Aldehydes, oxetane formation

The quantum yields for oxetane formation have not been determined in every case, and only a few relative rate constants are known. The reactivities of singlet and triplet states of alkyl ketones are very nearly equal in attack on electron rich olefins. 72> However, acetone singlets are about an order of magnitude more reactive in nucleophilic attack on electron-deficient olefins. 61 > Oxetane formation is competitive with a-cleavage, hydrogen abstraction and energy-transfer reactions 60 64> so the absolute rates must be reasonably high. Aryl aldehydes and ketones add to olefins with lower quantum yields, 66> and 3n-n states are particularly unreactive. 76>... 

Table IV summarizes the pertinent characteristics of some of the naphthyl carbonyl compounds. All of these compounds emit from a it,7T triplet very similar to that of naphthalene. Those that have been studied are resistant to photoreduction in isopropyl alcohol and photocycloaddition with 2-methyl-2-butene25 and isobutylene.17 Significant oxetane formation was, however, observed with the aldehydes, albeit with only moderate efficiency (quantum yield approximately one-tenth that of benzaldehyde).25...

Oxetane Formation—The Patemo-Bnchi Reaction. A large number of carbonyl compounds, primarily aldehydes, ketones, and quinones, form oxetanes by photocycloadditions to olefins.61-63 In general, it is observed that (/) carbonyl compounds which have low-lying (77, ) triplet states and which are photoreduced in isopropyl alcohol form oxetanes most readily, and (2) oxetane formation takes place when energy transfer from the carbonyl compound to the olefin is unfavorable because of the relative location of their triplet levels.64,65 Hence, oxetanes are most readily formed from simple olefins and allenes63,66 but are seldom formed from dienes.67 An extensive review by Arnold63 covers the mechanism and scope of this reaction. 

The double bonds in certain heterocyclic compounds, such as furans, Af-acylpyrroles and A-acylindoles are also susceptible to photoaddition of carbonyl compounds to form oxetanes (equation 106) (77JHC1777). A wide range of carbonyl compounds can be used, including quinones, a-diketones, acyl cyanides, perfluorinated aldehydes and ketones and esters. A remarkable case of asymmetric induction in oxetane formation has been reported from optically active menthyl phenylglyoxylate and 2,3-dimethyl-2-butene the oxetane product obtained after hydrolysis of the ester group had an optical purity of 53% (79AG(E)868). 

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... 

A highly diastereoselective oxetane formation was identified in the PB reaction of dihydropyridone with a m-hydroxybenzaldehyde derivative (Scheme 7.33). The chiral auxiliary, when bound to the aldehyde, offered a binding site to which the reaction partner could attach by two hydrogen bonds. In the hydrogen-bonded complex that was produced, the two enantiotopic faces of the alkene could be differentiated [52]... 

Funke, C. W., Cerfontain, H. Photochemical oxetane formation the Paterno-Biichi reaction of aliphatic aldehydes and ketones with alkenes and dienes. J. Chem. Soc., Perkin Trans. 2 1976, 1902-1908. 

The lowest excited singlet states of aliphatic aldehydes and ketones have lifetimes on the order of nanoseconds, but they can be trapped by alkenes in a diffusion-controlled bimolecular oxetane formation. According to a theoretical study, a C-atom attack mechanism is either a concerted process producing oxetane directly or it involves a C—C bonded transient singlet biradical intermediate that rapidly cyclizes.896 The O-atom attack, in contrast, represents a nonconcerted path, allowing conformational motion of the shortlived intermediate thereby formed. 

Oxetane Formation. Reactions of carbonyl compounds such as ketones and aldehydes with electron-rich olefins results in the formation of oxetanes. The oxetane formation involves the addition of the carbonyl oxygen to the olefinic n -system to produce a biradical intermediate, which then undergoes spin inversion to produce the oxetane (Hor-spool, 1976). Typical examples of oxetane formation include the photoreaction of furan compounds with aldehyde or alkene compounds as shown in Scheme 4 (Cantrell, 1977 Whipple and Evanega, 1968). 

It is also worthwhile comparing the intramolecular photochemical cycloaddition reactions of ethylenic aldehydes and ketones with free radical intramolecular additions. For instance, irradiation of 5-hexen-2-one (470) (Scheme 161) in the gas phase gives the oxetane 471 as only cyclized product, as expected from the known photochemical intermolecular reaction between olefins and ketones. If the irradiation is conducted in solution 470 gives 471 (26%) and 472 (18%). With other y,< -unsaturated ketones, the bicyclic compound analogous to 472 may become the major product. With 2-allylcyclanones such as 473 (Scheme 161) bicyclic compounds are obtained (80% yield) as a mixture of 474 and 475, with 475 being the major product, but such compounds are difficult to isolate. " In the same manner, selective irradiation of the carbonyl group of 2-acyl-2,3-dihydro-4/f-pyrans (476) leads exclusively (23% yield) to exo-brevicomin (477) (a sex attractant), neither oxetane formation nor Norrish type II reaction being observed. The formation of the compounds 472, 475, and 477 which was considered as unexpected... 

A-Substituted pyrroles, furans and dialkylthiophenes undergo photosensitized [2 + 2] cycloaddition reactions with carbonyl compounds to give oxetanes. This is illustrated by the addition of furan and benzophenone to give the oxetane (138). The photochemical reaction of pyrroles with aliphatic aldehydes and ketones results in the regiospecific formation of 3-(l-hydroxyalkyl)pyrroles (e.g. 139). The intermediate oxetane undergoes rearrangement under the reaction conditions (79JOC2949). 

Photocycloaddition Reactions of Carbonyl Compounds and Alkenes. Photocycloaddition of ketones and aldehydes with alkenes can result in formation of four-membered cyclic ethers (oxetanes), a process often referred to as the Paterno-Buchi reaction.196... 

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

Recent work by several groups has shown that formation of oxetanes from alkyl aldehydes and ketones can take place from the singlet as well as from the triplet state.66... 

The formation of 3-pyrrolylcarbinols (280) from the photochemically induced reaction of pyrrole, or its 1-alkyl derivatives, with aliphatic aldehydes and ketones is thought to proceed via an oxetane intermediate (279) (79JOC2949). In contrast, the analogous reaction of 1 -phenylpyrrole with benzophenone leads to the formation of the diphenyl(2-pyrrolyl)car-binol, whilst the oxetane (281) has been isolated from the photoaddition of 1-benzoylpyrrole and benzophenone (76JHC1037, B-77MI30500). 2-Benzoyl-1-methylpyrrole undergoes a normal Paterno-Buchi photocyclization with 2,3-dimethylbut-2-ene, via the n -> v triplet... 

The synthesis of oxetanes by the photochemical 1,2-cycloaddition of the carbonyl function in aldehydes and ketones to alkenes [Eq. (69)] was first reported by Paterno274 in 1909, and later reinvestigated by Biichi 275 in 1954. This reaction has recently been extensively reviewed.276, 277 The formation of the oxetane is apparently the result of addition of excited n, n triplet carbonyl to an alkene, although for certain aromatic aldehydes and ketones the mechanism is less clear.278... 

Stereopure epoxide 1 was prepared and treated with 3.6 equivalents of t-butyllithium in THF/HMPA at -78°C. The intention was that formation of the anion at the benzylic carbon would lead to a 4-exo-epoxide ring opening reaction a subsequent [l,2]-silyl shift (Brook rearrangement) would generate the oxetane 2 with stereocontrol at all three stereocentres. Anion formation proceeded smoothly at -78°C, then 1 ml of 1 M hydrochloric acid was added and the product isolated. Obtained pure in 40% yield, this was shown to be the aldehyde 3. No oxetane 2 was obtained. 

It has been demonstrated that optically active oxetanes can be formed from oxazolidinone 92, a crotonic acid moiety functionalized with Evans chiral auxiliary (Scheme 18) <1997JOC5048>. In this two-step aldol-cyclization sequence, the use of 92 in a deconjugative aldol reaction, with boron enolates and ethanal, led to formation of the syn-aldol 93. This product was then converted to the corresponding oxetanes, 94a and 94b, via a cyclization with iodine and sodium hydrogencarbonate. This reaction sequence was explored with other aldehydes to yield optically active oxetanes in similar yields. Unlike previous experiments using the methyl ester of crotonic acid, in an analogous reaction sequence rather than the oxazolidinone, there was no competing THF formation. 

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