Allenic zincs

SCHEME 30. Allylic and allenic zinc reagents via methylene homologation... [Pg.304]

SCHEME 31. Preparation and uses of allenic zinc reagents using lithium trialkylzincates... [Pg.305]

Interestingly, silylated propargylic zinc reagents, such as 342, may be better viewed as the allenic zinc reagent 343 which reacts with an aldehyde via a cyclic transition state, affording only the a f/-homopropargylic alcohol 344 with 90% yield (Scheme 91) 22. Alkylzinc halides react only sluggishly with aldehydes or ketones. This reactivity can be... [Pg.349]

Generation of an allenic zinc and its reaction with an aldehyde synthesis of (3/ , 4S )-2,4,7-trimethyloct-7-en-5-yn-3-ol (Structure 31 )11... [Pg.111]

Cool the flask to -85°C. Slowly add the solution of ZnCI2 over 15 min using a syringe pump, and then stir at this temperature for 1 h to prepare allenic zinc... [Pg.112]

The pure E stereochemistry of the vinylsilane 21 clearly shows that the carbocylization proceeds in a totally stereoselective fashion to give, under these conditions (20 °C, 5 min) the configurationally stable (Z)-alkenylzinc bromide [21] 20 via a syn addition of the allenic zinc bromide to the acetylenic moiety. This carbocyclization represents a new desymmetrization of diynes such as 18 which serves to deliver functionalized exo-... [Pg.421]

Remaifcable selectivities are observed in the addititm of allenic zinc chltxides to aldehydes, affOTding onn -homopropaigylic alcohols via a transition state of type (41) (see Scheme 21). An extension to 1-substituted trimethylsilylallenic zinc chltxides was also possible and gave the anh-alcohols (42) in 92-99% stereoistHneric purity. [Pg.221]

Allylstannanes result from stannylcupration of allenes, zinc mediated coupling of allylBr... [Pg.96]

G. Zweifel and G. Hahn, J. Org. Chem., 1984, 49, 4575. threo-Homopropargylic alcohols from allenic zincs and RCHO. [Pg.20]

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