Exploiting organocatalysis: Enantioselective synthesis of vinyl glycines by allylic sulfimide [2,3] sigmatropic rearrangement

Article abstract of DOI:10.1002/anie.200701459

(Chemical Equation Presented) Have it both ways: Enantiomerically enriched E vinyl glycines may be accessed from aldehydes in a concise sequence that combines an organocatalytic a sulfenylation, stereoselective olefination, a sulfimidation, and a [2,3] si

Full text of DOI:10.1002/anie.200701459

Angewandte
Chemie
DOI: 10.1002/anie.200701459
Asymmetric Synthesis
Exploiting Organocatalysis: Enantioselective Synthesis of Vinyl
Glycines by Allylic Sulfimide [2,3] Sigmatropic Rearrangement**
Alan Armstrong,* Lee Challinor, and Jennifer H. Moir
Amino acids—as fundamental building blocks of peptides and
proteins—are important targets for enantioselective synthe-
sis. In particular, efficient syntheses of non-proteinogenic
amino acids can provide access to modified peptides as
valuable biological probes. Several outstanding and versatile
techniques exist for the catalytic and enantioselective syn-
thesis of amino acids,[1a] and include the asymmetric hydro-
genation of amidoacrylates[1b] and the phase-transfer cata-
lyzed alkylation of glycine enolates.[1c] However, there are
several highly interesting types of amino acid to which these
Scheme 1. Chirality transfer in the allylic sulfimide rearrangement.[7]
Boc=tert-butoxycarbonyl, nHex=n-hexyl.
methods cannot generally be applied. One such class are the
b,g-unsaturated amino acids (vinyl glycines) 1. This motif is
present in several biologically significant targets, but can be
difficult to access, even in racemic form, particularly if the
stereocontrolled incorporation of the alkene is desired.[2] Few
catalytic asymmetric methods for their synthesis are currently
available.[3]
An important current trend in synthesis is the develop-
ment of transition-metal-free methods, particularly those in
which small organic molecules are used as catalysts (organo-
catalysis).[4] As well as avoiding the use of costly and
potentially toxic metal catalysts, these transformations often
have the practical benefits that they do not require rigorously
anhydrous or anaerobic reaction conditions.
process. These findings were important since earlier attempts
to effect amination/rearrangement on these esters had
afforded products with low yield and ee values.[6g] However,
a major limitation with this approach is that in all cases from
our studies and of others,[6g,8] the chiral a-branched sulfides
required for the rearrangement have been prepared from a
precursor from the chiral pool, (S)-methyl lactate 3. As well as
making one enantiomer of the amino acid product far more
accessible than the other, this has the severe limitation in that
the terminal alkene substituent is inevitably methyl.
In a continuation of our studies on transition-metal-free
reagents for heteroatom transfer, we have developed the
novel oxaziridine 2, which acts as an efficient source of
electrophilic nitrogen bearing a synthetically useful protect-
ing group (Boc).[5] Recently, we used this reagent to extend
the scope of the amination/[2,3] sigmatropic rearrangement
of allylic sulfides,[6] and showed for the first time (two
examples) that this transformation can be effected efficiently
on a,b-unsaturated esters (Scheme 1).[7] Essentially complete
1,3-chirality transfer was observed in the rearrangement
An alternative approach for the synthesis of the allylic
sulfide precursors would greatly expand the scope of the
process. Recently, Jorgensen and co-workers reported a
highly enantioselective a sulfenylation of aldehydes using
the proline-derived organocatalyst 7 and the triazole sulfide
8a as the electrophile.[9] The sensitive a-sulfenyl aldehyde 9
was reduced in situ to give the alcohol 10 with excellent
enantioselectivity (Scheme 1). We reasoned that, if the
intermediate aldehyde could be trapped by an in situ olefi-
nation,[10] this chemistry could potentially offer a one-pot,
organocatalytic route to a range of chiral a-branched allylic
sulfides 11 (Scheme 2). Herein we report that this concept can
indeed be realized, which led to a concise transition-metal-
free catalytic enantioselective synthesis of Boc-protected
vinyl glycines.
[*]Prof. A. Armstrong, L. Challinor
Department of Chemistry
Imperial College London
South Kensington Campus
London SW72AZ (UK)
Fax: (+44)20-7594-5804
Exploratory experiments revealed that allylic sulfides 11
with R2 = Bn, as would be generated using 8a, reacted with
oxaziridine 2 to give mixtures of sulfoxidation and sulfimida-
tion. In line with our earlier studies,[7] we were pleased to find
that chemoselective nitrogen transfer could be restored when
E-mail: A.Armstrong@imperial.ac.uk
Dr. J. H. Moir
Organon Newhouse
Lanarkshire ML15SH (Scotland)
À
the less sterically demanding S nHex reagent 8b was
[**]We thank the EPSRC and Organon (CASE award to LC) for their
support of this work, and Bristol–Myers Squibb, Pfizer, and Merck
Sharpe and Dohme for unrestricted funding.
employed. Use of 8b in the a-sulfenylation/in situ reduction
was found to proceed with comparable enantiomeric excess to
the reported values with 8a. We therefore carried out an
in situ olefination of the aldehyde 9 (R2 = nHex). After
screening several sets of conditions (see the Supporting
Supporting information (including experimental procedures and
compound characterization) for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 2: [2,3]Sigmatropic rearrangement of enantioenriched E allylic
sulfides.
Entry
R1
ee of
Yield of
ee of
(E)-11 [%][a]
12 [%][b]
12 [%][a]
1
2
3
4
5
6
Me
Et
iPr
Bn
Allyl
(CH2)2OTBS
93
93
91
88
90
91
79
79
81
85
87
81
93
93
91
87
89
91
Scheme 2. Asymmetric a sulfenylation/reduction of aldehydes.
a) 10 mol% 7, 8, toluene, RT, 3 h; b) NaBH4, MeOH; c) olefination.
Bn=benzyl, TMS=trimethylsilyl.
[a]Determined by HPLC on a chiral stationary phase [b]Yield of isolated
product after flash chromatography.
Information), we were able to effect the formation of 11 with
high E/Z selectivity and minimal racemization; use of a low
reaction temperature, CH2Cl2 as the solvent, and a phospho-
nate anion, generated with nBuLi, proved optimal.
With a successful one-pot synthesis of an enantiomerically
enriched allylic sulfide accomplished, we explored the scope
of the reaction by employing several commercially available
aldehydes (Table 1). The reaction was completely tolerant of
structures shown in Figure 1.[11] If we assume a concerted
suprafacial [2,3] sigmatropic rearrangement, the reaction can
occur via TS1, which would lead to (E)-12, or by TS2, which
Table 1: One-pot asymmetric a sulfenylation/olefination of aldehydes
6a–g.
Entry
R1
E/Z[a]
(E)-11 Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
Me (a)
Et (b)
iPr (c)
tBu (d)
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
52
79
70
61
64
73
62
93
93
91
89
88
90
91
Figure 1. Rationale for the stereochemical control observed in the
[2,3]sigmatropic rearrangement of allylic sulfimides.
Bn (e)
Allyl (f)
(CH2)2OTBS (g)[d]
[a]Determined by inspection of the 1H NMR spectrum of the crude
reaction mixture. [b]Yield of the isolated E alkene after flash chromatog-
raphy. [c]Determined by HPLC on a chiral stationary phase. [d]TBS =
tert-butyldimethylsilyl.
would lead to the product with Z-alkene geometry and the
opposite configuration at the newly formed stereocenter. The
essentially exclusive formation of the E product in the
rearrangement of (E)-11 suggests that TS1 is significantly
lower in energy than TS2, probably because of a destabilizing
allylic interaction between R1 and R3 in TS2. It is also worth
noting that the sulfimidation process produces a stereogenic
center at the sulfur atom. Generally, reaction of branched
(non-allylic) sulfides with 2 proceeds with low levels of
stereocontrol at the sulfur atom.[12] If we assume this is also
the case for the sulfimide intermediates here, the high yields
and E selectivity in the final product suggest that the
sulfimide configuration does not exert a significant influence
on the stereochemical outcome of the rearrangement.
the size of the b substituent on the aldehyde (Table 1,
entries 1–4), with even a tert-butyl group giving excellent
results (entry 4). Several other functional groups (such as
benzyl, allyl, and OTBS) also participated effectively with
consistently high asymmetric induction being obtained
(Table 1, entries 5–7), which demonstrates the robustness of
the method.
With a versatile route to a range of enantioenriched
E allylic sulfides 11 in hand, we next tested them in the
amination/rearrangement reaction. Pleasingly, all examples
reacted with 2 to give 12 with essentially complete chirality
transfer and only the E product was observed (Table 2).
The stereochemical outcome of these rearrangements
may be rationalized by considering the transition-state (TS)
An efficient entry into products with the opposite
absolute configuration is a valuable requirement for any
enantioselective synthetic method. Rather than employing
the opposite enantiomer of catalyst 7, which would be derived
from more expensive d-proline, the TS analysis in Figure 1
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À
suggested a more attractive alternative. With TS1 expected to
again be preferred to avoid R1–R3 interactions, the substrate
(Z)-11 should afford the opposite configuration at the newly
generated stereocenter in (E)-12 (R2 and R3 interchanged).
To explore this approach we required in situ olefination
conditions that would afford the Z allylic sulfide selectively.
Again, following optimization to prevent racemization, we
were able to accomplish this by using the diphenylphospho-
nate reagent reported by Ando et al.[13] with a good level of
Z/E selectivity (ca. 5:1; Table 3). As predicted, the Z isomer
cleanly underwent amination/rearrangement with 2, to give
the opposite enantiomeric series, again with very high
E selectivity and essentially complete chirality transfer.
to develop a one-pot amination/rearrangement/N S bond
cleavage sequence, which offered improved practicality and
overall yields (Table 4). Importantly, migration of the alkene
into conjugation was not observed, and HPLC analysis on a
chiral stationary phase in one example confirmed that
racemization had not taken place (Table 4, entry 4).
In conclusion, we have developed a novel concise
transition-metal-free catalytic enantioselective synthesis of
vinyl glycines, biologically important targets that are difficult
to access by using current synthetic technology. The proce-
dure combines an organocatalytic a sulfenylation of an
aldehyde with a stereospecific [2,3] sigmatropic rearrange-
ment. Either enantiomeric product series can be obtained
from the same chiral catalyst through the choice of E- or Z-
selective olefination. In principle, this strategy will also be
applicable to the enantioselective synthesis of a wide range of
other important nitrogen-containing building blocks by vary-
ing the olefination partner. Efforts to exploit this concept
further along these lines are currently underway.
Table 3: Asymmetric synthesis and [2,3]sigmatropic rearrangement of
enantioenriched Z allylic sulfides.
Received: April 4, 2007
Published online: June 11, 2007
Keywords: amino acids · organocatalysis ·
.
sigmatropic rearrangement · sulfimides · vinyl glycines
Entry
R1
E/Z[a]
Yield of
(Z)-11
[%][b]
ee of
Yield of
ent-12
[%][d]
ee of
(Z)-11
ent-12 [%][c]
[%][c]
[1] a) For a review of catalytic enantioselective amino acid syn-
thesis, see: J. A. Ma, Angew. Chem. 2003, 115, 4426; Angew.
Chem. Int. Ed. 2003, 42, 4290; b) for an example, see: M. J. Burk,
Acc. Chem. Res. 2000, 33, 363; c) M. J. OꢀDonnell, Acc. Chem.
Res. 2004, 37, 506; B. Lygo, B. I. Andrews, Acc. Chem. Res. 2004,
37, 518; K. Maruoka, T. Ooi, Chem. Rev. 2003, 103, 3013.
[2] a) For a recent review, see: D. B. Berkowitz, B. D. Charette,
K. R. Karukurichim, J. M. McFadden, Tetrahedron: Asymmetry
2006, 17, 869; b) for a recent synthetic approach to racemic
E vinyl glycines, see: P. A. Alexander, S. P. Marsden, D. M. M.
Subtil, J. C. Reader, Org. Lett. 2005, 7, 5433; c) for a recent
diastereoselective route to enantiomerically enriched quater-
nary E vinyl glycines, see: M. C. Jones, S. P. Marsden, D. M. M.
Subtil, Org. Lett. 2006, 8, 5509.
[3] a) For a recent method for catalytic enantioselective amino acid
synthesis that includes one example of a vinyl glycine, see: J.
Wolfer, T. Bekele, C. J. Abraham, C. Dogo-Isonagie, T. Lectka,
Angew. Chem. 2006, 118, 7558; Angew. Chem. Int. Ed. 2006, 45,
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Chem. Soc. 2003, 125, 5634.
1
2
Et
Allyl
1:5
1:5
64
73
93
92
82
82
93
92
[a]Determined by inspection of the 1H NMR spectrum of the crude
reaction mixture [b]Isolated yield of ( Z)-alkene after flash chromatog-
raphy [c]Determined by HPLC on a chiral stationary phase [d]Isolated
yield after flash chromatography.
For most synthetic applications, it is probable that
À
cleavage of the N S bond will be required; the presence of
the alkene, allylic C N bond and acidic a proton render this
transformation potentially problematic. We were able to
accomplish the desired desulfurization rapidly and cleanly on
stirring with triethylphosphite and triethylamine at room
temperature (Table 4). These mild conditions also allowed us
À
Table 4: Asymmetric synthesis of N-Boc-protected E vinyl glycine deriv-
atives.
[4] P. I. Dalko, L. Moisan, Angew. Chem. 2004, 116, 5248; Angew.
Chem. Int. Ed. 2004, 43, 5138.
[5] a) A. Armstrong, R. S. Cooke, Chem. Commun. 2002, 904; b) A.
Armstrong, R. S. Cooke, S. E. Shanahan, Org. Biomol. Chem.
2003, 1, 3142; c) A. Armstrong, L. H. Jones, J. D. Knight, R. D.
Kelsey, Org. Lett. 2005, 7, 713.
Entry
R1
Yield of 13
Yield of 13
from 12 [%][a]
from 11 [%][b]
[6] For reviews of sulfimide chemistry, see: a) P. C. Taylor, Sulfur
Rep. 1999, 21, 241; b) T. L. Gilchrist, C. J. Moody, Chem. Rev.
1977, 77, 409; for enantioselective sulfimidation of prochiral
allylic sulfides, see: c) H. Takada, Y. Nishibashi, K. Ohe, S.
Uemura, C. P. Baird, T. J. Sparey, P. C. Taylor, J. Org. Chem.
1997, 62, 6512; d) M. Murakami, T. Katsuki, Tetrahedron Lett.
2002, 43, 3947; e) M. Murakami, T. Uchida, B. Saito, T. Katsuki,
Chirality 2003, 15, 116; for recent work on the catalytic
formation of N-Boc-protected sulfimides, see: f) T. Bach, C.
1
2
3
4
5
Me
iPr
Bn
Allyl
88
94
94
92
93
85
78
81
74[c]
84
(CH2)2OTBS
[a]Yield of isolated 13 using a purified sample of 12 as starting material.
[b]Yield of isolated product from a one-pot reaction. [c]HPLC on a chiral
stationary phase indicated no racemization.
Angew. Chem. Int. Ed. 2007, 46, 5369 –5372
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Communications
Körber, Eur. J. Org. Chem. 1999, 1033; g) T. Bach, C. J. Körber,
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J. Org. Chem. 2000, 65, 2358; for the rearrangement of allylic
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[10] For recent examples of organocatalytic aldehyde a functional-
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Treweeke, J. Org. Chem. 2006, 71, 4028.
[8] For earlier applications of the allylic sulfimide rearrangement in
synthesis, see: a) R. E. Dolle, K. I. Osifo, C.-S. Li, Tetrahedron
Lett. 1991, 32, 5029; b) R. E. Dolle, C.-S. Li, R. Novelli, L. I.
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5372
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