Oxetanes, 2-alkoxy

The dimerization of the parent ketene gives the P-lactone. One molecule of ketene reacts across the C=C bond as a donor and the other molecule reacts across the C=0 bond as an acceptor. This is similar to the concerted [2+2] cycloaddition reaction between bis(trifluoromethyl)ketene and ethyl vinyl ether to afford the oxetane (Scheme 26) [127], A lone pair on the carbonyl oxygen in the ketene molecule as a donor activates the C=C bond as the alkoxy group in vinyl ether. 

Photocycloaddition of aromatic carbonyl compoimds with p,p-dimethylketene silylacetals gave 2-alkoxy-4-aryloxetanes <98JCS(P1)3253>. A similar photoaddition of derivatives of stilbene to chloranil gave the isomeric spiro oxetanes 16 in very high total yields <99JOC2250>. 

In einer Zweistufen-Reaktion lassen sich Oxetane in 3-Amino-alkanoIe iiberfiihren, indem man das Oxetan zunachst mit Cyan-trimethyl-silan in siedendem Dichlormethan in Gegenwart einer geringen Menge Zinkjodid umsctzt und das so erhaltene (3-Isocyan-alkoxy)-trimethy -silan mit Chlorwasserstofl -gesattigtem Methanol behandelt3. 

A number of other useful modifications of the intramolecular Williamson synthesis have been developed. Reaction of a,a-dialkyl-/3-tosyloxy aldehydes and ketones with potassium cyanide or with sodium alkoxide gives the corresponding 2-cyano- or 2-alkoxy-oxetanes... 

The (n,n excited state of a ketone has electrophilic character, similar to that associated with alkoxy radicals, and it is not surprising that these excited states readily attack carbon-carbon multiple bonds. The overall reaction that normally ensues is a cycloaddition, giving a four-membered oxygen heterocycle—an oxetane from an alkene addend (4.62), or an oxete from an alkyne addend (4.63). Some oxetanes are of interest in their own right, but many are useful intermediates in the synthesis of other compounds. 

The photo-cycloaddition of ethylene to 3-alkoxy chromones such as 315 has been applied to the synthesis of marine sesquiterpene filiformin and congeners (Scheme 43) <1996JOC4391>. Tandem [2+2] 7i-photo-cycloaddi-tion and y-hydrogen abstraction provided tetracyclic intermediate 316 which was converted to terpene 317 by subsequent oxetane ring reduction and acid-catalyzed rearrangement. 

It has been thought for some time that the interactions of triplet ketones with olefins involve a competition between hydrogen abstraction from, energy transfer to, and cycloaddition to the double bond 126>. Cycloaddition has generally been considered to proceed via a diradical intermediate. At first, the only evidence for a diradical was that the regioselectivity of oxetane formation often — but not always — is such as would be expected from the relative stabilities of the possible diradical intermediates 126>. Moreover, n,n ketone triplets are known to act like alkoxy radicals, which add to olefins, albeit less efficiently than do most other... 

Photooxetane formation is quite inefficient, a fact which usually points to the presence of an intermediate which can partially revert to ground state reactants. Cleavage of the diradical must be responsible for some of the inefficiency in oxetane formation 129>. However, in the past few years convincing evidence has appeared that a CT complex precedes the diradical iso.isi). The two most telling pieces of evidence are the relative reactivities of different alkenes 130> and the absence of any measurable secondary deuterium isotope effect on quenching rate constants 131>. Relative quenching rates of sterically un crowded olefins are proportional both to the ionization potentials of the donor olefins 130> and to the reduction potentials of the acceptor ketones 131>, as would be expected for a CT process. Inasmuch as n,n triplets resemble electron-deficient alkoxy radicals, such substituent effects would also be expected on direct radical addition of triplet ketone to olefin. However, radical addition would yield an inverse isotope effect (in, say, 2-butene-2,3-d2) and would be faster to 1,1-dialkylethylenes than to 1,2-dialkylethylenes, in contrast to the actual observations. 

The malic acid derivative 12 reacted with benzaldehyde to yield the oxetanes 13a,b with a diastereomeric excess of 80% (Scheme 4) [10]. It should also be mentioned that the regioselectivity and the exo/endo selectivity are complete. The favored formation of 13a is explained by the dominant conformation depicted in A, B. The syn approach of benzaldehyde excited in the 3n,7r state with respect to the alkoxy substituent (transition state A) is hindered by electrostatic repulsion between the substituent and the carbonyl group having a reversed polarity in the excited state. The addition of benzophenone to the furan derivative 14 was stereospecific [11]. In this case, however, the attack of the 3n,TT excited ketone occurred in a syn manner with respect to the hydroxy function to yield 15. The conformation indicated in the transition state C was supported by calculations. 

A similar application of a vinylcyclopropane in a stereoselective photoreaction has been repotted by Dtirr and coworkers. Scheme S shows the addition of a bicyclo[3.1.0]hexenol derivative and acetophenone, which afforded the strained 7-oxatricyclo[4.2.0.0]octane and a comparable amount of tetraphe-nylcyclopentadiene. Since the vinylcyclopropane is known to undergo photochemical isomerization and photofragmentation to the cyclopentadiene, it is conceivable that these reactions precede photocycloaddition. If this is the case, the major oxetane (4 1 selectivity) arises from addition of the alkoxy radical to... 

One may overcome this inpasse by changing the direction of approach of the arylsulfone carbanion to the ketone in IV from the endo to the exo side of the molecule, as depicted in IV (see Scheme 11.3). This is not only sterically feasible, but in addition the alkoxy and tosylate units would be parallel to one another at close to bonding distance. Their collapse to the oxetane postulated previously would occur easily as portrayed in XII - XIII. 

A spirothietane sulfone-oxetane is a comonomer in the preparation of polyethers. A polymer obtained from this sulfone in a solution of bis(3,3-chloromethyl) oxetane with phosphorus pentafluoride can be spun to drawable filaments. Thietane sulfone spirocyclic carbonates may be polymerized via the carbonate group to high-molecular-weight solids said to be useful in laminating. Thietane 1,1-dioxide improves the dye receptivity of poly (acrylonitrile), viscose, cellulose acetate, and poly(vinyl chloride). It is also reported to be a stabilizer for nitric acid in oxidizer mixtures for rocket motors. 2-Methylthietane 1,1-dioxide is claimed to be superior to sulfolane (thiolane 1,1-dioxide) in the liquid extraction of aromatic hydrocarbons from mixtures with saturated hydrocarbons. " A number of bis(3,3-alkoxy) thietane 1,1-dioxides have been proposed as intermediates in the preparation of cyanine dyes useful as photographic sensitizers. " ... 

The photoaddition of aldehydes or ketones to furan has been reported over the years. Griesbeck and his co-workers have established that the diaster-eoselectivity of the addition of benzaldehyde to furan is 212 1 for the formation of the exo and endo products (98) and (99). The study was extended to the addition of other carbonyl compounds (100) and the ratio of products from these additions is shown under the appropriate structure (101) and (102). L-Ascorbic acid and some of its derivatives (103) also undergo photochemical addition of aromatic aldehydes and ketones. With benzaldehyde and benzophenone the products obtained are the mixture of (104) and (105) with a preference for the formation of the former. The stereochemistry of the addition of the excited state carbonyl compound to ascorbic acid favours the path where the phenyl and the alkoxy groups are cis on the resultant oxetane. Benzaldehyde adds photochemically to the enamine (106) when the mixture is irradiated in acetonitrile solution. Three products are... 

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