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Fig. 1 Squaric acid and its derivatives [1-18]

a role of bioisostere (e.g., semisquaric acid is considered to have similarity to pyruvic acid in structure), research has pursued the possibility of 1 as a replaceable moiety for an acid part of natural products such as amino acids and nucleic acids, and for a certain part of pharmacoactive compounds (see later). As a synthetic tool, the use of 1 has recently been demonstrated in asymmetric reduction adopting the cyclobutenedione as a ligand with a chiral center [25,26].

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