| Research Abstract |
1) Attempted asymmetric addition of dialkoxytitanacycle to aldehydes. Titanacyclopentadiene prepared from a 1,6-diyne, Ti (O-i-Pr)_4, and i-PrMgCl was allowed to react with benzaldehyde in the presence of optically active binaphthol. However, no asymmetric induction was observed. It could be concluded that a large quantity of the magnesium salts present in the reaction media should hinder organization of a chiral titanium intermediate that is essential for the high level of asymmetric addition. Thus, we abandoned this project and turned our attention to substrate-controlled asymmetric cyclization. 2) Intramolecular asymmetric cyclization of allene and acetylene. It was reported last time that the titanacycles generated by the cyclization of optically active 5,6-dien-1-ynes react with aldehydes, ketones, or imines to give the corresponding adducts in a highly regio-, stereo-, and diastereoselective manner and with nearly complete retention of the enantiopurity of the starting material. In order to clarify the stereochemical course of this reaction, the absolute stereochemistry of the products was elucidated, which in turn, revealed that the addition of allyltitanium moiety in the titanacycle could be strictly expressed in terms of syn-S_E2' fashion. 3) Asymmetric cyclization of bis-unsaturated esters and its application to the synthesis of optically active terpenes. It was reported last year that the titanium-mediated tandem cyclization of olefins and acetylenes having a conjugated ester group affords 1-bicyclo [3.3.0] octen-3-ones (or-octan-3-ones) and bicyclo [3.1.0] hexane structures. This time, an allylic substituent in the starting material was found to nicely control the Chirality of the cyclization product, which was utilized in an asymmetric synthesis of d-sabinene, a monoterpene having a cyclopropane moiety.
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