Dibutylboron triflate/diisopropylethylamine mediated aldol-type cyclization provides an expedient route for the stereoselective synthesis of cyclic ethers in a single step. The method is highly efficient for the stereoselective synthesis of 4-cis-tetrahydropyranones. The reaction is proposed to proceed via an SN1-type mechanism through a chair-like transition state, in which both substituents occupy equatorial positions.
[Ishihara, Kazuaki; Hanaki, Naoyuki; Funahashi, Miyuki; Miyata, Mayumi; Yamamoto, Hisashi Bulletin of the Chemical Society of Japan, 1995 , vol. 68, # 6 p. 1721 - 1730]