2-Alkoxycarbonyl allylboronates

Elford, T. G. Arimura, Y Yu, S. H. Hall, D. G. Triflic Acid-Catalyzed Additions of 2-Alkoxycarbonyl Allylboronates to Aldehydes. Study of Scope and Mechanistic Investigation of the Reaction Stereochemistry. /. Org. Chem. 2007, 72,1276-1284. 

SCHEME 1 Synthesis and reactivity of 2-alkoxycarbonyl allylboronate 2 with aldehydes. 

In the last example, cis-carbocupration of alkynoic ester 14 provides a weakly nucleophilic and configurationally imstable 1-alkoxycarbonyl alkenylcopper intermediate (15) the presence of HMPA as additive was crucial to avoid erosion of the E/Z stereoselectivity in the alkylation step with electrophile 16 [39, 40]. Under these conditions, 3,3-disubstituted allylboronates 17 were prepared in over 20 1 selectivity, and these reagents were subsequently employed in the stereoselective preparation of quaternary carbon centers (Section 6.4.2.2). 

A variant of these reactions was optimized based on the Hosomi-Miyaura borylation of a,P-unsaturated carbonyl compounds with nucleophilic boryl copper, providing 2-alkoxycarbonyl (as well as 2-acyl- and 2-cyano) allylboronates such as 40 in high stereoselectivity (Equation 22) [60]. The resulting 2-alkoxycarbonyl reagents react with aldehydes to provide a-methylene-y-butyrolactones. 

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