A highly stereocontrolled synthesis of (2S,3R,5R,6S)-6-[(1E,3R)-1,3-dimethylhex-1-enyl]-2-ethyl-2-hydroxy-3,5-dimethyltetrahydro-4H-pyran-4-one has been achieved in 13 steps and an overall yield of 7.6% from (R)-(+)-4-benzyloxazolidin-2-one. This substrate-controlled asymmetric synthesis utilized Paterson’s lactate-derived chiral ketone (2S)-2-(benzoyloxy) pentan-3-one to generate the 5R and 6S stereocentres and alkylation of an Evans’ auxiliary to generate the remote side-chain 3R stereocentre. Furthermore, a novel, highly efficient, and selective strategy was used to generate an enol trimethylsilyl ether whose subsequent Mukaiyama aldol condensation gave the acyclic precursor to the final product. A thermodynamically controlled cyclization then gave (2S,3R,5R,6S)-6-[(1E,3R)-1,3-dimethylhex-1-enyl]-2-ethyl-2-hydroxy-3,5-dimethyltetrahydro-4H-pyran-4-one with control of the 2S and 3R stereocentres. The NMR spectroscopic data and optical rotation obtained for synthetic (2S,3R,5R,6S)-6-[(1E,3R)-1,3-dimethylhex-1-enyl]-2-ethyl-2-hydroxy-3,5-dimethyltetrahydro-4H-pyran-4-one were consistent with those reported for the hemiacetal isolated from Siphonaria australis, and thus, prove the absolute and relative configuration of the natural product.
Toward the total synthesis of ritterazine N
