Tryptophan-boron complexes

Corey has developed an interesting class of Lewis acidic boron complexes prepared from M-aroylsulfonyl-L-tryptophan 208. The active complex is conveniently prepared from 208 and BuB(OH)2 (Eq.42). This same complex catalyzes the enantioselective addition of methyl ketone derived trimethylsilylenol ethers and l-methoxy-3-trimethylsilyloxybutadiene (Eq.43) [118,119].Using 20 mol %... 

Among nonmetal chiral complexes, chiral oxazaborolidines have been found effective in many D-A reactions. The adduct obtained from the D-A reactions of 5-benzyloxymethyl-l,3-cyclopentadiene 98a with a-bromoacrolein in the presence of catalyst 98 (S-tryptophan-derived oxazaboroUdine) is an important intermediate in the synthesis of prostaglandins. The aldehyde group of the dienophile is bound to the catalyst by coordination with boron by Lewis interaction and the Lewis complex is stabilized by H-bonding. The upper face of the aldehyde is shielded by indole moiety of the catalyst. The benzyloxymethyl substituent of the cyclopentadiene produces a steric differentiation on the two faces of cyclopentadiene ring resulting the approach of the diene preferably from one face. 

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