Acid-Catalyzed Cyclization of Anthracenol Derivatives to Homotriptycenes

Abstract

10-Benzyl-9,10-dihydroanthracen-9-ols, having high electron densities in the benzene ring, exhibit in the presence of acid a transannular ring closure to the corresponding homotriptycenes in almost quantitative yields. Since the starting compounds are easily accessible from 9(10H)-anthracenone, this process represents the most facile route to such pentacyclic systems. An electron-releasing methoxy group enables the intramolecular electrophilic substitution in its para position. In the absence of such an activation, a number of alternative processes can occur, namely the acid-catalyzed dehydration to anthracene derivatives with (R not = H) or without (R = H) rearrangement or a disproportionation reaction of the secondary alcohol to the corresponding ketone and hydrocarbon.

Electrophilic Aromatic Substitution

“…34 Reaction of α-alkylcinnamaldehydes with ortho-esters, alcohols, or thiols in the presence of BF 3 .OEt 2 forms 1-alkoxy-2-alkylindenes [e.g. (22) from α-methylcinnamaldehyde and HC(OMe) 3 ].…”

“…39 Compound (33) is formed in 98% yield and 94% ee from the 2-acylimidazole (32) and pyrrole at −40 • C. A series of enantiomeree ically pure aziridin-2-ylmethanols has been tested as catalysts in the alkylation of N -methylpyrrole and N -methylindole by α,β-unsaturated aldehydes. 40 Enantiomeric ee excesses of up to 75% were observed for the alkylation of N -methylpyrrole by (E)-crotonaldehyde using (2S,3S)-3-methylazirin-2-yl(diphenyl)methanol TFA salt as catalyst to form (34).…”

Synthesis of Fréchet-type dendritic homotriptycenes

“…According to a method described earlier [33] , compound 4a was easily synthesized in 98% yield by the reduction of 3a with NaBH 4 in diglyme over night at room temperature. However, the desired product 4b could not be sufficiently obtained under the same reaction conditions.…”

“…The acid-catalyzed reactions of 10-benzyl-or 10, 10-dibenzyl-9-anthracenols can be affected by the substituents in the benzene rings, and can principally involve many competitive reactions, such as 1,4-elimination of H 2 O to anthracene, or disproportionation to ketone and hydrocarbon [33] . The desired transannular ring closure of carbenium ion 6 to homotriptycene required a high nucleophilicity in the position α of the substituted benzene ring (Scheme 2).…”

“…However, only few reports about homotriptycenes have been described [29][30][31] . We recently reported on a simple novel route for the synthesis of homotriptycenes from anthrancenol derivatives [32,33] .…”