Isoxazolo[2,3-d][1,4]benzoxazepine rings from 1,3-dipolar cycloaddition of [1,4]benzoxazepine N-oxides with acetylenic and olefinic dipolarophiles

1,3-Dipolar cycloaddition of [1,4]benzoxazepine N-oxides to triple and double bonded dipolarophiles affords di-and tetrahydro-isoxazolobenzoxazepines in the first reported synthesis of this tricyclic ring system. It is observed that in addition to the acetylenic dipolarophiles the diastereofacial selectivity varied both with the nature of the C-nitrone substituent as well as on the degree of substitution of the dipolarophile. Addition to alkene substrates occurred regiospecifically furnishing “5-substituted” isoxazolidine rings. For each olefinic dipolarophile the major cycloadduct arose via an endo-addition of the dipolarophile to the β -face of the dipole. Some of the new tricycles displayed sharp signals in their pmr spectra whilst signal duplication/broadening was evident for others indicating the adducts exist in solution as pairs of conformational isomers which reach coalescence at or close to room temperature. Preliminary investigations show the isoxazolidine ring of the tricycles can be cleaved under reductive and oxidative conditions affording novel benzoxazepines. The structure of the cycloadduct 6a formed between the C-phenyl nitrone 5a and dimethyl acetylenedicarboxylate and has been solved by single crystal xray analysis.


Introduction
The clinical importance of the 1,4-benzodiazepine drugs makes synthesis of analogues important and derivatives with an additional heterocyclic ring fused to the "a", "c" or "d" edges have been investigated [1][2][3][4]5 . Wehave previously contributed to this area in the synthesis of di-and tetrahydroisoxazolo [2,3-d] [1,4]benzeneodiazepinones from dipolar cycloaddition to a range of benzodiazepinone N-oxides 6,7 .We now report an investigation into the preparation of a related, yet hitherto unknown series of compounds viz tricyclic isoxazolo [2,3-d] [1,4]benzoxazepines 1.Given the structural similarity between the new tricycles and the tetracyclic isoxazolodibenzoxazepine 2 reported by Jannsen it is anticipated that the former may have interesting biological activity.Jannsen have filed a patent detailing the preparation of the tetracycle 2 by cycloaddition of the nitrone 4 to terminally unsaturated amines.The tetracycle is claimed to have biological activity -in the "elevated and illuminated plus maze test" 8 .In a second patent the same group claim preparation of azepine analogues, e.g. 3, which may have application in the treatment of CNS, cardiovascular or gastrointestinal disorders 9 .

Results and Discussion
The current approach to the targeted di-and tetrahydroisoxazolo [2,3-d] [1,4]benzoxazepines involves 1,3-dipolar cycloaddition of the benzoxazepine nitrones 5 to acetylenic as well as olefinic dipolarophiles.We have previously reported synthesis of the bicyclic nitrones beginning from hydroxybenzophenone and hydroxyacetophenone respectively 10 .The C-phenyl nitrone 5a reacted with dimethyl acetylenedicarboxylate (CHCl 3 for 32 h) to furnish a single cycloadduct, 6a (90%).The resonance signals for 6a (CDCl 3 , room temperature) were sharp suggesting either that the tricyclic framework adopts a single preferred conformation or that equilibration between conformational isomers is very rapid at room temperature.This observation is in line with our earlier finding that the isoxazolobenzodiazepinones 7 11 and the isoxazolopyrimidodiazepinone 8 12 have sharp pmr spectral signals, on the other hand Romeo's related tricycles, e.g. 9 (R = Et, Me) exist as a mixture of slowly interconverting diastereomers 13,14 .The two isomeric forms, 9-f folded and 9-e extended, result from the flexibility of the 7-membered ring, the barrier to inversion of which is calculated at ~17 kCal mol -1 .One significant structural feature uniting compounds 6a, 7 & 8 and dividing them from Romeo's adduct 9, is the presence of a C-5 substituent, accordingly we conclude that this substituent may be important in determining the conformational preferences of the tricycle at room temperature.NOEDS data for 6a (and all the dihydro-adducts in the series) was inconclusive, thus relative stereochemistry could not be assigned by spectral methods.However, single crystal xray structure analysis reveals 6a to have the 3-dimensional structure shown in Figure 1 [15][16][17] .In keeping with Romeo's terminology the tricyclic framework adopts a folded conformation in the solid state 6a-f.The boat like conformation of the 7-membered ring is such that it would be simplistic to describe the C-11b aryl ring and the C-5 groups simply as being cis or trans.However, it is clear that in the transition state leading to 6a that the dipolarophile must have approached the nitrone on the α−face, i.e. the face bearing the C-3 methyl substituent, Figure 2.  In attempting to account for the facial specificity of the cycloaddition we aimed firstly to establish the most likely conformation of the reacting dipole.A survey of the literature reveals that neither theoretical nor experimental data on the conformational preferences of the 2,3dihydro-1,4-benzoxazepine system are available.Accordingly, whilst neither is an ideal model for the dipoles 5, the closest are the diazepam-4-oxide 10 18 and the 5-phenyl-7chlorobenzodiazepine 11 19 .Crystal structure data for 10 and 11 indicate that the 7-membered ring adopts a boat conformation in the solid state.Messinger and Buss in a PM3-quantum mechanical study suggest that the more sterically hindered a 2,3-dihydro-1H-1,4-benzodiazepine the more favoured the boat over the half-chair conformers, for example the boat conformation of the 1,2,5-trimethyl substituted bicycle 12 is estimated to be 1.39 to 3.75 kCal mol -1 more stable that the alternate lowest energy chair forms 20 .Following from these reports and on inspection of Dreiding scale models it is suggested that the seven membered ring of 5 also adopts a boat conformation and one diastereomeric form, with the C-3 methyl in the less sterically hindered pseudo equatorial 21 position, is shown in Figure 2. In some of the pmr spectra of the nitrones 5a and 5b (room temperature, CDCl 3 ) there is evidence for the existence of two doublets representing the C-3 methyl protons, e.g. a pair of doublets centered about 1.38 ppm (major, J 6.4) and 1.37 ppm (minor, J 6.6) are visible for 5b, no duplication of other signals is observed.In other 1 H and in all the 13 C nmr spectra of the same molecule there is no signal duplication and all signals are sharp, these observations suggest that the bicyclic nitrones are present in solution as a pair of conformers which have almost reached coalescence at room temperature.In the transition state of a 1,3-dipolar cycloaddition the two reactants approach each other in parallel planes, the crystallographic data of 6a indicates it arose from an approach of the dipolarophile to the face of the dipole bearing the methyl group (α-face), Figure 2. Our explanation for the observed facial preference lies with an inspection of a Dreiding scale model of 5a which shows the pseudo equatorial methyl substituent to be almost co-planar with the dipolar moiety [CH 3 -C3-N4-C5 dihedral angle ~15 o ], thus, the steric impact of the methyl group is not significantly greater on the α-face than it is on the β-face of the 7-membered ring.Infact the β-face with the pseudo axial C-3 proton is more crowded.Thus, the dipolarophile adds to the α-face as it is less crowded than the β-face.
Facial specificity was lost upon reaction with methyl propiolate and the diastereomeric adducts 13a and 14a (~4:1) resulted after heating at reflux in CHCl 3 (36 h).Reaction of nitrones with electron poor acetylenic dipolarophiles is expected to yield 4-substituted isoxazolines 22 .That 13a and 14a have the expected regiochemistry is deduced from the resonance position of the vinylic proton which appears at 7.24 and 7.71 ppm.That cycloaddition to both the αand the β-faces of the dipole occurs with the monosubstituted dipolarophile is likely a simple reflection of the reduced steric demands of the dipolarophile.In the transition state for the formation of the "4-substituted" isoxazolines there is no destabilising steric interaction between the dipolarophile substituent and the H/CH 3 at the C-3 position of the dipole.
It is impossible to determine the relative stereochemistry of either adduct by NOEDS analysis.However, it is possible to discriminate between 13a and 14a following from a comparison of the resonance positions of their ring protons [H-5 and the methylenic pair H-6a/6b] with the data obtained for 6a where the relative stereochemistry is unambiguously known, selected data are summarised in Table 1.Thus, the resonance signals for H-5, H-6a, H-6b of the major adduct 13a appear at similar positions to the corresponding protons in 6a, whilst those of the minor adduct 14a show an upfield shift, of most significant magnitude for the H-6a (∆ 0.68 ppm) and H-6b protons (∆ 0.33 ppm).It is proposed that the major adduct, 13a, has the same relative configuration as 6a and that the minor adduct, 14a, has the C-11b Ph and the C-5 Me substituents of the 7-membered ring on the "same" face of the tricyclic skeleton.
It is of interest that the pmr spectral signals of the minor adduct 14a are sharp whilst those of 13a shows some signal duplication at room temperature, e.g. the signal for Me-5 is represented by overlapping doublets at 1.21 and 1.22 ppm suggesting 13a has a higher barrier to conformational inversion than either 6a or 14a.
The same dipole reacted with an excess of phenyl acetylene (30 equivalents) affording isomeric adducts 15 (40%) and 16 (55%).The formation of regioisomers is a consequence of a delicate balance between the steric factors promoting the 5-substituted isomer and the electronic factors favouring the 4-substituted product 22 .Discrimination between the regioisomers is based on the resonance position of the vinylic proton, for 15 H-1 appears as a singlet at 5.88 ppm whist H-2 of 16 resonates together with the aryl protons (m, 7.02-7.56ppm).There is insufficient similarity between the pmr positions for the ring protons of 15/16 with 6a (Table 1) to draw any conclusions with regard to the relative stereochemistry of the regioisomeric adducts.Cycloaddition of the C-methyl nitrone 5b with either of the acetylenic dipolarophiles occurred, with minimal facial selectivity.Reaction with dimethyl acetylenedicarboxylate furnished the diastereomeric cycloadducts 6b and 17 in an 11:9 ratio.It is thus apparent that the dipole C-substituent plays an important role in dictating facial preferences and with a methyl group in this position the two faces of the dipole 5b are almost equally attractive to the approaching dipolarophile.That the major adduct 6b has the same relative stereochemistry as 6a is concluded on the basis of the similarity between the resonance positions of the protons of the 7-membered ring in the two adducts (Table 1).The signals for H-5, H-6a, H-6b of 17 are shifted upfield with respect to 6a indicating 17 arises from addition to the β-face of the dipole.Both 6b and 17 have sharp resonance signals in their pmr spectra in accordance with conformational inversion on the nmr time scale at room temperature.
Two diastereomeric cycloadducts also resulted from addition of 5b to methyl propiolate.For each isomer the vinylic proton resonates at ~7.5 ppm.The 6a-H and 6b-H of the minor adduct 14b are both more shielded than the corresponding protons in 13b thus it is concluded that the major adduct, 13b (50%), arises from dipolarophile addition to the α-face and the minor adduct, 14b (38%), from addition to the β-face of the nitrone.The pmr spectral signals (CDCl 3 , room temperature) of 13b are sharp whilst those for 14b show some broadening and infact, two singlet signals at 7.554 and 7.549 represent the H-2 proton.These data suggest a high barrier to conformational inversion for 14b with respect to 13b.Cycloaddition of the nitrones 5 to unsymmetrical olefinic dipolarophiles leaves open the possibility for the formation of regio-and diastereomeric cycloadducts.Indeed Varlamov et al have noted that the related nitrone, 5-methyl-4,5-dihydro-3H-spiro[benz-2-azepine-3,1'cyclohexane] N-oxide reacts with acrylonitrile 23a with no selectivity, but with selectivity to trimethylvinylsilane 23b or styrene 23c .Reaction of 5b with each of phenyl vinyl sulfone, methyl acrylate and methyl vinyl ketone progressed slowly upon heating in boiling toluene (100 h, 110 o C, N 2 ).In all cases reaction occurred regiospecifically, however, facial and exo/endo selectivity varied with the dipolarophile.Addition to phenyl vinyl sulfone progressed with the least selectivity and addition to methyl vinyl ketone with the greatest selectivity.The major adduct in all three cases arose from an endo addition of the dipolarophile to the β-face of the dipole.
Reaction between 5b and phenyl vinyl sulfone furnished three diastereomeric adducts 18a, 18b and 18c isolated in 8, 14 and 20% yield respectively.That all adducts bear "5-substituted" isoxazolidine rings is evident from the resonance position of the H-2 proton; in the pmr spectrum of all three isomers of 18 the H-2 signal appears as a dd at ~5 ppm.The relative stereochemistry of each adduct has been assigned following analysis of NOEDS data, key enhancements are shown in Figure 3.The minor adduct arises from an exo addition of the dipolarophile to the βface of the dipole and the major product from an endo addition to the same face.The intermediate product, 18b is a result of an exo addition to the α-face of the dipole.Figure 3 Reaction of 5b with methyl acrylate with furnished, following purification by flash column chromatography, diastereomeric adducts 19a (56%) and 19b (20%).The former was obtained as an enriched sample with a trace amount of a third diastereomer.The resonance position of the H-2 proton in 19a and 19b indicates they each bear a 5-substituted isoxazolidine ring.NOEDS data for the major adduct suggests it arises by way of an endo attack on the β-face of the dipole.NOEDS data for the minor adduct, 19b, were inconclusive.The greatest facial selectivity was observed in the cycloaddition to methyl vinyl ketone and 20a, the major product (69%), results from an endo addition to the β-face of the dipole.NOEDS results for are shown in the drawing.The minor adduct, 20b (17%) arises from exo addition to the same face.For each adduct the H-2 proton appears in the pmr spectrum at ~4.4 ppm which characterises both the isoxazolidine rings as having a 5-substitution pattern.It is of interest that in the pmr spectra (CDCl 3 ) of adducts, 18-20, resulting from trapping of the benzoxazepine nitrone 5b with olefinic dipolarophiles, only 19a and 19b showed any evidence for signal broadening.In each of these adducts the signals representing 5-H, 6a-H and 6b-H were slightly broad in appearance.We have previously observed that the analogous isoxazolobenzodiazepinones 21 have sharp pmr and 13 C nmr spectral signals at room temperature 7,11 whilst similar adducts, e.g.22 from Aversa's laboratory 14 and the isoxazolopyrimidodiazepine 23 which we have recently reported 12 , display signal duplication and signal broadening respectively at room temperature.Upon perusal of the pmr spectral characteristics (room temperature) of the aforementioned molecules it is apparent that the conformational freedom of the 5,7,6-tricyclic framework is not easily predictable.However, it is apparent that the following factors likely to be important: In terms of the influence of the relative stereochemistry about the 7-membered ring on the resonance position of the adjacent protons it is interesting that for both diastereomeric pairs of adducts 6b, 17 and 13b, 14b, the C-11b Me protons of the adducts arising from addition to the βface of the dipole, appear ~0.1 ppm upfield of the corresponding protons in the adduct resulting from addition to the α-face of the nitrone.The same trend is noted amongst the isomeric adducts arising from phenyl vinyl sulfone or methyl vinyl ketone addition to 5b.Thus, the C-11b methyl protons of 18b resonate at 1.66 ppm, 0.2 ppm downfield of the corresponding protons in 18a and 18c, products of addition to the β-face.
Much of the rich contribution of nitrone chemistry to synthesis lies in post cycloaddition manipulation of the isoxazolidine/isoxazoline ring and in this respect we wished to explore the potential of the new tricycles to submit to oxidative and reductive ring cleavage.The cycloadduct 20a has two groups susceptible to reduction and on treatment with palladium (II) chloride under an atmosphere of H 2 , reaction products varied with duration of reaction.The peripheral ketone was the most vulnerable functionality and after 6 h reaction the hydroxyethyl derivative 24 was formed in 80% yield.After 24 h duration the isoxazolidine ring had also cleaved and the diol 25 could be isolated in 95% yield.It is of interest that following cleavage of the isoxazolidine ring the pmr signals of 25, most especially those of the seven membered ring, were very broad in their appearance.This may reflect a decrease in the rate of inversion of the seven membered ring and/or transient intramolecular H-bonding between N4 and the pendant OH groups.The cycloadduct 18c was investigated as a test case for an oxidative ring opening of the isoxazolidine nucleus.Following exposure to chromium trioxide the [1,4]benzoxazepine Noxide 26 was isolated as the only identifiable material, 36%.The presence of the additional site of unsaturation in the nitrone 26 with respect to the diol 25 is likely responsible for the conformational rigidity of the former bicycle.The ease with which the reduction of 20a and the oxidation of 18c occur assures the role of the new tricycles as platforms for the construction of a range of substituted benzoxazepines.

Conclusions
The tricyclic isoxazolobenzoxazepine ring system has been formed for the first time following 1,3-dipolar cycloaddition to benzoxazepine N-oxides.The regio-and stereochemical outcome of the reactions was sensitive to both the nature of the C-nitrone substituent as well as the structure of the dipolarophile.Thus, whilst dimethyl acetylenedicarboxylate added stereospecifically to the C-phenyl nitrone there was almost no selectivity when the C-methyl nitrone was trapped by the same dipolarophile.Regiospecific cycloaddition was noted in the addition to monosubstituted dipolarophiles with 4-substituted isoxazoline rings resulting from addition to acetylenic dipolarophiles and 5-substituted isoxazolidine rings from addition to alkenic dipolarophiles.The appearance of broad pmr signals or of signal duplication in the pmr spectra for some of the new cycloadducts suggests the tricyclic ring is in a state of conformational flux which is slow on the nmr timescale.The spectral signals for other adducts are sharp suggesting rapid equilibration at room temperature.A survey of the structural features of the adducts falling into each of these divisions indicates that the factors influencing conformational mobility are many and varied and that prediction of same is likely to be very difficult.That the new cycloadducts have synthetic potential is illustrated in the oxidative and reductive ring opening to novel compounds with a benzoxazepine scaffold.

Experimental Section
General Procedures.Mps. were determined on an Electrothermal melting point apparatus and are uncorrected.Elemental analyses were performed on a Perkin-Elmer model 240 CHN analyser. 1H and 13 C NMR spectra were recorded using a JOEL EX270 FT NMR spectrometer at probe temperatures with tetramethylsilane as internal reference and deuteriochloroform as solvent, J values are given in Hertz.Flash column chromatography was carried out on silica gel 60 (Merck 9385, 70-230 mesh) and analytical TLC plates were purchased from Merck.Samples were located by UV illumination using a portable Spectroline Hanovia lamp (λ, 254 nm) or by the use of iodine staining.

Xray crystal determination of (6a).
The structure was solved by direct methods, SHELXS-97 16 , and refined by full matrix least squares using SHELXL-97 17 .SHELX operations were automated using OSCAIL which was also used to obtain the drawings 18 .The XDS program was used for data reduction and the data was corrected for Lorentz and polarization effects but not for absorption.Hydrogen atoms were included in calculated positions with thermal parameters 30% larger than the atom to which they were attached.The non-hydrogen atoms were refined anisotropically.All calculations were performed on a Pentium PC.Crystal data for 6a -Table 2.

Figure 2
Figure 2 (i) the size of the C-11b substituent, [thus 13a inverts more slowly than 13b]; (ii) the relative configuration of the C-11b and C-5 substituents [thus 13a inverts more slowly than 14a]; (iii) the nature of the substituent at the C-2 position [thus 19a inverts more slowly than 20a]; (iv) the hybridisation state of the C-6 carbon atom [thus 19a inverts more slowly than 21]; (v) the degree of substitution at the C-5 position [thus the dihydro-adducts 9, and 22 both invert slowly on the nmr time scale].

Table 1 .
Selected pmr spectral data for adducts 6

Table 2 .
Crystal data and structure refinement for 6a