Squaric acids

Resolution Enhancement from MAS Using Single Crystals 

The assignment of the four peaks to the four respective C atoms in the SQA skeleton is shown at the top of Fig. 7. As discussed in details else- 

An enlarged view of the variation of 5iso in the vicinity of Tn is shown in Fig. 9. hi this plot, the low temperature side corresponds to the average of the data of the four peaks in Fig. 8. Notice that very close to Tn, the spectra are a superposition of those from both phases, but aroimd T = 373 K there is a sudden jump in the isotropic value, showing the onset of the near first-order phase transition. Here the whole plot is thus S-shaped, the jump in the isotropic value due to the phase transition is indicated as AS in the figure. The shape is consistent with the theoretical model developed in Chap. 1 in this volume. Some pertinent details are included here for completeness. 

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Hdfer P, Grupp A, Nebenfuhr H and Mehring M 1986 Hyperfine sublevel correlation (HYSCORE) spectroscopy a 2D ESR investigation of the squaric acid radical Chem. Phys. Lett. 132 279-82... 

Thermal electrocyclizations of perhalogenated 1,3-butadienes yield perhalogenated cyclobutenes which can be solvolysed to 3,4-dihydroxy-3-cydobutene-l,2-dione ( squaric acid") and its derivatives (G. Maahs, 1966 H. Knorr, 1978 A.H. Schmidt, 1978). Double CO extrusion from fused cyclobutenediones has been used to produce cycloalkynes, e.g., benzyne from benzocyclobutenedione by irradiation in an argon matrix (O.L. Chapman, 1973) and cyc/o-Ci8, cyclo-Cn, etc. by laser desorption mass spectroscopy of appropriate precursors (see section 4.9.8). 

Squalene epoxidation Squaraine Square 50 Square 80 Square permalloy Square Permalloy 80 Squaric acid [2892-51-5]... 

Ketene trimer can be recovered from the tarry residue of diketene distillation and converted into valuable building blocks like 1,3-cyclobutanedione and squaric acid [2892-51-5] (140,141), an important intermediate in the synthesis of pharmaceuticals and squaryHum dyes used in photostatic reproduction (142,143). 

Highly selective synthesis of polyfunctionalized heterocycles based on ring expansion of squaric acid derivatives 97YGK785, 98SL1167. 

The Dianion of Squaric Acid. The stability of this system is illustrated by... 

Ohno M, Eguchi S (2006) Directed Synthesis of Biologically Interesting Heterocycles with Squaric Acid (3,4-Dihydroxy-3-cyclobutene-l,2-dione) Based Technology. 6 1-37 Okino T (2006) Heterocycles from Cyanobacteria. 5 1-19... 

In general, symmetrical oxo-squaraines having the same end-groups are synthesized by reacting squaric acid with two equivalents of quatemized indolenine, 2-methyl-substituted benzothiazole, benzoselenazole, pyridine, quinoline [39, 45, 46] (Fig. 4) in a mixture of 1-butanol - toluene or 1-butanol - benzene with azeotropic removal of water in presence [39, 45] or absence [47] of quinoline as a catalyst. Other reported solvent systems include 1-butanol - pyridine [48], 1-propanol - chlorobenzene, or a mixture of acetic acid with pyridine and acetic anhydride [49]. Low CH-acidic, heterocyclic compounds such as quatemized aryl-azoles and benzoxazole do not react, and the corresponding oxo-squaraines cannot be obtained using this method [23, 50]. 

The key intermediates for the synthesis of unsymmetrical, heterocyclic oxo-squaraines are the mono-squaraines (semi-squaraines) shown in Fig. 5. These intermediates can be synthesized via condensation of dialkylsquarate with an equimolar amount of methylene base [51]. The obtained alkoxy-mono-squaraines are then reacted with the second methylene base to yield unsymmetrical oxo-squaraines. These mono-squaraine intermediates display a higher reactivity compared to squaric acid or its esters they allow the synthesis of the corresponding... 

Symmetrical, aniline-based, and aromatic oxo-squaraines are synthesized via a one-step reaction by heating two equivalents of the appropriate /V,/V-dialkylaniline or other reactive aromatic or heteroaromatic derivatives with squaric acid (Fig. 6) [38, 41]. Unsymmetrical aniline-type squaraines can be synthesized in two steps first one component is reacted with squaric acid dichloride to yield a mono-squaraine intermediate, which in a subsequent step is then reacted with the second component to yield the unsymmetrical squaraine dye [53]. 

Sprenger HE, Ziegenbein W (1966) Condensation products of squaric acid and tertiary aromatic amines. Angew Chem Int Ed Engl 5 894... 

Oguz U, Akkaya EU (1997) One-pot synthesis of a red-fluorescent chemosensor from an azacrown, phloroglucinol and squaric acid a simple in-solution construction of a functional molecular device. Tetrahedron Lett 38 4509 1512... 

In this context, divalent presentation of this promising candidate has been investigated by the authors using the fixed cone conformation of calix[4]arene to allow introduction of the molecules onto the upper rim, and their projection into the same portion of space, thus mimicking, to some extent, a small portion of the natural cell-surface ganglioside. The synthesis of the divalent neoglycoconjugate was based on initial functionalization of the corresponding diacid with an aminated spacer and further introduction of squaric acid moieties. 

Other electrophilic substitution reactions on aromatic and heteroaromatic systems are summarized in Scheme 6.143. Friedel-Crafts alkylation of N,N-dimethyl-aniline with squaric acid dichloride was accomplished by heating the two components in dichloromethane at 120 °C in the absence of a Lewis acid catalyst to provide a 23% yield of the 2-aryl-l-chlorocydobut-l-ene-3,4-dione product (Scheme 6.143 a) [281]. Hydrolysis of the monochloride provided a 2-aryl-l-hydroxycyclobut-l-ene-3,4-dione, an inhibitor of protein tyrosine phosphatases [281], Formylation of 4-chloro-3-nitrophenol with hexamethylenetetramine and trifluoroacetic acid (TFA) at 115 °C for 5 h furnished the corresponding benzaldehyde in 43% yield, which was further manipulated into a benzofuran derivative (Scheme 6.143b) [282]. 4-Chloro-5-bromo-pyrazolopyrimidine is an important intermediate in the synthesis of pyrazolopyrimi-dine derivatives showing activity against multiple kinase subfamilies (see also Scheme 6.20) and can be rapidly prepared from 4-chloropyrazolopyrimidine and N-bromosuccinimide (NBS) by microwave irradiation in acetonitrile (Scheme... 

Complexes of Squaric Acid and Its Monosubstituted Derivatives Lincoln A. Hall and David J. Williams... 

Figure 3.7 Phosphonic acid salt derived from squaric acid.

A variety of hydrophobic and hydrophilic squaraine rotaxane probes and labels such as 21a-21e c Rp and 22a-22e c Rp, containing reactive carboxylic functionalities and hydrophilic sulfo groups, are disclosed in a recent patent application [60]. It was shown that not only aniline-based squaraines 21a-21e but also heterocyclic squaraines 22a-22e can form stable pseudorotaxane complexes. The indo-lenine-based squaraine 22a forms rotaxane 22a C Rp. Importantly, also the sulfonated squaraine 22b could be successfully encapsulated in a Leigh-type, phenylene-based, tetralactam macrocycle to yield the water-soluble rotaxane 22b C Rp. Quatemized, indolenme-based squaraines do not form pseudorotaxanes probably because of sterical hindrance caused by /V-alkyl and 3,3 -dimethyl groups. On the other hand, quatemized benzothiazole (22c) and benzoselenazole (22d) squaraines could be embedded in a Leigh-type macrocycle to yield rotaxanes 22c C Rp and 22d C Rp, respectively. The hydrophilic, mono-reactive rotaxane 22e-NHS C Rp based on asymmetric squaraine, synthesized by a cross-reaction of squaric acid with the two different indolenines, was also obtained. 

As a possible source for new long-wavelength dye systems, biradicaloid dyes possessing energetically close-lying frontier molecular orbitals could be used. 146 An example of such dye systems already in extensive use in imaging are squaric acid derivatives like SQ. 147, 14S ... 

The same mechanism is operative for the preparation of squaric acid derivatives of type 112. Treatment of 3,4-dicMorocyclobutene-l,2-dione with two different zinc-copper reagents provides the double addition-elimination product 112 in 67% yield (Scheme 2.41) [87]. 

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