Study On The Synthesis Of Heterocyclic Homotriptycenes

Triptycene and its derivatives have attracted increasing interests in various fields, such as molecular machines, host-guest complexes, supramolecular chemistry, materials science and pharmaceutical properties owing to their unique rigid, aromatic 3D structure in recent 20 years. Triptycene chemistry has been a new dynamic and interdisciplinary research field. A homotriptycene is a homolog of the triptycene and also has unique rigid, aromatic 3D structure. However, homotriptycene has few reports due to the synthetic difficulties which badly limit the rapid development of homotriptycene chemistry. There were fewer reports on heterocyclic homotriptycenes.In recent years our group have found a simple novel route for the synthesis of homotriptycenes in high yields by an acid-catalyzed intramolecular 1,7-electrophilic dehydrations of 10,10-dibenzyl-9,10-dihydroanthracen -9-ols. In the present paper a series of novel heterocyclic homotriptycenes bearing furan, thiophen and pyridine ring 5a-f was synthesized by introducing rich-electron heterocyclic ring. The electron density in the heterocyclic ring (thiophene, furan, pyridine) is a key factor of the acid-catalyzed reaction to give heterocyclic homotriptycenes. When the heterocyclic ring linked in 10-position of anthracenols has a moderate electron density, such as such as 2-thenyl or 5-chloro-2-thenyl,two competitive reactions exist, which afford 9-monosubstituted anthracenes 6a,b by 1,4-elimination or 9,10-disubstituted anthracenes 7a,b by a rearrangement, respectively. The attached naphthalene ring has lower electron density, two competitive reactions change to be major reactions to afford 9-monosubstituted anthracenes 6m,n and 9,10-disubstituted anthracenes 7m,n; Electron-deficient pyridine heterocycles undergo a disproportionation to give oxidated prducts ketones 3i-l and reduced products hydrocarbons 15i-l. Specific content as follows.(1) A series of novel heterocyclic homotriptycenes bearing furan and thiophen ring 5a-e were synthesized.The dialkylation of anthrone was obtained by the reaction of anthrone and the chloro- or bromo- methyl five membered heterocyclic compounds 2a-e in acetone to obtain 10,10-bisheterocyclic methyl anthracenone (66-84%). The 9-anthracenol derivatives 4a-e were then synthesized in high yield (92-98%) by the reduction of 3a-e with NaBH4 at room temperature. 4a-e were treated with formic acid to obtain thiophen homotriptycenes 6a-c and furan homotriptycenes 6d,e. In the same time two kinds of by-products 9-monosubstituted anthracenes 6a,b and 9,10-disubstituted anthracenes 7a,b were separated. The mechanism research shows that homotriptycenes are afforded by intramolecular 1,7-electrophilic dehydrations of 4a-e. The acid-catalyzed reactions were closely related with electron density in the heterocyclic groups. The 1,7-elimination reaction activities are accord with the electron density (furan > thiophen) and the attack position (2-position > 3-position in thiophen and furan ring), so 4e > 4d > 4c > 4a,b. When the heterocyclic ring has a moderate electron density, such as 2-thenyl or 5-chloro-2-thenyl (compounds 4a,b), there exist two competitive reactions, namely the 1,4-elimination to monosubstituted anthracenes 6a,b and the rearrangement to 9,10-disubstituted anthracenes 7a,b. Some factors such as different acids and their amount, different solvents were changed to inhibit side reactions and enhance the selectivity of homotriptycenes 6d,e.(2) Two pyridine homotriptycenes 5f,g were synthesized.The electron-poor pyridine system must be introduced electron-releasing groups such as methoxy in order to obtain pyridine homotriptycenes. 5f was easily synthesized, the dialkylation of anthrone and 2-bromomethyl-6-methoxy pyridine 2f in acetone to obtain 10,10-bis(6-methoxypyridin-2-ylmethyl)-9(10H)anthrone 3f. The 9-anthracenol 4f were then synthesized by the reduction of 3f with NaBH4 at room temperature. 4f was treated with p-toluene sulfonic acid in CHCl3 to obtain pyridine homotriptycenes 5f in 90% yield. 5g was difficultly synthesized because its halogenated hydrocarbon is difficult to prepare, we firstly prepared 2-bromo-4-iodomethylpyridine 2i then 10,10-bis(2-bromo-4-pyridinylmethyl) -9(10H)anhracenone 3i, methoxy group was introduced to pyridine ring by nucleophilic substitution at 30 atm to give 10,10-bis(2- methoxypyridin-4-ylmethyl)- 9(10H)anthrone 3g, 3g was reduced by NaBH4 to give the anthronol 4g, 4g was treated with p-toluene sulfonic acid in ClCH2CH2Cl to obtain pyridine homotriptycenes 5g in 82%.(4) The disproportionation of the electron-poor pyridine anthracenols was researched.Picolyl chloride hydrochlorides are not active enough to react with anthrone in refluxing acetone, then improve the reaction temperature and reflux for 2 h in toluene to afford 10,10-bispyridinylmethyl-9(10H)-anthrones 3j-l in 63~68% yields. It is also difficult to reduce 3j-l to 9,10-dihydro-10,10-bispyridinylmethyl-9-anthronols 4j-l with NaBH4 at room temperature, then reduction of 3j-l at 80?for 4 h afforded anthronols 4j-l in 87~90% yields. Treatment of 4j-l with p-toluenesulfonic acid and BF3 in boiling toluene led to a disproportionation to oxidized product anthrones 3j-l and reduced products 9,10-dihydroanthracenes 15j-l. The disproportionation with p-TsOH as catalyst gave major products anthrones 3j-l. when BF3 as catalyst more reduced products, 9,10-dihydro-9,9-dipyridinylmethylanthracenes 15j-l, were gived Reduction of 3i at 40?with NaBH4 afforded anthronols 4i in 94% yield. Treatment of 4i with p-toluenesulfonic acid and BF3 in boiling toluene led to a disproportionation which was similar to 4j-l. Therefore, the disproportionation with BF3 as catalyst can prepare 9,10-dihydro-9,9-disubstituted anthracenes which are ofen difficult to prepare with other methods.(4) The acid-catalyzed reactions of 10,10-Bisnaphthalylmethyl-9,10-dihydro anthracen-9-ols 4m,n were researched.10,10-Bis(naphthalylmethyl)-9(10H)anthrones 3m,n were synthesized by dialkylation of anthrone and halomethyl naphthalene 2m,n in acetone in 70-85% yields . Reduction of 3m,n with NaBH4 at room temperature to give 10,10-Bisnaphthalylmethyl-9,10-dihydro anthracen- 9-ols 4m,n (97-99%). Treatment 4m,n with acids to obtain 9-monosubstituted anthracenes 6m,n by the 1,4-elimination and 9,10-disubstituted anthracenes 7m,n by the rearrangement. These anthracene derivatives have strong fluorescence and maybe implied in materials chemistry and photochemistry.