Phenols coupling, oxidative

In the first of these, the key step in the synthetic sequence involves an oxidative phenol coupling reaction patterned after the biosynthesis of the natural product. Preparation of the moiety that is to become the aromatic ring starts by methyla-tion of phloroglucinol (5) with methanolic hydrogen chloride to give the dimethyl ether (6). Treatment of that intermediate with sulfuryl chloride introduces the chlorine atom needed in the final product (7). 

Organic syntheses based on biosynthetic proposals are often extremely concise and elegant.6 Although the constitution and stereochemical complexity of carpanone may seem formidable, the sequential application of the Diels-Alder and oxidative phenolic coupling transforms7 to the natural product provides an exceedingly efficient solution. Chapman s striking synthesis of carpanone typi-... 

Schwartz MA, Holton RA (1970) Intramolecular oxidative phenol coupling. II. Biogenetic-type synthesis of (+-)-maritidine. J Am Chem Soc 92 1090-1092... 

Oxidative phenolic coupling. Biosynthesis of the alkaloid narwedine (3) is known to involve oxidative phenolic coupling of norbelladine derivatives (1), but the usual oxidants for such coupling in vitro convert 1(R = H) into the oxomaritidine skeleton (4) rather than 3. A new biomimetic synthesis of 3 involves the palladacycle 2, formed by reaction of 1(R = CH3) with Li2PdCl4, which is known to form complexes with allylic amines or sulfides (8,176-177). Oxidation of 2 with thallium(III) trifluoroacetate effects the desired coupling to give 3. 

Consistent with the results of this study is the outcome of the oxidation of 4-X-substituted phenols by use of PINO, generated from HPI with Pb(OAc)4 at 25 °C in MeCN containing 1% AcOH . The reactivity (fcn) of PINO towards phenolic O—H bonds (BDE 85-90 kcal moC ) was about one order of magnitude higher than that measured towards the C—H bond of benzyl alcohols (cf. Table 4). A p value of —3.1 was obtained from plotting log kn vs. for this reaction, where removal of H-atom from the phenolic O—H bond (which is weaker than the O—H bond of aliphatic or benzyl alcohols) induces an oxidative phenolic coupling with the PINO moiety. In view of the low redox potential of the substituted phenols (in the 0.8-1.1 V/NHE range), and of the substantial value of the kinetic isotope effect = 3.1-3.7 measured, ... 

The previous section has presented evidence that phenolic units are carefully positioned within the wall polymers. When these units undergo oxidative phenolic coupling reactions in the cell wall, the coupling reactions themselves are also remarkably specific. This can be illustrated by reference to the tyrosine residues of extensin. 

OXIDATIVE PHENOL COUPLING Ferric chloridc-Silica. 

Apart from the above-mentioned interest in m-octahedral species [M(bipy)2(anion)2] from the point of view of high-spin to low-spin transitions,72 this geometry is also of interest for the formation of reactive species, leading to, for example, dinuclear species with catalytic properties, such as the cobalt-peroxo compound shown in Figure 21. This compound is active in oxidative phenol coupling.132 With Cu the stoichiometry Cu(bipy)2X2 results in a variety of five-coordinate and distorted six-coordinate structures (see Figure 3).21... 

OXIDATIVE PHENOL COUPLING Bis(N-/i-propylsalicyUdeneaminate)cobalt(II). Vanadyl trichloride. 

Oxidative phenolic coupling.1 This cobalt complex and the related salcomine (2, 160, 3, 245 6, 507), both of which bind oxygen reversibly (102 2Co), catalyze the oxygenation of the phenol 1 to give carpanone 2 in 90-94% yield. Related complexes ol l c(ll) and Mn(II) are less effective. PdCl2 effects this reaction in 46% yield (4, 17(1) singlet oxygen is less efficient (29% yield). 

Oxidative phenolic coupling.1 A new hiomimetic approach to morphine alkaloids involves oxidative intramolecular coupling of the reticuline derivative 1 to a salutaridine derivative 2 with VOCl3 in ether.1 If the reaction is conducted in CH2C12 2 is obtained in markedly lower yield and the undesired aporphine 3 is formed as a major product. The dienone 2 has been converted into 2-hydroxycodeine (4). 

In a related study, the diphenolic substrate 409 was oxidized with a large excess of the iron complex [Fe(DMF)3Cl2][FeCl4] to give the spirodienone 413 in 35% yield (193). The oxidative phenolic coupling of 409 to furnish 413 using vanadium oxytrichloride had been previously reported, but the yield was slightly lower (176). Alkaline hydrolysis of 413 to cleave the A -trifluoroacetamide pro-... 

Oxidative phenolic coupling.1 The vancomycin antibiotics are polypeptides with bridging diphenyl ether groups. Evans et al. have shown in model systems such as 1 that cyclization to o-halophenolic peptides (2) can be accomplished by oxidation with thallium(III) nitrate in THF-methanol or CH2Cl2-methanol followed by CrCl2 reduction of a para-quinol intermediate (a). In three cases the yield of cyclic products was 40-48%. 

The hypervalent iodine reagents PIFA and PIDA have also been used in the synthesis of naturally occurring structures, primarily the amaryllidaceae alkaloids and related species. Work by White s group showed the feasibility of this method for the synthesis of 6a-epipretazettine and (-)-codeine [45, 46]. In the early 1990s, Rama Krishna and co-workers demonstrated that PIDA can promote the oxidative phenolic coupling of diaryl substrates 38a-e to deliver cyclohexadienones 39a-e, respectively, in consistent 30 % yields for all of the substrates examined (Scheme 10) [47]. 

Very recently, Node and co-workers improved the efficiency of this particular oxidative phenolic coupling in the context of a synthesis of (+)-galanthamine (51a) [50]. By using a trialkoxyarene as one of the aryl units, they were able to obtain yields of 56b of up to 90 % when the nitrogen was protected with a formyl group and the donor aryl s oxygen atoms were capped by benzyl moieties (Table 11). These authors were even able to isolate an interesting narwedine-type product 57a in low yield. 

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