Intramolecular Catalysis of Thiol Ester Hydrolysis
J . Org. Chem., Vol. 62, No. 14, 1997 4817
Sch em e 1
Exp er im en ta l Section
(a ) Ma ter ia ls a n d Gen er a l Meth od s. The following
materials were obtained from commercial suppliers: 4-ni-
trobenzoyl chloride (Aldrich), 5,5′-dithiobis(2-nitrobenzoic acid)
(DTNB; Ellman’s reagent, Sigma), 2-(dimethylamino)ethaneth-
iol:HCl (dimethylcysteamine:HCl, Aldrich), ethyl 2-mercap-
toacetate (Aldrich), thoiglycolic acid (Sigma), iodomethane
(BDH), glacial acetic acid (Fisher Scientific), sodium acetate
(General Intermediates of Canada), and triethylamine (BDH).
Buffers, MES [2-morpholinoethanesulfonic acid], MOPS [3-
morpholinopropanesulfonic acid], and EPPS [N-(2-hydroxy-
ethyl)piperazine-N′-3-propanesulfonic acid], were reagent grade
(Sigma) and were used as supplied. Purified deoxygenated
water from an Osmonics Aries water purification system was
used for buffer preparation. Acetonitrile was dried over 3 A
molecular sieves and distilled from phosphorus pentoxide
under argon prior to use. The pH was measured using a
Radiometer Vit 90 video titrator equipped with a GK2321C
combination electrode, standardized by Fisher Certified pH
2, 4, 7, and 10 buffers.
8a and 9a which subsequently undergo rapid hydro-
lysis. These findings fit nicely with the general rule
that thioesters are not particularly susceptible to attack
by oxygen nucleophiles,10 but are rapidly attacked by
nitrogen nucleophiles.10b,14 There are few examples
where intermolecular general base catalysis of thio-
1H and 13C NMR spectra were obtained using a Bruker AC-
200 or a Bruker AM-400 spectrometer. Infrared spectra were
obtained using a Bomem MB-120 FTIR spectrometer. High
resolution mass spectra were obtained using a Concept 2H
(Kratos) spectrometer. All melting points were obtained using
a Fisher-J ohns melting apparatus and are uncorrected.
(b) Syn th esis. The thioesters 10 and 11 were prepared
by the typical proceedure described below.
p -Nit r ob en zoa t e E st er of E t h yl 2-Mer ca p t oa cet a t e
(10). p-Nitrobenzoyl chloride (0.025 mol, 4.64 g) was dissolved
in 10 mL of CH3CN and then added to a mixture of 0.025 mol
(3.00 g) of HSCH2COOEt and 0.025 mol (2.52 g) of N(Et)3 in
15 mL of CH3CN. The reaction mixture was then stirred at
room temperature for 1 h after which the CH3CN was
evaporated to obtain reddish yellow crystals which were
dissolved in 50 mL of CH2Cl2. This solution was washed with
3 × 100 mL each of 1 N HCl, a saturated solution of NaHCO3,
and water. The CH2Cl2 layer was then dried with MgSO4 and
the solvent removed by rotary evaporation to obtain the crude
product (85% yield, 5.73 g). A 2 g portion of this was
recrystallized from acetone (10 mL)-hexane (a few drops) to
yield light yellow, needle-shaped crystals: mp 51-52 °C; IR
ester hydrolysis is documented11c,12,15 and, to our knowl-
edge, even fewer where intramoleular general base
catalysis is observed. Given the widely held view that
general base assistance of the hydrolysis of the cysteine
protease acyl enzymes is a facile process, it is surprising
that so few small molecule examples of this process
exist, the most notable being the hydrolyses of the
acetyl thioesters of 1 and 24,13 which can be taken as
reasonable models for the chemistry believed to occur
in the enzymatic active site. In order to investigate
the propensity for intramolecular assistance by a car-
boxylate and tertiary amine of the hydrolysis of thioesters
we have investigated the pH-dependent rate profile
for the cleavage of the p-nitrobenzoyl derivatives 10-
13. The following describes our findings which indicate
that 11 and 12 most probably exhibit general base
mechanisms for their hydrolyses in the neutral pH
domain, while the control compounds, 10 and 13, hydro-
lyze with specific base catalysis throughout the accessible
pH/rate range.
1
(KBr) 3113, 2982, 2935, 1730, 1667 cm-1; H NMR (CDCl3) δ
1.27 (t, 3H), 3.89 (s, 2H), 4.20 (q, 2H), 8.07-8.30 (m, 4H); 13
C
NMR (CDCl3) δ 13.98 (CH3), 31.67 (CH2), 62.00 (CH2), 123.84
(aromat CH) 128.28 (aromat CH), 140.53 (aromat C), 150.58
(aromat C), 167.89 (CdO), 188.60 (CdO). Exact mass, m/ z
calcd for C11H11NO5S: 269.03528; found: 269.03477 (4.1%).
Anal. Calcd for C11H11O5NS: C, 49.07, H, 4.09, N, 5.20, S,
11.90. Found: C, 49.26, H, 3.83, N, 5.21, S, 12.08.
p-Nitr oben zoyl Ester of Mer ca p toa cetic Acid (11).
This was prepared in 71% crude yield as above. A part (∼1.5
g) of the product was crystallized from acetone (15 mL)-
hexane (few drops) to give light yellow crystals: mp 155-156
°C; IR (KBr) 2727 -3363 (s, br), 3111, 2914, 1704, 1670 cm-1
;
1H NMR (CD3CN) δ 4.06 (s, 2H), 8.12-8.49 (m, 4H); 13C NMR
(CD3CN) δ 32.29 (CH2), 125.12 (aromat CH), 129.33 (aromat
CH), 141.54 (aromat C), 150.84 (aromat C), 169.67 (CdO),
189.90 (CdO). Anal. Calcd for C9H7O5NS: C, 44.81, H, 2.90,
N, 5.81, S, 13.28. Found: C, 44.81, H, 2.62, N, 5.79, S, 12.96.
p -Nit r ob en zoyl E st er of 2-(N,N-Dim et h yla m in o)et -
h a n eth iol (12). p-Nitrobenzoyl chloride (0.025 mol, 4.64 g),
HS(CH2)2 N(CH3)2 (0.025 mol, 3.54 g) and pyridine (0.025 mol,
3.96 g) were mixed using the above procedure. After 6 h, the
reaction mixture was filtered. The CHCl3 solution was
extracted with 1 N HCl, and the pH of the aqueous wash was
adjusted to 7 with NaOH (pH paper). The mixture was
extracted with CHCl3, this layer being dried over MgSO4,
filtered, and evaporated to give a yellow solid in 93% crude
yield (5.90 g). A part (∼2 g) of it was recrystallized from CHCl3
(11) (a) Fife, T. H.; DeMark, B. R. J . Am. Chem. Soc. 1979, 101,
7379. (b) Bruice, T. C. J . Am. Chem. Soc. 1959, 81, 5444. (c) Heller,
M. J .; Waller, J . A.; Klotz, I. M. J . Am. Chem. Soc. 1977, 99, 2780. (d)
Doi, J . T.; Carpenter, T. L.; Olmstead, M. M.; Musker, W. K. J . Am.
Chem. Soc. 1983, 105, 4684.
(12) (a) Fedor, L. R.; Bruice, T. C. J . Am. Chem. Soc. 1965, 87, 4138.
(b) Patterson, J . F.; Huskey, W. P.; Venkatasubban, K. S.; Hogg, J . L.
J . Org. Chem. 1978, 42, 4935.
(13) Street, J . P.; Brown, R. S. J . Am. Chem. Soc. 1985, 107,
6084.
(14) (a) Connors, K. A.; Bender, M. L. J . Org. Chem. 1961, 26, 2498.
(b) Weiland, T.; Lang, U. U.; Liebach, O. Ann. 1955, 597, 227, and
references therein.
(15) Bruice, T. C.; Fedor, L. R. J . Am. Chem. Soc. 1964, 86, 4880.