Oxetanes temperature effect

Reactions of 1,2-Diketones - A study of the ketoamides (217) has shown that their direct irradiation in benzene solution brings about efficient cycloaddition to give the bicyclic oxetanes (218). In the solid state, irradiation of (217) also affords these products. However, the syn anti ratio is higher in the solid state than in solution. Furthermore, there is a temperature effect and the syn. anti ratio is greater at lower temperatures. The ee is also affected by changes of temperature. 

Recently, a notable temperature-related effect was reported for site-selectivity (double-bond selectivity or chemoselectivity) in the PB reaction of unsymmetrically substituted furans (Scheme 7.14) [30]. For example, the selective formation of the more substituted oxetane, OX1, was observed during the PB reaction of 2-methyl-furan with benzophenone at a high temperature (61 °C). However, a 58 42 mixture of the oxetanes, 0X1 and 0X2, was reported at low temperature (—77 °C). This notable effect of temperature could be explained by the relative population of conformers of the intermediary triplet 1,4-biradicals, T-BR1 andT-BR2. The excited benzophenone was considered to attack the double bonds equally so as to produce a mixture of the conformers of T-BR1 and T-BR2 however, at low temperature the conformational change was suppressed. Thus, the site-random formation of oxetanes 0X1 and 0X2 was observed after the ISC process. Nonetheless, at high... 

In 1956 Rose (19) reported that oxetane and 3,3-dimethyl oxetane, when treated with typical cationic initiators, form mixtures of polymer and cyclic tetramer (16-membered rings). Increase of the temperature increases the proportion of tetramer in the reaction products. Oxetane for example gives 4% of tetramer at —80 °C but at 50 °C 66% of the reaction products was tetramer (total tetramer yield based on monomer 42%). It is not an equilibrium effect, for the final conversion of monomer to polymer and tetra-... 

When the bulk of the crystals of 68 (Table 13) in a test tube was irradiated at 15°C under an argon atmosphere, intramolecular [2 + 2] cyclization proceeded effectively without melting down, and two diastereomeric oxetanes, 69 and 70, were obtained in 95 and 5% yield, respectively. The enantiomeric purity of the main product 69 was determined as > 99%. When 68 was irradiated after dissolving in THF at various temperatures, optically active oxetanes were isolated below — 20°C, whereas the racemic oxetanes were naturally obtained from the photolysis above 0°C in THF. The photolysis in THF at — 60°C gave 87% ee of 69 and 62% ee of 70, in 76 and 24% chemical yields, respectively. The memory of... 

In the case of alkyl enol ethers the normal oie process competes with solvent incorporated and 1,2-di-oxetane products. Here however the ene process seems to be less inevitaUe when allylic protons are available and the product distribution may be effectively contndled by manipulation of solvent and temperature combinations. Best results are nonetheless achieved where the competitive processes are restricted. Thus enol ether (79) produces hydroxylated dimethoxy acetal (W) via direct incorporation of methanol or through reduction of the 1,2-dioxetane (81). 

We report on the reaction of 2,2-dimethyl-1,3-propanediol catalyzed by various solid acids in the gas phase at temperatures > 250 °C. Originally, we tried to synthesize four membered cyclic ethers since several oxetanes are of synthetic interest [7,8], E g. 3-hydroxy-oxetane can undergo ring opening polymerization, leading to a water soluble polymer Since 3-hydroxy-oxetane is not very stable, we choose 2,2-dimethyl-1,3-propanediol as model substrate. In this communication, we describe the effect of catalyst structure (various zeolites. 

Formation of cyclic oligomers in the polymerization of oxetane has already been noted by Rose, who found that in addition to polymer, cyclic tetramer was also formed in quantities depending on the temperature 13). Polymerization of OX with BF3 initiator at —80 °C gave 4% tetramer while at 100 °C as much as 50% cyclic tetramer was isolated. This work was later reinvestigated, and cyclic trimer in addition to cyclic tetramer was isolated from the polymerization products the quantities of both oligomers were dependent on the temperature and on the initiator used 32). The effect of the initiator structure is illustrated by the data in Table 5.6. 

The Paternd-Buchi reaction of 1,3-dimethylthymine and 1,3-dimethyl-uracil with benzophenones gives rise to two regioisomeric oxetanes (31). Substituent, temperature and heavy atom effects on the reaction are discussed in terms of entropy v.s. enthalpy control. Irradiation of uracil in frozen aqueous solution produces two diastereomeric (6-4) products. 

A practical and efficient asymmetric synthesis of 2-substituted oxetane-3-ones 46 has been developed by Shipman and co-workers by lithiation of SAMP/RAMP hydrazones of oxetane-3-one 45, followed by interception of the putative azaenolate lithiated intermediate with a range of electrophiles that include alkyl, aUyl and benzyl halides, and an aldehyde (Scheme 13) (2013JOC12243). As for the bases, w-BuLi and <-BuLi were found to be the most suitable for the metalation step providing adducts 46 in good yields and enantioselectivities (up to 84% ee), whereas LDA was less effective. Conversion of hydrazones 46 to the enantiomerically enriched 2-substituted oxetane-3-ones 47 can be achieved without detectable racemization using aqueous oxaHc acid at room temperature. 

Details of the procedures used in the preparation of commercial formaldehyde copolymers have not been fully disclosed. The principal monomer is trioxan and the second monomer is a cyclic ether such as ethylene oxide, 1,3-dioxolane or an oxetane ethylene oxide appears to be the preferred comonomer and is used at a level of about 2%. Boron trifluoride (or its etherate) is apparently the most satisfactory initiator, although many cationic initiators are effective anionic and free radical initiators are not effective. The reaction is carried out in bulk. The rapid solidification of the polymer requires a reactor fitted with a powerful stirrer to reduce particle size and permit adequate temperature control. The copolymer is then heated at 100°C with aqueous ammonia in this step, chain-ends are depolymerized to the copolymer units to give a thermally-stable product. The polymer is filtered off and dried prior to stabilizer incorporation, extrusion and granulation. 

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