Yamamoto asymmetric Claisen rearrangement

The only examples of asymmetric Claisen rearrangements catalyzed by a chiral aluminum catalyst are those published by Yamamoto and coworkers [24-27]. The Claisen rearrangement of allyl vinyl ethers of type 91 (Sch. 13) can proceed via either of the two enantiomeric chair transition states 92 or 94. If a chiral Lewis acid is used to activate the process, the transition states become diastereomeric and preferential formation of one enantiomer of the product is possible. This is complicated because coordination of a chiral Lewis acid to the ether oxygen of allyl vinyl ether produces a new chiral center as indicated in 96 and asymmetric induction might occur from a substituent on aluminum to the oxygen and then from the oxygen to the C-3 carbon in the product. In their initial report, Maruoka, Banno, and Yamamoto found that a catalyst prepared from the 3,3 -disilyl-substituted BINOL 97 and trimethylaluminum was effective in this transformation [24]. The catalyst 98 was shown to be monomeric by measurement of freezing point depression. 

K. Maruoka, K. Nonoshita, M. Banno and H. Yamamoto, J. Am. Chem. Soc., 1988, 110, 7922. Most recently the first asymmetric Claisen rearrangement catalyzed by a chiral organoaluminum reagent has been reported K. Maruoka, M. Banno and H. Yamamoto, J. Am. Chem. Soc., 1990, 112, 7791. 

Yamamoto and Maruoka have reported the first aluminum-promoted asymmetric Claisen rearrangement [5]. They used a chiral 3,3 -bis(triarylsilyl)-substi-tuted binaphthol (BINOL) aluminum reagent 1 to promote the enantioselective coordination to one of the two enantiomeric oxygen lone pairs of 2. Enantioselective discrimination is based on two chair-hke transition states A and B to give two enantiomeric products 3, respectively (Scheme 2.2). 

In 1990, Yamamoto and Maruoka reported the first example of chiral aluminum Lewis acid-catalyzed asymmetric Claisen rearrangement (Scheme 6.162) [192]. Since chiral Lewis acids (2a) and (2b) derived from 3,3 -substituted BINOL derivative (1) exists as a mononuclear complex, high catalytic activity was realized. In this reaction, simple allyl vinyl ethers gave only poor results, although silylated substrates resulted in good enantioselectivity. 

Later, Yamamoto and Maruoka reported improved Lewis acids (153b) and (153c), namely, aluminium tris (2-a-naphthyl-6-phenylphenoxide) (ATNP) and aluminium tris (l-a-naphthyl-3-phenylnaphthoxide) (ATBN), respectively (Scheme 6.163) [192d, 193]. These Lewis acids effectively catalyze asymmetric Claisen rearrangement of simple allyl vinyl ether substrates without a substituent on the vinyl group. 

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