title: Sequential Norrish type II photoelimination and intramolecular aldol cyclization of α-diketones: synthesis of polyhydroxylated cyclopentitols by ring contraction of hexopyranose carbohydrate derivatives.
abstract: The excitation of the innermost carbonyl of nono-2,3-diulose derivatives by irradiation with visible-light initiates a sequential Norrish type II photoelimination and aldol cyclization process that finally gives polyfunctionalized cyclopentitols. The rearrangement has been confirmed by the isolation of stable acyclic photoenol intermediates that can be independently cyclized by a thermal 5-(enolexo)-exo-trig uncatalyzed aldol reaction with high diastereoselectivity. In this last step, the large deuterium kinetic isotope effect found for the 1,5-hydrogen atom transfer seems to indicate that the aldol reaction runs through a concerted pericyclic mechanism. Owing to the ready availability of pyranose sugars of various configurations, this protocol has been used to study the influence of pyranose ring-substituents on the diastereoselectivity of the aldol cyclization reaction. In contrast with other pyranose ring contraction methodologies no transition-metal reagents are needed and the sequential rearrangement occurs simply by using visible light and moderate heating (0 to 60 °C).
authors.affiliation: Instituto de Productos Naturales, y Agrobiología del C.S. I.C. Carretera de la Esperanza 3, 38203 La Laguna, Tenerife, Spain.
authors.forename: Dimitri
authors.initials: D
authors.lastname: Alvarez-Dorta
authors.forename: Elisa I
authors.initials: EI
authors.lastname: León
authors.forename: Alan R
authors.initials: AR
authors.lastname: Kennedy
authors.forename: Angeles
authors.initials: A
authors.lastname: Martín
authors.forename: Inés
authors.initials: I
authors.lastname: Pérez-Martín
authors.forename: Concepción
authors.initials: C
authors.lastname: Riesco-Fagundo
authors.forename: Ernesto
authors.initials: E
authors.lastname: Suárez
mesh_terms: D000447:Aldehydes / Q000737:chemistry; D002241:Carbohydrates / Q000138:chemical synthesis* / Q000737:chemistry; D003500:Cyclization; D007659:Ketones / Q000138:chemical synthesis; D015394:Molecular Structure; D055668:Photochemical Processes
chemical_list: D000447:Aldehydes; D002241:Carbohydrates; D007659:Ketones; C116609:3-hydroxybutanal
keywords: carbohydrates; cyclopentitols; diketones; photochemistry; ring contraction