Diazo ketones conformations

The diazo ketone can exist in two conformations, called s-(E) and s-(Z). Studies have shown that Wolff rearrangement takes place preferentially from the s-(Z) conformation. 

There are several isolated examples of conformationally constrained a-diazo ketones that, under catalysis by copper salts, smoothly undergo intramolecular C-H insertion37. Although investigated in some detail, this cyclization in the simple acyclic series was not found to be a preparatively useful synthetic method38. 

Photolysis at 270 run of diazoacetone with ultrafast IR detection gives ketene generation in a concerted process, interpreted as showing a predominance of the syn-conformation of the diazo ketone (Scheme 7.48). Photolysis of azibenzil with UV-visible detection shows formation of the singlet benzoylcarbene in acetonitrile. The IR absorbance at 2100cm shows formation of the ketene by a concerted process arising from the syn[Pg.250]

The reaction between diazomethane and 3-oxo-steroids is catalysed by alu-mina. The 5 -3-ketone gave a 5 -A-homo-4-one, and the 5a-compound afforded an oxiran (50%) and A-homo-ketones. [The unusually high proportion of oxiran here, implying the intermediate with an equatorial diazomethyl group, probably results from an anti-conformation of the diazo- and hydroxy-groups... 

Condensation of l,2,5,6-tetrahydro-l,6-benzodiazocine (26 R = H) with aldehydes and ketones affords bridged aminal derivatives (23 R = R = H, Me R = H, R = Ph, p-Me2N-C6H4) in good yield, with the boat conformation and relative positions of bridge substituents indicated above demonstrated by NMR spectral analysis <90MRC423>. Electrochemical reduction of dicationic diazo-... 

---