Tandem Semipinacol-Type 1,2-Carbon Migration/Aldol Reaction toward the Construction of [5–6–7] All-Carbon Tricyclic Core of <i>Calyciphylline</i> A‑Type Alkaloids

A Lewis acid promoted tandem semipinacol-type 1,2-carbon migration/aldol reaction of trimethylsilane-protected vinylogous α-ketols with aldehyde or dimethyl acetals is reported. This reaction provides a direct and rapid way for the construction of 6-substituted spiro[4.5]decanes which extensively exist in Daphniphyllum alkaloids. By the use of this method, further construction of a [5–6–7] all-carbon tricyclic core of Calyciphylline A-type alkaloids was also completed.