Facilitated intramolecular conjugate addition of amides of 3-(3′,6′-dioxo-2′,4′-dimethyl-1′,4′- cyclohexadienyl)-3,3-dimethylpropionic acid. 2. Kinetics of degradation

Article abstract of DOI:10.1021/jo961069l

The chemical stability studies of amides of 3-(3′,6′-dioxo-2′,4′-dimethyl-1′,4′- cyclohexadienyl)-3,3-dimethylpropionic acid (Qa) [Qop(a-j)] were conducted in order to determine the utility of this redox-sensitive system as a potential prodrug promoiety o

Full text of DOI:10.1021/jo961069l

J . Org. Chem. 1996, 61, 6633-6638
6633
F a cilita ted In tr a m olecu la r Con ju ga te Ad d ition of Am id es of
3-(3′,6′-Dioxo-2′,4′-d im eth yl-1′,4′-cycloh exa d ien yl)-3,3-d im eth ylp r op ion ic
Acid . 2. Kin etics of Degr a d a tion
Michalis G. Nicolaou,† J anet L. Wolfe,†,‡ Richard L. Schowen, and Ronald T. Borchardt*
Department of Pharmaceutical Chemistry, The University of Kansas, Lawrence, Kansas 66047-2504
Received J une 6, 1996X
The chemical stability studies of amides of 3-(3′,6′-dioxo-2′,4′-dimethyl-1′,4′-cyclohexadienyl)-3,3-
dimethylpropionic acid (Qa) [Qop(a-j)] were conducted in order to determine the utility of this
redox-sensitive system as a potential prodrug promoiety or redox-sensitive protecting group in
organic synthesis. This study showed that quinone propionic amides of aniline derivatives [Qop-
(a-d)] underwent rapid degradation in mildly acidic conditions (pH 4.5-6.0) to yield degradation
products resulting from the intramolecular 1,2- or 1,4- conjugate addition of the amide nitrogen to
the quinone ring. This conjugate addition was found to be specific base-catalyzed and independent
of the para substituent on the aromatic ring of the amine. The predominant route of degradation
yielded a five-membered ring spirolactam. By altering the nature of the amine component of the
amide, these degradation reactions were prevented. Amides of Qa other than those of the aniline
type [Qop(e-j)] were found to be substantially more stable and were thus proposed as the more
suitable candidates for this potential redox-sensitive prodrug system and redox-sensitive protecting
group for amines and alcohols in organic synthesis.
Sch em e 1
In tr od u ction
The unique chemical properties of 3-(3′,6′-dioxo-2′,4′-
dimethylcyclohexa-1′,4′-dienyl)-3,3-dimethylpropionic acid
(Qa, Quinone acid), which include ease of reduction either
chemically or enzymatically to yield the hydroquinone
QH2a that rapidly lactonizes1 (Scheme 1), have made this
molecule useful as a redox-sensitive promoiety for amine-
containing drugs.2,3 In addition, Qa has been used as a
redox-sensitive protecting group for amines4,5 and alco-
hols6 by synthetic chemists.
For Qa to be a useful redox-sensitive promoiety or a
redox-sensitive protecting group, esters or amides of this
quinone propionic acid need to be chemically stable and,
upon reduction, to quantitatively release the amine or
the alcohol. In an earlier study from our laboratory,7 we
observed that when the p-anisidine amide derivative,
Qop(a), of this quinone propionic acid was dissolved in
mildly acidic or basic aqueous solutions, it rapidly
degraded to yield the products QAd(a), cy5en(a), cy5ke-
(a), and cy6ke(a) shown in Scheme 2 (QAd, Quinone
Adduct; cy5en, five-membered ring cyclic enol; cy5ke, five-
membered ring cyclic ketone; cy6ke, six-membered ring
cyclic ketone). These products arise from the 1,2- or 1,4-
conjugate addition of the amide nitrogen to the R,â-
unsaturated carbonyl system present in the quinone
propionic acid.
The objectives of this study were to determine more
extensively the aqueous stability of amides of Qa as a
function of pH and buffer concentration and to ascertain
how the structure of the amine affects the rates and the
extent of these degradation reactions. Information forth-
coming from this study should better define the limita-
tions of Qa as a redox-sensitive promoiety for drug
delivery and a redox-sensitive protecting group in organic
synthesis.
Resu lts
The kinetics of degradation of the quinone propionic
amide Qop(a) were determined in aqueous buffered
acetonitrile (90:10 v/v) in the pH range 4.5-6.0. Acetic
acid and sodium acetate were used in the concentration
range of 0.05-0.30 M. Scheme 2 illustrates the kinetic
scheme that adequately describes the degradation based
on the data generated in this study. The degradation of
the starting material Qop(a) and the appearance of the
degradation products, QAd(a), cy5en(a), cy5ke(a) and
cy6ke(a), were followed by HPLC. In Table S1 (support-
ing information) are provided the retention times for Qop-
(a-e) and their degradation products on the HPLC
system employed for analysis. Characterization of the
* To whom the correspondence should be addressed. Dr. Ronald T.
Borchardt: Tel: (913)-864-3427. Fax: (913)-864-5736. e-mail:
Borchardt@Smissman.hbc.ukans.edu.
† These authors contributed equally to this paper.
‡ Current Address: Department of Pharmaceutics, University of
Tennessee, Memphis, TN 38163.
X Abstract published in Advance ACS Abstracts, September 1, 1996.
(1) Borchardt, R. T.; Cohen, L. A. J . Am. Chem. Soc. 1972, 94, 9175-
9182.
(2) Amsberry, K. L.; Borchardt, R. T. Pharm. Res. 1991, 8, 323-
330.
(3) Carpino, L. A.; Triolo, S. A.; Berglund, R. A. J . Org. Chem. 1989,
54, 3303-3310.
(4) Wang, B.; Liu, S.; Borchardt, R. T. J . Org. Chem. 1995, 60, 539-
543.
(5) Carpino, L. A.; Nowshad, F. Tetrahedron Lett. 1993, 34, 7009-
7012.
1
degradation products by H NMR and UV spectroscopy
was described earlier.7 To further ascertain the identity
of these degradation products, the HPLC system was
(6) Wang, B.; Liu, S.; Borchardt, R. T. Unpublished results.
(7) Wolfe, J .; Vander Velde, D.; Borchardt, R. T. J . Org. Chem. 1992,
57, 6138-6142.
S0022-3263(96)01069-9 CCC: $12.00 © 1996 American Chemical Society

6634 J . Org. Chem., Vol. 61, No. 19, 1996
Nicolaou et al.
Ta ble 1. Ra te Con sta n ts for th e Degr a d a tion of th e
Qu in on e P r op ion ic Am id e, Qop (a ), a t Differ en t p H; in 0.1
M Aceta te Bu ffer (µ ) 0.3 M) a t 37 °C
Sch em e 2
k × 107 s-1
pH
kcy5
kcy6
kek
kke
4.50
5.00
5.50
6.00
285 ( 5
1742 ( 16
51 ( 2
77 ( 8
196 ( 9
763 ( 54
a
a
11622 ( 775
21822 ( 977
13537 ( 521
1753 ( 178
3761 ( 198
2742 ( 135
4101 ( 31
14943 ( 417
a
Due to the fast equilibration, kek and kke could not be
adequately estimated. The equilibrium constant obtained from
the ratio of the mole fractions of the keto and enol tautomers
cy5ke(a) and cy5en(a) was 3.0 ( 0.8.
reactant as illustrated by brackets in Scheme 2. The
disappearance of Qop(a)-QAd(a) was accompanied by
concomitant formation of the enol spirolactam cy5en(a)
and the proposed lactam cy6ke(a), whereas the keto
spirolactam cy5ke(a) appeared after a slight lag time
(Figure 1, Scheme 2).
Mole fractions of the starting material and the degra-
dation products were obtained from the relative HPLC
peak areas. The sum of all peak areas of the starting
material and the degradation products remained constant
throughout the time course of the experiment, suggesting
that all species absorbed similarly at a wavelength of 250
nm and that mass balance was maintained. The kinetic
rate constants kcy5, kcy6, kek, and kke (Scheme 2) were
estimated by nonlinear least-squares fits on the experi-
mentally obtained mole fraction-time data. The kinetic
equations describing the time dependence for the degra-
dation of Qop(a) and appearance of its degradation
products as well as the analytically derived integrated
rate equations are listed in Appendix 1 (supporting
information) (the analytically derived integrated rate
equations define each species involved in the reaction
with the letter S, followed by the specified species in
parentheses). The values for the rate constants shown
in Table 1, which were determined in different pH
conditions while maintaining the buffer concentration at
0.1 M, indicate that both kcy5 and kcy6 increase as the
reaction mixture becomes more basic. In addition, the
magnitude of kcy5 was consistently larger at all pH
values studied. The rate constants describing the enol-
keto equilibration (Table 1), kek and kke, do not show
equipped with a diode array spectrophotometric detector.
Table S2 (supporting information) lists the observed λmax
values for the different degradation products. The
structure of the product with retention time of 3.70 min
is proposed to be cy6ke.
Figure 1 illustrates time profiles for the disappearance
of the quinone propionic amide Qop(a) and its degrada-
tion products at the highest (6.0) and lowest (4.5) pH
values employed in this study. In all cases, Qop(a)
appeared to reach fast equilibration with the hydroxy
dienone QAd(a). Thus, it was not possible to isolate the
microscopic rate constants describing this equilibration,
and both species were treated as a pool of a single
F igu r e 1. Representative time profiles for the degradation of Qop-a (b) and the formation of cy5en-a ([), cy5ke-a (9), and cy6ke-a
(2) at 37 °C, 0.1 M acetate buffer and ionic strength (µ ) 0.3 M): A. pH ) 6.0, B. pH ) 4.5. Data points represent experimentally
obtained mole fractions, and lines are plots of equations from Appendix 1 with rate constants shown in Table 1.

Intramolecular Conjugate Addition of Amides
J . Org. Chem., Vol. 61, No. 19, 1996 6635
F igu r e 2. Plots for kcy6 (A) and kcy5 (B) as a function of buffer concentration. pH 6.0 (b), pH ) 5.5 (9), pH ) 5.0 (2), and pH
) 4.5 ([) at 37 °C.
F igu r e 3. Plots of average values of kcy5 (A) and kcy6 (B) as a function of the hydroxide ion concentration.
any systematic increase or decrease with the pH. At pH
4.5, the values for kek and kke were accompanied by
large standard deviations, and the equilibrium constant
obtained from the ratio of the mole fractions of cy5ke(a)
and cy5en(a) was reported instead.
The kinetics of degradation of Qop(a) were studied as
a function of different acetic acid/sodium acetate buffer
concentrations (0.05-0.30 M). Figure 2 illustrates that
kcy5 and kcy6 were observed to be pH-dependent (pH
range of 4.5-6.0) and buffer concentration-independent.
Average values for kcy5 and kcy6 were calculated with
reasonable standard deviations (<20% and 23%, respec-
tively). Figure 3 illustrates linear dependence in both
kcy5 and kcy6 with hydroxide ion concentration according
to the following relationship:
lated data at zero buffer concentrations were found to
be inconclusive to define pH dependencies for kek and
kke.
The effect of the amide structure was also addressed.
Different quinone propionic amides [Qop(a-j)] were
studied in order to determine structural features that
affect the rate of degradation of these types of compounds.
The kinetics of disappearance of Qop(a-j) were studied
at pH 5.0, 37 °C, constant ionic strength (µ ) 0.3 M),
and 0.1 M acetic acid/sodium acetate buffer. Analysis
was conducted by HPLC. Table S2 (supporting informa-
tion) shows retention times for all species of interest. The
characterization of the degradation products was con-
ducted by UV spectroscopy and λmax values for the
different species are shown in Table S1. First-order rate
constants for the disappearance of Qop(a-j) were ob-
tained by nonlinear least-squares fits on the HPLC peak
area-time data. Only Qop(a-d) underwent rapid deg-
radation. Disappearance rate constants were obtained
as follows: Qop(a), p-oMe substituent, 1819 ( 30 × 10-7
s-1; Qop(b), p-H, 2500 ( 233 × 10-7 s-1; Qop(c), p-Me,
872 ( 42 × 10-7 s-1 and Qop(d), p-F, 2000 ( 83 × 10-7
s-1. The change in reactivity is considered minimal. As
illustrated in the Hammett plot in Figure 4, the sub-
stituent on the para position of the aromatic ring does
not greatly affect the reactivity. By inserting a methyl-
ene group between the aromatic ring and the nitrogen,
e.g, Qop(e), solution stability (k ) 24 × 10-7 s-1) was
improved substantially. When steric bulk was introduced
kcy5,6 ) kOH × [OH-]
Second order rate constants (M-1 × s-1) obtained by
linear regression of the data shown in Figure 3 are kOHcy5
) 1.2 × 105 and kOHcy6 ) 8.5 × 103, relating the average
rate constants (kcy5 and kcy6) with the hydroxide ion
concentration, respectively.
Enol-keto equilibration data at different pH and
buffer conditions were also investigated. Rate constants
kek and kke were found to be buffer-dependent, and they
showed a systematic increase as buffer concentration
increased. However, the slopes of the buffer plots became
smaller as the pH increased, suggesting that acetic acid
is a more effective catalyst than is acetate ion. Extrapo-

6636 J . Org. Chem., Vol. 61, No. 19, 1996
Nicolaou et al.
centration independence (Figure 2) of these rate con-
stants suggests that the reactions in these two routes of
degradation are specific-base-catalyzed. Thus, the proton
abstraction by the hydroxide ion occurs prior to the rate-
determining step. Hydroxide ion-facilitated deprotona-
tion of the amide nitrogen promotes the development of
negative charge (on the nitrogen), forming a highly
reactive nucleophilic species, which by subsequent attack
at positions 1′ and 2′ of the quinone ring yields products
cy5en and cy6ke. The formation of the five-membered
ring product is the dominating degradation route of the
reaction. Such degradation occurs not only in the case
of amides, as illustrated in this study, but also in the
case of the carboxylic acid, Qa. Borchardt and Cohen
reported formation of a spirolactone, the product that
results from 1,4-conjugate addition of the carboxylic acid
oxygen to the 1′ position of the quinone ring, in equilib-
rium with Qa in acidic conditions.1 Other possible
degradation routes of Qa, such as attack at position 2′,
to form a product analogous to cy6ke or at position 6′ to
form QAd(a), were not reported. This degradation profile
was explained by the existence of a “trimethyl lock”,
which resulted in such conformation of the carboxylic acid
that favored the formation of the transition state, leading
to the five- instead of the six-membered ring spirolactone.
In a recent study,10 our laboratory determined the
crystal structures of quinone propionic acid and several
of its amides. The distance between the amide nitrogen,
in the case of the quinone propionic amides and the
carboxylic acid oxygen, in the case of the quinone propi-
onic acid, and the positions 1′ and 6′ of the quinone ring
is approximately 1 Å less than the distance from position
2′. Thus, in addition to unfavorable rotation that has to
take place in order to form the six-membered ring lactone,
simple propinquity of the reacting centers may explain
the dominance of the five-membered ring spirolactams.
In addition, the higher reactivity of the negatively
charged nitrogen compared to the corresponding car-
boxylic acid oxygen could be the driving force for the
formation of the less favorable degradation product
cy6ke. The reason for the slower degradation of Qop(a)
is that there is not enough base at low pH to promote
amide deprotonation.
F igu r e 4. Plot of the log kdis versus the Hammett value σ.
on the methylene group of compound Qop(e) to form Qop-
(f), as well as for compound Qop(g) and Qop(j), no
detectable degradation in solution occurred within 24 h.
When an amide was made from a secondary amine, i.e.
Qop(h), or when cyclohexylamine was used in the place
of aniline, i.e. Qop(i), no detectable degradation in solu-
tion occurred within 24 h. Finally, compounds Qop(i,j)
were found to be equilibrated with their corresponding
hydroxy dienones QAd(i,j), after 24 h in solution (Qop(i)
and Qop(j) contained 11% and 15% of QAd(i) and QAd-
(j), respectively).
Discu ssion
Scheme 2 illustrates the degradation of the compounds
of the Qop type. From this degradation scheme one may
realize that there are six sites on the quinone ring that
may undergo either 1,2 or 1,4 conjugate addition. How-
ever, only three sites undergo nucleophilic attack by the
nitrogen, and the products formed contain five- and six-
membered ring lactams. Attack at the other three sites
of the quinone ring would result in products containing
seven- and eight-membered ring lactams, which are
thermodynamically less favorable.8
Initially, the starting material Qop(a) undergoes a fast
intramolecular 1,2-conjugate addition by the amide ni-
trogen to yield the hydroxy-amide QAd(a). QAd(a) then
undergoes ring opening to form what seems to be the
thermodynamically more stable starting material Qop-
(a). This is consistent with an earlier observation of
Du¨rckheimer and Cohen where a 9-hydroxy-R-tocopher-
one, a cyclic hydroxy dienone, spontaneously underwent
ring opening and established a pH-dependent equilibrium
with the corresponding open quinone structure.9 Qop-
(a) can also undergo 1,4-conjugate addition by the amide
nitrogen at positions 1′ and 2′ on the quinone ring. The
major route of degradation in this case, however, is attack
at position 1′ to yield formation of the five-membered ring
spirolactam, which is present as both the enol and keto
tautomer, cy5en(a) and cy5ke(a), respectively. Attack at
the 2′ position is less favored, since <10% cy6ke(a) is
formed at equilibrium.
The predominant route of degradation, the formation
of the five-membered ring spirolactam, results in the
formation of the two possible tautomers: the enol form,
which forms first, and the keto form, which appears after
a brief lag time. Keto tautomers are generally found to
be the thermodynamically most stable tautomers,12-14
although rare exceptions have been observed.15 It is
interesting to note that approximately 10-15% of the
total degradation product was isolated as the enol tau-
tomer cy5en(a). This tautomer presents interesting
stabilization features: first, the enol is cyclic and the
enolic double bond is conjugated with another double
bond in the ring and, second, the methyl substitution â
to the enolic hydroxyl group provides the enol with
(10) Wang, B.; Nicolaou, M.; Liu, S.; Borchardt, R. T. Bioorg. Chem.
1996, 24, 39-49.
(11) Liu, S.; Wang, B.; Nicolaou, M.; Borchardt, R. T. J . Chem.
Crystallogr. 1996, 26, 209-214.
The combination of the linear dependence of the rate
constants kcy5 and kcy6 on the hydroxide ion concentra-
tion with zero intercept (Figure 3) and the buffer con-
(12) Chang, Y.; Kresge, A. J . Science 1991, 253, 395-400.
(13) Toullec, J . In The Chemistry of Enols; Rappoport, Z., Ed.; Wiley
and Sons: New York, 1990; pp 323-398.
(14) Keefe, J . R.; Kresge, A. J . In The Chemistry of Enols; Rappoport,
Z., Ed.; Wiley and Sons: New York, 1990; pp 399-476.
(15) Pratt, D. V.; Hopkins, P. B. J . Am. Chem. Soc. 1987, 109, 5553-
5554.
(8) Mandolini, L. Adv. Phys. Org. Chem. 1986, 22, 1-111.
(9) Durckheimer, W.; Cohen, L. J . Am. Chem. Soc. 1964, 86, 4388-
4393.

Intramolecular Conjugate Addition of Amides
J . Org. Chem., Vol. 61, No. 19, 1996 6637
additional stability.16 The keto-enol equilibrium con-
stant due to proton conservation was expected to be
independent of pH.12 Equilibrium constant data (Table
1 Kequil ) kek/kke) show some variation at different pH
values. This phenomenon may be due to possible ioniza-
tion of the enolic hydroxyl group and, in such a case, the
assumption of proton conservation does not hold.
Exp er im en ta l Section
All melting points were determined on a Meltemp apparatus
and are uncorrected. Mass spectral analyses were conducted
by The University of Kansas Mass Spectrometry Laboratory,
Lawrence, KS, and elemental analyses were determined by
Desert Analytics Organic Microanalysis, Tucson, AZ, or The
University of Kansas Elemental Analyses Laboratory. Column
chromatography was performed on silica gel (70-270 mesh)
purchased from Aldrich Chemical Co., Milwaukee, WI. Thin-
layer chromatography was performed with TLC plates consist-
ing of aluminum sheets precoated with silica gel 60 F254, which
were purchased from EM Science, Germany. 1H- and 13C-NMR
spectra were obtained in CDCl3 on a Bruker AM 500 or a
Varian XL-300 spectrophotometer. 1H-NMR spectra were
recorded at 500.13 or 300 MHz and chemical shifts were
recorded in δ ppm relative to TMS (0.0 ppm) or H2O (4.80
ppm). 13C-NMR spectra were recorded at 125.77 MHz and
chemical shifts were recorded in δ ppm relative to CDCl3 (77.0
ppm).
Gen er a l Meth od I for P r ep a r a tion of Am id es. 3-(3′,6′-
Dioxo-2′,4′-dimethylcyclohexa-1′,4′-diene)-3,3-dimethylpropi-
onic acid Qa (0.29 g, 1.23 mmol), whose synthesis was
previously described,2 was dissolved in 25 mL of dry CH2Cl2,
to which was added the amine (2 equiv). The mixture was
cooled to 0 °C, and dicylohexylcarbodiimide (DCC), (0.30 g, 2.60
mmol) and 1-hydroxybenzotriazole (0.18 g, 1.36 mmol) were
added. The mixture was shielded from light and allowed to
warm to room temperature. After 12 h, the insoluble white
crystalline side product dicyclohexylurea (DCU) was removed
by filtration and the CH2Cl2 removed under reduced pressure.
EtOAc was added to the residue, the mixture was again
filtered to remove DCU, and the solvent was again evaporated.
This procedure was repeated until no additional DCU could
be precipitated. The desired product was eluted from a silica
gel column with 30% EtOAc in hexane. The fractions were
concentrated and recrystallized in EtOAc/hexane.
Similar to that of Qop(a), degradation in aqueous
solution occurred with structures Qop(b-e). The kinetics
of disappearance of Qop(b-d) produced rate constants
that showed minimal dependence when plotted against
the Hammett σ value (Figure 4). This suggests that
substitution on the para position of the aromatic ring of
the amine group does not significantly alter the rate of
degradation of these compounds. The benzyl amide Qop-
(e), having one methylene between the amide nitrogen
and the aromatic ring, was expectedly found to degrade
significantly slower than the aromatic amine substituents
Qop(a-d). The minimal Hammett correlation (Figure 4)
suggests that there is no difference in the amount of
negative charge between the ground state of the starting
material and the transition state in the formation of the
different degradation products. This suggests that at the
transition state a strong carbon-nitrogen bond is formed,
resembling the structure of the products.
Compounds Qop(f-i) were found to be stable and did
not significantly degrade in aqueous solution at pH 5.0.
This suggests that this redox-sensitive system is stable
when the amine is secondary [Qop(h)], also supporting
the claim of the nitrogen deprotonation. Quinone pro-
pionic amides of aliphatic amines, such as Qop(g,i,j), did
not undergo significant degradation, and only a minor
amount of QAd(i,j) was observed. Finally, when the
amide nitrogen is sterically hindered, such as the case
of Qop(f), no degradation is observed.
Gen er a l Meth od II for P r ep a r a tion of Am id es. In a
Pierce reaction vial, the amine (2 equiv) was added to a
solution of the N-hydroxysuccinimide ester of Qa (0.1 g, 0.3
mmol), whose synthesis was previously described,3 in 2 mL
benzene. The vial was sealed and placed in an oil bath at 80
The “trimethyl lock” was found earlier to facilitate
acceleration of the rate of lactonization of hydroxy-
propionic acids such as Qa and amides such as Qop(a).1,2
In this case, the great strain relief to form products such
as cy5en(a), cy5ke(a), and cy6ke(a) is also of consider-
ation. In a recent publication, the crystal structure of the
aforementioned spirolactone revealed that the methyl
groups of the “trimethyl lock” are no longer in close
proximity as in the open structures; rather, the geminal
dimethyl group at position 3 points in the opposite
direction from the methyl group at position 2′.11 There-
fore, the major driving force for this reaction is possibly
the relief of the strain energy involved because the close
proximity between the methyl groups that form the
“trimethyl lock” is lost.
°C for 15 min and then cooled to room temperature.
A
crystalline white precipitate was formed. The reaction mixture
was then diluted with diethyl ether, washed with water (20
mL), 5% aqueous HCl (20 mL), water (20 mL), and dried over
anhydrous MgSO4. Evaporation of the solvent afforded a
yellow solid that was recrystallized in EtOAc/hexane.
3-(3′,6′-Dioxo-2′,4′-d im eth ylcycloh exa -1′,4′-d ien yl)-3,3-
d im eth ylp r op ion ic Acid Am id e p-An isid in e [Qop (a )]. The
synthesis for Qop(a) has been previously reported by this
laboratory using the general method
I described above.
Structural data for this compound are described elsewhere.2,7
3-(3′,6′-Dioxo-2′,4′-d im eth ylcycloh exa -1′,4′-d ien yl)-3,3-
d im eth ylp r op ion ic Acid An ilin e Am id e [Qop (b)]. Com-
pound Qop(b) was prepared and purified according to general
method I as described above. Analytically pure Qop(b) was
obtained in 44% yield. Structural data for this compound are
described elsewhere.2
Con clu sion
3-(3′,6′-Dioxo-2′,4′-d im et h ylcycloh exa -1′,4′-d ien e)-3,3-
d im et h ylp r op ion ic Acid p-Met h yla n ilin e Am id e [Qop -
(c)]. Compound Qop(c) was prepared and purified according
to general method I as described above. Analytically pure Qop-
(c) was obtained in 20% yield: mp 157-159 °C; UV (CH3CN)
λmax 250 nm (ꢀ ) 21876), 334 nm (ꢀ ) 954); MS (EI), m/e 325
In conclusion, the studies described here suggest that
the instability problems reported previously with amide
derivative Qop(a) of the quinone propionic acid Qa
generated from anisidine7 are unique to primary aromatic
amines [Qop(a-d)]. In contrast, amides of Qa generated
from secondary aromatic [Qop(h)] and aliphatic amines
[Qop(e,f,g,i,j)] are generally stable. This result suggests
that if amines are carefully chosen, derivatization with
Qa could lead to useful redox-sensitive prodrugs or redox-
sensitive protecting groups in organic synthesis.
1
(M); H NMR δ 1.50 (6 H, s, C(CH3)2), 1.98 (3 H, d, J ) 1.35
Hz, 4′-(CH3)), 2.19 (3 H, s, 2′-(CH3)), 2.28 (3 H, s, p-CH3), 3.00
(2 H, s, 2-CH2), 6.48 (1 H, d, J ) 1.38 Hz, 5′-H), 7.08, 7.25 (4
H, m, ArH); 13C NMR δ 14.80 (4′-C(CH3)), 15.95 (2′-C(CH3)),
21.22 (p-CH3), 29.82 (3C(CH3)2), 39.27 (3-C(CH3)2), 50.76 (2-
CH2), 120.51 (Ar), 129.82 (Ar), 134.33 (Ar), 135.43 (Ar), 135.49
(5′-(CH)), 139.98 (2′-C), 144.05 (4′-C), 152.54 (1′-C), 170.42
(amide carbonyl), 188.64 (3′-carbonyl), 190.93 (6′-carbonyl).
Anal. Calcd for C20H23NO3: C, 73.82; H, 7.12; N, 4.30.
Found: C, 73.67; H, 7.14; N, 4.12.
(16) Rappoport, Z.; Biali, S. E. Acc. Chem. Res. 1988, 21, 442-449.

6638 J . Org. Chem., Vol. 61, No. 19, 1996
Nicolaou et al.
3-(3′,6′-Dioxo-2′,4′-d im eth ylcycloh exa -1′,4′-d ien yl)-3,3-
d im eth ylp r op ion ic Acid p-F lu or oa n ilin e Am id e [Qop -
(d )]. Compound Qop(d) was prepared and purified according
to general method I as described above. Analytically pure Qop-
(d) was obtained in 22% yield: mp 126-128°C; UV (CH3CN)
λmax 248 nm (ꢀ ) 15953), 330 nm (ꢀ ) 846); MS (EI), m/e 329
14.68 (4′-C(CH3)), 15.65 (2′-C(CH3)), 23.15 (CH(CH3)2), 29.59
(3-C(CH3)2), 39.19 (3-C(CH3)2), 49.86 (2-CH2), 135.66 (5′-CH),
139.19 (2′-C), 143.41 (4′-C), 152.85 (1′-C), 171.19 (amide
carbonyl), 188.63 (3′ carbonyl), 190.43 (6′ carbonyl). Anal.
Calcd for C16H23NO3: C, 69.31; H, 8.30; N, 5.05. Found: C,
69.38; H, 8.68; N, 4.78.
1
(M); H NMR δ 1.48 (6 H, s, C(CH3)2), 1.95 (3 H, d, J ) 1.43
3-(3′,6′-Dioxo-2′,4′-d im eth ylcycloh exa -1′,4′-d ien yl)-3,3-
d im eth ylp r op ion ic Acid Cycloh exyla m in e Am id e [Qop -
(i)]. Compound Qop(i) was prepared and purified according
to general method II as described above. The analytically pure
Qop(i) was obtained in 95% yield: mp 142-143 °C; UV (CH3-
CN) λmax 256 nm (ꢀ ) 9198), 332 nm (ꢀ ) 650); MS (EI), m/e
317 (M); 1H NMR δ 0.90-1.90 (10 H, m, cyclohexyl CH2), 1.45
(6 H, s, 3-C(CH3)2), 1.98 (3 H, d, J ) 1.41 Hz, 4′-C(CH3)), 2.15
(3 H, s, 2′-C(CH3)), 2.78 (2 H, s, 2-CH2), 6.52 (1 H, d, J ) 1.53
Hz, 5′-CH); 13CNMR δ 14.14 (2′-C(CH3)), 15.48 (4′-C(CH3)),
24.69, 25.29, 33.05 (cyclohexyl CH2), 47.69 (HNCHCH2), 28.96
(3-C(CH3)2), 38.62 (3-C(CH3)2), 49.34 (2′-CH2), 135.20 (5′-CH),
138.46 (2′-C(CH3)), 143.12 (4′-C(CH3)), 152.45 (1′-C), 170.60
(amide carbonyl), 188.11 (3′ carbonyl), 189.85 (6′ carbonyl).
Anal. Calcd for C16H23NO3: C, 71.90; H, 8.50; N, 4.42.
Found: C, 72.19; H, 8.89; N, 4.69.
3-(3′,6′-Dioxo-2′,4′-d im eth ylcycloh exa -1′,4′-d ien yl)-3,3-
d im eth ylp r op ion ic Acid N-Meth yla n ilin e Am id e [Qop -
(h )]. 3-(3′,6′-Dioxo-2′,4′ dioxo-2′,4′-dimethylcyclohexa-1′,4′-
dienyl)-3,3-dimethylpropionic acid Qa (0.18 g, 0.76 mmol) was
dissolved in 50 mL of CH2Cl2, to which was added N-
methylmorpholine (0.23 g, 2.28 mmol). The mixture was
cooled to -55 °C in a dry ice-2-propanol bath. Isobutyl
chloroformate (0.092 g, 0.91 mmol) was then added, followed
by addition of N-methylaniline (0.24 g, 2.28 mmol) after 10
min. The reaction mixture was stirred for 30-40 min and
filtered. The residue obtained by evaporation of the solvent
under reduced pressure was dissolved in 30 mL CHCl3. The
solution was then washed with water (50 mL), 5% HCl (50
mL), 5% NaHCO3 (50 mL), and saturated NaCl (50 mL)
solutions. The product Qop(h) was eluted from a silica gel
column with 40% methyl tertiary butyl ether in hexane (25%
yield). All characterization data for this compound are de-
scribed elsewhere.2
HP LC Assa y Con d ition s. The kinetics of degradation of
the quinone propionic amides [Qop(a-j)] were monitored by
HPLC according to a previously described analytical method.7
The concentrations of the Qop(a-j) and their degradation
products [QAd(a-j), cy5en(a-j), cy5ke(a-j), cy6ke(a-j)] shown
in Scheme 2 were quantified by measuring peak areas.
Kin etics of Degr a d a tion . Degradation kinetics were
carried out using acetate buffers (pH 4.5-6.0) diluted with 10%
acetonitrile. The concentration of acetate was varied from 0.05
M to 0.03 M, while maintaining a constant ionic strength of
0.3 M with sodium chloride. Reactions were initiated by
adding 150 µL of 1.0 mM solutions of the quinone propionic
amide [Qop(a-j)] to 1.35 mL of acetate buffer. Samples were
incubated at 37 °C. Aliquots of reaction mixtures at pH 5.0,
5.5, and 6.0 were removed at T ) 2 min and every 13 min
thereafter, up to 236 min. For pH 4.5 reaction mixtures,
aliquots were removed at T ) 2 min and every 30 min
thereafter, up to 572 min.
Hz, 4′-(CH3)), 2.15 (3 H, s, 2′-(CH3)), 3.01 (2 H, s, 2-CH2), 6.46
(1 H, d, J ) 1.46 Hz, 5′-H), 6.93, 7.32 (4 H, m, ArH); 13C NMR
δ 14.39 (4′-C(CH3)), 15.56 (2′-C(CH3)), 29.37 (3-C(CH3)2), 38.79
(3-C(CH3)2), 50.22 (2-CH2), 115.43 (Ar), 115.61 (Ar), 121.76
(Ar), 121.83 (Ar), 130.55 (Ar), 133.53 (Ar), 135.00 (5′-(CH)),
139.52 (2′-C), 143.73 (4′-C), 151.72 (1′-C), 170.10 (amide
carbonyl), 188.17 (3′-carbonyl), 190.22 (6′-carbonyl). Anal.
Calcd for C19H20NO3F: C, 69.28; H, 6.12; N, 4.25; F, 5.77.
Found: C, 69.25; H, 6.33; N, 4.64; F, 6.08.
3-(3′,6′-Dioxo-2′,4′-d im eth ylcycloh exa -1′,4′-d ien yl)-3,3-
d im eth ylp r op ion ic Acid Ben zyla m in e Am id e [Qop (e)].
Compound Qop(e) was prepared according to general method
I as described above. Structural data for this compound are
described elsewhere.2
3-(3′,6′-Dioxo-2′,4′-d im eth ylcycloh exa -1′,4′-d ien yl)-3,3-
d im eth ylp r op ion ic Acid (S)-(-)-r-Meth ylben zyla m in e
Am id e [Qop (f)]. Compound Qop(f) was prepared and purified
according to general method I as described above. Analytically
pure Qop(f) was obtained in 24% yield as an orange oil: UV
(CH3CN) λmax 254 nm (ꢀ ) 8088), 336 nm (ꢀ ) 532); MS (EI),
m/e 339 (M); 1H NMR δ 1.40 (3 H, s, R-CH3), 1.41 (3 H, s,
-C(CH3)2), 1.48 (3 H, s, C(CH3)2), 1.96 (3 H, d, J ) 1.37 Hz,
4′-(CH3)), 2.12 (3 H, s, 2′-(CH3)), 2.84 (2 H, q, 2-CH2), 5.01 (1
H, m, R- proton), 6.42 (1 H, d, J ) 1.43 Hz, 5′-H), 7.22-7.30 (5
H, m, ArH); 13C NMR δ 14.80 (4′-C(CH3)), 15.95 (2′-C(CH3)),
21.22 (p-CH3), 29.82 (3-C(CH3)2), 39.27 (3-C(CH3)2), 50.76 (2-
CH2), 120.51 (Ar), 129.82 (Ar), 134.33 (Ar), 135.43 (Ar), 135.49
(5′-(CH)), 139.98 (2′-C), 144.05 (4′-C), 152.54 (1′-C), 170.42
(amide carbonyl), 188.64 (3′-carbonyl), 190.93 (6′-carbonyl).
Anal. Calcd for C21H25NO3: C, 74.31; H, 7.42; N, 4.13.
Found: C, 74.59; H, 7.71; N, 3.98.
3-(3′,6′-Dioxo-2′,4′-d im eth ylcycloh exa -1′,4′-d ien yl)-3,3-
d im eth ylp r op ion ic Acid L-P h en yla la n in e Eth yl Ester
Am id e [Qop (j)]. Compound Qop(j) was prepared and purified
according to general method I as described above. Analytically
pure Qop(j) was obtained in 27% yield as an yellow oil: UV
(CH3CN) λmax 256 nm (ꢀ ) 8663), 338 nm (ꢀ ) 425); MS (EI),
m/e 411 (M); 1H NMR δ 1.21 (3 H, t, J ) 7.11 Hz, CO2CH2CH3),
1.37 (3 H, s, 3-CH3), 1.41 (3 H, s, 3-CH3), 1.97 (3 H, d, J )
1.21 Hz, 4′-(CH3)), 2.12 (3 H, s, 2′-(CH3)), 2.86 (2 H, q, 2-CH2),
3.01 (2 H, m, CH2Ph), 4.12 (2 H, q, J ) 7.12 Hz, CO2CH2CH3),
4.78 (1 H, q, a-proton), 6.13 (1 H, d, amide proton), 6.47 (1 H,
d, J ) 1.33 Hz, 5′ proton), 6.98-7.19 (5 H, m, ArH); 13C NMR
δ 13.84 (CO2CH2CH3), 14.04 (4′-C(CH3)), 15.34 (2′-C(CH3)),
28.82 and 28.89 (3-C(CH3)2), 37.70 (3-C(CH3)2), 38.35 (NHCH),
48.81 (2-CH2), 52.54 (CH2Ph), 61.16 (CO2CH2CH3), 126.76 (Ar),
128.23(Ar), 129.03 (Ar), 134.97 (5′-(CH)), 135.68(Ar), 138.68
(2′-C), 143.06 (4′-C), 151.89 (1′-C), 171.16 (CO2CH2CH3), 171.28
(amide carbonyl), 187.96 (3′-carbonyl), 189.61 (6′-carbonyl).
Anal. Calcd for C23H27NO5: C, 70.05; H, 7.10; N, 3.40.
Found: C, 69.85; H, 7.36; N, 3.74.
3-(3′,6′-Dioxo-2′,4′-d im eth ylcycloh exa -1′,4′-d ien yl)-3,3-
d im eth ylp r op ion ic Acid Isop r op yla m in e Am id e [Qop -
(g)]. Compound Qop(g) was prepared and purified according
to general method II as described above. The analytically pure
Qop(g) was obtained in 95% yield: mp 121-122 °C; UV (CH3-
CN) λmax 262 nm (ꢀ ) 6757), 336 nm (ꢀ ) 570); MS (EI), m/e
Su p p or tin g In for m a tion Ava ila ble: Tables of HPLC
retention time data and of UV absorbance data; equations
describing the kinetics of the reaction in Scheme 2 (3 pages).
This material is contained in libraries on microfiche, im-
mediately follows this article in the microfilm version of the
journal, and can be ordered from the ACS; see any current
masthead page for ordering information.
1
277(M); H NMR δ 1.05 (6 H, d J ) 6.54 Hz, CH(CH3)2), 1.44
(6 H, s, C(CH3)2), 1.96 (3 H, d, J ) 1.41 Hz, 4′-(CH3)), 2.17 (3
H, s, 2′-(CH3)), 2.78 (2 H, s, 2-CH2), 3.95 (1 H, m, CH(CH3)2),
5.17 (1 H, d, NH), 6.52 (1 H, d, J ) 1.41Hz, 5′-H); 13C NMR δ
J O961069L