University of Groningen
Asymmetric Diels-Alder reactions with 5-menthyloxy-2(5H)-furanones
Jong, Johannes Cornelis de
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Jong, J. C. D. (2006). Asymmetric Diels-Alder reactions with 5-menthyloxy-2(5H)-furanones s.n.
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CHAPTER I
ASYMMETRIC DIELS-ALDER REACTIONS
Since its discovery in 1928 the Diels-Alder reaction has become one of the most
powerful tools in organic synthesis.' Together with the Robinson annulationZit has
become the standard methodology to generate six membered carbon ring systems. To
a large extent the great importance of this reaction derives from its very broad scope
and the ability to generate two carbon-carbon a-bonds and up to four contiguous
stereogenic centers in one synthetic operation. In an asymmetric cycloaddition
reaction there are at least four elements which govern the stereochemical course of
the reaction. These elements are: ck-addition, endo addition, and diastereoselectivities
of both the chiral diene and dienophile. There are several possibilities to achieve
T-
face differentiation in diastereoselective and enantioselective Diels-Alder reactions.
These include the (temporary) attachment of a chiral auxiliary to the diene or
dienophile or, more elegantly, the employment of a chiral catalyst. The chiral directing
group should meet a number of riter ria:^
1) provide a wide range of Diels-Alder adducts in high chemical yield with
virtually complete and predictable T-face stereodifferentiation.
2) both enantiomers or alternative topological counterparts should be readily
available.
3) be capable of efficient attachment and nondestructive removal from the
adduct with complete retention of the induced configuration.
4) permit facile purification of the major cycloadduct to almost 100% d.e.
5) impose crystallinity on intermediates and products.
In the following sections a review concerning asymmetric Diels-Alder reactions with
chiral dienophiles, chiral dienes and chiral Lewis acids will be given. No efforts will be
made to give a complete review since the main purpose of this chapter is to
demonstrate the wide variety of these reactions.
1.2 Chiral dienophiles
The asymmetric Diels-Alder chemistry4 started with the pioneering work of Korolev
and ~ u r : and Walborsky et
butadiene,)2.l(
(u)
with
followed by hydrolysis to remove the chiral auxiliary, led to (-)-4-
cyclohexene-1,2-dicarboxylicacid
go$0"i
Cyclization of (-)-dimenthy1 fumarate
(1.3)with an enantiomeric excess of ~ . 4 % . ~
c
0
1)
2) benzene,
4%
28hethanolic
67'C, KOH
6h
@OH
.."'If
On
0
1.1
-
1.2
-
1.3 5.4% e.e
-
Scheme 1.1
For the Diels-Alder reaction depicted in Scheme 1.1 it was reported that the absolute
configuration of the adducts was independent of the solvent. On the other hand Sauer
and Krede18 observed that in the reaction of acrylate esters
1.4with
cyclopentadiene
the absolute configuration of the preferred diastereoisomer changed by modifying the
1.4 react with cyclopentadiene to give, after reduction with
solvent. Chiral acrylates
LiAlH,, in high yield the chiral endo and exo alcohols 1.5 and
u. However,
the
diastereoselectivities obtained in these cycloaddition reactions are low.
H,C-CH-COOR*
1) cyclopentadiene
CH2C12. 35'C
w
2) LiAlH,
Scheme 1.2
The diastereoface differentiation in the uncatalyzed thermal Diels-Alder reaction was
considerably improved by Helmchen and Schmiererg using chiral methyl fumarates.
The chiral auxiliaries are readily accessible in enantiomerically pure form starting
from (+)-camphor. The presence of the N-phenylcarbamoyl group results in an
effective shielding of the fumarateand a high diastereoface differentiation.''
example, the reaction of
1.7 with
anthracene
For
(1.8)leads to the cycloaddition product
with 60% diastereomeric excess. The chiral auxiliary was removed by means of
reduction with LiAlH, to afford 11S,12S diol 1.R
1) toluene, llO'C,
2) LiAlH,
5%
0
Scheme 1.3
Tolbert and Ali studied the cooperativity in asymmetric induction of the uncatalyzed
Diels-Alder reaction. The enantiomeric ratio produced with a dienophile containing
two independent chiral moieties, e.g. dibornyl fumarate, can be predicted on the basis
of the asymmetric induction when a single chiral moiety is available."
When the
diene has two-fold symmetry the bischiral ratio is the square of the monochiral ratio.
1.10
-
Figure 1.1
Acrylate ester
1.10. prepared
from (-)-8-phenylmenth01~~*~~
has proven to be
superior in the chiral directing capacity compared to the corresponding menthyl ester.
The higher stereocontrol in this case has been attributed to the shielding of the GYre
face of the acrylate dienophile moiety by the phenyl ring of the auxiliary.
Consequently, the diene addition is more facile to the si face of the acrylate. Later,
several other auxiliary alcohols, which take advantage of the shielding effect of the
phenyl ring, were designed.14
Excellent diastereofacial selectivity (>100:1) was also achieved by Masamune
et ~ 1 . ' ~in~ the
' ~ uncatalyzed addition reaction of
1.11 and
cyclopentadiene. In this
case the high selection is attributed to the strong hydrogen bonding between the
hydroxyl and ketone functions in
1.11 which
results in the formation of a five-
membered chelate. Due to the diminished flexibility the two diastereotopic faces of
the enone system become highly distinguishable.
Figure 1.2
Very early it was found that Lewis acids exert a strong catalytic effect in Diels-Alder
reactions?' The acceleration of the [4+2] cycloadditions results from a lowering in
energy of the dienophile LUMO?' In general there is also an increase in stereoselectiviq and an enhancement of the regio~electivity.'~
Walborsky et aL6 observed that the addition of Lewis acids to the reaction mixture of
1.1 and butadiene)2.l(
-
(Scheme 1.1) raised the diastereoselectivity to 27-78%
depending on the reaction conditions. By complexation of the Lewis acid with the
carbonyl oxygen of the dienophile the s-trans conformation becomes the energetically
most favourable one. Lewis acid complexation results in an increased 7-bond order of
the enone single bond and thereby a more rigid complex is formed with less
conformational freedom. When diester 1.1 is allowed to react with anthracene, under
Lewis acid catalysis, complete asymmetric induction can be achieved. Also, in the
reaction of dienophile
1.7 with
anthracene
(128) (see
Scheme 1.3) the results were
considerably improved by the addition of Lewis acid. Complete diastereoselectivity
(d.e.>99%, 100% yield) was obtained with 2 equivalents of AlCl, at -30 "C in CH,Cl,.
Another interesting example is the Lewis acid catalyzed asymmetric Diels-Alder
reaction between acrylamides 1.12. derived from (2R,5R)-2,s-disubstituted pyrrolidines and cyclopentadiene
(m).The cycloadduct 1.14 was obtained with excellent
diastereoface ~electivity.~'With AIC1, as the Lewis acid endo-1.14 was prepared with
98% d.e. (endolexo 955).
a+o
t762
1.2eq
o l u e n Ae l, C O'C,
1,
*-.,
R'
Scheme 1.4
lh
Because of these outstanding results several of the Lewis acid catalyzed Diels-Alder
reactions have been explored in the synthesis of natural products or analogues.211"
The asymmetric Diels-Alder reactions with chiral butenolides are described in Chapter
11.
13 Chiral dienes
An alternative approach to face selectivity in Diels-Alder reactions is the introduction
of a stereogenic center within the diene or the attachment of a chiral auxiliary to the
diene. Some pertinent examples of this class of asymmetric Diels-Alder reactions are
reported in the following section. The first reports concerning chiral dienes were from
David et ~ 1 . ' ~The addition of n-butyl glyoxylate to carbohydrate derivative
1.15
afforded all four possible adducts 1.16a-1.16d. However, after the cycloaddition the
number of dihydropyrans can be reduced to two (1.16aand 1.16b)by epimerization of
the created acetal center under anhydrous acidic conditions. The pure cycloadducts
have been converted into optically active disaccharides." To obtain more insight in
the steric course of these reactions, compounds 1.17-1.19 were also prepared and
similarly subjected to ~ ~ c l o a d d i t i o nIn. ~each
~ ~ ~ ~case a strong preference was
obsewed for one of the P-faces of the butadienyl ether, but no great preference for
the endo addition was observed. A major drawback of the synthesis described above is
that the corresponding ck-dienes are inert towards glyoxylic esters. The more active
dienophile diethyl mesoxalate reacts with the cis and trans isomer of 1.15."
%0-
BnO-
BnO
BnO
OBn
OBn
Figure 1.3
OBn
OBn
Water soluble tram-butadienyl ethers were synthesized by using free glucose as the
hydrophilic part.28 With dienophiles like acrylaldehyde, methyl acrylate and
methacroleine complete endo selectivity was obtained. Fraser-Reid et a1.29330
developed another class of carbohydrate derived chiral dienes, in which the
carbohydrate is not only a chiral auxiliary, but also forms a part of the diene itself.
The a-face of compound 1.20 is rendered completely inaccessible by the 0isopropylidene ring and therefore the addition of a dienophile will be from the 8 face.
Reaction with maleic anhydride
(m)afforded the endo adduct 1.22 as the single
enantiomer. Diels-Alder reactions of analogous, six membered dienopyrano-sides 1.23
(R
=
H, OCH,), were investigated by Giuliano and B ~ z b ~ . ~ '
Figure 1.4
Reaction of
1.23 (R
= H) with maleic anhydride
(m)or maleimide gave a 1:l mix-
ture of two products. However, treatment of diene (R = OMe)
1.23 with maleimide
gave a single crystalline product which resulted from the endo addition of maleimide
to the 8-face of
1.23 (R
= OMe). Dienes of type
1.24. derived from glucose, were
'
anhydride and dimethyl acetylenedicarboxylate
examined by Lipshutz et ~ 1 . ~Maleic
each afforded a single cycloadduct, with complete endo selectivity. Also in the case of
the diazodienes diethyl azodicarboxylate and N-phenyl-1,2,4-triazoline-3,5-dioneonly
one product was formed in good yield (80% and 90%, respectively). In the group of
S e e b a ~ hdiene
~~
1.25 was
developed, which can be obtained from Lserine via a
multistep synthesis. The t-butyl substituent shields very effectively one side of this
compound. Reaction with maleic anhydride
with 95% d.e.
1.21 afforded
the exo-cycloadduct
1.26
1.25
1.21
Figure 1.5
Trost et
have observed that the boron trifluoride catalyzed addition of
1.27 to
acrolein gave 80% (3R,4S,6R)-U and 20% of the (3S,4R,6S)-isomer. The related diene 1.29 reacted with juglone to provide 1.30with complete asymmetric ind~ction.~'
CHC13, O ' C
98%
Scheme 1.5
The absolute configuration of the products has been rationalized in terms of a astacking model. The aromatic ring sewes as a steric steering group which directs the
incoming dienophile to one of the enantiotopic faces of the diene. An alternative
approach to T-face selectivity in Diels-Alder reactions is to incorporate a stereogenic
center within the diene at the allylic position (1.31-1.33).~~,~'.~~,~~
Two examples of conformational locked 1(E)-substituted 1,3-dienes containing a
stereogenic center at the allylic position are the compounds
cycloaddition reaction of
1.31 and 1.32. In
1.31with N-phenylmaleimide very high
the
diastereoselectivities
were obtained (upto 100%). These high selectivities were also obtained in the reaction
of N-phenylmaleimide and compound
functionality as directing group.
1.32. which
possesses a chiral sulfoxide
Complete P-face selectivity was attained in the Diels-Alder reaction of
1.33 with
N-phenylmaleimide to produce the cycloadduct 1.34 as the exclusive diastereoisomer.
The high stereoselectivity and regioselectivity of the intermolecular Diels-Alder
reaction may be attributed to the presence of the Z-alkoxy substituent, which forces
the reaction to proceed from the conformer with the Me-group perpendicular to the
plane of the P-system and the large trimethylsilyloxy group pointing away from the
MOM protecting group. This conformation results in the effective shielding of one of
the P-faces of the diene.*
1.33
Figure 1.6
1.4 C h i d Lewis acids
As has been described in Section 1.2, Lewis acids are able to promote the Diels-Alder
reaction4' by lowering the LUMO of the d i e n ~ p h i l eand
~ ~ to increase the regioselectivity. During the last 5 years there has been a growing interest in the design of
chiral Lewis acids and their application in the asymmetric Diels-Alder reaction.43 A
complexation of a chiral Lewis acid with one of the reaction components would yield
an asymmetric reagent with a potential for guiding a selective approach of the
remaining cycloaddition component. The advantage of using these chiral catalysts is
that the processes for the introduction and removal of the chiral auxiliary (either of
the diene or dienophile) can be omitted and that stoichiometric amounts of the chiral
auxiliary would no longer be necessary. The chiral Lewis acids can roughly be divided
in three main groups: Lewis acids based on aluminum, titanium, and boron. Their
achiral equivalents have shown to be excellent catalysts in Diels-Alder reactions with a
high regio- and stereo~electivit~?*~
1.4.1 Chiral Lewk acids based on aluminum
The first asymmetric Diels-Alder reaction catalyzed by chiral Lewis acids and with
considerable asymmetric induction (72% 0.p.) was reported by Koga et ~ 1 . ~The
' best
(1.35) and
(1.13)with menthyloxyaluminum dichloride as the chiral catalyst.
results were obtained in the reaction of methacrolein
II
1.13
-
1.35
-
cyclopentadiene
0.15 equiv.
69%
1.36 72%
-
0.p.
Scheme 1.6
The application of this type of chiral Lewis acids, with only one coordination to the
dienophile, has been very limited. With methyl acrylate the endo adduct of cyclopentadiene is obtained with very low enantiomeric excesses (6-9% e.e.). It was
postulated that the low chiral induction in the case of these conformationally mobile
catalysts was due to the possible rotation around the two Al-0 bonds in the Lewis
acid dienophile complex (Figure 1.7) and around the 0-menthyl bond.46 In any
reasonable conformation of the complexed s-trans dienophile the chiral moiety is
probably at too a great distance to effect a significant induction.
Figure 1.7
Much higher selectivities were obtained by Chapuis and ~urczak~'
with the bidentate
ligands 1.37 (R' = Me) and 1.39 (R' = Me). The advantage of these compounds over
e.g. methyl acrylate is their potential rigidity. Chelation of a Lewis acid would prevent
the rotation around the C(0)-N bond. The reaction of
1.39 (R
=
H,R'
= H) with
cyclopentadiene (1.13)was further improved by Corey et aL4' In the presence of 10
mol% of (S,S)-1.41 as a catalyst, 1.40 was obtained in 95% e.e. (endolexo > 50:l).
For 1.37 (R = Me) adduct 1.38was obtained in 96 % e.e.
CF,S02N,fiNS02CF,
CH,
1.41
10 no14
b
Scheme 1.7
1.4.2 Chiral Lewis acids based on titanium
In 1986 Narasaka reported a highly enantioselective Diels-Alder reaction between
.~~
the reaction
achiral dienes and dienophiles using a chiral titanium (IV) a l k ~ x i d e In
of cyclopentadiene
(1.13)with
(u),
catalyzed by 1
crotonoyl-1,3-oxazolidin-2-one
and 2 equivalents, respectively, of
1.44.the
endo adduct
1.43 was obtained in
75%
and 92% e.e.
Ph P h
P ~ ~ O ~ % O.Cl,
",c
He
u
0
Xo.Ti-Cl
Ph P h
2 equiv.
o e
93%
+
1.44
-
e 1
0
1.42
1.13
-
0
1.43
endo/exo 90 : 10
Scheme 1.8
Later it was found that these high e.e.'s could also be obtained by the use of a
catalytic amount of the chiral titanium (IV) complex 1
.44.'' In the presence of 4A
molecular sieves and with 10 mol% of
1.44. adduct 1.43 was
obtained in 91% e.e.
(endolexo 92:8). Also the solvent has a remarkable effect on the enantioselectivity of
this reaction. The optically purity of the adduct was greatly increased in the order of
benzene, toluene, xylenes and mesitylenes. The additional alkyl groups of the benzene
ring of the solent prevent ?r-stacking and therefore the interaction between the solvent
and the Ti-complex with the dienophile are minimized. The same increase in optical
purity could be effected by the addition of an apolar, non-coordinating solvent. When
the Diels-Alder reaction of
1.42 with
isoprene was performed in toluene the e.e. is
60%, whereas in toluene-petroleum ether (1:l) the e.e. increased to 94%'0.5lsS2
Chapuis and Jurczak4' also investigated chiral Ti(1V) catalysis using 3-crotonoyl- (1.37)
and 3-acryloyl-4,4-dimethyl-1,3-oxazolidin-2-one
(1.39).The best results were obtained
with 1.37, TiCI, and chiral ligand 1.45 in a 1:l:l ratio. With cyclopentadiene the addition product
1.38was obtained in 99% yield (endolexo
94:6) with an e.e. of >98%.
1.45
-
Figure 1.8
1.4.3 Chiral Lewk acids based on boron
Chiral Lewis acids based on boron were examined by Kelly et
induction was found in the reaction of juglone
diene
(w),which
(1.46)and
High asymmetric
l-methoxy-1,3-cyclohexa-
was catalyzed by a chiral Lewis acid prepared in sinr from
borohydride and (S)-3,3'-diphenyl-1,l'-bis-&-naphthol
(m).T o obtain the high e.e.
(>98%), 2 equivalents of this chiral Lewis acid are needed.
1.
-
Ph
BH,;
-78'C.
HOAc
< 2 min
b
70-905
1.46
-
1.47
-
Scheme 1.9
Also the chiral Lewis acids based on derivatives of tartaric acid show very high
e.e.'~?~," In the reaction of methacrolein
(1.35) with
cyclopentadiene
(1.13) (see
Scheme 1.10) the exo aldehyde 1.36was formed in 96% e.e. (endolexo 10:90).
SH
Hmch/cwH
1.50
+
CH2C12, O'C
BH,,.THF
b
O'fO
1.51
-
+
Q
10 -1%
Bh.
CH2C12, -78 C
85%
CHO
1.35
-
BL~*
1.13
-
Scheme 1.10
1.4.4 Miscellaneous
The iron (111) complex of the bis(dihydrooxazolyl)propane56
Alder reaction of cyclopentadiene
1.52 catalyzes the
Diels-
(m)with the bidentate dienophile 3-acryloyl-1,3-
oxazolidin-Zone (1.39).Corey et aL5' reported that treatment of 1.52 with FeCl, and
iodine resulted in formation of an active complex, presumably [m-FeCl,I], which
catalyzes the formation of the endo cycloaddition product with high selectivity. The
use of 10 mol% of the complex at -50 "C results in an e.e. of 86% (endolexo 99:l).
Scheme 1.11
The hetero Diels-Alder reaction between Danishefsky's diene
(1.54) is
(1.53)and benzaldehyde
catalyzed by a cationic ruthenium complex, CpRu(Ph,P),(CH,=CH,)PF,.
When the phosphorous ligand is replaced by chiral diphosphines such as CHIRAPHOS and DIOP, the asymmetric inductions are 25% and 16% r e ~ p e c t i v e l ~ ? ~
Scheme 1.12
1.5 Aims of this study and survey of the contents
At the beginning of the reseach described in this thesis the catalytic asymmetric DielsAlder reaction had scarcely been investigated. No good catalytic processes with high
enantiomeric excess were known at that time. At the same time the Diels-Alder
reactions with chiral dienophiles needed further improvement as the number of
diastereoselective cycloaddition reactions was limited to a few cases only. In most
cases no complete diastereoselectivity was obtained. Besides, most chiral auxiliaries
which were used were rather expensive or had to be synthesized by a multistep
syntheses. The inherent problem of the chiral dienophiles used sofar was their
conformational freedom leading to low selectivities, although the results were
considerably improved by means of Lewis acid complexation. The disadvantage of
Lewis acids is that they also catalyze the polymerization of dienes. Therefore, in most
catalyzed reactions the dienes are limited to the most simple and cheap ones like
butadiene or cyclopentadiene.
The general purposes of our research was: (i) to synthesize a chiral dienophile which
would meet all the criteria described in Section 1.1, i.e. both enantiomers should be
available, the absolute configuration of the Diels-Alder adducts should be predictable
and the starting material should be a crystalline compound; (ii) to synthesize a
number of substituted chiral dienophiles; (iii) to study these dienophiles in the thermal
Diels-Alder reaction. Furthermore these dienophiles should have low conformational
freedom, resulting in a high diastereoselectivity, and no Lewis acid catalysis should be
needed in the cycloaddition reactions with these dienophiles.
- In Chapter 1 a brief review is given of diastereoselective and enantioselective DielsAlder reactions with chiral dienophiles, chiral dienes and Diels-Alder reactions
catalyzed by chiral Lewis acids.
- The synthesis of enantiomerically pure 5-alkoxy-2(5H)-furanones is described in
Chapter 2. The chiral dienophile was modified by introducing substituents to the C,C
double bond of the furanone ring. These new chirsl dienophiles were studied in
thermal Diels-Alder reactions to investigate the T-face selectivity. Furthermore, it was
demonstrated that alkyl substituents on the 3- or Cposition of the furanone ring have
a dramatic effect on the reactivity of these systems in the [4+2] cycloadditions.
-
In Chapter 3 an alternative route for the Diels-Alder products of 4-alkyl-
5-menthyloxy-2(5H)-furanones is described. By alkylation of the products obtained
from the diastereoselective Diels-Alder reaction of 5-menthyloxy-2(5H)-furanon,
enantiomerically pure products with a quaternary stereogenic carbon were obtained in
high yield.
- The Diels-Alder reactions of 5-menthyloxy-2(5H)-furanone with relatively simple
dienes, as described in Chapter 2, were extended to the synthesis of polycyclic
compounds like decalones and hydroindanones. Up to four new stereogenic centers
with absolute stereocontrol and well defined absolute configuration could be
introduced.
- Based on the results obtained in the previous chapter, in Chapter 5 several attempts
are described for the synthesis of warburganal, a natural insect antifeedant. A new
route to an important intermediate for the synthesis of drimane related sesquiterpenes
has been developed.
- In the last chapter the development of an activated 5-menthyloxy-2(5H)-furanone is
described. The reactivity is increased by the introduction of a sulfonyl group at the
4-position of the furanone. Several examples are given which show the increased
reactivity compared to the unsubstituted 5-menthyloxy-2(5H)-furanone described in
Chapter 2. The thesis is concluded with a survey in English and in Dutch.
1.
2.
3.
4.
5.
Diels, 0.;
Alder, K. JUSW Liebigs Ann. Chem. 1928, 460, 98.
For reviews see: a) Jung, M.E. Tetrahedron 1976,32, 1.
b) Mundy, B.P. J. Chem. Ed. 1973,50, 110.
Oppolzer, W. Angew. Chem. Int. Ed. Engl. 1984, 23, 876.
For a review ahout asymmetric Diels-Alder reactions with chiral enoates as
dienophiles see; Helmchen, G.; Karge, R.; Weetman, J. "Modern Synthetic
Methods", Vol. 4; Scheffold, Ed.; Springer-Verlag: Berlin Heidelberg, 1986; p.
261.
Korolev, A.; Mur, V. Dokl. Akad. Nauk SSSR 1948, 59, 251; Chem. Absn:
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Walborsky, H.M.; Barash, L;Davis, T.C. J. Org.Chem. 1961, 26, 4778.
Walborsky, H.M.; Barash, L ; Davis, T.C. Tetrahedron Lett. 1963,19, 2333.
Sauer, J.; Kredel, J. Tetrahedron Lett. 1966, 6359.
Helmchen, G.; Schmierer, R. Angew. Chem. Int. Ed. Engl. 1981,20, 205.
Izumi, Y.; Tai, A. "Stereodifferentiation reactions"; Acadamic Press: New
York, 1977.
Tolbert, LM.; Ali, M.B. J. Am. Chem. Soc. 1984,106, 3806.
Corey, E.J.; Ensley, H.E. J. Am. Chem Soc. 1975, 97, 6908.
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Oppolzer, W.; Kurth, M.; Reichlin, D.; Chapuis, C.; Mohnhaupt, M.; Moffatt,
F. Helv. Chim. Acta 1981, 64, 2802.
Choy, W.; Reed, 111, L.A.; Masamune, S. J. Am. Chem. Soc. 1983,48, 1137.
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4441.
Inukai, T.; Kojima, T. J. J. Org. Chem. 1967,32, 872.
Trong Anh, N.; Seyden-Penne, J. Tetrahedron 1973,29, 3259.
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Kawanami, Y.; Katsuki, T.; Yamaguchi, M. Bull. Chem. Soc. Jpn. 1987, 60,
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For other acrylamides see: a) Waldman, H. J. Org.Chem 1988,53, 6133.
b) Evans, D.A.; Chapman, K.T.; Bisaha, J. J. Am. Chem. Soc. 1988, 110,
1238.
a) Katagiri, N.; Haneda, T.; Hayasaka, E.; Watanabe, N.; Kaneko, C. J. Org.
Chem. 1988,53, 226.
b) Katagiri, N.; Akatsuka, H.; Kaneko, C.; Sera, H. Tetrahedron Lett. 1988,29,
5397.
c) Helmchen, G.; Ihrig, K ; Schindler, H. Tetrahedron 1987, 28, 183.
For a review about Diels-Alder reactions in natural product synthesis see;
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