Photoinduced electron-transfer chemistry of the bielectrophoric N-phthaloyl derivatives of the amino acids tyrosine, histidine and tryptophan

The photochemistry of phthalimide derivatives of the electron-rich amino acids tyrosine, histidine and tryptophan 8–10 was studied with respect to photoinduced electron-transfer (PET) induced decarboxylation and Norrish II bond cleavage. Whereas exclusive photodecarboxylation of the tyrosine substrate 8 was observed, the histidine compound 9 resulted in a mixture of histamine and preferential Norrish cleavage. The tryptophan derivative 10 is photochemically inert and shows preferential decarboxylation only when induced by intermolecular PET

A New Directing Mode for Singlet Oxygen Ene Reactions: The Vinylogous Gem Effect Enables a ¹O₂ Domino Ene/[4 + 2] Process

The singlet oxygen reactions of 4-methyl-2,4-hexadienoates E,E- and E,Z-<b>4</b> proceed in a highly mode selective and regioselective domino process. The initial product is the allylic hydroperoxide <b>5</b> directed by a vinylogous gem effect. The subsequent ¹O₂ [4 + 2] cycloaddition delivers a 3:2 diastereoisomeric mixture of 1,2-dioxanes <b>8</b> in a one-pot process. The identical protocol delivers from the more reactive α-methylated substrates E,E-<b>10</b> and E,Z-<b>10</b> with excellent primary regioselectivity the 1,2-dioxane <b>13</b>

Singlet oxygen and natural substrates: functional polyunsaturated models for the photooxidative degradation of carotenoids

The primary chemical reactions of singlet molecular oxygen with polyunsaturated carotenoids are the focus of this research report. Model compounds that exhibit electronic properties and substituent pattern similar to natural carotenes, xanthophylls or apocarotenoids, respectively, were investigated with regard to photooxygenation reactivity. For dienes and trienes as substrates, high tandem reactivity was observed and hydroperoxy-endoperoxides were isolated as the secondary products of singlet oxygen reaction. The electronic gem-effect on the regioselectivity of the ene reaction is conserved also in vinylogous positions and thus appears to originate from a radical-stabilizing effect. In an attempt to combine different peroxide groups derived from natural products as a tool for new pharmaceutically active products, a dyade synthesis of an artemisinine-safranol with subsequent singlet oxygen addition was realized

ChemInform Abstract: Singlet Oxygen and Natural Substrates: Functional Polyunsaturated Models for the Photooxidative Degradation of Carotenoids

The primary chemical reactions of singlet molecular oxygen with polyunsaturated carotenoids are the focus of this research report. Model compounds that exhibit electronic properties and substituent pattern similar to natural carotenes, xanthophylls or apocarotenoids, respectively, were investigated with regard to photooxygenation reactivity. For dienes and trienes as substrates, high tandem reactivity was observed and hydroperoxy-endoperoxides were isolated as the secondary products of singlet oxygen reaction. The electronic gem-effect on the regioselectivity of the ene reaction is conserved also in vinylogous positions and thus appears to originate from a radical-stabilizing effect. In an attempt to combine different peroxide groups derived from natural products as a tool for new pharmaceutically active products, a dyade synthesis of an artemisinine-safranol with subsequent singlet oxygen addition was realized

Ene-Diene Transmissive Cycloaddition Reactions with Singlet Oxygen: The Vinylogous Gem Effect and Its Use for Polyoxyfunctionalization of Dienes

The singlet oxygen reactivities and regioselectivities of the model compounds 1b-d were compared with those of the geminal (gem) selectivity model ethyl tiglate (1a). The kinetic cis effect is k(E)/k(Z) = 5.2 for the tiglate/angelate system 1a/1a' without a change in the high gem regioselectivity. Further conjugation to vinyl groups enabled mode-selective processes, namely, [4 + 2] cycloadditions versus ene reactions. The site-specific effects of methylation on the mode selectivity and the regioselectivity of the ene reaction were studied for dienes 1e-g. A vinylogous gem effect was observed for the gamma,delta-dimethylated and alpha,gamma,delta-trimethylated substrates 1h and 1i, respectively. The corresponding phenylated substrates 1j-1 showed similar mode selectivity, as monomethylated 1j exhibited exclusively [4 + 2] reactivity while the tandem products 12 and 14 were isolated from the di- and trimethylated substrates 1k and 1l, respectively. The vinylogous gem effect favors the formation of 1,3-dienes from the substrates, and thus, secondary singlet oxygen addition was observed to give hydroperoxy-1,2-dioxenes 19 and 20 in an ene-diene transmissive cycloaddition sequence. These products were reduced to give alcohols (16, 17, and 18) or furans (24 and 25), respectively, or treated with titanium(IV) alkoxides to give the epoxy alcohols 26 and 27. The vinylogous gem effect is rationalized by DFT calculations showing that biradicals are the low-energy intermediates and that no reaction path bifurcations compete

Ene–Diene Transmissive Cycloaddition Reactions with Singlet Oxygen: The <i>Vinylogous Gem Effect</i> and Its Use for Polyoxyfunctionalization of Dienes

The singlet oxygen reactivities and regioselectivities of the model compounds <b>1b</b>–<b>d</b> were compared with those of the geminal (gem) selectivity model ethyl tiglate (<b>1a</b>). The kinetic cis effect is <i>k</i>_<i>E</i>/<i>k</i>_<i>Z</i> = 5.2 for the tiglate/angelate system <b>1a</b>/<b>1a′</b> without a change in the high gem regioselectivity. Further conjugation to vinyl groups enabled mode-selective processes, namely, [4 + 2] cycloadditions versus ene reactions. The site-specific effects of methylation on the mode selectivity and the regioselectivity of the ene reaction were studied for dienes <b>1e</b>–<b>g</b>. A vinylogous gem effect was observed for the γ,δ-dimethylated and α,γ,δ-trimethylated substrates <b>1h</b> and <b>1i</b>, respectively. The corresponding phenylated substrates <b>1j</b>–<b>l</b> showed similar mode selectivity, as monomethylated <b>1j</b> exhibited exclusively [4 + 2] reactivity while the tandem products <b>12</b> and <b>14</b> were isolated from the di- and trimethylated substrates <b>1k</b> and <b>1l</b>, respectively. The vinylogous gem effect favors the formation of 1,3-dienes from the substrates, and thus, secondary singlet oxygen addition was observed to give hydroperoxy-1,2-dioxenes <b>19</b> and <b>20</b> in an ene–diene transmissive cycloaddition sequence. These products were reduced to give alcohols (<b>16</b>, <b>17</b>, and <b>18</b>) or furans (<b>24</b> and <b>25</b>), respectively, or treated with titanium­(IV) alkoxides to give the epoxy alcohols <b>26</b> and <b>27</b>. The vinylogous gem effect is rationalized by DFT calculations showing that biradicals are the low-energy intermediates and that no reaction path bifurcations compete

Ene–Diene Transmissive Cycloaddition Reactions with Singlet Oxygen: The <i>Vinylogous Gem Effect</i> and Its Use for Polyoxyfunctionalization of Dienes

The singlet oxygen reactivities and regioselectivities of the model compounds <b>1b</b>–<b>d</b> were compared with those of the geminal (gem) selectivity model ethyl tiglate (<b>1a</b>). The kinetic cis effect is <i>k</i>_<i>E</i>/<i>k</i>_<i>Z</i> = 5.2 for the tiglate/angelate system <b>1a</b>/<b>1a′</b> without a change in the high gem regioselectivity. Further conjugation to vinyl groups enabled mode-selective processes, namely, [4 + 2] cycloadditions versus ene reactions. The site-specific effects of methylation on the mode selectivity and the regioselectivity of the ene reaction were studied for dienes <b>1e</b>–<b>g</b>. A vinylogous gem effect was observed for the γ,δ-dimethylated and α,γ,δ-trimethylated substrates <b>1h</b> and <b>1i</b>, respectively. The corresponding phenylated substrates <b>1j</b>–<b>l</b> showed similar mode selectivity, as monomethylated <b>1j</b> exhibited exclusively [4 + 2] reactivity while the tandem products <b>12</b> and <b>14</b> were isolated from the di- and trimethylated substrates <b>1k</b> and <b>1l</b>, respectively. The vinylogous gem effect favors the formation of 1,3-dienes from the substrates, and thus, secondary singlet oxygen addition was observed to give hydroperoxy-1,2-dioxenes <b>19</b> and <b>20</b> in an ene–diene transmissive cycloaddition sequence. These products were reduced to give alcohols (<b>16</b>, <b>17</b>, and <b>18</b>) or furans (<b>24</b> and <b>25</b>), respectively, or treated with titanium­(IV) alkoxides to give the epoxy alcohols <b>26</b> and <b>27</b>. The vinylogous gem effect is rationalized by DFT calculations showing that biradicals are the low-energy intermediates and that no reaction path bifurcations compete