Silver oxide with silyl enol ethers

The ring-opening of the cyclopropane nitrosourea 233 with silver trifiate followed by stereospecific [4 + 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-eyclohexen-l-ones [131], allylic oxidation of oc,/ -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

A 2 1 mixture of silver carboxy late and iodine is well known to be a good oxidant in the transformation of enol silyl ethers into the corresponding a-acyloxy carbonyl compounds (139). Five- and six-membered ring enol silyl ethers serve as the best substrates, with larger ring systems forming a-iodo carbonyl compounds as byproducts. 

Various methods for the a-hydroxylation of carbonyl compounds have appeared. lodosobenzene shows a high degree of selectivity in the preparation of acyloins, and benzeneseleninic anhydride appears to be an excellent reagent for the hydroxylation at the tertiary centre in a-alkylcycloalkanones. Enol silyl ethers give acyloins with osmium tetroxide and V-methylmorpholine V-oxide,and a-acyloxycarbonyl compounds with silver acetate and iodine. a-Acyloxyketones may also be prepared conveniently by treatment of oximes with acid chlorides [equation (42)]. ... 

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