**3. Conclusions**

38 Current Trends in X-Ray Crystallography

1*Z*,8*E*-Ac2AN→1*E*,8*E*-*anti*-Ac2AN 21.46

1*Z*,8*E*-Ac2AN→1*E*,8*E*-*syn*-Ac2AN 23.46

1*E*,8*E*-*anti*-Ac2AN→1*E*,8*E*-*syn*-Ac2AN 9.42

*C*2-*Z*-9,10-Ac2AN→*Ci*-*E*-9,10-Ac2AN 3.67

*C*s-*Z*-9,10-Ac2AN→*C2*-*E*-9,10-Ac2AN 4.36

*Cs*-2*Z*,7*Z*-Ac2AN→*Cs*-2*E*,7*Z*-Ac2AN 27.34 a enantiomerization barrier

and diacetylanthracenes

kJ/mol)

1*E*,8*E*-*anti*-Ac2AN→1*Z*,8*E*-Ac2AN 4.15 [90,8*E*-*anti*-Ac2AN] –844.80278079

1*E*,8*E*-*syn*-Ac2AN→1*Z*,8*E*-Ac2AN 6.22 [90,8*E*-*syn*-Ac2AN] –844.80231227

1*E*,8*E*-*syn*-Ac2AN→1*E*,8*E*-*anti*-Ac2AN 9.49 [1*E*,8*E*180-Ac2AN] –844.80296243

*Ci*-*E*-9,10-Ac2AN→*C*2-*Z*-9,10-Ac2AN 3.73 [*C*1-90-*anti*-9,10-Ac2AN] –844.79633239

*C*2-*E*-9,10-Ac2AN→*Cs*-*Z*-9,10-Ac2AN 3.44 [*C*1-90-*syn*-9,10-Ac2AN] –844.79605863

*Cs*-2*E*,7*Z*-Ac2AN→*Cs*-2*Z*,7*Z*-Ac2AN 31.63 [*C*1-90,7Z-Ac2AN] –844.81338779

Δ*G*<sup>298</sup> ΔΔ*G*<sup>298</sup> Δ*G*‡

Table 7. Energy barriers (Δ*G*‡, kJ/mol) for diastereomerizations of monoacetylanthracenes

1-AcAN *Z Cs* 15.79 0.00 19.52 1-AcAN *E C*1 28.80 13.01 6.51 2-AcAN *E Cs* 0.00 0.00 31.52 2-AcAN *Z Cs* 2.24 2.24 29.28 9-AcAN *– C*1 36.94 0.00 3.64 1,5-Ac2AN *ZZ C*2*<sup>h</sup>*27.69 0.00 19.93 1,5-Ac2AN *ZE C*1 40.48 12.79 7.14 1,6-Ac2AN *ZE Cs* 13.45 0.00 30.35 1,6-Ac2AN *ZZ Cs* 15.14 1.69 28.66 1,7-Ac2AN *ZE Cs* 12.34 0.00 31.54 1,7-Ac2AN *ZZ Cs* 15.83 3.50 28.05 1,8-Ac2AN *ZZ C*2 38.89 0.00 9.81 1,8-Ac2AN *EZ C*1 39.25 0.35 9.46 9,10-Ac2AN *E Ci* 71.57 0.00 3.73 9,10-Ac2AN *Z Cs* 71.63 0.06 3.67 2,7-Ac2AN *EZ Cs* 0.00 0.00 31.63

Table 8. Relative energies (Δ*G*298 and ΔΔ*G*298, kJ/mol) of selected monoacetylanthracenes and diacetylanthracenes and respective energy barriers for *E*,*Z*-diastereomerizations (Δ*G*‡, The monoacetylanthracenes and diacetylanthracenes under study adopt non-planar conformations in their crystal structures. The twist angles are maximal for the 9-acetyl groups (|τ9|=85.0–87.9°) and significant for the 1Z-acetyl groups (|τ1|=15.2–34.0°), but very small for 2-acetyl groups. The conformations in solution are in agreement with the X-ray crystal structure conformations, according to the NMR data. The crystal structures are stabilized by intermolecular interactions: aromatic–aromatic π–π interactions (1,6-Ac2AN and 1,7-Ac2AN), C...H-π interactions (2-AcAN, 1,5-Ac2AN, 2,7-Ac2AN and 9,10-Ac2AN), or π–π interactions between the anthracene unit and the carbonyl bond (1,8-Ac2AN). The B3LYP/6-31G(d) calculated conformations of the monoacetylanthracenes and diacetylanthracenes are in good agreement with the X-ray crystal structures. The acetyl groups in the crystal structures and the B3LYP/6-31G(d) calculated global minima of the monoacetylanthracenes and diacetylanthracenes preferentially adopts 1*Z* and 2*E* conformations. The order of stabilities of the diacetylanthracenes under study is 2,7- Ac2AN>1,7-Ac2AN≈1,6-Ac2AN>1,5-Ac2AN>1,8-Ac2AN>9,10-Ac2AN. The acetyl groups at positions 1, 5 and 8 destabilize the diacetylanthracenes because of the repulsive interactions between the carbonyl oxygen/methyl group and the aromatic *peri*-hydrogens, and because of the decreased resonance stabilization. This effect is even more pronounced for the acetyl groups at positions 9 and 10. The B3LYP/6-31G(d) calculated energy barriers for the E,Zdiastereomerizations show that the *E*,*Z*-diastereomerizations is swift on the NMR time scale (at room temperature), in accordance with the results of the NMR experiments. The present results of the crystallographic and theoretical study of monoacetylanthracenes and diacetylanthracenes contribute to our understanding of the motifs of reversibility and thermodynamic control in the Friedel–Crafts acyl rearrangements of these representative PAKs.
