Ring transformation of oxazoles to fused imidazoles. New synthetic route for 6-methyl-2,3-diphenyl-7,8-dihydroimidazo1,2-bpyridazine and 5-methyl-2,3-diphenyl-6,7-dihydro-5H-pyrrolo1,2-aimidazole, and their perhydrobenzo analogues

摘要

J. CHEM. SOC. PERKIN TRANS. I 1983 Ring Transformation of Oxazoles to Fused Imidazoles. New Synthetic Route for 6-Methyl-2,3-diphenyl-7,8-dihydroimidazo1,2-bpyridazineand 5-Methyl-2,3-diphenyl-6,7-dihydro-5H-pyrrolo1,2-aimidazole, and Their Perhydrobenzo Analogues Tadashi Sasaki," Masatomi Ohno, and Eikoh Ito Institute of Applied Organic Chemistry, Faculty of Engineering, Nag0ya University, Furo-cho, Chikusa- ku, Nag0 ya 464, Japan Fused imidazoles were synthesized by an intramolecular ring transformation of y-keto-oxazoles with hydrazine and by an intramolecular dehydration of y-amino-oxazoles. y-Keto-oxazoles (4) and (6) were prepared by the reaction of lithiated 2-methyl-4,5-diphenyloxazole(1) with the methyl enol ether of an a-bromo ketone, followed by hydrolysis.y-Keto-oxazoles (4) and (6) gave the 7,8-dihydroimidazo- I ,2-b pyridazine (7) and the 6,7,8,9,9a,1 0-hexahydroirnidazolr2-bcinnoline (8), respectively, on treatment with hydrazine hydrate in acetic acid. The transformed fused imidazoles (7) and (8) were further converted into the corresponding tetrahydroimidazo 1,2-b pyridazine (9) and octahydroimidazo- lr2-bcinnoline (1 0), respectively, by reduction with NaBH4. y-Amino-oxazoles (1 1 ) and (1 2) were prepared from (4) and (6), respectively, on reduction with NaBH,CN in the presence of ammonium acetate. The pyrolysis of (1 1 ) and (1 2) provided the cyclodehydrated 6,7-dihydro-5H-pyrrolol,2-a-imidazole (1 3) and 5,6,7,8,8a,9-hexahydro-4aH-imidazol,2-aindole (1 4),respectively. A variety of ring transformations of heterocycles are known,' but few have been applied to the synthesis of bicyclic hetero- cycles.-f An intramolecular ring transformation strategy could potentially provide an attractive route to a variety of bridgehead nitrogen heterocycles.We have recently applied this strategy to the synthesis of fused a~oles.~,~ This paper deals with the synthesis of fused imidazoles such as 7,8- dihydroimidazol,2-hpyridazines e.g. (7) and (S) and 6,7- dihydro-5H-pyrrolol,2-aimidazolese.g. (1 3) and (14), in which the intramolecular replacement of the oxygen in an oxazole ring with nitrogen is the key step (see Scheme). Imidazopyridazines are obtained by the reaction of y-keto- oxazoles with hydrazine. Pyrroloimidazoles are obtained by intramolecular dehydration of y-amino-oxazoles. Results and Discussion Preparation of Oxazokes (4) and (6).--The preparation of the y-keto-oxazoles (4) and (6) was envisaged to occur via coupling of the bromide (2) with an enolate anion.However, compound (2) could not be obtained by bromination of the 2- methyloxazole (1) or the lithium derivative of the 2-methyl- *q5 oxazoIe.6 An alternative method involving the reaction of lithiated (I) with a suitable eiectrophile was more successful. The methyl enol ether of the a-bromo ketone solved the problem. Treatment of lithiated (1) with 2-methoxyallyl bromide (3) 'in tetrahydrofuran (THF) at -78 "C, followed by hydrolysis of the resulting methyl enol ether with IM HC1, provided the ketone (4)in 59 yield after purification on a silica gel column and subsequent recrystallisation.Similarly, 2-(2-oxocyclohexyl)methyl-4,5-diphenyloxazole(6) was pre- pared in 96 yield by treatment of lithiated (1) with 3-bromo-t One is the synthesis of cycloalkanoapyrroles by the dehydration of furylalkylamines: J. M. Patterson, J. Brasch, and P. Drenchko, J. Org. Chem., 1962, 27, 1652; F. Sorm and Z. Arnold, Collect. Czech. Chem. Comtniiti., 1947, 12,467. The other is the synthesis of pyrrolizines by the similar dehydration of 2-(3-aminoalkyl)tetra- hydrofurans: A. A. Ponomarev and I. M. Skvortsov, Zlt. Ohshch. Khim., 1962, 32, 97 (Cliem. Abstr., 1962, 57, 12409). $ Although Lipshutz recommended the reaction temperature to be -100 "C, we found that we could carry out the lithiation at -78 "C.2-methoxycyclohexene (5).8 The i.r. spectra showed a strong carbonyl absorption at 1 720 cm-' for (4) and 1 710 cm-' for (6), and the 'H n.m.r. spectra exhibited methyl and methylene proton signals for (4) at 6 2.55 and 2.95-3.20, and complicated cyclohexane ring and methylene proton signals for (6) at 6 1.20-3.65. Transformation of (4) and (6) into Fused Imidazo1es.-Reaction of the y-keto-oxazole (4) with hydrazine hydrate (5-fold excess) in acetic acid at room temperature for 3 days gave the 7,8-dihydroimidazol,2-bpyridazine (7) in 80 yield. Similarly, (6) was transformed into the corresponding tricyclic hexahydroimidazol ,2-bcinnoline (8) in 76 yield.However, higher temperatures with shorter reaction times lowered the yields of both (7) and (8). The structures of (7) and (8) were based on elemental analyses and spectral data: the i.r. spectra of (7) and (8) showed no carbonyl absorption and the 'H n.m.r. spectra exhibited methyl and methylene proton signals for (7) at 6 2.12 and 2.30-3.25, and compli- cated ring proton signals for (8) at 6 1.20-3.50. The mechan- ism for these transformations is considered to be that pro- posed previously, with cyclisation of the hydrazones to give the spiro derivatives (1 5 a and b), which collapse to the ketones (1 6 a and b). The final cyclisation of (16 a and b) provides the fused imidazoles (7) and (8) respectively equation (l).Compounds (7) and (8) underwent smooth reduction with NaBH, to give the corresponding tetrahydro derivative (9) and octahydro derivative (1 0) in 95 and 94 yield, respectively. The amino function in these products was indicated by the strong absorption at 3 180 cm-I for (9) and 3 195 cm-I for (10) in the i.r. spectra and the appearance of D20-exchangeable N-H signals at 6 4.00 for (9) and 3.80 for (10) in the 'Hn.m.r. spectra. The structure of (9) was further supported by a methyl doublet (J6.8 Hz) at 6 1.20 and the chemical shift of the ring protons at 6 1.50-2.40 (2 H, m, 7-H2)and 2.80-3.50 (3 H, m, 6-H and 8-H2), being in good agreement with those of the 5,6,7,8-tetrahydroimidazo1,2-bpyridazine ring system re-ported in the literature.' The I3C n.m.r.spectrum of (10) showed two doublets at 6, 59.0 and 38.1 p.p.m. assignable to C-5a and C-9a, respectively, and five triplets due to C-6, -7, -8, -9, and -10. No other minor signals were observed, indicating that the product is a single stereoisomer. We tentatively 3028 J. CHEM. SOC. PERKIN TRANS. I 1983 Ph NaBH4 ~ Ph "3 HN N7H4OAci. Br 1 Ph 'O))CH2Li'tNJ Ph NaBHL (1) R=H (2) R=Br (6 1 Scheme. H . 1 2a; R =H, R =CH3 b; R' R2= C H2amp; a; R' = H, R2= CH3 b; R'R' = CHamp; (4) +(15a) (16a) ---t (7)(6) -+ (15b) -+ (16b) +(8) assigned the stereochemistry of (10) as trans because axial An alternative, two-step synthetic route was considered; first, approach of hydride ion is commonly accepted in the reduc- conversion of the y-keto-oxazole into a y-amino-oxazole and tion of cyclohexanone.1deg; second, cyclodehydration to a 6,7-dihydro-SH-pyrrolo 1,2-a-Attempted aromatization of (7) and (8) with 2,3-dichloro- imidazole. The reduction of the oxime of the y-keto-oxazole 5,6-dicyano-l,4-benzoquinone(DDQ) in refluxing methanol with LiAlH., in refluxing THF was examined first but gave a led to no reaction.mixture of inseparable products. The desired conversion was Treatment of (4) and (6) with ammonium acetate in re- achieved by treating compounds (4) and (6) with NaBH3CN fluxing acetic acid did not provide the desired pyrrolol72-u- in the presence of ammonium acetate in methanol." After imidazoles, but resulted in the recovery of the starting material. purification on an alumina column, y-amino-oxazoles (1 1) J.CHEM. SOC. PERKIN TRANS. I 1983 and (12) were obtained in 49 and 63 yield, respectively. Their structures were indicated by the appearance of broad N-H absorptions at 3 600-3 200 cm-' for (11) and 3 500-3 200 cm-I for (12)in the i.r. spectra and by DzO-exchange- able N-H signals at 6 1.77 for (1 1) and 1.88 for (12)in the 'H n.m.r. spectra. The 13C n.m.r. spectrum of (12)revealed two pairs of doublets at 6c 44.9 and 54.6 p.p.m. and at 6c 40.3 and 49.1 p.p.m., suggesting (12) to be a mixture of stereo- isomers. The former pair, observed at lower field, was char- acterized as due to C-1 and C-2of the cyclohexane ring of the trans isomer and the latter as those of the cis isomer.* Thus the ratio of cis to trans isomers of (12)was determined by the integral ratio to be 1 :4, but the isomers could not be separ- ated by column chromatography.For the cyclodehydration of y-amino-oxazoles (1 1) and (1 2) to the dihydropyrroloimidazole (1 3) and the hexahydroimid- azoindole (14), respectively, we examined various acidic conditions including treatment with (a) conc. HzS04 at room temperature, (b) PPA (polyphosphoric acid) at 150 "C, (c) Pz05 in refluxing benzene, and (d) PzOsCH3S03H at 70 "C," but these reactions resulted in the recovery of the starting material. No reaction was also observed under the anionic conditions using BunLi in refluxing THF.However, pyroly- sis accomplished the desired cyclodehydration. Crude (1 l), after the reductive amination, was directly heated at 280deg;C under reduced pressure (5 mmHg) to give the cyclodehydrated dihydropyrroloimidazole (1 3) in 46 overall yield from (4) after purification on an alumina column. Under the same con- ditions, crude (12) was transformed into the tricyclic hexa- hydroimidazoindole (14)in 23 overall yield from (6). The structures of (1 3) and (14)were determined from elemental and spectral analyses: the 'H n.m.r. spectrum of (13) showed ring proton signals at 6 1.80-3.20 (4 H, m, 6-and 7-H2) and 4.10-4.70 (1 H, m, 5-H). These chemical shifts are in accord with those of the 6,7-dihydro-SH-pyrrolo 1 ,2-aimidazole ring system reported in the 1iterat~re.I~ The 'H n.m.r.spectrum of (14)exhibited two C-4a proton signals due to cis and trans isomers at 6 3.90-4.40 and 3.20-3.90, respectively. The I3C n.m.r. spectrum of (14)revealed, as expected, two pairs of doublets due to C-4a and C-8a of each isomer at tic 64.1 and 50.8 p.p.m. (trans) and at 6c 56.4 and 40.1 p.p.m. (cis). The intensity of these signals showed (14)to be a 1 : 5 mixture of cis and trans isomers, although the separation of these two isomers was not performed by column chromatography. The lack of appreciable change in the cis : trans ratio from (12)to (14)suggests that the cyclodehydration was not substantially affected by the stereochemistry of (12). Experimental M.p.s were measured with a Yanagimoto micromelting point apparatus and are uncorrected.1.r. spectra were obtained on a JASCO-IRA-1 spectrometer. 'H N.m.r. and I3C n.m.r. spectra were recorded on a JEOL JMN-C-60HL instrument at 60 MHz and a JEOL-FX-60 FT spectrometer at 15.04 MHz, res- pectively. Chemical shifts are reported in p.p.m. (6) relative to Me4Si as internal standard. Microanalyses were performed with a Perkin-Elmer 240B elemental analyser. Pyrolysis was carried out with Sibata Glass Tube Oven GTO-250. 2-(3-Oxobutyf)-4,5-diphenyloxazole(4).-To a stirred solu- tion of (1) (3.0g, 12.8mmol) in dry THF (40ml) was added *The 13C n.m.r. chemical shifts cf the ring carbons appear at higher field in cis-l,2-disubstituted cyclohexanes than in the trans isomers: H.B. Kagan, ' Stereochemistry,' Georg Thieme Publishers, Stuttgart, 1977, vol. 1, pp. 105-108. a solution of BunLi in hexane (1.6~; 10 ml, 16.0 mmol) dropwise during 30 min at -78 "C under a current of Nz. After the resulting deep orange-red suspension had been stirred at -78 "C for an additional 15 min, compound (3) '(2.5ml, 19 mmol) was added. The resulting mixture was slowly warmed to room temperature and quenched with phosphate buffer (70 ml; pH 7). The organic layer was extracted with CHC13 (3 x 50 ml). The combined extracts were washed in turn with 1~ HCI (100 ml) and water (3 x 100 ml), and dried over MgS0,. Removal of the solvent yielded an orange oil which was chromatographed on a silica gel column AcOEt-n- hexane (1 : 2) to give crude (4).Subsequent recrystallisation from ethanol gave pure ketone (4) (2.2 g, 59), m.p. 69-71 "C (Found: C, 78.4; H, 5.95; N, 4.85. Cl9Hl7NOZrequires C, 78.33;H, 5.88; N, 4.81); vmax.(KBr) 3 050, 2930, 1 720, 1 600, 1 585, 1 500, 1 440, and 1 420 cm-'; 6H (CDCI,) 2.25 (3H, s, CH3),2.95-3.20 (4 H, m, 2 x CHz), and 7.20-7.80 (10 H, m, Ar). 2-(2-Oxocycfohexyl)methyl-4,5-diphenyloxazole(6).-By the same procedure as employed for the preparation of (4), compound (6)was obtained from (1) and (9.'Purification was performed on a silica gel column using CHC13 as eluant: 96 yield; yellow oil (Found: C, 79.6;H, 6.5;N, 4.25.C22H21N0z requires C, 79.73; H, 6.39;N, 4.23); vmX. (film) 3 050, 2 940, 2 860, 1 710, 1 600, 1 565, 1 500, and 1 445 cm-'; 6H (CDC13) 1.20-3.65 (11 H, m), and 7.20-7.80 (10 H, m, Ar).6-Methyf-2,3-diphenyl-7,8-dihydroimidazo1,2-blpyridazine (7).-A solution of compound (4) (1.5 g, 5.2 mmol) and hydra- zine hydrate (1.3g, 26 mmol) in acetic acid (20ml) was stirred at room temperature for 3 d. Then the resulting mixture was poured into saturated brine (200ml). Filtration of the precipi- tates gave compound (7) (1.2 g, 80). An analytical sample was obtained by recrystallisation from ethanol, m.p. 173-176 "C (Found: C,79.4;H, 6.05; N, 14.55.C19H17N3requires C, 79.41;H, 5.96;N, 14.62); vmax. (KBr) 3 070, 2 960, 1 650, 1 605, 1 530, 1445, 1 385, and 1 350 cm-'; 6" (CDC13) 2.12 (3 H, s, CH,), 2.30-3.25 (4 H, m, 7-and 8-Hz), and 7.10-7.70 (10H, m, Ar).2,3- Diphenyl-6,7,8,9,9a, 1 0-hexahydroimidazo 1,2-bJcinno-line @).--In a similar way to (7),compound (8)was obtained from (6)in 67 yield, m.p. 184-187 "C (from EtOH) (Found: C, 80.45;H, 6.5;N, 12.65. CZ2HZ1N3requires C, 80.70;H, 6.46;N, 12.83); vmX. (KBr) 3 100,2 945,2 850,l 620,l 600, 1 535, and 1440 cm-l; 6H(CDC13) 1.20-3.50 (11 H, m) and 7.10-7.70 (10 H, m, Ar). 6-Methyl-2,3-diphenyl-5,6,7,8-tetrahydroimidazo1,2-b-pyridazine (9).-To a stirred solution of NaBH, (280 mg, 7.4 mmol) in dry ethanol (7ml) was added dropwise a solution of compound (7) (210 mg, 0.7 mmol) in dry ethanol (20ml). The mixture was stirred for an additional 20 h at room temperature and the unchanged NaBH4 was then decomposed with 6~ HCI solution (pH 2).The mixture was then made basic with 10 NaOH solution (pH 9)and evaporated to dryness under reduced pressure, and the residue was extracted with hot CHCl, (3 x 10 ml). The combined extracts were dried over MgS0, and concentrated to give crude (9),which was purified on a silica gel column using CHC1,-EtOH (30 : 1) as eluant to give pure compound (9) (200mg, 9573,m.p. 126-129 "C (Found: C,79.15;H, 6.75;N,14.15.C19H19N3requires C, 78.86;H, 6.61;N,14.52); vmaX.(KBr) 3 180, 3 000, 2 980, 2 940, 1 600, 1 505, 1 440,and 1 440 cm-'; SH (CDCI,) 1.20 (3 H, d, J 6.8 Hz, CH,), 1.50-2.40 (2 H, m, 7-Hz), 2.80-3.50 (3 H,m, 6-Hand 8-Hz), 4.00 (1 H,br s, D,O-exchangeable, NH), and 7.05-7.62 (10 H, m, Ar). 3030 2,3-Diphenyl-5,5a,6,7,8,9,9a,10-octahydroimidazo1,2-bcinn-oline (lo).-By the same reduction as above, compound (10) was obtained from (8).Purification was performed on a silica gel column using CHCI, as eluant: 94 yield, m.p. 200- 201 "C (Found: C, 80.45; H, 7.05; N, 12.7. CZ2H2,N3 requires C, 80.21; H, 7.04; N, 12.76); vmnx,(KBr) 3 195, 3 060,2 925, 2 850, 1 600, 1 510, and 1 440 cm-'; amp;, (CDCI,) 1.00-3.50 (12 H, m), 3.80 (1 H, br s, DzO-exchangeable, NH), and 7.10-7.50 (10 H, m, Ar); 6c (CDCI,) 24.9 (t), 25.4 (t), 30.0 (t), 30.2 (t), 32.1 (t), 38.1 (d, C-9a), 59.0 (d, C-ja), 125.1 (s), 126.1 (d), 126.9 (d), 127.7 (d), 128.0 (d), 128.4 (d), 129.9 (s), 130.5 (d), 134.6 (s), 134.9 (s), and 139.6 p.p.m. (s). 2-(3-Aminobutyl)-4,5-diphenyloxazole(1 1).-A solution of the ketone (4) (870 mg, 3 mmol), NaBH,CN (1.3 g, 21 mmol), and ammonium acetate (2.3 g, 30 mmol) in dry methanol (30 ml) was heated under reflux for 4 h.The resulting mixture was acidified with conc. HCl solution (pH 2) and then basified with 20 NaOH solution. After extraction with CHCl, (3 x 20 ml), the combined extracts were dried over MgS04. Removal of the solvent yielded crude (11) as a yellow oil, which was purified on an alumina column CHCl,-EtOH (30: l) to give the amine (11) (430 mg, 49) (Found: C, 78.15; H, 6.9; N, 9.5. C19H20N20 requires C, 78.05; H, 6.90; N, 9.58); vnlax.(film) 3 600-3 200 br, 3 050,2 960, 1 600, 1 570, 1 500, and 1 445 cm-'; 8H (CDCI3) 1.13 (3 H, d, J 6.7 Hz, CH3), 1.77 (2 H, br s, D20-exchangeable NH,), 1.87 (2 H, q, J7.5 Hz, CH,), 2.70-3.20 (3 H, ni), and 7.20-7.80 (10 H, m,Ar).2-(2-Aminocyclohcxyl)methyl-4,5-diphenyloxazole(1 2).-By the same reductive amination as above, compound (12) was obtained from (6): 63 yield; yellow oil (Found: C, 79.4; H, 7.7; N,8.1. CZ2H2,N2O requires C, 79.48; H, 7.28; N,8.43); vmax.(film) 3 500-3 200br, 3 040, 2 920, 2 840, 1 600, 1 570, 1 500, and 1 445 cm-'; amp; (CDC1,) 0.90-3.50 (12 H, m), 1.88 (2 H, br s, DzO-exchangeable, NH2), and 7.20-7.90 (10 H, m, Ar); 6c (CDCI,) trans: 25.6 (t), 25.9 (t), 31.5 (t), 32.0 (t), 37.1 (t), 44.9 (d, C-1 of cyclohexane), and 54.6 p.p.m. (d, C-2 of cyclohexane); cis: 20.7 (t), 24.7 (t), 26.8 (t), 28.2 (t), 33.3 (t), 40.3 (d, C-1 of cyclohexane), and 49.1 p.p.m.(d, C-2 of cyclohexane). The other complicated aromatic carbon signals were observed at amp; 126.4-162.9 p.p.m. 5-Methyl-2,3-diphenyl-6,7-dihydro-5H-pyrrolol,2-aimid-azole (13).-Crude compound (11) obtained from (4) (1.0 g, 3.4 mmol) was heated at 280 "C under 5 mmHg pressure for 20 min in a glass tube oven with a trap bulb heated at 200 "C. The brown oil trapped in a bulb immediately solidified on J. CHEM. SOC. PERKIN TRANS. 1 1983 being cooled. Purification on an alumina column with CHCIJ as eluant gave compound (13) 430 mg, 46 from (4), m.p. 141-143 "C (Found: C, 83.2; H, 6.75; N,10.05. CI9Hl8N2 requires C, 83.18; H, 6.61; N, 10.21); vmaX.(KBr) 3 040, 2 970, 1 602, 1 535, 1 500, and 1 445 cm-'; 8" (CDCI,) 1.20 (3 H, d, J 6.7 Hz, CH3), 1.80-3.20 (4 H, m, 6- and 7-H2), 4.10-4.70 (1 H, m, 5-H), and 7.05-7.70 (10 H, m, Ar).2,3-Diphenyl-5,6,7,8,8a,9-kexahydro-4aH-imidazo1,2-a-indole (14).-1n the same way as above, compound (14) was obtained by the pyrolysis of crude (12). Purification was per- formed on a silica gel column with AcOEt as eluant: 23 yield from (6), m.p. 160-163 "C (Found: C, 83.9; H, 7.25; N,8.85. C2,H2,N2 requires C, 84.04; H, 7.05; N, 8.91); vnlsX. (KBr) 3 050, 2 930, 2 850, 1 600, 1 535, and 1 445 cm-'; 6" (CDCl,) 0.80-3.20 (11 H, m), 3.20-3.90 (m, 4a-H of trans isomer), 3.90-4.40 (m, 4a-H of cis isomer), and 7.00-7.75 (10 H, m, Ar); Sc (CDC13) trans: 24.4 (t), 25.7 (t), 29.1 (t), 29.8 (t), 30.4 (t), 50.8 (d, C-8a), and 64.1 p.p.m.(d, C-4a); cis: 21.1 (t), 22.0 (t), 26.9 (t), 27.8 (t), 29.1 (t), 40.1 (d, C-8a), and 56.4 p.p.m. (d, C-4a). The other complicated aromatic signals were observed at 6c 125.6-152.9 p.p.m. References 1 H. C. van der Plas, ' Ring Transformation of Heterocycles,' Academic Press, New York, 1973, vols. Iand 2. 2 T. Sasaki, E. Ito, and I. Shimizu, J. Org. Chem., 1982, 47, 2757. 3 T. Sasaki, E. Ito, and 1. Shimizu, Heterocycles, 1982, 19, 2119. 4 D. Davidson, M. Weiss, and M. Jelling, J. Org. Chem., 1937, 2, 328. 5 D. L. Aldous, J. L. Riebsomer, and R. N. Castle, J. Org. Chem., 1960,25, 1151. 6 B. H. Lipshutz and R. W. Hungate, J. Org. Chem., 1981, 46, 1410. 7 R. M. Jacobson, R. A. Raths, and J. H. MacDonald 111, J. Org. Chem., 1977,42,2545. 8 E. W. Garbisch, Jr., J. Org. Chem., 1965, 30, 2109. 9 P. K. Kadaba, B. Stanovnik, and M. Tisler, J. Heterocycl. Chem., 1976, 13, 835. 10 H. 0.House, ' Modern Synthetic Reactions,' W. A. Benjamin, Menlo Park, 1972, pp. 54-70. 11 R. F. Borch, M. D. Bernstein, and H. D. Durst, J. Am. Chenz. SOC.,1971, 93, 2897. 12 D. L. Boger, J. Org. Chem., 1978,43, 2296. 13 1. Antonini, P. Franchetti, and M. Grifantini, J. Heterocycl. Chern., 1976, 13, 11 1 ; C. B. Kanner and U. K. Pandit, Tetra-hedron, 1981, 37, 3519. Received 1st June 1983; Paper 31879