Role of .sigma.-Donation in the Stereocontrol of Hydrogen-Transfer Reactions Involving Acyclic Radicals

Guindon, Yvan; Slassi, Abdelmalik; Rancourt, James D.; Bantle, G.; Bencheqroun, Mohammed; Murtagh, Lorraine; Ghiro, Élise; Jung, Grace

doi:10.1021/jo00107a002

Cited by 41 publications

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“…The radical substitution reaction has been reported to yield preferentially the trans/syn and cis/syn diastereoisomers. [46,47] Assigning the relative configurations of the products obtained by the radical substitution process (Scheme 7) based on the literature report is compatible with the chemical correlation.…”

Section: Ring-opening/ring-closing Sequence For the Alkylationsupporting

confidence: 71%

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Thione Esters as Substrates for the Stereoselective Alkylation of Model Compounds of Nonactic Acids

Loiseau¹,

Kholod²,

Neier³

2010

Eur J Org Chem

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The naturally occurring macrotetrolide antibiotic nonactin is used in ammonium ion selective electrodes. To increase the lifetime of nonactin in the semipermeable membrane of these sensors we have developed methods for the introduction of hydrophobic side‐chains. Simple model compounds for nonactic acid such as 5 and 7 were synthesised. Maintaining the cis arrangement of the substituents in the 2,5‐disubstituted tetrahydrofurans proved to be difficult. Three different routes were studied. The enolates obtained by treatment with NaHMDS or KHMDS could be alkylated with benzyl or allyl iodide as electrophiles. Under these conditions a cis/trans isomerisation of the substituents on the tetrahydrofuran ring occurred. A multistep methodology was developed as a synthetic alternative. The sequence consisted of a selective retro‐Michael reaction, a 5‐exo‐tet iodocyclisation and a radical substitution. The products obtained by the two methodologies could be correlated, and thereby a tentative assignment of the relative configurations could be achieved. In the third route the thione ester 18 was deprotonated with tBuOK. At temperatures below –78 °C, the enolate maintained the cis configuration of the substituents at the tetrahydrofuran ring. The alkylated product 12 could be isolated with a satisfactory cis/trans ratio of 85:15. The model compound 12 could be successfully transformed into more hydrophobic derivatives by using either the Heck coupling or the cross‐coupling metathesis transformation.

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Section: Ring-opening/ring-closing Sequence For the Alkylationsupporting

confidence: 71%

“…To achieve the radical substitution, we applied the excellent work of Guindon and co-workers (Scheme 7) [46,47] who described this reaction for a monosubstituted tetrahydrofuran. According to their report, the syn product is favoured in the absence of Lewis acids.…”

Section: Ring-opening/ring-closing Sequence For the Alkylationmentioning

confidence: 99%

Thione Esters as Substrates for the Stereoselective Alkylation of Model Compounds of Nonactic Acids

Loiseau¹,

Kholod²,

Neier³

2010

Eur J Org Chem

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“…Namely, the opposition of the phosphinyl group and electronegative O-atom in the furanose ring reduces intramolecular electrostatic repulsion [14]. Moreover, the alignment of the s(C(4)ÀC (3)) bond with the radical p orbital stabilizes the transition state [15]. Although the mechanistic proposals have been reported for the radical-mediated reduction of a-bromo-b-alkoxycarboxylates [16], no report seems to exist, to the best of our knowledge, for the corresponding balkoxyphosphonate derivatives.…”

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confidence: 99%

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Hanaya

Yamamoto

2002

HCA

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Dedicated to Professor Wolfgang Pfleiderer on the occasion of his 75th birthday Starting from methyl 2,3-O-isopropylidene-a-d-mannofuranoside (5), methyl 6-O-benzyl-2,3-O-isopropylidene-a-d-lyxo-hexofuranosid-5-ulose (12) was prepared in three steps. The addition reaction of dimethyl phosphonate to 12, followed by deoxygenation of 5-OH group, provided the 5-deoxy-5-dimethoxyphosphinyla-d-mannofuranoside derivative 15a and the b-l-gulofuranoside isomer 15b. Reduction of 15a and 15b with sodium dihydrobis(2-methoxyethoxy)aluminate, followed by the action of HCl and then H 2 O 2 , afforded the dmannopyranose (17) and l-gulopyranose analog 21, each having a phosphinyl group in the hemiacetal ring. These were converted to the corresponding 1,2,3,4,6-penta-O-acetyl-5-methoxyphosphinyl derivatives 19 and 23, respectively, structures and conformations ( 4 C 1 or 1 C 4 , resp.) of which were established by 1 H-NMR spectroscopy.

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“…Table 3 illustrates some of the results obtained during the course of hydrogen-transfer reactions with conformationally restricted substrates. 18 The size of the distant alkyl group R 1 had little impact (entries 1 to 3) on the stereochemical outcome. There was however a significant difference in selectivity observed when R 1 was an alkyl group versus when R 1 was a hydrogen (entry 4).…”

Section: Methodsmentioning

confidence: 99%

Hydrogen and Allylation Transfer Reactions in Acyclic Free Radicals

Guindon¹,

Jung²,

Guérin³

et al. 1998

Synlett

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Asymmetric induction involving radicals that are flanked by an ester and a stereogenic center bearing a heteroatom has been demonstrated in hydrogen-transfer reactions and allylations. The roles of both structural and electronic features in controlling the facial selectivity of these radical reactions are best rationalized by a transition state model, which resembles the ground state conformation of the radical supported by calculations and ESR studies.

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Role of .sigma.-Donation in the Stereocontrol of Hydrogen-Transfer Reactions Involving Acyclic Radicals

Cited by 41 publications

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Thione Esters as Substrates for the Stereoselective Alkylation of Model Compounds of Nonactic Acids

Thione Esters as Substrates for the Stereoselective Alkylation of Model Compounds of Nonactic Acids

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Hydrogen and Allylation Transfer Reactions in Acyclic Free Radicals

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