Configurational stability of allenic anions, use of a vitamin D derived CD fragment as a stereochemical probe and the stereoselective synthesis of precursors

KRUCHTEN, E. M. G. A. VAN; Haces, Alberto; Okamura, William H.

doi:10.1016/s0040-4039(00)88230-1

Cited by 13 publications

(4 citation statements)

References 22 publications

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“…An early report concerns 8P - Li , in which the 1 H NMR AA‘BB‘ pattern of the cyclopropyl group in benzene at room temperature coalesced and became an A 4 pattern as THF was added. Okamura and co-workers metalated diastereomeric allenes ( 49 and its isomer) and found that deuteration gave identical products . However, treatment of sulfoxide 48 and its isomer with MeLi to give 49 , a reaction that probably goes through an allenyllithium intermediate, proceeded stereospecifically with retention of configuration, implying some stereochemical lifetime of the lithium reagent before protonation occurs …”

Section: Discussionmentioning

confidence: 99%

Solution Structure and Stereochemistry of Alkyl- and Silyl-Substituted Allenyl-Propargyllithium Reagents

et al. 1999

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Analysis of 13 C chemical shifts and Li-C couplings showed that allenyllithium (5A-Li) was a mixture of monomer and dimer in THF, both with an allenyl structure. Similarly, the metalation products of 2-butyne (6A-Li), 4-methylpentyne (9A-Li), and 4,4-dimethylpentyne (21A-Li) in THF, as well as several 1, 3-dialkyl (32A-Li, 33A-Li, 34A-Li) propargyl-allenyllithiums formed by metalation or Li/Sn exchange were all monomeric lithioallenes in THF. Compound 6A-Li was shown to have less than 5% and 21A-Li less than 3% of the propargyllithium isomers (6P-Li, 21P-Li) present from analysis of residual broadening of the propargyl carbon by Li-C coupling. The reagent prepared by metalation of dicyclopropylacetylene (8P-Li) has a propargyl structure, but two related reagents with a cyclobutane spanning the 3,3-positions (37A-Li and 39A-Li) had allenyl structures. Several triorganosilyl-substituted reagents were also investigated. Those with silyl groups at the allenyl position (28A-Li, 31A-Li) are allenyllithiums, those with silyl groups at the propargyl position (22-Li to 26P-Li) showed chemical shifts intermediate between those of allenyl and propargyl isomers, and the shifts were strongly temperature-dependent under some conditions. These compounds are probably equilibrating mixtures of allenyl and propargyllithiums or equilibrating mixtures of unsymmetrically π-complexed structures, with barriers to interconversion (∆G q -150 ) below 4 kcal/mol. Several of the organolithium reagents studied had diastereotopic carbon signals (SiMe 2 , CMe 2 , or CPh 2 groups), which allowed determination of barriers to configurational inversion of the chiral allenyl fragment. Barriers from 6.1 kcal/mol (26A-Li in dimethyl ether) to 14.5 kcal/mol (39A-Li in 3:2 THF/ether) were measured.

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Section: Discussionmentioning

confidence: 99%

Solution Structure and Stereochemistry of Alkyl- and Silyl-Substituted Allenyl-Propargyllithium Reagents

et al. 1999

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“…No starting allene was observed by GC after workup of the reactions in which the acetylene zipper conditions were used (e.g., entries 1, 2). In contrast, the metalation of allene 1 using n -BuLi (THF, −78 to 0 °C) and quenching the reaction at 0 °C with aqueous ammonium chloride gave a 1:1 mixture of starting allene to terminal alkynes . The 2 : 3 selectivity in this reaction was measured at 4:1, an increase in selectivity that prompted us to examine the allene isomerization in THF.…”

Section: Resultsmentioning

confidence: 99%

Stereogenic Propargylic CentersviaBase-Mediated Terminal Allene Isomerization

et al. 1996

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A study of alkali metal amide-mediated isomerizations of terminal allenes is described. The isomerizations of substituted ethenylidenecyclohexanes to form diastereomeric mixtures of terminal alkynes have been conducted to determine factors which may influence the stereochemistry at the newly formed propargylic centers. An initial base screen revealed that potassium N-methylbutylamide (KMBA) exhibits the highest level of equatorial to axial alkyne diastereoselectivity. With the severely hindered terminal allene 26, the use of potassium 3-aminopropylamide is required to effect isomerization. A general synthesis of deuterated terminal allenes has also been achieved, and a mechanistic study using d(2)-allenes 18a,b has revealed the involvement of a propargylic anion in the course of the KMBA-mediated isomerizations.

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“…Conversion of allene 10 [2,12] to the lithium cornpound followed by addition of diphenyl diselenide treatment with a suitable organocopper reagent would afford the desired allenes and, indeed, at a time when only 3b and 3c were available, (6R)-allenes related to la could be readily prepared [2] . There has been some question as to whether the organocopper mediated conversion of propargylic esters into the corresponding allenic compounds proceeds either by a syn or an anti attack of the copper reagent as in Scheme 1 [2].…”

Section: Propargylic Substratesmentioning

confidence: 99%

“…Depending upon conditions and substrates, however, the degree of stereoselectivity is highly variable. Furthermore, some care must be exercised to minimize epimerization afforded primarily the selenide 19 [12]. m-Chloroperoxybenzoic acid (MCPBA) oxidation and hydrolysis of Ig-2g resulted in 4a (60%) along with small amounts of 3a (6%).…”

Section: Reactantsmentioning

confidence: 99%

On the Stereochemistry of Organocopper Mediated Conversion of Propargylic Esters to Allenes

Haces

KRUCHTEN

Okamura

1985

Israel Journal of Chemistry

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Abstract. Using a vitamin D-related steroidal CD fragment as a stereochemical probe, the effect of various factors on the stereochemical course of the organocopper induced conversion of propargylic esters to allenes has been studied. In all cases, an anti mode of attack by the organocopper species was found to be preferred. The effectiveness of Gilman-type reagents (R 2CuLi) versus that of the Lipshutz-type higher-order mixed cuprate systems (R 2CuCNLi 2 ) in promoting the reaction was also studied.

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Configurational stability of allenic anions, use of a vitamin D derived CD fragment as a stereochemical probe and the stereoselective synthesis of precursors

Cited by 13 publications

References 22 publications

Solution Structure and Stereochemistry of Alkyl- and Silyl-Substituted Allenyl-Propargyllithium Reagents

Solution Structure and Stereochemistry of Alkyl- and Silyl-Substituted Allenyl-Propargyllithium Reagents

Stereogenic Propargylic CentersviaBase-Mediated Terminal Allene Isomerization

On the Stereochemistry of Organocopper Mediated Conversion of Propargylic Esters to Allenes

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