Synthesis of Some 3-Aryl-1 , 2 , 4-oxadiazoles Carrying a Protected L-Alanine Side Chain

1,2,4-Oxadiazoles are an important class of compounds. Many of them have been found to possess biological activity. For example, some are analgesics, anti-inflammatory agents, antimicrobials, antivirals, pesticides and insecticides. Some have pronounced β-adrenoreceptor blocking activity combined with moderate α-adrenoreceptor blocking properties among others. Recently, emphasis has been given to synthesize oxadiazoles having novel functional groups attached either to C-3 or C-5 of the 1,2,4-oxadiazole ring. Sokolov et al. prepared 1,2,4-oxadiazoles by the reaction of lactone 1,4benzodioxin-2(3H)-one with amidoximes in an aprotic polar solvent such as DMSO or dioxane at 90-140 °C esp. 100-105 °C. A generalized and efficient synthesis of 1,2,4oxadiazoles from 1,2,5 oxadiazoles has also been described by Buscemi and collaborators. Synthesis and reaction of lithiated oxadiazoles have been reviewed by Grimmet and Iddon. Improved synthesis of oxadiazoles under microwave irradiation conditions was also studied by Oussaid et al. and Srivastava and colaborators. In our continuing program to discover more biologically potent 1,2,4-oxadiazoles, we attempted to synthesize oxadiazoles having an amino acid function attached at C-5 from benzamidoximes 1a-d with Nand Oprotected aspartic acid having a terminal carboxyl function free (2). These products might be potential compounds for biological activity tests. A literature search revealed that no such oxadiazoles have yet been prepared. This paper therefore describes the synthesis of four oxadiazoles 4a-d having an alanine moiety attached to C-5 of the heterocyclic ring.

In our continuing program to discover more biologically potent 1,2,4-oxadiazoles 12b,c , we attempted to synthesize oxadiazoles having an amino acid function attached at C-5 from benzamidoximes 1a-d with N-and O-protected aspartic acid having a terminal carboxyl function free (2).These products might be potential compounds for biological activity tests.A literature search revealed that no such oxadiazoles have yet been prepared.This paper therefore describes the synthesis of four oxadiazoles 4a-d having an alanine moiety attached to C-5 of the heterocyclic ring.

Results and Discussion
When amidoximes 1a-d were reacted with N-and Oprotected aspartic acid 2 in the presence of dicyclohexylcarbodiimide (DCC) in dichloromethane at room temperature, the starting amidoxime was consumed in a short time as evidenced by thin-layer chromatography.
Purification by liquid chromatography on a silica gel column using n-hexane-ethyl acetate (9:1) as eluent provided the products presumably 3a-d with have Rf values (≤ 0.6), slightly higher than benzamidoximes (Rf ≤ 0.4).Compounds 3a-d were obtained as solids.However, no effort was made to identify them with precision.It is common that an amidoxime forms an O-acyl product when allowed to react with a carboxylic acid 13 .Therefore, it is safe to assume that the structures of 3a-d are the ones as shown in the Scheme 1.
An examination of the 1 H-NMR spectrum of compound 4a showed the following signals: H-2' at δ 5.42 as a broad multiplet.This is due to the coupling with two protons attached on C-3' and the NH proton.The benzylic protons appeared at δ 5.12 as a singlet.The protons on C-3' are not equivalent and provided two sets of signals -one at δ 3.29 (ddd, 1H, J ≈ 17.00 Hz, J ≈ 3.00 Hz and J ≤ l.00 Hz) and the other at δ 3.12 (dd, 1H, J = 17.00Hz, J = 5.04 Hz).The NH proton gave a broad unresolved doublet at δ 5.75 indicating its coupling with H-2'.Addition of D2O caused the disappearance of this signal thus confirming its identity.1 H-NMR chemical shifts of compounds 4a-d are given in Table 1.
It is necessary to comment about compounds 4a-d.All four compounds gave negative specific rotations (Table 2).Three of them, 4a-c, have rotations between -17.0° to -21.5°, but 4d showed [α]D 25 equal to -6.4°.Since the intermediates 3a-d were heated at an elevated temperature for cyclization, there existed the posibility of either partial or total racemization of 4a-d.In order to clarify this point, we carried out the experiment by adding the chiral shift reagent, tris[3-(trifluoromethylhydroxymethylene(+)cam-phorato]3 europium derivatiave, directly in the NMR tube and obtained the spectrum each time after adding the shift  reagent.The object was to see if H-2 gives two signals after the complexation occurs.The H-2' signal of compound 4a moved 45.0Hz downfield after two such additions.The NH proton also moved to lower field by 21.0 Hz.At each small addition of the shift reagent, we tried to amplifly the region between δ 5.6-5.0 ppm.However, no separation of the H-2' signal was observed.Compound 4d showed similar downfield shift without any separation of the H-2' signal.With this observation, we feel that there was no racemization of compounds 4a-d.

Experimental
Melting points were determined with a Thomas Hoover apparatus and are uncorrected.Elemental analyses of compounds 4a,b,d were performed in the Laboratoire de Spectrométrie de masse de 1' Université de Montpellier II, France, and 4c was done by Luzia Narimatsu of Instituto de Química da Universidade de São Paulo, SP.Infrared spectra were recorded on a Bruker spectrophotometer Model IFS66.300 MHz 1 H-NMR spectra were recorded on a Varian Unity plus instrument, using CDCl3 as solvent and TMS as internal reference.Thin-layer chromatography (tlc) was done on plates coated with silica gel having flluorescent indicator (Merck) and the spots were detected under ultraviolet light.Specific rotations were measured on JASCO polarimeter Model DIP-370.

Arylamidoximes
These compounds were obtained by the method reported in the literature 14 .

O-(N-t-Butyloxycarbonyl-O-benzyl-L-alanylcarbonyl) arylamidoximes (3a-d)
The appropriate arylamidoxime (2.13 mmol) in dry dichloromethane (10 mL) was allowed to react with N-butoxycarbonyl-O-benzyl-L-aspartic acid 15 (2.13 mmol), in the presence of dicyclohexylcarbodiimide (2.35 mmol) for l h at room temperature.The product obtained was chromatographed on a silica gel column using hexane-ethyl acetate (6:4) as eluent.The fractions having the desired product were combined and the solvent removed under reduced pressure.The yields were approximately 80%.

Conclusions
We have been able to show that O-and N-protected aspartic acid having a free terminal carboxyl function reacts with arylamidoximes at room temperature to give the intermediates 3a-d.These intermediates are easily transformed to 4a-d by heating at 100-110 °C.It is also concluded that the heating conditions which we employed did not cause any noticiable racemization.

Table 2 .
Some physical data of compounds 4a-d.
+The yields are based ou the conversion of compouds 3a-d to 4a-d.