Vinyllithiums olefinic

B. Cyclization Reaction of Heteroatom-substituted Olefinic Vinyllithiums... 

The Shapiro reaction provides a convenient, easy and straightforward method to convert ketones into a plethora of olefinic substances in high yields. Many of these vinyllithium derivatives are useful for further synthetic manipulations. No attempt is made in this chapter to cover all the applications of the Shapiro reaction and only few representative examples will be described. A variety of polyolefins such as 119, used for cation olefin cyclization, can be stereospecifically formed in a concise and modular approach in a single step from the components shown in equation 42 via the Shapiro reaction . 

It is a modification of the Shapiro olefin synthesis3 which allows the vinyl anion intermediate to be trapped with primary halides and other electrophiles. Use of triisopropylbenzenesulfonylhydrazones as the vinyllithium precursor4 is an improvement over previously4 used toluenesulfonylhy-drazones,5 6 which can be employed in the sequence provided excess sec-butyllithium (typically 4.5 equiv) and alkyl halide (3.0 equiv) are used. Methyl ketones (e.g., acetone, acetophenone, 2-octanone) can also be used and can be converted into their dianions using 2.2 equiv of the weaker base, n-butyllithium. The conditions described above, with the slight modifications noted, have been used for a variety of ketones as shown in Table I. 

An enantioselective route to a-amino acid derivatives has been developed which utilizes addition of a scalemic vinyllithium species to an N-sulfonyl imine [99]. Metallation of dibromide 287, derived from enantiomerically pure lactic acid, is regioselective and affords lithiated species 288 which adds to a variety of N-mesitylsulfonyl imines to give adducts 289 in diastereomeric excesses above 90% (Scheme 48). Ozonolysis of the olefinic moity of purified adducts 289 then afforded enantiomerically pure a-amino acid derivatives 290. 

Quenching the vinyllithium compound derived from ketone tosylhydrazones with water constitutes a facile procedure for the preparation of olefins from ketones as shown in equations 60 and 6155. 

Addition of vinyllithium reagent 636 to 632 occurs largely from the re side (Felkin—Anh selectivity) to give the anti diastereomer 637 (85% de) [193]. Lithium—bromine exchange at — 78 °C followed by protonation affords 638 in 89% yield with complete retention of olefin geometry. Ozonolysis of 638 followed by lithium aluminum hydride reduction of the intermediate aldehyde furnishes protected triol 639 in 78% yield. 

The cases that form the basis for the above discussion involve additions to chiral C=N electrophiles. The alternative process, in which the chirality resides in the nucleophile, is not well represented. However, Braun has examined a remarkable case with chiral vinyllithium species 47 (Scheme 11.5) [59]. The vinyl anion was shown to participate in additions to N-sulfonyl al-dimines such as 48, to furnish the corresponding adduct 49 in this case as a 96 4 mixture of diastereomers. Ozonolytic cleavage of the olefinic product provided the corresponding a-amino ester 50 with no observed deterioration in optical purity. It was suggested that the positioning of the MEM-acetal in 47 led to stabilization of the carbanion by chelation and, significantly, provided a conformationally locked system capable of differentiating between the two C=N faces. 

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