Straightforward access to protected syn α-amino-β-hydroxy acid derivatives

Article abstract of DOI:10.1002/anie.200800860

(Chemical Equation Presented) Titanium stability: syn α-Amino-β- hydroxy acid derivatives were prepared in excellent diastereoselectivities by an aldol reaction with chiral N-(azidoacetyl)thiazolidin-2-thione derivatives (see scheme; NMP = N-methylpyrrolidinone). Titanium enolates of these derivatives are stable and provide a new and efficient method to access chiral amino acids.

Full text of DOI:10.1002/anie.200800860

Communications
DOI: 10.1002/anie.200800860
Synthetic Methods
Straightforward Access to Protected syn a-Amino-b-hydroxy Acid
Derivatives**
Jignesh Patel, Guillaume ClavØ, Pierre-Yves Renard, and Xavier Franck*
syn a-Amino-b-hydroxy acids are the key structures of many
natural products exhibiting a wide range of biological
activities. For example, syn a-amino-b-hydroxy acids are
found in vancomycin[1] or polyoxins[2] (antibiotics), cyclo-
marins[3] (cytotoxic, anti-inflammatory), ustiloxins[4] (antibi-
otic, antimitotic), and exochelins[5] (iron chelator). Many
studies have been devoted to the synthesis of this unit and
most of them rely on an aldol reaction between a glycine
equivalent[6] and an aldehyde. Among these glycine equiv-
alents, the most effective are those bearing an isothiocyanate
unit as a masked amino group, which proved to be very
effective in either diastereoselective or enantioselective aldol
reactions.[7] However, recovering the free amino alcohol
requires hydrolysis of the resulting oxazolidin-2-thione; this is
not a trivial step as prior transformation of the oxazolidin-2-
Scheme 1. Isothiocyanate vs azide as a masked amino group. a) Iso-
thione into the more easily hydrolyzed oxazolidin-2-one is
needed (Scheme 1).[7c,f] The need for a more straightforward
and flexible approach to the syn a-amino-b-hydroxy acid
moiety justifies the use of the azide goup as a masked amine.[8]
This azide group is a convenient protecting group (Scheme 1)
because it is readily converted into an amine. Nevertheless, it
is well known that enolates of a-azido ketones or esters are
not stable and that they spontaneously decompose into a-
imino ketones or esters.[9] A few reports, however, show that
these enolates can be trapped by electrophiles such as
aldehydes when EtONa or DBU (DBU = 1,8-diazabicyclo-
[5.4.0]undec-7-ene) is used as a base in substoichiometric
amounts to give racemic aldol products in both moderate
yields and diastereoselectivities.[9] To our knowledge, these
are the only examples of using enolates of a-azido ketones or
esters as aldol precursors. We believe that the potential of this
reaction has long been underexploited because of the
instability of the intermediate enolate. We have recently
shown that titanium enolates derived from N-acyl-oxazolidin-
2-thiones were stable and could be used in diastereoselective
aldol reactions by forming the reputedly unstable a-CF3
enolates.[10] Moreover, we have shown that N-acyl-thiazoli-
thiocyanate as a masked amino group; see reference [7]. b) Azide as a
masked amino group; this work. *CpN=oxazolidin-2-one, oxazolidin-
2-thione, or thiazolidin-2-thione.
din-2-thiones[11] could be easily replaced by an ester or an
amide by the simple addition of the corresponding alcohol or
amine, respectively, in the presence of a slight excess of
imidazole. The thiazolidin-2-thiones can act as a chiral
auxiliary, as well as an activated ester.
Herein we report that the a-azido enolates (2) derived
from N-acyl-thiazolidin-2-thione substrates (1) can also be
used in diastereoselective aldol reactions, providing a con-
venient method to access protected syn a-amino-b-hydroxy
acid derivatives (Table 1).
Preparation of (R)-N-2-azidoacetyl-4-phenylthiazolidin-
2-thione (1) was achieved either by direct coupling of 2-
azidoacetic acid[12] with (R)-4-phenylthiazolidin-2-thione
(prepared from d-phenylglycine) in the presence of DCC
(DCC = dicyclohexylcarbodiimide),[10,13] or by the prelimi-
nary formation of the corresponding acid chloride of 2-
azidoacetic acid and subsequent coupling with (R)-4-phenyl-
thiazolidin-2-thione in the presence of Et3N (yields were
usually slightly better than the first method, 70–80%). Aldol
reactions were conducted by using reported procedures:[13,14]
compound 1 in CH2Cl2 was cooled to À788C and treated with
TiCl4 (1.05 equiv), and stirred for 15 minutes. iPr2NEt
(1.1 equiv) was then added to the reaction mixture and
stirred for 1 hour. NMP (2equiv) was added and then the
reaction mixture was stirred for 15 minutes, after which the
aldehyde (1.5 equiv) was added.
[*] Dr. J. Patel, G. ClavØ, Prof. Dr. P.-Y. Renard, Dr. X. Franck
UniversitØ de Rouen
INSA de Rouen
CNRS UMR 6014, C.O.B.R.A.—I.R.C.O.F.
1 Rue Tesniere; 76131 Mont-Saint-Aignan cedex (France)
Fax: (+33)2-3552-2959
E-mail: xavier.franck@insa-rouen.fr
[**] We gratefully acknowledge the CEA for a fellowship to J.P. and the
RØgion Haute Normandie for financial support. Dr. B. Figad›re
(Châtenay-Malabry, France) is also acknowledged for his interest in
this research.
Aldehydes 3a–g readily afforded the syn-aldol products
(4) as single diastereomers (as evaluated by 1H NMR analysis
of the crude reaction mixture) without noticeable degradation
of the enolate (Table 1).[15]
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
Table 1: Diastereoselective aldol reaction and transesterification.
performed on rac-5b and (2R, 3S)-5b, obtained with the
chiral auxiliary 1, showed > 99% ee for (2R, 3S)-5b
(Scheme 3).
Entry Aldehyde (3a–g)
4 [%][a]
5 [%][b] d.r. 5
1
2
3
4
CH3CHO (3a)
PhCHO (3b)
p-NO2PhCHO (3c)
N-tBoc-indol-3-carboxalde-
hyde (3d)
n.a. (4a) 61 (5a) >98:2
75 (4b) 64 (5b) >98:2
76 (4c) 70 (5c) >98:2
70 (4d) 66 (5d) >95:5[c]
Scheme 2. Determination of absolute and relative configurations of
aldol products. CSA=camphorsulfonic acid.
=
5
6
7
CH2 CHCHO (3e)
62 (4e) 68 (5e) >98:2
60 (4 f) 68 (5 f) >95:5[c]
n.a. (4g) 70 (5g) >98:2
=
(E)-C3H7CH CHCHO (3 f)
=
(E)-C13H27CH CHCHO (3g)
As a proof of concept for the efficacy of our methodology,
we synthesized two simple natural products and advanced
intermediates for drug or complex natural product targets.
Indeed, the aldol reaction with aromatic aldehydes provides
entries to aryl-b-hydroxy-a-amino acids that can be found in
numerous biologically active compounds.
[a] Yield of aldol product after chromatography. [b] Yield of ester without
prior purification of the aldol product. [c] Epimerization occurred during
methanolysis (10 equiv imidazole for 5d and 3 equiv imidazole for 5 f);
measured by HPLC and 1H NMR methods. n.a.=not applicable;
product not isolated. NMP=N-methylpyrrolidinone.
For example, the long chain aliphatic aldehyde hexadec-
2en-1-al (3g) provided an attractive route to the threo-
sphingosine skeleton (Scheme 4). Indeed, (2S,3S)-azido-
sphingosine (10g) can be obtained efficiently (58% yield)
The titanium enolate of 1 reacts with either aliphatic,
aromatic, or a,b-unsaturated aldehydes to give the syn-aldol
products (4) in good yields and diastereoselectivities. The
products proved to be quite sensitive to hydrolysis during the
workup, upon standing, and during column chromatography
(particularly for those derived from acetaldehyde or hexadec-
2-en-1-al; Table 1, entries 1 and 7, respectively).[15] Therefore,
we found it beneficial to directly submit the crude mixture to
methanolysis (MeOH/imidazole, 3 equiv of imidazole) to
afford the methyl esters (5) in good yields. Some degree of
epimerization occurred during methanolysis (5% with 5 f and
15% with 5d). Notably, 4 undergoes epimerization, whereas 5
does not or less; the transesterification of 4d with 10 equiv-
alents of imidazole decreased the epimerization to only 5%
(instead of 15% with 3 equiv). Thus, epimerization can be
minimized by using additional amounts of imidazole. The
relative and absolute stereochemistry of the aldol products
were secured by chemical correlation and shown to corre-
spond to that of the Evans aldol product as expected.[13,14]
Indeed, ester 5a was silylated with TBDMSCl (TBDMS =
tert-butyldimethylsilyl) to give 6a, the optical rotation of
which compared well with the literature data.[8] As additional
proof, 4b was directly reduced with diisobutylaluminum
hydride (DIBAL-H) to the diol, which was then protected as
the acetonide (7b) and assigned a cis configuration based on
the measurement of the coupling constants: the measured
value (J < 1 Hz) is typical for Hax.–Heq. coupling constants in
an acetonide (Scheme 2).
Scheme 3. Preparation of rac-5b.
from 1 after the aldol reaction and subsequent reduction
(temporary protection of the hydroxy group is required to
obtain good yields).[16] Reduction of the azido group by
classical Staudinger reaction conditions should lead to known
l-threo-sphingosine,[17] thereby illustrating that this aldol
reaction represents one of the simplest and most efficient
methods, reported so far, for the synthesis of a sphingosine
core.[18]
As another example, we used 4c (R = p-NO2Ph) for the
synthesis of amino alcohol 12c, the enantiomer of a known
precursor of the antibiotic chloramphenicol (Scheme 4). 12c
was prepared in 67% yield over three steps from 1 with
excellent enantiomeric purity (99% ee). Aldol product 4c was
reduced to 11c by using DIBAL-H and the the reduction of
the azido group was accomplished by using a Staudinger
reaction. This is one of the most efficient methods to access
chloramphenicol.[18a,19]
Enantiomeric purity was additionally checked by chiral
HPLC analysis. rac-9b was prepared by using achiral
thiazolidi-2-thione 8 and transesterified to rac-5b. Chiral
HPLC (Daicel, Chiralcel OD-H, 250 ” 4.6 mm, 5 mm) analysis
A last example is provided by derivative 5d, the
enantiomer of which can be found in the cyclomarins, and
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4225

Communications
into a peptidic sequence or reduced to the diols as precursors
to numerous biologically active compounds.
Received: February 12, 2008
Published online: April 24, 2008
Keywords: aldol reaction · amino acids · azides ·
.
synthetic methods · titanium
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