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Novel Synthesis of 2‐Alkylquinolizinium‐1‐olates and Their 1,3‐Dipolar Cycloaddition Reactions with Acetylenes
Author(s) -
Gebert Andreas,
Barth Max,
Linden Anthony,
Widmer Ulrich,
Heimgartner Heinz
Publication year - 2012
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.201200038
Subject(s) - chemistry , cycloaddition , mesoionic , regioselectivity , pyridine , ketone , medicinal chemistry , 1,3 dipolar cycloaddition , amide , methyl vinyl ketone , nitrile , organic chemistry , catalysis
Several 2‐alkylquinolizinium‐1‐olates 9 , i.e. , heterobetaines, were prepared from ketone 11 , the latter being readily available either from pyridine‐2‐carbaldehyde via a Grignard reaction, followed by oxidation with MnO 2 , or from 2‐picolinic acid (=pyridine‐2‐carboxylic acid) via the corresponding Weinreb amide and subsequent Grignard reaction. Mesoionic heterobetaines such as quinolizinium derivatives have the potential to undergo cycloaddition reactions with double and triple bonds, e.g. , 1,3‐dipolar cycloadditions or DielsAlder reactions. We here report on the scope and limitations of cycloaddition reactions of 2‐alkylquinolizinium‐1‐olates 9 with electron‐poor acetylene derivatives. As main products of the reaction, 5‐oxopyrrolo[2,1,5‐ de ]quinolizines (=‘[2.3.3]cyclazin‐5‐ones’) 19 were formed via a regioselective [2+3] cycloaddition, and cyclohexadienone derivatives, formed via a DielsAlder reaction, were obtained as side products. The structures of 2‐benzylquinolizinium‐1‐olate ( 9a ) and two ‘[2.3.3]cyclazin‐5‐ones’ 19i and 19l were established by X‐ray crystallography.