3.3- disubstituted allylboronates

The haloboration-coupling sequence provided simple access to the stereochemically pure 3,3-disubstituted allylboron compounds 116 via one carbon homologation of the alkenylboron intermediates (Equation (20)).196 Bromoboration of allene afforded 117 which was converted into a tartrate for asymmetric allylboration (Equation (21)).197... 

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). 

Diastereoselective addition of 2-carboxyester 3,3-disubstituted allylboronates to both aromatic and aliphatic aldehydes using a catalytic amount of Sc(OTf)3... 

Tablet. 1,2-Disubstituted 3-Butenols by Diastereoselectivc Reaction of Aldehydes and Substituted Allylboronates ... 

The addition to 1,3-dienes afforded a new class of allylboron compounds. The platinum(0)-phosphine catalysts stereoselectively yielded or-1,4-addition products 131233 234 and 133216 235 for 2,3-disubstituted butadiene, 1,3-cyclohex-adiene, and 1,3-pentadiene by Txr-coordination of a diene to a platinum catalyst (Equations (30) and (31)). In contrast, phosphine-free Pt(dba)2 resulted in the selective formation of a 1,2-addition product 134216 for 1,3-pentadiene (Equation (31)). The corresponding chiral allyl boronates were synthesized by diboration of dienes with 123 or 124.234 235... 

The diboration of allenes afforded another series of allylboron compounds that each have a boryl group at the vinyl carbon. The addition had a strong tendency to occur at the internal double bond of terminal allenes such as 1,2-heptadiene (Equation (33)).237 However, steric hindrance in both allenes and phosphine ligands forced the addition toward the terminal double bond of dimethylallene (Equation (34)).237 On the other hand, addition to the terminal double bond occurred selectively for both monosubstituted and 1,1-disubstituted allenes when Pd(dba)2 was used in the presence of a co-catalyst (RI) such as I2, Arl, and iodoalkenes (Equation (35)).238 The role of co-catalyst was attributed to in situ formation of I-Bpin intermediate, which undergoes oxidative addition and insertion leading to 2-boryl-7r-allylpalladium intermediate. 

The intramolecular allylboration of an aldehyde function leads selectively to cir-disubstituted cyclic ethers. It has been shown that both the reactive aldehyde and the allylboronate moiety can be initially generated in situ in a masked form and then liberated simultaneously by hydrolysis of the precursor functions <1997JA7499>. This methodology was successfully applied to the one-pot synthesis of the oxocene 82, a precursor of (-l-)-laurencin (Scheme 13). A DIBAL reduction of the Weinreb amide 80, metalation with r f-butyllithium, borylation with the pinacol borate ester, and, finally, liberation of both the aldehyde and the allylboronate function by aqueous pH 7 buffer solution generated the reactive 81, which cyclized in 38% overall yield to the oxocene 82. Only the all-cis-diastereomer is formed, which means that the cyclization proceeds under high asymmetric induction from the resident stereogenic center present in 80. 

Z- or -Configured allylboronates 46b bearing a trisubstituted olefin moiety could also be employed in the developed reaction sequence affording access to optically active fS,y-disubstituted derivatives 29 (Scheme 4.12). Importantly, the reactions with both aldehydes 38 and imines 47 proved fully diastereospecific. While reactions with Z-46b led to diastereoselective formation of trans-29, the allylboration of -46b gave CIS-configured 29 exclusively. Excellent enantioselectivities (>95% ee) obtained in the synthesis of lactams 29b (X = NMe) are worth noticing. 

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