Synthesis of (+)-(1R,2R,4R,6S)-1,6-epoxy-4-benzyloxycyclohexan-2-ol, a key precursor to inositol monophosphatase inhibitors, from (−)-quinic acid

Abstract

A new and efficient route to homochiral (+)-(1R,2R,4R,6S)-1,6-epoxy-4-benzyloxycyclohexan-2-ol and its 2-benzyl ether derivative is described, starting from (−)-quinic acid. The compounds are key intermediates in the solution-phase and solid-phase synthesis of inhibitors for inositol monophosphatase. The pivotal step involves a La³⁺-induced reversal of the diastereoselectivity for the borohydride reduction of an intermediate cyclohexan-4-one. (1R,2R,4R,6R)-(O⁶-Propyl)cyclohexane-1,2,4,6-tetraol 1-phosphate, predicted to be a submicromolar competitive inhibitor of inositol monophosphatase, was prepared from the title epoxide in 5 steps in good overall yield. The compound proved to be a competitive inhibitor and displayed the expected potency confirming the stereochemical requirements for inhibition. The O²-benzylated epoxide derivative could be stereospecifically alcoholysed using either BF₃·(OEt)₂ or Yb(III)(OTf)₃ as catalysts without appreciable levels of benzyl ether protecting group cleavage. The preparation of the alcoholysis products (1S,2R,4S,6R)-2,4-bis(benzyloxy)-6-isopropyloxycyclohexanol and (1S,2R,4S,6R)-2,4-bis(benzyloxy)-6-(phenethyloxy)cyclohexanol, and the synthesis and evaluation of the inhibitor (1R,2R,4R,6R,2′S)-6-(1′-hydroxy-3′-phenylpropan-2-yloxy)-2,4-dihydroxycyclohexyl phosphate and its diastereomer (1R,2R,4R,6R,2′R)-6-(1′-hydroxy-3′-phenylpropan-2-yloxy)-2,4-dihydroxycyclohexyl phosphate are described.