Toward the development of a general chiral auxiliary 10: Enhancement of diastereoselectivity in Diels-Alder reactions of imides derived from a new bicyclic lactam chiral controller molecule

Article abstract of DOI:10.1055/s-2004-829091

Based upon X-ray structural information derived from a series of camphor-derived bicyclic lactam chiral controller molecules, a new auxiliary has been designed which affords superior diastereoselectivity (>95:5) in Diels-Alder reactions with common dienes but of opposite π-facial selectivity than previously observed with this class of chiral controller. A rationale for the selectivity based on subtle structural changes in the chiral controller is discussed.

Full text of DOI:10.1055/s-2004-829091

LETTER
1399
Toward the Development of a General Chiral Auxiliary 10: Enhancement of
Diastereoselectivity in Diels–Alder Reactions of Imides Derived from a New
Bicyclic Lactam Chiral Controller Molecule
I
mprove
d
E
nantioslectivity in
b
A
symmetric
e
D
iels–Ald
r
er React
tions K. Boeckman, Jr.,* Lisa M. Reeder
Department of Chemistry, University of Rochester, P.O. Box 270216 Hutchison Hall, Rochester, New York 14627-0216, USA
Fax +1(585)7560210; E-mail: rkb@rkbmac.chem.rochester.edu
Received 17 May 2004
This paper is dedicated to Professor and Dean Clayton Heathcock on the occasion of his retirement from the University of California,
Berkeley.
ably few studies have sought to establish the relationship
Abstract: Based upon X-ray structural information derived from a
between auxiliary structure and the observed level of dia-
series of camphor-derived bicyclic lactam chiral controller mole-
stereoselectivity within a set of auxiliaries of the same
cules, a new auxiliary has been designed which affords superior di-
structural class other than by empirical observations.6,7
astereoselectivity (>95:5) in Diels–Alder reactions with common
dienes but of opposite p-facial selectivity than previously observed
As part of ongoing studies we aimed to uncover the struc-
with this class of chiral controller. A rationale for the selectivity
tural origin of the unusual selectivity imparted by the class
based on subtle structural changes in the chiral controller is dis-
of bicyclic lactam chiral controller molecules of general
cussed.
structure 1 (Figure 1), developed in our laboratories.6 In
Key words: asymmetric cycloaddition, Lewis acid promoted,
order to improve the selectivities observed with these aux-
chiral controller, bicyclic lactams, structure vs. selectivity
iliaries, we were led to consider additional modifications
of the lactam core structure. Our prior studies had suggest-
ed that the origin of the selectivity (or lack thereof) may
The development of asymmetric synthetic methodology
in significant part be derived from subtle bond angle
in which bond formation is mediated either by covalently
changes within the bicyclic framework, which modulate
bound or catalytic chiral controller molecules has seen
the interaction between the reactive N-acyl sidechain and
enormous progress in the last 15 years, driven in no small
the proximal bridgehead position in the auxiliary.6 This
part by the increasing need to prepare enantiomerically
interaction is responsible in large part for controlling the
pure materials for a variety of applications.1,2
rotamer population about the sidechain Ca-Cb bond. Cou-
Along with asymmetric catalysis, use of covalently bound
chiral auxiliaries to facilitate the preparation of enantio-
merically pure substances has been firmly established as
an important general strategy.3 Appropriate controllers
have been developed for a diverse group of C-C bond
forming and functional group transformations, including
many of the most powerful and widely used synthetic
transformations such as cycloaddition, aldol condensa-
tion, conjugate addition, and alkylation.3,4
pled with chelation control over the N-Ca, control over the
Ca-Cb rotamer population is critical to achieving high p-
facial selectivity in cycloaddition, conjugate addition, and
Claisen rearrangement of chiral imides bearing a,b-unsat-
urated N-acyl side chains.
Based upon this hypothesis, we targeted lactams such as
2, which were modified by substitution on the C8 methyl
group proximal to the nitrogen bearing the reactive
sidechain. Molecular mechanics calculations suggested
that introduction of an aryl group at that position would
result in opening the C8-C5-C9 angle resulting in a reduc-
tion of the C1-C5-C4 angle and the effective movement of
the C4 carbon and its attached hydrogen inward lessening
the interaction between the bridghead hydrogen and the
reactive sidechain attached to the lactam nitrogen. This
should then allow a,b-unsaturated acyl sidechains to as-
sume the more thermodynamically preferred S-cis confor-
mation about the Ca-Cb bond. Furthermore, the
interactions of the C8 arylmethyl group with the C9 and
C10 methyl groups results in the most stable conformation
with the aryl group lying parallel and bisecting N-Cabond.
9
8
5
9
8
10
6
O
2
5
1
R1
6
1
R
10
O
2
3
N
O
4
7
C α
H
3
4
N
O
7
β C
C α
H
H
H
β R2
H
1
2
Figure 1
Camphor derivatives have occupied a central place among
this group of compounds.5 Although, a number of ratio-
nally designed camphor derivatives have been introduced,
including examples from our own laboratories,6 remark-
We prepared the required 8-phenyl camphor lactam 3 in
seven steps from the known 8-bromocamphor (4),8 itself
readily available from commercial 3-bromocamphor, as
shown below in Scheme 1. Owing to its hindered nature,
ketalization of the carbonyl group in 4 requires use of the
TMSCl–ethylene glycol protocol as decribed by Money
SYNLETT 2004, No. 8, pp 1399–1403
0
1.
0
7.
2
0
0
4
Advanced online publication: 22.06.2004
DOI: 10.1055/s-2004-829091; Art ID: Y03004ST
© Georg Thieme Verlag Stuttgart · New York

1400
R. K. Boeckman, Jr., L. M. Reeder
LETTER
affording the bromo ketal 5 in 79% yield.8 The subsequent in 92% yield employing SeO2 in xylenes at reflux. Further
nickel-catalyzed coupling is the key transformation in the oxidation of diketone 7 to the related diacid by basic hy-
sequence and should be amenable to the introducton of a drogen peroxide was complicated by the low solubility of
structurally diverse group of substituents, requiring only 7 in aqueous media and the insolubility of the resulting di-
compatibility of the substrate halide with formation of the acid in organic solvents. However, oxidation of 7 with
required Grignard reagent and the absence of b-hydro- aqueous NaOCl in THF proceeded smoothly.10 After de-
gens. Although modeled on the work of Scott,9 the cou- struction of excess oxidant with thiosulfate, acidification,
pling required significant optimization to limit formation and partial concentration, the resulting aqueous slurry of
of the major side product resulting from reduction of 5. diacid was then diluted with DMF and treated with K2CO3
Many such couplings can be successfully conducted in and excess CH3I affording diester 8 in 70% overall yield.
THF at reflux, however in the case of 5, use of these con- The remainder of the sequence follows the route devel-
ditions results exclusively in reduction. We attributed that oped during our earlier studies.6,11 Selective partial sapon-
result to the ready formation of radicals by homolysis of ification affords the less hindered mono acid 9
the Ni-C bond at higher temperatures and the ease of hy- quantitatively. An effectively one-step Curtius rearrange-
drogen atom donation from THF. Control experiments ment of 9 then affords the urethane 10 in near quantitative
supported this hypothesis since reduction did not occur yield. Finally, ring closure proceeds smoothly using 1
upon exposure of 5 to either the nickel catalyst or the equivalent of freshly prepared anhydrous NaOCH3 in
Grignard reagent independently. Use of diethyl ether vir- THF at room temperature providing crystalline 8-phenyl
tually completely suppressed reduction. The other main lactam (3, mp 128–130 °C) in 70% yield. Since lactam 3
side reaction is homo coupling of the phenylmagnesium is somewhat susceptible to cleavage, the presence of hy-
bromide to afford in this case biphenyl. This side reaction, droxide or excess base, and elevated temperatures must be
while unimportant in this particular case, would be a sig- avoided. Although the sequence is somewhat lengthy, it
nificant problem for less readily available halides.
can be conducted with minimal manipulations and purifi-
cations, proceeds in excellent overall yield, and is easy to
perform on multigram scale. The auxiliary is generally
readily recovered and recycled after use.
Br
Br
O
O
TMSCl
OH
79%
O
HO
4
5
Ph
O
n-BuLi
Ph
O
PhMgBr
(Ph3P)2NiCl2
Et2O
Ph
O
N
O
Ph
O
Cl
O
O
SeO2
88%
11
then 10% HCl
THF
HN
xylenes, ∆
Ph
3
n-BuLi
O
Cl
6
7
O
92%
68%
O
N
Ph
1.
NaOCl
Ph
THF/H2O
KOH
81%
12
H3CO2C
HO2C
H3CO2C
H3CO2C
CH3OH/H2O
∆
95-100%
2.
K2CO3
CH3I, DMF
70%
Scheme 2
8
9
Ph
Ph
O
With the desired lactam 3 in hand, we set out to examine
the reactivity and diastereofacial selectivity of chiral imi-
des derived from 3 in Lewis acid promoted Diels–Alder
reactions with various alkyl dienes. We prepared the a-
methacryloyl and crotonoyl imides 11 and 12 (Scheme 2)
in order to facilitate structural and stereochemical com-
parison with the products prepared via the first generation
auxiliaries.6,11 Treatment of lactam 3 in THF with n-BuLi
in hexanes at 0 °C followed by addition of either a-meth-
acryloyl chloride or crotonoyl chloride afforded the ex-
pected imides 11 and 12 in 88% and 81% yields,
respectively.
1. EtO2CCl, TEA
2. NaN3, H2O
3. toluene, ∆
NaOCH3
THF, r.t.
70%
H3CO2C
H3CO2CN
H
HN
4. CH3OH, TEA
10
3
95-100%
Scheme 1
Although homocoupling cannot be entirely overcome, it
can be minimized by keeping the concentration of the
Grignard reagent low through slow addition over at least
4 hours, via a syringe pump, of a relatively dilute solution
of the Grignard reagent to the Ni catalyst and halide at re-
flux in diethyl ether. In practice, the coupling reaction
mixture was directly treated with 10% aqueous HCl after
dilution with THF to afford 8-phenylcamphor (6) after pu-
rification. Under optimized conditions, 60–80% overall
yield of 6 from 5 were obtained. Mercury(II)-catalyzed
oxidation of 8-phenylcamphor (6) directly to the cam-
phoric acid using dilute nitric acid is precluded in this case
by competing electrophilic nitration of the aromatic ring.
However, oxidation to the diketone 7 proceeds smoothly
Our prior work had suggested that the best of the Lewis
acid promoters would be CH3AlCl2 and (CH3)2AlCl. We
began with cyclopentadiene owing to its high reactivity.
Precomplexation of 11 with 1.5 equivalents of CH3AlCl2
in CH2Cl2 at –78 °C followed by treatment with excess
cyclopentadiene (ca. 20 equiv) slowly over 30 minutes
quite surprisingly afforded a mixture (16:4:4:1) of all four
diastereomeric products 13–16 in variable yields (up to
61%, Scheme 3). The observed exo/endo selectivity was
Synlett 2004, No. 8, 1399–1403 © Thieme Stuttgart · New York

LETTER
Improved Enantioselectivity in Asymmetric Diels–Alder Reactions
1401
determined to be 4:1 and the p-facial selectivity also 4:1 When imide 11 was treated with 1.5 equivalents of
1
by H NMR analysis employing the well-resolved C4 CH3AlCl2 and ≤1.0 equivalents of (CH3)3Al in CH2Cl2 at
bridgehead proton resonances. The ratios were quantified –78 °C to –55 °C followed addition of excess diene, the
by capillary GC. This surprisingly mediocre result was yields improved significantly (>70%) but interestingly no
troubling since it was worse than the comparable first increase in diastereoselectivity was observed. However,
generation auxiliary devoid of the phenyl group.11 This when >1.0 equivalents of (CH3)3Al was employed, a dra-
pattern of poor yields (30–50%) and p-facial selectivity matic increase in diastereoselectivity was observed. As
(2-4:1) was repeated upon reaction of 11 with selection of can be seen from the entries in Table 1, after some optimi-
other acyclic butadienes.
zation, we observed that conducting the cycloaddition in
the presence of 1.5 equivalents of CH3AlCl2 and 2.0
equivalents of (CH3)3Al afforded very good to excellent
yields, superior levels of p-facial selectivity (>93:7) and,
where relevant, excellent endo/exo selectivity (>96:4).
The sense of asymmetric induction was proven to be op-
posite that of the first generation series by chemical corre-
lation of the major adducts with those prepared previously
or with other materials of known configuration.6,11
Ph
O
CH3AlCl2 (1.5 equiv)
CH2Cl2, –78 °C
24 h
N
O
11
Ph
Ph
O
O
N
N
+
:
We can only speculate as to the origin of this effect. Con-
trol experiments ruled out promotion by (CH3)3Al alone
as no reaction was observed under the reaction conditions
in the absence of CH3AlCl2. We believe that there are two
effects observed as a result of the addition of the
(CH3)3Al. The first, seen when substoichiometric amounts
of (CH3)3Al are present, results from scavenging of
protons leading to suppression of Brønsted acid catalyzed
polymerization of the diene resulting in increased yields.
O
O
13
14
16
4
Ph
Ph
O
N
O
N
O
O
15
16
:
1
4
Scheme 3
Table 1 Conducting the Cycloaddition in the Presence of 1.5 Equiv-
alents of CH3AlCl2 and 2.0 Equivalents of (CH3)3Al
During these experiments, it was noted that substantial
polymerization of the dienes was occuring during at-
tempts to effect cycloaddition. The formation of sub-
stantial amounts of polymer hindered isolation and
purification of the adducts, and was even considered to be
sequestering the Lewis acid resulting in reaction via
mono-coordinated Lewis acid complexes, which were
known to exhibit poor p-facial selectivity in cycloaddi-
tions.12,13 Thus, we felt it important to investigate the ori-
gin of the polymerization and find ways to suppress this
process.
Ph
O
diene (excess)
O
N
CH3AlCl2 (1.5 equiv)
XcN
(CH3)3Al (2.0 equiv)
CH2Cl2, –78 °→55 °C
24 h
O
NXc
11
Diene
Major adduct
p-Facial Endo:exo Yield (%)
selectivity
O
95:5
95:5
93:7
99:1
95:5
96:4
–
75
XcN
In our earliest studies, it was found that strictly anhydrous
conditions were helpful in suppressing unwanted poly-
merization of the diene.11 Thus, we hypothesized that the
polymerization might be principally catalyzed by Brøn-
sted acid(s) rather than the Lewis acid promoter. In prior
unpublished studies of cycloaddition with electron rich
silyloxybutadienes, we had employed Brønsted acid scav-
engers such as triethylaluminum in an effort to improve
yields. However, on that occasion we found interference
from radical-mediated 1,4-addition of ethyl residues to the
dienophile. We considered that radical-mediated side re-
actions using methyl aluminum reagents should be less
likely. Thus, we were led to consider such a solution in
this instance.
73
O
XcN
–
78
O
XcN
99:1
–
>99 (50)a
70
O
XcN
O
XcN
a 50% Conversion (unoptimized).
Synlett 2004, No. 8, 1399–1403 © Thieme Stuttgart · New York

1402
R. K. Boeckman, Jr., L. M. Reeder
LETTER
124.1, 60.3, 57.3, 51.4, 38.1, 30.7, 26.4, 18.4, 14.5, 9.6. HRMS:
-
-
(CH3)2AlCl2
CH3
(CH3)2AlCl2
CH3
m/z calcd for C19H23NO2 [M+]: 297.1729. Found: 297.1732.
Cycloaddition Procedure: Reaction with Isoprene:
+
+
O
N
O
N
Al
Al
A solution of 141 mg (0.47 mmol) of phenylcamphor imide 11 in
0.6 mL of CH2Cl2 was cooled to –55 °C. A 1.0 M solution of
(CH3)3Al in hexanes (475 mL, 0.95 mmol) was slowly added down
the side of the cooled reaction vessel, followed by 711 mL (0.71
mmol) of a 1.0 M solution CH3AlCl2 in hexanes. After 15 min, 0.5
mL (4.74 mmol) of freshly distilled isoprene was added over 30 min
and the resulting yellow solution stirred at –55 °C for 24 h. After
this period, 0.6 mL of 1 M aq HCl was carefully added and the re-
action mixture warmed to r.t. After separation, the organic phase
was diluted with 1.0 mL of EtOAc, 500 mg of Celite was added, and
the mixture stirred at r.t. for 30 min. The solids were removed by fil-
tration through silica gel and the filtrate concentrated in vacuo. The
crude product mixture was purified on silica gel eluting with 5%
EtOAc–hexanes to afford 126 mg (73%) of diastereomers in a 95:5
ratio by 1H NMR (500 MHz).
CH3
CH3
O
O
CH3
minor
major
Figure 2
The second and more significant effect arises from forma-
tion of a different reactive complex than that obtained
from imide 11 and CH3AlCl2. It is known that alkyl alu-
minum reagents are aggregated in solution and undergo
facile ligand exchage leading to rapid disproportion-
ation,14 and that cycloadditions of this type are promoted
by formation of ion pairs in which the reactive species in-
volve bidentate cationic aluminum chelates of the im-
ide.12,13 Thus, we speculate that disproportionation to
(CH3)2AlCl occurs to a significant extent under our reac-
tion conditions leading to the cycloaddition occurring via
the formation of the reactive ion pair chelate shown above
Major Cycloadduct:
1H NMR (500 MHz, CDCl3): d = 7.34–7.17 (m, 3 H), 7.03–6.98 (m,
2 H), 5.37 (br s, 1 H), 4.34, (br s, 1 H), 3.00 (br d, J = 17.7 Hz, 1 H),
2.60 (d, J = 13.2 Hz, 1 H), 2.30 (d, J = 13.2 Hz, 1 H), 2.25–2.06 (m,
2 H), 1.88–1.83 (m, 2 H), 1.68 (s, 3 H), 1.80–1.63 (m, 5 H), 1.40 (s,
3 H), 1.17 (s, 3 H), 0.81 (s, 3 H). 13C NMR (75 MHz, CDCl3): d =
177.4, 176.8, 137.8, 133.4, 129.7, 128.0, 126.1, 119.3, 62.5, 57.3,
in Figure 2. The difference between excess CH3AlCl2 and 50.7, 42.8, 37.6, 33.3, 31.8, 31.0, 27.9, 26.0, 23.1, 21.1, 14.2, 9.7.
IR (film): 2963, 1743, 1674, 1328 cm–1. HRMS: m/z calcd for
the mixture of (CH3)3Al and CH3AlCl2 [or (CH3)2AlCl] is
the substitution of a second methyl group on the alumi-
C24H31NO2 [M+]: 365.2354. Found: 365.2349.
num (replacing a smaller chlorine) in the reactive ion pair.
The buttressing results in an interaction with the methyl
group in the reactive side chain resulting a larger shift in
the rotamer population favoring the S-cis reactive rotam-
er, which correlates with the observed sense of asymmet-
ric induction and increase in selectivity. Experimental
support for this tentative interpretation was obtained by
conducting the reactions shown in Table 1 employing 1.5
equivalents of (CH3)2AlCl and of 0.5 equivalents of
(CH3)3Al. These reagents qualitatively duplicated the re-
sults reported in Table 1.
Acknowledgment
We wish to thank the NIGMS of the NIH (GM-29290 and GM-
30345) and the NSF CHE-0305790 for their generous support of
these studies.
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Somewhat surprisingly, preliminary studies with imide 12
have not delivered increased selectivity in this case rela-
tive to the first generation auxiliaries. Further studies of
imides 11 and 12 and other applications of this class of in-
triguing chiral controller molecules including applications
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Characterization Data for Lactam 3 and Imide 11:
3: [a]D25 –28.2 (c 1.3, CHCl3); mp 128–130 °C. 1H NMR (400 MHz
CDCl3): d = 7.31–7.16 (m, 5 H), 6.46 (br s, 1 H), 3.34 (s, 1 H), 2.90
(d, J = 13.25 Hz, 1 H), 2.25 (d, J = 13.25 Hz, 1 H), 1.81–1.77 (m, 2
H), 1.56–1.52 (m, 2 H), 1.15 (s, 3 H), 0.86 (s, 3 H). 13C NMR (100
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54.6, 37.6, 30.1, 28.2, 14.9, 8.9. HRMS: m/z calcd for C15H19NO
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Synlett 2004, No. 8, 1399–1403 © Thieme Stuttgart · New York