Oxetane structure

In 1954 this reaction was more extensively investigated by Buchi,. Inman, and Lipinsky,(81) who confirmed the oxetane structure of the photoproduct. To credit their initial work on this interesting reaction, it commonly is referred to as the Paterno-Buchi reaction. 

BAMO is also copolymerized with nitratomethyl methyl oxetane (NIMO) to formulate the energetic liquid polymer BAMO-NIMO. Since NIMO is a nitrate ester containing an -O-NO2 bond in its molecular structure, BAMO-NIMO copolymer is more energetic than BAMO-THF copolymer. The chemical structures of BAMO and NIMO are both based on the oxetane structure, and the structure of the BAMO-NIMO copolymer is shown in Fig. 4.10. 

The photocycloaddition of a carbonyl compound to an olefin with the formation of an oxetane 3 was first observed by Paterno and Chieffi in 1909.1 They reported obtaining a compound in good yield from the irradiation of benzophenone in a petroleum mixture rich in 2-methyl-2-butene, for which they proposed the oxetane structure. Although Paterno and Chieffi had suggested the correct structure, it was not until 1954 that Buchi and his collaborators reinvestigated this reaction and... 

This technique has been a great value in assigning oxetane structures, particularly because the a-protons in oxetanes have much higher chemical shift values than in any other class of ethers. For the parent compound the chemical shifts are at 4.73 and 2.72 p.p.m. in deuteriochloroform. Proton NMR spectroscopy is very useful for 2-oxetanones also, but has not been given as much study. Table 2 gives data for several representative oxetanes. 

Another characteristic of photocycloaddition to electron-rich alkenes is the loss of any stereochemistry of the starting alkene in the oxetane structure. An example is the formation of practically the same mixture of geometric isomers of 2,2-diphenyl-3,4-dimethyloxetane from benzophenone and either cis- or trans- 2-butene (equation 103). This is understandable on the basis of the diradical intermediate having a sufficiently long lifetime for bond rotations to occur. 

Irradiation of /3- and -y-ketoalkenes may produce some highly strained and unusual polycyclic oxetanes. For example, from 7,8-dihydro-/3-ionone (62) at 25 °C in pentane solution there was obtained a 70% yield of (63), while in glyceryl triacetate solution at 60 °C a 10% yield of (64) was obtained (as well as other products, of course). This is an example of the possibilities for rather complex cycloadditions to occur, in the course of generating an oxetane structure (80HCA154). 

Feast. Dibenzoylbenzene derivative (114) was irradiated with tetramethylallene for 3.5 minutes in benzene to produce a polymeric structure in which there were 90% oxetane structures incorporated along the backbone. This process constitutes an example of step-growth photopolymerization via the triplet state of an aromatic carbonyl moiety. The products are thermally unstable and readily degraded by acids, but are quite soluble in organic solvents and may possess useful properties as new materials. 

The stereoselective synthesis of unsaturated oxetanes has recently been achieved by Feigenbaum and coworkers.Previous studies have indicated that photochemical cis-trans isomerization of enals is rapid and results in the formation of equivalent amounts of stereoisomeric alkene adducts. " For example, irradiation of rran.r-crotonaldehyde and 2,6-dimethylfuran produced a 1 1 mixture of alkenic isomers (174) and (175) in 64% yield. Irradiation of 4-trimethylsilylbutyn-2-one and furan provided with S 1 stereoselectivity the bicyclic oxetane (176) in which the methyl group occupies the exo position, presumably because of the small steric requirement of the triple bond. Desilyation of the protected al-kyne produced an alkynic oxetane which was hydrogenated under Lindlar conditions to bicyclic vinyl-oxetane (177) attempts to use the unprotected butyn-2-one gave low isolated yields of oxetane because of extensive polymerization. The stereochemical outcome thus broadens previous alkynyloxetane syn-theses and makes possible the preparation of new oxetane structures that may be synthetically useful. 

A minor product of the photolysis of the hypoiodite derived from AT-acetyljervine (19) was originally assigned an oxetan structure (20). The structure of this product has been reassigned by the original authors and is now formulated as the acetal... 

Clementeins. Sesquiterpene lactones with the rare oxetane structure (e.g., taxol) from Centaurea cana-riensis (syn. C. clementei), e.g., C. A CjiHjeO, Mr 390.41, mp. 193-195°C. 

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