2- azetidine, ring enlargement

Regiospecific, Pd(II) catalysed, cycloaddition reactions of azetidines to yield ring-enlarged products include the formation of tetrahydro-2-iminopyrimidines (e.g. 10) by reaction with carbodi-imides (95JOC253) and tetrahydro-l,3-thiazin-2-imines (e.g. 11) from isothiocyanates (95JOC3092). 

Anderson and Wills U report ylid intermediates in the reaction of azetidines with potassium amide in liquid ammonia. The major product resulting from the reaction with 1,1,3,3-tetramethylazetidinium bromide, 40, is the ring enlarged product 41 in 70% yield. Both an ion pair mech-... 

As described later, however, attempted reduction of an isoxazole to an isoxazolidine in most cases causes ring opening. Cyclization of open-chain systems at the 1 5 bond is often a good method (c), but for the isoxazolidine ring this method is not particularly convenient. A few examples of ring enlargement of azetidine /V-oxides are reported (d). Almost all the isoxazolidines so far reported have been prepared by process a. 

Some azetidine-ZV-oxides are known to form isoxazolidines by thermal ring-enlargement.168... 

The oxetane /-amides 222 undergo a ring expansion-contraction sequence in the presence of a Lewis acid to azetidine derivatives 223 (Equation 60) <2000JOC2253>. The overall reaction sequence has been described as double isomerization . The four-membered oxetane ring first enlarged to a [2.2.2]-dioxazabicycle, which in turn rearranged to the final azetidine derivatives. 

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