Oxacarbene formation

This excludes a-cleavage which would be expected to result in equilibration at C-13. 

Cleavage of Bonds Attaching Atoms or Groups to the a-Carbon of Ketones 

The transformations are not sensitive to the presence of oxygen or changes in the solvent (isopropanol, ethanol, dioxane, benzene, 1,3-pentadiene). Results from studies with aliphatic and monocyclic epoxy ketones  

The initially formed ]5-dicarbonyl compounds are subject to further photo-transformations. One example is provided in the case of epoxy ketone (88), where the resulting /5-diketone (89) undergoes partly a-cleavage and acyl-alkyl 

Irradiations of the unsaturated epoxy ketone (92) in the presence of tri- -butylstannane at 2537 A (in dioxane) or 3100 A (in dioxane or benzene) produce qualitatively similar mixtures containing products (71), (101), 

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The photochemistry of cyclobutanones differs from that of less strained larger cycloalkanones. Fragmentation to ethylene and ketene (derivatives), decarbonylation and rearrangement to oxacarbenes predominate here. The oxacarbene formation, which occurs with retention of the configuration of the... 

Photochemical oxacarbene formation, 307 Photochemical rearrangements of cross-conjugated cyclohexadienones, 330 Photochemical rearrangements of enol esters and enol lactones, 339... 

Only epimerization was observed on photolysis of 2-methoxynorbomane, in contrast with the Norrish II eliminations known to occur with other a-alkoxycyclo-alkanones. The suppression of enal formation by a suitable substitution pattern and molecular geometry is responsible for the observed photochemical ring expansion of the camphor derivative (432), in methanol containing sodium bicarbonate, to the mixture of acetals (433). The reaction involves oxacarbene formation the ketene-derived product is also formed, but in a minor reaction pathway. A second short paper considers the effects of structure and substitution pattern upon ketene versus oxacarbene formation in the photolysis of bridged 2-norbomanones. ... 

Scheme 10.14 rationalizes the divergent behavior of the two catalytic systems in these selective transformations of pent-l-yn-ols. The presence of phosphine ligands promotes the formation of ruthenium vinylidene species which are key intermediates in both reactions. The more electron-rich (p-MeOC6Fl4)3P phosphine favors the formation of a cyclic oxacarbene complex which leads to the lactone after attack of the N-hydroxysuccinimide anion on the carbenic carbon. In contrast, the more labile electron-poor (p-FC6H4)3P) phosphine is exchanged with the N-hydroxysuccinimide anion and makes possible the formation of an anionic ruthenium intermediate which liberates the cyclic enol ether after protonation. 

The photochemical formation of ring-expanded acetals from ketones in alcoholic solvents was originally suggested by Yates and Kilmurry (9,38) to proceed by way of oxacarbenes (e.g., Scheme II) (39). The inter-... 

The formation of [90] is most easily explained by intramolecular trapping of oxacarbene [91]. 

More work on the nature of product formation from cyclobutanones, such as [79a], in inert solvents is required in order to better understand the fate of the presumed oxacarbene intermediate. At any rate, it is clear that ring expansion can occur in the absence of hydroxylic solvents, thereby eliminating a special role for protonic solvents. 

An interesting question which has not been clarified concerns whether formation of the oxacarbene intermediate is reversible, i.e., whether the reaction [31] [26a]... 

These results can be interpreted as true "solvent effects".on the partition of some intermediate or, more specifically, as evidence for reversible formation of an oxacarbene, and its potential participation in decar-bonylation. In any case, some revealing information might be given by experiments in which the yields for all products and quantum yields for cyclobutanone disappearance are measured in an "inert" solvent as a function of added methanol. 

The photochemical behavior of cyclobutanone (IS) contrasts sharply with that of other ketones. Cyclobutanone undergoes a cleavage also from the (n, r ) state, with subsequent fragmentation to ketene and olefin, decarbonylation to cyclopropane or cyclization to oxacarbene (16), whose concerted formation has also been proposed on the basis of stereochemical observations (Stohrer et al., 1974). In contrast, cyclohexanone cleaves exclusively from the triplet state and undergoes disproportionation reactions. The photochemical activity of cyclobutanone persists even at low temperatures (77 K) where cyclohexanone is photostable. 

In the presence of pyridine, photolysis of benzocyclobutene-l,2-dione (196) leads to the formation of pyridine ylide (197), and this is thought to arise by reaction with bisketene formed from the substrate dione rather than from an oxacarbene (198). 3-(2-Hydroxy-4-methoxyphenyl)-4-phenyl-2(5H)-fura-none, one of the photoproducts of 6-methoxybenzofuran-2,3-dione and styrene, has now been synthesised. ... 

The propensity of cyclopropylideneamines to give stable adducts with alcohols was also reflected in reactions in which the strained iminocyclopropanones were generated as nonisolable intermediates. Photolysis of 3-iminocyclobutanones 13 in alcohols resulted in decarbonylation giving 2,2,3,3-tetramethylcyclopropylideneamines 14 which were isolated as the stable alcohol adducts 15. This reaction was accompanied by the formation of an a-oxacarbene 16, which was transformed into the functionalized tetrahydrofurans 17. In an inert solvent, no photolytic decarbonylation was observed, which precluded possible isolation of the corresponding tetramethyl-cyclopropylideneamine 14. ... 

As frequently noted before, the formation of the silicon-oxygen bond occurs here also with virtually complete retention of configuration based on studies with (+ )-l-naphthylphenylmethylbenzoylsilane. Similar photochemical formation of oxacarbenes have recently been reported by several workers 65-68). The role of base and acid in leading to these apparently unrelated pathways is not yet understood. Qualitative rate studies indicate that conversion of the mixed acetal to the alkoxysilane and acetal under comparable acid conditions is much slower than their direct photochemical formation so that the mixed acetal does not appear to be an intermediate in the reactions containing acid. 

The formation of 2-alkoxytetrahydrofurans from oxacarbenes by the photolysis of cyclobutanones in alcohol is a well known reaction. It has recently been used as a key step (Scheme 41) in the synthesis of the prostaglandin precursor (126). ... 

Best results were obtained with Rh(PR3)3Cl and [Rh(COD)Cl]2 catalysts in the presence of an excess of electron-poor triaryl phosphines to avoid undesirable dimerization/oligomerization processes. The proposed reaction mechanism involves the formation of the Rh-vinylidene complex followed by the intramolecular endo-dig cyclization. The protodemetallation of intermediate I seems to be the more plausible path, whereas the formation of the Rh-oxacarbene complex II was excluded because all attempts to generate lactones by using N-hydroxysuccinimide failed and the cycloisomerization product was the only product obtained (Scheme 10). 

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