Analyse conformationnelle a l'aide de la notation des angles de torsion—V : Cours sterique de la cis-addition de reactifs sur les olefines de cycles petits et moyens; le cas des derives cyclopenteniques

The steric course of suprafacial cis-addition to cyclopentenes can be interpreted or predicted using the following assumptions: (a) cis-addition to an olefin is not a synchronous process; (b) the reaction involves the least amount of conformational distortion from the reactant to the primary product; (c) the addition takes place so as to give preferentially the axial product. Using the torsion angle notation,^2,3 the sign sequence of torsion angles, taken clockwise before and after the double bond, is related to the direction of cis-addition; the (+0?) sequence corresponds to β-cis-addion, above the mean plane of the ring, and the (?0+) sequence corresponds to α-cis-addition, below the mean plane of the ring.