Pyrrolidine ring geometry

In the case of poly-L-proline II (the amide group being in the planar trans conformation), the angle is fixed by the rigid geometry of the pyrrolidine ring hence, the conformational energy depends only on tfi. 

Theoretical calculations showed that in a-helices trans proline peptide bonds can best be accepted up to the fourth position within the helix because there is neither a disruption of the hydrogen bonding network in this region nor does the bulk of the pyrrolidine ring seriously interfere with the regular helix geometry. Analyses of the Brookhaven database in 1991 also showed that the highest fre-... 

Mukaiyama and Kobayashi have postulated the Sn(ll) diamine complex 118 to possess square pyramidal geometry. The optically active diamine ligand forms a chelate to tin with the stereochemically relevant lone-pair residing in the sterically demanding position proximal to the pyrrolidine ring. Aldehydes are proposed to bind at the site trans to the lone pair and syn to the aminonaphtha-... 

Five-membered Rings.—Pyrrolidines constitute an important class of compounds, and much synthetic effort is devoted to them each year. Cycloaddition reactions figure prominently among the many methods that are available for their preparation, and the number has increased this year. Trimethylsilyl triflate is an efficient catalyst for C-C-bond-forming reactions, and has now been shown to catalyse 1,3-dipolar cycloadditions of the ylide (94) with olefinic and acetylenic dipolarophiles, to give pyrrolidines. The geometry of the products depends... 

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