Cyclization oxetane ring formation

The above synthetic methods for oxetane all involve formation of a new C—O bond. Cyclization by formation of a new C—C bond has been applied with compounds having benzylic or alkylic CH groups. Recent examples of this type of ring closure are the rearrangement of trans- 2,3-epoxycyclohexyl allyl ether by means of s-butyllithium and the dehydrochlorination of a-cyanobenzyl 2-chloroethyl ether with aqueous base and phase transfer catalyst (equation 86). Both reactions probably involve carbanion intermediates (76TL2115, 75MIP51300). 

Formation of the four-membered ring of fluorinated oxetanes as for their hydrocarbon congeners is normally accomplished by one of the two general approaches the cyclization of y-substituted alcohols or [2 + 2] cycloaddition reaction. 

The results are consistent with the formation of zwitterionic intermediate 35 (See Figure 10). Selective formation of the threO isomer 37 directly from 35 implies a preferred geometry for 35. Although chelation is not possible, electrostatic interactions could lead to a preferred geometry that would lead to the selective formation of 37. This result cannot be rationalized, even a posteriori, from examination of models. An alternative explanation, which is supported by circumstantial evidence, involves the reversible cyclization of 35 to give the more stable /r /w-substituted oxetane 36 followed by ring opening of the acetal with inversion by an Sn2 mechanism. 

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