Diazo compounds, ketoesters from

Three potential routes from 14 to 2, shown in Scheme 2.6, were identified and evaluated. Option A was the original plan of preparation. Hydroboration of the carbon-carbon double bond in 14 followed by oxidation provided primary alcohol 19 (P=H). Beta-ketoester 19 was converted to the corresponding diazo compound... 

The azetidinonyl iodonium ylide 290 is a safe and convenient alternative to the related diazo compound for synthesis of the carbacephalosporin precursor 291 (Scheme 82) (97TL6981). The ylide was prepared in 88% yield from the corresponding /3-ketoester and was cyclized to 291 with a Rh(II)-catalyst, presumably via a Rh-carbenoid intermediate. 

The rhodium-catalyzed conversion of a-diazo-p-hydroxy carbonyl into P-dicarbonyl compounds (Table 23, Entries 6-8) in general seems to be preferable to the acid-catalyzed reaction because of higher yields and absence of side-reactions 37S,377). From a screening of 20 metal salts and complexes, Rh2(OAc)4, RhCl(PPh3)3, PdCl2 and CoCl2 emerged as the most efficient catalysts for the transformation of a-diazo-P-hydroxy esters into P-ketoesters 376). This reaction has become part of... 

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