Rearrangements of Ring

Reaction of the epimeric 4,20-epoxides of 5-0-(triethylsilyl)taxinine A with boron trifluoride etherate gave the acetates 6.4.4 and 6.4.5 from the /J-epoxide and 6.4.6 from the a-epoxide 231). 

Studies of the chemistry of the phenylisoserine side chain of taxol continue to be made, reflecting the importance of this structural unit for the synthesis of taxol from baccatin III and the potential of preparing improved analogs of taxol by modification of the side chain. Synthetic 

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Generally, Claisen rearrangement of ring-bearing substrates with remote asymmetric centers has led to higher stereoselectivities than in the acyclic series. Kurth and cowoikers used amino acid derived chiral auxiliaries such as (302) for the preparation of enantiomerically pure carboxylic acids. The excellent facial selectivity in these systems was hampered, however, by poor chair vs. boat selectivities. 

Fig. 2.9 2.10 Enzymatic formation of uro-gen III. Note intramolecular rearrangement of ring D.

The base-catalysed rearrangement of ring-substituted benzoins in aqueous methanol is reported to be initiated by rate-determining a-hydrogen abstraction rather than a mechanism with initial hydroxyl hydrogen abstraction as in the general a-ketol rearrangement (Scheme 93). ... 

Rearrangement of ring skeleton Coordination with Lewis-acid Thermolysis or metal complexes... 

Manganese.—Improved methods of preparation of salts of the type [(i7-RCeH4X)-Mn(CO)3]+ [PFfi]" have been described, including routes involving intramolecular hydrogen-shift rearrangements of ring-substituted derivatives of [(ly-cyclohexa-... 

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