Selective cleavage with iodotrimethylsilane

The selective and facile cleavage of the benzylic ether linkages of 1,2,3 or 4 is accomplished by treatment with iodotrimethylsilane to form the corresponding benzylic iodide. Further addition to these iodide derivatives of 1 affords dendrimers of generation 1 with phosphonium ion sites at the periphery. Such a strategy is conducted up to generation 3 with a phosphine or a phosphonium core (Scheme 3). 

Thus two successive oxidations of 429 (Scheme 59) involving the Jones reagent and dichlorodicyano-p-benzoquinone respectively led to the enone 430. A similar reductive cleavage as above of the derived dimethyl acetal 431 with diisobutylaluminium hydride occurred from the less hindered a-side to furnish the P-methoxyether 432 as the major isomer. Selective O-debenzylation of432, achieved with iodotrimethylsilane led to ( )-coccinine (413). 

Botta and coworkers recently developed an Oxone cleavage methodology for the solid-phase synthesis of substituted uracils (equation 57)165. Whereas some common methods of thioether cleavage, such as reduction with Na/NH3, acid catalyzed hydrolysis, heavy metal ions followed by treatment with hydrogen sulfide and iodotrimethylsilane proved to be unsuccessful for this reduction, Oxone was shown to be an efficient and selective reagent for cleavage of polymer bound thiouracils. 

When exposed to iodotrimethylsilane at room temperature, 59 underwent C-O bond cleavage along with iodination to produce the silyl ester 60 (Fig. 11). In situ cyclization was carried out by treating 60 in methanol at room temperature to provide (+)-mono-morine I (62) and its C-3 epimer (63) in 42% and 40% yields from 59, respectively. An appreciable improvement of the diastereo-selectivity in the cyclization to monomorine I was obtained by the following sequence. With compound 59 in hand, the ketone 61 was prepared by Collins oxidation in 94% yield. On catalytic hydrogenation of 61, ( )-monomorine I (62) was obtained in 71% yield, along with ( )-3-epimonomorine I (63) as a minor isomer (15%). 

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