Oxetane ring stability

Diversity of synthesis methods because of oxetane ring stability under alkali conditions. 

A mechanistic scheme involves intramolecular single-electron transfer (SET) from the enamino moiety toward carbon 5 of the ring, and subsequent transannular interaction in zwitter-ionic biradical 259 with formation of stabilized zwitter-ion 260. Opening of the oxetane ring in 260 leads to cyclobutene 256, while secondary photolysis of the latter gives olefin 257 and alkyne 258 (Scheme 100). 

Cyclopropane analog " 98 showed microtubule disassembly inhibitory activity comparable with paclitaxel, but lower than docetaxel. The author thus demonstrated that the oxetane ring is not essential for the interaction of paclitaxel analogs with microtubules when the C-ring conformation is locked by cyclopropane, but the oxygen atom in the D ring of paclitaxel may participate in the stabilization of a dmg-tubulin complex. 

Exocyclic unsaturation can stabilize small ring heterocycles. In three-membered rings it is difficult to separate the contributions from increased angle strain and from electronic interactions between the unsaturation and the heteroatom. In four-membered rings such separation has been done 74PMH(6)199, p. 235). The CRSEs change from oxetane... 

The stability of cyclic ethers toward hydrolytic agents is largely a function of the size of the ring and, hence of the internal strain. Thus, whereas the oxiranes (epoxides) are particularly sensitive to these reagents,95,96 the oxetanes are relatively more stable.97 Oxolanes of the tetrahydrofuranol type seem, in particular, little affected by... 

Exocyclic unsaturation can stabilize small ring heterocycles. In three-membered rings it is difficult to separate the contributions from increased angle strain and from electronic interactions between the unsaturation and the heteroatom. In four-membered rings such separation has been done (74PMH(6)199, p. 235). The CRSEs change "from oxetane (106 kJ mol-1) by -11 kJ mol-1 to oxelan-2-one (95 kJ mol-1) (corrected for electronic effects) and 4-methyleneoxetan-2-one (95 kJ mol" ). In contrast, an increase of 10 kJ mol 1 over the value for cyclobutane (111 kJ mol-1) is observed on going to both methylenecyclobutane and l,3-bis(methylene)cyclobutane. 

The ring opening of 2,2 -diphenyloxetane has been achieved using cerium(iv) ammonium nitrate (CAN) as a redox catalyst (Equation 7) <2003TL4585>. The first step involves oxidation of the oxetane by Ce(rv) to a cyclic radical cation. Equilibration to the ring-opened distonic version 42 with a stabilized cation, then quenching of the cation with methanol and reduction of the alkoxide radical by Ce(lll) to the anion, completes the catalytic cycle. 

Bos and coworkers have developed several applications of alkyne-ketone photoadditions to organic synthesis. 2 Irradiation of a solution of acetone and 1-methylthio-l-propyne gives as a major product the cycloadduct (67) this results from selective oxetane formation due to radical stabilization by the methyl-thio group, followed by electrocyclic ring opening. Similarly, photoreaction of benzil and 1-t-butylthio-1-propyne gave the adduct (68) in 45% yield, which was transformed in 90% yield to the furan (69). 

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