Lead tetraacetate–iodine oxidation of 23-spirostanols

Synthesis and application of steroidal 22,16β-carbolactones: A review

In the plant kingdom, steroidal lactones occur as glycosides, compounds consisting of a sugar moiety linked to a steroid aglycone. Steroidal lactones consist of five fused rings, with a total of 22 carbon atoms. Numerous methods for the preparation of steroidal lactones take advantage of the fact that steroid spirostanes may be degraded from six- to a five-rings structure. One of the most striking features common to reactions of steroid sapogenins is the C₂₂-lactone formation. In the review, different methods for the preparation of steroidal lactones are presented with consideration of the structure of starting material. In addition, examples of lactones used in the synthesis of biologically active compounds and their analogues are described.

Studies on the reactivity of 23E-benzylidene spirostanes

A study of the reactivity of 25R and 25S 23E-benzylidene spirostanes that includes epoxidation, catalytic hydrogenation as well as Lewis or Brønsted acid-catalyzed rearrangements is described. Exhaustive NMR characterization of the obtained compounds is presented. Additionally the structures of some of the obtained compounds were confirmed by single crystal X-Ray Diffraction studies.

Synthesis of dimeric spirostanols linked through a 1,4-dimethylidenebenzene moiety by double BF3·Et2O-catalyzed aldol condensation of steroid sapogenins and terephtalaldehyde

BF₃·Et₂O-catalyzed double aldol condensation between acetylated steroid sapogenins and terephtalaldehyde led to acetylated dimeric spirostanols linked through a 1,4-dimethylidenebenzene moiety in moderate to good yields. The E configurations of the introduced double bonds were corroborated by NOE experiments. Saponification of the dimeric steroids led to the corresponding dimeric spirostanols.

Baeyer-Villiger reaction of steroid sapogenins by CF3COOH-H2O2. A short cut to pregnan-3β,16β,20-triol 3-monoacetates

Treatment of steroid sapogenins with H₂O₂ in CF₃COOH for 15min followed by reflux in CH₃OH/H₂O afforded good yields of pregnan-3β,16β,20-triol 3-monoacetates. When the hydrolysis step was carried out with KOH in refluxing methanol excellent yields pregnantriols were obtained. The resulting compounds were characterized by their melting points and NMR spectral data. An X-ray diffraction analysis of compound 3a confirmed the proposed structure and provided detailed information about the bond lengths, bond angles and conformation.

Mechanistic insights on the reactivity of furospirostanes with the 16β,22:22,25-diepoxy-23-acetoxymethyl-24-methyl side chain

F-ring opening in spirostanes with the 16β,22:22,25-diepoxy-23-acetoxymethyl-24-methyl side chain produces a Δ(22)-intermediate with an allylic acetoxy group. For this reason the reactivity profile of these compounds deviates from that observed in other naturally occurring or synthetic spirostanes and furospirostanes.

Cephalostatins and ritterazines

This review article is a tribute to the numerous chemists whose relentless effort for the last quarter of a century resulted in the isolation, identification, and finally the chemical synthesis of a family of bis-steroidal pyrazine alkaloids of marine origin. In the task of defeating cancer, the search for bioactive substances among the naturally occurring compounds is, without any doubt, a preferential approach. The remarkable contribution of Petitt, Fusetani, and their coworkers allowed to discover this family of marine alkaloids that emerge as potential therapeutic anticancer agents, although there is still a long way to go. The challenging and dangerous task of collecting living organisms from deep-waters was followed by a laborious isolation, elucidation of the complicated structures and biological tests. The outcome of this paramount effort was the identification of 45 compounds that stand, to date, as some of the most potent anticancer agents. The intriguing structures of the isolated alkaloids drew the attention of synthetic chemists, valiant enough to undertake the challenging task of synthesizing some of the most active members of the family. Fuchs, Heathcock, Winterfeldt, Suarez, Shair, and their associates pioneered in the establishment of feasible synthetic routes for the preparation of some of the naturally occurring compounds and a large number of synthetic analogs, allowing to establish SAR criteria that have guided the design of new synthetic analogs. Numerous analogs have been prepared to investigate the mechanism of action of bis-steroidal pyrazines, e.g. cephalostatin analogs bearing a strained spiroketal moiety. However, the mechanism of action and the biological target of these compounds remain far from being understood. Therefore, the rational design of simpler, yet highly active analogs seems at the current stage elusive. It is still 1 to clear why these compounds need to be dimeric to show high biological activity. Furthermore, it is not known whether the central pyrazine ring is simply a linker or has some additional function. This could be tested by examining the biological activity of steroidal dimers with other linkers, e.g. with a benzene ring. Such analogs have been actually prepared but without functional groups necessary for biological activity. The clinical trials of cephalostatins have got stuck due to a shortage of material. There is an urgent need to provide highly active, yet not too complex analogs, which could be available in substantial amounts for advanced pharmacological studies.

On the reactivity of 23-methoxycarbonyl furospirostanes

Brønsted and Lewis acid-catalysed reactions of the 23-methoxycarbonyl furospirostanic side chain are described. While bromination, deuteration and BF(3)·Et(2)O/AcOH treatment involve regioselective F-ring opening with exclusive participation of Δ(22)-furostenic intermediates, BF(3)·Et(2)O/Ac(2)O treatment leads to irreversible E- or F-ring cleavage.

Studies on the BF3·Et2O catalyzed Baeyer-Villiger reaction of spiroketalic steroidal ketones

During reactions of 23-oxosapogenins and the corresponding isomeric 22-oxo-23-spiroketals with MCPBA in the presence of BF(3)·Et(2)O, equilibration occurs between the ketones. The Baeyer-Villiger type oxidation is followed by fragmentation to the dinorcholanic lactones and 3-methylbutyrolactone. The mechanistic aspects of these reactions in the 25R and 25S series are discussed.

On reactions of steroidal 23-oxo and 23,24-epoxysapogenins with Lewis acids

The reaction of 23-oxotigogenin acetate with TMSOTf in THF afforded the corresponding bisnorcholanic lactone in 60% yield. The analogous reactions carried out in dichloromethane or benzene gave the rearranged products--the isomeric spirostanic ketone (10-15%) and bisfuran (40-42%). Similar products were also obtained upon treatment of 23,24-epoxysapogenins with BF(3). The epoxides treated with TiCl(4) afforded mostly chlorohydrins and no rearranged products were detected.

Beckmann reactions of steroidal spirocyclic oximes derived from the 16beta,23:23,26-diepoxy-22-oxo moiety

The Beckmann rearrangement of the syn and anti isomers of the spirocyclic oxime derived from a 16beta,23:23,26-diepoxy-5beta-cholestan-22-one was studied. Whereas the anti isomer always follows the Beckmann fragmentation course, the syn isomer, depending on the reaction conditions, follows the normal Beckmann rearrangement course and/or the isomerization to the anti isomer followed by the fragmentation course.

1H and 13C NMR characteristics of synthetic derivatives of steroid sapogenins. Part III. 16Beta,23:23,26-diepoxy side chains

The full assignments of the (1)H and (13)C NMR signals of steroids bearing the 16beta,23:23,26-diepoxy side chain are provided. Differentiation of the diasterotopic H-26 pair was achieved with the aid of NOESY experiments. The main substituent and steric effects associated with this moiety and their influence on the chemical shifts of the neighboring atoms are discussed.

Efficient and selective removal of methoxy protecting groups in carbohydrates

[reaction: see text] The selective removal from carbohydrate substrates of methoxy protecting groups next to hydroxy groups is reported. On treatment with PhI(OAc)(2)-I(2), the methoxy group is transformed into an easily removable acetal. The mild conditions of this methodology are compatible with many functional groups, and good to excellent yields are usually achieved.