Chemistry of Squaric Acid with 3Cyclobutene12Dione Skeleton

Squaric acid (1) is categorized as an oxocarbon having a four-membered ring [1] (Fig. 1). Despite being a small molecule, it possesses unique 2n-pseudoaromaticity [2-5], which brings high acidity (pKa1 = 0.52, pKa2 = 3.48) as an organic acid, and polyfunctionality, including two hydroxyl and two carbonyl groups conjugated across a double bond. Peculiar hydrogen-bonded network and chelated structures in some acid derivatives have been occasionally discussed [6-12]. The unique structure is utilized in electronic devices, for example, as a donor-acceptor triad called "squaraine" (2) [1,1315]. The dicationic nature of the cyclobutene ring necessary for aromatic character is combined with the donating nature of aromatic and heteroaro-matic rings to produce SHG properties, for example [16]. Dimer 3 is a new candidate designed for extension of conjugation plane [17,18].

On the other hand, the unique structure of 1 has also been applied in organic synthesis as an attractive C4-synthon. The relief of ring strain can serve as a significant driving force in its ring-transformation reaction and this is in fact accomplished by two processes. The first is conversion of the stable aromatic cyclobutenedione system to the more reactive hydroxycyclobutenone system; where required or desired substituents can be introduced into the ring system. The second is ring expansion from a four-membered ring to five ~ seven-membered rings in either concerted or stepwise manner. This methodology has been exploited in the synthesis of various bioactive carbo-and heterocycles [19-23]. Another feature of using 1 to develop bioactive compounds is based on variation of substituents on squaric acid esters and amides, where the cyclobutenedione ring is still retained. In fact, semisquaric acid (4), which is known as moniliformin, is a primitive derivative with biological activity (mycotoxin) [24]. According to the concept, for 1 to play

HO N0 1

HO N0 1

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