Model study for steroidal D/C ring closure in olefinic cyclization reactions; a MINDO/3 study

MINDO/3 molecular orbital calculations are performed for the ring closure of (Z)- and (E)-allkenes as a model for the D/C ring closure to steroids. It is shown that a lower activation energy is needed for cyclizations in the chair configuration, than for cyclizations in the boat configuration. An analogous picture is found by comparison of (E)- and (Z)-alkenes. Cyclization of the (E)-alkene in the chair or boat configuration is energetically favoured over the ring closure of (Z)-alkenes. These results are in agreement with the Stork-Eschenmoser hypothesis.