Formation of a Four-Membered Ring Oxetanes

All of these findings clearly indicate that the demand for synthetic oxetanes is high. There are basically three methods for preparing oxetanes  

The thermal and photochemical [2 + 2] cycloaddition reaction of alkenes with 

Before describing the regioselective, site-selective and stereo-selective preparation of oxetanes via the PB reaction, the mechanism of the photochemical reaction will be briefly summarized. The reason for this is that an understanding of the reaction 

The Generally Accepted Mechanism of the Patemo-Biichi Reaction 

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This chapter deals with [2 + 2]cycloadditions of various chromophors to an olefinic double bond with formation of a four-membered ring, with reactions proceeding as well in an intermolecular as in an intramolecular pattern. Due to the variety of the starting materials available (ketones, enones, olefins, imines, thioketones, etc.. . .), due to the diversity of products obtained, and last but not least, due to the fact that cyclobutanes and oxetanes are not accessible by such a simple one-step transformation in a non-photo-chemical reaction, the [2+2]photocycloaddition has become equivalent to the (thermal) Diels-Alder reaction in importance as for ring construction in organic synthesis. 

Oxetanes are the cycloadducts from a carbonyl compound and an olefin. This one step photochemical formation of a four membered ring heterocycle has been named the Paterno-Buchi reaction 489a> b). Oxetanes are important synthetic intermediates as they can fragment into the carbonyl-olefin pair by which they were not formed (a so termed carbonyl-olefin metathesis). Two examples of such oxetan cracking reactions are shown below in (4.76)490) and in (4.77)491) in this last example the oxetane was used as a precursor for the pheromone E-6-nonenol,... 

The formation of a i6/S,i8 oxide (26) [ 144] (tetrahydrofuran derivative) is another example of internal substitution of a tosylate group with suitable stereochemistry, and illustrates the preference for formation of a five-membered ring rather than a four-membered (i6j8,20) oxetane. 

Another important class of cycloaddition reactions is the formation of oxetane rings between a photoexcited carbonyl compound and an unsaturated molecule. These reactions also occur probably through an exciplex although these exciplexes are non-fluorescent as they are formed from the triplet state of the ketone or aldehyde. The formation of the four-membered oxetane ring is an interesting example of a typical photochemical reaction... 

Spectroscopic data has been obtained that is consistent with the formation of four-membered ring adducts 61/62 as the kinetic products of the reaction. The step leading to these cyclic products has been shown to be reversible prolonged exposure of 61/62 to the reaction conditions led to the conversion of these metastable oxetanes to 63, the thermodynamic product of the reaction. The investigators have speculated that the formation of oxetane adducts in this study is a consequence of a slow silyl transfer step 60—>63. Thus, these observations highlight the fine balance that can exist between the various reaction pathways available to the adduct of the C-C bond-forming step (cf 60). 

Catalysis by (6—4) photolyase must accomplish two chemical tasks cleavage of the C6—C4 sig a bond, and transfer of the OH (or —NH2) group from the C5 of the 5 base to the G4 of the 3 base. Because formation of the (6—4) photoproduct is presumed to proceed through a four-mem-bered oxetane or azetidine intermediate, it has been proposed that (6—4) photolyase first converts the open form of the (6—4) photoproduct to the four-membered ring by a thermal reaction, and then the four-mem-bered ring is cleaved by retro [2+2] reaction photochemically (Kim et al, 1994 Zhao et aL, 1997). A site-directed mutagenesis study has identified two histidine residues in the active site that may participate in conversion of the (6-4) photoproduct to the oxetane intermediate by general acid-base catalysis (Hitomi et al, 2001). A current model for catalysis by (6-4) photolyase is as follows (Fig. 8) The enzyme binds DNA and flips out the... 

Cycloreversion Reactions A cycloreversion reaction is the reverse of a cycloaddition reaction and leads to the formation of the starting reactants through the cleavage of two bonds in the ring [18], A typical example is the formation of C2H4+ and neutral C2H4 from the cyclobutane radical cation. As shown in reaction (6.37), this reaction proceeds through the intermediacy of a distonic ion. The radical cations of a variety of other four-membered cyclic compounds, such as cyclobutanones (3), diketene (4), oxetane (5), cyclobutylamine (6), and thiocyclobutane (7), are known to participate in cycloreversion reactions [27]. 

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