We have recently introduced cobalt-mediated intramolecular achiral enediyne [2+ 2+ 21 cycloadditions as a synthetic method in chiral polycyclic diene construction.'We have now found that chiral substrates A (Scheme I) may in some cases [eg, R,= Si (CH3),, R,= CH3, C6H5CH2, or CH30CH2] undergo the same reaction with remarkable diastereoselectivity, out of the four possible products basically only B (major) and C (minor) being formede2 (1) ...