Oxetanes synthesis from oxiranes

In a new method of ring enlargement of oxiranes to oxetanes, the intramolecular Williamson reaction is the essential last step. Reaction of an oxirane with selenomethyl-lithium reagents, followed by bromination, gives y-bromo alcohols, which can be cyclized by base to oxetanes. Since the oxiranes can be prepared readily from ketones and the selenomethyllithium reagent, this is also a synthesis of oxetanes from ketones (80TL585). 

A general method has been elaborated for the synthesis of oxetanes from oxiranes by means of carbene insertion, with an a-selenoalkyllithium reagent that has also been utilized for the regioselective preparation of the oxirane itself from a carbonyl compound (Eq. 185). ... 

Alcohols with a nucleofuge leaving group in the r-position can be cyclized to give oxetanes. Thus the cyclodehydrohalogenation of halo alcohols occurs in an analogous way to the oxirane synthesis from y -halogenated alcohols (see p 20). Oxetanes can be prepared from 1,3-diols via monoarene sulfonates ... 

Carbon-Oxygen Bond Formation. CAN is an efficient reagent for the conversion of epoxides into /3-nitrato alcohols. 1,2-cA-Diols can be prepared from alkenes by reaction with CAN/I2 followed by hydrolysis with KOH. Of particular interest is the high-yield synthesis of various a-hydroxy ketones and a-amino ketones from oxiranes and aziridines, respectively. The reactions are operated under mild conditions with the use of NBS and a catalytic amount of CAN as the reagents (eq 25). In another case, N-(silylmethyl)amides can be converted to A-(methoxymethyl)amides by CAN in methanol (eq 26). This chemistry has found application in the removal of electroauxiliaries from peptide substrates. Other CAN-mediated C-0 bondforming reactions include the oxidative rearrangement of aryl cyclobutanes and oxetanes, the conversion of allylic and tertiary benzylic alcohols into their corresponding ethers, and the alkoxylation of cephem sulfoxides at the position a to the ester moiety. 

A very promising new method for converting oxiranes, as well as ketones, into oxetanes has recently been reported. This method uses the carbanion of dimethyl(N-tosyl)sulfoximine and gave good yields in the several cases reported. When this reagent is employed with ketones, oxirane formation is presumably an intermediate stage, but the oxirane is not isolated. The method thus provides an excellent synthesis of spiro-oxetanes from ketones, as the example with camphor in equation (85) shows (79JA6135). 

This section covers the synthesis of aziridines, oxiranes, /J-lactams and oxetanes. Aziridines are fairly important moieties in bioactive molecules and thus new routes for their synthesis are constantly being developed. /1-Lactams are probably the most important heterocyclic compounds that contain a single nitrogen atom, due to their importance in penicillin and cephalosporin chemistry. Their synthesis and chemistry has received much attention and much of this work has been reviewed544. The oxygen-containing heterocycles are much less commonly synthesized from double-bonded functional groups. 

Iodocyclization.1 This reagent converts various unsaturated alcohols into ethers and unsaturated carboxylic acids into lactones. It is particularly useful for synthesis of 2-(l-iodomethyl)oxiranes from allylic alcohols and of 2-(l-iodomethyl)oxetanes from homoallylic alcohols. 

It is relatively easy to ring-open ethylene oxide, more difficult to open oxetane, and tetra-hydrofuran is relatively unreactive. The ease of ring opening oxirane favors its use in macrocycle synthesis. In addition, though, the placement of the heteroatoms that results from its use is particularly favorable. 

Besides the oxiranes, the respective four-membered heterocyclic oxetanes have been studied as monomers in ROPs. Vandenberg et al obtained a linear and highly crystalline polymer from oxetanes and other authors detailed the synthesis of hyperbranched polyethers from hydroxyl-functional oxetanes.Mostly cationic initiators have been used in the ROP of oxetanes, primarily because of the higher basicity compared to three-membered oxiranes, which are prevalently polymerized by anionic techniques. 

---