A catalytic and stoichiometric approach to the synthesis of the steroid B-ring en route to estratrienes

Zirconocene catalyzed cyclization of 2-(5-methoxy-3-penten-1-yl)styrene in the presence of organomagnesium reagents was studied. The cyclization proceeded in high isolated yields (up to 84%) with excellent trans-selectivity (> 98%), which is unusual for the formation of cyclohexane derivatives. Catalytic cyclization in the presence of Cp2ZrCl2 proceeded as well with similar results. The reaction with (R,R)-(EBTHI)(2)ZrCl2 gave a cis/trans mixture of 5 in low yield and poor ee. (c) 2007 Elsevier Ltd. All rights reserved.