Synthetic Strategies Directed Towards 5a‐Carbahexopyranoses and Derivatives Based on 6‐endo‐trig Radical Cyclizations

Abstract

Several synthetic strategies directed towards 5a-carbahexopyranoses and based on 6-endo-trig radical cyclization of unsaturated carbohydrate derivatives have been devised. Three elements for regiocontrol to optimize the 6-endo/5-exo ratio have been incorporated, and their efficiencies in directing 6-endo cyclizations have been evaluated. These elementsnamely: i) the incorporation of a substituent at C-5 (radical numbering), ii) the use of a vinyl (rather than alkyl) radical,

Catalytic N-radical cascade reaction of hydrazones by oxidative deprotonation electron transfer and TEMPO mediation

“…Notably, it has been recently documented by MacMillan 36 37 , Knowles 24 38 39 and our group 28 40 that the addition of a suitable Brønsted acid, Lewis acid or base could facilitate some otherwise inaccessible photocatalytic event by weakening chemical bonds of reactants and co-catalysts or modulating their redox potential. It has also been demonstrated by López and Gómez that complete 6- endo -selectivity over 5- exo ring closure in radical cyclization of C-centred radicals can be controlled by the radical property, substitution pattern at C-5 or ring strain of substrate 34 35 . Quite recently, the MacMillan group also first integrated elementary hydrogen atom transfer (HAT) process into H-bond catalysis, and achieved a highly selective photoredox α-alkylation/lactonization cascade of alcohols 41 .…”

“…To realize the target catalytic N-radical cascade reaction of unsaturated hydrazones as shown in Fig. 1b , several major challenges would probably be encountered, such as the controlled formal homolysis of the recalcitrant N–H bond for the formation of the neutral N-centred hydrazonyl radical, regioselectivity of the N-radical cyclization step (for example, 6- endo and 5- exo , path a versus path b) 34 35 and selective homolytic activation of aza-allylic C–H bond in C-centred radical intermediate. Notably, it has been recently documented by MacMillan 36 37 , Knowles 24 38 39 and our group 28 40 that the addition of a suitable Brønsted acid, Lewis acid or base could facilitate some otherwise inaccessible photocatalytic event by weakening chemical bonds of reactants and co-catalysts or modulating their redox potential.…”

Alkenes as Alkyne Equivalents in Radical Cascades Terminated by Fragmentations: Overcoming Stereoelectronic Restrictions on Ring Expansions for the Preparation of Expanded Polyaromatics

“…In particular, intermolecular radical addition to alkynes opens access to reactive vinyl radicals that can participate in reaction cascades . A number of literature reports, a few of which are summarized in Scheme , suggest that, in multifunctional systems, Sn radical addition to terminal alkynes is preferred over reaction with an alkene. − Overall, selective addition in these systems proceeds in a Markovnikov fashion to form the more stable vinyl radical. However, the present case (enyne) corresponds to attack at an internal alkyne.…”

Cross-Coupling Reactions Using Samarium(II) Iodide

“…In 2011, Gómez and López reported a 5- exo -trig aldehyde/alkene cyclization of functionalized sugar derivatives (Scheme ) . Under the SmI 2 –HMPA– t -BuOH reaction conditions, a 72:28 ratio of products arising from the 5- exo -trig cyclization and competing intermolecular pinacol dimerization was observed.…”