D
A. A. Tsygankov et al.
Letter
Synlett
the novel protocol were similar to those of the classical
two-step route, the one-step procedure is operationally
more convenient and less time-consuming. The resulting
chiral amines were used as organocatalysts in two-step
syntheses of α-hydroxy γ-keto esters from arenes; this per-
mitted the preparation of methyl (S)-2-hydroxy-4-oxo-2-
phenylpentanoate in 96.5:3.5 e.r. The absolute configura-
tion of the product was determined by means of anomalous
X-ray scattering. Some of the prepared diamines were also
tested as chiral ligands for Henry and Meervein–Ponndorf–
Verley reactions.
(i) Clegg, W.; Harington, R. W.; North, M.; Pizzato, F.; Villuendas,
P. Tetrahedron: Asymmetry 2010, 21, 1262. (j) Zhu, X.; Lin, A.;
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(10) Diamines 3a–k; One-Step General Procedure
Acknowledgment
The work was supported financially by the Russian Science Founda-
tion (Grant # 16-13-10393). We thank Dr. Dmitry Usanov for his
feedback on this manuscript.
A glass vial in a 10-mL stainless-steel autoclave was charged
with Rh2(OAc)4 (0.7 mg, 1.60 μmol, 1 mol%), (1R,2R)-cyclohex-
ane-1,2-diamine dihydrochloride (30.0 mg, 0.160 mmol, 100
mol%), K2CO3 (26.6 mg, 0.192 mmol, 120 mol%), i-PrOH (0.1 mL),
H2O (0.1 mL), and the appropriate aldehyde (0.320 mmol, 200
mol%). The autoclave was then sealed, flushed three times with
CO (5 atm), and pressurized with CO (50 atm). The reactor was
placed in a preheated oil bath (140 °C). After 4 h, the reactor was
cooled to r.t. and depressurized. The residue was collected by
filtration and analyzed by 1H NMR.
Supporting Information
Supporting information for this article is available online at
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References and Notes
(11) Schiff Bases 2a–k; General Procedure
A mixture of (1R,2R)-cyclohexane-1,2-diaminium (S)-tartrate
(1.0 equiv), K2CO3 (1.0 equiv), and H2O (0.66 mL per mmol of
K2CO3) was stirred until the solids were completely dissolved
and then MeOH (5.2 mL/mmol of tartrate) was added. The
mixture was heated to 65 °C and a solution of the appropriate
aldehyde (2 equiv) in MeOH (2.2 mL/mmol of tartrate) was
added over 30 min. The mixture was refluxed for an additional
4 h, then cooled to r.t. and concentrated in vacuo. The residue
was dissolved in EtOAc (4 mL/mmol of tartrate) and the solution
was washed with H2O (2 × 1 mL/mmol of tartrate), dried (Na2-
SO4), and concentrated in vacuo to give a beige-colored crude
product. The crude product containing some starting aldehyde
was used in the next step without further purification.
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(1R,2R)-2,2′-[Cyclohexane-1,2-diylbis(nitrilomethy-
lylidene)]diphenol (2i)
Yellow oil; yield: 3.1 g (82%). 1H NMR (300 MHz, CDCl3): δ = 8.32
(s, 2 H), 7.34–7.26 (m, 2 H), 7.21 (dd, J = 7.6, 1.5 Hz, 2 H), 6.95 (d,
J = 8.2 Hz, 2 H), 6.89–6.82 (m, 2 H), 3.43–3.32 (m, 2 H), 2.06–
1.88 (m, 4 H), 1.87–1.68 (m, 2 H), 1.62–1.46 (m, 2 H).
(12) Diamines 3a–k; General Procedure from Schiff Bases 2a–k
NaBH4 (2.1 equiv) was added portionwise over 40 min to a solu-
tion of the appropriate Schiff base 2 (1.0 equiv) in MeOH (4
mL/mmol of Schiff base) at r.t. The mixture was refluxed with
stirring for 1 h then cooled to r.t. H2O (5 mL/mmol of Schiff
base) was added, the mixture was extracted with CH2Cl2 (3 × 4
mL/mmol of Schiff base), and the organic layer was concen-
trated. If any aldehyde remained in the mixture, the residue was
dissolved in 35% aq HCl (1 mL/mmol of Schiff base) and the
solution was washed with CH2Cl2 (3 × 4 mL/mmol of Schiff
base). Excess K2CO3 (4.5 equiv) was added to the aqueous phase,
which was extracted with CH2Cl2 (3 × 3 mL/mmol of Schiff
base). The organic layers were combined, dried, and concen-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E