Synthesis of Substituted Benzocrowns

Because mono and dibenzocrowns were among the first crowns to be synthesized, and because the syntheses of these molecules were relatively straightforward, quite a number of modifications in the basic skeleton have been reported. Some of these modifications have utilized substituted catechols as starting materials and others have involved transformations on the existing crown. 

Of the many methods which have been published so far for the substitution of existing crowns, probably the most straightforward are Friedel-Crafts alkylation or acylation reactions. Cygan, Biernat and Chadzynski have reported the successful di-t-butylation of dibenzo-24-crown-8 using t-butanol as alkylating agent s . The crown was heated at 100° for 4 h in the presence of excess t-butanol and 85% phosphoric acid. The product was obtained as a crystalline (mp 52—74°) solid in 93% yield. The alkylated crowns are presumably a mixture of isomers substituted once in each ring as illustrated in Eq. (3.14). 

Tusek has reported the Clemmensen reduction of acylated crowns such as shown above. This approach constitutes a useful two-step alkylation procedure, although the yields reported for this sequence are not as high as one might wish. Other examples of closely related acylation approaches can be found in recent work by Tashmukhamedova and coworkers ° ° and by Kauer °. 

We have noted above that benzocrowns may be nitrated quite readily. This approach was used in the formation of a photoresponsive bis-crown (see Sect. 3.8) wherein the nitrobenzo crowns reductively dimerize to the corresponding azobenzene. Kikukawa, Nagira and Matsuda have utilized 4-nitrobenzo-15-crown-5 in a somewhat different way during the synthesis of 4 -vinylbenzo-l 5-crown-5Nitration is effected using nitric acid in a mbcture of chloroform and acetic acid. 

Hydrogenation reduces the nitro group to amino which is then diazotized using sodium nitrite and tetrafluoroboric acid. The diazotized crown was not isolated but the aq. solution was treated directly with sodium acetate and bis(dibenzylideneacetone)-pal-ladium(O) in acetonitrile solution. Ethylene was then introduced to the autoclave and the solution was allowed to stir for 2 days. 4 -Vinylbenzo-15-crown-5 was isolated (30% from 4 -nitrobenzo-15-crown-5) as a colorless solid (mp 43.5—44.2°) °. The synthesis is illustrated in Eq. (3.16). 

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