Anthracenes benzannulated

Danishefsky et al. succeeded in preparing the benz[a] anthracene core structure 111 of angucycline antibiotics by performing a benzannulation reaction with the cycloalkynone 109 [69]. Deprotonation of the naphthoquinone 110 with DBU yields the desired anthraquinone 111 (Scheme 49). 

The benzannulation of naphthyl carbene complexes may be used as a direct route to functionalized phenanthrenes (Scheme 34) [37a]. While it is obvious that 1-naphthyl carbene complexes such as 87 lead to phenanthrenes 88 (see also Section 8.3.2, Scheme 19), chromium 2-naphthylcarbenes 89 offer two alternatives for annulation, giving either phenan-threne or anthracene derivatives. Generally, angular benzannulation affording phenanthrenes 90 is favored over linear benzannulation to give anthracene derivatives 91. A rationale for this regiopreference is the higher electron density at C-l compared with that at... 

C-3 of the naphthalene nucleus that controls the electrophilic ring-closure of the vinyl ketene intermediate moreover, the degree of aromaticity of the angular rings in the phenanthrene skeleton exceeds that in the anthracene analogues. This regioselectivity observed in the benzannulation of chromium carbenes is paralleled by results observed for 2-naphthyl cyclo-butenones [77] and for the palladium-catalyzed cyclocarbonylation of 2-naphthyl allyl acetates [78]. 

A [4+2] benzannulation between acetylenic ketones 248 and a benzenediazo-nium 2-carboxylate proceeds effectively in the presence of a catalytic amount of AuCl, yielding functionalized anthracenes 250 in good yields (Scheme 12.67) [137]. It is suggested that the reaction involves a reverse electron demand Diels-Alder reaction between benzyne and the benzopyrylium aurate complex 249. 

Benzannulated anthracenes 110a, 110b, and 110c (Fig. 31) are highly twisted with end-to-end twists of 66°, 61°, and 70°, respectively [94, 95]. Similarly, crowded anthracenes 111 and 112 show twists of 60° and 57°, respectively [96]. 

Pascal and coworkers found unexpectedly that pyrolysis of phencyclones 424 forms diaryltetrabenz[ ,c,/r,7 ]anthracenes 425 (Scheme 6.112) [261]. The compounds 425 possess large twists of the polyacene units (66, 61, and 70, respectively), because benzannulated acenes with phenyl groups positioned to clash with the benzo hydrogens give molecules with the largest end-to-end twists. 

Asao, Sato, and co-workers reported that AuCl-catalyzed benzannulation of o-alkynylphenyl ketones 11 (R H) with benzenediazonium 2-carboxylate 51 gave a variety of anthracene derivatives 52. On the other hand, the reaction of o-alkynylbenzaldehyde 11 (R = H) afforded triptycyl ketones 53 (Scheme 15.21) [33]. The reaction proceeded most probably through [4 - - 2] cycloaddition between ben-zopyrylium intermediate and benzene. Zhu et al. investigated the mechanism of this reaction by theoretical calculation and reported that AuCl catalyst is more effective than AuCls, due to the lower activation free energy of the rate-determining step [34],... 

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