CH2Cl2) to give 14 (3.77 g, 58%) as a white solid; Rf 0.55 (1:19
(400 MHz, CDCl3)
d
1.49 (dt, J¼7.4 Hz, 2H), 1.64 (dt, J¼7.6 Hz, 2H),
1.84 (br s, 1H), 2.32 (t, J¼7.4 Hz, 2H), 2.59 (t, J¼7.2 Hz, 2H), 3.24 (d,
J¼5.6 Hz, 2H), 3.97 (t, J¼7.8 Hz, 2H), 4.48 (t, J¼7.8 Hz, 2H), 5.11 (d,
J¼10.4 Hz, 1H), 5.18 (d, J¼17.2 Hz, 1H), 5.86e5.96 (m, 1H), 7.47 (t,
J¼7.4 Hz, 2H), 7.55 (t, J¼7.6 Hz, 2H), 8.01 (d, J¼8.4 Hz, 2H), 8.34 (d,
J¼9.6 Hz, 2H), 8.38 (s, 1H); 13C NMR (100 MHz, CDCl3)
d 22.8, 27.5,
29.5, 34.3, 48.9, 52.5, 64.3, 116.4, 124.3, 124.6, 125.1, 126.2, 126.9,
129.4, 130.5, 131.7, 136.7, 173.9; FT-ICR-MS calcd for C24H28NO2þ
[MþH]þ m/z 362.2115, found 362.2141.
EtOAc/hexanes); mp¼73e76 ꢀC; 1H NMR (400 MHz, CDCl3)
d 1.27
(s, 9H), 3.46 (br s, 2H), 3.81 (br s, 2H), 3.87 (br s, 2H), 5.01e5.13 (m,
2H), 5.71e5.77 (m, 1H), 7.70 (br s, 2H), 7.82 (br s, 2H); 13C NMR
4.3. Representative amino carbonate synthesis
(100 MHz, CDCl3) d 28.2, 44.5, 49.3, 50.2, 80.1, 117.2, 123.4, 132.3,
133.9, 134.2, 155.3, 168.3.
4.3.1. 3-(Allyl(tert-butoxycarbonyl)amino)propyl 1H-imidazole-1-
carboxylate
(12.2). N,N-Diisopropylethylamine
(566
mL,
4.4.2. 2-(Anthracen-9-yl)ethyl N-(2-(allylamino)ethyl)carbamate (3,
3.25 mmol) was added to a solution of alcohol 11.2 (399 mg,
1.85 mmol) in dry CH2Cl2 (46 mL) at 0 ꢀC. 1,10-Carbonyldiimidazole
(527 mg, 3.25 mmol) was then added to the cooled solution and the
cooling bath was removed to allow the reaction mixture to slowly
warm to rt. After 24 h, the reaction mixture was washed with water
(2ꢃ20 mL), brine (20 mL), and then dried (Na2SO4). After filtration
and concentration in vacuo, the residue was purified by column
chromatography (SiO2, EtOAc) to give 12.2 (543 mg, 95%) as a col-
TFA salt). Hydrazine monohydrate (147 mL, 3.03 mmol) was added
to a solution of 14 (217 mg, 0.66 mmol) in 2:1 CH2Cl2/EtOH (6 mL)
with stirring at 0 ꢀC. The reaction mixture was stirred for 18 h,
allowing it to warm slowly to rt. The white precipitate was then
filtered and the filter cake was washed with CH2Cl2 and concen-
trated in vacuo. The concentrate was diluted with CH2Cl2 and the
precipitate was filtered, the cake washed with CH2Cl2, and con-
centrated in vacuo again to give the crude amine (light yellow oil,
122 mg, 92%), which was used in the next step without further
purification. Rf 0.47 (10:2:88 MeOH/NH4OH/CH2Cl2); 1H NMR
orless oil; Rf 0.50 (EtOAc); 1H NMR (400 MHz, CDCl3)
d 1.43 (s, 9H),
2.01 (dt, J¼6.6 Hz, 2H), 3.34 (br s, 2H), 3.82 (br s, 2H), 4.43 (t,
J¼6.4 Hz, 2H), 5.09e5.14 (m, 2H), 5.74e5.81 (m, 1H), 7.06 (s, 1H),
(500 MHz, CDCl3)
d
1.40 (s, 2H), 1.43 (s, 9H), 2.79 (t, J¼5.0 Hz, 2H),
7.41 (s, 1H), 8.12 (s,1H); 13C NMR (100 MHz, CDCl3)
d 27.7, 28.6, 43.5,
3.22 (br s, 2H), 3.81 (br s, 2H), 5.09e5.12 (m, 2H), 5.74e5.79 (m,1H);
50.3, 66.4, 80.2, 116.7, 117.3, 130.9, 134.2, 137.3, 148.8, 155.6; FT-ICR-
13C NMR (100 MHz, CDCl3)
d 28.6, 40.8, 50.1, 79.8, 116.4, 134.3,
MS calcd for C15H24N3Oþ4 [MþH]þ m/z 310.1761, found 310.1765.
156.0; FT-ICR-MS calcd for C10H21N2Oþ2 [MþH]þ m/z 201.1598,
found 201.1599.
4.3.2. 2-(Anthracen-9-yl)ethyl 3-((allyl)amino)propyl carbonate (2.2,
TFA salt). Alcohol 20 (131 mg, 0.59 mmol) was added to a mixture
of 12.2 (210 mg, 0.65 mmol) and KOH (s, 1 pellet) in dry toluene
(3 mL) at 60 ꢀC. After 5 h, the reaction mixture was concentrated in
vacuo and the residue was diluted with CH2Cl2 (5 mL). The solution
was washed with water (3ꢃ5 mL), dried over Na2SO4, filtered, and
concentrated in vacuo. The crude material was purified by column
chromatography (SiO2, 1:19 EtOAc/CH2Cl2) to give Boc-protected
2.2 as an orange oil (93 mg, 34%). Rf 0.63 (1:19 EtOAc/hexanes);
The amine (188 mg, 0.94 mmol) was added dropwise with
stirring to a solution of ClC(O)OCH2CH2(9-anthracenyl) (303 mg,
1.06 mmol) in dry CH2Cl2 (3.5 mL) at 0 ꢀC. After 10 min, Et3N
(148 mL, 1.06 mmol) was added dropwise to the reaction mixture,
causing the solution to darken. After stirring for 17 h, the reaction
mixture was quenched by addition of satd NH4Cl (5 mL) and the
aqueous phase was extracted with CH2Cl2 (5 mL). Combined or-
ganic phase was dried over Na2SO4, filtered, and concentrated in
vacuo. The crude material was purified by column chromatography
FTIR: 3017, 2971, 1739, 1229 cmꢂ1; 1H NMR (400 MHz, CDCl3)
d
1.47
(SiO2, 5:1:4 CH2Cl2/hexanes/EtOAc) to give Boc-protected
(171 mg, 41%) as a yellow gum; Rf 0.33 (1:19 EtOAc:CH2Cl2); FTIR:
3449, 3058, 2971, 1724, 1514 cmꢂ1; 1H NMR (400 MHz, CDCl3)
1.46
3
(s, 9H), 1.92 (br s, 2H), 3.30 (br s, 2H), 3.83 (br s, 2H), 4.05 (t,
J¼8.0 Hz, 2H), 4.19 (t, J¼6.2 Hz, 2H), 4.49 (t, J¼8.2 Hz, 2H), 5.13 (d,
J¼11.2 Hz, 2H), 5.74e5.84 (m, 1H), 7.46e7.50 (m, 2H), 7.54e7.58 (m,
2H), 8.02 (d, J¼8.0 Hz, 2H), 8.34 (d, J¼8.8 Hz, 2H), 8.40 (s, 1H); 13C
d
(s, 9H), 3.13 (br s, 2H), 3.36 (br s, 2H), 3.83 (br s, 2H), 3.97 (t,
J¼8.0 Hz, 2H), 4.43 (br s, 2H), 5.10e5.15 (m, 2H), 5.75e5.81 (m, 1H),

R.J. Knipp et al. / Tetrahedron 70 (2014) 3422e3429
3427
7.44e7.48 (m, 2H), 7.52e7.56 (m, 2H), 8.00 (d, J¼8.4 Hz, 2H),
8.35e8.37 (m, 3H); 13C NMR (100 MHz, CDCl3)
28.1, 28.5, 40.5,
(d, J¼9.2 Hz, 2H), 8.38 (s, 1H), 9.47 (br s, 2H); 13C NMR (100 MHz,
d
CDCl3) d 26.5, 27.4, 29.4, 32.1, 36.1, 37.1, 43.2, 49.9, 64.4, 124.1, 124.3,
46.2, 50.6, 64.6, 80.3, 116.6, 124.4, 125.1, 126.1, 126.8, 129.3, 130.5,
131.7, 134.0, 155.3, 157.0; FT-ICR-MS calcd for C27H32N2NaO4þ
[MþNa]þ m/z 471.2254, found 471.2256.
125.2, 126.2, 127.0, 127.9, 129.2, 129.5, 130.5, 131.7, 174.1; FT-ICR-MS
calcd for C27H34NOþ2 [MþH]þ m/z 404.2584, found 404.2588.
Trifluoroacetic acid (0.50 mL, 6.53 mmol) was added to a solu-
tion of Boc-protected 3 (20 mg, 0.045 mmol) in dry CH2Cl2
(0.50 mL) at 0 ꢀC. After stirring for 1 h, the volatiles were removed
in vacuo to afford the TFA salt of 3 (18 mg, 88%) as an oil; 1H NMR
4.6. Synthesis of gem-dimethylcarbonate 6 (Scheme 5)
4.6.1. Methyl 4-(allyl(tert-butoxycarbonyl)amino)-2,2-
dimethylbutanoate (17). DIC (837
(pinch) were added to a mixture of amino acid 8.1 (631 mg,
2.59 mmol) and dry MeOH (173 L, 4.28 mmol) in CH2Cl2 (32 mL) at
mL, 5.34 mmol) and DMAP
(400 MHz, CDCl3)
d 3.09 (br s, 2H), 3.48e3.56 (m, 4H), 3.90 (t,
J¼8.0 Hz, 2H), 4.36 (t, J¼8.0 Hz, 2H), 5.35e5.47 (m, 2H), 5.81e5.88
m
(m, 1H), 7.41e7.51 (m, 4H), 7.97 (d, J¼8.4 Hz, 2H), 8.27 (d, J¼8.8 Hz,
rt. After 12 h, the precipitated solids were filtered and the filter cake
was washed with CH2Cl2. The combined filtrate was concentrated
in vacuo and the resulting residue was purified by column chro-
matography (SiO2, 1:1:8, EtOAc/THF/hexanes) to give methyl 4-
(allyl(tert-butoxycarbonyl)amino)butanoate as a pale yellow oil
(532 mg, 80%); Rf 0.65 (1:1:8, EtOAc/THF/hexanes); 1H NMR
2H), 8.34 (s, 1H), 9.47 (br s, 2H); 13C NMR (100 MHz, CDCl3)
d 27.8,
37.8, 47.2, 50.2, 65.1, 124.3, 124.6, 125.2, 126.2, 126.9, 127.5, 129.4,
130.5, 131.7, 134.3, 157.5; FT-ICR-MS calcd for C22H25N2Oþ2 [MþH]þ
m/z 349.1910, found 349.1913.
4.5. Synthesis of gem-dimethyl ester 5 (Scheme 4)
(400 MHz, CDCl3)
d
1.44 (s, 9H), 1.82 (dt, J¼7.0 Hz, 2H), 2.30 (t,
J¼7.4 Hz, 2H), 3.20 (br s, 2H), 3.65 (s, 3H), 3.79 (br s, 2H), 5.10 (d,
4.5.1. 6-(Allyl(tert-butoxycarbonyl)amino)-4,4-dimethylhexanoic
acid (16). Amino acid 15 (912 mg, 3.52 mmol) was added to a slurry
of 60% NaH (703 mg, 17.6 mmol) in dry THF (18 mL) at 0 ꢀC. After
J¼12.8 Hz, 2H), 5.72e5.80 (m, 1H); 13C NMR (100 MHz, CDCl3)
d
23.7, 28.6, 31.4, 45.9, 49.7, 51.8, 79.8, 116.7, 134.3, 155.7, 173.8.
The methyl ester (532 mg, 2.06 mmol) was dissolved in dry THF
stirring for 1 h, allyl bromide (913
m
L, 10.5 mmol) was added
(10 mL) and cooled to ꢂ78 ꢀC with stirring. A solution of lithium
bis(trimethylsilyl)amide (LiHMDS) in THF (6.20 mL of 1 M solution,
6.20 mmol) was added dropwise to the reaction solution. After
dropwise. After 24 h, the reaction mixture was cooled to 0 ꢀC and
quenched with water until the reaction mixture became trans-
parent. The reaction mixture was acidified to pH w3 by addition of
1 M HCl and the layers were separated. The aqueous phase was
extracted with EtOAc (3ꢃ10 mL) and the combined organic phase
was washed with brine (15 mL), dried over Na2SO4, filtered, and
concentrated in vacuo. The crude material was purified by column
chromatography (SiO2, 1:1 EtOAc/hexanes with 0.5% AcOH) to give
16 (712 mg, 68%) as a light yellow oil; Rf 0.43 (1:1 EtOAc/hexanes
stirring for 1 h, methyl iodide (772
mL, 12.4 mmol) was added
dropwise and the reaction mixture was stirred overnight while
slowly warming to rt. After 22 h, the reaction mixture was cooled to
0 ꢀC and quenched with water (5 mL), followed by 1 M HCl (5 mL).
The phases were separated and the aqueous phase was extracted
with Et2O (3ꢃ15 mL). The combined organic phase was washed
with NaHCO3 (10 mL) and brine (10 mL) and then dried over
Na2SO4, filtered, and concentrated in vacuo. The crude material was
purified by column chromatography (SiO2, 1:4 EtOAc/hexanes) to
give a yellow oil 17 (397 mg, 67%); Rf 0.48 (1:4, EtOAc/hexanes); 1H
with 0.5% AcOH); 1H NMR (400 MHz, CDCl3)
d 0.88 (s, 6H),
1.39e1.44 (m, 11H), 1.56 (t, J¼8.0 Hz, 2H), 2.31 (t, J¼8.2 Hz, 2H), 3.14
(br s, 2H), 3.80 (br s, 2H), 5.11 (d, J¼11.6 Hz, 2H), 5.71e5.81 (m, 1H);
13C NMR (100 MHz, CDCl3)
d
26.8, 28.7, 29.5, 32.0, 36.4, 39.5, 42.8,
NMR (400 MHz, CDCl3)
(br s, 2H), 3.65 (s, 3H), 3.79 (br s, 2H), 5.10 (d, J¼10.8 Hz, 2H),
5.70e5.79 (m,1H); 13C NMR (100 MHz, CDCl3)
25.3, 28.6, 38.1, 41.1,
d 1.18 (s, 6H), 1.44 (s, 9H), 1.74 (br s, 2H), 3.11
49.6, 79.8,116.8,134.6,155.6,180.2; FT-ICR-MS calcd for C16H28NO4ꢂ
[MꢂH]ꢂ m/z 298.2024, found 298.2024.
d
43.1, 49.4, 51.9, 79.7, 116.1, 134.2, 155.5, 178.0; FT-ICR-MS calcd for
4.5.2. 2-(Anthracen-9-yl)ethyl
4,4-dimethyl-6-(allylamino)-hex-
C
15H27NNaOþ4 [MþNa]þ m/z 308.1832, found 308.1836.
anoate (5, TFA salt). To a mixture of carboxylic acid 16 (370 mg,
1.24 mmol) and 8 (249 mg, 1.12 mmol) in dry CH2Cl2 (10 mL) at 0 ꢀC
4.6.2. tert-Butyl
allyl(4-hydroxy-3,3-dimethylbutyl)carbamate
were added DIC (264
m
L, 1.69 mmol) and DMAP (pinch). After 16 h,
(18). LiBH4 (45 mg, 2.08 mmol) was added to a solution of 17
(265 mg, 0.93 mmol) in dry THF (21 mL) at 0 ꢀC. After 5 min of
stirring, the reaction mixture was warmed to rt and stirred over-
night. The reaction mixture was then carefully quenched with
NH4Cl (25 mL) and extracted with Et2O (3ꢃ10 mL). The combined
organic phase was washed with brine (15 mL), dried over Na2SO4,
filtered, and concentrated in vacuo. The crude material was purified
by column chromatography (SiO2, 1:1 EtOAc/hexanes) to give 18
(175 mg, 73%) as a colorless oil; Rf 0.47 (1:1, EtOAc/hexanes); 1H
the precipitated white solids were filtered and the filter cake was
washed with CH2Cl2. The combined filtrate was condensed in vacuo
and the crude material was purified by column chromatography
(SiO2, 0:100 to 1:19 EtOAc/CH2Cl2 gradient) to give Boc-protected
gem-dimethyl ester 5 as a yellow oil (494 mg, 87%); Rf 0.58 (1:19
EtOAc/CH2Cl2); FTIR 3058, 2963, 1728, 1684 cmꢂ1
;
1H NMR
0.87 (s, 6H), 1.39 (br s, 2H), 1.46 (s, 9H), 1.51 (t,
(400 MHz, CDCl3)
d
J¼8.4 Hz, 2H), 2.28 (t, J¼8.2 Hz, 2H), 3.12 (br s, 2H), 3.81 (br s, 2H),
3.98 (t, J¼7.8 Hz, 2H), 4.48 (t, J¼7.8 Hz, 2H), 5.12 (d, J¼11.6 Hz, 2H),
5.73e5.82 (m, 1H), 7.46e7.49 (m, 2H), 7.53e7.57 (m, 2H), 8.02 (d,
J¼8.4 Hz, 2H), 8.34 (d, J¼9.2 Hz, 2H), 8.39 (s, 1H); 13C NMR
NMR (400 MHz, CDCl3)
1H), 3.13e3.17 (m, 2H), 3.32 (s, 2H), 3.78 (br s, 2H), 5.09e5.13 (m,
2H), 5.71e5.81 (m,1H); 13C NMR (100 MHz, CDCl3)
24.4, 28.6, 34.7,
d 0.86 (s, 6H), 1.43e1.49 (m, 11H), 2.89 (br s,
d
(100 MHz, CDCl3)
d
26.8, 27.5, 28.7, 29.8, 32.0, 36.6, 39.2, 42.8, 49.6,
36.5, 42.9, 50.4, 70.8, 79.8, 116.4, 134.6, 155.8; FT-ICR-MS calcd for
53.6, 64.3, 79.6, 116.6, 124.3, 125.2, 126.2, 127.0, 129.5, 130.5, 131.7,
134.7, 155.5, 174.5; FT-ICR-MS calcd for C32H41NNaOþ4 [MþNa]þ m/z
526.2928, found 526.2927.
Trifluoroacetic acid (0.50 mL, 6.53 mmol) was added to a solu-
tion of Boc-protected 5 (11 mg, 0.022 mmol) in dry CH2Cl2
(0.50 mL) at 0 ꢀC. After stirring for 1 h, the volatiles were removed
in vacuo to afford the TFA salt of 5 (10 mg, 85%) as an oil; 1H NMR
C
14H27NNaOþ3 [MþNa]þ m/z 280.1883, found 280.1886.
4.6.3. 2-(Anthracen-9-yl)ethyl 2,2-dimethyl-4-(allylamino)butyl car-
bonate (6, TFA salt). N,N-Diisopropylethylamine (304 L,
m
1.74 mmol) was added to a solution of alcohol 18 (251 mg,
0.98 mmol) in dry CH2Cl2 (25 mL) at 0 ꢀC. 1,10-Carbonyldiimidazole
(283 mg, 1.74 mmol) was then added to the solution followed by
warming to rt. After 24 h, the reaction mixture was washed with
water (2ꢃ10 mL), brine (10 mL) and then dried over Na2SO4. After
filtration and concentration in vacuo, the crude residue was puri-
fied by column chromatography (SiO2, EtOAc) to give the imidazole
(400 MHz, CDCl3)
d
0.86 (s, 6H), 1.47 (t, J¼8.4 Hz, 2H), 1.58 (t,
J¼8.6 Hz, 2H), 2.24 (t, J¼8.4 Hz, 2H), 2.88 (br s, 2H), 3.54 (br s, 2H),
3.96 (t, J¼7.6 Hz, 2H), 4.46 (t, J¼8.0 Hz, 2H), 5.40e5.47 (m, 2H),
5.84e5.94 (m, 1H), 7.44e7.56 (m, 4H), 8.00 (d, J¼8.8 Hz, 2H), 8.32

3428
R.J. Knipp et al. / Tetrahedron 70 (2014) 3422e3429
carbamate as a colorless oil (316 mg, 92%); Rf 0.61 (EtOAc); 1H NMR
(400 MHz, CDCl3)
122 mg, 100%) that was used in the next step without further pu-
d
1.01 (s, 6H), 1.42 (s, 9H), 1.56 (t, J¼7.8 Hz, 2H),
rification. Rf 0.20 (1:9, MeOH/CH2Cl2); 1H NMR (400 MHz, CDCl3)
3.21 (br s, 2H), 3.76 (br s, 2H), 4.12 (s, 2H), 5.07e5.09 (m, 2H),
d
1.39 (s, 9H), 1.50e1.56 (m, 4H), 2.40 (t, J¼6.6 Hz, 2H), 3.12 (t,
5.70e5.79 (m, 1H), 7.07 (s, 1H), 7.42 (s, 1H), 8.13 (s, 1H); 13C NMR
J¼7.0 Hz, 2H), 3.66 (s, 6H), 3.73 (br s, 2H), 4.26 (br s, 2H), 4.39 (br s,
2H), 4.64 (br s, 2H), 4.77 (br s, 2H), 5.06 (d, J¼12.0 Hz, 2H),
5.68e5.74 (m, 1H), 7.76e7.81 (m, 4H); 13C NMR (100 MHz, CDCl3)
(100 MHz, CDCl3) d 24.2, 28.6, 33.7, 36.7, 42.5, 50.0, 75.9, 79.8, 116.4,
117.2, 130.9, 134.5, 137.2, 148.9, 155.4.
A solution of alcohol 20 (191 mg, 0.86 mmol) in dry THF (1 mL)
was added dropwise to a slurry of NaH (103 mg of 60% in mineral
oil, 2.57 mmol) in dry THF (5 mL) at ꢂ5 ꢀC and stirred for 30 min. A
solution of the above imidazole carbamate (316 mg, 0.90 mmol) in
dry THF (1 mL) was then added dropwise to the reaction mixture.
The reaction mixture was stirred overnight, then the mixture was
filtered through Celite and the filter cake was washed with Et2O.
The filtrate was washed with water (2ꢃ10 mL) and the combined
aqueous layers were extracted with Et2O (3ꢃ10 mL). The combined
organic phase was washed with brine, dried over Na2SO4, filtered,
and concentrated in vacuo. The crude material was purified by
column chromatography (SiO2, 1:19 EtOAc/CH2Cl2) to give Boc-
protected 6 (207 mg, 48%) as a yellow oil; Rf 0.64 (1:19, EtOAc/
d
21.9, 27.5, 28.5, 46.2, 49.9, 53.2, 57.8, 63.2, 64.9, 72.4, 79.5, 116.2,
124.1, 128.6, 134.3, 135.2, 155.6, 163.2, 172.5.
Methyl hydrazine (24 L, 0.46 mmol) was added to a stirred
m
solution of the crude ammonium salt (49 mg, 0.076 mmol) in 1:1
CH2Cl2/EtOH (2 mL) at ꢂ40 ꢀC. After 1.5 h, the solution was con-
centrated in vacuo and diluted with CH2Cl2, causing a white pre-
cipitate to form. The solid was filtered and the filter cake was
washed with CH2Cl2. The filtrate was concentrated in vacuo to give
22 (yellow gum, 39 mg, 100%) that was used in the next step
without further purification; Rf 0.14 (1:9, MeOH/CH2Cl2); 1H NMR
(400 MHz, CDCl3)
d
1.41 (s, 9H), 1.51e1.59 (m, 4H), 2.39 (t, J¼7.0 Hz,
2H), 3.14 (t, J¼6.4 Hz, 2H), 3.48 (s, 6H), 3.74 (br s, 2H), 4.02 (br s, 2H),
4.08 (br s, 2H), 4.19 (br s, 2H), 4.57 (br s, 2H), 5.07 (d, J¼15.6 Hz, 2H),
CH2Cl2); FTIR 3008, 2974, 1743, 1685, 1256 cmꢂ1
(400 MHz, CDCl3)
;
1H NMR
5.70e5.74 (m, 1H); 13C NMR (100 MHz, CDCl3)
d 22.0, 27.6, 28.6,
d
0.98 (s, 6H), 1.46 (s, 9H), 1.53 (t, J¼7.8 Hz, 2H),
33.8, 46.2, 49.9, 53.1, 57.8, 63.9, 64.2, 69.2, 79.7, 116.3, 134.3, 155.7,
172.6; FT-ICR-MS calcd for C19H38N3Oþ5 [M]þ m/z 388.2806, found
388.2807.
3.19 (br s, 2H), 3.77e3.82 (m, 2H), 3.90 (s, 2H), 4.05 (t, J¼8.0 Hz, 2H),
4.50 (t, J¼8.4 Hz, 2H), 5.10e5.13 (m, 2H), 5.74e5.80 (m, 1H),
7.45e7.49 (m, 2H), 7.53e7.57 (m, 2H), 8.01 (d, J¼8.4 Hz, 2H), 8.34 (d,
J¼9.2 Hz, 2H), 8.40 (s, 1H); 13C NMR (100 MHz, CDCl3)
d
24.1, 27.6,
4.7.2. Procedure for loading 22 onto PDMS microchannel. Ester 22
(39 mg, 0.076 mmol) was placed into a pressure tube with a stir-bar
and the headspace was purged with nitrogen. Catalytic PtO2 was
28.7, 33.5, 37.1, 42.5, 49.7, 67.3, 76.2, 79.7, 116.2, 124.1, 125.2, 126.4,
127.2, 128.3, 129.5, 130.5, 131.7, 134.5, 155.5, 155.7; FT-ICR-MS calcd
for C31H39NNaOþ5 [MþNa]þ m/z 528.2720, found 528.2720.
Trifluoroacetic acid (0.50 mL, 6.53 mmol) was added to a solu-
tion of Boc-protected 6 (12 mg, 0.024 mmol) in dry CH2Cl2
(0.50 mL) at 0 ꢀC. After stirring for 1 h, the volatiles were removed
then added, followed by triethoxysilane (14 mL, 0.076 mmol). The
headspace was purged with nitrogen and the pressure tube was
sealed and heated to 80 ꢀC. After 2 days, the reaction mixture was
cooled to rt and the solution was filtered under nitrogen through
Celite and the filter cake was washed with dry CH2Cl2. The filtrate
was concentrated in vacuo to give a moisture-sensitive residue,
triethoxylsilane intermediate 23, which was immediately loaded
onto the PDMS microchannel without further purification by first
1
in vacuo to afford the TFA salt of 6 (11 mg, 92%) as an oil; H NMR
(400 MHz, CDCl3)
d
0.96 (s, 6H),1.70 (t, J¼8.4 Hz, 2H), 2.99 (br s, 2H),
3.86 (br s, 2H), 4.03 (t, J¼8.0 Hz, 2H), 4.49 (t, J¼8.0 Hz, 2H),
5.40e5.48 (m, 2H), 5.81e5.91 (m, 1H), 7.45e7.60 (m, 4H), 8.00 (d,
J¼8.4 Hz, 2H), 8.32 (d, J¼8.4 Hz, 2H), 8.39 (s, 1H), 9.06 (br s, 2H); 13
C
dissolving in CH3CN (0.8 M) and then injecting the solution (10
80 mol) into the microchannel. After 1 h, the microchannel was
L) and placed in 110 ꢀC oven for 15 min.
mL,
NMR (100 MHz, CDCl3)
d
24.0, 27.4, 33.6, 34.8, 43.3, 50.0, 67.5, 75.3,
m
124.1, 124.3, 125.2, 126.4, 127.2, 127.5, 128.3, 129.5, 130.5, 131.7,
155.5; FT-ICR-MS calcd for C26H32NOþ3 [MþH]þ m/z 406.2377,
found 406.2379.
washed with CH3CN (5ꢃ10
m
On cooling, the loaded microchannel was stored at rt in a sealed bag
until needed.
4.7. Synthesis of ester 22 and PDMS loading procedure
Acknowledgements
4.7.1. 2-((5-(Allyl(tert-butoxycarbonyl)amino)pentanoyl)oxy)-N-(2-
((1,3-dioxoisoindolin-2-yl)oxy)ethyl)-N,N-dimethylethanaminium io-
dide (22). Amino acid 8.2 (98 mg, 0.38 mmol) and the 2-(2-((2-
hydroxyethyl)(methyl)amino)ethoxy)-2,3-dihydro-1H-isoindole-
1,3-dione mono-O-phthalimide of N-methyl-diethanolamine27
This work was partially supported by a grant from NIH (1 R01
ES022191-01). We thank Olivetta Uradu for her assistance with
substrate syntheses.
Supplementary data
were dissolved in dry CH2Cl2 (1.7 mL) with stirring. DIC (82 mL,
Experimental details, 1H and 13C NMR spectra. Supplementary
data associated with this article can be found in the online version,
at http://dx.doi.org/10.1016/j.tet.2014.03.092.
0.52 mmol) was added to the reaction solution followed by cat.
DMAP. After 2 h, the white solids were filtered out and the filter
cake was washed with CH2Cl2. The filtrate was condensed in vacuo
and the crude material was purified by column chromatography
(SiO2, 3:1 to 1:0, EtOAc/hexanes gradient) to give the corresponding
ester as a light yellow oil (96 mg, 55%). Rf 0.55 (EtOAc); 1H NMR
References and notes
(400 MHz, CDCl3)
d
1.43 (s, 9H), 1.49e1.62 (m, 4H), 2.33 (t, J¼7.4 Hz,
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2H), 2.39 (s, 3H), 2.77 (t, J¼6.0 Hz, 2H), 2.91 (t, J¼5.6 Hz, 2H), 3.16 (br
s, 2H), 3.78 (br s, 2H), 4.16 (t, J¼5.8 Hz, 2H), 4.30 (t, J¼5.4 Hz, 2H),
5.09 (d, J¼11.2 Hz, 2H), 5.72e5.78 (m, 1H), 7.72e7.75 (m, 2H),
7.76e7.84 (m, 2H); 13C NMR (100 MHz, CDCl3)
d 22.4, 28.0, 28.6,
34.1, 42.9, 46.3, 55.9, 60.6, 62.3, 76.1, 79.6, 116.5, 123.7, 129.2, 134.6,
155.7, 163.6, 173.6.
To the ester (96 mg, 0.19 mmol) in dry CH2Cl2 (1 mL) in a pres-
sure tube was added iodomethane (24 mL, 0.38 mmol). The tube
was sealed and heated to 60 ꢀC for 18 h. The solution was then
concentrated in vacuo to give a crude ammonium salt (yellow gum,

R.J. Knipp et al. / Tetrahedron 70 (2014) 3422e3429
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