Rearrangements Involving the B-Ring

Photochemistry of taxine B gave the bridged product 6.2.2 in which the dimethylamino substituent on the C-5 side chain was also lost reductively (279). The isomer of 6.2.2 with the acetate at C-10 was also obtained. 

Treatment of the 7-xanthate of 10-deacetoxybaccatin III with Bu3SnH and AIBN at 80°C gave the expected 7-deoxyproduct, but also gave a new product 4.1.2.7 formed by a radical coupling reaction. When (TMS)3SiH was used as a hydrogen source, the three internally cross-linked products 4.1.2.7, 4.1.2.8, and 4.1.2.9 were all obtained, as previously discussed (220). 

An alternate B-nortaxol analog was prepared from 7-(triethylsilyl)-13-oxobaccatin III (6.2.7). Treatment of this substrate with trifluoro-acetic anhydride in pyridine gave a product 6.2.6 related to those discussed in the previous section. Treatment with DBU, however, yielded the B-nor product 6.2.8. This product was postulated to arise by an initial C-2 to C-1 benzoyl migration, followed by a pinacol-type rearrangement (222). 

---