Cyclization by radical displacement on ester groups. Conversion of acetals to lactones by radical abstraction with stereoelectronic control of bond scission

Rynard, Carolyn M.; Thankachan, C.; Tidwell, Thomas T.

doi:10.1021/ja00499a025

Cited by 29 publications

(5 citation statements)

References 3 publications

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“…Such radicals may cleave in the exocyclic mode to give the aglycone radical and a lactone, or in the endocyclic mode to give esters, as observed here. 2- Methoxy-2-tetrahydropyranyl radicals were initially thought to undergo preferential fragmentation in the exocyclic mode to give δ-valerolactone: a preference which was explained in terms of stereoelectronic effects with the required coplanarity between the singly occupied orbital and the σ*-orbital of the scissile bond being more readily accessible in the exocyclic mode ). 2a, Increased substitution at C-6 of the 2-tetrahydropyranyl system (corresponding to C-5 in the carbohydrate numbering scheme) favors endocyclic fragmentation (Scheme ) …”

Section: Resultsmentioning

confidence: 99%

Chemistry of 1-Alkoxy-1-glycosyl Radicals: The Manno- and Rhamnopyranosyl Series. Inversion of α- to β-Pyranosides and the Fragmentation of Anomeric Radicals

1996

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The formation and stereoselective quenching of 1-mannopyranosyl radicals by a tributyltin hydride-mediated intramolecular 1,5-hydrogen abstraction sequence is described. A competing process is 1,4-hydrogen atom abstraction leading principally to glucopyran-2-ulosides. Fragmentation of the anomeric radical resulting in the formation of ring opened products is a problem in certain series. The chemistry is dictated to a considerable extent by the nature of the protecting groups employed with the 4,6-benzylidene series and, for rhamnose, the Ley 3,4-dispiroketal, being particularly susceptible to the 1,4-hydrogen atom abstraction but less to the fragmentation. Photochemical conditions are described, in which these side reactions are practically eliminated, and applied to the inversion of an alpha- to a beta-mannoside in a disaccharide.

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Section: Resultsmentioning

confidence: 99%

Chemistry of 1-Alkoxy-1-glycosyl Radicals: The Manno- and Rhamnopyranosyl Series. Inversion of α- to β-Pyranosides and the Fragmentation of Anomeric Radicals

1996

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“…An attempt was made to trap the amino-anhydride intermediate 5 in the reaction of 27 with 1 (acetonitrile solution, −30 °C, 10 min) by adding the nucleophilic combination hydrogen peroxide / pyridine. 27 However, the amino diacid hydrolysis product 34 was not observed; only the familiar cis delta-lactam 28 was isolated.…”

Section: Resultsmentioning

confidence: 98%

Stereochemistry and reactivity of the HPA-imine Mannich intermediate

Polyak

Phung

Liu

et al. 2017

Tetrahedron Letters

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Homophthalic anhydride (HPA) typically reacts rapidly with benzalimines to afford the formal [4+2] adduct, a 1,2,3,4-tetrahydroisoquinolin-1-one-4-carboxylic acid. The stereochemical outcome of this reaction is consistent with an open transition state comprising an iminium species and enolized HPA, leading to a short-lived amino-anhydride intermediate. In the case of N-tert-butylbenzalimine, this Mannich-type intermediate, which would normally cyclize at low temperature to a single isomer of the delta-lactam, is intercepted by base treatment to afford beta-lactam products. A pathway featuring ketene formation followed by ring closure is implicated.

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“…As was described under section “Thermal Homolysis”, the efficiency of these processes can be strongly influenced by the rate of in‐cage recombination reactions. Figure illustrates the competition for capture of the benzylic radical released from an aryl‐substituted analog of tert ‐butyl peracetate …”

Section: Reactionsmentioning

confidence: 99%