Cycloaddition with oxygen

The tricarbonylchromium complex 6 of methyl 1/f-azepine-l-carboxylate undergoes photoin-duced [6 + 4] 7t-cycloadditions with dienes, e.g. 7, to give, after decomplexation with oxygen, azabicyclo[4.4.1]undecanes, e.g. 8, in high yields.276... 

Singlet oxygen undergoes [4 + 2] cycloaddition with dienes. 

Several unusual cycloaddition reactions of 9 with unsaturated ketones should be mentioned in conclusion the heterocumulene generated photolytically from 7 undergoes [8 + 2]-cycloaddition with tropone to form 33 (40%) the structure of the product has been unequivocally established by X-ray structure analysis 22,23). Once again, the affinity of phosphorus for oxygen is manifested an entirely analogous cycloaddition reaction is known for diphenylketene 26). 

Structure B is of most interest. It is responsible for the activity of nitronates as 1,3-dipoles in [3+ 2]-cycloaddition reactions. This is the most important aspect of the reactivity of nitronates determining the significance of these compounds in organic synthesis (see e.g., Ref. 267). In addition, this structure suggests that nitronates can show both, O -nucleophilic properties, that is, react at the oxygen atom with electrophiles, and a-C-electrophilic properties, that is, add nucleophiles at the a-carbon atom. 

The positional selectivity of the cycloadditions of 351 is of particular interest, since the [2 + 2]-cydoadditions proceed exclusively at the double bond bearing the oxygen atom, whereas the [4 + 2]-cycloadditions occur at the double bond remote of the oxygen atom with the exception of the major product resulting from 1,3-cydohexadiene (369, Scheme 6.75). Supposing diradical mechanisms, these findings require that... 

Table 7 Diastereoselectivity of the [4+2] cycloaddition with singlet oxygen as dienophile...

The acyclic precursor is an oc, 3-unsaturated amido aldehyde that was condensed with iV-methylhydroxylamine to generate the nitrone ( )-48, which then underwent a spontaneous cycloaddition with the alkene to afford the 5,5-ring system of the isoxazolidinyl lactam 47. The observed product arises via the ( )-nitrone transition state A [or the (Z)-nitrone equivalent] in which the position of the benzyl group ot to the nitrone effectively controls the two adjacent stereocenters while a third stereocenter is predicted from the alkene geometry. Both transition states maintain the benzyl auxiliary in an equatorial position and thus avoid the unfavorable 1,3-diaxial interaction with the nitrone methyl or oxygen found in transition state B. Semiempirical PM3 calculations confirm the extra stability, predicting exclusive formation of the observed product 47. Related cycloadducts from the intramolecular reaction of nitrones containing ester- rather than amide-tethered alkene functionality are also known (83-85). 

Intramolecular ylide formation with the lactone carbonyl oxygen (53) in 145 provided a carbonyl ylide 146 that was trapped with Al-phenyl maleimide to give cycloadduct 147. Likewise (54), carbonyl yhde 149, derived from ester 148, suffers intramolecular cycloaddition with the tethered alkene to deliver acetal 150 in 87% yield. An enantioselective version of this process has also been described (Scheme 4.33). 

Kato et al. (113) have had much better success in performing 1,3-dipolar cycloadditions with isomiinchnones than with miinchnones (see above). Thus, the room temperature union of isomiinchnone 51a with benzocyclopropene (249) leads to a syn-cycloadduct 250. The latter is remarkably stable and is recovered unchanged upon heating to 300°C. It is also impervious to the action of tributylpho-sphine, in Kato s abortive attempt to excise the bridging oxygen, which would have led to a methanooxonine. 

While both hydrogenation and epoxidation reactions of (7) (and substituted forms) occur on the oxepin valence tautomer, cycloaddition reactions proceed more readily on the arene oxide form (where the diene is closer to planarity). Thus the dienophiles DM AD and maleic anhydride (MA) readily yielded [4 + 2] cycloadducts with (7) as shown in Scheme 22 (67AG(E)385). A similar type of singlet oxygen cycloaddition reaction gave an unstable endoperoxide (106) which upon heating yielded trans-benzene trioxide quantitatively (equation 14). (75JOC3743). 

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