Ammonium ylides Subject

Recently, Padwa s group has encountered an interesting Stevens rearrangement sequence which occurred when a-diazo ketoamide 142 was subjected to Rh(II)-catalyzed conditions.66 In the presence of DMAD, the expected product 146 derived from the trapping of dipole 145 was isolated (57%) together with the unexpected lactam 144 (23%). In the absence of DMAD, lactam 144 was the only product obtained in 62% yield. The formation of 144 was attributed to the initial generation of ammonium ylide 143 followed by a [l,2]-benzyl shift. 

A derivative of (S)-prolinol 10.23, quartemized by CICH2CN and then transformed into the ammonium ylide by KO-fert-Bu in DMSO, has been subjected to a [2,3]-sigmatropic rearrangement at -90°C. After hydrolysis of the aminoni-trile formed in this way, an a-chiral P,y-unsaturated aldehyde is formed with an excellent selectivity [261,290, 1008, 1062] (Figure 10.9). 

The employment of an ion-supported phosphonium ylide allows the alkene product to be purified by simple filtration of the reaction mixture and subsequent removal of the solvent from the filtrate. In this regard, a sulfonate anion [145] and an ammonium cation [146] have been introduced to the phosphonium salt by Chan and Togo, respectively (Scheme 31). The recovered ion-supported phosphine oxide byproduct is subjected to reduction to regenerate the corresponding phosphine, which can be reused for the Wittig reaction. 

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