Amines review-PAC2006

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Amines review-PAC2006,
 
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Pure Appl. Chem.
, Vol. 78, No. 7, pp. 1397–1406, 2006.
doi:10.1351/pac200678071397
© 2006 IUPAC
Recent developments in the chiral synthesis of
homoallylic amines via organoboranes*
P. Veeraraghavan Ramachandran
‡
and Thomas E. Burghardt
†
Herbert C. Brown Center for Borane Research, Department of Chemistry,
Purdue University, 560 Oval Dr., West Lafayette, IN 47907-2084, USA
-pinene-based versatile reagents for the synthesis of such amines con-
firmed that very high enantioselectivity and outstanding diastereoselectivity can be readily
achieved. Addition of the “allyl”boron reagents to various N-substituted imines provided the
desired amine products in high yields and high to very high ee. The discovery that an addi-
tion of 1.0 equiv of methanol or water to the “allyl”boration reaction with
N
-masked imines
is critical allowed for higher yields and noticeably improved ee. The use of
N
-aluminoimines,
which are not only easy to prepare by a partial reduction of nitriles, but are also relatively sta-
ble for both enolizable and non-enolizable substrates, considerably expanded the scope of the
reactions. In this review, the developments in the syntheses of chiral homoallylic amines
using organoboranes, with the particular accent on the reagent-controlled reactions, are sum-
marized. Additionally, the novel methodology for the crotyl- and alkoxyallylboration of
imines using trialkylboron “ate” complexes is described.
α
Keywords
: chiral homoallylic amines; N-substituted imines; allylboron reagents; organo-
boranes; homoallylic amines;
N
-aluminoimines.
INTRODUCTION
The homoallylic amine moiety is not widely present in natural products, however, compounds like
eponemycin [1], which exhibits potent activity against B16 melanoma cells, or a depsipeptide crypto-
phycin 337 [2], which is an analog of a potent anti-tumor compound cryptophycin, contain this sub-
unit (Fig. 1).
Fig. 1
Natural products containing homoallylic amine moiety.
*Paper based on a presentation at the 12
th
International Meeting on Boron Chemistry (IMEBORON-XII), Sendai, Japan,
11–15 September 2005. Other presentations are published in this issue, pp. 1299–1453.
‡
Corresponding author
†
Current address: Johns Manville, 10100 West Ute Ave., Littleton, CO 80127, USA
1397
Abstract
: Among the plethora of protocols for the preparation of chiral homoallylic amines,
the use of boron-based reagents remains relatively undeveloped. However, the recent ad-
vances in the use of
1398
P. V. RAMACHANDRAN et al.
On the other hand, homoallylic amines are excellent building blocks for the synthesis of numer-
ous nitrogen-containing natural products [3]. Chiral homoallylic amines were utilized as the key inter-
mediates in the preparation of many natural products, such as an amino-sugar vancosamine isolated
from vancomycin [4], a spirocycle alkaloid halichlorine isolated from a Japanese sponge
Halichondria
okadai
Kadota [5], an alkaloid from
Prosopis africana
, desoxoprosopinine [6], and many others.
Additionally, a variety of other nitrogen-containing compounds, including amino alcohols,
β
- and
-lactams, and many other compounds can be easily accessed from
homoallylic amines (Fig. 2). With the utilization of ring-closing metathesis methodology, numerous
piperidine alkaloids can be easily prepared from the corresponding aminodienes [7]. Preparation of
β
-amino acids,
γ
-amino alcohols,
γ
-amino acids is of particular interest to medicinal and bioorganic chemists as various
β
-amino acid
-lactam antibiotics, HIV-protease inhibitors, and other compounds.
Moreover, high proteolytic stability of
β
β
-amino acids makes them excellent substrates for the peptido-
mimetics [8].
Fig. 2
Utility of homoallylic amines.
Hence, preparation of homoallylic amines in an optically pure form is a very important task in or-
ganic synthesis, leading to an abundance of protocols for their preparation that can be found in the lit-
erature [3,8]. Diastereoselective addition of allyl organometallic compounds to N-substituted aldimines
is a widely recognized procedure for the preparation of homoallylic amines [9]. While the use of sub-
strate-controlled asymmetric additions is quite commonly employed, the use of reagent control remains
undeveloped [9]. However, in spite of aldimines being very attractive starting materials for the synthe-
ses of homoallylic amines, the inherent instability of unsubstituted aldimines and the low reactivity of
the more stable N-substituted ones significantly limit their use [9,10]. Furthermore, attempted 1,2-ad-
ditions to aldimines quite often fail due to the tendency of enolizable compounds to undergo deproto-
nation instead of addition [9d]. Among the developments in allylations of aldimines, organoboron com-
pounds have found significant synthetic utility due to the Lewis acidity of boron, which allows it for the
coordination with nitrogen, thus improving the electrophilicity of the aldimines [9].
The
α
© 2006 IUPAC,
Pure and Applied Chemistry
78, 1397–1406
γ
moieties can be found in taxoids,
-pinene-based versatile reagents for asymmetric allyl-, crotyl-, and alkoxyallylboration that
were developed by Brown and coworkers are being commonly used for the synthesis of homoallylic al-
cohols [11]. Due to their ease of preparation and use, ability to reliably deliver all isomers of the homo-
allylic products in very high ee and de, and low cost, they became very attractive reagents for the prepa-
ration of homoallylic amines as well.
Chiral synthesis of homoallylic amines
1399
The purpose of this review is to examine the developments in the utilization of the organoboranes
for the asymmetric allylations of aldimines. Preparation of the homoallylic amines using a variety of
other procedures has been recently summarized [12].
SUBSTRATE-CONTROLLED ADDITIONS
-optically pure N-substituted aldimines using
B
-“allyl”-9-BBN
was first described by Yamamoto and coworkers [13]. These reactions proceeded in high diastereose-
lectivity due to the sterically influenced chirality according to the Cram rules. With the use of properly
matched N-protecting groups, up to 100 % de was obtained in the case of allylboration (eq. 1).
However, application of the same concept to the crotylboration provided the products with only low dia-
stereoselectivity [14].
α
(1)
The use of an
N
-borylimine containing a chiral auxiliary, followed by the addition of alkyllithium
reagents, gave only poor enantioselectivity [15]. A method for the preparation of functionalized homo-
allylic amines from N-chirally modified imines was also developed [16]. As one of the isomers was ob-
tained by the use of methallyl or prenallyl zinc reagents, the other required the use of an allylboronate.
Among the recent developments in the substrate-controlled additions, the reaction of a chiral
N
-
tert
-
butanesulphinamide with potassium allyltrifluoroborate in the presence of catalytic BF
3
(2)
REAGENT-CONTROLLED ALLYLATIONS
Initial developments
-pinene on
N
-trimethylsilylbenzaldimine was done (Fig. 3).
Hence, assorted homoallylic amines were obtained in moderate to good enantioselectivities [1].
Modification of the chiral ligands revealed that while
B
-allyldiisopinocampheylborane (
1
) provided
high ee, norephedrine-derived allyloxazaborilidine (
2
) gave even better results in terms of yield and ee.
Other evaluated reagents, based on Roush’s tartrate (
3
), and camphor-derived sulfonylamino alcohol
(
4
) were inferior as compared to the first two. In further studies, the evaluation was expanded to methal-
lylboration, which proceeded satisfactorily as well [19].
-amino alcohols, and
α
© 2006 IUPAC,
Pure and Applied Chemistry
78, 1397–1406
Allylboration and crotylboration of
OEt
2
, devel-
oped by Batey and coworkers, furnished, after hydrolysis, the desired homoallylic amine in high ee
(eq. 2) [17].
The first report about the utilization of chiral organoboranes for the synthesis of homoallylic amines
came from Itsuno’s group. Having established high reactivity of
N
-trimethylsilylaldimines with triallyl-
boron, successive study of allylations using various chiral “allyl”boron reagents derived from tartrate
esters, diols,
α
1400
P. V. RAMACHANDRAN et al.
Fig. 3
Reagents for the asymmetric allylboration of
N
-silylimines.
The use of non-enolizable
N
-borylimines, obtained by the partial reduction of nitriles with
LiEt
3
BH, followed by the addition of
1
,
2
, or
3
provided the desired amine products [20]. In this case,
the reagent
1
afforded the best results. Interestingly, no influence of the reaction temperature on the
enantioselectivity was observed (eq. 3) [20].
(3)
Allylboration of the
N
-trimethylsilylaldimines was limited to the non-enolizable substrates.
However, this inefficiency was avoided by expansion of the protocol to
N
-aluminoimines. Thus, partial
reduction of nitriles with DIBAL-H provided
N
-aluminoimines, which reacted with
1
to yield the de-
sired amine products. Due to the higher stability of enolizable
N
-aluminoimines, the allylations were
quite successful even in the case of aliphatic substrates [21]. Additionally, the reactions run on poly-
meric support provided the desired amines as well [22].
The addition of camphor-derived functionalized chiral allylboronate to
N
-trimethylsilyl and
N
-alkylaldimines provided the desired homoallylic amines in moderate to good yields and high ee,
however, prolonged reaction times were required (Scheme 1) [23]. In situ formation of
α
-methylene-
-lactams was observed in the cases of the more reactive substrates and with the labile N-protecting
groups.
Scheme 1
Synthesis of α-methylene-γ-lactams.
© 2006 IUPAC,
Pure and Applied Chemistry
78, 1397–1406
γ
Chiral synthesis of homoallylic amines
1401
Kobayashi’s group recently reported a three-component coupling between benzaldehyde, ammo-
nia and Hoffmann’s chiral allylboronate providing the corresponding chiral homoallylic amine,
albeit
,
with low enantioselectivity (eq. 4) [24].
(4)
Investigation of the rate of allylations of
N
-trimethylsilylimines with the reagent
1
with
11
B NMR
spectroscopy revealed that the reactions would not proceed unless a molar equivalent of water was
added to the reaction [25]. Indeed, the yields of several homoallylic amines prepared with the inclusion
of water exhibited improved yields, higher enantioselectivities, and shortened reaction times (Fig. 4,
Table 1). Further study showed that methanol could replace water [26]. Later, the reaction was expanded
to
N
-aluminoimines, which in the reactions with
1
required 1.0 equiv of methanol as well, to provide
homoallylic amines (Fig. 4) [27]. In the case of the less reactive aliphatic substrates, increased reaction
temperature allowed for improved yields, however, the observed ee was somewhat lower.
Fig. 4
Asymmetric allylboration of aldimines with (–)-
B
-allyldiisopinocampheylborane in the presence of 1.0 equiv
of water.
Table 1
Allylboration of
N
-trimethylsilylimines with
1
in the presence and absence of
water at –78 °C.
R
Yield (%)
ee (%)
Without water
a
With water
b
Without water
a
With water
b
Ph
73 (59)
c
90 (90)
c
73 (33)
c
92 (88)
c
2-Cl-(C
6
H
4
)
62
69
46
82
4-MeO-(C
6
H
4
) 65
74
52
92
a
The values obtained from refs. [18b,21].
b
The values obtained from ref. [25].
c
The values in parentheses are for
N
-aluminoimines.
In the postulated reaction mechanism, the addition of methanol allowed for the liberation of a
“naked” C=NH aldimine, which rapidly reacted with the organoboron reagent (Fig. 5). Coordination
between boron and the aldimine’s nitrogen was found to be critical, as evidenced by the relative stabil-
ity of the aldimines to methanolysis [26,27].
© 2006 IUPAC,
Pure and Applied Chemistry
78, 1397–1406
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