Polycyclic carbon skeletons syntheses

Anionic-anionic cascades are also effective for the rapid assembly of carbon skeletons. For example, double Barbier cyclisations were used by Cook for the synthesis of tetracyclic polyquinenes.21 Molander has shown that such sequences can be used to access a range of fused polycyclic systems (Scheme 6.17).13... 

The key step in the enantioselective synthesis of a polycyclic target is the establishment of the first cyclic stereogenic center. Further construction can then be chrected by that initial center. This principle is nicely illustrated by a synthesis of (+)-estrone methyl ether (145), reported by Taber (Scheme 8). The specifically designed naphthylbomyl ester (141) is used to direct C—insertion selectively toward one of the two diastereotopic C—H bonds. The new ternary center so created then directs the formation of the adjacent quaternary center in the course of the alkylation. Finally, the chiral skew in the product cy-clopentanone (144) directs the relative and absolute course of the intramolecular cycloaddition, to give the steroid carbon skeleton. 

Complex phosphonates and phosphonium ylides have been used in a variety of syntheses. For example an olefination reaction of the phosphonate (244) has been used to construct the carbon skeleton in a convergent synthesis of the structural fragment (245) of the marine metabolite halichondramide 27 and Wittig reactions with the ylide (246) have been used to prepare the polycyclic cervinomycin A -trimethyl ether and A2-methyl ether. 2 Both complex phosphonates (247) and phosphonium ylides (248) have been investigated for use in the synthesis of calyculin A. Use of the phosphonate (247) was not successful due to a competing elimination reaction and the ylide (248) was used in the actual synthesis. 29... 

Sterols comprise a very important class of compounds in plant and animal systems. Bacteria and other organisms utilize cholesterol or related sterols in the synthesis and maintenance of their cell walls. True sterols are present in cyanobacteria to only a very small degree they are largely replaced with a steroid-like class of triterpenoid known as the hopanoids. The carbon frameworks of sterols and bacterial hopanoids are synthesized in essentially the same manner as other terpenes, however, the cyclization reactions to yield their polycyclic skeletons are unique. A variety of sterols and hopanoids are found in cyanobacteria and they are believed to have important cellular functions in cell wall structure and function. " As discussed in Section 2.06.5, many of the hopanoids also possess a polyalcohol chain derived from a sugar. ... 

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