Ring transformations analogical approaches

In the alternate and unsuccessful approach to haemanthidine (382), which entailed the construction of the D ring prior to the functionalization of the C ring, the ester 458 was converted to the unsaturated keto lactam 468 by a straightforward route analogous with the one discussed above for the transformation of 461 to 463. The carbonyl group at C-6 was then reduced with sodium borohydride to alleviate concern over its reactivity, but epoxidation of the double bond at C-2 and C-3 of 469 afforded a mixture of diastereomeric a- and (3-epoxides. Owing to the lack of stereoselectivity in this crucial step, this route was abandoned (202). 

Metalation of sulfinylferrocene (338) to give the lithiated species, followed by transmetalation to give either the isolable boron analog (348) or the nonisolable zinc analog (349), is an approach used to access yet another class of ferrocene derivatives (equation 79)Palladium-catalyzed coupling of (348) or (349) with aryl iodides affords planar chiral aryl ferrocenes that are converted by standard transformations into planar chiral monophosphine aryl ferrocenes (350) that are also useful ligands in asymmetric catalysis. With appropriate ort/ o-substitution on the aryl ring, ferrocenes that possess axial chirality as well as planar chirality are prepared if the orfAo-substitutent is a second phosphine unit, access to a bisphosphine aryl ferrocene is achieved. ... 

There are several approaches to the synthesis of heterochain PCSs, which can be divided into three large groups chain ring opening and vinyl polymerizations, step-growth reactions (polymerization and polycondensation), and polymer-analogous transformations. 

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