5- -2,2-dimethyl Irradiation

Pyran-2-one, 5,6-dibromo-5,6-dihydro-reactions, 3, 735 Pyran-2-one, 5,6-dihydro-allylic bromination, 3, 799 dehydrogenation, 3, 724, 799 H NMR,3, 581 synthesis, 3, 841, 843 Pyran-2-one, 4,6-dimethyl-irradiation, 3, 677 photochemistry, 3, 678 Pyran-2-one, 5,6-dimethyl-chloromethylation, 3, 680 conformation, 3, 631 Pyran-2-one, 5-f ormyl-IR Spectra, 3, 595 Pyran-2-one, 6-formyl-IR spectra, 3, 595 Pyran-2-one, 5-halo-synthesis, 3, 799 Pyran-2-one, 3-hydroxy-IR spectra, 3, 595 Pyran-2-one, 4-hydroxy-methylation, 2, 57 3, 676 pyran-4-one synthesis from, 3, 816 reactions with phosphorus oxychloride, 2, 57 synthesis, 3, 792, 794, 795, 798 tautomerism, 2, 56 3, 642 Pyran-2-one, 4-hydroxy-6-methyl-methylation, 3, 692 reactions... 

Photochemical decomposition of riboflavin in neutral or acid solution gives lumichrome (3), 7,8-dimethyl all oxazine, which was synthesized and characterized by Karrer and his co-workers in 1934 (11). In alkaline solution, the irradiation product is lumiflavin (4), 7,8,10-trimethyhsoalloxazine its uv—vis absorption spectmm resembles that of riboflavin. It was prepared and characterized in 1933 (5). Another photodecomposition product of riboflavin is 7,8-dimethy1-10-foTmylmethy1isoa11oxazine (12). 

In some instances the solvents may react with the substrate during the irradiation. For example, 3,6-dichIoropyridazine, when irradiated in acidified methanol, gives a mixture of raonoraethylated (56), dimethylated (57) and hydroxyraethylated (58) compounds. Further transformation of the hydroxymethyl compound (58) results in the formation of y-Iactones (59) and succinates (60 Scheme 20). 

There is a scattered body of data in the literature on ordinary photochemical reactions in the pyrimidine and quinazoline series in most cases the mechanisms are unclear. For example, UV irradiation of 4-aminopyrimidine-5-carbonitrile (109 R=H) in methanolic hydrogen chloride gives the 2,6-dimethyl derivative (109 R = Me) in good yield the 5-aminomethyl analogue is made similarly (68T5861). Another random example is the irradiation of 4,6-diphenylpyrimidine 1-oxide in methanol to give 2-methoxy-4,6-diphenyl-pyrimidine, probably by addition of methanol to an intermediate oxaziridine (110) followed by dehydration (76JCS(P1)1202). 

The photolysis of 4-substituted 2,3-dimethyl-3-isoxazolin-5-ones has been studied. Irradiation in methanol or ethanol with a 100 W high-pressure mercury lamp through a Pyrex filter of a 4-phenylthio compound produced a semithioacetal (Scheme 5). In contrast, an H, Cl or OPh moiety gave no reaction. The use of alkylthio substitution gave similar products. Cyclic compounds yielded cyclic products (Scheme 5), and the photolysis of (29) in benzene... 

The thermal or photolytic fragmentation of furazans to nitriles and nitrile Af-oxides has been reported (73JOC1054, 75JOC2880). The irradiation of dimethylfurazan (419) in the presence of cyclopentene, and benzofurazan (420) in the presence of dimethyl acety-lenedicarboxylate, gave isoxazoline (421) and isoxazole (422), respectively, in good yields. The thermolysis of acenaphtho[l,2-c]furazan (423) in the presence of phenylacetylene gave isoxazole (424) in 55% yield. 

Nitrile ylides derived from the photolysis of 1-azirines have also been found to undergo a novel intramolecular 1,1-cycloaddition reaction (75JA3862). Irradiation of (65) gave a 1 1 mixture of azabicyclohexenes (67) and (68). On further irradiation (67) was quantitatively isomerized to (68). Photolysis of (65) in the presence of excess dimethyl acetylenedicar-boxylate resulted in the 1,3-dipolar trapping of the normal nitrile ylide. Under these conditions, the formation of azabicyclohexenes (67) and (68) was entirely suppressed. The photoreaction of the closely related methyl-substituted azirine (65b) gave azabicyclohexene (68b) as the primary photoproduct. The formation of the thermodynamically less favored endo isomer, i.e. (68b), corresponds to a complete inversion of stereochemistry about the TT-system in the cycloaddition process. 

Silacyclopentadiene, 1,1 -dimethyl-2,3,4,5-tetraphenyl-irradiation, I, 618 structure, I, 617 Silacyclopentadiene, 1-methyl-synthesis, I, 615 Silacyclopentadienes... 

Initial C-perfluoroalkylation of p diketonesoccurs dunng thenUV irradiation in the presence of perfluoroalkyl iodides in liquid ammonia Pluorinated enami-noketones are obtained by subsequent ammonolysis of a difluoromethylene group and removal of the acetyl group [131] C-alkylation of dimethyl malonate takes... 

Dimethyl-1,3-dioxolane, microwave irradiation. Montmorillonite KSF, 38-95% yield.""... 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

When N,A/ -dimethyl-Af,N -diphenyl-p-phenylenediamine (231) underwent irradiation in die presence of oxygen, a remarkably regioselective reaction was observed, from which the /V,A/ -dimediylindolo[2,3-f ]carbazole (190) was obtained, without coformation of die other possible isomeric product, indolo[3,2-/)]carbazole (4). The expected intermediate 232, where only one ring closure had taken place, could not be detected. The authors attributed this fact to a two-photon autosensitization mechanism in which die conversion of 231 into 190 is sensitized by 232, permitting accumulation of 232 only in extremely small concentrations (83JA6268). 

The same rearrangement was observed somewhat later by the irradiation of 5,5-dimethyl-A -pyrroline oxide (13)/ The product (11% yield) was shown to be identical with the oxazirane (14) synthesized from 5,5-dimethyl-A -pyrroline and hydrogen peroxide. 

The most striking feature of electrocyclic reactions is their stereochemistry. For example, (2 ,4Z,6 )-2,4,6-octatriene yields only c/s-5,6-dimethyl-l,3-cyclo-hexadiene when heated, and (2 ,4Z,6Z)-2,4,6-octatriene yields only trnns-5,6-dimethyl-l,3-cyclohexadiene. Remarkably, however, the stereochemical results change completely when the reactions are carried out under what are called photochemical, rather than thermal, conditions. Irradiation, or photolysis,... 

Highly substituted dimethyl 2,4-diphenyl-3-benzoxepin-l,5-dicarboxylate was converted by irradiation to an indene derivative 4 which had lost one benzoyl group by acidic hydrolysis.146... 

Curiously, prolonged irradiation of 1,3-dimethyl-5//-pyrazolo[l,2-a]benzotriazolyl-4-ium (108) in benzene in the presence of dipropylamine produces the 2//-azepine 109 rather than the 3H-isomer.192... 

Irradiation of dimethyl ,2.7-trimethyl-l//-azcpinc-3,6-dicarboxylate (5) yields methyl 1,2,5-trimethylpyrrole-3-carboxylate (7) most probably by loss of methyl propiolate from the bicyclic valence tautomer 6 (cf. Houben-Weyl, Vol.6a, p719).239... 

Irradiation of 5-acetyldibenz[i>,/]azepine (19) in the presence of. V-methylmaleimide results in a mixture of the homo dimer 20 and the mixed dimer 21. No [2 + 2] cycloadducts are obtained, however, with other alkenes such as styrene, acetylenedicarboxylic acid, dimethyl fumarate or acrylonitrile.32 Later studies have demonstrated that the dimerization occurs via the excited triplet state.196... 

A mixture of ethyl 3,5-dimethyl-l/f-thieno[2,3-c]-1,2-diazepine-l-carboxylate (6) (10-15%) and the isomeric ethyl 2,5-dimethyl-3//-thieno[2,3-t/]-l, 3-diazepine-3-carboxylate (8) (50-60%) is obtained when a solution of the thienopyridine A-iminc 4 in benzene is irradiated with a 400-W high-pressure mercury lamp for 2-3 hours at 20rC. It has been suggested that product 6 is formed from the intermediate 5. The isomeric intermediate 7 undergoes two consecutive walk rearrangements, followed by valence tautomerization, to give the major product 8.168... 

Azetidines 19 and 21 resulted (81M12) from photocycloaddition of 1,3-dimethyl-6-azathymine 17 with 1-heptene. The reaction could also be extended to the irradiation of 17 with 2-heptene, cyclohexene, and 1-, 3-, or 4-methylcyclohexene to give 20 and 22, respectively (87MI2). Photocycloaddition of dihalomaleimides 23 with 18 gave tricyclic 24 (83AG156) (Scheme 6). 

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