Adição de nucleofilos a ions oxocarbenio ciclicos derivados de y-Lactois 5-Substituidos. Formação diastereosseletiva de sistemas tetraidrofuranicos 2,5-trans- e 2,5-cis-Dissubstituidos

AUTOR(ES)
DATA DE PUBLICAÇÃO

2001

RESUMO

2,5-Disubstituted tetrahydrofurans moieties are found in many biologically important natural products including pheromones, polyether antibiotics and acetogenins. We have demonstrated that the synthesis of 2,5-disubstituted tetrahydrofuran derivatives can be achieved in good yields and divergent diastereoselectivity either by intermolecular or intramolecular replacement of the hydroxyl group of 5-substituted g-Iactols by the allyl group. 5-Substituted lactol 51 was converted to 2,5-disubstituted tetrahydrofuran derivatives by Lewis acid promoted reaction with allylsilanes. High trans selectivity (12:1) was obtained when hindered allylsilane 37 was employed. 5-Substituted lactol 74 was transformed into 2,5-cis-disubstituted tetrahydrofuran 68 (6:1 ratio) by a TiCl4-promoted intramolecular allyl transfer process. Furthermore, 2,5-cis-disubstituted tetrahydrofuran derivatives were obtained in good yields and diastereoselectivities after alkyllithium addition to the lactone 48, followed by Et3SiH/BF3.OEt2 reduction of the corresponding hemiketals. The absolute configurations of the isomers were established by nOe experiments. Additionally, 2,5-disubstituted tetrahydrofuran rings were prepared through the addition of titanium(lV) enolate derived from oxazolidinone 94a-g to the oxocarbenium ion derived from lactol 51. After deprotection with HF/CH3CN, the mixtures were separated to afford the adducts in moderate to good yields and diastereoselections. The absolute configurations of the isomers were established by X-ray diffraction analysis and spectroscopic correlation of the corresponding diols. These strategies provide a new synthetic opportunity for the synthesis of biologically active natural preducts.

ASSUNTO(S)

reações quimicas titanio ions metalicos litio ions reação nucelofilica

ACESSO AO ARTIGO

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