Dialkyl-2.3-dihydrobenzimidazoles have been produced by reduction of the aromatic parent compounds with lithium aluminium hydride and also directly by cyclization of the appropriate nitrenes generated from azides or from nitro-compounds by deoxyyenation with trialkyl phosphites. The factors influencing the stability of the title compounds have been studied. of Salford, Salford M5 4WT, Lancs.THE imidazole ring in benzimidazole (I; R1 = R2 = H) and its N-alkyl derivatives (I; R = alkyl) shows considerable stability towards reduction since hydrogenation over platinum catalysts affects only the benzene ring to produce the tetrahydrobenzimidazoles (11) ; analogously the naphthimidazoles give the corresponding tetraliydronaphthimidazoles.2 With platinum oxide in acetic acid 1,3diacetyl-2,3-dihydrobenzimidazole (I11 ; R1 = R2' = Ac) can be made from benzimidazole (I; R1 = R2 = H) which demonstrates that hydrogenation of the imidazole ring can occur but in the absence of a trapping agent the dihydro-compound reverts to the aromatic structure. Bohlmann claimed to have prepared the 2,3-dihydrobenzimidazole (111; R1 = R2 = H) by reduction of benzimidazole (I ; R1 = R2 = H) with lithium aluminium hydride. However, attempts by us to repeat this reaction failed and it is felt that the evidence for structure (111) which was based on the colour and analysis of its picrate is unsatisfactory. In fact it has been shown that even fairly stable dihydrobenzimidazoles are dehydrogenated by picric acid.The resistance of 2,3-dihydrobenzirnidazoles to aromatization is expected to increase with progressive substitution of its hydrogens with, for instance, alkyl or acyl groups. Thus, the dihydro-compound (111; R1 = R2 = H) can be expected to be much less stable than its
Alkenylalumination of substituted styrene oxides with [alpha-(ethoxycarbonyl)alkenyl]diisobutylaluminum, in the presence of BF(3).Et(2)O, affords the corresponding (Z)-alpha-alkylidene-gamma-aryl-gamma-hydroxy esters in 81-100% Z-selectivity. Chromatographic separation of isomers, followed by lactonization with trifluoroacetic acid, provides isomerically pure (Z)-alpha-alkylidene-gamma-aryl-gamma-butyrolactones in 53-78% overall yield. Isomerization of the (Z)-alkylidene hydroxyl esters using LDA, followed by protonation using a bulky proton source, such as BHT, provides a simple route to the corresponding alpha-(E)-alkylidene-gamma-phenyl-gamma-hydroxy esters in 72-78% yield, which were cyclized to obtain the corresponding (E)-butyrolactones in 78-85% yield.
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