The bis(stilbenyl)squaraines 6d − j,d‘,j‘ represent a novel class of NIR pigments. Their synthesis was performed by the regioselective 2-fold condensation of highly nucleophilic 3,5-dihydroxystilbenes 4d − j,d‘,j‘ with squaric acid (5). Depending on the substitution with alkoxy groups, the absorption maxima range in solution from 680 to 735 nm. Reflexion measurements in the solid state reveal an exciton splitting with maxima at about 670 and 1000 nm.

The susceptibility of glycal-derived carbinols to acid-catalyzed ring expansion is described. In the systems prepared from cyclopentanone, Ferrier ionization precedes the pinacol-like Wagner-Meerwein shift, thermodynamic control operates, and high stereoselectivity is seen if a C(6) substituent is present. In contrast, the adducts to cyclobutanone exhibit release of ring strain under kinetically controlled conditions and intercept the oxonium species reversibly formed via direct proton transfer. The results show that the substituents positioned on the glycal ring have a pronounced influence on whether a chair-like or twist-boat transition state geometry is adopted primarily. The composite reaction profiles reveal for the first time the fundamental importance of exothermicity and of substitution in these spiro glycosidation reactions. Since optical activity is preserved in all instances, the utility of this chemistry for the synthesis of bis-C,C-glycosides and more complex oxacyclics appears promising.The field of carbohydrate chemistry has witnessed remarkable growth in recent times as a consequence of intense interest in the synthesis of C-glycosides. The discovery of biologically active members of this important class of sugars in nature has resulted in the development of many methods for their stereoselective construction. 1 Particularly noteworthy is the wide range of approaches which have proven serviceable. These extend from the utilization of both electrophilic and nucleophilic carbohydrates to the adoption of free radical techniques, Wittig olefinations, palladium-mediated couplings, and concerted processes. Notwithstanding these advances, spirocyclic bis-C,C-glycosides have not found similar favor. Only in connection with the recent independent discoveries by Descotes 2 and by Vasella 3 of the feasibility of preparing anomeric carbenes have reports of the capture of these reactive intermediates with olefins to give cyclopropanes made their appearance.Recent work in these laboratories has illustrated that the acid-catalyzed rearrangement of dihydropyranylcarbinols such as 1 to spirocyclic ketones can be highly diastereoselective. 4 This efficient process results in the generation of a new stereogenic center by means of controlled pinacol-like 1,2-migration to a cyclic oxonium ion. Under conditions where the starting carbinols would be derived from glycals and would therefore be enantiomerically pure, this transformation would provide a convenient means for establishing the absolute configuration of a fully substituted carbon while delivering bis-C,C-glycosides. Presented here are the results of an extended study involving a variety of carbohydrate-based glycals substituted in diverse ways. A preliminary investigation 5 had previously uncovered divergent responses from systems amenable to operation of the Ferrier rearrangement. 6 These variances provided important insight with regard to the ranking of competing reactions available to six-membered oxonium ions and, accordingly, have been evaluated more exte...

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Extended Conjugation in Stilbenylsquaraines Unsymmetrical 1‐aryl‐3‐stilbenylsquaraines 2a–e and 1,3‐bis(stilbenyl)squaraines 3a–c were prepared by CC coupling reactions of the corresponding substituted arenes with derivatives of squaric acid (Schemes 2 and 3). Dialkylamino and alkoxy groups enhance the solubility of these dyes and enlarge the intramolecular charge transfer of these donor–acceptor–donor systems. The extended conjugation of the stilbene units – in comparison with arene building blocks – leads to significant bathochromic shifts in the Vis/NIR absorption spectra.

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