"Endo" and "exo" bicyclo[4.2.0]-octadiene isomers from the electrocyclization of fully substituted tetraene models for SNF 4435C and D. control of stereochemistry by choice of a functionalized substituent.

[reaction: see text] A tandem electrocyclic closure, perceived as the key step in a biomimetic approach to SNF 4435C and D, was tested with 1,1,8-trisubstituted tetraene substrates. The ratio of endo:exo products could be controlled by the choice of the RZ substituent at C-1. On the basis of these results, a short stereoselective route to an advanced SNF 4435 intermediate was devised.