Oxetanes scission

This is a widely observed type of reaction for oxetanes and 2-oxetanones and has many variations. The typical pattern involves electrophilic attack on the ring oxygen atom of oxetanes to form an unstable oxetanium ion. The latter either undergoes C—O ring scission to a carbonium ion which then combines with a nucleophile, or nucleophilic attack by even very weak nucleophiles at the a-carbon atom. For clarity, these reactions will be arranged according to the nature of the overall process. 

The estimated values of fe 14 for oc, /3, 7 and 6-H transfer are given in Table 3 (p. 275) from which it can be seen that the reverse reaction is faster than the forward reaction. However, it can be seen by compEirison with Table 4 (p. 278) that the reverse isomerization of 7 and 5-hydro-peroxyalkyl radicals is much slower than their decomposition to tetra-hydrofurans and tetrahydropyrans, respectively. Reactions (—147) and (—145) will therefore be of little significance. On the other hand, the reverse isomerization of a- and j3-hydroperoxyalkyl radicals would be expected faster than either their decomposition to oxirans and oxetans, respectively or than their decomposition to the conjugate alkene and j3-scission products respectively. Reactions (—14a) and (—14j3) would thus be expected to be important. As discussed in Sect. 3.2.2, however,... 

In contrast, compounds arising from /3-scission of C—C bonds are the major products formed during the oxidation of 3-ethylpentane, their yields being ca. 3 times as large as those of the corresponding oxetans. /3-Scission decomposition of /3-hydroperoxyalkyl radicals competes effectively, therefore, with their decomposition by cyclization to oxetans. 

A relevant extension of the ring opening of epoxides with a titanium salt can be seen in the concomitant scission of the C-C bond of cyclobutane illustrated in Eqs (285) and (286) [648]. Choice of each diastereoisomer enables highly selective construction of the trisubstituted double bond which finds application in terpene synthesis. The tandem cleavage of the C-O and C-C bonds was similarly feasible in the combination of oxetane and cyclobutane as illustrated in the stereoselective synthesis of a homoallyl alcohol (Eq. 287) [648]. 

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