Scaleable catalytic asymmetric Strecker syntheses of unnatural α-amino acids

Article abstract of DOI:10.1038/nature08484

α-Amino acids are the building blocks of proteins and are widely used as components of medicinally active molecules and chiral catalysts. Efficient chemo-enzymatic methods for the synthesis of enantioenriched α-amino acids have been developed, but it is still a challenge to obtain non-natural amino acids. Alkene hydrogenation is broadly useful for the enantioselective catalytic synthesis of many classes of amino acids, but it is not possible to obtain α-amino acids bearing aryl or quaternary alkyl α-substituents using this method. The Strecker synthesisthe reaction of an imine or imine equivalent with hydrogen cyanide, followed by nitrile hydrolysisis an especially versatile chemical method for the synthesis of racemic α-amino acids. Asymmetric Strecker syntheses using stoichiometric amounts of a chiral reagent have been applied successfully on gram-to-kilogram scales, yielding enantiomerically enriched α-amino acids. In principle, Strecker syntheses employing sub-stoichiometric quantities of a chiral reagent could provide a practical alternative to these approaches, but the reported catalytic asymmetric methods have seen limited use on preparative scales (more than a gram). The limited utility of existing catalytic methods may be due to several important factors, including the relatively complex and precious nature of the catalysts and the requisite use of hazardous cyanide sources. Here we report a new catalytic asymmetric method for the syntheses of highly enantiomerically enriched non-natural amino acids using a simple chiral amido-thiourea catalyst to control the key hydrocyanation step. This catalyst is robust, without sensitive functional groups, so it is compatible with aqueous cyanide salts, which are safer and easier to handle than other cyanide sources; this makes the method adaptable to large-scale synthesis. We have used this new method to obtain enantiopure amino acids that are not readily prepared by enzymatic methods or by chemical hydrogenation.

Full text of DOI:10.1038/nature08484

Vol 461
|
15 October 2009
|
doi:10.1038/nature08484
LETTERS
Scaleable catalytic asymmetric Strecker syntheses of
unnatural a-amino acids
Stephan J. Zuend1, Matthew P. Coughlin1, Mathieu P. Lalonde1 & Eric N. Jacobsen1
a-Amino acids are the building blocks of proteins and are widely
In mechanistically and synthetically guided efforts to identify
used as components of medicinally active molecules and chiral simpler small-molecule H-bond donors for enantioselective imine
catalysts1–5. Efficient chemo-enzymatic methods for the synthesis
of enantioenriched a-amino acids have been developed, but it is still
achallengetoobtainnon-naturalaminoacids6,7. Alkene hydrogena-
tion is broadly useful for the enantioselective catalytic synthesis of
many classes of amino acids8,9, but it is not possible to obtain
a-amino acids bearing aryl or quaternary alkyl a-substituents
using thismethod. The Strecker synthesis—the reactionofanimine
or imine equivalent with hydrogen cyanide, followed by nitrile
hydrolysis—is an especially versatile chemical method for the syn-
thesis of racemic a-amino acids10,11. Asymmetric Strecker syntheses
using stoichiometric amounts of a chiral reagent have been applied
successfully on gram-to-kilogram scales, yielding enantiomerically
enriched a-aminoacids12–14. In principle, Strecker syntheses employ-
ing sub-stoichiometric quantities of a chiral reagent could provide a
practical alternative to these approaches, but the reported catalytic
asymmetric methods have seen limited use on preparative scales
(more than a gram)15,16. The limited utility of existing catalytic
methods may be due to several important factors, including the
relatively complex and precious nature of the catalysts and the
requisite use of hazardous cyanide sources. Here we report a new
catalytic asymmetric method for the syntheses of highly enantio-
merically enriched non-natural amino acids using a simple chiral
amido-thiourea catalystto control the key hydrocyanation step. This
catalyst is robust, without sensitive functional groups, so it is com-
patible with aqueous cyanide salts, which are safer and easier to
handle than other cyanide sources; this makes the method adaptable
to large-scale synthesis. We have used this new method to obtain
enantiopure amino acids that are not readily prepared by enzymatic
methods or by chemical hydrogenation.
hydrocyanation, we discovered that amido-thiourea derivatives lack-
ing the diaminocyclohexane moiety in 1 (refs 19, 20) are efficient
catalysts for the hydrocyanation of N-benzhydryl-protected imines
using HCN generated in situ from TMSCN and MeOH (Table 1)21.
Whereas simple amido-thiourea 4a induced low levels of enantioselec-
tivityin hydrocyanation ofbothaliphatic and aromaticimines(entry1
in Table 1), the corresponding phenyl-substituted amido-thioureas
proved more effective (entries 2–4). The highest enantioselectivities
were observed in reactions catalysed by N-benzhydryl-substituted
catalyst 4e (entry 5). This catalyst contains a single stereogenic centre
and is prepared in three steps from commercially available reagents
(74% overall yield on the 5-g scale), and is thus readily accessible
compared with chiral Strecker catalysts identified previously15,16
.
Amido-thiourea derivative 4e proved effective and highly enantio-
selective for the hydrocyanation of imines derived from alkyl (Table 2,
entries1–5), aryl(entries7–16), heteroaryl (entries17–19), and alkenyl
aldehydes (entries 20–22). Lower enantioselectivities are obtained with
less sterically demanding imines (entries 6 and 23). In general, (R)-a-
aminonitriles are obtained from the catalyst derived from (S)-tert-
leucine. This fortuitous outcome introduces an important practical
feature of this method, because (S)-tert-leucine is readily available
inexpensively by enzymatic methods6, but practical catalytic methods
H
N
t-Bu
X
R
N
N
H
H
O
N
HO
1a: X = S, R = polystyrene
1b: X = O, R = Ph
t-Bu
OCO2t-Bu
The urea- and thiourea-catalysed Strecker synthesis of (R)-tert-
leucine developed several years ago by our group17,18 illustrates many
of the factors that have limited the application of catalytic asymmet-
ric imine hydrocyanation methods towards routine preparative-scale
syntheses (Figs 1 and 2). Although this synthesis provides (R)-tert-
leucine in high yield and enantiomeric excess, the hydrocyanation
reaction is run at cryogenic temperatures and uses a hazardous cyanide
source: either trimethylsilyl cyanide (TMSCN)/methanol (MeOH) or
HCN. In addition, the syntheses of either polystyrene-bound catalyst
1a or homogeneous analogue 1b require eight chemical steps, and the
conversion of the a-aminonitrile to the a-amino acid requires four
chemical steps.
O
a
b
HCN, 1a (4 mol.%),
c
H2SO4
toluene
N
N
Ph
H
N
Ph
CN
Ac2O, HCO2H
t-Bu
t-Bu
O
d
e
HCl
H2, Pd/C, MeOH
NH2 • HCl
H
Ph
t-Bu
CO2H
t-Bu
CO2H
84% yield
99% e.e.
2.1-g scale
NH2
O
NH
NH2
CN
H3O+
NH3
HCN
OH
Figure 2
|
First-generation thiourea-catalysed asymmetric Strecker
R
R
R
R
synthesis of tert-leucine. a, HCN (1.3 equiv.), 1a (4 mol.%), toluene, –75 uC,
15 h. b, Acetic anhydride (Ac2O), HCO2H, 5 min. c, 65% aqueous H2SO4,
45 uC, 20 h. d, Concentrated HCl, 13 h. e, H2, Pd/C (0.1 equiv.), methanol
(MeOH), 8 h. Ph, phenyl; t-Bu, tert-butyl; e.e., enantiomeric excess.
O
Figure 1
organic substituent).
|
Strecker synthesis of a-amino acids. (R 5 aliphatic or aromatic
1Harvard University, Department of Chemistry and Chemical Biology, Cambridge, Massachusetts 02138, USA.
968
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NATURE
|
Vol 461
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15 October 2009
LETTERS
Figure 3
|
Potassium cyanide-
CF3
mediated Strecker synthesis.
a, Catalyst 4e (0.5 mol.%), KCN
(2 equiv.), acetic acid (AcOH,
1.2 equiv.), H2O (4 equiv.), toluene,
0 uC, 4–8 h. b, Aqueous H2SO4 and
HCl, 120 uC, 44–68 h. c, NaOH,
NaHCO3. d, Di-tert-butyl
t-Bu
Me
N
S
(0.5 mol.%)
CF3
Ph
b
c
d
H2SO4, HCl, 44–68 h
NaOH, NaHCO3
Boc2O, dioxane, 16 h
N
N
CHPh2
CN
CHPh2
H
H
NHBoc
CO2H
Ph
O
HN
N
a
4e
R
R
R
KCN, AcOH, H2O, toluene, 0 °C, 4–8 h
e
Recrystallize
dicarbonate (Boc2O, 2.5–3 equiv.),
dioxane, 16 h. e, Recrystallize
directly from hexanes/diethyl ether
or as the tert-butylamine (t-
BuNH2) salt from tetrahydrofuran/
ethanol. Me, methyl; Boc 5 tert-
butoxycarbonyl.
87–90% e.e.
98–99% e.e.
NHBoc
CO2H
NHBoc
NHBoc
CO2H
• t-BuNH2
CO2H
t-Bu
5a
5b
5d
62–65% yield
6 to 14-g scale
50–51% yield
3.5-g scale
48–51% yield
4-g scale
do not exist for the synthesis of (R)-tert-leucine and related (R)-amino protected amino acid derivatives 5a, 5b and 5d (Fig. 3). The requisite
acids22.
imines 2a, 2b and 2d (Table 2) were prepared on multi-gram scales in
High yields and enantioselectivities are obtained in these imine one or two steps from commercially available aldehydes24. The hydro-
hydrocyanations catalysed by 4e; however, TMSCN is expensive, and
the stoichiometric HCN generated upon combination with methanol
introduces serious practical and safety liabilities that limit application
on preparative scale. Potassium cyanide (KCN) and sodium cyanide
(NaCN)represent inexpensive, alternative cyanide sourcesfor Strecker
syntheses13,14,23, but these reagents have found limited application in
catalytic asymmetric imine hydrocyanations developed to date. This
may be attributed to the poor solubility of cyanide salts in organic
solvents, and the incompatibility of known catalysts to aqueous
media. In contrast, catalyst 4e lacks any sensitive functional groups,
and therefore might be adaptable to use under aqueous or biphasic
conditions. Indeed, treatment of toluene solutions of imine 2a
with KCN, acetic acid, water and catalyst 4e led to the formation of
a-aminonitriles with enantioselectivity similar to that observed in the
homogeneous,TMSCN/MeOH-mediatedreaction(Fig. 3).Onlysmall
decreases in enantioselectivity were observed at higher temperatures
and concentrations, and reactions carried out under these more prac-
tical conditions proceeded at substantially higher rates.
cyanation reaction mixtures were treated with aqueous K2CO3 before
workup to quench any unreacted HCN generated under the reaction
conditions. The enantiomerically enriched a-aminonitriles were
isolated in crude form by routine extraction and solvent removal pro-
cedures, and converted to the corresponding tert-butoxycarbonyl-
protected (R)-a-amino acids by a two-step sequence involving
N
O
O
t-Bu
OH
Ph
O
H
H
N
PPh2
N
N
N
OMe
MeO
N
N
t-Bu
H
H
O
t-Bu
O
(S)-t-Bu-phox
(ligand for asymmetric catalysis)
Atazanavir (HIV protease inhibitor)
Figure 4
from tert-leucine.
|
Examples of a pharmaceutical product and a chiral ligand derived
Theefficiencyofthisreactionisrelativelyinsensitivetosmallchanges
in reagent and catalyst concentration: using an optimized protocol,
hydrocyanation experiments using 0.5 mol.% catalyst were executed
reproducibly andsafely atthe25–100mmolscale for the preparation of
Table 2
|
Scope of asymmetric imine hydrocyanation
CHPh2
CN
CHPh2
TMSCN (2 equiv.), MeOH (2 equiv.),
HN
N
4e, toluene (0.2 M),
Table 1
catalyst structure
|
Dependence of imine hydrocyanation enantioselectivity on
R
R
–30 °C, 20 h
2
3
CHPh2
CN
CHPh2
N
TMSCN (2 equiv.), MeOH (2 equiv.),
Entry Imine 2 (R 5 )
4e (mol.%) Yield (%) Enantiomeric excess (%)
HN
1
2
3
4
5
6
7
8
9
tert-butyl (a)
Et2MeC (b)
PhMe2C (c)
(1-methyl)cyclohexyl (d)
1-adamantyl (e)
cyclohexyl (f)
p-OMeC6H4 (g)
p-MeC6H4 (h)
C6H5 (i)
2
2
2
2
2
2
2
2
2
2
2
10
2
2
2
2
2
2
2
2
99
99
97
99
99
99
99
98
98
97
98*
96*
98
93
96
85
95
93
74
99
98
98
98
96
93
99
88
97
92
93
97
95
95
91
95
73
4 (5 mol.%), toluene (0.2 M),
R
R
–30 °C, 20 h
2
3
CF3
t-Bu
Me
N
S
Me
N
N
CF3
H
H
O
4a
10 p-ClC6H4 (j)
11 p-CF3C6H4 (k)
12 p-CNC6H4 (l)
13 p-BrC6H4 (m)
14 o-OMeC6H4 (n)
15 o-BrC6H4 (o)
16 m-BrC6H4 (p)
17 2-furyl (q)
18 3-furyl (r)
19 2-thiophenyl (s)
20 1-cyclohexenyl (t)
21 (E)-1-methyl-pent-1-enyl (u) 2
22 (E)-cinnamyl (v)
23 (E)-hexenyl (w)
Me
Me
N
Me
Me
N
Ph
N
Ph
Ph
N
Ph
97
O
Me
4c
O
Me
4d
O
Ph
4e
O
96*
97*
99
98
97*
99
4b
Entry
Catalyst
Enantiomeric excess (%),
R 5 tert-Bu
Enantiomeric excess (%),
R 5 C6H5
1
2
3
4
5
4a
4b
4c
4d
4e
–
14
30
58
77
93
41
86
90
97
98
98
99
97
2
2
Isolated yields of 3 after silica gel chromatography of reactions run on 1.0 mmol scale.
Enantiomeric excess determined by chiral HPLC analysis using commercially available columns.
*Reactions run at 0 uC. Et, ethyl.
Enantiomeric excess determined by chiral HPLC analysis using commercially available columns.
Me, methyl.
969
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LETTERS
NATURE
|
Vol 461
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15 October 2009
Figure 5
|
Proposed catalytic mechanism. The
Ph
H
S
Ph
S
structure in brackets is the iminium/cyanide ion
pair intermediate that directly precedes C–C
bond formation, as calculated using the B3LYP/
6-31G(d) level of density functional theory
within Gaussian 03. Bond distances are shown in
angstroms (italic numbers). Three-dimensional
coordinates are included in the Supplementary
Information.
H
N
H
N
N
H
t-Bu
N
H
H
t-Bu
Me
1.92
N
O
Me
CHPh2
CN
CHPh2
N
N
2.00
O
HN
t-Bu
N
Ph2HC
C
Ph2HC
1.80
+ HCN
3.52
H
N
t-Bu
t-Bu
Ph2HC
H
8. Knowles, W. S. Asymmetric hydrogenation. Acc. Chem. Res. 16, 106
–
112 (1983).
9. Najera, C. & Sansano, J. M. Catalytic asymmetric synthesis of a-amino acids.
Chem. Rev. 107, 4584 4671 (2007).
10. Block, R. J. The isolation and synthesis of the naturally occurring a-amino acids.
Chem. Rev. 38, 501 571 (1946).
11. Miller, S. L. Production of some organic compounds under possible primitive earth
conditions. J. Am. Chem. Soc. 77, 2351 2361 (1955).
H2SO4/HCl-mediated hydrolysis followed by treatment of the result-
ing aqueous amino acid solutions with di-tert-butyl dicarbonate
(Boc2O)25,26. The highly enantiomerically enriched, sterically demand-
ing protected a-amino acidswerethen isolatedonmulti-gram scalesby
recrystallization. In each case, the synthetic sequence required no chro-
matographic purification or specialized equipment.
a-Amino acids bearing quaternary alkyl substituents, especially
tert-leucine, are common components of pharmaceuticals3 or medi-
cinal chemistry targets27, and their derivatives have been found to be
highly effective as components of chiral ligands5 and organocata-
lysts28 used in small-molecule asymmetric catalysis (Fig. 4). Use of
these a-amino acids has largely been limited to (S)-tert-leucine,
which may be prepared efficiently by enzymatic methods6. The
method described in this paper allows for efficient access both to
the (R)-enantiomer of tert-leucine and to more sterically demanding
analogues, thereby expanding the pool of a-amino acids that can be
used in medicinal and other applications.
A detailed experimental and computational29 analysis of the hydro-
cyanation reaction catalysed by 4e points to a mechanism involving
initial amido-thiourea-induced imine protonation by HCN to generate
a catalyst-bound iminium/cyanide ion pair (Fig. 5). Collapse of this ion
pair andC–C bondformation to formthea-aminonitrilethen occurs in
a post–rate-limiting step. Complete details of this novel mechani-
stic hypothesis and the basis for enantioselectivity will be reported
separately30.
´
–
–
–
12. Harada, K. Asymmetric synthesis of a-amino acids by the Strecker synthesis.
Nature 200, 1201 (1963).
13. Kuethe, J. T., Gauthier, D. R. Jr, Beutner, G. L. & Yasuda, N. A concise synthesis of
(S)-N-ethoxycarbonyl-a-methylvaline. J. Org. Chem. 72, 7469
14. Shu, L. & Wang, P. Synthesis of enantiopure Fmoc-a-methylvaline. Org. Process
Res. Dev. 12, 298 300 (2008).
15. Gro¨ger, H. Catalytic enantioselective Strecker reactions and analogous syntheses,
Chem. Rev. 103, 2795 2827 (2003).
–7472 (2007).
–
–
´
´
16. Merino, P., Marques-Lopez, E., Tejero, T. & Herrara, R. P. Organocatalyzed
Strecker reactions. Tetrahedron 65, 1219 1234 (2009).
17. Sigman, M. S., Vachal, P. & Jacobsen, E. N. A general catalyst for the asymmetric
Strecker reaction. Angew. Chem. Int. Edn Engl. 39, 1279 1281 (2000).
18. Doyle, A. G. & Jacobsen, E. N. Small-molecule H-bond donors in asymmetric
catalysis. Chem. Rev. 107, 5713 5743 (2007).
19. Wenzel, A. G., Lalonde, M. P. & Jacobsen, E. N. Divergent stereoinduction
mechanisms in urea-catalyzed additions to imines. Synlett 1919 1922 (2003).
20. Reisman, S. E., Doyle, A. G. & Jacobsen, E. N. Enantioselective thiourea-catalyzed
additions to oxocarbenium ions. J. Am. Chem. Soc. 130, 7198 7199 (2008).
21. Mai, K. & Patil, G. Alkylsilyl cyanides as silylating agents. J. Org. Chem. 51,
3545 3548 (1986).
–
–
–
–
–
–
22. Be´langer, E., Pouliot, M.-F. & Paquin, J.-F. Use of 5,5-(dimethyl)-i-Pr-PHOX as a
practical equivalent to t-Bu-PHOX in asymmetric catalysis. Org. Lett. 11,
2201–2204 (2009).
23. Ooi, T., Uematsu, Y. & Maruoka, K. Asymmetric Strecker reaction of aldimines
using aqueous potassium cyanide by phase-transfer catalysis of chiral quaternary
ammonium salts with a tetranaphthyl backbone. J. Am. Chem. Soc. 128,
2548–2549 (2006).
24. Anderson, J. C., Denton, R. M., Hickin, H. G. & Wilson, C. Synthesis of
METHODS SUMMARY
Reactions were carried out in round-bottomed flasks under nitrogen, unless
otherwise noted. Commercially available reagents were purchased and used as
received unless otherwise noted. Catalysts, imines and a-amino acids were char-
acterized by nuclear magnetic resonance (NMR) and infrared spectroscopy, and
by mass spectrometry. The enantiomeric excess of chiral, non-racemic a-amino
acids was determined by chiral high-performance liquid chromatography
(HPLC) analysis of the benzyl ester derivatives. a-Aminonitriles were character-
ized by NMR and infrared spectroscopy, and the enantiomeric excesses were
determined by chiral HPLC analysis. For experimental details and spectroscopic
characterization data, chiral HPLC traces of racemic and non-racemic a-ami-
nonitriles and benzyl esters of a-amino acids, 1H and 13C NMR spectra of catalyst
4e and a-amino acids, and the geometry of the calculated intermediate, see the
Supplementary Information.
dibenzofuran-1,4-diones using the Dotz benzannulation. Tetrahedron 60,
2327–2335 (2004).
25. Corey, E. J. & Grogan, M. J. Enantioselective synthesis of alpha-amino nitriles from
N-benzhydryl imines and HCN with a chiral bicyclic guanidine as catalyst. Org.
Lett. 1, 157–160 (1999).
26. Krueger, C. A. et al. Ti-catalyzed enantioselective addition of cyanide to imines. a
practical synthesis of optically pure alpha-amino acids. J. Am. Chem. Soc. 121,
4284–4285 (1999).
27. Arasappan, A. et al. Practical and efficient method for amino acid derivatives
containing beta quaternary center: application toward synthesis of hepatitis C
virus NS3 serine protease inhibitors. Tetrahedr. Lett. 48, 6343–6347 (2007).
28. Mita, T. & Jacobsen, E. N. Bifunctional asymmetric catalysis with hydrogen
chloride: enantioselective ring-opening of aziridines catalyzed by a
phosphinothiourea. Synlett 1680–1684 (2009).
Full Methods and any associated references are available in the online version of
the paper at www.nature.com/nature.
29. Frisch, M. J. et al. Gaussian 03 Revision E.01 (Gaussian, Inc., 2004).
30. Zuend, S. J. & Jacobsen, E. N. Mechanism of amido-thiourea catalyzed
enantioselective imine hydrocyanation: transition state stabilization via multiple
non-covalent interactions. J. Am. Chem. Soc. doi:10.1021/ja9058958 (in the press).
Received 18 May; accepted 28 August 2009.
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Supplementary Information is linked to the online version of the paper at
www.nature.com/nature.
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3. Tsantrizos, Y. S. Peptidomimetic therapeutic agents targeting the protease
enzyme of the human immunodeficiency virus and hepatitis C virus. Acc. Chem.
Acknowledgements This work was supported by the NIH, and through fellowships
from the American Chemical Society and Roche (to S.J.Z.) and the Natural
Sciences and Engineering Research Council of Canada (to M.P.L.).
Res. 41, 1252
4. Davie, E. A. C., Mennen, S. M., Xu, Y. J. & Miller, S. J. Asymmetric catalysis
mediated by synthetic peptides. Chem. Rev. 107, 5759 5812 (2007).
5. Helmchen, G. & Pfaltz, A. Phosphinooxazolines—a new class of versatile, modular
P,N-ligands for asymmetric catalysis. Acc. Chem. Res. 33, 336 345 (2000).
–1263 (2008).
–
Author Contributions S.J.Z. and M.P.L. synthesized and evaluated the catalysts;
M.P.C. evaluated the scope of the TMSCN-mediated reaction; S.J.Z. developed the
KCN-mediated syntheses and the large-scale procedures; S.J.Z. and E.N.J. wrote
the manuscript; E.N.J. guided the research.
–
6. Bommarius, A. S., Schwarm, M. & Drauz, K. Comparison of different
chemoenzymatic process routes to enantiomerically pure amino acids. Chimia 55,
Author Information Reprints and permissions information is available at
www.nature.com/reprints. Correspondence and requests for materials should be
addressed to E.N.J. (jacobsen@chemistry.harvard.edu).
50
7. Breuer, M. et al. Industrial methods for the production of optically active
intermediates. Angew. Chem. Int. Edn Engl. 43, 788 824 (2004).
–59 (2001).
–
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doi:10.1038/nature08484
mixture was allowed to warm to room temperature over 5 min. The septum
was removed, and the reaction mixture was treated with 50 ml of a 0.2 g ml21
aqueous K2CO3 solution. The mixture was transferred to a 250-ml separatory
funnel in a fume hood, the reaction flask was rinsed with diethyl ether (3 3 5 ml),
and the rinses were added to the separatory funnel. The organic and aqueous
layers were thoroughly mixed, and the aqueous layer removed. The organic layer
was washed with another 50 ml of K2CO3 solution and then with brine (50 ml).
The aqueous layers were disposed in a waste container that was maintained at
basic pH and stored in a fume hood. The clear, colourless organic layer was dried
over Na2SO4, decanted into a 500-ml round-bottomed flask, rinsing with diethyl
ether (3 3 5 ml), and concentrated to a volume of approximately 100 ml using a
rotary evaporator. The flask was then charged with a 2-cm-long stir bar, placed in
a 25 uC water bath, and concentrated to a volume of approximately 15 ml by
vacuum transfer into a dry ice/acetone bath. A sample of the clear, colourless
liquid residue was analysed by chiral HPLC analysis (Chiralpak AS-H column,
Chiral Technologies), 1 ml min21, 5% iso-propanol/hexanes, 220 nm): retention
time tR(minor) 5 6.18 min, tR(major) 5 9.09 min, 87–88% enantiomeric excess
(range of three experiments). The liquid was transferred to a 100-ml round-
bottomed flask, rinsing with CH2Cl2 (3 3 4 ml). The solution was concentrated
to a mass of 12 g under reduced pressure (30 torr down to 1 torr). 1H-NMR
analysis of the clear, colourless oil revealed approximately 30 mol.% remaining
toluene. Crude 3a was used in the next step without further purification.
Experimental procedures for the hydrolysis of 3a and for the isolation of
(R)-Boc-tert-leucine are provided in the Supplementary Information.
METHODS
Preparation of a-aminonitrile 3a by KCN-mediated hydrocyanation. Caution!
HCN is produced. The experiment should be executed in a well-ventilated fume
hood.
A 250-ml round-bottomed flask containing a 4-cm-long stir bar was charged
with KCN (5.21 g, 80 mmol, 2.0 equiv.) and toluene (76 ml), capped with a virgin
rubber septum, and cooled at 0 uC for 10 min under N2. Acetic acid (2.75 ml,
48 mmol, 1.2 equiv.) and water (2.88 ml, 160 mmol, 4.0 equiv.) were added
sequentially via syringe, and the N2 inlet was removed. The resulting white,
heterogeneous mixture was stirred vigorously at 0 uC. After 5 min the upper
organic layer had become a clear, colourless solution, and the lower aqueous
layer contained a chunky, white precipitate. After stirring for 20 min, the N2 inlet
was restored, and a freshly prepared stock solution of 2a (9.79–9.93 g, prepared
from 40 mmol of aminodiphenylmethane as described in the Supplementary
Information) and 4e (116 mg, 0.20 mmol, 0.0050 equiv.) in toluene (24 ml)
was added via syringe in 10-ml portions over 1 min. The flask containing the
stock solution was rinsed with additional toluene (2 3 3 ml), and the rinses were
added to the reaction. The N2 inlet was removed, and the mixture was stirred at
0 uC. The reaction was monitored as follows: a 100 ml aliquot was removed via
syringe, filtered through a 1-cm-high plug of Na2SO4, rinsed with hexanes
(2 3 3 ml), and concentrated under reduced pressure. The sample was dissolved
in 600 ml CDCl3 and analysed by 1H-NMR spectroscopy. After 2.5 h, conversion
was estimated to be 95% by integration of the benzhydryl resonances of the
starting material (5.4 p.p.m.) and product (5.2 p.p.m.). After 4 h, the reaction
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