Boronic hemiesters

The first enantioselective organocatalytic oxo-Michael reaction to enones was reported by Falck [109]. Falck developed the intramolecular addition of boronic acids hemiesters to enones catalyzed by thiourea catalysts. Alkyl, aryl, and heteroaryl y-hydroxy enones react with phenylboronic acid to furnish the corresponding boronic hemiesters 118, which after oxidative cleavage of the resulting dioxaboro-lane renders the chiral 1,2 diols 119 in excellent yields and enantioselectivities (Scheme 33.34). 

Scheme 6.141 Mechanistic proposal for the 121-catalyzed asymmetric intramolecular Michael addition exemplified for the model substrates ( )-4-hydroxy-l-phenyl-2-buten-l-one (n = 0) and ( )-5-hydroxy-l-phenyl-2-buten-l-one (n = 1) 121 functions as push/pull-type bifunctional catalyst inducing the cyclization of boronic acid hemiester (1) to form intermediate (2) release ofdiol product (3) by oxidation.

Scheme 33.34 Intramolecular addition of boronic acid hemiesters to enones.

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