Long-Range Through-Bond Photoactivated σ Bond Cleavage in Steroids. Intramolecular Sensitized Debromination¹

Abstract

[structure: see text] The photolysis of 17alpha-bromo-3alpha-(triphenylsilyloxy)-5alpha-androstane (2; 3alphaTPSO/17alphaBr) and 17alpha-bromo-3alpha-(triphenysilyloxy)-5-androstan-6-one (3; 3alphaTPSO/6ketone/17alphaBr) is described. Irradiation of 2 with 266 nm light leads to debromination via intramolecular transfer of triplet excitation energy with a quantum efficiency of 0.0011. Photolysis of 3 with both 266 and 308 nm light leads to debromination with quantum efficiencies of ca. 0.0066. The debromination of 3 is attributed to activation via the ketone excited singlet state, with singlet energy transfer from C6 to C17 ca. 35% efficient and occurring with a rate constant of 1.4 x 10(8) s(-1).

Synthesis of ergostane-type brassinosteroids with modifications in ring A

“…Both 3α- and 3β-alcohols of type 10 and 11 ( Scheme 1 ) can be obtained in this way depending on the reducing agent used. Thus, the reaction of 3,6-diketones with K-selectride was shown to afford 3α-hydroxy-6-ketones [ 31 ]. On the other hand, treatment of the diketone 31 with NaBH 4 followed by acetate deprotection led to 24-epiteasterone ( 34 ) having a 3β-hydroxy group on the A-ring ( Scheme 7 ).…”

A novel copper framework with amino tridentate N‐donor ligand as heterogeneous catalyst for ring opening of epoxides

“…[45] Isopropylamine is apparently less reactive, which may arise from the higher basicity of aliphatic amines resulting in the occupation of the uncoordinated catalytic site and the catalyst poisoning. [46][47][48] Furthermore, when aniline individually reacted with alkyl oxirane tert-butyl glycidyl ether and 1-bromo-2,3-epoxypropane (Table 3, entries 5 and 6), the yields were manifested as 96% and 90% and isomer B turned into the major product. It is probably because of the prior steric effect, since the carbon atom with less substituted functional groups becomes much more vulnerable to be attacked by the nucleophilic amine owing to the disappearance of the arylmethyl carbocation.…”

Unexpected reactivity difference between iodo-alkene moieties of steroids possessing remote lactame or cycloalkane structural units: a theoretical approach

“…In a recent paper, the long-range j-bond cleavage of C(17)-Br functionality by intramolecular sensitization has been discussed. The debromination reaction was explained by the activation via the ketone excited singlet state, with singlet energy transfer from C-6 to C-17 [8]. (In addition to the above papers aiming at the deeper understanding of these phenomena, many works are referring to unexpected effects as dlong-range interactionT without discussing it in detail.…”