Dienes oxetanes

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-vinyl-7-laCtone 4I9[380], The /i,7-titisaturated 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]. 

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

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. 

Fig. 2. Schematic representation of paclitaxel biosynthesis. Dimethylallyl-diphosphate and isopentenyl-diphosphate are condensed through geranylgeranyl diphosphate synthase activity to render geranylgeranyl-diphosphate (GGPP). GGPP is converted into taxa-4(5), 11 (12)-diene in a reaction catalyzed by the taxane synthase (TS). A series of reactions catalyzed by cytochrome P450 monoxygenases lead to the production of a taxane intermediate that is further converted to baccatin III through enzymes-driven oxidation and oxetane ring formation. The side chain moiety of paclitaxel is derived from L-phenylalanine. Three consecutive arrows mean multiple steps. Ac, acetyl Bz, benzoyl.

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... 

If the triplet energy of the carbonyl compound is below that of the diene, triplet-triplet transfer will become inefficient and photocycloaddition may occur. For example, the triplet energy of 1,4-benzo-quinone is about 50 kcal mole - V41 and photocycloaddition to dienes can indeed occur.42 The products are spiro-pyrans 13 (not vinyl oxetanes), which may arise via the allylic radical intermediate 12. 

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... 

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. 

Turro69 has reported that several alkyl ketones react with 1,2-dicyano-ethylene (12) by nucleophilic attack of the ketone (rr, n) singlet state to give oxetanes stereospecifically. Addition of diene quenchers which are known to deactivate ketone triplets do not affect the rate of oxetane formation. This... 

With conjugated dienes, photocycloaddition of carbonyl compounds occurs at one of the double bonds to give vinyloxetanes. An interesting example is the reaction of acetone with 2-methyl-l,3-butadiene, which gave the two oxetanes (60) and (61) in a ratio of 3 1 and a total yield of about 20% (72JA8761). Other alkenes which have been used for photosynthesis of oxetanes include enol ethers, ketene acetals, enamines, allenes and diketene, with the reaction of the last compound with benzaldehyde illustrated in equation (105) (75CPB365). 

A novel synthesis of an oxetane from a 1,4-diene by means of the new reagent PhSeOH has been reported. From 2,4-dimethyl-1,4-pe ntadiene there was obtained 2,4-dimethyl-2,4-bis(phenylselenomethyl)oxetane (65) in 34% yield. This compound can be deselenated with Raney nickel to 2,2,4,4-tetramethyloxetane (79JOC1742). 

Intramolecular photoaddition in cis-5,5,6-trimethylhepta-3,6-dien-2-one (284) takes a different course,296 yielding not the oxetane but the two dihydropyrans (285 and 286). This is in contrast to the inter-molecular cycloaddition of a, jS-unsaturated aldehydes to alkenes which affords only oxetanes, and has been accounted for in terms of diradical intermediates (287 and 288) formed from the s-cis conformation (284) of the dienone. The intermolecular equivalent is thought to occur by addition to the s-trans conformation. 

Reaktion nicht vielmehr addiert es sich an das Olefin zu einem Oxetan. Bestrahlung von Benzonitril in Gegenwart von Athylvinylather, Vinyl-acetat und cis-Dichlorathylen fuhrte nur im Falle des Athylvinylathers zu einem 1 1-Addukt, bei dem es sich vermutlich ebenfalls um ein Deri-vat des Bicyclo[4,2,0]octa-2,4-diens handelt (14). 

The nonstereospecific (using Zimmerman s definition) [108] nature of this reaction has been demonstrate by the use of stereoisomeric substrates which lead to oxetanes 114 in identical diastereomeric ratios. Another possibility for obtaining stereoisomerically pure oxetanes is the use of alkenes which simultaneously serve as quenchers for the carbonyl triplets, e.g., dienes. The photocycloaddition of acetone and 2-methyl-2,4-hexadiene (115) represents such a process leading to two regioisomeric oxetanes 116 and 117 where the substrate configuration is retained in 117 [75]. 

A triplex has been proposed as an intermediate in some photoreactions. Yang and co-workers [54] found that addition of 1,3-dienes to anthracene excimers leads to different products than does its adition to monomeric excited arenes. Lewis and co-workers [55, 56] found that stilbene excimers can be intercepted by dimethyl fumarate to give an oxetane through a presumed triplex. More recently, Schuster and co-workers [57,58] studied the Triplex Diels-Alder reaction of 1,3-dienes with enol, alkene, and acetylenic dienophiles. Take 9,10-dicyanonaphthalene (DCN)/indene (IN)/l,3-cyclohexadiene (CHD) system as an example ... 

Structures (316) with an oxetan ring, recently proposed250 for two iodo-compounds obtained during the hypoiodite reaction of cholesterol, have now been revised.251 Spectroscopic and X-ray data show the compounds to be iodo-acetals (317), converted by de-iodination and reaction with BF3-acetic anhydride into the dihydropyran (318). Photolysis of the oxime of a l,5-dien-3-one gave mainly rearranged ketonic products, isomers of type (319), which are also obtained by photo-rearrangement of the l,5-dien-3-one.252... 

Dowd and coworkers have also published results which seemingly support the intermediacy of an acyl alkyl biradical (19). When cyclobutanone [21] was irradiated at -78°C in neat 1,3-butadiene, an equimolar mixture of 3-vinylcyclohexanone [33] and oxetane [34] was produced. The formation of [34] is a characteristic bimolecular reaction of aliphatic ketones and 1,3-dienes and has ample precedent (20). The formation of cyclohexanone [33], on the other hand, is novel and has been suggested to arise... 

Finally let us consider the very unusual bimolecular addition of dienes to cyclobutanones (34), a flagrant violation of Rule 7. The reactions which were run at -78 °C can be understood if we assume that the rate of cleavage of to a biradical is slowed down because of a decrease in the available thermal activation. However, a moderate yield of oxetane is formed from cyclobutanone and butadiene (19) at 10°C. From work on the quenching of fluorescence by alkanones (53), we anticipate the rate of... 

Mazzochi and Khachik, 1981). 1-Aminoanthraquinone forms oxetanes with conjugated dienes presumably via an excited singlet exciplex (Inoue et al., 1979). Evidence has been presented for the formation of exciplexes between arylketones and norbornadiene (Barwise et al., 1980). 

The use of dienes such as piperylene in the quenching of triplet states of carbonyl compounds is well known. However, in many photochemical studies, attempts to quench the carbonyl Ti state are often complicated by the formation of oxetane photoproducts.Since the photosensitized dimerization of diene triplets is well known, synthetic applications utilizing dienes in the Patemo-BUchi photocycload dition are rather limited. However, the diene addend not only allows for rapid assemblage of functionality but remains a continuing challenge in terms of experimental efficiency and generality. 

Likewise, photocycloaddition of acetaldehyde to ( )-penta-1,3-diene gave oxetanes (84) and (85) as major products (51%, 25%), indicating that attack at the more substituted double bond is preferred with high orientational selectivity. The formation of (86) and (87) from (Z)-penta-1,3-diene (48%, 27%) implicates a singlet intermediate because of the observed retention of stereochemistry. ... 

Further evidence for singlet 1,4-diradical intermediates has been obtained in the case of photocycloaddition of cyclopropanecarbaldehyde with ( )-penta-1,3-diene, in which the formation of the cyclopropyl-oxetane (PS) dominated. This is in contrast to the case of aromatic carbonyl compounds and cyclopropylethylene in which the formation of considerable amounts of ring-opened products and ox-epins, formed by cyclopropylcarbinyl-allylcarbinyl rearrangement, implicates the intermediacy of a triplet 1,4-diradical. ... 

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