text: The resonance structures below show the 1,3-dipole contribution, in which the two carbon atoms adjacent to the nitrogen have a negative or positive charge. The most common representation of azomethine ylides is that in which the nitrogen is positively charged, and the negative charge is shared between the two carbon atoms. The relative contributions of the different resonance structures depend on the substituents on each atom. The carbon containing electron-withdrawing substituents will have a more partial negative charge, due to the ability of the nearby electron-withdrawing group to stabilize the negative charge.
  Three different ylide shapes are possible, each leading to different stereochemistry in the products of 1,3-dipolar cycloaddition reactions. W-shaped, U-shaped, and S-shaped ylides are possible. The W- and U-shaped ylides, in which the R substituents are on the same side, result in syn cycloaddition products, whereas S-shaped ylides result in anti products. In the examples below, where the R substituent ends up in the product depends on the substituent's steric and electronic nature (see regioselectivity of 1,3 dipolar cycloadditions). The stereochemistry of R and R  in the cycloaddition product is derived from the dipole. The stereochemistry of R is derived from the dipolarophile—if the dipolarophile is more than mono-substituted (and prochiral), up to four new stereocenters can result in the product.