Dynamic kinetic asymmetric allylic amination and acyl migration of vinyl aziridines with imido carboxylates

Article abstract of DOI:10.1002/anie.200700835

(Chemical Equation Presented) An atom-economical method has been developed for the preparation of chiral vicinal diamines through a dynamic kinetic asymmetric allylic amination and acyl-group migration of vinyl aziridines with imido carboxylates. Application of the asymmetric transformation enabled the concise synthesis of the azepane core 1 of (+)-balanol and its syn analogue.

Full text of DOI:10.1002/anie.200700835

Angewandte
Chemie
DOI: 10.1002/anie.200700835
Asymmetric Catalysis
Dynamic Kinetic Asymmetric Allylic Amination and Acyl Migration of
Vinyl Aziridines with Imido Carboxylates**
Barry M. Trost,* Daniel R. Fandrick, Tobias Brodmann, and Dylan T. Stiles
The palladium-catalyzed asymmetric allylic
alkylation (AAA) has proven to be a versatile
method for the preparation of synthetically
useful materials.[1] Vinyl epoxides have shown a
broad utility in related dynamic kinetic asym-
metric transformations (DYKATs), wherein a
racemic starting material is converted into an
enantioenriched product.[2] Only recently has
the cycloaddition of vinyl aziridines with iso-
cyanates to afford chiral imidazolidinones been
reported.[3] Although this methodology pro-
vides the valuable chiral vicinal diamine moiety[4] in high ee,
its synthetic utility has been limited owing to the difficult
differentiation of the resulting amines. In recent studies with
imide nucleophiles, we discovered an atom-economical[5]
DYKAT for the efficient preparation of useful orthogonally
protected chiral vicinal diamines through use of imido
carboxylates that undergo a facile in situ acyl migration.
Application of the AAA enabled the formal synthesis of the
potent PKC inhibitor balanol and its
Regioselectivity can be rationalized by an analogous hydro-
À
gen-bonding interaction between the H N group of the imide
nucleophile and the amine of the h3-allyl PdII intermediate, as
well as the directing effect of the ligand.[1]
When benzoyl imido carboxylates were examined as
nucleophiles, a facile in situ acyl migration occurred, furnish-
ing the protected N1-benzoyl-N2-tert-butoxycarbonyl vicinal
diamine 7 in high enantioselectivity and yield [Eq. (2)]. The
cis diastereomer.
By analogy to the DYKAT reac-
tions with vinyl epoxides,[6] we first
examined the asymmetric addition of
phthalimide to the parent vinyl azir-
idine 1 with the use of diphosphine
ligand 3 [Eq. (1)]. The addition of
catalytic acetic acid, which was ben-
eficial in the previous cycloadditions
with isocyanates,[3] gave an increase in ee (47 to 82% ee) with
a concomitant drop in yield (99 to 55%). The use of catalytic
triethylamine instead provided a balance in improving the ee
while affording a practical yield. A reasonable rationalization
for the high enantioselectivity is the Curtin–Hammett kinetic
amination of one diastereomeric h3-allyl PdII complexfrom a
pair which rapidly interconvert through a p–s–p mechanism.
isomeric Boc acyl migration product was not observed,
demonstrating the high selectivity for migration of the more
electrophilic benzoyl group. Contrary to previous palladium-
catalyzed DYKATs with vinyl aziridines,[3] the enantioselec-
tivity was high without the use of an additive.
This dynamic kinetic asymmetric allylic amination and
acyl migration proved general for a variety of benzoyl imido
carboxylates to afford Boc-, Cbz-, ethoxycarbonyl-, and Troc-
protected products (Table 1). Under current conditions, the
reaction with acyl imido carboxylates affords only low yields.
A similar highly enantioselective DYKAT and acyl migration
was obtained with butadiene monoepoxide (Scheme 1).
Saponification of ester 17 provided the known amino alcohol
18[7] and established the absolute stereochemistry of the
reaction. The products from the vinyl aziridine substrates
were initially assigned by analogy. Subsequently, the absolute
stereochemistry of the diamine products was confirmed by
the synthesis of the azepane core of (+)-balanol (see below).
In an effort to furnish both amines protected with readily
cleavable protecting groups, the use of imido dicarboxylates
was examined. Similar to the previous reactions, the acyl
[*] Prof. B. M. Trost, Dr. D. R. Fandrick, T. Brodmann, D. T. Stiles
Department of Chemistry
Stanford University
Stanford CA 94305 (USA)
Fax: (+1)650-725-0002
E-mail: bmtrost@stanford.edu
[**] We thank the National Science Foundation and the National
Institutes of Health, General Medical Sciences Institute (GM-
13598), for their generous support of our programs. Mass spectra
were provided by the Mass Spectrometry Facility of the University of
California-San Francisco, supported by NIH Division of Research
Resources.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
Table 1: Dynamic kinetic asymmetric allylic amination with benzoyl
imido carboxylates.[a]
Scheme 1. Dynamic kinetic asymmetric allylic alkylation with butadiene
monoepoxide. a) 2 mol% [{(h3-C3H5)PdCl}2], 6 mol% (S,S)-3, CH2Cl2,
358C, 18 h (72%, 90% ee). b) LiOH, H2O, THF(86%).
Entry
1
Aziridine
R2
Product
ee[b]
(yield)[c]
Table 2: Dynamic kinetic asymmetric allylic amination with imido
dicarboxylates.[a]
89%
(90%)
1
EtOCO
93%
(84%)
Entry
Aziridine
R2,
R3
Product
ee[b]
2
3
4
5
6
7
1
1
1
1
8
9
Troc
Boc
Cbz
Boc
Boc
Boc
(Yield)[c]
Boc,
EtOCO
94%
(71%)
1
2
3
1
1
8
89%
(95%)
Cbz,
Cbz
90%
(67%)
90%
(75%)
Cbz,
Cbz
88%
(68%)
92%
(81%)
[a] Reaction conducted in dichloromethane (0.15m) with 1.05 equiv
imide. [b] Enantiomeric excess determined by chiral HPLC. [c] Yield of
isolated product.
86%
(97%)
To demonstrate the utility of the asymmetric amination,
we turned our efforts towards the synthesis of the potent
protein kinase C (PKC) inhibitor balanol (Scheme 2).[9] Ring-
closing metathesis (RCM)[10] of the DYKAT product 21
provided the tetrahydroazepine intermediate 22. Unlike
other approaches towards balanol,[11] the stereochemistry at
the hydroxy-bearing carbon was established from the olefin
functionality created in the RCM. Our first approach relied
on a regio- and diastereoselective hydroboration rationalized
by the inductive and steric effects[12] resulting from the allylic
carbamate. Optimal conditions furnished the azepane core in
a 2:1 d.r. and 3:1 regioselectivity, from which the anti isomer
was isolated in reasonable yield. Hydrogenolysis of the
desired heterocycle completed the synthesis of the azepane
intermediate 23 employed in the total synthesis of (+ )-
balanol reported by Lampe et al.[13]
87%
(90%)
[a] Reaction conducted in dichloromethane (0.15m) with 1.05 equiv
imide. [b] Enantiomeric excess determined by chiral HPLC. [c] Yield of
isolated product. Boc=tert-butoxycarbonyl. Bn=benzyl. Cbz=benzyl-
oxycarbonyl. DMB=2,4-dimethoxybenzyl. Troc=2,2,2-trichloroethoxy-
carbonyl.
migration occurs through elimination of the carbamate
functionality from a tetrahedral intermediate. However, the
yields for this dynamic kinetic asymmetric allylic amination
were slightly lower than with the analogous benzoyl imido
carboxylates. This difference is likely a result of competitive
fragmentations of the tetrahedral intermediate.[8] Generally,
the AAA with ethyl, tert-butyl, and bisbenzyl imido dicar-
boxylates proceeded in high enantioselectivity and with
reasonable yields. Entry 1 in Table 2 demonstrates that the
chemoselectivity of the acyl migration is also controlled by
steric factors. For example, with tert-butyl ethyl imido
dicarboxylate, the ethyl carbamate functionality selectively
migrated over the Boc.
Since the syn disubstituted azepane core[14] provides an
alternative strategy to the trans stereochemistry of balanol as
well as access to the Z analogues, which also are of interest,
we evolved an efficient sequence to diastereomer 26. After
acylation of carbamate 22 with Boc anhydride, iodocycliza-
tion with NIS furnished oxazolidinone 24 in high yield.
Dehalogenation with a tin-free protocol[15] and subsequent
global deprotection completed the synthesis of the syn
azepane 26 in quantitative yield, and in 77% overall yield
from the metathesis product 21. Performing the esterification
of the hydroxy group with inversion of configuration will
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Angewandte
Chemie
Keywords: asymmetric catalysis · balanol ·
.
homogeneous catalysis · palladium · vinyl aziridines
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Scheme 2. Formal synthesis of balanol and epi-C4-balanol. a) 5 mol% Grubbs II
catalyst, CH2Cl2, 358C (90%), b) BH3·THF, then NaBO3·H2O, 08C (47%), c) H2,
Pd(OH)2, MeOH, then HCl (83%), d) (Boc)2O, DMAP, CH3CN, e) NIS, DCE, hn
(79%, 2 steps), f) lauroyl peroxide, cyclohexane, hn (99%), g) HCl, H2O, reflux
(99%). DMAP=4-dimethylaminopyridine, NIS=N-iodosuccinimide, DCE=di-
chloroethane.
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allow this isomer to be converted into the natural diastereo-
mer as well.
In conclusion, we have developed an efficient method for
the preparation of chiral vicinal diamines and amino alcohols
through a dynamic kinetic asymmetric allylic amination and
acyl migration of vinyl aziridines and epoxides with imido
carboxylates. The discovery of the facile acyl migration
greatly expands the product scope and utility in natural
product synthesis. Accordingly, the methodology provided a
platform for the formal synthesis of balanol and analogues.
Experimental Section
Typical conditions for the dynamic kinetic asymmetric allylic
amination of vinyl aziridines and epoxides with imido carboxylates:
The catalyst solution was prepared by stirring under argon a mixture
of [{(h3-C3H5)PdCl}2] (2 mol% based on electrophile) and ligand
(S,S)-3 (6 mol% based on electrophile) in anhydrous and degassed
dichloromethane (0.15m based on electrophile) for 15 min at ambient
temperature. The aziridine or epoxide was added to the catalyst
solution by syringe, and the solution was transferred by cannula into
an argon-purged vessel containing the nucleophile (1.05 equiv based
on electrophile). The reaction mixture was stirred at 358C for 18 h, at
which point the electrophile has been consumed by TLC. Concen-
tration of the reaction mixture in vacuo gave an oil, and purification
by silica gel chromatography (ether/hexanes) provided the desired
product.
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Received: February 24, 2007
Published online: July 10, 2007
Angew. Chem. Int. Ed. 2007, 46, 6123 –6125
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