Cyclopentadiene, formylation

Under the conditions of the Vilsmeyer reaction, 2,3,4-triphenyl-cyclopentadiene triphenylarsorane (8) was converted to its formylated product (23) (65). 

Cyclic acetals 7 and 9 with one exocyclic oxygen atom react with bis(dimethylaluminum)selenide in the presence of cyclopentadiene at 100 JC giving the cycloadducts 8 and 10, each having a hydroxy group at the terminal position108. Similarly, the reaction of dihydropyranyl methyl acetal 11 affords the Diels-Alder adduct 12 with a side chain having a terminal formyl group108. 

The mechanism of the photochemically induced conversion of vinylazides into 2H-azirines has been examined using ab initio MO calculations. A new approach to the elusive thionitrosobenzene system has been reported. Direct irradiation of the 3-azido-2,l-benzisothiazole (88) affords a transient 2-cyanothionitrosobenzene (89) which can be trapped as the cycloadduct (90) with cyclopentadiene. Nitrene cyclization is the preferred pathway on irradiation of the 5-formyl- or 5-benzoyl-6-azidouracils (91) and yields the isoxazolo[3,4-d]pyrimidines (92). In contrast, 5-phenyl- and 5-benzyl-6-azidouracils were converted into pyrimido[4,5-b]indoles and pyrimido[4,5-ti Jquinolines, respectively, by a pathway involving photochemically induced loss of nitrogen and intramolecular nitrene insertion. 

The absolute stereochemical course of the enantioselective Diels-Alder reactions can be rationalized on the basis of the mechanistic model previously described for the reaction of a,/3-enals and cyclopentadiene with a catalyst in which a formyl CH - O hydrogen-bond provides additional organization of the transition state [25,35]. On the basis of that analysis, there are two possible approaches of cyclopentadiene to the... 

Xylitol has been derived from the product of photo-oxidation of cyclopentadiene [204], (Z)-(4RS)-4,5-epoxypent-2-enal (Scheme 13.105). Chemoselective reduction of the formyl group gives cis-hydroxyepoxypentene 448, which is directly acetylated into 449. Treatment of 449 with tetrabutylammonium acetate in AC2O opens the epoxide with formation of 450. De-O-acetylation gives 451, the epoxidation of which with p-nitroperbenzoic acid generates a 3 7 mixture of epoxides 452 and 453, isolated as peracetates. The major epoxide 453 is hydrolyzed into xylitol via the orthoester 454. 

I = cyclopentadiene II = pyrrole III = 2,5-dimethylpyrrole IV = 2,5-di(trifluoromethyl)pyrrole V = 2,5-diformylpyrrole VI = 2,5-disulfonylpyrrole VII = 1-methylpyrrole VIII = 1-trifluoromethylpyrrole IX = 1-formylpyrrole X = 1-sulfonylpyrrole XI = l-formyl-2,5-dimethylpyrrole XII = lithium-pyrrole XIII = pyrrolium ion A = LUMOgcetylene HOMOdienel B = LUMOdiene HOMOacetylene 0 = LUMOdiformylacetylene HOMOdiene D = LUMOdiene HOMOdiformylacetylene E = LUMOdimethoxyacetylene HOMOdienei F = LUMOdiene HOMOmethoxyacetylene. 

The reaction of ODC6 DO to form a five member ring ODY(C5 )CDO radical has the lowest barrier of only 15 kcal mol This ODY(C5 )CDO radical can beta scission (eliminate) a formyl radical with a 37 kcal mof barrier to form cyclopentadiene-one (ODY(C5)). It is important to note that resonantly stabilized radicals of cyclopentadiene and cyclopentadiene-ones are formed in the oxidation steps of aromatic systems. Their thermochemistry, reaction paths and kinetics all need to be analyzed for the inclusion of aromatics in combustion mechanisms. 

The high ion yield of benzene and other aromatic molecules found in low pressure gas phase studies suggests the use of these ions in preparative phtochemistry in solution. The absorption spectrum of benzene iy wat r as a solvent shows a weak band around 250 nm (E = 200 1 mole cm ) corresponding to the S S transition (see sec fon II). The photochemistry fo this molecule exhibits a great manifold of processes. Pure benzene can be transformed by irradiation in this spectral region into various ring isomers, open chain valence isomers and other minor products. In aqueous solution benzene may be photochemically oxidized to formyl-cyclopentadiene-1,3. This oxidation is believed to proceed via the formation of benzvalene in the first step which is rapidly oxidized - with and without oxygen, eqn. (1) - to the aldehyde... 

Pyrolysis of the trimethylsilyl enol ether of norbornanone is smooth at 600 C, giving 2-trimethylsilyloxycyclopentadiene. Alkylation of norbornanone thus enables the preparation of 2-alkylcyclopentenones, since the cyclopentadiene formed on pyrolysis is readily hydrolysed to the enone (Scheme 52). Trimethyl orthoformate in the presence of a Lewis acid has been employed to formylate olefins related to Vitamin A, and a Vilsmeier-Haack formylation of limonene, using N,iV-dimethylchloromethaniminium phosphodichloridate, has been reported. No comments on the generality of the methods were noted. 

Enantiopure 2-formyl-3,4-dimethylphosphaferrocene has been prepared by column chromatography of the aminals from reaction with (i ),(i )-l,2-di(iV-methylamino)cyclohexane. Chiral pinene-fused cyclopentadienyl-containing phosphaferrocenes have also been prepared directly from the iron tricarbonyl complex PCp Q CO)2 z /-butyl-phosphole, where PCp = pinene-fused cyclopentadiene. A crystal structure of the product phosphaferrocene is shown as... 

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