ASYMMETRIC INDUCTION IN REDUCTIVELY INITIATED [2,3]-WITTIG AND RETRO [1,4]-BROOK REARRANGEMENTS OF SECONDARY CARBANIONS

The synthesis of O,S-acetals and the lithium naphthalenide initiated rearrangement reactions thereof are described. O,S-Acetal 8a resulted from trapping of the 1,4-dipole 7 with thiophenol. O,S-Acetals 16a and b were obtained from aldehydes 14a/b by a one-pot reaction with (trimethylsilyl)prenol, (trimethylsilyl)thiophenol, and trimethylsilyl triflate. Upon reduction with lithium naphthalenide all 0,S-acetals delivered alpha-lithiated ethers. They rearranged either in a 12,3]-Wittig mode furnishing the 1,3-diol derivatives 20a-h/21 with moderate stereoselectivity (syn:anti = 35:65 to 78:22) or underwent retro [1,41-Brook rearrangements yielding the alpha-silyl ethers 23i-k/24 with still less stereocontrol (syn:anti = 34:66 to 50:50). The mechanistic implications are discussed.