Sequential Norrish Type II Photoelimination and Intramolecular Aldol Cyclization of α-Diketones: Synthesis of Polyhydroxylated Cyclopentitols by Ring Contraction of Hexopyranose Carbohydrate Derivatives

• Published in: Chemistry : a European journal 2013-07, Vol.19 (31), p.10312-10333
• Main Authors: Alvarez-Dorta, Dimitri, León, Elisa I., Kennedy, Alan R., Martín, Angeles, Pérez-Martín, Inés, Riesco-Fagundo, Concepción, Suárez, Ernesto
• Format: Article
• Language: English
• Subjects: Aldehydes - chemistry; carbohydrates; Carbohydrates - chemical synthesis; Carbohydrates - chemistry; Cyclization; cyclopentitols; diketones; Ketones - chemical synthesis; Molecular Structure; Photochemical Processes; photochemistry; ring contraction
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201301230

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). Just light and heat! Visible‐light photoexcitation of the innermost carbonyl of 2,3‐diulose triggers an unprecedented Norrish type II photoelimination and uncatalyzed aldol sequential rearrangement through the thermal 5‐(enol‐exo)‐exo‐trig cyclization of the acyclic photoenol intermediate. The process involves a pyranosyl ring contraction and the formation of a new type of cyclopentitol derivatives with high diastereoselectivity (see scheme; R=acyl, alkyl, silyl group).