The Bestmann–Ohira reagent is a unique and versatile reagent in organic synthesis. It is a well‐known homologating agent for the conversion of aldehydes into the corresponding terminal alkynes. Recently, it has also been used as a potent cycloaddition partner in 1,3‐dipolar cycloaddition reactions. The Bestmann–Ohira reagent can be readily accessed from commercially available sources. It has found tremendous application in the regioselective synthesis of a variety of organic synthons, including alkynes, triazoles, triazolines, pyrazoles, oxazoles, thiazoles, isoquinoline N‐oxides, indazoles, pyrroles, and enol ethers in good‐to‐very‐good yields.
Triazoles have always occupied a pivotal position in the field of medicinal and pharmaceutical chemistry. Recently, the area of chiral fused triazoles has gained attention. The synthetic routes to access chiral fused triazole may involve either a natural or synthetic chiral starting material with a suitably functionalized azide and alkyne component or isomer resolution to achieve chirality. Asymmetric synthesis methods using chiral complexes and organocatalysis have also emerged for obtaining chiral fused triazoles. Herein, we have described an up-to-date overview of different synthetic protocols applied to construct chiral fused 1,2,3-triazoles. Scheme 1. Basic methodologies for triazole construction. 2 3 4 5 6 7 8 . Yanai's stereo-divergent synthesis of ido-triazole ent-6 a and gluco-triazoles 7 a. Scheme 3. Yanai's diastereo-divergent synthesis of manno-, gulo-, galacto-and altro-triazoles. . Konda's synthesis of tricyclic triazolo-oxazepines 62. Scheme 15. Cobb's synthesis of spirocyclic triazolooxazine nucleosides 67. Scheme 16. Sen's synthesis of tetracyclic triazolo-oxazepines 69. to azide group and protected amine was propargylated to access azido-alkynes 122 c-h for triazole formation 124 (Scheme 29). The copper catalyzed version of this step was also carried out that gave almost approximately same yields in lesser Scheme 25. Chrandrasekaran's synthesis of triazolo-pyrazines and pyrazinones from amino acid derived sulfamidates. Scheme 26. Chrandrasekaran's synthesis of triazolo-pyrazinone 110 having pendant amino acid.structural features of triazoles, the authors carried out one pot hydrogenation and insitu Boc-protection of 155 a, followed by azidation and propargylation to afford azido-alkyne 160. Subsequently, thermal azide-alkyne cycloaddition furnished the triazole product 161 in good yield (Scheme 34).Allenyl silanes are extensively used in synthetic organic transformations and provide ready access to enantiopure pyrroles, oxiranes, furans and vinyl silanes. In lewis acid assisted reaction, allenyl silanes work as propargyl anion equivalents. Allenyl silane forms regioselective homopropargylic alcohol when treated with aldehyde and ketone. The stabilization is Scheme 31. Thomas's synthesis of steroidal fused 1,2,3-triazoles.Scheme 32. Thomas's synthesis of steroidal fused NH-1,2,3-triazoles and steroidal 4,5-fused triazolo-alkynes.
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