Ring contractions, photolysis diazoketones

The photolysis of cyclic a-diazoketones results in ring contraction to a ketene, which can be isolated as the corresponding ester. 

To synthesize the very strained tricyclic system of 4 (Fig. 3.12), a photochemical Wolff rearrangement was chosen when MM predicted that the skeleton of 4 should be about 109 kJ mol less stable than that of the available 5. Photolysis of the diazoketone 6 gave a high-energy carbene which lay above the carbon skeleton of 4 and so was able to undergo Wolff rearrangement ring contraction to the ketene precursor of 4. 

Further syntheses of oxetanocin have been described. A branched-chain tetroside whose synthesis is covered in Chapter 14 was converted in standard steps to oxetanocin (17) together with its o(-anomer. Another sequence started from adenosine, and proceeded via a 3 -deoxy analogue to the diazoketone (18), which underwent the necessary ring contraction to give (19) on photolysis.An oxetanocin analogue derived from 3,5-anhydro-D-xylofuranose is mentioned in Chapters 20 and 22. 

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