Anodic oxidation of N-protected 4-methoxy anilines: Improved synthesis of quinone imine acetals

Full text of DOI:10.1021/jo991398o

J . Org. Chem. 2000, 65, 1231-1234
1231
as N-acyl-p-quinol imine ethers,11 are easily available
through anodic oxidation of p-methoxy or p-alkyl substi-
tuted anilides.12 The former could be considered as
masked quinone imines.
An od ic Oxid a tion of N-P r otected
4-Meth oxy An ilin es: Im p r oved Syn th esis of
Qu in on e Im in e Aceta ls
In connection with a program directed toward the
synthesis and applications of chiral sulfinyl p-quinol
derivatives,13 we were interested in carbamate-protected
quinone imine ketals, which could be the precursors of
amino p-quinol analogues. Among the carbamate groups,14
we chose the tert-butylcarbamoyl because it is stable
enough to allow synthetic manipulation and is more
easily hydrolizable than other. Despite the general ac-
cessibility of N-acylated quinone imine acetals, few
examples of carbamate-protected ones have been re-
ported. Swenton10a described the anodic oxidation of
N-BOC and N-methoxycarbonyl-p-methoxy anilines us-
ing both platinum anode and cathode. Apparently, the
use of a copper cathode did not allow oxidation to be
completed. Although good yields of isolated products were
obtained, reaction conditions appear critical, and the use
of the platinum cathode introduced a serious limitation
due to the cost of this material. We report herein the
anodic oxidation of differently N-substituted p-methoxy
anilines 1 based on the use of a copper cathode and show
that N-tert-butylcarbamoyl-protected quinone imine ac-
etals can be prepared in good yields using a very simple
experimental procedure. This electrochemical oxidation
has been also checked in other N-substituted derivatives
such as acetanilides, p-toluenesulfonamides, and phen-
ylhydrazines, as well as N-alkyl and N-sulfinyl anilines.
M. Carmen Carren˜o* and Mar´ıa Ribagorda
Dpto. Quı´mica Orga´nica (C-I) Universidad Auto´noma,
Cantoblanco, 28049 Madrid, Spain
Received September 3, 1999
Recent synthetic applications of quinone imines1 and
bisimines2 have focused on nucleophilic additions pro-
moted by both acids and bases.3 These reactions require
the use of stable N-arylsulfonyl derivatives of quinone
imines to avoid decomposition, which occurs with more
labile protecting groups. However, the strongly acidic
conditions necessary to remove the sulfonyl group,4 after
transformation of the quinone imine, limit their synthetic
potential. Although N-acyl derivatives are also available,2c
similar problems arise upon amide hydrolysis. Simple
p-benzoquinone imines are extremely unstable com-
pounds and are subject to rapid hydrolysis. With the
exception of N-aryl quinone imine acetals,5 they should
be generated in situ for further studies or synthetic
manipulation.6 The difficulties found in the synthesis and
isolation of such derivatives could be the origin of their
limited synthetic use.7 The most general method for the
preparation of both quinone derivatives stems on the
chemical oxidation of p-methoxy anilides or p-phenylene
diamides,2a,b mainly effected with Ce(IV) salts8 or Pb-
(AcO)4.4,9 In all cases the presence of an imide (N-acyl or
N-arylsulfonyl group) is required to prevent decomposi-
tion of the resulting imine in the conditions where these
chemical oxidations are performed. Although electro-
chemical oxidation of p-amino phenols6 has served for the
in situ preparation of simple quinone imines, their
isolation was subjected to the same limitations as chemi-
cal methods. N-Acylated quinone imine acetals,10 as well
Resu lt a n d Discu ssion
Starting N-tert-butoxycarbonyl-p-methoxy anilines 1a-f
were obtained by direct treatment of adequately substi-
tuted commercially available anilines with (BOC)2O
following the reported procedure for aliphatic amines.15
Sulfonamide 1g, acetanilide 1h , and propynylaniline 1i
were prepared by conventional procedures involving
reaction of p-methoxy aniline with p-toluenesulfonyl
chloride, acetic anhydride, and propargyl bromide, re-
spectively. Synthesis of 1j was achieved by hydrogenation
(Al/Hg) of the 4-methoxy azobenzene precursor, and
preparation of 1k (63% yield) was carried out by sequen-
tial treatment of p-methoxy aniline with n-BuLi and
menthyl p-toluene sulfinate.16 This procedure gives acces
to enantiopure (S)-N-(p-tolyl sulfinyl)-4-methoxy aniline
1k (Scheme 1).
(1) For applications of p-quinone monoimines, see (a) Engler, T. A.;
Chai, W.; Lynch, K. O., J r.; Meduna, S. P. Tetrahedron Lett. 1995, 36,
7003. (b) Engler, T. A.; Chai, W.; LaTessa, K. O. J . Org. Chem. 1996,
61, 9297 and references therein. (c) Engler, T. A.; Chai, W. Tetrahedron
Lett. 1996, 37, 6969.
(2) For applications of p-quinone bisimines, see: (a) Engler, T. A.;
Wanner, J . Tetrahedron Lett. 1997, 38, 6135. (b) Engler, T. A.; Meduna,
S. P.; LaTessa, K. O.; Chai, W. J . Org. Chem. 1996, 61, 8598. (c) Boger,
D. L.; Zarrinmayeh, H. J . Org. Chem. 1990, 55, 1379. (d). Boger, D.
L.; Coleman, R. S. J . Am. Chem. Soc. 1988, 110, 4796. (e) Holmes, T.
J ., J r.; Lawton, R. G. J . Org. Chem. 1983, 48, 3146.
(3) For Lewis acid promoted cycloadditions with styrenyl systems,
see: (a) Engler, T. A.; Scheibe, C. M.; Iyengar, R. J . Org. Chem. 1997,
62, 8274. (b) Engler, T. A.; Lynch, K. O., J r.; Chai, W.; Meduna, S. P.
Tetrahedron Lett. 1995, 36, 2713.
(4) (a) Adams R.; Looker, J . H. J . Am. Chem. Soc. 1951, 73, 1145.
(b) Adams, R.; Whitaker, L. J . Am. Chem. Soc. 1956, 78, 658.
(5) (a) Carren˜o, M. C.; Cuerva, J . M.; Echavarren, A.; Ribagorda,
M. Angew. Chem., Int. Ed. Engl. 1999, 38, 1449. (b) Coutts, I. G. C.;
Southcott, M. R. J . Chem. Res. 1988 (S) 241; (M) 1921.
(6) (a) Harmalker, S. P.; Sawyer, D. T. J . Org. Chem. 1984, 49,
3579-85. (b) Young, T. E.; Beidler, W. T. J . Org. Chem. 1984, 49, 4833.
(c) Young, T. E.; Babbit, B. W. J . Org. Chem. 1983, 48, 562.
(7) Reviews: (a) The Chemistry of the Carbon-Nitrogen Double
Bond; Patai, S., Ed. Interscience Publishers: New York, 1970; pp 633-
729. (b) The Chemistry of Quinonoid Compounds; Patai, S., Rappoport,
Z., Eds.; J ohn Wiley & Sons: New York, 1988; Vol. II, Parts 1 and 2.
(c) Grundmann, C. Methoden der Organischen Chemie (Houben-Weyl);
Thieme: Stuttgart, 1977: Vol. 7/3b, Chapter V.
(10) (a) Swenton, J . S.; Bonke, B. R.; Chen, C.-P.; Chou, C.-T. J .
Org. Chem. 1989, 54, 51. (b) Swenton, J . S.; Bonke, B. R.; Clark, W.
M.; Chen, C.-P.; Martin, K. V. J . Org. Chem. 1990, 55, 2027.
(11) (a) Novak, M.; Helmick, J . S.; Oberlies, N.; Rangappa, K.; Clark,
W. M.; Swenton, J . S. J . Org. Chem. 1993, 58, 867. (b) Swenton, J . S.;
Biggs, T. N.; Clark, W. M. J . Org. Chem. 1993, 58, 5607.
(12) Chen, C.-P.; Chou, C.-T.; Swenton, J . S. J . Am. Chem. Soc. 1987,
109, 946.
(13) (a) Carren˜o, M. C.; Pe´rez Gonza´lez, M.; Fischer, J . Tetrahedron
Lett. 1995, 36, 4893. (b) Carren˜o, M. C.; Pe´rez Gonza´lez, M.; Ribagorda,
M.; Fischer, J . J . Org. Chem. 1996, 61, 6758. (c) Carren˜o, M. C.; Pe´rez
Gonza´lez, M.; Houk, K. N. J . Org. Chem. 1997, 62, 9128. (d) Carren˜o,
M. C.; Pe´rez Gonza´lez, M.; Ribagorda, M.; Houk, K. N. J . Org. Chem.
1998, 63, 3687.
(14) Greene, T. W.; Wuts, P. G. Protective Groups in Organic
Synthesis, 3rd ed.; Wiley-Interscience: New York, 1999.
(15) (a) Stahl, G. L.; Walter, R.; Smith, C. W. J . Org. Chem. 1978,
43, 2285.
(8) Engler, T. A.; Chai, W.; Lynch, K. O., J r. Tetrahedron Lett. 1995,
36, 7003.
(9) Adams, R.; Reifschneider, W. Bull. Soc. Chim. Fr. 1958, 23.
(16) Solladie´, G.; Hutt, J .; Girardin, A. Synthesis 1987, 173.
10.1021/jo991398o CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/02/2000

1232 J . Org. Chem., Vol. 65, No. 4, 2000
Notes
Sch em e 1
Sch em e 2
Anodic oxidation of compounds 1 was effected in a
single cell apparatus with a 5 cm diameter × 5 cm 45-
mesh Pt anode and a copper wire (1.5 mm × 10 cm)
situated inside as a cathode, using a methanol solution
of 1 in the presence of lithium perchlorate as electrolyte
and an added base, which is necessary to avoid decom-
position of the final product. As previously observed for
anilides,10a reaction products and yields were dependent
on an array of experimental variables, namely, current
density, added base and workup of the reaction, as well
as the nature of the substituent existent on the nitrogen.
The best results were obtained when a methanolic
solution of 1 (0.1-1 equiv), LiClO4, and pyridine (1 equiv)
was subjected to a constant current of 0.1 A (2 V) at 0
°C. Although pyridine was reported to promote the
formation of pyridinium derivatives in the electrochemi-
cal oxidation of p-methoxy-benzanilide,17 in our case,
analogous pyridinium salts were not detected. The amount
of LiClO4 necessary in each case was different. The
electrolyte was added to the methanol solution until the
current rose to 0.1 A. Addition of NaHCO3 instead of
pyridine as a base did not allow completion of the
reaction, probably as a consequence of its low solubility
in methanol. Although 2,6-lutidine was soluble enough
and gave yields similar to those obtained with pyridine,
further removal of the former was inconvenient, which
made the isolation process difficult. Workup was carried
out by removal of the solvent at reduced pressure,
extraction of the product with ethyl acetate, and washing
with brine. Although the resulting quinone imine acetals
2 were stable enough to be purified by flash chromatog-
raphy (with the exception of 2e), they must be stored in
a freezer below 0 °C. The results of these reactions are
indicated in Table 1.
and in accordance with the proposed mechanism for
similar anodic oxidations.19 N-BOC-4-methoxynaphthyl-
amine 1e behaved similarly, affording pure quinone
imine acetal 2e in 97% crude yield (entry 5). In this case
chromatographic purification gave a complex mixture of
decomposition products. When a Cl subtituent is present
in the starting anilide 1f, quinone imine acetal 2f could
be obtained in 68% yield and characterized as a syn/anti
mixture if the crude reaction mixture was directly
purified by flash chromatography. If the usual workup
was followed (washing with brine), 3-chloro-p-benzo-
quinone dimethyl monoacetal 3 was the isolated product.
The higher electrophilicity of the imine carbon in the
initial oxidation product 2f, due to the electron-with-
drawing effect of the Cl, should facilitate the formation
of 3, even in the presence of pyridine.
N-p-Toluenesulfonyl-p-methoxyaniline 1g and p-meth-
oxyacetanilide 1h also gave the quinone imine acetals
2g ad 2h in good yields (Table 1, entries 7 and 8)
following this oxidation procedure.
Finally, under similar conditions (E, MeOH, LiClO4,
Py), compounds 1i, 1j, and 1k , lacking an amide or
carbamate N-protecting group, evolved into p-benzo-
quinone dimethyl bisacetal 420 in good yields. The un-
stability of the p-benzoquinone imine acetals that should
result in these reactions would explain their further
evolution to 4 (Scheme 2)
It is important to note that this electrochemical oxida-
tion allows the synthesis of N-BOC-protected quinone
imine acetals 2 following an experimentally simple
procedure that avoids the consumption of an expensive
Pt cathode. The use of pyridine as an added base prevents
the experimental complication arising when NaHCO3 was
the base of choice, which forces the periodical washing
of the electrodes during the electrolysis.10a
In summary, a simple synthesis of N-protected quinone
imine acetals that improves the method previously
reported has been developed. Transformation of 2 into
different interesting amino derivatives will be described
in future papers.
N-BOC-p-methoxy aniline 1a reacted in 6 h to give
derivative 2a after removal of the methanol and pyridine
under vacuum in quantitative yield. Although the crude
1
product 2a was >97% pure by H NMR, flash column
chromatography allowed its isolation pure in 70% yield
(Table 1, entry 1). The oxidation of 2- and 3-methoxy-
substituted derivatives 1b and 1c (entries 2 and 3), as
well as that of N-BOC-4-methoxy-2-methyl aniline 1d ,
gave rise to the formation of quinone imine acetals 2b,
2c, and 2d , respectively, in good isolated yields. Com-
pound 2c was characterized as a mixture of the syn and
anti imines. As expected, substrates 1b-d bearing
electron-donating substituents at the aromatic ring (OMe
and Me) were also easily oxidized as a consequence of
the effect of the substituents on the oxidation potential18
Exp er im en ta l Section
Gen er a l. All reactions were monitored by TLC, which was
performed on precoated silicagel 60 F254 plates. Flash column
chromatography was effected with silicagel 60 (230-240 mesh).
1H NMR spectra were recorded at 200 or 300 MHz. 13C NMR
were recorded at 50 or 75 MHz. Chemical shifts are reported in
ppm relative to TMS in CDCl3. All NMR spectra were obtained
in CDCl3 at room temperature. HRMS were measured at 70 eV.
(17) Ikenoya, S.; Masui, M.; Ohmori, H.; Sayo, H. J . Chem. Soc.,
Perkin Trans. 2 1974, 571. (b) Masui, M.; Ohmori, H.; Sayo, H.; Ueda,
A. J . Chem. Soc., Perkin Trans. 2 1976, 1180.
(18) Electron-donating substituents decrease the oxidation potential
of aromatic derivatives (see: Weingberg, H. L. Technique of Electro-
organic Synthesis Part II; J ohn Wiley: New York, 1975.)
(19) (a) Dolson, M. G.; Swenton, J . S. J . Am. Chem. Soc. 1981, 103,
2361. (b) Swenton, J . S. Acc. Chem. Res. 1983, 16, 74.
(20) (a) Belleau, B.; Weinberg, N. L. J . Am. Chem. Soc. 1963, 85,
2525. (b) Weinberg, N. L.; Belleau, B. Tetrahedron 1973, 29, 279.

Notes
J . Org. Chem., Vol. 65, No. 4, 2000 1233
Ta ble 1. An od ic Oxid a tion of N-P r otected p-Meth oxy An ilin es 1
All reagents were purchased from Aldrich and were used without
further purification.
30 min of stirring, 5 mL of a saturated solution of NH4Cl was
added, and the organic phase was washed (NaCl saturated
solution, 2 × 10 mL), dried (Na2SO4), and evaporated to dryness.
The resulting oil crystallized on addition of an ether/hexane
mixture (671 mg, 63% yield). White solid. Mp: 98-101 °C
(S)-N-(p-Tolylsu lfin yl)-4-m eth oxya n ilin e (1k ). A 2.5 M
solution of n-BuLi in hexane (3.25 mL, 8.12 mmol, 2 equiv) was
slowly added to a mixture of 4-methoxyaniline (500 mg, 4.06
mmol, 1 equiv) and 10 mL of THF at -78 °C. After 15 min of
stirring at this temperature, the resulting mixture was added
to a solution of (-)-menthyl p-toluene sulfinate (1.49 g, 5.07
mmol, 1.25 equiv) in 3 mL of THF at room temperature. After
(hexane); [R]20 ) +203 (c ) 1, CHCl3). 1H NMR: δ 7.62-7.28
D
(AA′BB′ system, 4H), 7.03 (d, 1H, J ) 6.6 Hz), 6.69 (d, 1H, J )
6.6 Hz), 6.10 (bs, 1H), 3.75 (s, 3H), 2.41 (s, 3H). 13C NMR: 156,
141, 133, 129 (2C), 125 (2C), 122.4, 122.2 (2C), 114 (2C), 55, 21

1234 J . Org. Chem., Vol. 65, No. 4, 2000
Notes
(3C). Anal. Calcd for C14H15NO2S: C, 64.34; H, 5.79; N, 5.36; S,
12.27. Found: C, 64.60; H, 5.52; N, 5.15; S, 12.06.
vacuum and extraction with AcOEt (97% crude yield). Compound
2e decomposes to a black oil if the organic phase was washed
with brine and upon standing at room temperature or even at 0
°C after 24 h. It could not be purified by flash chromatography.
1H NMR (syn and anti): δ 8.6 (brs, 1H), 8.21 (dd, 1H, J ) 1.4
and 7.9 Hz), 7.71 (dd, 1H, J ) 1.6 and 8 Hz), 7.59 (dt, 1H, J )
1.3 and 7.6 Hz), 7.43 (dt, 1H, J ) 1.6 and 8 Hz), 6.73 (d, 1H,
J ) 10.2 Hz), 6.59 (d, 1H, J ) 10.2 Hz), 3.13 (6H, s), 1.6 (s, 9H).
13C NMR: 162.2, 155.3, 148.8, 139.9, 137.3, and 136.6, 132.0
and 130.9, 128.7, 126.2, 125.5, 125.2 and 123.8, 95.2, 82.9, 50.8
and 49.8 (2C), 27.7 (3C).
N-(ter t-Bu t oxyca r b on yl)-3-ch lor o-p -b en zoq u in on e Im -
in e Dim et h yl Acet a l (2f). Compound 2f was obtained from
N-(tert-butoxycarbonyl)-3-chloro-4-methoxylaniline 1f (317 mg
1.23 mmol, 1 equiv), using 100 µL of pyridine (1.23 mmol, 1
equiv), 83 mg of LiClO4 (0.6 mmol, 0.4 equiv), and 100 mL of
MeOH. After removal of MeOH and extraction with AcOEt, the
crude product was directly purified by flash column chromatog-
raphy (hexanes/ethyl acetate 6:1, 240 mg, 68% yield, colorless
oil). 1H NMR (syn and anti): δ 6.67 (d, 1H, J ) 2.1 Hz), 6.55
(dd, 1H, J ) 2.1 and 10.2 Hz), 6.0 and 5.89 (2d, 1H, J ) 10.2
Hz), 3.22 and 3.18 (2s, 6H), 1.53 and 1.42 (2s, 9H). 13C NMR:
160.4, 156.2 and 156, 148.5, 146, 139.7and 139.0, 132.4 and
132.1, 124.9 and 123.9, 95.1, 83.1, 51.2 and 51.0 and 50.1 (2C),
27.8 and 27.5 (3C). HRMS: (EI) m/z calcd for M+ 287.0924, found
287.0929.
Gen er a l P r oced u r e for An od ic Oxid a tion . Electrolysis
was carried out in a single cell apparatus in reagent grade
methanol, using a circular platinum gauze anode (5 cm × 5 cm
in diameter) 45-mesh, a copper wire cathode (1.5 mm × 10 cm),
and a AMEL (model 549) power supply. A solution of the
corresponding aniline derivative 1 (1 equiv), pyridine (1 equiv),
and LiClO4 (0.3-1 equiv) in MeOH was anodically oxidized at
0 °C, under constant current (0.1 A, 2 V). The reaction was
monitored by TLC. The methanolic solution was evaporated in
a vacuum, extracted with AcOEt, washed with brine solution,
dried over Na2SO4, filtered, and concentrated in a vacuum. Flash
column chromatography afforded compounds 2.
N-(ter t-Bu toxycar bon yl)-p-ben zoqu in on e Im in e Dim eth -
yl Aceta l (2a ). Compound 2a was obtained from N-(tert-
butoxycarbonyl)-4-methoxyaniline 1a (3 g, 0.01 mol, 1 equiv)
using 1 mL (0.01 mol, 1equiv) of pyridine, 1.4 g of LiClO4 (0.01
mol, 1 equiv), and 250 mL of MeOH as a colorless oil (hexanes/
ethyl acetate 10:1, 1.7 g, 70%). 1H NMR: δ 6.46 (brd, 2H, J )
10.2 Hz), 6.33 (d, 2H, J ) 10.2 Hz), 3.26 (s, 6H), 1.48 (s, 9H).
13C NMR: 160.4, 155.6, 139.9, 138.5, 129.0, 121.8, 92.2, 81.7,
49.1 (2C), 27.8 (3C). HRMS: m/z calcd for M+ 253.1314, found
253.1319.
N-(ter t-Bu toxyca r bon yl)-2-m eth oxy-p-ben zoqu in on e Im -
in e Dim eth yl Aceta l (2b). Compound 2b was obtained from
N-(tert-butoxycarbonyl)-2,4-dimethoxyaniline 1b (1 g, 3.9 mmol,
1 equiv) using 337 µL (3.9 mmol, 1 equiv) of pyridine, 200 mg of
LiClO4 (1.58 mmol, 0.4 equiv), and 250 mL of MeOH. After
removal of MeOH and extraction with AcOEt, the crude product
was directly purified by flash column chromatography (hexanes/
ethyl acetate 4:1, 62%) as a light yellow oil. 1H NMR: δ 7.0 (brd,
J ) 10.2 Hz, 1H), 6.15 (dd, J ) 10.2 and 1.7 Hz, 1H), 5.54 (d,
J ) 1.7 Hz, 1H), 3.69 (s, 3H), 3.02 (6H, s), 1.28 (s, 9H). 13C NMR:
167.8, 152.9, 140.6, 129.2 (2C), 103.6 (2C), 80.79, 79.9, 59.9, 55.8,
51.0, 27.8 (3C). HRMS: m/z calcd. for M+ 283.1419, found
283.1421.
N-(ter t-Bu toxyca r bon yl)-3-m eth oxy-p-ben zoqu in on e Im -
in e Dim eth yl Aceta l (2c). Compound 2c was obtained from
N-(tert-butoxycarbonyl)-3,4-dimethoxyaniline 1c (300 mg, 1.2
mmol) using 100 µL (1.2 mmol) of pyridine, 76 mg of LiClO4 (0.72
mmol, 0.6 equiv), and 100 mL of MeOH as a light yellow oil
(AcOEt/hexane 2:1, 235 mg, 70% yield). 1H NMR (syn and
anti): δ 6.45 (brd, J ) 10.2 Hz, 1H), 6.31 (brd, J ) 10.2 Hz,
1H), 5.68 (brs, 1H), 3.75 (s, 3H), 3.23 (6H, s), 1.51 (s, 9H). 13C
NMR: δ 164.2 (br), 161.9 (br), 158.6, 148.7, 137.2 (br), 136.9 (br)
124(br), 94.2, 82.6, 55.5, 51.1 and 49.8 (2C), 27.8 and 27.7 1 and
27.6 (3C). HRMS: m/z calcd for M+ 283.1419, found 283.1415.
N-(ter t-Bu toxyca r bon yl)-2-m eth yl-p-ben zoqu in on e Im -
in e Dim eth yl Aceta l (2d ). Compound 2d was obtained from
N-(tert-butoxycarbonyl)-4-methoxy-2-methylaniline 1d (2.1 g, 8.8
mmol, 1 equiv), using 714 µL of pyridine (8.8 mmol, 1 equiv),
336 mg of LiClO4 (3.1 mmol, 0.3 equiv), and 250 mL of MeOH
as a light red oil (hexanes/ethyl acetate 6:1, 1.7 g, 71%). 1H NMR:
δ 6.48 (dd, 1H, J ) 2.3 and 10.3 Hz), 6.33 (dq, 1H, J ) 1.4 and
2.3 Hz), 6.32 (d, 1H, J ) 10.3 Hz), 3.29 (s, 6H), 1.97 (d, 3H, J )
1.4 Hz), 1.53 (s, 9H). 13C NMR: 161.6, 156.4, 138.7, 136.2, 134.8,
122.8, 93.1, 82.2, 49.7 (2C), 27.8 (3C), 16.9. HRMS: m/z calcd
for M+ 267.1470, found 267.1470.
When washed with brine and extracted with ethyl acetate the
crude mixture resulting from 1f was quantitatively transformed
into 3-chloro-p-benzoquinone dimethyl acetal 3. 1H NMR: δ 6.79
(d, 1H, J ) 10.2 Hz), 6.61 (d, 1H, J ) 2.1 Hz), 6.45 (dd, 1H, J )
2.1 and 10.2 Hz), 3.31 (s, 6H). 13C NMR: 183, 152.6, 143.7, 131.8,
131.4, 94.8, 51.4 (2C).13C NMR: 184.1, 152.2, 144, 132.8, 132,
94.2, 52.1 (2C).
N-(p-Tolu en esu lfon yl)-p-ben zoqu in on e Im in e Dim eth -
yla ceta l (2g). Compound 2g was obtained from N-(p-toluene-
sulfonyl)-4-methoxyanilide 1g (670 mg, 2.41 mmol) using 194
µL (2.41 mmol) of pyridine, 256 mg of LiClO4 (1.4 mmol, 0.6
equiv), and 100 mL of MeOH. Compound 2g was isolated pure
as a purple solid after crystallization with ether (573 mg, 77%).
Mp 111.4-111.9. 1H NMR: δ 7.85 and 7.32 (AA′BB′ system, 4H),
7.62 (dd, 1H, J ) 2 and 12.8 Hz), 6.76 (dd, 1H, J ) 2.7 and 12.8
Hz), 6.69 (dd, 1H, J ) 2.7 and 10.2 Hz), 6.34 (dd, 1H, J ) 2 and
10.2 Hz), 3.34 (s, 6H), 2.48 (s, 3H). 13C NMR: 162.9, 143.9, 143.3,
142.0, 137.2, 130.2, 129.3 (2C), 127.0 (2C), 122.8, 91.8, 50.0 (2C),
21.3. HRMS: m/z calcd for M+ 307.0878, found 307.0875.
N-Acetyl p-Ben zoqu in on e Im in e Dim eth yla ceta l (2h ).10a
Compound 2h was obtained from 4-methoxyacetanilide 1h (200
mg, 1.2 mmol) using 180 µL (1.2 mmol) of pyridine, 127 mg of
LiClO4 (1.2 mmol, 1 equiv) and 100 mL of MeOH. Compound
1
2h was isolated pure as a yellow oil (195 mg, 69%). H NMR: δ
6.54 (d, J ) 10.8 Hz, 1H), 6.38 (d, J ) 10.8 Hz, 1H), 3.35 (s,
6H), 2.20 (s, 3H).
Ack n ow led gm en t. We thank Ministerio de Educa-
cio´n y Cultura for the award of a predoctoral fellowship
to M.R. This research was supported by the Direccio´n
General de Investigacio´n Cient´ıfica y Te´cnica (DGICYT),
grants PB95-0174 and PB98-0062.
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra of compounds 2a -g and 3 and 1H NMR spectrum of
2h . This material is available free of charge via the Internet
at http://pubs.acs.org.
N-(ter t-Bu toxyca r bon yl)-1,4-n a p h th oqu in on e Im in e Di-
m eth yl Aceta l (2e). Compound 2e was obtained from N-(tert-
butoxycarbonyl)-4-methoxynaphthylamine 1e (1 g, 3.6 mmol, 1
equiv) using 295 µL of pyridine (3.6 mmol, 1 equiv), 383 mg of
LiClO4 (3.6 mmol, 1 equiv), and 250 mL of MeOH as a light
yellow oil after removal of the methanol and pyridine under
J O991398O