O-Vinylamidoximes 2 are synthesized from amidoximes 1 and acetylene in superbase systems (KOH/DMSO, KOH/NMP, NaOH/CsF/NMP) in a preparative yield of up to 80%. They decompose explosively above 150°C, being resistant to solvolysis in the KOH/DMSO system (100°C, 3 h) and to polymerization (AIBN, UV). Trifluoroacetylation of O-vinylbenzamidoxime 2b with (CF 3 CO) 2 O/pyridine gives 5-trifluoromethyl-3-phenyl-1,2,4-oxadiazole (3) in 49% yield.
Dendrons 8, 13, and 17 were subsequently attached to (E)-2,3-bis[(triisopropylsilyl)ethynyl]but-2-ene-1,4-diol (18) [4] [21] using the Mitsunobu reaction [22] to give the dendritic silyl-protected monomers 19 ± 21, respectively (Scheme 2). The yield of the third-generation compound 21 was very low (4%), possibly due to steric hindrance of the reacting COOH group by the bulky dendritic wedges in dendron 17. After deprotection with Bu 4 NF in wet THF, the free (E)-enediynes 22 ± 24 were obtained. The dendritic wedges substantially stabilize the usually rather unstable free (E)enediynes [23], and compounds 22 ± 24 can be stored in the air at ambient temperature for months without decomposition. Oxidative Hay coupling [24] of 22 ± 24 in the presence of PhCCH as an end-capping reagent [21] provided the oligomeric PTAs as solids. The first-generation compound 22 afforded separable oligomers up to the pentamer (1a ± e), which extend in length from 19.4 (1a) to 49.4 (1e) [11] [21]. For steric reasons, the second-generation derivative 23 yielded isolable oligomers only up to the trimer (2a ± c). Finally, due to severe steric overcrowding, conversion of the third generation enediyne 24 gave only end-capped monomer and dimer (3a, b) in pure form and sufficient yields.Analytical gel-permeation chromatography (GPC) proved to be extremely useful for monitoring the purification of our compounds. The separation of the oligomers was achieved by preparative GPC (Bio-Beads S-X1; CH 2 Cl 2 ), and the purity of the fractions was determined by analytical GPC because, specifically for the higher oligomers, thin-layer chromatography (TLC) was useless. With other techniques such as 1 H-and 13 C-NMR spectroscopy, we could not detect impurities (lower and higher oligomers) with less than 10% abundance, while analytical GPC was sensitive for impurities with an abundance of less than 1%. The analytical GPC traces of the firstgeneration dendritic oligomers 1a ± e are shown in Fig. 4. Optical detection occurred at 300 nm in the absorption region of the aromatic wedges. GPC Separations of the higher-generation compounds were equally efficient.The molecular constitution of the dendritic rods was confirmed by matrix-assisted laser-desorption-ionization mass spectra (MALDI-TOF-MS; matrix: 9-nitroanthracene), which depicted either the [M Na] or [M K] ions as base peaks, and NMR spectra. In the 1 H-NMR spectra of the centrosymmetrical oligomers, the number of the t-Bu resonances increases with the number of monomeric units ( Table 1). Similar behavior was observed for the aromatic protons positioned between the two alkoxy substituents of the central benzene rings that serve as linkers between PTA backbone and dendritic wedges.
The Friedel-Crafts sulfonylation for the synthesis of diaryl sulfones derivatives was achieved by using triflic acid (TfOH) as the acidic catalyst with aryl sulfonyl chlorides and arenes at elevated temperatures. Different sulfones could be synthesized with this method in good to excellent yield and with high selectivity. In contrast to the previous methodology reported for this sulfonylation, the catalyst and the excess of arene could be easily recovered and re-used in further batches.
Pyrazine derivatives
Pyrazine derivatives R 0550Cyclization of N-(1,1-Dimethyl-2-propynyl)acetamide (I) to 2,2,3,5,5,6-Hexamethyl-2,5-dihydropyrazine (II). -(VASIL'TSOV, A. M.; SHMIDT, E. Y.; MIKHALEVA, A. I.; ZAITSEVA, A. B.; ARNDT, J.-D.; HENCKELMANN, J.; TROFIMOV, B. A.; Chem. Heterocycl.
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