Cycloreversions, oxidative

In contrast to other furoxans, the cycloreversion of 3,4-dinitrofuroxan to nitro-formonitrile oxide was observed even at room temperature. The nitrile oxide could be trapped in situ with electron-deficient nitriles (Scheme 149) (95MC231). Attempts to obtain cyclo adducts with styrene, phenylacetylene, rran.s-stilbene, and cyclohexene failed. 

Interestingly, in the inverse-electron-demand Diels-Alder reactions of oxepin with various enophiles such as cyclopentadienones and tetrazines the oxepin form, rather than the benzene oxide, undergoes the cycloaddition.234 236 Usually, the central C-C double bond acts as dienophile. Oxepin reacts with 2,5-dimethyl-3,4-diphenylcyclopenta-2,4-dienone to give the cycloadduct 6 across the 4,5-C-C double bond of the heterocycle.234 The adduct resists thermal carbon monoxide elimination but undergoes cycloreversion to oxepin and the cyclopenta-dienone.234... 

In a sequence of cycloaddition and cycloreversion, 3-phenyl-l, 2,4-triazine 1-oxides react with benzyne, generated from 2-aminobenzoic acid (see Houben-Weyl, Vol. 5/2 b, p 622 ff), to give 2-phenyl-l, 3-benzoxazepines in moderate yield.419... 

TV-Substituted l,4-dihydro-l,4-diazocines 6 can be obtained by [TC2S + 2S + 2S] cycloreversion from. mi-benzene diimine (cA-bisazirinofa. c]benzene, diaza-c-bishomobenzene) derivatives 5 at room temperature or slightly elevated temperatures.2 - 5 The syn-benzene diimines (3,8-dia-zatricyclo[5.1.0.02,4]oet-5-enes), which are required for the valence isomerization, are available by two methods from benzene oxide derivatives. 

The retro Diels-Alder reaction is strongly accelerated when an oxide anion substituent is incorporated at positions 1 and 2 of the six-membered ring which has to be cycloreversed, namely at one terminus carbon of the original diene or at one sp carbon of the dienophile [51] (Equation 1.22). 

The first example of an oxide-anion accelerated retro Diels Alder reaction was reported by Papies and Grimme [52]. The adduct 19 (Equation 1.23) treated with tetra-w-butylammonium fluoride (TBAF) in THE at room temperature is immediately converted into 20, in contrast to the parent 21 (Equation 1.24) which undergoes cycloreversion into 22 at 100 °C. The dramatic oxide-anion acceleration (> 10 ) was ascribed to the loss of basicity of about 8pK, units in the transformation of alcoholate ion of precursor 19... 

Cyclodiphosphazanes(III) 27 shown in Scheme 16 undergo oxidation reactions to give the cyclodiphosphazanes(V) of type 28. These are prospective ligands in catalysis since these ligands due to lack of phosphorus lone-pairs are less susceptible to the destructive cycloreversion of the ligands. Hence they could prevent catalyst deactivation in the process. When treated with trimethyl aluminum the cyclodiphosphazanes form symmetrically substituted bimetallic species of type 29 [90]. Characterization by single-crystal X-ray studies show... 

In yet another approach towards the synthesis of cyclocarbons by cycloreversion, Adamson and Rees [71] prepared the 1,2,3-triazole-fused dehydroannulenes 42 - 44, as mixtures of regioisomers in ca. 30 % overall yield, by oxidative Hay coupling of the protected 4,5-diethynyl-l,2,3-triazole 41 (Scheme 7). No investigations have yet been reported on the thermal or mass spectrometric [3-1-2] cycloreversions of 42-44, with loss of the triazole moieties and ultimate formation of the cyclocarbons Cis, C24, and C30, respectively. 

The cycloreversion experiments showed a clean Tf=T-DNA to T/T-DNA transformation. No by-products were detected, which supports the idea that DNA may be more stable towards reduction compared to oxidation. Even heating the irradiated DNA with piperidine furnished no other DNA strands other then the repaired strands, showing that base labile sites - indicative for DNA damage - are not formed in the reductive regime. The quantum yield of the intra-DNA repair reaction was therefore calculated based on the assumption that the irradiation of the flavin-Tf=T-DNA strands induces a clean intramolecular excess electron transfer driven cycloreversion. The quantum yield was found to be around 0=0.005, which is high for a photoreaction in DNA. A first insight into how DNA is able to mediate the excess electron transfer was gained with the double strands 11 and 12 in which an additional A T base pair compared to 7 and 8 separates the dimer and the flavin unit. 

Cycloreversion with nitrile oxide formation is known not only in furoxans but also in isoxazolines, 1,2,4-oxadiazoles, furazans, and some other live-membered heterocycles (76). Such process, eliminating nitrile oxide fragment 3-R CeHiC N+Cr ", was observed mass spectrometrically in 3a,4,5,6-tetrahydro-[ 1,2,4 oxadiazolo[4,5-a J [ 1,5 benzodiazepine derivatives 11 (83). 

Dimethyl-3-methylenepyrrolidine-2-thione, which reacts with nitrones regio- and stereoselectively at its exocyclic C=C bond to give only spirocy-cloadducts 116, behaves more complicatedly with nitrile oxides. The latter undergo 1,3-dipolar cycloaddition both to the exocyclic C=C and C=S double bonds with subsequent cycloreversion and formation of spiro-lactams 117 (281). 

Snyder and coworkers followed a completely different path to canthin-6-one (Fig. 23). Earlier they had shown that indole-substituted 1,24-triazine 66 could be heated in refluxing triisopropylbenzene (bp = 232 °C) to give /3-carboline 67 via an intramolecular cycloaddition/cycloreversion reaction [58]. Selective oxidation of 67 at C-6 was achieved through the use of triethylbenzylammonium permanganate [59]. Success of the reaction proved to be very sensitive to the solvent chosen. Heating 67 for 4 h at 70 °C in a 5 1 mixture of dichloromethane and acetic acid gave a 65% yield of 63, yet use of increasing amounts of dichloromethane slowed the reaction down (no reaction occurred in pure dichloromethane), while use of pure acetic acid led to an intractable mixture. 

ET-induced cycloadditions of polycyclic olefins and cycloreversions of cyclobutane species have been studied by ESR spectroscopy [266]. Upon chemical and electrochemical reduction, 2,2 -distyrylbiphenyl rearranges by intramolecular coupling into a bis-benzylic dihydrophenanthrene dianion (Scheme 1), which can be either protonated to a 9,10 -dibenzyl-9,10-dihydrophenanthrene or oxidatively coupled to a cyclobutane species. It is interesting to note that the intramolecular bond... 

In respect of cycloreversion, cyclobutane-fused fullerenes derived from acyclic enones [342] are less stable than their bycycKc equivalents (e.g. 293, Scheme 4.55). For the addition of mesityl oxide the equilibrium constant is so small that a 1000-fold excess of the enone is necessary to complete the reaction. The product of 302 is more stable and requires only a 100-fold excess of the enone. Reaction of 302... 

Ozonolysis of alkenes in participating solvents such as alcohols often leads to trapping of intermediates. Most commonly, an alcohol will react with the carbonyl oxide zwitterion, generated from cycloreversion of the primary ozonide (Section 4.16.8.2), to give an alkoxy hydroperoxide. The secondary ozonide (1,2,4-trioxolane) is usually more stable to nucleophilic attack from alcohols. 

Representations of those possibilities that have been exemplified are given in Table 10. The more synthetically useful processes are discussed in greater detail in Section 4.16.9 and reference is given to the relevant subsection in the table. (Note Cycloreversion of a 1,2,3-trioxolane to give a carbonyl oxide, which can undergo [3 + 2] reactions with carbonyl derivatives, is included in Section 4.16.8.)... 

The 1,2,4-diazaarsoles are colorless oils or crystals. The unsubstituted compound is deprotonated by butyllithium and subsequently alkylated or acylated at N-1 <86TL2957>. The 1-acyl derivatives (9) (R = Me, Ph) readily undergo a regiospecific cycloaddition of nitrones, nitrile oxides and diazoacetic esters (Scheme 1). The cycloaddition of diphenyl nitrile imine is more general in respect to the 1-substituent (R = H, Me, Ph, COMe). The cycloreversion of the adduct (10) at higher temperatures provides an in situ access to the 1,3-diphenyl diazaarsole (11) which immediately enters another cycloaddition (Scheme 2) <86TL2957>. 

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