Tautomerism, photochemical valence

The aziridine derivative 117, a photochemical valence tautomerization product of berberinephenolbetaine, was also converted into the benzindan-oazepines by regioselective C-14—N bond cleavage (Scheme 24)." The treatment of 117 with p-toluenesulfonic acid in benzene under reflux gave rise to the benzindenoazepine 118 whose N-methylation with dimethyl sulfate in HMPA in the presence of sodium hydride afforded the n-methyl derivative 119." Since 119 has already been converted" to ( )-c -alpini-genine (120) and ( )-ds-alpinine (121), this synthesis constitutes a formal synthesis of these alkaloids. 

The similar conversion of dihydroepiberberine (122) to ( )-fumaritrine (127) was achieved by the same author (Scheme 25). Epiberberinephenol-betaine (123), derived from 122 by oxidation with w-chloroperbenzoic acid, was irradiated in methanol with a high-pressure mercury lamp to give the aziridine 124. Reduction of 124 with sodium borohydride in methanol yielded the alcohol 125 stereoselectively, which was further converted to the d -benzindenoazepine 126 by treatment with j7-toluenesulfonic acid in methanol, followed by methylation with methyl iodide. Successive mesyla-tion of 126 and reduction of the mesylate afforded ( )-fumaritrine (127) in 71% yield. Thus the conversion of berberines to benzindenoazepines was successfully accomplished by the formation of aziridine derivatives as key intermediates by photochemical valence tautomerization. 

A spirobenzylisoquinoline alkaloid, ochrobirine (112) has been synthesized from its biogenetic precursor, coptisine (111) (Scheme 43). The key reaction presented in this biomimetic and stereoselective conversion involves a regioselective C8—N bond fission of the aziridine (113), which is formed from the betaine by photochemical valence tautomerization <79TL3749>. 

A. Padwa, and E. Vega, Photochromic aziridines. On the photochemical valence tautomerization and cycloaddition reactions of substituted indano[l,2-h]aziridine,/. Org. Chem 40, 175-181 (1975). 

Pyridine and the diazines are capable of thermal and photochemical valence tautomerizations in analogy to benzene. 

Mild thermolysis of acetoxydithiocin (176 R = Ac) affords phenyl acetate in unreported yield <78T3631>. As discussed above, this reaction is proposed to proceed via thermally induced valence tautomerization, followed by sulfur extrusion <78T363l>. The photochemical valence tauto-merization of this 1,4-dithiocin and of its benzo analogue is discussed above. 

The reactivity of pyridine is determined by its character as an electron-deficient aromatic azine system. Therefore, electrophiles should attack preferably at the N-atom, but also at the ring-C-atoms in SnAr reactions, while nucleophiles should attack at the ring carbons and undergo SNAr reactions. In analogy to benzene, thermal as well as photochemical valence tautomerizations can be expected. 

In 1966, a photochemical rearrangement by ultraviolet (UV) irradiation of hexafluorobenzene to hexafluorobicyclo[2 2 0]hexa-2,5-diene was achieved Since then, many reactions analogous to the valence tautomerism of benzene and bicy clo[2 2 0]hexadiene (1 e, Dewar benzene), as well as of fluonnated benzvalene and... 

The following types of thiopyran isomerizations have been reported in the last decade valence-bond tautomerism, endocyclic hydrogen shifts and migration of nonhydrogen substituents. Thermal processes will be mentioned here and photochemically induced isomerizations will be discussed in Section V,I. 

The valence tautomerism 251 257 has been proposed to account for the formation of iV-methylthiobenzamide by irradiation (2537 A) in methyl cyanide solution. The valence tautomer 257, R = Me, = Ph, has been detected spectroscopically (p , 2060 cm —N=C=S) either during photolysis at 25° or by heating at 160°. Photochemical oxidative cyclization of 4,5-diaryl derivatives (251, R = R = Ar) analogous to the formation of phenanthrene from stilbene has been reported. Thus, irradiation of the 4,5-diphenyl derivative 251, R = R = Ph, yields the tetracyclic meso-ionic compound (258). ... 

Dimerization of pyridinium-3-olates (86) is also induced photochemically. In accord with theory the structure of the photodimers differs from that of the thermal dimers. A-Phenylpyridinium-3-olate (101) upon irradiation gives dimer 103 together with the bicyclic valence tautomer 102 (Scheme 4). This valence tautomerism (101 - 102) is analogous to that of pyrylium-3-... 

Irradiation of troponoid systems leads to a variety of photochemical reactions which include valence tautomerization, deep-seated rearrangement and dimerization. The photochemical transformations of troponoid systems as a general rule proceed readily in Pyrex vessels and thus must involve excitation of the intense, long wavelength absorption band (320-350 m/t) of the troponoid system. 

The photochemical transformations of simple tropolone methyl ethers can be more complex than simple valence tautomerization. Irradiation of a-tropolone or its methyl ether (Formulas 16a,b) in aqueous solution in quartz vessels gives 4-oxo-2-cyclopentenylacetic acid (Formula 17a) or the methyl ester (Formula 17b) (15,16). 

Of the cyclic 1,3,5-trienes, only 1,3,5-cycloheptatriene and compounds derived from this ring system have been studied extensively. In almost all the cases the primary photochemical process has been a valence tautomerization which, in cycloheptatriene itself, can be written as ... 

Barton has suggested 35> that in photochemical reactions of cyclic conjugated olefins, ring fission will predominate in rings of n annular atoms containing (n/2)-l double bonds and valence tautomerization will occur in other systems. Consequently, it was anticipated that 1,3,5-cyclo-octatriene upon photolysis should give rise to an acyclic tetraene in the same manner that 1,3-cyclohexadiene opens 35> 36> to 1,3,5-hexatriene. When 1,3,5-cyclooctatriene (55) was irradiated in solution (ether or pentane) 37> 39> two isomerization products were isolated, bicyclo[4.2.0]-octa-2,7-diene (56) and tricyclo[5.1.0.0 4> 8]oct-2-ene (57). The formation of 56 is not exceptional. The formation of 57 has been visualized by... 

The valence tautomeric diaziridines 33 are formulated as intermediates in these rearrangement reactions. At normal temperatures, the equilibrium diazepine diaziridine is exclusively on the side of the diazepine. Thermal rearrangement of pyridine A-imines to diaziridines or diazepines has not yet succeeded 67 such reactions are accessible by irradiation (see Section IV, F). These reactions are not controlled exclusively by orbital symmetry. The Hoffmann-Woodward rules68 allow both thermal and photochemical... 

Pyridines show a number of thermal and photochemical transformations which are analogous to the valence tautomerization of benzene, i.e. formation of Dewar benzene, prismane and benzvalene. 

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