70 h, H Br 1 37 % S J 100'. 17 h. 2. (Ph)3P=CHCOOEt, CH~CII. r.t., 29 % overall yield ' 'OOE~ quant. 4 racemic mixture, cisltmns 70:30
However, this protocol was limited by several drawbacks in its original version; e.g., when using 2-allylpiperidine in the one-pot ring opening, the cyclized products were formed with poor stereoselectivity. Long reaction times were required for the (1,3-dithiolan-2-y1)methyl-substituted amines to ring-open 1. The conversion of 1 -[(2E,4Z)-4-bromohexa-2,4-dienyl]-2-[(1,3-dithiolan-2-yl)methyl]piperidine (3) into its corresponding a,P-unsaturated ester 4 was accomplished only in modest yield. However, the last step in the sequence showed promising features: quantitative yields and good stereoselectivities were observed when triene 4 was cyclized thermally; excellent stereoselectivities (cislfrans ratio 6:94) were obtained when catalyzing the intramolecular Diels-Alder addition with TiCl,. This encouraged us to develop a new and general method for the synthesis of various PA, IA, and QA. In this paper, we introduce amino alcohols as versatile starting materials for the ring-opening reaction, and by using L-prolinol, five asymmetric centers could be formed stereoselectively in the final products.