Novel double functional protection of cephalostatin analogues using a gas-free chlorination method.

Herewith, we report on a method that allows to simultaneously protect both the ∆14,15 bond and the carbonyl group of the symmetrical bis-steroidal diketone 2. We found that environmentally friendly and gas-free chlorination is ideally suited to achieve this goal. This method was discovered during our efforts to methoxylate 2 in a solution of dichloromethane and basic methanol in the presence of diacetoxy iodobenzene. Unexpectedly, the ∆14,15 bonds were chlorinated once as well as twice in a statistical manner. Interestingly, the singly dichlorinated desymmetrized product is an ideal precursor for conduction a series of position selective transformations. Importantly, the carbonyl group present in the nonchlorinated hemisphere can be selectively reduced, olefinated or oximated, while the other carbonyl group stays unaltered. A structurally related "monomeric" steroid derivative undergoes ∆14,15 chlorination and 11-position methoxylation under same conditions. These findings represent a powerful entry for preparing new nonsymmetrical cephalostatin derivatives.