Hydroxy esters from boron enolates

Dialkylboron trifluoromethanesulfonates (Inflates) are particularly useful reagents for the preparation of boron enolates from carbonyl compounds, including ketones, thioesters and acyloxazoiidinones. Recentiy, the combination of dicylohexyiboron trifluoromethanesulfonate and triethyiamine was found to effect the enolization of carboxyiic esters. The boron-mediated asymmetric aldoi reaction of carboxyiic esters is particuiariy usefui for the construction of anti p-hydroxy-a-melhyl carbonyl units. The present procedure is a siight modification of that reported by Brown, et ai. ... 

Polymeric Evans auxiliary 76 was also applied to aldol reactions2 4 (Scheme 1.6.37). In a model reaction, immobilised dihydrocinnamic acid was converted into the boron enolate and reacted with isovaleraldehyde. Products were released by treatment of the resin with NaOMe. The p-hydroxy ester 79 was obtained in a 20 1 diasteromeric ratio, along with some ester 80 derived from unreacted starting material. 

The synthesis began with Prins qrchzation of the symmetric vinylo-gous ester 273, prepared from heptadienol 272, followed by hydrolysis of the resulting trifluoroacetate and benzylation, to afford the desired 2,6-cis-tetrahydropyran 274 with 92 8 diastereoselection at C5 [113], By this novel desymmetrization, 2,4,6-all-ci5 trisubstituted pyran was efficiently provided. Boron-enolate aldol reaction, as Carreira did, of the methyl ketone 274 with aldehyde 275 gave hydroxy ketone 271 as a single isomer. In contrast to Car-reira s result, samarium-catalyzed intramolecular Tishchenko reduction [114]... 

Ester enolates replace bromide from a-bromo boronic esters with remarkable diastereoselcctiv-ity. (Dibromomethyl)lithium is generated by addition of lithium diisopropylamide to dibro-momethane in the presence of a boronic ester at — 78 "C to produce an a-bromo boronic ester. Reaction of the a-bromo boronic ester with lithium 1-tert-butoxy-Tpropen-l-olate yields a product that is almost exclusively the threo-isomer (d.r. = 15 1 to 60 1), as shown by conversion to the / -hydroxy carboxylic ester24. It is worth noting the facility with which a-bromo boronic esters racemize in the presence of halide ions72. 

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