Four-membered rings 2- oxetanes

The mechanism of Grignard addition to oxetane is the same as the mechanism of Grignard addition to epoxides, described in Section 18.6. The reaction proceeds at a reduced rate because oxetane is less reactive than ethylene oxide. The four-membered ring oxetane is less strained, and therefore more stable, than the three-membered ethylene oxide ring. 

Among cyclic ethers, three-membered ring epoxides - with the exception of some four-membered ring oxetanes - are the only ones that can be polymerized by an anionic or a related nucleophilic polymerization mechanism. Larger cyclic ethers polymerize exclusively by a cationic or electrophilic ring-opening mechanism. 

Because of the high ring strain of the four-membered ring, even substituted oxetanes polymerize readily, ia contrast to substituted tetrahydrofurans, which have tittle tendency to undergo ring-opening homopolymerization (5). 

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... 

Asymmetric alcoholyses catalyzed by lipases have been employed for the resolution of lactones with high enantioselectivity. The racemic P-lactone (oxetan-2-one) illustrated in Figure 6.21 was resolved by a lipase-catalyzed alcoholysis to give the corresponding (2S,3 S)-hydroxy benzyl ester and the remaining (3R,4R)-lactone [68]. Tropic acid lactone was resolved by a similar procedure [69]. These reactions are promoted by releasing the strain in the four-membered ring. 

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,... 

T. K. Wu The largest ring used so far was oxepane, but we are planning to go in the other direction, using four membered rings. However, in this case complications arise because the strained oxetane ring does not undergo equilibrium polymerization. 

C. Four-Membered Ring Heterocycles 1. Oxetanes and Spiro-oxetanes... 

What effect the four-membered ring has upon the chemical reactions of azetidine, oxetane and thielane, together with those exhibited by their partly unsaturated analogues... 

The PE spectra of several oxetanes and related small ring compounds have received careful study. The first ionization potential for oxetane occurs at 7.65 eV as a sharp, adiabatic transition. This is at significantly lower potential than for oxirane (10.57 eV) or for acyclic ethers (dimethyl ether, 10.04 eV), showing the potential-lowering effect of the four-membered ring. This is seen also in azetidines and thietanes. 

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. 

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