Sy//-Periplanar

To explain this phenomenon, there are two seperate processes to consider. The first and most important one for this reason is the formation of the oxaphosphetane. The addition of the ylide to the aldehyde can, in principle, produce two isomers with either a Z or substituted double bond. The following elimination step is stereospecific, with the oxygen and phosphorus departing in a sy -periplanar transition state. With unstabilized ylides the syn diastereomer of the oxaphosphetane similar to 61 is formed preferentially. This step is kinetically controlled and therefore iiTeversible, and predominantly the Z-alkene 62 that results reflects this. 

All the observation made in the case of275 can be applied in the case of the seven-membered ring orthoesters 216-219. However, due to the much floppier structures of these compounds and also to the fact that sy -periplanar lone pairs must also be at work, application of stereoelectronic principles becomes an awkward task. 

Cullman, O., Vogtle, M., Stelzer, E, and Prinzbach, H., Proximate, sy -periplanar bisdiazenes/ bisdiazeneoxides — synthesis, photochemistry. Tetrahedron Lett., 39, 2303,1998. 

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