Homopropargylic methyl

Upon treatment with lithium diisopropylamide (LDA) followed by the addition of an electrophile (water, trimethylsilyl chloride, or methyl iodide), 2-methylene oxetanes were converted into homopropargylic alcohols, in good yields (Equation 26) <1998JOC6782>. 

Reactions of chiral allenes proceed with a preference for the formation of the syn diastereomer. The stereochemical outcome of these reactions can be rationalized by invoking an open transition state model for the addition reactions (Figure 12), which depicts an antiperiplanar orientation of the chiral allenylsi-lane to the aldehyde carbonyl. In this model, steric repulsion between the allenyl methyl and the aldehyde substituent is most likely responsible for the destabilization of transition state (B), which leads to the anti (minor) stereoisomer. This destabilizing interaction is minimized in transition state (A). Table 5 illustrates representative examples and summarizes the scope of the regiocontrolled synthesis of homopropargylic alcohols using allenylsilanes. 

Insight into the mechanism of the reaction of allenylboranes with aldehydes is available from the reaction of enantiomerically enriched allenylborane (5)-236 with benzaldehyde produced the homopropargylic alcohols 237 and 238 (Scheme 10-78) [136]. Although the relative diastereoselectivity was poor syn anti, 3/1), the induced selectivity at the methyl-bearing center is conserved. Moreover, the configuration of the methyl-bearing center indicates that the reaction proceeded via a syn Se pathway through a cyclic transition structure. 

Figure 11-18. Reagents for the enaiitioselective synthesis of syn a-methyl homoallylic and homopropargylic alcohols.

Enantiomerically pure allenylzinc and indium reagents are readily available through oxidative transmetallation of transient allenylpalladium intermediates derived from sulfonic esters of (/ )- or (5)-3-butyn-2-ol derivatives, such as 250. When these transmetaUations are conducted in the presence of aldehydes, anti homopropargylic alcohol adducts 251 are formed with high diastereoselectivity (Scheme 10.49). The preference for anti over syn adducts can be assigned to an unfavorable eclipsing interaction between the methyl group of allenyl moiety and the... 

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