1.12- Dimethylbenzo phenanthrene

Nitration of 2,4-dimethylbenzo[/i]quinoline gives the yields at the 7-and 9-positions shown in 11.79 (77MI1), neither position being conjugated with nitrogen. The reason for the higher yield of the 9-isomer is not obvious, especially since in phenanthrene the positional order in nitration (9 > 1 > 3 > 2 > 4) predicts the 7-position of 11.79 to be most reactive. 

In thermolysing (93) one observes, by the evolution of dimethylamine, the first cyclization step at 160°, no doubt to the intermediate phenanthrene (94), clearly differentiated from the second one at 220°. The latter cyclization leads like the above (89) to a (1 Immixture (90%) of two isomeric dinitriles, the 2,11-dimethyl-benzo(c)phenanthrene-4,9-dicarbonitrile (95) and the 2,9-dimethylbenzo(a)-anthracene-4,11-dicarbonitrile (96), mp. 312—313°. 

A better chromatographic separation was achieved after hydrolysis and decarboxylation to the hydrocarbons, the 2,1 l-dimethylbenzo(c)phenanthrene, mp. 

The history of the chemistry of l,12-dimethylbenzo[c]phenanthrene (2) resembles that of 4,5-dimethylphenanthrene (1). The first symthesis of 2 was reported by Newman and Wolf in 1952 [9]. The synthetic sequence included preparation of diacid 67 for the Friedel-Crafts acylation reactions to form diketone 68 (Scheme 15). Reduction followed by dehydration and dehydrogenation then produced 2. 

Similarly, l,12-dimethylbenzo[c]phenanthrene-5-acetic acid (69) (Fig. 17) was later synthesized and resolved [62]. However, unlike 4,5,8-trimethyl-l-phenanthre-neacetic acid (9a), which undergoes rapid helix inversion at room temperature, 69 is optically stable in refluxing mesitylene at 163°C for several days. The methyl ester of 69 was also found to be optically stable at temperature up to 250°C for 30 min. 

Several 5,8-disubstituted l,12-dimethylbenzo[c]phenanthrenes, including diacid 70 [63, 64], diacetylene 71 [65], diamine 72 [66], and the tetranitro derivative 73 [66], have also been synthesized and resolved. The charge transfer complex between (M)-73 and electron-rich (M)-72 was found to be more stable than the complex between (M)-73 and (P)-12. 

For substituted benz[a]anthracenes and benzo[cj-phenanthrenes, there is related destabilization. Examples include the 3,9- and l,12-dlmethylbenz[a]-anthracenes with their gas-phase enthalpies of formation of 188.7 3.9 and 251.5 3.9 kJ/mol, although why the latter should be more stable than the 7,12-isomer with its enthalpy of formation of 277.7 4.4 kJ/mol is not apparent. The same trends apply to both, qualitatively and quantitatively, in the solid state. This is fortunate because the corresponding examples for dimethylbenzo[c phenanthrene, the... 

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