Among the reactions available to synthetic chemists for the construction of new C--C bonds, the Claisen rearrangement is one of the most powerful, elegant, and well-characterized methods. A genuinely new variant, the Bellus-Claisen rearrangement came to light a quarter of a century ago: The reaction of an allylic ether, thioether, or amine with a ketene leads through a [3,3] sigmatropic bond reorganization of a zwitterionic intermediate to an E unsaturated ester, thioester, or amide. When applied to cyclic allylic substrates, a ring-enlargement by four carbon atoms in one step provides medium-ring unsaturated E-configured lactones, thiolactones, and lactams. The scope of the Bellus-Claisen rearrangement and the optimum reaction conditions will be discussed in this Minireview.

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A novel type of ketene-Claisen rearrangement in which the precursor of the rearrangement is generated in situ by reaction of optically active allyl thioethers with dichloroketene is described. A characteristic feature of this rearrangement is the excellent chemoselectivity in favor of allyl thioethers vs. allyl ethers, i.e., exclusive chirality transfer of the allylic sulfur moiety is observed with 12, 13, and 25-27. The cyclic, optically active allyl thioethers (+ )-(R)-4 and (-)-(S)-4 and the open-chain allyl thioethers 11-13 rearrange with in sifu generated dichloroketene to the optically active thioesters ( -)-(S)-28, (+)-(R)-ZS, and 31-33, respectively. A chirality-transfer of > 99% in the cyclic cases (+)-(R)-4 and (-)-(S)-4, and 96-98% in the open-chain cases 11-13 is observed. Furthermore, the dichloroketene-Cluisen rearrangement is characterized by a high asymmetric 1,2-induction. The chiral allylic sulfides 25-27 give the optically active thioesters 36-38 with a 1,2-induction > 99% as determined by NMR-shift experiments.

Oxazolomycin A and neooxazolomycin were firstly isolated in 1985 by the group of Uemura et al. from the Streptomyces sp. bacteria.

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