Heterocycles from carbodiimides

Carbodiimides readily undergo reactions across their C=N bonds to form heterocycles. For example, in their di- and trimerization reactions four and six membered ring N-hetero-cycles are obtained (see Section 2.4.1). Also, numerous four, five and six membered ring heterocycles are formed in the cycloaddition reactions of carbodiimides, often in excellent yields (see Section 2.4.2). Nucleophilic reactions of carbodiimides sometimes give rise to 

When carbodiimides are generated in the proximity to suitably substituted OH, SH, or NHR groups, subsequent cyclization can give rise to the formation of heterocycles. This reaction is used to synthesize heterocycles 602 from ortho substituted thioureas 601 (X = O, S, NH).562 

Similarly, generation of the carbodiimide in proximity to a carboxyl group leads to cyclization. For example, the N-o-carboxyphenyl-N -phenyl urea 603 upon reaction with HgO affords the carbodiimide 604 which undergoes intramolecular cyclization to give the heterocycle 605. ° 

The cyclization of 603 is also achieved when the urea precursor is treated with a polymer supported EDC.  

The heterocyclic urea derivative 606 (R = OCOCHaPh or H) with an o-substituted carboxyl group, on treatment with DCC, also cychzes with formation of 607 

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Cuprous chloride 2-Imino-l,3-0,N-heterocyclics from carbodiimides and diols... 

Attempts to synthesize N-heterocyclic substituted carbodiimides from 1,2,4-triazine imino-phosphoranes 19 and isocyanates result in the isolation of the corresponding... 

N-N Bond scission and synthesis of N-heterocycles 86UK1785. Carbodiimides, formation of N-heterocycles from 82UK848 81AG855. Catalytic synthesis of N-heterocycles 83KGS1587, 83YGK604. Condensed N-heterocycles, syntheses using modified Japp-Klingemann... 

Iminophosphoranes derived from readily available ot-azidocinnamates react with aroyl chlorides to give the corresponding 2-aryl-4-arylidene-5(4//)-oxazolones 37 2 500,501 AjteiTjatively, these iminophosphoranes are converted to the corresponding 2-arylamino-4-arylidene-5(4//)-oxazolones 373 via heterocyclization of an intermediate carbodiimide as shown in Scheme 7.119 (Table 7.32, Fig. 7.43). ° ... 

Ethylenimines or aziridines [lb] can be considered cyclic imines and are only briefly covered in Section 5. The preparation of heterocyclic imine systems, semicarbazones, hydrazones, azines, and oximes, is omitted from this chapter. The synthesis of carbodiimides is presented in Chapter 9. 

The nitrene 28 is not produced from the azide precursor, but from heterocycles via photolysis and thermolysis as shown in Sch. 11 [20]. Iminoacyl nitrenes react intramolecularly giving benzimidazoles with good yields (Sch. 11), and, dependending on the precursor used and the reaction conditions, varying amounts of carbodiimides are obtained. The reactivity of the acyl nitrenes is influenced by the substituent connected to the acyl group (see Sch. 10), however all acyl nitrenes are quite reactive and therefore rather unselective. Apart from cycloaddition reactions with Tt-bonds, insertion reactions into a-bonds, additions to lone pair electrons of... 

A less usual type of 2-aza-l,3-butadienes is the class of C=C-conjugated carbodiimides like 3-59. They are readily available from iminophosphoranes and react smoothly with carbo- and hetero dienophiles to yield the desired heterocycles, e.g. 1,3-oxazine 3-60 (Fig. 3-18) [286-288]. 

Also, the one pot reaction of heterocyclic bis-iminophosphoranes with one or two equivalents of aryl isocyanates leads to selective formation of fused heterocycles. For example, from the pyrazole derivative 17 either [l,3]-diazepines 18 or tricyclic ring systems 19 are formed. The latter reaction involves an intramolecular [2-1-2] cycloaddition of both carbodiimide substituents. ... 

Table 3.2 Heterocyclic Compounds Obtained from Enyne Carbodiimides... 

A heterocyclic ring annulated carbodiimide is generated from a bisaryliminophosphorane and trapped with phenyl isocyanate. ... 

The Wittig-type reactions of iminophosphoranes with isocyanates and related compounds have also been extensively used in heterocyclic synthesis. Examples include the preparations of the mesoionic [l,3,4]thiadiazolo[2,3-c][l,2,4]triazines (210) from (209), 0 bicyclic guanidines, e.g. (212), from (211), naphthypyridines (215), (216), and (217) from (213) and (214), 2 pyrido[l,2-f]pyrimido-[4,5-d)pyrimidines (218), 7H-pyrido-[4,3-c]- (219) and 10H-pyrido[3,4-b]- (220) carbazoles, tricyclic fused 2,4-diimino-l,3-diazetidines (222) from the bisiminophosphorane (221), benzotriazepines (225) from (223) and (224), 6 and mesoionic thiazolo-[2,3-b]-1,3,4-thiadiazoles (227) and N,N-bisheteroarylamines from the iminophosphorane (226), derived from 3-amino-4-phenylthiazole-2(3H)-thione. The carbodiimides (229), prepared from the iminophosphorane (228), can be converted into quinolines or a-carboline derivatives depending on the nature of the isocyanate used in the reaction with (228) and the reactions of iminophosphoranes (230) and (231) with aryl and styryl isocyanates provide one-pot syntheses of quinoline, a-carboline, and quinindoline derivatives. 9... 

Reaction with carbodiimides. The adducts (2) from reaction of this phosgenelike salt with carbodiimides or N,N -disubstituted ureas are useful for synthesis of several heterocycles (scheme I). ... 

Thus, reaction of carbonimidoyl dihalides with a wide variety of nitrogen-containing substrates can be achieved, and the products obtained range from linear chloroformamidines, guanidines, and carbodiimides to five- and six-membered heterocycles. 

In numerous [4-1-2] cycloaddition reactions the diene, such as an a-oxoketene, is generated in situ and trapped by the carbodiimide. The functional ketenes are generated from diketene or from masked heterocyclic precursors. Also, o-substituted aromatic carboxylic acids are used as precursors for the functional ketenes. 

Five-membered heterocycles were obtained when ammonia or amine nucleophiles reacted with carbodiimides, generated in situ from phosphazenes by a cascade process... 

The first parallel solid-phase 1,3,5-triazino-annulation S5mthesis has been achieved by Hoesl et al. [28]. Aza-Wittig reaction of an iminophosphorane generated in situ from a resin-bound 2-aminobenzimidazole (under Mitsunobu conditions) resulted in the formation of a highly reactive carbodiimide intermediate, which subsequently underwent intramolecular heterocyclization to afford the corresponding triazinobenzimidazoles (Scheme 7). 

Heteroaromatic azides often show photochemistry that is very similar to the photochemistry of simple homoaromatic azides. Ring expansion to cycHc ketenimines is observed, if the photolysis is performed at ambient temperature, and triplet nitrenes are formed upon low-temperature irradiation. If a nitrogen atom is placed adjacent to the nitrene center, such as in 2-pyridylnitrene 100, addition generally takes place away from the nitrogen atom. In the case of the photolysis (or pyrolysis) of the 2-azidopyridine 99, the reaction leads to the cycHc carbodiimide 101. A comphcation arises from the fact that derivatives of 2-azidopyridine and related heterocycles frequently undergo a thermal isomerization to tetrazolopyridines, which also undergo photochemical dediazotation, albeit with a low quantum yield. ... 

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