Synthetic Applications of Oxetane Formation

The reaction of carbonyl compounds to olefins often yields products difficult to obtain synthetically by other routes. The excellent yields obtainable under proper conditions make this reaction of definite preparative interest. Examples of some synthetic applications of oxetane formation follow  

Irradiation of benzaldehyde in the presence of S-decyne results in the formation of the above a,j3-unsaturated ketone, presumably from decomposition of an intermediate oxetene which may be formed in a vibrationaUy excited (hot) ground state.  

Acetylcyanide adds to olefins to form oxetanes having a useful functional group directly attached to the trimethylene oxide ring  

Similarly to aldehydes, esters add to acetylenes to produce unstable oxetenes which decompose to j8-alkoxy-a-benzoylstyrene derivatives when diphenylacetylene is used. ° For example. 

Laage and Bosch reported the interesting intramolecular oxetane formation shown below. Reduction with lithium aluminum hydride afforded trflitf-9-decalol (32%)  

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Synthetic application of Paterno-Bilchi reaction of simple dienes with carbonyl compounds is rare. While seemingly an extension of the photocycloaddition of olefins and carbonyl compounds, the reaction between dienes and carbonyls is often complicated by the fact that triplet excited states of carbonyl compounds are quenched by dienes, although the formation of oxetanes can be observed during these reactions. Recall also that the photosensitized dimerization of diene triplet excited states is also a well known reaction vide infra), these two observations would seem to naturally limit the synthetic potential of this process. Kubota and coworkers found that irradiation of propanal in the presence of 1,3-cyclohexadiene produced oxetanes 164a and 164b in a 4 1 ratio (Scheme 37). ... 

The formation of thietanes from thiones and olefins has been less exploited for synthetic purposes than the corresponding oxetane-forming reaction. It should be remarked that thiocarbonyl compounds very often undergo efficient photoreactions from the second excited singlet state S2 U4). One interesting synthetic application is found in the photochemical preparation of quinolines from N-thioamides (4.84)498). The primary photochemical step is assumed to be the intramolecular thietane formation. 

The use of dienes such as piperylene in the quenching of triplet states of carbonyl compounds is well known. However, in many photochemical studies, attempts to quench the carbonyl Ti state are often complicated by the formation of oxetane photoproducts.Since the photosensitized dimerization of diene triplets is well known, synthetic applications utilizing dienes in the Patemo-BUchi photocycload dition are rather limited. However, the diene addend not only allows for rapid assemblage of functionality but remains a continuing challenge in terms of experimental efficiency and generality. 

As can be seen from the foregoing, phthalimide photocyclizations have provided useful synthetic routes to a variety of heterocyclic products. Machida et al. have used the reaction to yield the spiro compounds (325) from the irradiation (in methanol) of the phthalimide derivatives (326). The products are presumed to be formed via bond formation in the biradical produced by the addition of methanol to the radical cation/r ulical anion pair. The indoles (327) undergo photochemical (2-f-2)-addition with A methyl phthalimide to afford the oxetan adducts (328). This work has also been the subject of a patent application. " ... 

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