Facile synthesis of 4 H -naphtho [ 2,3 -e ] derivatives of 1,3-thiazines and 1,3-selenazines and naphtho [ 2',3':4 , 5 ] derivatives of selenolo[2,3-b ]pyridines and thieno[2,3-b ]pyridines via 2,3-didehydronaphthalene

Thiaazadienes ( 2a – e ), selenoazadienes ( 6a – f ), Barton selenium esters ( 8a – e ) and Barton sulphur esters (10a–d) react with 2,3-didehydronaphthalene (4) generated from (phenyl) [ (3-trimethylsilyl)-2-naphthyl ] iodonium triflate (3) at 0°C to give 4H-naphtho [ 2,3-e ] -1,3-thiazines ( 5a - e ), 4H-naphtho[2,3-e]-1,3-selenazines (7a-f), naphtho[2',3':4,5]selenolo[2,3-b]pyridines (9a-e) and naphtho [ 2',3':4,5 ] thieno[2,3-b]pyridines ( 11a - d ).

The required (phenyl)[(3-trimethylsilyl)-2-naphthyl]iodonium triflate (3) was prepared from 2,3-dibromonaphthalene. 13 The thiaazabutadienes (2a-e) 14 and the selenoazadienes (6a-f) 15 were prepared by literature procedures.With the exception of thiaazadiene (2e) the thiaazadienes (2ad) are new compounds and their % yields are shown in Scheme 1.The Barton selenium esters (8a-e) were prepared from 2-chloropyridines 12 and the Barton sulfur esters (10a-d) were prepared by the Barton method. 16ith the dienes on hand, the reactions listed in Scheme 2 were performed.The generation of 2,3-didehydronaphthalene (4) was carried out by simply adding Bu4NF to a solution of the precursor 3 at    Possible mechanisms for the reactions reported here are worthy of discussion.The reaction of benzyne ( 4) with the esters 8 and 10 giving 9 and 11, respectively, probably proceeds by the usual cycloaddition pathway previously reported. 11However, the reaction of 4 with thiaazabutadienes (2) and with selenoazadienes (6) to give 5 and 7 is worthy of discussion.We 12 have shown that the reaction of selenoazadienes with benzynes generated by the reaction of 2trimethylsilylphenyl triflates with CsF at room temperature gave extremely polar compounds which LC/MS showed to be N-oxide derivatives.By adding NaBH 4 , the desired 4H-

Scheme 3
Thus, it is unlikely that the proposed reduction mechanism in Scheme 3 is operative.One of the referees of this paper suggested the possibility that the electrons in 12 flow the other way in which the heterocyclic ring is reduced to 14 and the dimethylamine moiety is oxidized to CH 3 N=CH 2 (15).This possibility is illustrated in Scheme 4. A 1,3-hydride shift would lead to the observed product 7. Exactly the same process could apply to the reaction of 4 +2 going to 5. To date we have not been able to trap 15 and have insufficient information to support this mechanism.However, it appears to be a reasonable mechanism, and we are studying its validity.In summary, we have synthesized a large number of naphthalene derivatives using 2,3bis(trimethylsilyl)naphthalene with a PhI(OAc) 2 /TfOH-reagent system which behaves as a highly efficient 2,3-didehydronaphthalene precursor.Compared with currently available aryne precursors, the hypervalent iodine reagent 3 not only generates arynes under neutral conditions at low temperatures but also reacts with dienes to give adducts in very good yields.This process represents a very powerful tool for the preparation of complex molecules, which might be difficult to prepare by any other present methodology.

Experimental Section
General Procedures.Melting points were taken on a Mel-Temp capillary apparatus and are uncorrected with respect to stem correction. 1 H-, and 13 C-NMR Spectra were recorded on a 400MHz Bruker AVANCE DRX-400 Multi-nuclear NMR spectrometer.Chemical shifts are reported in reference to TMS as internal standard.Elemental analyses were obtained from SMU Analytical Laboratories.HRMS analyses were provided by the Washington University Mass spectrometry Resource, an NIH Research Resource (Grant No. P41RR0954).Barton esters were stored in an amber bottle in a refrigerator.The glassware was heated overnight in an oven at 125 o C prior to use.All the benzyne reactions were done under an atmosphere of dryO 2 -free Ar via balloon.

ISSN 1424-6376 Page 53 © ARKAT USA, Inc SiMe 3
reduced by NaBH 4 giving the observed product.However, in this study we found that by generating benzyne (4) from the reaction of (phenyl)[(3-trimethylsilyl)-2naphthyl]iodonium triflate (3) with Bu 4 NF, the desired products 5 and 7 are obtained without the aid of the NaBH 4 reducing agent.This dilemma is shown in Scheme 3.
1,3benzoselenazines were obtained.We proposed a somewhat controversial mechanism involving the formation of a Diels-Alder adduct intermediate which could aromatize by the electron release from S and NR 2 displacing the NMe 2 anion.What is left behind is a reasonably stable aromatic cation which gets