Synthesis of Spirostanic Analogues of Brassinosteroids via Homogeneous Permanganate Dihydroxylation

Homogeneous non-aqueous permanganate dihydroxylation is used to synthesize spirostanic analogues of brassinosteroids bearing a variety of oxygen functions on ring B.

The multiple multicomponent approach to natural product mimics: tubugis, N-substituted anticancer peptides with picomolar activity

The synthesis of a new generation of highly cytotoxic tubulysin analogues (i.e., tubugis) is described. In the key step, the rare, unstable, and synthetically difficult to introduce tertiary amide-N,O-acetal moiety required for high potency in natural tubulysins is replaced by a dipeptoid element formed in an Ugi four-component reaction. Two of the four components required are themselves produced by other multicomponent reactions (MCRs). Thus, the tubugis represent the first examples of the synthesis of natural-product-inspired compounds using three intertwined isonitrile MCRs.

First total synthesis of tubulysin B

The first total synthesis of tubulysin B is described. The aziridine route to tubuphenylalanine (Tup) of the tubulysin D/U-series could not be transferred to the synthesis of tubutyrosine (blue moiety). Therefore, tubutyrosine (Tut) was synthesized by a Wittig olefination/diastereoselective catalytic reduction sequence. Interestingly, the C-2 epimer of tubulysin B has a cytotoxic activity almost identical to the natural diastereomer.

Studies on the regioselectivity of the Baeyer-Villiger reaction of 3-keto steroids: conformational effects determine the migration aptitude

A detailed study of the Baeyer-Villiger reaction of 3-ketosteroids has been performed by using m-chloroperoxybenzoic and trifluoroperoxyacetic acids as oxidants. The process was fully regiospecific for 3-keto-5alpha-steroids with the employ of both peracids, and only partially regioselective for 3-keto-5beta-steroids by using trifluoroperoxyacetic acid. Interestingly, the reaction resulted completely unselective for 3-keto-5beta-steroids by using m-chloroperoxybenzoic acid. Theoretical studies were performed to explain the regiochemistry of this process, which is suggested to be controlled by conformational effects in the transition state of the Criegee rearrangement.