Gold(III) Chloride Catalyzed Synthesis of Chiral Substituted 3‐Formyl Furans from Carbohydrates: Application in the Synthesis of 1,5‐Dicarbonyl Derivatives and Furo[3,2‐c]pyridine |
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Authors: | Kanchan Mal Abhinandan Sharma Dr Indrajit Das |
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Institution: | Chemistry Division, CSIR‐Indian Institute of Chemical Biology, 4, Raja S. C. Mullick Road, Jadavpur, Kolkata 700032 (India), Fax: (+91)?33‐2473‐5197 |
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Abstract: | This report describes a gold(III)‐catalyzed efficient general route to densely substituted chiral 3‐formyl furans under extremely mild conditions from suitably protected 5‐(1‐alkynyl)‐2,3‐dihydropyran‐4‐one using H2O as a nucleophile. The reaction proceeds through the initial formation of an activated alkyne–gold(III) complex intermediate, followed by either a domino nucleophilic attack/anti‐endo‐dig cyclization, or the formation of a cyclic oxonium ion with subsequent attack by H2O. To confirm the proposed mechanistic pathway, we employed MeOH as a nucleophile instead of H2O to result in a substituted furo3,2‐c]pyran derivative, as anticipated. The similar furo3,2‐c]pyran skeleton with a hybrid carbohydrate–furan derivative has also been achieved through pyridinium dichromate (PDC) oxidation of a substituted chiral 3‐formyl furan. The corresponding protected 5‐(1‐alkynyl)‐2,3‐dihydropyran‐4‐one can be synthesized from the monosaccharides (both hexoses and pentose) following oxidation, iodination, and Sonogashira coupling sequences. Furthermore, to demonstrate the potentiality of chiral 3‐formyl furan derivatives, a TiBr4‐catalyzed reaction of these derivatives has been shown to offer efficient access to 1,5‐dicarbonyl compounds, which on treatment with NH4OAc in slightly acidic conditions afforded substituted furo3,2‐c]pyridine. |
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Keywords: | carbohydrates chirality furans gold homogeneous catalysis |
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