A Facile Oxidation / Deprotection of Electron Rich Silyl Ethers Using DDQ

  • Paterson I
  • Cowden C
  • Rahn V
 et al. 
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Abstract

The selective oxidation/deprotection of allylic and benzylic silyl ethers to give aldehydes can be achieved by hydride abstraction with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) under neutral conditions. This reaction is possible in the presence of a variety of silyl protecting groups, as in 2 → 3, and even PMB ethers and PMP acetals, as in 14 → 15, which are normally labile under DDQ oxidation conditions. As the complexity of synthetic targets increases, so too does the need for new protecting groups. In this respect, a large number of different silyl ethers are now available which offer a range of reactivity based upon either steric or electronic properties. 1,2 An alternative approach is to use the structure of the substrate to differentiate between similar protecting groups. For example, the selective removal of one tert-butyldimethylsilyl (TBS) ether in the presence of another is possible using LiAlH 4 when a nearby polar group can direct the reductive cleavage. 3 For many years, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) 4 has been used for the oxidation of allylic alcohols 5 and, more recently, allylic ethers 6,7 to α,β-unsaturated carbonyl compounds. In our studies towards the synthesis of aplyronine A, 8 a 24-membered cytotoxic macrolide isolated from the sea hare Aplysia kurodai, a protecting group strategy was selected which relied heavily upon the use of silyl ethers. For instance, in intermediate 2 silyl protection was chosen for four hydroxyl groups. In preparation for a macrolactonisation reaction, deprotection of the C 1 TBS ether and then oxidation to a carboxylic acid were required. Conventional methods for the selective removal of the C 1 TBS ether proved unsatisfactory and instead a novel oxidation/ deprotection strategy was developed which employed DDQ under neutral conditions. In practice, 9 treatment of substrate 2 with DDQ (1.10 equiv., CH 2 Cl 2 ,

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Authors

  • Ian Paterson

  • Cameron J Cowden

  • Volker S Rahn

  • Michael D Woodrow

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