Steroid ABCD-ring system

The steroid hydrocarbon structure may incorporate functional groups such as alcohols, ketones and olefinic linkages, either in the four-ring system or on the side chain originating at C17. Sterol is a term commonly used to describe all steroidal alcohols. Stenols refer to sterols with A 5 or A 7 double bonds, whereas stanols have a fully saturated ABCD ring system (Gagosian and Heinzer, 1979). 

New synthetic approaches to the ABCD ring systems of C-nor-D-homo-steroids have also been devised (see Figure 12). An A- to D-ring sequence by Brown and Lebreton uses hydrazone 167 (accessible from the classical Wieland—Miescher ketone, structure not shown) as a starting material. The C-ring was introduced via the aUylation of the corresponding hydra-zone with chloride 168, ozonolysis, aldol condensation, and olefin... 

The selectivity for two-alkyne annulation can be increased by involving an intramolecular tethering of the carbene complex to both alkynes. This was accomplished by the synthesis of aryl-diynecarbene complexes 115 and 116 from the triynylcarbene complexes 113 and 114, respectively, and Danishefsky s diene in a Diels-Alder reaction [70a]. The diene adds chemoselectively to the triple bond next to the electrophilic carbene carbon. The thermally induced two-alkyne annulation of the complexes 115 and 116 was performed in benzene and yielded the steroid ring systems 117 and 118 (Scheme 51). This tandem Diels-Alder/two-alkyne annulation, which could also be applied in a one-pot procedure, offers new strategies for steroid synthesis in the class O—>ABCD. 

There are several other types of glycosteroid systems, which differ in their steroidal moiety, with variations in the hydrojqrlation pattern around the ABCD traditional steroid scaffold, in some the type of the additional E-ring with substituents on the E ring system, or its replacement by an open chain appendage, with the presence of double bonds, of... 

Scheme 1-5 Virtual synthetic pathways toward the steroid skeleton with rings A, B, C, and D. Top row stepwise conversion of a ring A (B, C, or D)-building block into the ABCD system middle row expansion in a...

C-nor-D-homo- or [14(13 2)-abeo] systems. The latter, also referred to as Solanum alkaloids, share a common ABCD steroid skeleton, i.e., six-membered C-rings and five-membered D-rings. The Veratrum alkaloids are further divided into three subgroups (see Figure 1) the jervanine, the veratramine, and the cevanine. The Solanum alkaloids on the other hand, are further divided into the solanidine and verazine subgroups (see Figure 1). 

The steroidal alkaloids of the jervanine subgroup are composed of a hexacyclic framework with a tetrahydrofuran E-ring, spiro-cormected to the C-nor-D-homo ABCD-system and fused to a piperidine F-ring (see F pre 1). 

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