- Boron-Catalyzed Double Hydrofunctionalization Reactions of Unactivated Alkynes
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Tandem hydroalkoxylation/hydroallylation and hydroalkoxylation/hydrocyanation reactions of alkyl-substituted unactivated alkynes by catalytic systems based on B(C6F5)3·nH2O and silyl nucleophiles were developed.
- Shibuya, Masatoshi,Okamoto, Masaki,Fujita, Shoji,Abe, Masanori,Yamamoto, Yoshihiko
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- Synthese de polyethers polycycliques. II. Synthese stereoselective de dimethyl-2,5, dimethyl-2',5 et trimethyl-2,2',5 bis-2,2'-tetrahydrofurranes
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Our interest in natural polycyclic ethers of ionophoric type in wich several alkyl ligands are located on the oxygen rings has led us to further extend our previous study on 2,2'-bistetrahydrofurans. A series of γ,δ-unsaturated tetrahydrofuryl alcohols were prepared by reaction of a hydride or an organometallic compound with 2-formyl or 2-acetyltetrahydrofurans.In contrast with our previous work, the syntheses are (with one exeption) stereoselective and the Cram rule applies.In fact, stereoselectivity is observed only when there is enough steric strain, i.e. when there are at least two alkyl groups among the three ligands R1, R2 and R3 (see Scheme 1).Erythro and Threo isomers of secondary and tertiary alcohols were thus prepared and their configurations determined by NMR correlations and comparison with molecules prepared unambiguously by hydrogenation (previous work) or stereospecific synthesis.The alcohols were cyclysed by known method using mercuric acetate, giving 2,2'-bisterahydrofurans with known relative configrations of the two rings.NMR correlations and independent synthesis of 2-furyltetrahydrofurans followed by their hydrogenation enabled us to determine the cis or trans configuration of the second ring.Cyclisation takes place by trans mechanism : for the major isomer, in the second ring, CH3-5 is trans relative to the first ring.In agreement with previous work, stereoselectivity reflects the competition arising from the steric hindrance of the two substituents located on the carbinolic carbon atom (H, CH3, THF or CH3-2THF) and grows from 64 percent to 87 percent.
- Amouroux, Roger,Chastrette, Francine,Chastrette, Maurice
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p. 293 - 302
(2007/10/02)
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