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. © 201

Transfer hydrogenation of aldehydes catalyzed by silyl hydrido iron complexes bearing a [PSiP] pincer ligand

The synthesis and characterization of a series of silyl hydrido iron complexes bearing a pincer-type [PSiP] ligand (2-R(2)PC(6)H(4))(2)SiH(2) (R = Ph (1) and (i)Pr (5)) or (2-Ph(2)PC(6)H(4))(2)SiMeH (2) were reported. Preligand 1 reacted with Fe(PMe(3))(4) to afford complex ((2-Ph(2)PC(6)H(4))SiH)Fe(H)(PMe(3))(2) (3) in toluene, which was structurally characterized by X-ray diffraction. ((2-(i)Pr(2)PC(6)H(4))SiH)Fe(H)(PMe(3)) (6) could be obtained from the reaction of preligand 5 with Fe(PMe(3))(4) in toluene. Furthermore, complex ((2-(i)Pr(2)PC(6)H(4))Si(OMe))Fe(H)(PMe(3)) (7) was isolated by the reaction of complex 6 with 2 equiv. MeOH in THF. The molecular structure of complex 7 was also determined by single-crystal X-ray analysis. Complexes 3, 4, 6 and 7 showed good to excellent catalytic activity for transfer hydrogenation of aldehydes under mild conditions, using 2-propanol as both solvent and hydrogen donor. α,β-Unsaturated aldehydes could be selectively reduced to corresponding α,β-unsaturated alcohols. The catalytic activity of penta-coordinate complex 6 or 7 is stronger than that of hexa-coordinate complex 3 or 4

Geländer Molecules with Orthogonal Joints: Synthesis of Macrocyclic Dimers

Orthogonal joints, understood as connections with an angle of 90°, were introduced in the design of the “Geländer” model compounds 1 and 2. The banister, consisting of a conjugated carbazole dimer linked by either 1,3-butadiyne (2) or a single thiophene (1), wraps around an axis composed of a phthalimide dimer due to the dimensional mismatch of both subunits, which are interconnected by phenylene rungs. The “Geländer” structure was assembled from a monomer comprising the 1,4-diaminobenzene rung with one amino substituent as part of a 4-bromo phthalimide subunit forming the orthogonal junction to the axis, and the other as part of a masked 2-ethynyl carbazole as orthogonal joint to the banister. The macrocycle was obtained by two sequential homocoupling steps. A first dimerization by a reductive homocoupling assembled the axis, while an oxidative acetylene coupling served as ring-closing reaction. The formed butadiyne was further derivatized to a thiophene, rendering all carbons of the model compound sp2 hybridized. Both helical structures were fully characterized and chirally resolved. Assignment of the enantiomers was achieved by simulation of chiroptical properties and enantiopure synthesis

Geländer Molecules with Orthogonal Joints: Synthesis of Macrocyclic Dimers

Orthogonal joints, understood as connections with an angle of 90°, were introduced in the design of the “Geländer” model compounds 1 and 2. The banister, consisting of a conjugated carbazole dimer linked by either 1,3-butadiyne (2) or a single thiophene (1), wraps around an axis composed of a phthalimide dimer due to the dimensional mismatch of both subunits, which are interconnected by phenylene rungs. The “Geländer” structure was assembled from a monomer comprising the 1,4-diaminobenzene rung with one amino substituent as part of a 4-bromo phthalimide subunit forming the orthogonal junction to the axis, and the other as part of a masked 2-ethynyl carbazole as orthogonal joint to the banister. The macrocycle was obtained by two sequential homocoupling steps. A first dimerization by a reductive homocoupling assembled the axis, while an oxidative acetylene coupling served as ring-closing reaction. The formed butadiyne was further derivatized to a thiophene, rendering all carbons of the model compound sp2 hybridized. Both helical structures were fully characterized and chirally resolved. Assignment of the enantiomers was achieved by simulation of chiroptical properties and enantiopure synthesis