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Asymmetric induction in the inverse diels-alder reaction of chiral 2-methylene-imidazolidines
Tetrahedron Letters,Vo1.28,No.24,pp Printed in Great Britain
ASYMMETRIC
INDUCTION OF CHIRAL
Ursula Institut
IN THE
inverse
Gruseck
Diels-Alder
which
active
Since
with
have
been
can smoothly
of chiral
2-methylene-imidazolidines
temperatures be cleaved
lead to diastereomeric
by dilute
acids
with N,N-
to optically
by Korolev
and Murla'
asymmetric
Diels-Alder
cyclo-
thoroughly
investigated by many groups. These efforts have in the last years 1) . However, Diels-Alder reactions 2) usually being optidemand were examined, the dienophiles
derivatives
catalystslbfd'
der Universitgt
Miinchen 2
success
electron
active
reactions
Heuschmann*
Chemie
D-8000
at ambient
report
remarkable
normal
cally
REACTION
cyclohexenones.
the first
additions met with
DIELS-ALDER
and Manfred
fiir Organische
2,4_hexadienoates acetals,
INVERSE
2-METHYLENE-IMIDAZOLIDINES
Karlstr.23,
Summary:
oo40-4039/87 $3.00 + .oO Pergamon Journals Ltd.
2681-2684,1987
of acrylic
acidla-d),
in some cases
chiral
dienes lb) or
used. Only one diastereomer was isolated in the inverse 2) Diels-Alder reaction of an optically active enol ether le) , enantioselective inverse hetero-Diels-Alder reactions If) are known. We report here on a rare example
were
of an enantioselective
Diels-Alder
synthesis
with
inverse
electron
demand. Optically undergo room which
active
2-methylene-imidazolid.ines
cycloadditions
temperature slowly
with methyl
or below
rearrange
!. 3, like their
2,4_hexadienoates
to form diastereomeric
to their
tautomers
achiral
z4) in inert
mixtures
analogues
solvents
at
of the N,N-acetals
2,
_5 and the enamines 5, 4 (eq. I).
Me
Me
+
.4
R’;-q;R2
(es.
Me
E = C02Me,
Substituents
RlA4
see Table
2681
1
1)
2682
Treatment affords whole
of this mixture
the optically
sequence
ketene
from
active
cyclohexenone
system
with
5 or enol
dilute
aqueous
1 (eq. 2). Thus,
1 to s/z the 2-methylene-imidazolidines
acids
in the
are used
as chiral
equivalents.
The chiral
amines
treat-lent with
8 can easily
concentrated
can then be converted For
in a two phase
(S)-1~ and
occured
during
from the aqueous
hydroxide
and extraction
to the corresponding
comparison
phase with
by
ether
2-methylene-imidazolidines
of rotations
showed
that no racemization
and 1 6) . had
one cycle.
H+ 3AS
potassium
back
(SS)-11
be isolated
-
H2$4
;TR3 M21; "Q.R~ +
Hz0
+o*
Foe
FoM
s
7
9
ii
&N,n,,
l
(es. 2) &2H+
y
‘0
(9
1”” (R)
i: 10 % Pd/C,
H2; ii: 1) KOH/MeOH,
It was not possible chiral
shift
reagents
palladium
on charcoal
refluxing
aqueous
carbazon known
in Table
the optical
to determine
can be drawn yields
(S)-If,
with
reduction
of $11 with
potassium
hydroxide
and isolation-as
yield')
by comparison
with
The results
compounds.
in
the semithe
are compiled
Thus,
(RR - and
(2E,4E)-
tion with
(SS)-11 and
is stereospecific
is also
(SS)-11
hexadienoates
(R)-lh and
with
of different
each produce
geometry.
different
(2E,42)-hexadienoates
with
regard
signs
2, respec-
to the geometry
of the
2.
stereospecific
enantiomeric
As expected,
results:
control in controlled. With thermodynamic 10) one particular chiral imidazolidine
with methyl
(S)-lq,
the reaction
first
cycloaddition
for the products
2. the cycloaddition
with
regard
to the chiral
2-methylene-imidazolidines
form products
with
opposite
like
dieno-
(R)- and
signs of rotation
(S)-la
in the reac-
(2E,4E)-2.
3. lower
reaction
changing
from +20°C
can not be further -2OOC
pure
from these
I-doub 1e bond of the hexadienoates
or
the optical
are kinetically
the same product
(S)-la,
of rotation
phile.
degradation
to 3-methylcyclohexanone
not concerted
yield
However, tively
But catalytic
Yb(TFC)3).
to 2') and further
h/I with
1.
a reversible, should
(Eu(TFC)3,
2) H2N-CO-NH-NH2;
the e-e.' s of the products
of the enantiomerically
Some conclusions 1.
to determine
methanol
1Q8) allowed
rotations
30 min reflux,
temperatures to -2OOC exploited
for 85% completion
lead to higher increases
as the reaction
in toluene).
e.e.'s.
the e.e.
In the case of
(S)-Ia,
from 7% to 2l%, but this effect
rate becomes
too slow
(two month
at
2683
1: Reaction
Table
conditions
with dienoates
of the cycloadditions
2 and chemical
in parenthesis)
of the hydrolysis
2-Methylene-imidazolidine
of methylene-imidazolidines
and optical
yields
products
methyl
6
solvent
temperature
hexadienoate
(S)-la
R'
R2
R3
R4
Me
Me
iPr
H
1
(e.e.; R/S assignment
yielda)
e.e.
(OC)
(%)
+20
54
7 (S)
+2
45
10 (S)
toluene
-20
49
21 (S)
2E,4E
toluene
+30b)
71
3 (R)
2E,42
benzene
+2
51
16 (R)
2Z,4E
benzene
+2
39
3 (S)
2E,4E
toluene
2E,4E 2E,4E
(%)
(R)-la
Me
Me
iPr
H
2E,4E
toluene
+25
52
8 (R)
(S)-lb
Me
Et
iPr
H
2E,4E
benzene
+25
48
16 (R)
(S)-lc --
Me
cHex
iPr
H
2E,4E
benzene
+2
39
18 (R)
(S)-l'l
Et
Me
iPr
H
2E,4E
+2
35
13 (S)
(S)-le --
Ph
Me
iPr
H
2E,4E
+25
53
38 (S)
(S)-lf
Me
Me
Bz
H
(S)-lg
Ph
Me
Bz
Me
Ph
H
(R)-lh
Ph
(S)-11
Ph
(RR)-!&
Me
Me
(SS)-11
Me
Me
Ph
Ph
(RR)-11
Me
Me
Ph
Ph
(CH2)3
H
H
(CH2)4
c-hexane
2E,4E
toluene
+2
56
11 (S)
2E,4Z
toluene
+2
53
46 (R)
2E,4E
c-hexane
+25
47
44 (S)
2E,42
c-hexane
+25
50
10 (R)
2E,4E
benzene
+25
53
15 (R)
2E,4Z
c-hexane
+25
54
26 (S)
2E,4E
c-hexane
+25
48
27 (R)
2E,4E
c-hexane
+25
39
11 (R)
2E,4E
benzene
+25
51
15 (S)
2E,4Z
benzene
+25
57
19 (R)
2E,4E
benzene
+25
48
14 (S)
a) isolated yields after distillation b) cycloaddition run in a pressure vessel
4. pressure
does
other
Diels-Alder
lower
rotation
high
not have a positive
the opposite
instead
It was of no avail atoms
to achieve
2-methylphenyl achiral
effect
reactions Ig). The product
with
(3000 bar)
at 3000 bar").
better
sign
of atmospheric to introduce
2-methylene-imidazolidines
of
(S)-la and
if the cycloaddition
yields
like
(2E,4E)-2
it did
shows
is performed
a
under
pressure.
bigger
diastereoselection.
or 2,6_dimethylphenyl
on the optical
groups
substituents With
on one of the nitrogen
isopropyl,
on N-l
no cycloaddition
tert-butyl,
in different product
chiral
could
or
be isolated.
in
2684 REFERENCES la) A. Korolev and V. Mur, Dokl. Akad. Nauk SSSR, 59, 251 (1948); C.A. $2, 6776 (1948); b) reviews: P. Welzel, Nachr. Chem. Techn. Lab. ---_I 1983 979; L. A. Paquette in J. D. Morrison: Asymmetric Synthesis, Vol. 3A, Chapt. 4, Academic Press, New York 1984; H. Wurzinger, Kontakte (Darmstadt) _---I 1984 Nr. 2, p. 3; W. Oppolzer, Angew. Chem. 26, 840 (1984), Angew. Chem., Int. Ed. Engl. 22, 876 (1984); J. W. ApSimon and T. L. Collier, Tetrahedron 42, 5157 (1986); c) T. Poll, J. 0. Metter, and G. Helmchen, Angew. Chem. 92, 116 (1985), Angew. Chem., Int. Ed. Engl. 24, 112 (1985); W. Oppolzer, C. Capuis, and G. Bernardinelli, 42, 4035 (1986); J. Mulzer and M. Helv. Chim. Acta 62, 1397 (1984), Tetrahedron Kappert, Tetrahedron Lett. 26, 1631 (1985); H. Takayama, K. Hayashi, and T. Koizumi, ibid. 22, 5509 (1986); K. Furuta, K. Iwanaga, and H. Yamamoto, ibid. 21, 4507 (1986); S. G. Davies and J. C. Walker, J. Chem. Sot., Chem. Commun. 1986, 609; 0. De Lucchi, V. Lucchini, C. Marchioro, G. Valle, and G. Modena, J. Org. Chem. 5l, 1457 (1986); D. A. Evans, K. T. Chapman, and J. Bisaha, J. Am. Chem. Sot. 106, 4261 (1984); W. Ando, Y. Hanyu, and T. Takata, J. Org. Chem. 5l, 2122 (1986); R. S. Glass, K. Reineke, and M. Shanklin, ibid. $9, 1527 (1984); d) T. R. Kelly, A. Whiting, and N. S. Chandrakumar, J. Am. Chem. Sot. IQ@, 3510 (1986); K. Maruoka, M. Sakurai, J. Fujiwara, and H. Yamamoto, Tetrahedron Lett. 21, 4895 (1986); K. Narasaka, M. Inoue, and N. Okada, Chem. Lett. 1986, 1109; K. Narasaka, M. Inoue, and T. Yamada, ibid. 1986, 1967; e) G.H. Posner and D.G. Wettlaufer, J. Chem. Sot., Chem. Commun. _---I 1985 1786, Tetrahedron Lett. 22, 667 (1986), J. Am. Chem. Sot. 108, 7373 (1986); f) L.-F. Tietze, S. Brand, and T. Pfeiffer, Angew. Chem. ?I, 790 (1985), Angew. Chem., Int. Ed. Engl. 24, 784 (1985); g) J. Jurczak, T. Bauer, S. Filipek, M. Tkacz, and K. Zygo, J. Chem. Sot., Chem. Commun. 1983, 540; J. Jurczak, T. Bauer, and S. Jarosz, TetraLett. hedron 42, 5045, 6477 (1986); W. G. Dauben and R. A. Bunce, Tetrahedron 21, 4875 (1982). 2) J. Sauer and R. Sustmann, Angew. Chem. 92, 773 (1980), Angew. Lett. 19_11, 2717. Engl. 19, 779 (1980); R. Sustmann, Tetrahedron 3) M. Heuschmann,
publication
Chem.
Int. Ed.
in preparation.
4) (2E,4E)-2: F. D. Lewis, D. K. Howard, S. V. Barancyk, and D. Oxman, J. Am. Chem. Sot. 1Q8, 3016 (1986); (2Z,4E)-2: U. Eisner, J. A. Elvidge, and R. P. Linstead, J. Chem. Sot. 1953 ----, 1372; (23,42)-z: S. Tsuboi, T. Masuda, H. Makino, and A. Takeda, Tetrahedron Lett. 23, 209 (1982), this product was a mixture containing 10% of (2E,4E)-2. 5) 2-Methylenimidazolidines without C2-axis form additional isomers of 4, where R1/R4 and R2/R3 are exchanged. 2, 4, and 3 were never isolated. The structures and by were assigned on the basis of 'H- and 13C-NMR spectra of the mi$ures comparison with the cycloaddition products of achiral analogues of 1. 6) Treatment of amines 8 with orthoacetate and HBF4 leads to dihydroimidazoliumwhich can be deprotonated to 1 with sodium hydride in THF. tetrafluoroborates, 7) E. J. Eisenbraun, G. H. Adolphen, Chem. 16, 414 (1971).
K. S. Schorno
and R. N. Morris,
J. Org.
8) J. K. Shillington, G. S. Denning, Jr., W. B. Greenough, III, T. Hill, Jr., and 0. B. Ramsay, J. Am. Chem. Sot. @(I, 6551 (1958); N. L. Allinger and C. R. Riew, J. Org. Chem. Ul, 1316 (1975). 9) The stereochemical
assignment
(R,S) is the same for 6, 9, and lg.
10) Diels-Alder cycloadditions of an achiral analogue of 1 with 2 could be shown3) to be not -_- concerted and totally reversible at elevated temperatures. 11) We are indebted (Received
in Germany
to Prof.
Tiltscher,
17 February
1987)
Erlangen,
for his help.
ASYMMETRIC
INDUCTION OF CHIRAL
Ursula Institut
IN THE
inverse
Gruseck
Diels-Alder
which
active
Since
with
have
been
can smoothly
of chiral
2-methylene-imidazolidines
temperatures be cleaved
lead to diastereomeric
by dilute
acids
with N,N-
to optically
by Korolev
and Murla'
asymmetric
Diels-Alder
cyclo-
thoroughly
investigated by many groups. These efforts have in the last years 1) . However, Diels-Alder reactions 2) usually being optidemand were examined, the dienophiles
derivatives
catalystslbfd'
der Universitgt
Miinchen 2
success
electron
active
reactions
Heuschmann*
Chemie
D-8000
at ambient
report
remarkable
normal
cally
REACTION
cyclohexenones.
the first
additions met with
DIELS-ALDER
and Manfred
fiir Organische
2,4_hexadienoates acetals,
INVERSE
2-METHYLENE-IMIDAZOLIDINES
Karlstr.23,
Summary:
oo40-4039/87 $3.00 + .oO Pergamon Journals Ltd.
2681-2684,1987
of acrylic
acidla-d),
in some cases
chiral
dienes lb) or
used. Only one diastereomer was isolated in the inverse 2) Diels-Alder reaction of an optically active enol ether le) , enantioselective inverse hetero-Diels-Alder reactions If) are known. We report here on a rare example
were
of an enantioselective
Diels-Alder
synthesis
with
inverse
electron
demand. Optically undergo room which
active
2-methylene-imidazolid.ines
cycloadditions
temperature slowly
with methyl
or below
rearrange
!. 3, like their
2,4_hexadienoates
to form diastereomeric
to their
tautomers
achiral
z4) in inert
mixtures
analogues
solvents
at
of the N,N-acetals
2,
_5 and the enamines 5, 4 (eq. I).
Me
Me
+
.4
R’;-q;R2
(es.
Me
E = C02Me,
Substituents
RlA4
see Table
2681
1
1)
2682
Treatment affords whole
of this mixture
the optically
sequence
ketene
from
active
cyclohexenone
system
with
5 or enol
dilute
aqueous
1 (eq. 2). Thus,
1 to s/z the 2-methylene-imidazolidines
acids
in the
are used
as chiral
equivalents.
The chiral
amines
treat-lent with
8 can easily
concentrated
can then be converted For
in a two phase
(S)-1~ and
occured
during
from the aqueous
hydroxide
and extraction
to the corresponding
comparison
phase with
by
ether
2-methylene-imidazolidines
of rotations
showed
that no racemization
and 1 6) . had
one cycle.
H+ 3AS
potassium
back
(SS)-11
be isolated
-
H2$4
;TR3 M21; "Q.R~ +
Hz0
+o*
Foe
FoM
s
7
9
ii
&N,n,,
l
(es. 2) &2H+
y
‘0
(9
1”” (R)
i: 10 % Pd/C,
H2; ii: 1) KOH/MeOH,
It was not possible chiral
shift
reagents
palladium
on charcoal
refluxing
aqueous
carbazon known
in Table
the optical
to determine
can be drawn yields
(S)-If,
with
reduction
of $11 with
potassium
hydroxide
and isolation-as
yield')
by comparison
with
The results
compounds.
in
the semithe
are compiled
Thus,
(RR - and
(2E,4E)-
tion with
(SS)-11 and
is stereospecific
is also
(SS)-11
hexadienoates
(R)-lh and
with
of different
each produce
geometry.
different
(2E,42)-hexadienoates
with
regard
signs
2, respec-
to the geometry
of the
2.
stereospecific
enantiomeric
As expected,
results:
control in controlled. With thermodynamic 10) one particular chiral imidazolidine
with methyl
(S)-lq,
the reaction
first
cycloaddition
for the products
2. the cycloaddition
with
regard
to the chiral
2-methylene-imidazolidines
form products
with
opposite
like
dieno-
(R)- and
signs of rotation
(S)-la
in the reac-
(2E,4E)-2.
3. lower
reaction
changing
from +20°C
can not be further -2OOC
pure
from these
I-doub 1e bond of the hexadienoates
or
the optical
are kinetically
the same product
(S)-la,
of rotation
phile.
degradation
to 3-methylcyclohexanone
not concerted
yield
However, tively
But catalytic
Yb(TFC)3).
to 2') and further
h/I with
1.
a reversible, should
(Eu(TFC)3,
2) H2N-CO-NH-NH2;
the e-e.' s of the products
of the enantiomerically
Some conclusions 1.
to determine
methanol
1Q8) allowed
rotations
30 min reflux,
temperatures to -2OOC exploited
for 85% completion
lead to higher increases
as the reaction
in toluene).
e.e.'s.
the e.e.
In the case of
(S)-Ia,
from 7% to 2l%, but this effect
rate becomes
too slow
(two month
at
2683
1: Reaction
Table
conditions
with dienoates
of the cycloadditions
2 and chemical
in parenthesis)
of the hydrolysis
2-Methylene-imidazolidine
of methylene-imidazolidines
and optical
yields
products
methyl
6
solvent
temperature
hexadienoate
(S)-la
R'
R2
R3
R4
Me
Me
iPr
H
1
(e.e.; R/S assignment
yielda)
e.e.
(OC)
(%)
+20
54
7 (S)
+2
45
10 (S)
toluene
-20
49
21 (S)
2E,4E
toluene
+30b)
71
3 (R)
2E,42
benzene
+2
51
16 (R)
2Z,4E
benzene
+2
39
3 (S)
2E,4E
toluene
2E,4E 2E,4E
(%)
(R)-la
Me
Me
iPr
H
2E,4E
toluene
+25
52
8 (R)
(S)-lb
Me
Et
iPr
H
2E,4E
benzene
+25
48
16 (R)
(S)-lc --
Me
cHex
iPr
H
2E,4E
benzene
+2
39
18 (R)
(S)-l'l
Et
Me
iPr
H
2E,4E
+2
35
13 (S)
(S)-le --
Ph
Me
iPr
H
2E,4E
+25
53
38 (S)
(S)-lf
Me
Me
Bz
H
(S)-lg
Ph
Me
Bz
Me
Ph
H
(R)-lh
Ph
(S)-11
Ph
(RR)-!&
Me
Me
(SS)-11
Me
Me
Ph
Ph
(RR)-11
Me
Me
Ph
Ph
(CH2)3
H
H
(CH2)4
c-hexane
2E,4E
toluene
+2
56
11 (S)
2E,4Z
toluene
+2
53
46 (R)
2E,4E
c-hexane
+25
47
44 (S)
2E,42
c-hexane
+25
50
10 (R)
2E,4E
benzene
+25
53
15 (R)
2E,4Z
c-hexane
+25
54
26 (S)
2E,4E
c-hexane
+25
48
27 (R)
2E,4E
c-hexane
+25
39
11 (R)
2E,4E
benzene
+25
51
15 (S)
2E,4Z
benzene
+25
57
19 (R)
2E,4E
benzene
+25
48
14 (S)
a) isolated yields after distillation b) cycloaddition run in a pressure vessel
4. pressure
does
other
Diels-Alder
lower
rotation
high
not have a positive
the opposite
instead
It was of no avail atoms
to achieve
2-methylphenyl achiral
effect
reactions Ig). The product
with
(3000 bar)
at 3000 bar").
better
sign
of atmospheric to introduce
2-methylene-imidazolidines
of
(S)-la and
if the cycloaddition
yields
like
(2E,4E)-2
it did
shows
is performed
a
under
pressure.
bigger
diastereoselection.
or 2,6_dimethylphenyl
on the optical
groups
substituents With
on one of the nitrogen
isopropyl,
on N-l
no cycloaddition
tert-butyl,
in different product
chiral
could
or
be isolated.
in
2684 REFERENCES la) A. Korolev and V. Mur, Dokl. Akad. Nauk SSSR, 59, 251 (1948); C.A. $2, 6776 (1948); b) reviews: P. Welzel, Nachr. Chem. Techn. Lab. ---_I 1983 979; L. A. Paquette in J. D. Morrison: Asymmetric Synthesis, Vol. 3A, Chapt. 4, Academic Press, New York 1984; H. Wurzinger, Kontakte (Darmstadt) _---I 1984 Nr. 2, p. 3; W. Oppolzer, Angew. Chem. 26, 840 (1984), Angew. Chem., Int. Ed. Engl. 22, 876 (1984); J. W. ApSimon and T. L. Collier, Tetrahedron 42, 5157 (1986); c) T. Poll, J. 0. Metter, and G. Helmchen, Angew. Chem. 92, 116 (1985), Angew. Chem., Int. Ed. Engl. 24, 112 (1985); W. Oppolzer, C. Capuis, and G. Bernardinelli, 42, 4035 (1986); J. Mulzer and M. Helv. Chim. Acta 62, 1397 (1984), Tetrahedron Kappert, Tetrahedron Lett. 26, 1631 (1985); H. Takayama, K. Hayashi, and T. Koizumi, ibid. 22, 5509 (1986); K. Furuta, K. Iwanaga, and H. Yamamoto, ibid. 21, 4507 (1986); S. G. Davies and J. C. Walker, J. Chem. Sot., Chem. Commun. 1986, 609; 0. De Lucchi, V. Lucchini, C. Marchioro, G. Valle, and G. Modena, J. Org. Chem. 5l, 1457 (1986); D. A. Evans, K. T. Chapman, and J. Bisaha, J. Am. Chem. Sot. 106, 4261 (1984); W. Ando, Y. Hanyu, and T. Takata, J. Org. Chem. 5l, 2122 (1986); R. S. Glass, K. Reineke, and M. Shanklin, ibid. $9, 1527 (1984); d) T. R. Kelly, A. Whiting, and N. S. Chandrakumar, J. Am. Chem. Sot. IQ@, 3510 (1986); K. Maruoka, M. Sakurai, J. Fujiwara, and H. Yamamoto, Tetrahedron Lett. 21, 4895 (1986); K. Narasaka, M. Inoue, and N. Okada, Chem. Lett. 1986, 1109; K. Narasaka, M. Inoue, and T. Yamada, ibid. 1986, 1967; e) G.H. Posner and D.G. Wettlaufer, J. Chem. Sot., Chem. Commun. _---I 1985 1786, Tetrahedron Lett. 22, 667 (1986), J. Am. Chem. Sot. 108, 7373 (1986); f) L.-F. Tietze, S. Brand, and T. Pfeiffer, Angew. Chem. ?I, 790 (1985), Angew. Chem., Int. Ed. Engl. 24, 784 (1985); g) J. Jurczak, T. Bauer, S. Filipek, M. Tkacz, and K. Zygo, J. Chem. Sot., Chem. Commun. 1983, 540; J. Jurczak, T. Bauer, and S. Jarosz, TetraLett. hedron 42, 5045, 6477 (1986); W. G. Dauben and R. A. Bunce, Tetrahedron 21, 4875 (1982). 2) J. Sauer and R. Sustmann, Angew. Chem. 92, 773 (1980), Angew. Lett. 19_11, 2717. Engl. 19, 779 (1980); R. Sustmann, Tetrahedron 3) M. Heuschmann,
publication
Chem.
Int. Ed.
in preparation.
4) (2E,4E)-2: F. D. Lewis, D. K. Howard, S. V. Barancyk, and D. Oxman, J. Am. Chem. Sot. 1Q8, 3016 (1986); (2Z,4E)-2: U. Eisner, J. A. Elvidge, and R. P. Linstead, J. Chem. Sot. 1953 ----, 1372; (23,42)-z: S. Tsuboi, T. Masuda, H. Makino, and A. Takeda, Tetrahedron Lett. 23, 209 (1982), this product was a mixture containing 10% of (2E,4E)-2. 5) 2-Methylenimidazolidines without C2-axis form additional isomers of 4, where R1/R4 and R2/R3 are exchanged. 2, 4, and 3 were never isolated. The structures and by were assigned on the basis of 'H- and 13C-NMR spectra of the mi$ures comparison with the cycloaddition products of achiral analogues of 1. 6) Treatment of amines 8 with orthoacetate and HBF4 leads to dihydroimidazoliumwhich can be deprotonated to 1 with sodium hydride in THF. tetrafluoroborates, 7) E. J. Eisenbraun, G. H. Adolphen, Chem. 16, 414 (1971).
K. S. Schorno
and R. N. Morris,
J. Org.
8) J. K. Shillington, G. S. Denning, Jr., W. B. Greenough, III, T. Hill, Jr., and 0. B. Ramsay, J. Am. Chem. Sot. @(I, 6551 (1958); N. L. Allinger and C. R. Riew, J. Org. Chem. Ul, 1316 (1975). 9) The stereochemical
assignment
(R,S) is the same for 6, 9, and lg.
10) Diels-Alder cycloadditions of an achiral analogue of 1 with 2 could be shown3) to be not -_- concerted and totally reversible at elevated temperatures. 11) We are indebted (Received
in Germany
to Prof.
Tiltscher,
17 February
1987)
Erlangen,
for his help.