Stereoselectivity corrins

Aratani has rationalized the stereoselectivity of his catalysts by a model based on a metallacyclobutene intermediate (cf. Scheme 4, reaction b) [18]. For semi-corrin and related ligands, formation of a metallacyclobutane seems less attractive because of steric hindrance, and a different pathway, analogous to reaction a in Scheme 4, has been proposed [36,40,53b]. Fig. 2 shows the postulated ster-... 

Stereoselectivity. See Asymmetric induction Axial/equatorial-, Cis/trans-, Enantio-, Endo/exo- or Erythro/threo-Selectivity Inversion Retention definition (e.e.), 107 footnote Steric hindrance, overcoming of in acylations, 145 in aldol type reactions, 55-56 in corrin synthesis, 261-262 in Diels-Alder cyclizations, 86 in Michael type additions, 90 in oiefinations Barton olefination, 34-35 McMurry olefination, 41 Peterson olefination, 33 in syntheses of ce-hydrdoxy ketones, 52 Steric strain, due to bridges (Bredt s rule) effect on enolization, 276, 277, 296, 299 effect on f3-lactam stability, 311-315 —, due to crowding, release of in chlorophyll synthesis, 258-259 in metc-cyclophane rearrangement, 38, 338 in dodecahedrane synthesis, 336-337 in prismane synthesis, 330 in tetrahedrane synthesis, 330 —, due to small angles, release of, 79-80, 330-333, 337... 

Cor mth stereoselective olefin synthesis 14,717 Corrins s. Semicorrins Conmarans... 

The total synthesis of vitamin Bn is arguably the most difficult challenge so far presented to synthetic organic chemists. It is therefore surprising that the application of a photocyclization reaction to a solution of this problem has escaped the attention of photochemistry reviewers. Irradiation of the optically active seco-corrin cadmium complex (120) in methanol gave the corrin (121), isolated as its Co complex, with a stereoselectivity of 95% (Scheme 38). Model... 

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