THE "GAS-SOLID-PHASE" 2,5-DIOXOPIPERAZINE SYNTHESIS. CYCLIZATION OF VAPOROUS DIPEPTIDES ON SILICA SURFACE

Article abstract of DOI:10.1135/cccc19940461

The "gas-solid-phase" method is used for the preparation of both symmetric and asymmetric 2,5-dioxopiperazines via cyclization of vaporous linear dipeptides in the presence of silica.

Full text of DOI:10.1135/cccc19940461

Short Communication
461
THE “GAS–SOLID-PHASE” 2,5-DIOXOPIPERAZINE SYNTHESIS.
CYCLIZATION OF VAPOROUS DIPEPTIDES ON SILICA SURFACE
Vladimir A. BASIUK and Taras Yu. GROMOVOY
Institute of Surface Chemistry,
Academy of Sciences of the Ukraine, Prospekt Nauki 31, Kiev 252650, Ukraine
Received November 26, 1992
Accepted April 19, 1993
The “gas–solid-phase” method is used for the preparation of both symmetric and asymmetric 2,5-di-
oxopiperazines via cyclization of vaporous linear dipeptides in the presence of silica.
Recently we have reported on the “gas–solid-phase” synthesis of 2,5-dioxopiperazines
(DOPs) via cyclodimerization of vaporous amino acids on silica surface1. The pre-
requisite for realization of this method is the ability of amino acids to sublime in va-
cuum without decomposition, intermolecular condensation and loss of optical activity2.
Besides amino acids themselves, this property is characteristic also for some simple
dipeptides, i.e. dipeptides derived from aliphatic bifunctional amino acids2. Although
DOPs can be detected in dipeptide sublimates, their yields, according to Gross and
Grodsky2, do not exceed 7%.
Taking into account that in the course of amino acid cyclodimerization on silica
surface dipeptides must be formed as intermediates1,3, we supposed that dipeptides
themselves can be used as starting materials for the “gas–solid-phase” DOP synthesis.
By this method it would be possible to obtain both symmetric and asymmetric DOPs.
In the present communication we report on the synthesis of several DOPs, including
optically active ones, via cyclization of vaporous linear dipeptides in the presence of
silica.
EXPERIMENTAL
HPLC analyses were performed on Milikhrom 1A microcolumn chromatograph (Nauchpribor, Orel,
Russia) with UV detection at 220 – 240 nm. Stainless steel microcolumns, 64 × 2 mm, were packed
with Silasorb 600 silica gel (4 µm) and reversed phase Separon C18 (5 µm), both from Chemapol
(Prague, The Czech Republic). Ligand-exchange chromatography of amino acid enantiomers was per-
formed on a reversed phase coated with N-octyl-L-proline (see ref.4 and literature cited herein). Ace-
tate buffer solution (pH 5.21)–acetonitrile, 4 : 6 (v/v), and 10−4 M Cu(NO3)2 in acetate buffer solution
of pH 5.21 were used as mobile phases. Optical rotations were determined on Perkin–Elmer 141
spectropolarimeter. Fast-atom-bombardment mass spectra were obtained on MX 1310 mass spec-
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Basiuk, Gromovoy:
trometer (Nauchpribor, Orel, Russia); DOP samples were in the form of suspensions in glycerol. En-
ergy of bombarding argon atoms was 3.5 keV; intensity of bombarding beam, about 1 µA; angle of
incidence to the target, 70° to the normal.
Dipeptides H-Gly-Gly-OH, H-Gly-DL-Ala-OH, H-Gly-DL-Leu-OH, H-Gly-L-Leu-OH, H-Gly-L-Val-OH,
DL-H-Ala-DL-Ala-OH, L-H-Ala-L-Ala-OH and L-H-Val-L-Val-OH were purchased from Reanal (Buda-
pest, Hungary). Silokhrom C-120 macroporous silica from Reakhim (Russia), specific surface area
about 120 m2/g, was preliminarily heated at 400 °C in the air for 3 h to remove adsorbed contamina-
tions.
The “gas–solid-phase” DOP synthesis was performed in the simple vacuum reactor used earlier1,4
.
General Procedure
Dipeptide (0.5 – 2.0 g) and silica (2 – 5 g) are placed into the reactor connected to a vacuum source
and pumped out to vacuum of about 10 Pa. The bottom zone of the reactor is heated to 180 – 210
°C for 1 – 3 h. Sublimate, containing DOP and unreacted dipeptide, condenses in the reactor cold
zone as a crust. To increase the conversion of linear dipeptide to DOP, the sublimate is returned to
the reaction zone, and the procedure is repeated; duration of the second sublimation cycle is 1 – 2 h.
Crude product is extracted from the reactor and recrystallized from water or water–ethanol.
For the ligand-exchange HPLC analyses of enantiomeric composition, DOP samples are hydro-
lyzed with 6 M HCl for 24 h at 100 °C in evacuated ampules.
RESULTS AND DISCUSSION
As starting materials only dipeptides derived from aliphatic amino acids containing no
other functional groups can be used (otherwise the synthesis can be complicated by the
formation of by-products in appreciable amounts due to increasing a number of reaction
channels on the hot silica surface). The dipeptides were sublimed in the presence of
macroporous silica at reduced pressure and temperatures of 180 – 210 °C. After the
first sublimation cycle, DOPs condensed contain small amount of unreacted dipeptides.
The process should be repeated once more to increase the conversion of dipeptides and
to obtain pure DOPs. Indeed, HPLC analyses of crude products did not reveal appreci-
able amounts of the starting dipeptides.
Mechanism of the cyclocondensation, similarly as for amino acids3, includes tran-
sient formation of a surface ester of orthosilicic acid and may be represented by
Scheme 1.
The method proposed can be considered as a rough analogue to the solid-phase DOP
synthesis5 – 8, from which any solvent and protection–deprotection steps are excluded.
The system, which is considered in the present case, has the following important
feature. At temperatures about 200 °C the silica surface still contains appreciable
amount of adsorbed water and catalyzes not only condensation of amino acids and
peptides, but also hydrolysis of the latter3. If the main process may be represented by Eq. (A)
(A)
A and A′, amino acids fragments
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463
the following undesireable reactions leading to two symmetric DOPs are possible in
principle (Eqs (B) – (D)).
(B)
(C)
(D)
We have tested such possibility by means of HPLC and have found that the correspond-
ing by-products are formed in negligible amounts. For cyclo(-Gly-L-Leu-) the content
of cyclo(-L-Leu-L-Leu-) and cyclo(-Gly-Gly-) made up less than 1% even in the crude
sublimate (Fig. 1). Similar results were obtained also for other asymmetric DOPs spe-
cified in Table I.
Crude products were recrystallized from water or water–ethanol mixtures. Yields of
purified DOPs, melting points, optical rotation values in comparison with literature data
and corresponding peaks in fast-atom-bombardment mass spectra are presented in
Table I. For cyclo(-Gly-L-Val-) and cyclo(-L-Ala-L-Ala-) optical rotation values are
fairly close to those reported in literature. For cyclo(-L-Val-L-Val-) and cyclo(-Gly-L-
Leu-) the obtained optical rotations were found to be somewhat lower than the lit-
erature values that points to the racemization. In order to characterize this possibility in
more detail, optically active DOPs prepared have been hydrolyzed to amino acids, and
enantiomeric composition of the latter has been examined by the ligand-exchange chro-
SCHEME 1
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matography on C18 reversed phase coated with N-octyl-L-proline4; for comparison the
same procedure has been performed with the starting dipeptides. The results are
presented in Fig.2. In the case of cyclo(-Gly-L-Leu-) appreciable racemization is ob-
FIG. 1
The HPLC control of cyclo(-Gly-L-Leu-) purity. Column: 64 × 2
mm; stationary phase: Silasorb 600, 4 µm; mobile phase: acetate
buffer (pH 5.21)–acetonitrile, 4 : 6 (v/v); flow rate: 100 µl/min;
temperature: ambient; detection: UV at 230 nm. 1 cyclo(-L-Leu-L-
Leu-); 2 cyclo(-Gly-L-Leu-); 3 cyclo(-Gly-Gly-)
FIG. 2
Enantiomeric composition of hydrolyzates of optically active DOPs and starting linear dipeptides by
the ligand-exchange HPLC. Hydrolysis: evacuated ampules, 6 M HCl, 24 h, 100 °C; column: 64 × 2
mm; stationary phase: Separon C18 (5 µm) coated with N-octyl-L-proline; mobile phase: 10−4
M
Cu(NO3)2 in acetate buffer solution pH 5.21; flow rate: 100 µl/min (A) and 50 µl/min (B, C); tem-
perature: ambient; detection: UV at 240 nm. Hydrolyzates: A: H-Gly-L-Leu-OH (a) and cyclo(-Gly-
L-Leu-) (b);
1 H-Gly-OH; 2 L-H-Leu-OH; 3 D-H-Leu-OH. B: H-Gly-L-Val-OH (a) and
cyclo(-Gly-L-Val-) (b); 1 H-Gly-OH; 2 L-H-Val-OH. C: L-H-Val-L-Val-OH (a) and cyclo(-L-Val-L-
Val-) (b); 1 L-H-Val-OH; 2 D-H-Val-OH
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served (about 20% of initial L-H-Leu-OH is present in the D-form); in the case of
cyclo(-L-Val-L-Val-) approximately equal amounts of D-H-Val-OH are present both in
the DOP and in commercially available L-H-Val-L-Val-OH, i.e., racemization is insig-
nificant. For cyclo(-Gly-L-Val-) racemization is not observed. Alanine enantiomers (for
cyclo(-L-Ala-L-Ala-)) are not resolved using chromatographic conditions described.
The authors are grateful to A. M. Glukhoy and Dr V. G. Golovaty (L. V. Pisarzhevsky Institute of
Physical Chemistry, Academy of Sciences of the Ukraine) for performing the fast-atom-bombardment
mass spectra.
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