Further studies in the acyl-type radical additions promoted by SmI 2 : mechanistic implications and stereoselective reduction of the keto-functionality

Mikkelsen, Lise Munch; Jensen, C. E.; Høj, Bettina; Blakskjær, Peter; Skrydstrup, Troels

doi:10.1016/j.tet.2003.07.016

Cited by 23 publications

(23 citation statements)

References 40 publications

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“…30 A mechanistically analogous addition/elimination sequence effecting the allylation of B-alkylcatecholboranes was reported by Schaffnner and Renaud. 31 Examples of acyl-type radical additions, promoted by SmI 2 , were reported by Skrydstrup et al 32,33 In these systems, reaction of a CLO or CLN bond with SmI 2 generates a radicaloid species (eqn. ( 12)) which can subsequently add to unsaturated systems.…”

Section: Addition Reactionsmentioning

confidence: 95%

12 Reaction mechanisms : Part (i) Radical and radical ion reactions

Tanko

2004

Annu. Rep. Prog. Chem., Sect. B: Org. Chem.

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This chapter provides an overview of the 2003 literature pertaining to the mechanism and kinetics of reactions involving neutral free radicals and radical ions in solution. This overview begins by highlighting significant contributions to our understanding of fundamental classes of reactions: atom abstraction, addition, and fragmentation. Afterwards, rearrangement reactions, which are generally intramolecular variants of these fundamental classes, are examined. Quite noteworthy is the fact that a significant number of papers in 2003 involved the use of computational chemistry, density functional theory (B3LYP) in particular, to examine mechanisms of free radical and radical ion reactions. The ability of these methods to reproduce experimental data is impressive, giving workers in this area the confidence to make some intriguing predictions. A few examples of the use of radical reactions in synthesis are interspersed throughout this report. However, the coverage of this important area is very much limited to truly unusual transformations (from a mechanistic perspective). Hydrogen atom transfer (HAT) reactionsA great deal of the 2003 radical literature deals with hydrogen atom transfer reactions from a variety of perspectives. These reactions are of interest in several fields including biochemistry and medicine (e.g., mechanisms of enzymatic oxidations, oxidative degradation of biomaterials, reactive oxygen species, and antioxidants), environmental chemistry, materials science, and organic synthesis. A considerable amount of effort has been expended to determine (and predict) 1 the rate constants for this important class of reactions. This section highlights noteworthy advances in this area.H/D isotope effects were reported for hydrogen abstractions by hydroxyl radical (HO . ) from ethanol and methylamine in aqueous solution. For ethanol, the major process (w90%) is hydrogen abstraction from the a-carbon, and the observed k H /k D is 1.96. For methylamine, the major process is hydrogen abstraction from nitrogen to

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Section: Addition Reactionsmentioning

confidence: 95%

12 Reaction mechanisms : Part (i) Radical and radical ion reactions

Tanko

2004

Annu. Rep. Prog. Chem., Sect. B: Org. Chem.

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“…[3][4][5][6][7][8] Its ease of preparation, high oxophilicity, functional group selectivity and moderate oxidation potential (which is tunable via the addition of additives), generally gives clean, high yielding reactions with excellent diastereoselectivities. [3][4][5][6][7][8] In 2002, a new reaction involving the addition of an 'acyl-like' radical to activated olefins was unveiled, [9,10] and is a convenient method to prepare modified biomolecules, such as peptide isosteres, modified amino acids and peptide derivatives thereof. The reactions of acyl radicals comprise an important class of C-C bond forming reactions, [11,12] however the reactions are generally limited by the fact that many such acyl radicals undergo (reversible) decarbonylation.…”

Section: Radical Carbonylation: Set Using Smimentioning

confidence: 99%

“…thioesters of amino acids with stoichiometric amounts of acylamides and acrylates at low temperature, gave good yields of γ-ketoamides and -esters, respectively (Scheme 1). [9,10] These products are akin to acyl radical addition products, but notably no decarbonylation products were observed. This can be rationalised by considering the proposed mechanism for the coupling (Scheme 2) in which no formal acyl radical is formed.…”

Section: General Introductionmentioning

confidence: 99%

Radicals by Design

Croft¹,

Lindsay²,

Renaud³

et al. 2008

Chimia

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Radical reactions offer many advantages over conventional chemical reactions, lending themselves to numerous bond forming processes and rearrangements that are, clean, rapid, and tolerant of a wide range of functional groups. This review provides a brief overview of the design concepts behind new reactions illustrating, in particular, their relevance for the efficient synthesis of biomolecules. Tw o general reactions are highlighted: single electron transfer (SET) using SmI 2 and hydrogen transfer using reagents such as those based on tin, thiols and phosphorous reagents. Intermolecular influences that may direct radical reactions, such as partial protonation and radical complexation, are also discussed.

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“…49, 50 Hence, as illustrated in Scheme 14, reduction of the c-ketoamide with either LiAl(Ot-Bu) 3 H or S-Alpine hydride leads to either diastereomer with high selectivity and good yields. 27 With the cketoester, reduction is followed by internal cyclisation generating the corresponding lactone. 28 Other researchers have demonstrated the importance of chiral c-butyrolactones as important precursors for the synthesis of hydroxyethylene isosters due to their ability to introduce stereoselectively alkyl sidechains in the a-position of the ring.…”

Section: Scheme 12mentioning

confidence: 99%

“…However, several observations reject this pathway. First, the acrylamide or acrylate are only slowly reduced by SmI 2 under the reaction conditions used 27 and second, reduction of the thioester to the corresponding aldehyde was seen in many of these coupling reactions as a minor byproduct implying that electron transfer was taking place with the thioester carbonyl group. 28,29 Whereas this acyl-like radical addition reaction was successful with many of the amino acids tested, it also suffered from several limitations.…”

Section: Scheme 12mentioning

confidence: 99%