Miscellaneous routes to azetidin-2-ones

There are several mechanistically related ring expansion reactions of cyclopropanones which lead to /3-lactams. The conversion of cyclopropanone to /3-lactam (174) via the cyclopropanolamine (173) (75JOC1505) is just one modification, but it illustrates the strategy of this type of approach (73TL4855, 69JA2375) which has been applied to the synthesis of 3-amino-nocardicinic acid (81JOC2999). 

Oxidative ring expansion occurs when the hydrazine (175) is treated with manganese dioxide (73TL4091). Diphenylcyclopropenone (176) reacts with ammonia and methylamine to give /3-lactams (69BCJ1777). Initial attack could occur at either C-1 or C-2 of the cyclopropenone. 

The aziridinecarboxylate (177) undergoes ring expansion in good yield, possibly via a mixed anhydride, on treatment with thionyl or oxalyl chlorides in benzene (69JA4590). 

Several ring contraction routes to /3-lactams have been developed. One of the most important is the photochemical Wolff rearrangement of 3-diazopyrrolidine-2,4-diones (178), which provides a general route to 3-carboxyazetidin-2-ones (179). Lack of stereoselectivity is a problem, but facile epimerization is possible because of the electron withdrawing 3-substituent (78T1731). 

A variety of mono- and bi-cyclic /3-lactams have also been prepared by oxidative ring contraction of pyrrolidine-2,3-diones (180) (75JOC2356), and by the photolysis of pyrazolidin-3-ones (181) (78T1731, 75JOC3510, 75JOC3502, 75CC725). 

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