Free Radical Chemistry of β-Lactones. Arrhenius Parameters for the Decarboxylative Cleavage and Ring Expansion of 2-Oxetanon-4-ylcarbinyl Radicals. Facilitation of Chain Propagation by Catalytic Benzeneselenol

Abstract

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1,5-Hydrogen Atom Transfer/Surzur–Tanner Rearrangement: A Radical Cascade Approach for the Synthesis of 1,6-Dioxaspiro[4.5]decane and 6,8-Dioxabicyclo[3.2.1]octane Scaffolds in Carbohydrate Systems

“…The 1-alkyl radical (III) continues the cascade sequence by the two mechanisms mentioned before: pure radical β-fragmentation to give unlabeled 31 , through a non-isolated olefin, and S–T rearrangement through the radical-polar intermediate (IV). 45 When phosphate is used as LG, the reaction is directed toward two competitive pathways: cine and ipso intramolecular cyclization by the primary alcohol that now acts as a nucleophile. The minor ipso cyclization affords the bis(pyran) (1- 2 H) 36 through radical (V).…”

“…The alkoxyl radical (I) initiated two competitive abstraction processes: 1,5-HAT of the 5H and 1,6-HAT of one hydrogen of the methoxyl group at C2. The radical II leads to (OCH 2 - 2 H) 52 , whereas the radical at C5 (III) may be stabilized by three different mechanisms: reduction with inversion of configuration giving rise to l -altrose derivative (5- 2 H) 50 (path a), radical β-fragmentation of the LG that can explain the formation of olefin 46 and the unlabeled 45 (path b), 45 or continuing the sequence by the radical–ionic mechanism that finally provided (4- 2 H) 45 through the cine cyclization step (path c).…”

The Total Synthesis of ( - )-Tetrahydrolipstatin

“…The use of 9-borabicyclo[3.3.1]nonane [49] under an inert atmosphere as initiator was also investigated, however this also gave variable results. Finally we investigated the radical reduction conditions developed by Crich and Mo [25] using tributyltin hydride in combination with diphenyldiselenide and di-tertbutylperoxyoxalate (DBPO, CAUTION!) [50] as the initiator.…”

“…[22][23][24] We also anticipated that the electronic influence of the adjacent allylic alcohol functionality would favour formation of β-lactone regioisomers over their γ-lactone counterparts. The recent evaluation of the kinetics associated with the radical reduction of related brominated β-lactones by Crich [25] pointed to the viability of the subsequent radical dehalogenation step. The pivotal intermediate in our approach, hydroxy acid (4), could be obtained from cyclic sulfite (5) in two steps by diorganocuprate-mediated S N reaction followed by ester hydrolysis.…”

Disulfides and Boryl Sulfides Serve as both Initiators and Precatalysts in Radical Reductions of Halides by an N‐Heterocyclic Carbene⋅Borane

“…This process is typically called polarity reversal catalysis [7] because a mismatched polarity in step (2) (both reactants are nucleophilic) is reversed to two matching polarities (one nucleophilic, one electrophilic) in steps (3) and (4).…”