Ring Closure Reactions of Substituted 4-Pentenyl-1-oxy Radicals. The Stereoselective Synthesis of Functionalized Disubstituted Tetrahydrofurans
摘要:
N-(Alkyloxy)pyridine-2( VT)-thiones 3 and benzenesulfenic acid O-esters 5 have been synthesized from substituted 4-pentenols 1 or the derived tosylates. Compounds 3 and 5 are efficient sources of free alkoxy radicals 6 which undergo synthetically useful fast ring closure reactions 6 --> 8 [k(exo) = (2 +/- 1) x 10(8) s(-1) to (6 +/- 2) x 10(9) s(-1) (T = 30 +/- 0.2 degrees C)]. Tetrahydrofurfuryl radicals 8 can be trapped with, e.g., hydrogen or chlorine atom donors to afford either trans- or cis-disubstituted tetrahydrofurans 10 or 12 depending on the substitution pattern of the 4-pentenyloxy radical. Substituted tetrahydropyrans 11 or 13 are formed in the minor 6-endo-trig cyclization. According to the data of competition kinetics, the observed stereoselectivities in free alkoxy radical cyclizations arise from steric interactions between the substituents in the transition state of the ring closure reactions. Alkyl substituents cause small differences in the measured relative rate constants of B-exo cyclizations which are reminiscent of the data obtained from the rearrangements of alkyl-substituted 5-hexenyl radicals. Likewise, a stereochemical model for oxygen radical cyclization is proposed where the pentenyloxy chain adopts a six-membered, chairlike transition state with the alkyl substituents preferentially situated in the pseudoequatorial positions leading to 2,5-trans-, 2,4-cis-, and 2,3-trans-substituted tetrahydrofurfuryl radicals 8 as the major intermediates.
Understanding the effect of allylic methyls in olefin cross-metathesis
摘要:
A series of NMR spectroscopy experiments have been conducted with both the model compound, 3-methyl-1-pentene and the corresponding ADMET monomer 3,6,9-trimethylundeca-1,10-diene (11) to better understand the effect of allylic methyls during olefin metathesis chemistry. Traditional ADMET catalysts such as Schrock's molybdenum (1), and Grubbs' ruthenium 1st and 2nd generation (2 and 3) were examined under cross-metathesis and ADMET conditions. Regardless of catalyst selection, 50% or less metathesis conversion was observed for all reactions, especially in the case of the more sterically encumbered diene. With Schrock's molybdenum catalyst 1, the reaction leads to an accumulation of the non-productive metallacyclobutane, trapping the catalyst in an inactive form. With Grubbs' ruthenium catalysts 2 and 3, the substrate coordinates to the metal center primarily to yield non-productive metathesis, which results in a build-up of the methylidene complex leading to catalyst decomposition. These results are directly correlated to the orientation of the substrate's bulk during the metallacyclobutane formation, the alkyl branch being adjacent to the metal center in the case of the molybdenum catalyst 1, and opposite to it in the case of ruthenium catalyst 2 and 3. (c) 2005 Elsevier B.V. All rights reserved.
Determining the Scope of the Organolanthanide-Catalyzed, Sequential Intramolecular Amination/Cyclization Reaction: Formation of Substituted Quinolizidines, Indolizidines, and Pyrrolizidines
作者:Gary A. Molander、Shawn K. Pack
DOI:10.1021/jo035205f
日期:2003.11.1
The scope of the lanthanide-mediated, intramolecular amination/cyclization reaction was determined for the formation of substituted quinolizidines, indolizidines, and pyrrolizidines. A methyl group was installed at diverse positions in the substrates to determine the sense and magnitude of diastereoselection. High diastereoselectivity (>20:1) was achieved for the formation of some quinolizidines and