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In only a few steps, the synthesis of the BCD part of methyl-substituted steroids such as 1 is possible from the aldehyde 4. The crucial step of the synthesis is the tandem reaction of 2 to 3 named in the title, which is mediated by Me3SiOTf or EtAlCl2—with different stereoselectivities.
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Tietze et al. (1992) studied this question.
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