Asymmetrical Naphthol Dearomatization

Fig. 22 Quideau s asymmetric hydroxylative naphthol dearomatization via an in situ w-CPBA-mediated generation of a chiral binaphthyl iodane...

Several other mechanistically distinct metal-catalyzed dearomatization procedures have been reported, and almost all involve phenol or naphthol derivatives undergoing dearomatization via intramolecular transformations. Intramolecular Pd- and Rh-catalyzed C4-arylation and alkylation of /)ara-substituted phenols has been used to construct compounds of general structure 82 (Fig. 15.1) [86]. These reactions rely on generation of electrophilic aryl or alkyl o-metal complex intermediates that participate in tandem C4 metalation-reductive elimination with an attached phenol. Ruthenium- and Pt-catalyzed reactions of naphthalenes and alkynes deliver spirocyclic products such as 83 [87, 88]. An asymmetric intramolecular naphthalene dearomatization catalyzed by Pd(0)-phosphine complexes has been used to prepare carbazole derivatives 84 in good enantiomeric excess from l-(AI-2-bromophenyl)aminonaphthalene precursors [89]. 

Phenolic oxidations are pivotal steps frequently involved in the biosynthesis of natural products, which possess a variety of important biological activities. Therefore, a continuing interest exists in such transformations, in particular in asymmetric oxidative protocols. Kita et al. performed asymmetric dearomatization of naphthols 43 mediated by chiral hypervalent iodine(III) reagents, 33 and 45 having a rigid spirobiindane backbone (Scheme 20) [66, 67]. A series of other ortho-functionalized spirobiindane reagents of type 46 were synthesized. Intramolecular oxidative substitution of 43 afforded five-membered spirolactone 44 with good levels of enantioselectivity (up to 92% ee). Conformationally flexible iodoarenes employed in this study produced almost racemic products. Catalytic use of these chiral catalysts with wCPBA as cooxidant afforded the chiral spirolactones without detrimental effects on the ee values. 

Figure 19.11 Chiral organo-iodine catalysts for asymmetric dearomatization of naphthols.

Scheme 19.16 Asymmetric dearomatization of naphthols using hypervalent iodine catalysts.

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