The total synthesis of sphingofungin F through the Overman rearrangement of an unsaturated ester, which is known to be an unsuitable substrate under standard conditions due to the competitive aza-Michael reaction, is described. The developed conditions enabled the ester to be compatible with the original Overman rearrangement, providing quick access to α,α-disubstituted amino acids by minimizing extra protecting group manipulations and redox reactions.

This article describes the details of two new types of Overman rearrangement from allylic vicinal diols. Starting from identical diols, both bis(imidate)s and cyclic orthoamides were selectively synthesized by simply changing the reaction conditions. Whilst exposure of the bis(imidate)s to thermal conditions initiated the double Overman rearrangement to introduce two identical nitrogen groups in a single operation (the cascade-type Overman rearrangement), the reaction of cyclic orthoamides resulted in a single rearrangement (the orthoamide-type Overman rearrangement). The newly generated allylic alcohols from the orthoamide-type reaction can potentially undergo a variety of further transformations. For instance, we demonstrated an Overman/Claisen sequence in one pot. The most conspicuous feature of this method is that it offers precise control over the number of Overman rearrangements from the same allylic vicinal diols. This method also excludes the tedious protecting-group manipulations of the homoallylic alcohols, which are necessary in conventional Overman rearrangements. All of the performed rearrangements proceeded in a completely diastereoselective fashion through a chair-like transition state.

The development of a two-step synthesis for β-hydroxy-α,α-disubstituted amino acid derivatives from cyclic orthoamide is reported. The first step is the orthoamide-type Overman rearrangement of an α,β-unsaturated ester to give a sterically hindered α,α-disubstituted amidoester. The α,β-unsaturated ester is known to be a challenging substrate in the conventional Overman rearrangement due to the competitive aza-Michael reaction. However, suppression of the aza-Michael reaction is realized by two factors; 1) the high reaction temperature, and 2) an alkyl substituent at the α-position. The second step is stereodivergent intramolecular SN2′ reaction for the installation of a hydroxy group at the β-position. Either syn- or anti-type SN2′ reaction is possible by simply changing the reaction conditions. The developed method can provide all four possible stereoisomers of the β-hydroxy-α,α-disubstituted amino acid, and is successfully applied to the total synthesis of sphingofungin F.

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In the title compound, C11H14Cl3NO6, the fused five-membered oxazoline and tetra­hydro­furan rings are essentially planar with maximum deviations of 0.069 (1) and 0.031 (1) Å, respectively, and make a dihedral angle of 64.23 (11)° with each other. In the crystal, mol­ecules are linked by O—H⋯O and C—H⋯O hydrogen bonds, forming chains along the b-axis direction. Further C—H⋯O hydrogen bonds are observed between the chains.