Intramolecular oxidative biaryl

Another example of intramolecular oxidative biaryl coupling in the synthesis of allocolchicine-type structures was provided by the total synthesis of the allocolchicinoidjerusalemine 9, Fig. (13) [31]. 

In the intermolecular mode, this reaction has been utilized for the preparation of products 28 from various nucleophiles, including C-nucleophiles (e. g. (3-dicarbonyl compounds). A similar reaction in the intramolecular mode provides a powerful synthetic tool for the preparation of various polycyclic compounds via oxidative biaryl coupling [21,27 - 30]. Several examples of these C-C bond forming reactions are shown in Schemes 13-15. Specifically, various dibenzoheterocyclic compounds 30 have been prepared by the oxidation of phenol ether derivatives 29 with [bis(trifluoroacetoxy)iodo]benzene in the presence of BF3-etherate in dichloromethane (Scheme 13) [27-29]. 

Not many catalyzed processes involving free radicals are known with these metals. Some vanadium-catalyzed pinacol coupling reactions were developed (reviews [129, 171], [172, 173] and cited ref, [174]). Niobium and tantalum complexes were applied in pinacol coupling reactions [130]. Vanadium(IV) [175-179] and vanadium(V) ([129], reviews [180-186]) complexes are known to catalyze asymmetric oxidative dimerizations of phenols and naphthols in moderate to excellent ees applying oxygen as the terminal oxidant. Biaryls are accessible by intramolecular coupling of sodium tetraarylborates, catalyzed by EtOVOCl2 in the presence of air [187]. 

Tab. 2. Substituent effects on the PIFA-mediated intramolecular oxidative cyclization of a,co-biaryls.

Liegault B, Lee D, Huestis MP, Stuart DR, Fagnou K (2008) Intramolecular Pd(II)-catalyzed oxidative biaryl synthesis under air reaction development and scope. J Org Chem 73 5022-5028... 

A simplification of the classical Baker-Venkataraman 3-step route uses DBU to effect a one-pot synthesis of 2,8-disubstituted chromones <97SYN195>. Solid state phenol coupling of a 2 -hydroxyacetophenone is the key step in a total synthesis of the natural atropisomer of a biflavone <97JOC7222>. An alternative approach involves an asymmetric intramolecular oxidative coupling to produce the biaryl precursor <97TL1087>. 

Additional examples of intramolecular oxidative coupling of phenolic ethers include the oxidative biaryl coupling of various N-substituted 1-benzyltetrahydroisoquinolines 313 to the corresponding aporphines 314 [374], the oxidative cyclization of 3,4-dimethoxyphenyl 3,4-dimethoxyphenylacetate (315) leading to the seven-membered lactone 316 [375] and the conversion of phenol ether derivatives 317 into the products of... 

An intramolecular SrnI reaction was used as key step in the synthesis of the alkaloid 0-demethyleupoularamine 3. The intermediately formed SrnI product is readily transformed by photochemical oxidative biaryl formation and subsequent methylation to the target compound 3 in good yield (Equation 13.7) [23]. The SrnI cyclization of l-(2-bromobenzyl)-l,2,3,4-tetrahydroisoquinolin-7-ol derivatives 4 was recently applied to the synthesis of aporphine alkaloids 5 (Equation 13.8) [24] and, by using the same approach, homoaporphine alkaloids can also be synthesized. 

An interesting chemical oxidative methodology, first reported many years ago (the Scholl reaction), has gained significant interest in recent years, due to its potential in the synthesis of several TT-conjugated materials. This reaction is an intramolecular oxidative C—C bond formation, mediated by various metal-based oxidants, between two benzenoid rings to produce a biaryl linkage and it has been extensively utilized for the synthesis of planar polycyclic aromatic compounds. 

Chan has discovered a completely atropdiasteroselective synthesis of a biaryl diphosphine by asymmetric intramolecular Ullmann coupling or Fe(m)-promoted oxidative coupling. A chiral atropisomeric biaryl bisphosphine ligand 2 was synthesized through this central-to-axial chirality transfer.38 Recently, a xylyl-biaryl bisphosphine ligand, Xyl-TetraPHEMP, was introduced by Moran, and is found to be effective for the Ru-catalyzed hydrogenation of aryl ketone.39... 

Isoquinolines have been prepared on insoluble supports by radical-mediated cycli-zations and by intramolecular Heck reaction (Table 15.25). Entry 1 in Table 15.25 is a rare example of the formation of a biaryl by intramolecular addition of an aryl radical to an arene. Oxidative aromatization was achieved by using a large excess of AIBN. 

Phenolic or non-phenolic oxidative coupling methods have been extensively used since the 1970 s to synthesize polyoxygenated bridged biaryl compounds, as will be illustrated later in the case of steganes. Remarkably enough, the use of transition metal oxidants is particularly suited to perform intramolecular biaryl coupling whereas most other methods are better suited to intermodular coupling. 

Table 3 shows the application of these intramolecular biaryl-forming conditions to the indirect synthesis of acyclic biaryls using a temporary tether strategy (Scheme 9). Silaketal tethers (Y = SiR2 and X = O, 31a-f) react efficiently to give the biaryl unit. A sulfide derivative (Y — S, X = CH2, 31g) led to many oxidized products, but sulfoxides (Y = SO,... 

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