TI(II)-MEDIATED ALLENYNE CYCLIZATION AS A NEW TOOL FOR GENERATION OF CHIRAL ORGANOTITANIUM COMPOUNDS - ASYMMETRIC GENERATION OF CYCLIC ALLYLTITANIUM REAGENTS WITH NO CHIRAL AUXILIARY

Treatment of a variety of 1,2-dien-7-ynes with a slight excess of (eta (2)-propene)Ti(O-i-Pr)(2) (1), prepared in situ from Ti(O-i-Pr)(4) and 2 equiv of i-PrMgCl in ether, afforded 1-alkenyl-2-alkylidenecyclopen tanes in moderate to good yields after aqueous workup. The intermediat e titanabicycle such as 7 was identified by deuterolysis, which gave d (2)-8 with exclusive deuterium incorporation. The use of an optically active allene such as 19 (less than or equal to 83% ee) realized a hig hly efficient axial to centered chirality transfer to give 21 (80-83% ee). Analogously, the cyclization of 22 (less than or equal to 86% ee) with 1 followed by carbonylation (under ca. 1 atm of CO) afforded the optically active bicyclic ketone 25 (86% ee). When a homologous 1,2-d ien-6-yne 27 was subjected to the cyclization as above, a different ty pe of titanabicycle 28 was generated, which was identified by hydrolys is giving 29 or the following carbonyl addition. Upon reaction with no nanal, 28 afforded the alcohol 32 with very high regio-(with respect t o the allylic system), stereo-(the diene moiety), and diastereoselecti vities (the hydroxy group). Various 1,2-dien-6-ynes underwent the cycl ization followed by the addition to carbonyl compounds in comparable s electivities as above. Chiral 1,2-dien-6-ynes 46 (less than or equal t o 83% ee) achieved a nearly complete chirality transfer to generate a chiral titanabicycle 48, which, in turn, reacted with nonanal, acetone , or N-benzylidenepropylamine to afford the alcohols 51 and 52 (both i n 80% ee) or the amine 53 (81% ee) in good yields with a very small lo ss of the enantiopurity.