Condensation vinyl-Grignard reagents

Of particular importance for the synthesis of carotenoids is the preparation of vinyl alcohols by the condensation of aldehydes and ketones with vinyl magnesium halides, frequently the commercially available bromides, in THF [12]. The reaction is conducted by adding a solution of the aldehyde or ketone in anhydrous THF to the vinyl Grignard reagent in THF below 0°C, and is exemplified by the preparation of the vinyl-p-ionols 20 [13] and 27 [14] from p-ionone (3) and (3/ )-3-hydroxy-p-ionone (22), respectively (Scheme 6). Table 2 lists some vinyl alcohols that have been prepared from vinyl magnesium halides. 

When dibromides from alkyl vinyl ethers were condensed with Grignard reagents and the resulting )0-bromoethers were treated with zinc, 53-84% of olefins could be yielded, which is 10-30% higher than the traditional procedure from aldehydes. ... 

The Bartoli-indole synthesis utilizes the [3,3]-rearrangement of an 0-vinyl-iV-arylhydroxylamine intermediate to form indoles through a similar rearrangement and condensation process as the Fischer-indole synthesis f Scheme 14.7a). The rearrangement precursor 59 is generated from the addition of a vinyl Grignard reagent to a nitroarene. The... 

Condensations - Methyl ketones are converted to the corresponding vinyl compounds in high yield by treatment with paraformaldehyde in the presence of N-methylaniline trifluoroacetate in THF or dioxane. The formamide of 2-(N-methyl)aminopyrldine has been found to be an efficient reagent for the transfer of formyl groups to Grignard reagents. 

The required 2-(4-methoxybenzocyclobutenyl)-ethyl iodide 79 was prepared by a multistep process from 2-bromo-5-methoxybenzaldehyde via l-cyano-4-methoxybenzocyclobutene 77 as illustrated. 2-Methylcyclohexenone was then converted into 2-methyl-3-vinyl-6-n-butylthiomethylenecyclohexanone 80. Blocking C-6 was considered necessary to insure regiospecihc condensation of 79 at C-2. However, the condensation reaction afforded adduct 81 in only 16% yield. In subsequent work (with 2-methylcyclopentenone) 1 4 addition of a vinyl Grignard or lithium reagent and coupled alkylation of the intermediate enolate occurred at C-2 without the need to block C-6 (Scheme 1, Scheme 14 - ). 

Michael-Peterson Condensation. 3-Trimethylsilyl-3-buten-2-one also undergoes smooth Michael addition with Grignard reagents (R = Me, n-Pr, i-Pr, f-Bu, Ph), generating magnesium enolates which are then trapped with benzaldehyde to give ( )-and (Z)-enone isomers after Peterson condensation (eq 6). For example, treatment of the a-silyl vinyl ketone with methylmagne-sium iodide followed by reaction with benzaldehyde yields a 7 1... 

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