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).

Photomediated ring contraction of saturated heterocycles

“…A more functional-group-compatible, albeit similarly specific, variant of this type of transfor-mation was reported by Suárez and co-workers for a-diketonyl sugars (e.g., 1b), which undergo light-mediated ring contraction with a subsequent hemiketalization to give fused [5,5] ring systems (i.e., 2b) (Fig. 1D) (21,22). Notably, the a-diketone moiety required for the reaction is derivatized in the resulting product.…”

Chemical Exchange Saturation Transfer (CEST) Agents: Quantum Chemistry and MRI

“…Seltener synthetisch genutzte photochemische Reaktionen erlebten eine kleine Renaissance. So wurde die Norrish‐Typ‐II‐Reaktion in Zusammenhang mit einer anschließenden Aldol‐Reaktion für die Ringverengung von Kohlenhydraten genutzt 161. Die 1,3‐Acylwanderung des Ketons (126) zu Cyclobutanon (127) bildete den Schlüsselschritt in der Synthese des Naturstoffs (+)‐Armillarivin (128) (Abbildung 59) 162…”

Aldehydes as Photoremovable Directing Groups: Synthesis of Pyrazoles by a Photocatalyzed [3+2] Cycloaddition/Norrish Type Fragmentation Sequence

“…Herein, we report a sequential dipolar photoclick reaction followed by a Norrish type deformylation step under green-light irradiation using eosin Y as a photoredox catalyst. To the best of our knowledge, there are no photocatalytic examples for this fragmentation besides the important advantages of this approximation. , …”