RESUMO
This study represents the first examples of stereoselective radical cyclizations on soluble supports. A stereocontrol element consisting of a polymer-imbedded (+)-isosorbide chiral auxiliary was used in each monomer subunit. A survey of various Lewis acids was also examined. The best results gave very high distereoselectivites of >100:1 in a hepa-1,3-dienenyl radical cyclization using zinc chloride as a Lewis acid. [reaction: see text]
RESUMO
[reaction: see text]. Free radical reactions were performed on soluble ring-opening metathesis (ROM) polymers. These polymers have high substrate loading, short reaction times, and the benefit of a facile purification. All reactions on these supports were obtained in good yields as white crystalline-like materials readily separated from tin byproducts.
RESUMO
[figure: see text] Carbohydrates as removable chiral scaffolds for free radical cyclizations were examined for the first time. This investigation illustrates the utility of two inexpensive carbohydrate derivatives as sources of asymmetry for 5-hexenyl radical cyclizations. Diastereomeric ratios as high as 100:1 were achieved with an ester-appended (+)-isosorbide hexose and 70:1 for a diol-protected D-xylose pentose. Temperature dependence, Lewis acids, and solvents were all examined. By correlation with known compounds, the newly generated chiral centers were of the (S)-configuration.
Assuntos
Alcenos/química , Carboidratos/química , Radicais Livres , Hexoses/química , Indicadores e Reagentes , Conformação Molecular , Estrutura Molecular , Pentoses/química , EstereoisomerismoAssuntos
Frutose/análogos & derivados , Compostos Heterocíclicos com 2 Anéis/química , Monossacarídeos/síntese química , Configuração de Carboidratos , Frutose/química , Frutose/metabolismo , Compostos Heterocíclicos com 2 Anéis/metabolismo , Espectroscopia de Ressonância Magnética , Monossacarídeos/química , Espectrofotometria Infravermelho , EstereoisomerismoRESUMO
[reaction: see text] A new allylstannane reagent on non-cross-linked polystyrene was developed for the first time. This support differs markedly from standard cross-linked polymers because it is completely soluble in organic solvents; moreover, the reactions can be conveniently monitored directly by standard 1H NMR methods. The allylstannane underwent a free radical reaction with an alkyl halide to form a new allyl appendage. Tin byproducts can be easily recovered from cold methanol as white crystalline solids.