Oxidative cyclization, asymmetric

Permanganese is a common oxidative reagent, the application of which to the asymmetric oxidative cyclization of 1,5-dienes has been reported by Brown (Scheme 3.14). The addition of acetic acid is quite important for the reaction to proceed, and highly functionalized tetrahydrofurans are obtained in a range of 58 to 75% ee, in diastereoselective manner [35]. Another oxidative transformation using KMn04 with a chiral ammonium salt has been investigated. Scheme 3.15 illustrates the asymmetric dihydroxylation of electron-deficient olefins to chiral diols in the... 

Diastereoselective asymmetric oxidative cyclization of m-l,2-divinylcyc ohexane is achieved if chiral carboxylic acids are employed as nucleophiles instead of acetic acid 2l 22 8b. Addition of various types of molecular sieves has a decisive influence on the oxidative cyclization of m-1,2-di-vinylcyclohexane, resulting in enhanced diastereoselectivities (up to 62 %), as well as in opposite absolute configuration [(lS,3a/, 7a5 ) or (l/ ,3aS.7a7 )] of the newly created centers in aryl-sub-stituted hexahydro-3-methylene-l-(l-oxo-2-phenoxypropoxy)indans 921. The origins of these effects, although not without precedence, are unclear. 

Hosokawa T, Uno T, Inui S, Murahashi S-I. Palladium (II)-catalyzed asymmetric oxidative cyclization of 2-aUylphenols in the presence of copper(II) acetate and molecular oxygen. Study of the atCalysis of the Wacker-t) pe oxidation. J. Am. Chem. Soc. 1981 103 2318-2323. 

Phenol, Oms-2-(2-butenyl)-asymmetric intramolecular oxidative cyclization, 6,365 Phenol, 4-cyano-hydrolysis... 

Wacker cyclization has proved to be one of the most versatile methods for functionalization of olefins.58,59 However, asymmetric oxidative reaction with palladium(II) species has received only scant attention. Using chiral ligand 1,1 -binaphthyl-2,2 -bis(oxazoline)-coordinated Pd(II) as the catalyst, high enantioselectivity (up to 97% ee) has been attained in the Wacker-type cyclization of o-alkylphenols (66a-f) (Scheme 8-24). 

Furoxans are not formed by direct oxidation of furazans, but they can readily be prepared by ring closure or cycloaddition pathways. The most synthetically useful routes are the oxidative cyclization of 1,2-dioximes, the dehydration of a-nitroketoximes and, for symmetrically substituted furoxans, dimerization of nitrile oxides. For asymmetrically substituted analogues care must be taken in selecting the route in order to avoid formation of mixtures of 2- and 5-oxide isomers. 

In fact, the role of copper and oxygen in the Wacker Process is certainly more complicated than indicated in equations (151) and (152) and in Scheme 10, and could be similar to that previously discussed for the rhodium/copper-catalyzed ketonization of terminal alkenes. Hosokawa and coworkers have recently studied the Wacker-type asymmetric intramolecular oxidative cyclization of irons-2-(2-butenyl)phenol (132) by 02 in the presence of (+)-(3,2,10-i -pinene)palladium(II) acetate (133) and Cu(OAc)2 (equation 156).413 It has been shown that the chiral pinanyl ligand is retained by palladium throughout the reaction, and therefore it is suggested that the active catalyst consists of copper and palladium linked by an acetate bridge. The role of copper would be to act as an oxygen carrier capable of rapidly reoxidizing palladium hydride into a hydroperoxide species (equation 157).413 Such a process is also likely to occur in the palladium-catalyzed acetoxylation of alkenes (see Section 61.3.4.3). 

An oxidative cyclization has been exploited as the key step in the elegant asymmetric syntheses of both (+)- and (-)-galanthamine (291) (Scheme 29) (163). This biomimetic synthesis commenced with the secondary amine 311, which was readily prepared by the reductive amination of 3,5-dibenzyloxy-4-... 

A series of dibenzofuran-l,4-diones illustrated in the following scheme were constructed via a DDQ-mediated intramolecular oxidative cyclization of quinone-arenols <070L2807>. Dibenzo[b] furans were also found to be made from 2//-pyran-2-ones and 6,7-dihydro-577-benzo[Z>]furan-4-one <07T1610>. The asymmetric synthesis of chiral furo-fused BINOL derivatives was achieved via copper(II)-mediated oxidative coupling from naphthofuranol in the presence of chiral phenylethylamine <07TL317>. 

An asymmetric permanganate-promoted oxidative cyclization of 1,5-dienes using a chiral phase-transfer catalyst was recorded <2001AGE4496>, and a diastereoselective permanganate-mediated oxidative cyclization with an Oppolzer sultam has been employed in the total synthesis of m-solamin <20020L3715>. In a metal-oxo-mediated approach to the synthesis of 21,22-di- /)7-membrarollin based on the use of a camphor-derived Oppolzer sultam as... 

A Pd-catalyzed oxidative cyclization of phenols with oxygen as stoichiometric oxidant in the noncoordinating solvent toluene has been developed for the synthesis of dihydrobenzo[ ]furans (Equation 136). Asymmetric variants of this Wacker-type cyclization have been reported by Hayashi and co-workers employing cationic palladium/2,2 -bis(oxazolin-2-yl)-l,l -binaphthyl (boxax) complexes <1998JOC5071>. Stoltz and co-workers have reported ee s of up to 90% when (—)-sparteine is used as a chiral base instead of pyridine <2003AGE2892, 2005JA17778>. Attempts to effect such a heteroatom cyclization with primary alcohols as substrates, on the other hand, led to product mixtures contaminated with aldehydes and alkene isomers, which is in contrast to the reactions with the Pd(ii)/02 system in DMSO <1995TL7749>. 

Trend, R. M., Ramtohul, Y. K., Ferreira, E. M., Stoltz, B. M. Palladium-catalyzed oxidative Wacker cyclizations in nonpolar organic solvents with molecular oxygen A stepping stone to asymmetric aerobic cyclizations. Angew. Chem., Int. Ed. Engl. 2003,42, 2892-2895. 

Although this cyclization is not yet general, there is potential for expanding the scope of this reaction to other systems. At present, the details of asymmetric oxidations are not as well characterized as those of hydrogenation, but the range of synthetically useful possibilities for transition metal catalyzed oxidation is larger. 

Investigations by both Katsuki and Che have highlighted the application of chiral Mn(III) salen complexes for asymmetric N-atom transfer reactions (Fig. 17) [79-81]. In the former case, the use of [Mn(salen)PF6] together with PhI= Ts affords modest product yields and enantiomeric ratios for amination of simple benzylic and allylic starting materials. A high-valent Mn imido species is presumed to be the active oxidant. Methodological and mechanistic studies conducted by Che have found that the same types of Mn(III) salen complexes will induce oxidative cyclization of sulfamate esters. Although these catalysts afford... 

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