Oxidation of the Carbonyl Functionality

Vogel also used (-)-5, prepared from 2,4-dimethylfuran, to show that a sequence involving stereoselective functionalization, fragmentation via Baeyer-Villiger oxidation and exhaustive reduction constitutes a quick assembly of optically pure polypropionate arrays with four contiguous stereocenters, Eq. 82 [14,132]. 

A variation of this strategy employed the Beckmann rearrangement to insert nitrogen, eventually leading to a synthesis of a muscarine analogue, Eq. 83 [133]. 

Ring scission which is complementary to that using the Baeyer-Villiger oxidation-hydrolysis sequence has also been developed. The naked sugar derivative 111 was converted to its silyl enol ether and then ozonolyzed and reduced to give 112 which was transformed to the C-nucleoside, cordecepin C, Eq. 84 [109a]. 

Ketone 113 was similarly cleaved via its corresponding silyl enol ether, eventually leading to the synthesis of tiazofurin, a potent antiviral and antitumor agent, Eq. 85 [134]. 

Oxabicyclic ketones have also been further derivatized to the alpha-oxidation products which are in turn cleaved, offering still another option for carbon-carbon bond scission. For example, hydroxyketone 114, available in 98 % ee from the parent ketone, was cleaved by lead tetraacetate to afford an excellent yield of the hydroxyester 115, a key intermediate in Noyori s synthesis of showdomycin, Eq. 86 [121,129]. In this case, the ozonolysis of the silyl enol ether of the parent ketone led to complex mixtures, demonstrating the complementarity of these approaches. 

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