| 11. | The ring closure is carried out at high temperatures ( 200 300 �C, 473 573 K ).
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| 12. | A rearrangement reaction of the carbocation follows with ring closure to "'9 " '.
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| 13. | In addition, the electronic structure of the transition state differs from that of other conrotary ring closures.
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| 14. | It was proposed that the methylene of ( + )-Costunolide is reduced before the second ring closure.
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| 15. | For this step, the Schiff base must be heated to ~ 250 �C for the ring closure to occur.
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| 16. | The ring closure can even be driven by stereocenters adjacent to the carbon-carbon multiple bond as shown below.
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| 17. | Subsequent ring closure leads to the formation of the ?-lactam and the regeneration of the chiral nucleophilic organocatalyst.
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| 18. | The mechanism of ring closure is complex, but involves an aldol condensation at C-2 and C-7.
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| 19. | This allows for strictosidine s formation under the subsequent ring closure via electrophilic substitution, as shown in the adjacent image.
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| 20. | Compound XII was formed from condensation of [ carbonyl-14C-]-2-hydroxy-acetophenone with dimethyl oxylate and ring closure.
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