Oxetanes unsaturated

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... 

Two different alkenes can be brought to reaction to give a [2 -I- 2] cycloaddition product. If one of the reactants is an o, /3-unsaturated ketone 11, this will be easier to bring to an excited state than an ordinary alkene or an enol ether e.g. 12. Consequently the excited carbonyl compound reacts with the ground state enol ether. By a competing reaction pathway, the Patemo-Buchi reaction of the 0, /3-unsaturated ketone may lead to formation of an oxetane, which however shall not be taken into account here ... 

The a,( -unsaturated aldehyde 452 is generated from the unstable spiro-oxetane 451, and hydrogen abstraction from the aldehydic C-H bond by 3449 gave a triplet radical pair 453 and 454. Intersystem crossing and radical recombination followed by intramolecular nucleophilic attack of the hydroxyl group toward the ketene functionality furnish the diastereomeric products 54 and 55 (Scheme 102) <20000L2583>. 

The unsaturated aldehyde in (2.2) very often undergoes intramolecular photocycloaddition with formation of an oxetane (cf, chapter 4.3.6). The... 

It can be concluded that the [3+2] pathway seems to be the only feasible reaction pathway for the dihydroxylation by permanganate. The study on the free activation energies for the oxidation of a. P unsaturated carboxylic acids by permanganate shows that the [3+2] mechanism is in better agreement with experimental data than the [2+2] pathway. Experimentally determined kinetic isotope effects for cinnamic acid are in good agreement with calculated isotope effects for the [3+2] pathway, therefore it can be concluded that a pathway via an oxetane intermediate is not feasible. 

Carbonyl Compounds, The Photocycloaddition of, to Unsaturated Systems The Syntheses of Oxetanes (Arnold). 

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... 

VII. Tabular Survey of Photocycloaddition Reactions of Carbonyl Compounds to Unsaturated Systems which Yield Oxetanes. 349... 

Two different pathways have been proposed for the photocycloaddition reaction. One depicted in step 4, 3a, p. 304, involves addition of the lone (j) electron of the oxygen atom to an electron-rich unsaturated system yielding a diradical intermediate. Subsequent closure of the diradical intermediate leads to the observed product oxetane. 

The irradiation of , 3-unsaturated carbonyl compounds in the presence of olefins does not usually lead to oxetanes. In some cases, however, a photocycloaddition reaction takes place, yielding a cyclobutane ring. This has proved to be a useful reaction which has warranted recent review.71 These carbonyl compounds typically are not reduced upon irradiation in isopropanol, nor do they show any phosphorescence emission. The mechanism of this reaction has been discussed 72,73 however, the nature of the excited state involved (n,n or 7r,7r singlet or triplet) is still in question. 

There are some cases where both types of photocycloaddition take place. For example, cinnamaldehyde and crotonaldehyde yield, upon irradiation with 2-methyl-2-butene, both the oxetane and the cyclobutane products.26 In marked contrast, mesityl oxide, as similar as it would appear to be to crotonaldehyde (Table I), is stable to irradiation in the presence of both isobutylene and isopropanol.37,74 These differences in reactivity of a,/9-unsaturated carbonyl compounds have been attributed to conformational (that is, s-cis or s-trans) differences.74... 

For carbonyl compounds that ordinarily would react in the n,n triplet state, an important reaction which can compete with photocycloaddition is energy transfer to the unsaturated system. When the triplet energy of the unsaturated system is below that of the carbonyl compound, triplet-triplet transfer may take place to the exclusion of oxetane formation. Such is the case when irradiation of a carbonyl compound in the presence of a diene results in isomerization and dimerization of the diene, while no direct photocycloaddition takes... 

VII. TABULAR SURVEY OF PHOTOCYCLOADDITION REACTIONS OF CARBONYL COMPOUNDS TO UNSATURATED SYSTEMS WHICH YIELD OXETANES... 

Carbonyl compound Unsaturated system Oxetane (Yield, Comments Refs. 

Bishop and Hamer found that acyclic a,/8-unsaturated 1,2-diketones form cyclopentanol derivatives in high yield, while / ,y-unsaturated derivatives form oxetanes by internal cycloaddition.114 Unexpectedly, the y,S-unsatu-rated derivatives also gave oxetanes after an initial migration of the double bond to the /8,y position. The formation of oxetanes such as 38 was observed in the camphorquinone sensitized dimerization of butadiene.115 Photocycloadditions of a-diketones to various olefins have been studied by several groups.116... 

The reaction of alkenyl mercurials with alkenes forms 7r-allylpalladium intermediates by the rearrangement of Pd via the elimination of H—Pd—Cl and its reverse readdition. Further transformations such as trapping with nucleophiles or elimination form conjugated dienes[379]. The 7r-allylpalladium intermediate 418 formed from 3-butenoic acid reacts intramolecularly with carboxylic acid to yield the 7-vinyI-7-lactone 419[380]. The /3,7-unsaturated amide 421 is obtained by the reaction of 4-vinyl-2-azetidinone (420) with an organomercur-ial. Similarly homoallylic alcohols are obtained from vinylic oxetanes[381]. 

Oxetanes can be formed by intramolecular reaction between a carbonyl group and an alkene, and this has been used (4.74) in making analogues of thromboxane A, (one of the compounds responsible for the control of blood clotting), albeit usually as the minor product. A special case of intramolecular reaction is seen for a,p-unsaturated carboxylic acids 14.75), where the product is an oxete that is tautomeric with a p-lactone. Oxetes may also be formed by photocycloaddition of ketones or aldehydes with alkynes the oxete normally ring-opens at room temperature to give an a,p-unsaturated carbonyl compound (4.76), but at lower temperatures its spectral... 

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. 

Photocycloadditions are common and usually involve diradical intermediates photo-excited ketones react with a variety of unsaturated systems (Scheme 1). Both the singlet and the triplet (n, it ) excited states of the ketones will form oxetanes with electron-rich alkenes. With electron-deficient alkenes only the singlet states give oxetanes. Diradicals are the immediate precursors to the oxetanes in all cases, but the diradicals are formed by different mechanisms, depending on the availability of electrons in the two components. 

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