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I. L. Rodionov et al. / Tetrahedron 58 (2002) 8515–8523
and dried over magnesium sulfate. The drying agent was
filtered and washed with EtOAc (3£75 ml) and the
combined organic solutions were concentrated under
reduced pressure to give a solid residue, which was
crystallized from ether or ether/hexane.17 The pure product
was collected and washed with cold ether: yields 80–95%
(Table 5).
azine 25–48 (30 mmol) was dissolved in 10% HCOOH–
AcOH (200 ml). An equal weight of 20% palladium
hydroxide on charcoal was added carefully and the reaction
mixture was stirred overnight.11 The catalyst was filtered
and washed with 2£10 ml of 50% acetic acid. The combined
filtrate and washes were evaporated in vacuum to oil. This
was further re-evaporated with water (5£20 ml) to ensure
complete removal of acids. The residue was dissolved in
water (50 ml) and left over activated charcoal for 12 h. The
charcoal was removed by filtration and washed with water
(3£20 ml); the combined aqueous solutions were neutral-
ized with 5% solution of ammonium hydroxide and
concentrated in vacuum to a small volume. As a rule,
crystallization of the product started spontaneously at this
stage, otherwise the process was initiated by addition of
methanol and after keeping the solution at þ48C overnight,
crystalline 1–24 were obtained 83–47% yields (Tables 3
and 4). Samples for analytical purposes were recrystallized
from water–methanol and vacuum dried in Fisher apparatus
over KOH pellets.
1.3.2. Protected dipeptides Boc-D-Gln-Aax(Bzl)-OBzl
(73–76). The title compounds are prepared using a standard
mixed anhydride method starting from Boc-D-Gln-OH
(57 mmol) in 100 ml of DMF,18 freshly distilled isobutyl-
chloroformate (57 mmol), NMM (57þ57 mmol) and
p-toluenesulfonate of dibenzyl ester of Asp or Glu
(57 mmol) dissolved in 100 ml of DMF. Temperature of
the reaction mixture is kept at 2108C throughout dropwise
addition of the reagents and during the next 2 h. After a
standard work-up and crystallization from EtOAc–Et2O–
hexane peptides 73–76 (Table 5) homogeneous by TLC are
obtained in 82–89% yields.
1.3.3. Boc-D-Dab(Z)-Aax(Bzl)-OBzl (68–71). To a stirred
solution of freshly prepared PIFA (85 mmol) in DMF–H2O
(500 ml, 1:1 v/v), Boc-D-Gln-Aax(Bzl)-OBzl 73–76
(55 mmol) is added in one portion at room temperature.
After 15 min pyridine (110 mmol) is added, and stirring is
continued for 3 h.12 The solvents are evaporated in vacuum
and the residue is dissolved in ethyl acetate (700 ml). The
solution is washed extensively with water, 10% sodium
bicarbonate, brine and dried over MgSO4. Solvent is
removed in vacuum and crude product Boc-D-Dab(pTFA)-
Aax(Bzl)-OBzl thus obtained is used in the next step
without further purification. To a stirred solution of the latter
salt (55 mmol) in acetonitrile (300 ml) at 08C, N-methyl-
morpholine (55 mmol) and Z-Cl (55 mmol) are added
consequently in three portions.17 After stirring for 1 h at
08C, the reaction mixture is left overnight at room
temperature. The solvent is removed by evaporation under
reduced pressure and the residue is redissolved in EtOAc
(700 ml) and the solution is washed with 10% sodium
bicarbonate, 5% sodium hydrosulfate, water, brine and is
dried over MgSO4. After solvent removal in vacuum, the
crude dipeptides 68–71 (Table 5) are crystallized from
EtOAc–Et2O–hexane. Yields: 80–84%.
Acknowledgements
The financial support of this study from AFFYMAX
Research Institute (Palo Alto, CA, USA) is gratefully
acknowledged. Authors wish to thank Drs T. Muranova and
L. Markova for amino acid analyses.
References
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from the solution on cooling already after 2 h of heating.
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and precipitated protected DKP was separated by filtration.
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separately, washed with methanol and ether and then
recrystallized from s-BuOH. Total yields of 25–48 were
in the range 50–80% (Table 2).
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1.3.5. Diketopiperazines (1–24). A protected diketopiper-
Rodionov, Igor L
