Alkylation, aldol, and related reactions of O-alkanoyl- and 2-alkenoylTEMPOs (2,2,6,6-tetramethylpiperidine-N-oxyl): Insight into the reactivity of their anionic species in comparison with esters and amides

Article abstract of DOI:10.1021/jo0617316

(Chemical Equation Presented) The lithium anionic species generated from O-alkanoylTEMPOs upon treatment with LDA were first employed as a nucleophile for alkylation, Michael addition, direct aldol reaction, and others. The alkylation occurred smoothly at

Full text of DOI:10.1021/jo0617316

Furthermore, the choice of the acyl substituent is important for
the ensuing elaboration of the acyl functionality to a more
versatile form. For instance, the aldol derivatives comprised of
N-methoxy-N-methylamides (Weinreb amide), accessible either
directly by aldol reaction3 or indirectly by transamidation of
â-hydroxycarboxylic derivatives,4 are often utilized as a precur-
sor of aldehydes or ketones through the reduction or alkylation
process.5 In this context, we succeeded in selective reduction
of O-acylTEMPOs,6 extended carboxylic acid derivatives with
a hetero-hetero bond-like peroxyesters7 and Weinreb amides,8
to the corresponding aldehydes upon treatment with DIBALH.
In our continuing studies on exploring TEMPO substitution
as a reaction-controlling element,9 we examined the reactivity
of O-alkanoylTEMPOs A, easily accessible by acyl substitution
with the TEMPO anion, in the C-C bond-making reaction at
the C2 position including alkylation, direct aldol reaction,
Michael addition, as well as related reactions. Thus far scant
attention has been paid to characterization of the anionic species
B and/or C possibly accessible by deprotonation of A, though
compound A poses as carboxylic derivatives (Scheme 1).10 In
the meantime, we also discussed the effect of 2,2,6,6-tetra-
methylpiperidine-1-oxyl (TEMPO) as an auxiliary on the
stereochemistry of the aldol reactions.
Alkylation, Aldol, and Related Reactions of
O-Alkanoyl- and 2-AlkenoylTEMPOs
(2,2,6,6-Tetramethylpiperidine-N-oxyl): Insight
into the Reactivity of Their Anionic Species in
Comparison with Esters and Amides
Tsutomu Inokuchi*,† and Hiroyuki Kawafuchi‡
Department of Bioscience and Biotechnology, Faculty of
Engineering, Okayama UniVersity, Tsushima-naka, Okayama,
700-8530, Japan and Department of Chemical and Biochemical
Engineerings, Toyama National College of Technology,
Hongo-machi, Toyama, 939-8630, Japan
inokuchi@cc.okayama-u.ac.jp
ReceiVed August 21, 2006
As shown in Table 1, deprotonation of the O-acylTEMPO
2a was easily achieved on treatment with LDA in THF at
-78 °C, and reaction of the resulting anionic species with
iodomethane (MeI) and benzyl and allyl bromides led to the
corresponding 3a,b,c (E ) Me, PhCH2, allyl) in 74-87% yields
(entries 1-3). The same alkylation was also achieved with tert-
butyl bromoacetate as an electrophile at 0 °C, affording the
desired mixed tert-butyl/TEMPO-1-yl succinate 3d (E ) CH2-
CO2But) in 64% yield (entry 4).11 On the other hand, our
attempts for the alkylation of 2,2-dialkylated 3a were unsuc-
cessful. Furthermore, silylation of the anionic species from
O-aceylTEMPO (1), 2a, and 3a with TMSCl at -10 °C resulted
in the recovery of the starting O-acylTEMPOs, which is different
from that of tert-butyl isobutyrate, forming the corresponding
enol silyl ether even at -78 °C.12
The lithium anionic species generated from O-alkanoylTEM-
POs upon treatment with LDA were first employed as a
nucleophile for alkylation, Michael addition, direct aldol
reaction, and others. The alkylation occurred smoothly at
the methylene carbon, and no alkylation was found in the
isobutyryl analogue, while silylation was scarcely attainable.
Substitutions of the heteroatom were achieved by reaction
with PhSSPh and DEAD. The reactivity of these anionic
species is successfully extended to aldol reactions in which
moderate anti or syn selectivity was executed with propionyl
derivatives. Tandem Michael addition of lithium amide
followed by aldol reaction was performed on the O-
crotonoylTEMPOs.
Subsequently, we examined the reactivity of this anionic
species toward other electrophiles such as nitroolefin and
(3) (a) Pe´rez, M.; del Pozo, C.; Reyes, F.; Rodr´ıguez, A.; Francesch,
A.; Echavarren, A. M.; Cuevas, C. Angew. Chem., Int. Ed. 2004, 43, 1724-
1727. (b) Andre´s, J. M.; Pedrosa, R.; Pe´rez-Encabo, A. Tetrahedron 2000,
56, 1217-1223. (c) Palomo, C.; Aizpurua, J. M.; Aurrekoetxea, N.; Lo´pez,
M. C. Tetrahedron Lett. 1991, 32, 2525-2528.
(4) (a) Evans, D. A.; Trenkle, W. C.; Zhang, J.; Burch, J. D. Org. Lett.
2005, 7, 3335-3338. (b) Blakemore, P. R.; Browder, C. C.; Hong, J.;
Lincoln, C. M.; Nagornyy, P. A.; Robarge, L. A.; Wardrop, D. J.; White,
J. D. J. Org. Chem. 2005, 70, 5449-5460. (c) Brinkmann, Y.; Carreno, M.
C.; Urbano, A.; Colobert, F.; Solladie, G. Org. Lett. 2004, 6, 4335-4338.
(d) Baker-Glenna, C.; Ancliff, R.; Gouverneur, V. Tetrahedron 2004, 60,
7607-7619.
Enolates of carboxylic acid derivatives such as esters,
thioesters, and amides are one of the pivotal intermediates in
the carbon-carbon bond-making process in alkylation, aldol
reaction, Michael addition, and others,1 and their potential is
well demonstrated by the key role played in the synthesis of
biologically significant compounds such as taxol and epothilones.2
† Okayama University.
(5) Nahm, S.; Weinreb, S. M. Tetrahedron Lett. 1981, 22, 3815-3818.
(6) Inokuchi, T.; Kawafuchi, H. Tetrahedron 2004, 60, 11969-11975.
(7) Lawesson, S.-O.; Yang, N. C. J. Am. Chem. Soc. 1959, 81, 4230-
4233.
‡ Toyama National College of Technology.
(1) Carey, F. A.; Sundberg, R. J. AdVanced Organic Chemistry, 4th ed.;
Kluwer Academic and Plenum Publishers: New York, 2001; part B, pp
1-89. (b) No´gra´di, M. StereoselectiVe Synthesis, 2nd ed.; VCH: Weinheim,
1995; pp 184-208. (c) Heathcock, C. H. ComprehensiVe Organic Synthesis;
Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford, 1991; Vol. 2, pp
181-238.
(2) (a) Carey, F. A.; Sundberg, R. J. AdVanced Organic Chemistry, 4th
ed.; Kluwer Academic and Plenum Publishers: New York, 2001; part B,
pp 881-896. (b) In Modern Aldol Reactions; Mahrwald, R., Ed.; Wiley-
VCH: Weinheim, 2004.
(8) Weinreb, S. M.; Folmer, J. J. Encyclopedia of Reagents for Organic
Synthesis; John Wiley & Sons: New York, 1995; Vol. 3, pp 2083-2086.
(9) (a) Inokuchi, T.; Kawafuchi, H. J. Org. Chem. 2006, 71, 947-953.
(b) Inokuchi, T.; Kawafuchi, H.; Nokami, J. Chem. Commun. 2005, 537-539.
(10) O-AcetylTEMPO (1) shows CdO absorption at 1759 cm-1 in IR
spectra, which is distinguishable from that of Weinreb amide at 1668 cm-1
and ethyl acetate at 1741 cm-1
.
(11) He, M.; Dowd, P. J. Am. Chem. Soc. 1998, 120, 1133-1137.
10.1021/jo0617316 CCC: $37.00 © 2007 American Chemical Society
Published on Web 01/25/2007
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J. Org. Chem. 2007, 72, 1472-1475

TABLE 1. Alkylation, Michael Addition, and Sulfenylation of
SCHEME 2. Reaction of O-AcylTEMPOs with CS2 and Mel
O-PropionylTEMPO with LDAa
b
entry
electrophile (E-X)
conditions
product
%
1
2
3
4
5
6
7
Mel
PhCH2Br
allyl-Br
t-BuO2CH2Br
PhCHdCHNO2
(EtO2CNd)2
PhSSPh
-78 to -10 °C/3.5 h
-78 to -25 °C/4 h
-70 to -20 °C/2 h
-70 to 0 °C/8 h
-70 to -7 °C/2.5 h
-78 to -15 °C/4.5 h
-70 to +6 °C/3.5 h
3a
3b
3c
3d
3ed
3ff
3gg
74c
87c
83c
64
83e
65
69
SCHEME 3. Aldol Reactions of O-AcylTEMPOs with
a Carried out by reaction of 2a (1 mmol) with LDA (1.5-2 equiv) at
-78 °C for 45 min followed by addition of the electrophiles (2 equiv).
b On the basis of isolated products. c Contaminated with 1-3% of 2a by
1H NMR analyses. d E ) PhCHsCH2NO2. e A 1:1 diastereomeric mixture.
f E ) EtO2CNHsN(CO2Et). g E ) PhS.
Various Aldehydes and Ketones
SCHEME 1. Possible Anionic Species from O-AcylTEMPOs
and Their Reactions
hetero-hetero atoms bonds. Thus, 1,4-addition of this anionic
species to nitroolefin,13 (E)-PhCHdCHNO2, smoothly pro-
ceeded, giving the corresponding adducts 3e as a 1:1 diaster-
eomeric mixture in 83% yield (entry 5). However, our efforts
for the 1,4-addition to tert-butyl acrylate and acrylonitrile by a
similar treatment as described above were unsuccessful.
Introduction of hetero atoms such as nitrogen and sulfur at
the C2 carbon of 2a was also achieved by treatment of the
anionic species with diethyl azodicarboxylate (DEAD)14 and
diphenyl disulfide ((PhS)2),15 giving the corresponding 3f (E
) EtO2CNH-N(CO2Et)) and 3g (E ) PhS) in 65% and 69%
yields, respectively (entries 6 and 7).
The reaction of ester enolate with carbon disulfide (CS2) is
known to undergo either C- or O-alkylation depending on the
carbon-skeleton of the C2 position,16 that is, a methylene or
methine structure. Accordingly, anionic species from O-acylTEM-
POs 1 and 2 were submitted to the reaction with CS2 followed
by methylation with MeI, giving the desired 2-bis(methylthio)-
methylenealkanoylTEMPOs 4 and 5 in moderate to good yields.
In this case, a trace amount of the O-alkylation, that is 6c,
besides the C-alkylated 5c as a major product was found in the
case of methoxyacetylTEMPO 2c (Scheme 2).
Subsequently, we examined aldol reactions of O-acetylTEM-
PO (1) with various aldehydes 7 and ketones 9 using LDA as
a base. As shown in Scheme 3, it turned out that the deproton-
ated species from 1 reacted smoothly with common aldehydes
and ketone such as acetaldehyde (7a), anisaldehyde (7b), and
cylohexanone (9a), giving the corresponding aldols 8a and 8b
in 90% and 97% yields and 10a in 88% yield, respectively. The
reaction of 1 with sterically hindered pinacolone (9b) was slight-
ly sluggish, but a fairly good yield (67% yield, 85% conversion)
of the adduct 10b was obtained by raising the temperature to
between -40 and -35 °C for 2 h. On the other hand, our
attempts for the aldol reaction of O-isobutyrylTEMPO (3a), the
2,2-dimethylated analogue of 1, with 7a and 7b were unsuc-
cessful, resulting in recovery of 3a. Thus, although no significant
steric hindrance of the TEMPO group was observed in aldol
reaction of the anionic species from O-acetylTEMPO (1), the
disubstituents at the C2 carbon, that is 3a, exerted depression
on their reactivity in aldol and related alkylation processes.
We next examined the chemoselective aldol reaction of 1
with various functionalized carbonyl compounds 9c-e, since
the inertness of the O-acylTEMPO moiety toward LiAlH4 at
low temperature and to Grignard reagents would allow further
elaboration of the aldol products into functionally diverse
derivatives with chemo- and regioselectivity.9b Thus, the aldol
reaction of 1 with 2-cyclohexenone (9c) afforded the 1,2-adduct
10c in 79% yield with no 1,4-adduct being detected. Diacid
alkyl/TEMPO-1-yl esters 10d and 10e were obtained by the
aldol reaction of 1 with keto esters such as pyruvate 9d and
levulinate 9e, respectively (Scheme 4).
(12) (a) Otera, J.; Fujita, Y.; Sato, T.; Nozaki, H.; Fukuzumi, S.; Fujita,
M. J. Org. Chem. 1992, 57, 5054-5055. (b) Seebach, D.; Amstutz, R.;
Laube, T.; Schweizer, W. B.; Duniz, J. D. J. Am. Chem. Soc. 1985, 107,
5403-5409.
(13) Lee, V. J. In ComprehensiVe Organic Synthesis; Trost, B. M.,
Fleming, I., Eds.; Pergamon Press: Oxford, 1991; Vol. 4, pp 169-137.
(14) (a) Stoner, E. J. Encyclopedia of Reagents for Organic Synthesis;
John Wiley & Sons: New York, 1995; Vol. 3, pp 1790-1793. (b) Williams,
R. M. Synthesis of Optically ActiVe R-Amino Acids; Pergamon Press:
Oxford, 1989; pp 167-176.
(15) Grieco, P. A.; Reap, J. J. Tetrahedron Lett. 1974, 15, 1097-
1100.
(16) (a) Ali, S. M.; Tanimoto, S. Chem. Commun, 1989, 684-685.
(b) Ali, S. M.; Tanimoto, S. J. Org. Chem. 1989, 54, 5603-5607.
J. Org. Chem, Vol. 72, No. 4, 2007 1473

TABLE 2. Anti/Syn Selectivity of Aldol Reactions of
SCHEME 4. Chemoselective Aldol Reactions of
O-AcetylTEMPO (1) with Various Ketones
O-AcylTEMPOsa
entry
2a,c Y
RCHO 7
product, %b
anti/sync
1
2
3
4
5
6
Me
Me
Me
Me
Me
MeO
c, Me2CHCHO
a, MeCHO
d, (E)-MeCHdCHCHO
e, PhCHO
b, 4-MeOC6H4CHO
b, 4-MeOC6H4CHO
20c, 91
20a, 86
20d, 82
20e, 92
20b, 95
21b, 41
9/1(10/1)
2.5/1(2/1)
2.3/1
5.4/1(3/1)
7/1
1/3
a Carried out using YCH2COT (2a, Y ) Me; 2c, Y ) MeO, 2 mmol),
RCHO (7, 2.0-2.5 equiv), and LDA (1.5 equiv) in THF at -78 °C for 15
min: 20, Y ) Me; 21, Y ) MeO, T ) TEMPO. b Yields are based on
SCHEME 5. Competitive Aldol Reactions of 1 vs 11 and 1
vs 14 with LDA as a Base
1
isolated products. c Determined by H NMR. Numbers in parentheses are
of data of alkoxyalkyl propionate taken from ref 20a.
SCHEME 6. Regioselective Aldol Reactions of Glutaric
Derivative Bearing O-AcylTEMPO and Weireb Amide
In order to estimate the relative reactivity of the O-
acylTEMPOs in comparison with usual alkyl esters and amides,
competitive aldol reactions of 1 vs tert-butyl acetate (11) or 1
vs Weinreb amide 14 were attempted (Scheme 5). Thus, a 1:1
mixture of 1 and 11 was treated with 1 equiv of LDA at
-78 °C for 1.5 h, and then the resulting reaction mixture was
allowed to react with an excess amount of either anisaldehyde
(7b) or cyclohexanone (9a). In each case, the products were a
ca. 1:1 mixture of the corresponding aldol adducts, 8b + 12b,
and 10a + 13a, as well as the starting carboxylic derivatives 1
and 11. Thus, these results tell that the acidities of 1 and 11 are
similar. Furthermore, similar results were also obtained in a
competitive aldol reaction of 1 vs 14. Thus, O-acetylTEMPO
(1) can be used as a viable alternative of carboxylic deriv-
atives in terms of its comparable nucleophilicity on the C2
carbon.
Subsequently, the competitive aldol reaction was extended
to diacid derivatives comprised of O-acylTEMPO and the
Weinreb amide to compare the reactivity of the anionic species
from the viewpoint of its regioselectivity.17 Thus, treatment of
glutaric derivative 17 with LDA (2.5 equiv) at -70 °C for 3 h
followed with anisaldehyde (7b) at -70 to -30 °C for 3 h
afforded the aldol product 18b as a major product18 along with
several byproducts (Scheme 6).19 Similar regioselectivity was
found in the reaction of 17 with 7c and 9a, producing 18c and
19a, respectively. Contrary to our expectation based on the
acidity of the R-methylene of O-acylTEMPO compared with
that of Weinreb amide, the aldol reaction occurred at the C2
position of the Weinreb amide predominantly. This can be best
explained by formation of a dianion species, generated on both
R positions of the two carbonyl groups of 17 due to excess LDA
followed by a fast addition reaction at the enolate of the amide,
since the enolate of the Weinreb amide is considered to be more
reactive than the anionic species available from the O-al-
kanoylTEMPO moiety.
We next examined the aldol reaction of O-propionylTEMPO
(2a) or 2-hetero-substituted O-acetylTEMPO 2c with various
aldehydes 7 in order to explore the effect of the TEMPO group
toward anti/syn selectivity.20 As shown in Table 2, aldol
reactions of 2a proceeded smoothly to afford the corresponding
adducts 20 as an anti/syn diastereomeric mixture in good yields.
1
The anti/syn ratio was determined by H NMR; the anti-aldol
20b, for example, showed a doublet at 4.74 ppm (J ) 8.2 Hz)
due to the â-CH, while the syn-isomer of 20b was at 5.05 ppm
(J ) 4.7 Hz). Almost the same or higher selectivities compared
with that obtained from other alkyl esters bearing an internal
chelating group were attained with O-acylTEMPO 2a with
aliphatic aldehydes (entries 1-3).20a Similar anti selectivity was
also obtained by the reaction with aromatic aldehydes 7e and
7b (entries 4 and 5). Aldol reaction of methoxyacetylTEMPO
(17) According to absorptions due to CO groups in IR spectra of 17, the
R-proton of O-acylTEMPO (1762 cm-1) is considered to be more acidic
than that of the Weinreb amide (1668 cm-1).
(18) Compound 18b was confirmed by transformation to the correspond-
ing enal ii by the successive treatment with LiAlH4, giving i, followed with
MsCl and DBU.
(19) Decomposition of Weinreb amide with a strong base was found in
the reaction products: Graham, S. L.; Scholz, T. H. Tetrahedron Lett. 1990,
31, 6269-6272.
(20) (a) Meyers, A. I.; Reider, P. J. J. Am. Chem. Soc. 1979, 101, 2501-
2502. (b) Pirrung, M. C.; Heathcock, C. H. J. Org. Chem. 1980, 45, 1727-
1728. (c) Evans, D. A.; McGee, L. R. Tetrahedron Lett. 1980, 21, 3975-
3978.
1474 J. Org. Chem., Vol. 72, No. 4, 2007

SCHEME 7. Aldol Reactions of CrotonoylTEMPOs
reactions of the resulting anionic species lead to the three-
component combined products smoothly.
Experimental Section
General Procedure for Aldol Reaction of O-AcylTEMPOs
Using LDA. To an LDA solution, prepared from BuLi (1.58 N in
hexane, 1.9 mL) and (Me2CH)2NH (0.42 mL, 3.0 mmol) in THF
(5 mL), cooled to -78 °C, was added dropwise over 5 min a
solution of O-propionylTEMPO (2a, 426 mg, 2.0 mmol) in THF
(3 mL). After stirring at -78 °C for 5 min, was added a solution
of acetaldehyde (7a, R ) Me, 370 mg, 8.4 mmol) in THF (2 mL).
The mixture was stirred at the same temperature for 15 min, the
reaction was quenched with cold aqueous NH4Cl, and the products
were extracted with AcOEt. The extracts were washed with brine,
dried (MgSO4), and concentrated. The products were analyzed by
1H NMR and purified by LC (SiO2, hexane-AcOEt 5:1, by
increasing the gradient from 10:1 to 1:2) to give 440 mg (86%) of
the aldol 20a as a 2.5:1 diastereomeric mixture. An analytical
sample of the anti isomer (contaminated with a small amount of
the syn isomer) was obtained by recrystallization from hexane-
AcOEt. anti- 20a (Rf ) 0.50, hexane-AcOEt 2:1): mp 105-106
°C (from hexane-AcOEt); IR (KBr) 3419, 3012, 2941, 1741, 1456,
1383, 1367, 1257, 1163, 1115, 1099, 999, 905 cm-1; 1H NMR (500
MHz) δ 1.07 (s, 6H), 1.16, 1.17 (s, 6H), 1.25 (d, J ) 6.4 Hz, 3H),
1.28 (d, J ) 7.3 Hz, 3H), 1.38-1.44 (m, 1H), 1.50-1.75 (m, 5H),
2.50 (quin, J ) 7.3 Hz, 1H), 2.82 (brs, 1H), 3.90 (m, 1H); 13C
NMR (75.5 MHz) δ 14.8, 16.9, 20.51 (2C), 20.75, 31.79 and 31.91,
38.95 and 39.07, 46.1, 59.96 and 60.19, 69.3, 175.6. HRMS (EI)
calcd for C14H27NO3 257.1991, found 257.2025.
2c with 7b proceeded smoothly to give the corresponding
adducts 21b with a reverse anti/syn ratio of 1:3 (entry 6).
In the course of our examination on the γ-alkylation or
R-alkylation with the vinylogous anionic species accessible from
O-2-alkenylTEMPOs,21 we encountered facile Michael addition
of LDA,22 employed as a base, to the crontonyl derivative 22
and subsequent aldol reaction of the resulting anionic species
with aldehydes 7a and 7b, giving the corresponding adducts
23a (R ) Me) in 82% yield and 23b (R ) 4-MeOC6H4) as a
separable 3:1 diastereomeric mixture in 71% yield (Scheme 7).
On the other hand, treatment of 24 with bulky lithium amide
from 2,2,6,6-tetramethylpiperidine (TMP) as a base for depro-
tonation followed by reaction with 7a afforded the R-addition
products 25a as a ca. 4:1 diastereomeric mixture in 37% yield
(Scheme 7). In this case, no γ-addition was found.
In summary, O-alkanoylTEMPOs are easily deprotonated
with LDA at -78 °C to generate the corresponding lithium
anionic species, which are a potent nucleophiles for carbon-
carbon bond-making reactions such as alkylations, Michael
additions, aldol reactions, etc. Fairly good anti or syn selectivities
were attained in aldol reactions of O-propionoyl derivatives
owing to the TEMPO moiety as an auxiliary, whose selectivities
are comparable with those reported.20 CrotonoylTEMPOs, more
electron deficient than usual alkyl crotonates, are prone to
undergo 1,4-addition of lithium amide, and the subsequent aldol
Acknowledgment. We appreciate the support by Okayama
University (T.I.), the Okayama Foundation for Science and
Technology (T.I.), the Wesco Scientific Promotion Foundation
(T.I.), and a Grant-in-Aid for Scientific Research from the Japan
Society for the Promotion of Science (H.K., No. 16550104) for
this work. We are grateful to the SC-NMR laboratory of
Okayama University for high-field NMR experiments. We are
indebted to Professor Junzo Nokami, Okayama University of
Science, for HRMS analyses and thankful to Koei Chemical
Co., Ltd. for a research sample of TEMPO•.
Supporting Information Available: Spectral data including IR,
1H NMR, and 13C NMR spectra of 3a-g, 4, 5a-c, 8a,b, 10a-e,
and 17 and starting material of 17, 18b,c, 19a, 20a-d, 21b, 23a,b,
and 25a. This material is available free of charge via the Internet
at http://pubs.acs.org.
(21) Saito, S.; Shiozawa, M.; Yamamoto, H. Angew. Chem., Int. Ed. 1999,
38, 1769-1771.
(22) (a) Rathke, M. W.; Sullivan, D. Tetrahedron Lett. 1972, 13, 4249-
4252. (b) Herrmann, J. L.; Kieczykowski, G. R.; Schlessinger, R. H.
Tetrahedron Lett. 1973, 14, 2433-2436. (c) Hase, T. A.; Kukkola, Synth.
Commun. 1980, 10, 451-455. (d) Little, R. D.; Dawson, J. R. Tetrahedron
Lett. 1980, 21, 2609-2612.
JO0617316
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