Intramolecular carbenoid displacement

A special method involves the carbenoid displacement reaction with diazocarbonyl compounds75, which is also used in an intramolecular fashion76. 

A slightly different approach to 850 takes advantage of an intermolecular carbenoid displacement and intramolecular alkylation to construct the pyrrolizidine ring (Scheme 128) [193,194]. The precursor is prepared from 875 (R==OH — OEt —> SPh) and, for convenience, the protecting acyl group is replaced with a MOM group, which gives 876 as an 8 1 mixture of trans and cis phenyl sulfides. The major ra 5-sulfide is subjected to a carbenoid dis-... 

An intermolecular carbenoid reaction followed by intramolecular displacement of acetate gives the clavulanic acid derivative (112) in one step from 4-acetoxyazetidin-2-one (91) (80CC1257). Carbene-induced reactions of penicillins and cephalosporins have been reviewed (75S547, 78T1731). 

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