Bridged Polycyclic Targets

General Procedure for the Preparation of Compound 184 [68] An oven-dried round-bottom flask (10 mL) equipped with a magnetic stir bar was charged with 4 A molecular sieves (300 mg), and sealed with a rubber septum. Then acetal 182 

Povarov-type imine addition followed by trapping the oxocarbenium ion intermediate with an internal nucleophile, particularly electron-rich aromatic systems. 

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The bridged polycyclic target sarain (47) (Scheme 6.39) provides an even greater challenge for synthesis. The initial approaches cxrnctenfrated on inroads to the tricychc core (47i) of sarain. 

Many examples of polycyclic alkene osmylations have been reported in the literature in connection with the syntheses of specific target molecules such as alkaloids, prostanoids, or steroids. However, carefully determined diastereomer ratios are usually not available. Due to the varied nature of these substrates, it is not possible to formulate definite rules for diastereo-face differentiation except in specific cases. Thus, for example, the exclusive exo reactivity of bridged systems such as bicyclo[2.2.1]heptene derivatives (norbornene-type) is well known as Alder s rule of exo addition63. 

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