ChemInform Abstract: 1,3‐DIPOLAR ADDITION OF PYRIDINE N‐IMINE TO ACETYLENES AND THE USE OF CARBON‐13 NMR IN SEVERAL STRUCTURAL ASSIGNMENTS

Abstract

N‐Amino‐pyridinium‐iodid (I) addiert über das N‐Imin (II) Acetylencarbonsäureester (III) zu den Addukten (Iva), aus denen verschiedene funktionelle Derivate (IVb)‐(IVd) sowie die Decarboxylierungsprodukte (V) zugänglich sind.

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“…The key bicycle 25 was then prepared by a [3 þ 2] cycloaddition of the N-aminopyridinium salt with an alkynoic ester. 29 Hydrogenation of the benzyl ester 25, followed by decarboxylation in 48% HBr, gave the required 2-phenylpyrazolo-[1,5-a]pyridine-7-carboxamide (27) after amide formation. A similar synthetic strategy was employed for the formation of 2-phenyl[1,2,4]triazolo[1,5-a]pyridine-8-carboxamide (30), in which case the N-aminopyridinium salt was condensed with benzaldehyde to elaborate the key heterocyclic intermediate 29 that was subsequently transformed to 30.…”

“…The synthesis of the pyrazolo[1,5- a ]pyridine 27 started from ethyl 2-picolinate ( 23 ) which upon treatment with O -mesitylenesulfonylhydroxylamine gave the N -aminopyridinium salt 24 . The key bicycle 25 was then prepared by a [3 + 2] cycloaddition of the N -aminopyridinium salt with an alkynoic ester . Hydrogenation of the benzyl ester 25 , followed by decarboxylation in 48% HBr, gave the required 2-phenylpyrazolo[1,5- a ]pyridine-7-carboxamide ( 27 ) after amide formation.…”

“…Methyl 2-Phenyl[1,2,4]triazolo[1,5-a]pyridine-8-carboxylate (29). O-Mesitylenesulfonylhydroxylamine (236 mg, 1.1 mmol) was added to a stirred solution of methyl 2-aminopyridine-3carboxylate (28) (152 mg, 1 mmol) in dioxane (10 mL) at room temperature.…”

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“…Ultimately, the use of a highly reactive N-aminating reagent, O -mesitylensufonylhydroxylamine (MSH), , was required to effect the desired transformation . Intermediate 11 underwent a 1,3-dipolar cycloaddition reaction with dimethylacetylene dicarboxylate (DMAD) to give the pyrazolopyridine 12 in low-to-moderate yields. , Heating 12 in the presence of H 2 SO 4 led to ester hydrolysis followed by selective decarboxylation at C(3) of the pyrazolopyridine . Cleavage of the methyl ether followed by Fischer esterification of the acid gave 13 in 73% yield for the three-step sequence.…”