Regioselective transformation of 6/5-fused bicyclic isoxazolidines to second-generation cyclic aldonitrones

The cycloaddition reactions of 4-(2-hydroxy-2-propyl)-3,4,5,6-tetrahydropyridine 1-oxide with mono-and di-substituted alkenes have been found to be highly stereo-as well as face-selective. In solution, the 6/5 fused bicyclic cycloadducts remain solely as the cis -fused invertomers in order to accommodate the bulky tertiary substituent 2-hydroxy-2-propyl in the equatorial orientation. The cycloadducts, upon peracid oxidation, leads to the exclusive formation of synthetically important second-generation cyclic aldonitrones. The stereo-and face-selectivity of the cycloaddition reactions of these second-generation nitrones bearing substituents at C(4) and C(6) have been briefly examined. One interesting finding was that treatment of the first generation nitrone i.e., 4-(2-hydroxy-2-propyl)-3,4,5,6-tetrahydropyridine 1-oxide, with mercury(II) oxide afforded a novel bicyclic nitrone, 1-oxa-5,6-dehydro-6-aza-bicyclo[3,2


Introduction
2][3] The pyrrolidine-and piperidine-based alkaloids, which are widespread in nature, can be accessed through the cycloaddition reaction of the parent five-1 and six-membered cyclic nitrones, 4 or the secondgeneration aldonitrones 3 and 5, respectively (Scheme 1). 5 The five-membered aldonitrones 3 can be accessed regiospecifically by peracid-induced ring opening of the bicyclic isoxzolidines 2 (nitrone 1alkene cycloaddition products).It has been suggested that the orientation of the nitrogen lone pair in 2 dictates the formation of the N-oxide intermediate on the -face of the nitrone; the subsequent ring opening leads the C(2)-O -to abstract the nearby Hb immediately, thereby leading to the exclusive formation of the aldonitrones 3. 6 However, the proper utilization of the second-generation six-membered aldonitrones 5 has been severely hampered by the lack of selectivity 7 for the oxidation process in 6/5-fused isoxazolidines 4a,b (R 1 =H) (Table in Scheme  1), where the synthetically less important ketonitrones 6a,b are obtained as the major products.

5: Aldonitrone
Scheme 1.While the geometric compulsion makes sure that the 5/5-ring system in 2 remains cis-fused, its corresponding 6/5 ring system in cycloadducts 4 exists in three different conformations/configurations: the cis pair A and B, in rapid equilibrium by chair inversion (CI), and its trans invertomer C, in a relatively slow equilibrium with cis invertomer B by nitrogen inversion process (NI).It has been suggested that the higher activation barrier to nitrogen inversion (G # , ~70 kJ/mol) 7b than the oxidation process does not permit the Curtin-Hammett principle 8 to apply; as such the invertomer ratio reflects the ratio of the products keto-and aldonitrones.While the cis invertomer leads to aldonitrones 5 via intermediate D, the trans invertomer affords the synthetically less important ketonitrones 6 via E. As evident from the Table included in the Scheme 1, the cycloadduct 4a having a cis/trans invertomer ratio of 24:76 afforded the alod-5/keto-6 nitrones in an almost identical ratio of 23:77. 9Likewise, 4b having a cis/trans invertomer ratio of 22:78 affords the aldo-5/keto-6 nitrones in a similar ratio of 35:65.7b Note that the placement of a substituent R 1 at C(5) in cycloadducts 4 would favour the cis invertomer A at the expense of B and the trans invertomer C, both of which places the C(5)R 1 in the unfavourable axial orientation.Exploring this idea, greater percentages of the aldonitrones 5 are obtained from peracid-induced oxidation of the isoxazolidines 4c and 4d. 10,11In our continuing endeavor to obtain the aldonitrones 5 regiospecifically, we intended to place at C(5) in 4 a very bulky substituent that would ascertain the exclusive presence of the invertomer A and exclude the C(5) axially-oriented R 1 in cis B and trans invertomer C. The current work describes our attempt to test the above proposition and confirm the mechanism of the peracid oxidation process.

Results and Discussion
The synthesis of nitrone 12, having a bulky CMe2OH at C(4) is outlined in Scheme 2. Amine 10 upon hydrogen peroxide oxidation in the presence of sodium tungstate 12 in water afforded a mixture of nitrone 12 and hydroxylamine 11 which upon treatment with NaBH4 afforded the hydroxylamine 11 in pure form.The required nitrone 12 was then prepared by mercury(II) oxide oxidation of 11.
Next, we pursued the addition reaction of nitrone 12 with various alkenes.The addition of monosubstituted alkene styrene 13a was found to be stereo-, as well as face-selective; a single adduct 14a was obtained in 80% yield.The 1 H NMR analysis of the crude as well as purified product failed to reveal the presence of any minor isomer.Likewise, the addition reaction of 1hexene 13b also afforded a single isomer 14b.The configuration of the adduct 14a and 14b was based on the sterically favourable exo approach (Scheme 2) of the Ph and Bu groups from the less hindered face (i.e. face) of the nitrone. 11Such a high selectivity is surprising since the C(4)-CMe2OH group, imparting the facial difference, is positioned at the furthest point from the nitrone functionality in 12, yet a surprisingly high selectivity in the addition reactions were observed.
The addition of disubstituted alkenes methyl methacrylate 15 to the nitrone 12 also demonstrated a very high face-and stereoselectivity (Scheme 2); a nonseparable mixture of adducts 16 and 17 in a respective ratio of 95:5 was obtained.The major adduct 16 was obtained via -exo (Me) approach.The stereochemistry is based on the precedent lietrature 2athe parent nitrone 3,4,5,6-tetrahydropyridine 1-oxide is known to give major and minor adducts in a ratio of 96:4 as a result of a favourable secondary orbital interaction via an endo-oriented methoxycarbonyl group in the transition state.

Scheme 2
Since the stereochemistry of the ring fusion dictates the regiochemical outcome of the peracid oxidation process leading to the second-generation nitrones (vide supra) (Scheme 1), we have examined the conformational aspects as well as composition of the nitrogen invertomers (if any) by NMR spectroscopy.The presence of −N−O− moiety in an organic molecule has a distinctive place in conformational analysis; [13][14][15] oxygen being next to nitrogen raises the barrier to nitrogen inversion to such an extent that the individual invertomers can be identified by NMR spectroscopy. 16At ambient temperature, the 1 H and 13 C NMR spectra of these cycloadducts show sharp signals indicating the presence of a single invertomer for each of the compounds 14a, 14b and 16 as well as their corresponding acetate derivatives 18-20 obtained by reacting the former compounds with acetic anhydride in the presence of 4-(N,N-dimethylamino)pyridine (DMAP) (Scheme 3).With respect to the six-membered ring, both cis-fused B and trans-fused C have the bulky CMe2OH(Ac) substituent axially-oriented, while the tertiary group is equatorially oriented in cisfused A (Scheme 3).As such the major cycloadducts 14a, 14b and 16a, as well as their acetates 18-20, are expected to remain exclusively in the invertomeric form of cis-fused A. Note that for compound 4a, the parent 6/5 fused bicyclic isoxazolidine, a cis/trans ratio of 24:76 translates into a G o value (determined at -50 °C) of 2.11 kJ mol -1 favoring the 4a-trans-fused invertomer, while for a cis/trans ratio of 22:78 for cycloadduct 4b, G o value (determined at +25 °C) becomes 3.13 kJ mol -1 (Scheme 1).t Butyl group is well known to have a conformational enthalpy (H o ) difference of 21 kJ mol -1 .Comparing cis-fused A of 14a with its trans-fused C, the bulky tertiary substituent CMe2OH (akin to a t butyl group) at C( 5) is expected to destabilize the latter invertomer by an approximate H o of 21 kJ mol -1 , thereby implying an overall free energy (G o ) advantage of about 18 kJ mol -1 (i.e.21-3.13) for the cis-invertomer.(Note that the entropy difference (S o ) between the two invertomeric forms is assumed to be zero since both the invertomers remain as dl-pairs and have no axis of rotation).Such an astronomical energy difference would predict the complete absence of the trans invertomer as far as NMR detection limit is concerned.That the stable invertomers have the configuration of cis-fused A as depicted in Scheme 3 get further credence from 1 H NMR spectroscopy.While the C(3a)H is equatorially oriented in cis-fused A, it is axially oriented in trans-fused C. The equatorially and axially oriented protons are known 11 to appear at the chemical shift values of 3.8 and 3.3 ppm, respectively; the observed chemical shifts of ~3.8 for the current compounds thereby ascertain the equatorial orientation of the C(3a)H in the exclusive invertomer cis-fused A. Further credence to the conformational assignments came from NOESY experiment.Based on COSY correlation, the signals of Ha, Hb, Hc, Hd and He of 14a (Scheme 3) were found to appear at 3.25, 2.83, 5.40, 2.75, 2.03 ppm, respectively.A strong NOE peak was observed between the protons Hb and Hc as a result of their proximity, possible only in the conformer A.

Exclusive
Assertion of the cis fusion of the ring juncture predicts that the synthesis of the desired second-generation aldonitrones regiospecifically may be achieved by the peracid oxidation process mentioned earlier.To our relief and delight, the isoxazolidines 18-20, on treatment with m-chloroperbenzoic acid (MCPBA) gave the aldonitrones 21-23 exclusively and in almost quantitative yields (Scheme 3).This is the first time a series of 6/5-fused isoxazolidines have been shown to generate the synthetically important aldonitrones regiospecifically.
The peracid oxidation was also carried out in protic solvent ethanol in the hope that it will be able to intercept the intermediate B to obtain its protonated species C which would then generate both the aldo-and ketonitrones by general base catalysed abstraction of the proton Ha or Hb and Hc, respectively (Scheme 4).The oxidation of 20 with MCPBA in methanol did indeed generate two nitrones 23 and 24 in a respective ratio of 80:20.While the general base catalyzed proton abstraction would favour the formation of more substituted ketonitrone 24, its formation as a minor isomer certifies a certain degree of concertedness as depicted in intermediate A as well as a competitive abstraction of proton Hb by RO -in B versus the protonation leading to C. The nitrones are readily identified by the 1

Scheme 4
Next, we explored the cycloaddition reaction of the second-generation nitrone 21 with the alkene 13; a nonseparable mixture of three adducts in a respective ratio of 89:8:~1) was obtained in 91% yield (Scheme 5).The addition was thus found to be highly face selective.The stereochemistry of the major adduct was based on the approach of the alkene from the -face of the nitrone to give C(3a),C(7)-trans substituted adduct 25; in the -face approach, the CO2Me group is expected to experience severe steric crowding in the transition state.The face selectivity is thus dictated by the steric influence of the substituent at C(6) so as to force the alkene to approach from the -face of the nitrone whereby the smaller Hs at the unsubstituted end of the alkene are in a better position to negotiate with the steric encumbrance of C(4) substituent.The adduct 25 is expected to be equilibrating between the two invertomers in both of which the bulky tertiary substituent is placed at equatorial orientation.The 1 H as well as 13 C NMR spectra at ambient temperatures did indeed reveal broad signals.
For the addition reaction of styrene 13a with nitrone 21, a mixture of isomers 26 and 27 was obtained in a respective ratio of 1:3; the face selectivity is thus dictated by the steric influence of the substituent at C(4) so as to force the alkene to a preferable approach from the -face of the nitrone.The endo-oriented H of styrene will have very little discomfort in compare to the endooriented carbomethoxy as far as the steric encumbrance of the oriented substituent at C( 6) is concerned.The stereochemical analyses thus revealed that the mono-13a and disubstituted 15 alkenes prefer to approach the and -face of the nitrone, respectively, and the experimental findings are rationalized in terms of the transition state structures depicted in Scheme 5.The stereochemistry of the addition reaction was confirmed by chemical conversion of 27 into the ring opened product 28 by cleaving the N-O bond of the cycloadducts with zinc/acetic acid.The NMR spectra of the amine 28 (C26H35NO4), obtained from adduct 27, confirmed its symmetric nature; as expected the 13 C NMR spectrum revealed the presence of 13 carbon signals.The two benzylic protons appeared identical as displayed by a single signal at 4.96-4.94(2H, m); even the two phenyl rings appeared identical as displayed by three types of proton at 7.14 (4H, d, J 7.3 Hz), 7.24 (4H, t, J 7.3 Hz), 7.14 (2H, t, J 7. 3 Hz).
The very idea of having a 4-hydroxymethyl substituent in the current cyclic nitrone was our desire to synthesize a nitrone with an unusual bicyclic system as shown in Scheme 6.To our surprise, while the mercury(II) oxide oxidation of 11 in protic solvent ethanol afforded cleanly the monocyclic nitrone 12, the oxidation in aprotic solvent chloroform gave the novel bicyclic nitrone 29 in almost quantitative yield.The polar functionality of nitrone 12 is strongly solvated in ethanol; as a result, the internal aminalization of the nitrone moiety to the N-hydroxy compound 30 is discouraged.In an aprotic solvent, further oxidation of the intermediate 30 led to the bicyclic nitrone 29.Work is in progress on the cycloaddition reactions of this type of nitrone(s) with the aim of constructing and elaborating the unusal bicyclic system found in a novel inhibitor of tyrosyl tRNA synthetase. 17The study has confirmed the mechanism of the peracid induced ring opening of the isoxazolidine, and led to the synthetically important second-generation cyclic aldonitrones, for the first time, with a complete regioselectivity.The bulkier tertiary substituent at C(5) in the cycloadducts has, to our advantage, frozen the invertomer exclusively in the cis-fused form and thus led to the observed regioselectivity.The synthesis of the novel bicyclic nitrone 29 has paved the way to study its cycloaddition reactions to incorporate and elaborate this interesting and unusal bicyclic system.

Experimental Section
General.Elemental analysis was carried out on a Perkin Elmer Elemental Analyzer Series 11 Model 2400.IR spectra were recorded on a Perkin Elmer 16F PC FTI.R spectrometer. 1 H and 13 C NMR spectra were measured in CDCl3 at +25°C using TMS as internal standard on a JEOL LA 500 MHz spectrometer.Mass spectra were recorded on a GC/MS system (Agilent Technologies, 6890N).Silica gel chromatographic separations were performed with Silica gel 100 from Fluka Chemie AG (Buchs, Switzerland).4-methoxycarbonylpiperidine 7, 1-hexene, styrene, methyl methacrylate, m-chloroperbenzoic acid, from Fluka Chemie AG (Buchs, Switzerland) were used as received.All solvents were of reagent grade.Dichloromethane was passed through alumina before use.All reactions were carried out under N2.

N-Benzyl-4-(2-hydroxy-2-propyl)piperidine (9).
To a stirring solution of amine 8 (10 g, 42 mmol) in THF (50 mL) at 0 °C was added dropwise a 3M solution of methyl magnesium bromide (30 mL, 90 mmol).The mixture was then stirred at room temperature for 6 h.After addition of a saturated solution of ammonium chloride (20 mL), the aqueous layer was extracted with CH2Cl2 (4×30 mL).The combined organic layers was dried (Na2SO4), concentrated and the residual liquid was purified by chromatography over silica using 1:1 ether/methanol mixture as eluant to give the aminoalcohol 9 as a white solid (7.3   (10).Protected aminoalcohol 9 (10 g, 42 mmol) in ethanol (50 mL) containing Pd/C (1 g) was hydrogenated at 20 °C under 50 psi pressure for 3 h.The reaction mixture was filtered over celite and washed with ethanol (2×10 mL).After removal of the solvent, the residue was crystallized from acetone to give pure aminoalcohol 10 as a white solid (

2-Phenyl-5-(2-hydroxy-2-propyl)hexahydro-2H-isoxazolo[2,3-a]pyridine (14a).
A solution of nitrone 12 (10 mmol) in EtOH (40 mL) containing styrene 13a (5 mL) was heated at 90°C for 4 h under N2 in a closed vessel.After removal of the solvent and excess alkene the residual crude mixture was purified by chromatography over silica using 85:15 ether/methanol as eluant to give a single adduct 14a as a white solid (2.0 g, 80%). 1   (16 and 17).A solution of nitrone 12 (10 mmol) in EtOH (40 mL) containing methyl methacrylate (15) (6 mL) was heated at 50°C for 3 h under N2 in a closed vessel.After removal of the solvent and excess alkene the residual crude mixture was separated by chromatography over silica using 95: 5 DCM/methanol as eluant to give a nonseparable mixture of adducts 16 and 17 in a respective ratio of 95:5 as a colorless liquid (2.1 g, 83%).The presence of the minor adduct was revealed by the presence of a CO2Me singlet at   (N,N-dimethylamino)pyridine] (0.12 g, 1 mmol) at 70 ºC for overnight.After removal of the solvent and excess acetic anhydride, the residual liquid was purified by chromatography over silica gel using 1:1 ether/hexane as eluant to give acetate 18 as a white solid (1.24 g, 95%  (15).Nitrone 21 [prepared by MCPBA oxidation of adduct 18 (1.0 mmol)] in CH2Cl2 (10 mL) was treated with methyl methacrylate (15) (1.0 mL) and the mixture was stirred at 45ºC for overnight.After removal of the solvent and excess alkene, the residual liquid was purified by chromatography over silica gel using 9:1 ether/hexane as eluant to give a nonseparable mixture of three adducts (as indicated by the presence of three CO2Me singlets at 3.77, 3.80 and 3.84 ppm in a respective ratio of 89:8:~1 as a colourless liquid (380 mg, 91%).The major adduct was assigned the stereochemistry of 25. )] in CH2Cl2 (10 mL) was treated with styrene (2.0 mL) and the mixture was stirred at 45ºC for overnight.After removal of the solvent and excess alkene, the residual liquid was separated by chromatography over silica gel using 7:1 ether/hexane as eluant to give the minor isomer 26 as a colorless liquid (90 mg).Continued elution gave a mixture of 26 and 27.Finally, Scheme 1 Scheme 3 Scheme 5.