Rearrangements Involving Both A- and B-Rings

Treatment of protected 10-deacetylbaccatin III or 14/3-hydroxy-lO-deacetylbaccatin III with Mn02 gave the A-nor-B-homo product 6.3.5 (225). The most likely pathway for the formation of 6.3.5 involves oxidation of the A-ring of the baccatin to the 13,14-diketone, followed by a base-catalyzed migration of C-15 to the C-14 carbonyl carbon. 

Methylation of 13-oxobaccatin III with methyl iodide and sodium hydride led to an interesting series of rearranged products. The initial product 6.3.6 is presumably formed by a homoretroaldol type of reaction initiated by methylation of the C-13 carbonyl group. Continued treatment with methyl iodide and base gave rise to an intramolecular aldol reaction, leading to the tetracyclic product 6.3.7 and two less methylated intermediates (226). 

A similar retroaldol type of reaction was observed on enzymatic dehydration of 10-deacetylbaccatin III with Pseudomonas cepacia lipase, which led to the seco analog 6.3.8 (227). A related reaction 

Protection of 13-acetyl-9-dihydrobaccatin III as its 7,9-ketal, followed by hydrolysis of the C-13 acetate, oxidation at C-13, and treatment with sodium methoxide gave the bridged ketal 6.3.11. Continued treatment with base then led to the A/B secobaccatin 6.3.12, in which a homo-retroaldol reaction is followed by cleavage of a diketone and lactone formation (228). 

The A-nor-B-bisnor derivative of baccatin III 6.3.13 was formed by treatment of 10-dehydro-10-deacetylbaccatin III with trichloroacetic acid. Interestingly, treatment of 7-epi-10-dehydro-10-deacetylbaccatin III under the same conditions gave the less rearranged product 6.3.14. Finally, treatment of either epimer with base led to the extensively rearranged product 6.3.15 (229). 

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