Polyfunctionality effect

A one-pot sequential Michael-Michael ring closure (MIMIRC) reaction using 2-cyclo-hexenone as the initial Michael acceptor allowed an effective construction of various polyfunctionalized polycyclic compounds (Scheme 50)210. 

The necessity to have more than one component in a catalyst arises from many needs those linked to the polyfunctionality often required for the different steps in a reaction, the need to enhance the rate of some reaction steps, inhibition of unwanted side reactions, provision of adequate thermal stability, to take advantage of observed synergetic effects. From a fundamental point of view, the presence of several metal elements in a common structure permits the adjustment of the local electronic properties, imposes well defined coordinations, limits the extent of oxidation-reduction phenomena, and may stabilize the whole catalyst by retarding sintering. Mixed oxide catalysts are used as such, or as precursors of active catalysts, for a whole range of important industrial processes, a representative selection of which is given in Table 1. 

Arnone et al. studied the epoxidation of various olefins 220 with perfluorinated oxaziridine 80 (Equation 10) <1996JOC8805>. Alkyl-substituted olefins are epoxidized with this oxaziridine under particularly mild conditions. Electron-deficient substrates can also be epoxidized, and the more electron deficient the double bond is, the more severe the reaction conditions become. The reaction is chemoselective and stereoselective, with air-alkenes affording air-epoxides. Various complex and polyfunctionalized substrates of natural origin (monoterpenes, sesquiterpenes, and steroids) have been epoxidized effectively with this reagent (Table 18). 

En route to the synthesis of mensacarcin (104), a polyfunctionalized hexahy-droanthracene, showing cytostatic and cytotoxic activity, Tietze et al. [49] devised a Mizoroki-Heck cyclization for the formation of the tricyclic core. Several substrates with different protecting groups and substitution patterns were tested, out of which 102 turned out to be the best, affording 103 in 94% yield under optimized reaction conditions (102 103, Scheme 5.20). A similar strategy was pursued by Banerjee and coworkers [50] for the synthesis of tetrahydroanthracenes leading to umbrosone (107). Variation of the substituents in 105 had a minor effect, the reactions proceeded smoothly in 84-86% yield (105 106), and subsequent elimination of H2O provided the tetrahydroanthracene core. When diene 108 was employed the aromatic system in 109 was directly installed by double-bond migration (108 109). 

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