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Boranes crotyl

The acid portion of 1 was assembled by enantio-and diastereocontrolled addition of Z-crotyl borane to the aldehyde 8, following the Brown protocol. Hydroboration and oxidation led to 9, which was condensed with the allenyl silane 10 to give 11 with high diastereocontrol. Conversion of the alcohol to the iodide followed by three-carbon homologation by the Myers procedure then led to 1, which was cyclized with > 10 1 regio- and diasterocontrol to give 12. Ozonolysis and methylenation of the less hindered ketone then delivered 3. [Pg.52]

Allyl and crotyl boranes react using the double bond... [Pg.1284]

The six-member ed transition state for the reaction of an allylic borane or boron ate is very reminiscent of the cyclic transition state for the aldol reaction you met in Chapter 34. In this case the only change is to replace the oxygen of the enolate with a carbon to make the allyl nucleophile. The transition state for the aldol reaction was a chair and the reaction was stereospecific so that the geometry of the enolate determined the stereochemistry of the product aldol. The same is true in these reactions. -Crotyl boranes (or boronates) give anti homoallylic alcohols and Z-crotyl boranes (or boronates)... [Pg.1285]

Less is known about the reaction of allylboron compounds however, allylation will occur smoothly in the presence of a base and a palladium catalyst (Scheme 40). The reaction of tri(crotyl)borane with iodobenzene in THF in the presence... [Pg.47]

Simple alkyl boranes do not react with aldehydes and ketones but allyl boranes react rapidly because they can use a six-membered cyclic transition state 68 not unlike that of the Claisen rearrangement. The evidence for this is that the allyl group is rearranged in the product. In this example a crotyl borane 66 adds to an aldehyde to give anti-70 since both R and Me prefer to be equatorial in the transition state 68. [Pg.513]

Brown s crotyl borane 158 (chapter 24) provides reagent control through a chair-like six-membered ring in the formation of 159. Oxidation of the alkene (ozone with oxidative workup) gives the free acid marked in 160 and on removal of the benzylic group the freed amine (circled) cyclises to it to give the pyrrolidone required (147 as its methyl ester). The synthesis can easily be completed from there. [Pg.739]

Scheme 5.3. (a) Stereospecificity (within experimental error) of crotyl borane additions to aldehydes, R = Me, Et, i-Pr, Ph [26]. (b) Transition structures for stereospecific addition of crotyl boron compounds to aldehydes. [Pg.164]

A few examples showcasing double stereodifferentiation phenomena are outlined below. In the case of crotylation of a-alkoxy-substituted aldehyde 72, Roush observed a reversal of facial selectivity with either enantiomer of the chiral ( )-crotylboronate reagent 29 (Scheme 5.13) [48]. Similarly, Brown found that the pinene-derived crotyl boranes 76 and 77 provide access to all four stereotriads 78-81 with impressive stereoselectivity with use of either the ( )- or the (Z)-crotyl reagent (Scheme 5.14) [77]. [Pg.165]


See other pages where Boranes crotyl is mentioned: [Pg.105]    [Pg.1285]    [Pg.398]    [Pg.1287]    [Pg.1287]    [Pg.95]    [Pg.514]    [Pg.514]    [Pg.275]    [Pg.1285]    [Pg.454]   


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