Received: 20 August 2001 / Uploaded 21 August 2001
 
 

We have studied the cyclization of N-(2-vinyl)aryl-N’-aryl carbodiimides 1 and N-(2-vinyl)aryl-N’-styryl carbodiimides 3 under thermal conditions to give, respectively, 2-aminoquinolines 2 and pyrido[2,3-b]indoles (?-carbolines) 4 [1]. The conversion 1?2 is just a 6?-electrocyclization, whereas the formation of pyrido[2,3-b]indoles 4 involves a [4+2] cycloaddition whereby the unsaturated carbodiimide has functioned as a 2-azadiene (using a cumulative C=N double bond and the styryl C=C double bond), and the C=C double bond of the ortho-vinyl substituent has taken the role of the dienophile. This work showed for the first time an intramolecular [4+2] cycloaddition involving a carbodiimide as the dienic component.

Also, a modification on the nature of the heterocumulenic part was considered and the cyclization of ketenimines was studied. The thermal treatment of ketenimines 5 gave the quinolines 6 by an electrocyclic ring closure.

In this communication we describe the thermal behaviour of the N-arylcarbodiimides 7 and 8 and the N-arylketenimines 9 bearing an ortho-allyl substituent on the benzene ring. We selected this substrates with the aim of obtaining the products derived from a [4+2] cycloaddition involving the C=C bond of the allyl substituent as dienophile and the heterocumulene fragment as diene.

In these new heterocumulenic systems, when compared with the compounds 1, 3 and 5, the presence of the methylene should preclude their evolution via an electrocyclic ring closure.

Staudinger reaction of 2-allyl-1-azidobenzene [2] 10 with triphenylphosphine gives the corresponding triphenylimino-phosphorane 11, used as common starting material for the preparation of compounds 7, 8 and 9 by aza-Wittig reaction with isocyanates or ketenes.

Our first attempt was the reaction of iminophosphorane 11 with 4-methylphenylisocyanate to generate N-(2-allyl)phenyl-N’-(4-methyl)phenyl carbodiimide 7, which by thermal treatment unexpectedly produced the quinoline 13. Carbodiimide 7 does not undergo the expected intramolecular [4+2] cycloaddition but probably the allylic side-chain converted by an 1,3-hydrogen shift into a vinyl substituent thus allowing a 6?-electrocyclic ring closure in the proposed intermediate 12 to give quinoline 13.

We then considered the reaction of 11 with trans-cinnamyl isocyanate to prepare N-(2-allyl)phenyl-N’-styryl carbodiimide 8. We reasoned that an intramolecular [4+2] cycloaddition in compound 8 involving the cumulenic C=N bond and the adjacent C=C bond should be easier, as in this case no aromaticity would be destroyed, when compared with the similar intramolecular cycloaddition of carbodiimides 7.

The thermal treatment of the N-styrylcarbodiimide 8 gave, in fact, a [4+2] product, the pyrido[2,3-b]indole 15, but involving again the initial rearrangement of the allyl side-chain to a vinyl one.

By contrary, when N-(2-allyl)phenyl-C,C-diphenylketenimine 9 (R=C₆H₅), prepared by reaction of the iminophosphorane 11 with diphenylketene, was heated at 160ºC for 16 h, it converted into the tetrahydrobenzo[b]acridine 16. Oxidation of this last compound yielded the benzo[b]acridine 17.

The conversion 9 (R=C₆H₅)?16 can be explained as a formal [4+2] cycloaddition where the ketenimine fragment acts as an all-carbon diene, involving the cumulative C=C bond and a double carbon-carbon bond of the phenyl ring on the sp² carbon terminus.

However, a similar thermal treatment of the ketenimine 9 (R=CH₃) failed to give the [4+2] cycloadduct 18. Instead of, a very complex mixture resulted. It seems that the presence of two phenyl groups in the C-terminus of the ketenimine fragment (as in 9, R=C₆H₅) is mandatory for allowing the intramolecular cycloaddition to occur.

We have succeeded in our objectives only partially: the [4+2] cycloaddition took place with success only in the case of the C,C-diphenyl ketenimine but neither in the C-methyl-C-phenyl ketenimine nor in the N’-aryl and N’-styryl carbodiimides prepared. The [4+2] cycloadduct obtained is a tetrahydrobenzo[b]acridine, that converted easily in a fully planar aromatic benzo[b]acridine.

Currently we are exploring the application of this reaction for the preparation of a variety of benzo[b]acridines and related compounds.
 
 

[1]- Molina, P.; Alajarín, M.; Sánchez-Andrada, P. J. Org. Chem. 1992, 57, 929.

[2]- Smith, P.A.S.; Chon, S.P. J. Org. Chem. 1981, 46, 3970.
 
 

This work was supported by the Dirección General de Enseñanza Superior (Project PB95-1019), Fundación Séneca-CARM (project PB/2/FS/99) and Acedesa (a division of Takasago). Mª del Mar Ortín-Avilés thanks the Fundación Séneca for a fellowship, Fulgencio Tovar also thanks the MECD for a fellowship.