3888 J. Am. Chem. Soc., Vol. 120, No. 16, 1998
Zhu and Bennet
added to precipitate the product 7d. This procedure was repeated twice,
and the final white powder was crystallized from H2O to give colorless
crystals of 7d (0.43 g, 65%). Mp: 86-91 °C. 1H NMR (400 MHz,
D2O): δ 2.10 (s, 3 H, CH3), 2.12 (s, 3 H, CH3), 2.17 (s, 3 H, CH3),
2.73 (ddd, 1 H, J2S,1 ) 4.2 Hz, J2S,2R ) 15 Hz, J2S,3 ) 7.0 Hz, H-2S),
2.99 (ddd, 1 H, J2R,1 ) 6.9 Hz, J2R,3 ) 3.9 Hz, H-2R), 4.29-4.35 (m,
2 H, H-5, H-6a), 4.61 (dd, 1 H, J6b,5 ) 6.9, Hz, J6b,6a ) 13 Hz, H-6b),
5.13 (t, 1 H, J4,3 + J4,5 ) 12 Hz, H-4), 5.30 (m, 1 H, H-3), 6.59 (dd,
1 H, H-1), 8.12 (m, 1 H, Ar-H), 8.37 (bd, 1 H, Ar-H), 8.48 (bd, 1 H,
Ar-H), 8.52 (m, 1 H, Ar-H), 9.05 (d, 1 H, Ar-H), 9.91 (bs, 1 H,
Ar-H).
2-Deoxy-r-D-arabino-hexopyranosyl 4′-Bromoisoquinolinium Tet-
rafluoroborate (4d). The acetylated salt 7d (0.14 g, 0.25 mmol) was
dissolved in cold methanol (3 mL, -16 °C), and following the dropwise
addition of tetrafluoroboric acid (54% w/v in ether, 2 mL), the resulting
solution was kept at around -20 °C in a freezer. After 10 days an
analytically pure sample of 4d had crystallized from the acidic solution
(0.09 g, 82%). Mp: 95-97 °C. 1H NMR (400 MHz, D2O): δ 2.54
(ddd, 1 H, J2S,1 ) 4.9 Hz, J2S,2R ) 16 Hz, J2S,3 ) 11 Hz, H-2S), 3.13
(ddd, 1 H, J2R,1 ) 4.1 Hz, J2S,3 ) 3.2 Hz, H-2R), 3.49 (ddd, 1 H, J5,4
) 8.4 Hz, J5,6a ) 3.2 Hz, J5,6b ) 5.4 Hz H-5), 3.69 (dd, 1 H, J4,3 ) 9.1
Hz, H-4), 3.86-3.97 (m, 3 H, H-3, H-6a, H-6b), 6.51 (bt, 1 H, H-1),
8.12 (m, 1 H, Ar-H), 8.37 (m, 1 H, Ar-H), 8.50 (m, 1 H, Ar-H),
8.54 (m, 1 H, Ar-H), 9.08 (bs, 1 H, Ar-H), 9.92 (bs, 1 H, Ar-H).
Anal. Calcd for C15H17BBrF4NO4: C, 40.76; H, 3.88; N. 3.17.
Found: C, 40.65; H, 3.74; N, 3.16.
Kinetics. Hydrolysis of 4d (≈ 0.2 mM)12 at 65 °C was monitored
by following the rate of decrease in absorbance at 346 nm using a
Cary3E UV-vis spectrophotometer equipped with the Cary six-cell
Peltier constant-temperature accessory. The reaction was initiated by
the injection of an aqueous stock solution of the glucoside (10 µL, 60
mM) into a 1-cm quartz cuvette that contained 3.0 mL of the required
buffer which had been preequilibrated for 5 min at 65 °C. Clean
isosbestic points were observed at 262, 295, 300, and 327 nm, and the
rate constant for hydrolysis was calculated by performing a standard
nonlinear least-squares fit of the absorbance (346 nm) versus time data.
The hydrolyses of 4a-c were monitored in an analogous fashion, except
that the reactions were initiated by injection of an aqueous stock solution
of the glucoside (6.5 µL, 30 mM) into a 1-cm quartz cuvette containing
1.0 mL of buffer solution. For the reactions in which the pH was greater
than 6.0 the change in absorbance was monitored at 261, 260, and 338
nm for 4a-c, respectively, whereas at a pH value of 4.42, the hydrolysis
reactions were monitored at 270, 264, and 280 nm for 4a-c,
respectively.
equilibrated, and, to this end, results are presented from kinetic
and product studies on the reactions of four 2-deoxy-R-D-
glucopyranosyl pyridinium salts (4a-d) in the presence of added
anionic nucleophiles. In addition, to determine if nucleophilic
attack by the solvent occurs with equal propensity on the two
diastereotopic faces of the proposed reaction intermediate (SSIP:
M), the reactions of 1 and 4d were conducted in 20% aqueous
methanol, and the resulting product distributions were deter-
mined.
Materials and Methods
The buffers 2-(N-morpholino)ethanesulfonic acid (MES), 3-(N-
morpholino)propanesulfonic acid (MOPS), N-tris(hydroxymethyl)-
methyl-3-aminopropanesulfonic acid (TAPS), 2-(N-cyclohexylamino)-
ethanesulfonic acid (CHES), and 3-(N-cyclohexylamino)propanesulfonic
acid (CAPS) as well as sodium azide (SigmaUltra) were purchased
from Sigma and used without further purification. All other salts used
in the hydrolysis studies were of “Analar” grade and were used without
further purification. Milli-Q water (18.2 MΩ cm-1) was used for the
kinetic experiments. The NMR spectra were acquired on a Bruker
AMX-400 spectrometer. Melting points are reported as uncorrected
values. Full experimental details for the synthesis of 4a, 4b, 4c, and
8 (13C-labeled 4d) are given in the Supporting Information.
5,5-Dimethyl-2-thiono-(3,4,6,-tri-O-acetyl-2-deoxy-â-D-arabino-
hexopyranosylthio)-1,3,2-dioxaphosphorinane (6). Sodium hydride
(0.18 g, 60% in mineral oil) and 5,5-dimethyl-2-thiolo-2-thiono-1,3,2-
dioxaphosphorinane (0.89 g)11 were mixed in anhydrous THF (2 mL)
at -15 °C, under an inert atmosphere. When hydrogen evolution was
complete, a solution of 3,4,6-tri-O-acetyl-2-deoxy-R-D-arabino-hexo-
pyranosyl bromide5 (1.03 g) in THF (1 mL) was added to the solution
and stirred for an additional 10 min at -15 °C. After removal of the
solvent by reduced pressure, the residue was dissolved in CH2Cl2 (50
mL), and this solution was then washed with H2O (50 mL), dried
(MgSO4), and filtered. Volatile components were removed under
reduced pressure, and the resulting syrup (90% crude yield) was then
crystallized from EtOAc/hexane to give a colorless solid, mp ) 131-
133 °C. 1H NMR (400 MHz; CDCl3) δ 0.91 (s, 3 H), 1.26 (s, 3 H),
1.96-2.09 (m, 1 H, H-2a), 2.03 (s 3 H), 2.04 (s, 3 H), 2.07 (s, 3 H),
2.54 (ddd, 1 H, J1,2e ) 2 Hz, J2a,2e ) 13 Hz, J2e,3 ) 5 Hz, H-2e), 3.69
(ddd, 1 H, J5,6a ) 5.5 Hz, J5,6b ) 2 Hz, J4,5 ) 9.5 Hz, H-5), 3.85-4.00
(m, 2 H), 4.04 (dd, 1 H, J6a,6b ) 12 Hz, H-6b), 4.12-4.30 (m, 2 H),
4.31 (dd, 1 H, H-6a), 4.98 (t, 1 H, J3,4 + J4,5 ) 19 Hz, H-4), 5.06 (ddd,
1 H, J2a,3 ) 11 Hz, J2e,3 ) 5 Hz, H-3), 5.18 (ddd, 1 H, J1,2a ) 13 Hz,
2JH,P ) 12 Hz, H-1). Anal. Calcd for C17H27O9PS2: C, 43.40; H, 5.78.
Found: C, 43.48; H, 5.82.
The solvolysis studies in the presence of added anions were all
performed in 0.01 M (1:1 Na2HPO4:NaH2PO4) phosphate buffer at a
pH of around 6.8 (i.e., in the pH independent region of the hydrolysis
reaction).
Product Studies. The UV-vis spectrum (240-380 nm) of a
completely hydrolyzed (>10 half-lives) sample of 4d (2.00 × 10-4
M) in 0.01 M phosphate buffer (µ ) 0.02, NaClO4) at 65 °C was
identical within 0.038 AUs (maximum absorbance ) 0.95 AU) of a
spectrum of 4-bromoisoquinoline (2.00 × 10-4 M). Identification of
(12) Initial kinetic measurements on 4d were performed with a sample
that contained about 4% of the â-anomer (1). However, since the rate of
hydrolysis for 1 is approximately 9-fold less than that of 4d (ref 5), following
four-half-times for hydrolysis of 4d, approximately 1% of the total
hydrolyzed material originated from 1.
3,4,6-Tri-O-acetyl-2-deoxy-r-D-arabino-hexopyranosyl 4′-Bro-
moisoquinolinium Tetrafluoroborate (7d). Silver tetrafluoroborate
(0.23 g, 1.18 mmol) and 4-bromoisoquinoline (1.09 g, 5.23 mmol) were
added in one portion, with stirring, to an ice-cold solution of
dioxaphosphorinane 6 (0.55 g, 1.17 mmol) in dichloromethane (1 mL).
Stirring of the solution was continued for 120 min, at which point
addition of methanol (100 mL) induced precipitation of silver salts.
After the precipitate had been removed by filtration, the volatiles were
evaporated under reduced pressure. The resulting residue was dissolved
in a minimum amount of methanol, and diethyl ether (250 mL) was
(11) Edmundson, R. S. Tetrahedron 1965, 21, 2379-2387.