Five-membered rings with two or more nitrogen atoms

In what the authors suggest might be the first example of a [1 + 3]dipolar cycloaddition, mesoionic-4 thiazolones (130) give imidazolinones (132) upon reaction with ethyl azidoformate, in a multistep process which may proceed via the strained bicycle (131). A similar loss of sulphur occurred during an attempted synthesis of the thienodiazepinone (133), the isolated product being the pyrrolo[l,2-a]imidaz-2(3//)-one derivative (134)/  

Two papers from Gnichtef demonstrate the utility of oximes of a-aminoketones [e.g. (135)] for preparing small heterocycles [e.g. (136)—(138)]. In both examples imidazoline iV-oxides can be formed if the anft-forms of the oximes are reacted with carbonyl compounds as shown. 

The first report of the reaction of 2//-l-benzopyran (139) with various 1,3-dipoles has appeared/ Good yields of single cycloaddition products (140) were obtained upon reaction with nitrile-imines, whereas nitrile oxides gave 1 1 mixtures of the regioisomers (141) and (142), The reactions with diazomethane gave rather inconclusive results, and phenyl azide and diphenyl nitrone failed to react. Similar types of reactions have been carried out using a variety of enones in which the overall conclusion appears to be that simple aliphatic or alicyclic enones produce mixtures of regioisomers upon reaction with nitrile oxides and nitrile imines whereas enones of the chalcone type show only one product. Experts in FMO theory will probably declare these results 

Diazomethane reacts in a 1,3-dipolar fashion across the C=C of thioketene S -oxides to give good yields of the 1 1 adducts (145). In contrast, thioketenes are known to react across the C=S bond. 

An investigation of the reactions of vinyl azides with enamines has been published, in which the results are given extensive discussion with consideration 

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Five-membered rings with two or more nitrogen atoms... 

FIVE-MEMBERED RINGS WITH TWO OR MORE NITROGEN ATOMS... 

Five-membered Rings with two or more Nitrogen Atoms.—Conventional syntheses of imidazolines (128) normally require nitriles or imino-ethers as starting materials. Only in selected cases can ethylene diamines and acids or esters lead to the required products. The use of thiols and nitriles goes some way to circumventing the problems, but under the conditions described by Neef and co-workers, a wide range of esters is readily transformed to imidazolines and their benzo-analogues (Scheme 7). 

Five-membered rings with two or more heteroatoms are usually good at electrophilic substitution as one of the heteroatoms must be either O or S or a pyrrole-like nitrogen atom any of which supply lone pair electrons. Pyrazole 12, imidazole 13, oxazole 17, thiazole 18, isoxazole 19 and isothiazole 20 are examples. Multiple substitution can be a problem but is less so than for pyrrole because there are fewer carbon atoms available and the pyridine-like nitrogen atoms deactivate the ring. 

In the solid phase, five-membered heteroatomic compounds with two or more nitrogen atoms in the ring have a broad absorption at 2800-2600 cm (3.57-3.85 pm) due to the NH N bond. 

Five-membered aromatic ring compounds with two or more hetero atoms are deprotonated more easily than the one-hetero atom systems [1], The ease of this proton removal is in part due to inductive electron withdrawal by the additional hetero atom(s), and in some cases coordination of the lithiating reagent by (sp2) nitrogen may contribute (e.g. in 1,3-thiazole and 1-substituted imidazoles). Some examples of aromatic systems with more hetero atoms are ... 

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