2- oxetanes, preparation

Khusnutdinova JR, Newman L, Zavalij PY et al (2008) Direct C(ip )-0 reductive elimination of olefin oxides from Pt" -oxetanes prepared by aerobic oxidation of Pt olefin derivatives (Olefin = cw-Cyclooctene, Norbomene). J Am Chem Soc 130 2174—2175... 

Scheme 6.20 Synthesis of chloroalcohols by ionic opening polycyclic oxetanes prepared by Patemb-Blichi cycloaddition [63]...

Economic Aspects. Oxetanes are expensive monomers and are not readily available in commercial quantities. Commercial production of PBCMO has been discontinued its end uses were not able to support its comparatively high selling price. Energetic polymers prepared from appropriately substituted oxetanes have opened a new market for their use to prepare soHd rocket propeUants and explosives. Should this specialty market result in the large-scale production of these oxetanes even at current (1996) high prices and/or in a cheap synthetic route to oxetanes, this economic picture could change. 

Krespan ° has prepared a number of macrocycles, having both aza- and oxa-linkages in them, based on the 3,3-dimethyleneoxetane unit (see also Sect. 8.4 and Eq. 8.12). Typically, 3,3-bis(chloromethyl)oxetane is treated with a diol as shown in Eq. (3.40), in the presence of base. Once the bicyclic system is formed, further treatment with other nucleophiles (e.g., ammonia) can lead to opening of the 4-membered ring. 

This is one of the few methods available for the direct and efficient conversion of an acid, via the acid chloride, to an ortho ester. The preparation of the oxetane is straightforward, and a large number of oxetanes have been prepared [triol, (EtO)2CO, KOH]." In addition, the -butyl analogue has been used for the protection of acids. During the course of a borane reduction, the ortho ester was reduced to form a ketal. This was attributed to an intramolecular delivery of the hydride. ... 

Bis(3-nitrofurazanoxymethyl)oxetane 221 was synthesized in 52% yield by base-promoted ring closure of the corresponding 3-hydroxy-l-propyl triflate, 219, which is readily available from the diol and triflic anhydride. Oxetane 221 can also be prepared in 74% yield by treatment of the trifurazanyl ether 220 with DBU. Polymerization and copolymerization reactions of oxetane 220 have also been investigated (97MI7) (Scheme 148). 

Preparation of oxetanes by [2- -2]-photocycloaddition reactions of ketones and dienes 98S683. 

By reaction with the appropropriate aryl halides can be prepared a variety of aryltin compounds that are not accessible from the reactions involving arylmagnesium halides and organotin halides (88,89) there is evidence that an aryne intermediate may be involved (90). However, for some purposes, such as the addition to carbonyl compounds, ox-iranes, and oxetanes, to give hydroxyalkyltin compounds, the Sn-Mg reagents may have advantages (see Section II,E) (91-93). 

A completely different way of preparing isocyanides involves the reaction of epoxides or oxetanes with trimethylsilyl cyanide and zinc iodide, for example, ... 

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. 

In order to investigate the single electron donation process from a reduced flavin to a pyrimidine dimer or oxetane lesion, the photolyase model compounds 1-4 depicted in Scheme 4 were prepared [41, 42]. The first model compounds 1 and 2 contain a cyclobutane uracil (1) or thymine (2) dimer covalently connected to a flavin, which is the active electron donating subunit in photolyases. These model systems were dissolved in various solvents... 

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 ... 

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 enantioselective preparation of trans-2,4-disubstituted azetidines 4 by treatment of 3 with methanesulfonyl chloride and triethylamine followed by benzylamine at 45 °C has been reported. A-Arylation of the debenzylated 4 has given 5 in yields of 32-96% by use of rac-Binap and moderate reaction temperatures to suppress racemization of the amines . Azetidines can also be formed from certain oxetanes (see 4.2.3) and from P-Iactams (see 4.3 and 4.6) <99JOC9596>. 

The same process shown in Scheme 88 starting from different 2-substituted oxetanes and using biphenyl as the electron-carrier catalyst under THF reflux has been used to prepare regioselectively substituted primary alcohols. On the other hand, the combination of a DTBB-catalyzed ca 20%) lithiation with triethylaluminium in TFIF at —78 °C has been used for the transformation of strained oxetanes to substituted di- and triquinanes through a rearrangement process . An example is given in Scheme 89 for the transformation of oxetane 299 into the product 302 through radicals 300 and 301. 

Carbon dioxide is a widely available, inexpensive, and renewable resource. Hence, its utilization as a source of chemical carbon or as a solvent in chemical synthesis can lead to less of an impact on the environment than alternative processes. The preparation of aliphatic polycarbonates via the coupling of epoxides or oxetanes with CO2 illustrates processes where carbon dioxide can serve in both capacities, i.e., as a monomer and as a solvent. The reactions represented in (1) and (2) are two of the most well-studied instances of using carbon dioxide in chemical synthesis of polymeric materials, and represent environmentally benign routes to these biodegradable polymers. We and others have comprehensively reviewed this important area of chemistry fairly recently. Nevertheless, because of the intense interest and activity in this discipline, regular updates are warranted. 

Photocycloaddition of thiones to alkenes is the most popular and fruitful method for the preparation of the thietane system. In analogy to the formation of the oxetanes by cycloaddition of the electronic excited ( ,tc ) carbonyls, thietanes can be expected to arise photochemically from aromatic thioketones and substituted olefins as well as 1,2- and 1,3-dienes. ° Thiobenzophenone serves as a source of a sulfur atom and, because of its blue color, which disappears on photocycloaddition, permits exact control over the reaction time. A mixture of thiobenzophenone and a-phellandrene must be irradiated for 70 hr before the blue color disappears (Eq. 2) and... 

In the years since these reviews were written the usefulness of the photocycloaddition reaction for the preparation of oxetanes has certainly been demonstrated. The yields can be high, sometimes nearly quantitative, and the starting materials are readily available. Photocycloaddition is frequently the method of choice for the preparation of oxetanes since they can be difficult to prepare by more classical methods. 

The conditions for the photocycloaddition (discussed in detail in a later section of this review) can be relatively mild. There is usually a small probability of the oxetane being destroyed in dark reactions which would probably preclude isolation after preparation by any method. One mode of decomposition of oxetanes is fragmentation, either back to the starting materials or to the other possible carbonyl compound and olefin. For example, the oxetane from 4,4 -dimethoxybenzophenone and isobutylene forms readily and is easily detected and characterized by infrared and NMR spectroscopy. All efforts to purify it, however, have led to its decomposition into formaldehyde and the diarylethy-lene.17 37 In some cases, as with fluorenone and isobutylene37 or 2-methyl-2-butene,25b the oxetane is apparently too unstable for detection, but the presence of the olefin 96 attests to its formation. 

There are a wide variety of other synthetic methods for the preparation of oxetanes however, most of these are not very general. They frequently require starting materials which are difficult to prepare, and rarely give high yields (never as high as the better photochemical preparations). It is clear then that in comparison with the alternative methods of oxetane synthesis, the photocycloaddition reaction is more generally useful and convenient. This synthetic utility justifies a brief description of experimental conditions. 

Many of the compounds of this type have been prepared by interaction of cumulene ylides witli heterocumulenes and these are covered in a review by Bestmann <77AG(E)349>. The oxetane system 5 results from reaction of Ph3P=C=C=S with aryl isocyanates, ArNCO <77AG(E)349>. An unusual approach to an oxetane ylide is the reaction of DMAD with CO2 and two equivalents of methyl phosphite which gives 6 <81CC607>. 

Carbohydrate oxetanes have been prepared from deoxyiodo sugars without die use of strongly alkaline reagents. Thus, treatment of 5-deoxy-5-iodo-l,2-0-isopropylidene-a-D-xylofuranose with silver fluoride in cold pyridine afforded 3,5-anhydro-l,2-0-isopropylidene-... 

The synthesis of an alditol having a 4-membered (oxetane) ring was first reported by Ustyuzhanin and coworkers,50 who prepared 1,3-anhydro-5,6-di-0-methyl-2,4-0-methylene-D-glucitol by saponification of the 1-p-toluenesulfonate of the corresponding derivative of D-... 

Previous general reviews of oxetanes and of 2-oxetanones were published in 1964 (64HC(19-2)983, 64HC( 19-2)729), covering the literature to about 1962. Additional surveys of methods of preparation and reactions of these classes of compounds were published in 1963 and 1965 <63HOU(6/2)511, 65HOU(6/3)489>. 

Bromo-, 3-chloro-, 3-fluoro- and 3-iodo-oxetanes have all been prepared in good yield by the reaction of 3-oxetanyl tosylate with alkali metal halides in hot triethylene glycol (equation 70). Substitution reactions of the halogen atom have not been reported, except for the reaction of 3-iodooxetane with diethylamine. A low yield of 2-diethylaminooxetane was obtained from this reaction at 200 °C, but its chemical properties are not known (73JOC2061). 

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