Enantioselective catalytic approach to the C23–C28 subunit of 24α-methyl steroids

Asymmetric Total Synthesis of Dankasterones A and B and Periconiastone A Through Radical Cyclization

“…Next, deacetylation of 10 occurred smoothly to give the diol, which was followed by oxidation to generate the labile aldehyde, an advanced intermediate posed for side‐chain installation. Chemoselective Julia‐Kocienski olefination [25a] of the aldehyde with 39 afforded dankasterone B ( 1 b ). The other three carbonyl groups remained untouched.…”

Synthesis of Swinhoeisterol A, Dankasterone A and B, and Periconiastone A by Radical Framework Reconstruction

“…For the synthesis of swinhoeisterol A ( 1 ) we employed rearrangement product 9 , being aware that the ergostane-derived stereoconfiguration of remote C24 was inverse to the one in the natural product and, thus, had to be adjusted . Ozonolytic cleavage of steroid side chains and Julia–Kocienski olefination of so-obtained aldehydes with sulfones of type 22 (Scheme B) have been reported . The dense functionality of 9 as well as of several downstream intermediates, though, required careful consideration and probing for the right opportunity, if tedious redox or protecting group operations were to be minimized.…”

“…Protonation during workup took place selectively from the β-face. Controlled ozonolysis of the Δ 22 bond then gave an aldehyde, which was successfully merged with sulfone 22 to yield exclusively ( Z )-olefin 14 in 63% yield over two steps …”

“…22 Ozonolytic cleavage of steroid side chains and Julia−Kocienski olefination of so-obtained aldehydes with sulfones of type 22 (Scheme 2B) have been reported. 23 The dense functionality of 9 as well as of several downstream intermediates, though, required careful consideration and probing for the right opportunity, if tedious redox or protecting group operations were to be minimized. The required sulfone 22 23 was accessible through standard transformations starting from (R)-Roche ester (19), shortening a previously reported sequence to tosylate 21 24 via diol 20.…”

“…23 The dense functionality of 9 as well as of several downstream intermediates, though, required careful consideration and probing for the right opportunity, if tedious redox or protecting group operations were to be minimized. The required sulfone 22 23 was accessible through standard transformations starting from (R)-Roche ester (19), shortening a previously reported sequence to tosylate 21 24 via diol 20. 25 Attempted Julia−Kocienski olefination on most downstream intermediate-derived aldehydes revealed mainly aldol reactivity, an intramolecular process between readily enolizable C7 and the C22 aldehyde (see Supporting Information).…”