Imines reactions with allenylsilanes

The allenylsilane ene reaction is also well suited for the synthesis of cyclohexane rings. Jin and Weinreb have described the process of Eq. 13.55 in a synthesis of 5,11-methanomorphanthridine, an Amaryllidaceae alkaloid [64], Conversion of aldehyde 163 to imine 164 with piperonylamine took place in situ. Heating the solution of imine at reflux in mesitylene for 2 h led to cyclization through the conformer shown. The yield of 165 from aldehyde 163 was 66%. 

Allenylsilanes undergo intramolecular additions to appropriately positioned aldehydes, imines, conjugated esters and alkenes to afford various alkynylcyclopentane and cyclohexane derivatives (Eqs. 9.66-9.70) [66]. The reactions are promoted by SnCl4 or by thermolysis. The stereochemistry of these cyclization reactions is consistent with a concerted sigmatropic process as illustrated in Scheme 9.17. 

Allenylsilanes 159 (Eq. 13.53) and 161 (Eq. 13.54) differ in the axial stereochemistry of the allene function. In each case, formation of the benzyl imine, followed either by treatment with tin(IV) chloride in benzene at room temperature or heating in toluene, leads to diastereomeric products 160 and 162 [63], Significantly, there is no crossover, pointing to a concerted (or fast, stepwise) process. Since the absolute stereochemistry of the allenylsilanes is easily controlled, the methodology is ideal for applications in total synthesis. Weinreb and co-workers have used the reaction for his synthesis of the marine natural product (-)-papuamine. 

In the Cu(i) catalyzed reaction of the 1-alkyl substituted allenylsilane 325 with tosyl imines in refluxing benzene a 92 % yield of the [3+2] cycloadduct 326 is obtained... 

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