Iodine(III)-Mediated Ring Contraction Reactions: Synthesis of Oxygen- and Nitrogen-Substituted Indanes

Abstract

The synthesis of oxygen- and nitrogen-substituted indanes was successfully performed by iodine(III)-mediated ring contraction of 1,2-dihydronaphthalenes. Acetoxy and benzoyloxy alkenes afforded indanes in 60-71% yield, irrespective of their position on aromatic ring. Similarly, the nitrogen containing substrates protected with 9-fluorenylmethyloxycarbonyl (Fmoc) and benzoyl (Bz) groups smoothly undergoes ring contraction giving indanes in 64-77% yield. The tosyl-protected substrate resulted only in addition products.

Keywords:
indane; hypervalent iodine; ring contraction; 1,2-dihydronaphthalenes

Introduction

Indane skeleton occurs in many biologically active natural products^1,2 and pharmaceuticals, constituting an important target in organic synthesis and in drug discovery.³ Numerous synthetic methodologies have been developed to construct indane core including cyclization,^4-6 cycloaddition,^7-9 and rearrangements.^10-13 Indanes substituted in the aromatic ring by nitrogen^14-18 or by oxygen^2,19-22 are present in compounds with promising biological activities. Hypervalent iodine reagents play a substantial role in chemical synthesis, promoting efficiently carbon-carbon bond formations, rearrangements and functional group interconversions, including asymmetric versions for many reactions.^23-28 The iodine(III)-mediated ring contraction of 1,2-dihydronaphthalenes gives functionalized indanes (Scheme 1, 1a).^13,29 This transformation was successfully employed in total syntheses, such as (+)-mutisianthol,³⁰ (±)-indatraline,²⁹ and (+)-trans-trikentrin A.³¹ Although the reactivity of several substrates was investigated under many conditions, the tolerance for substitution at aromatic ring was not high. For example, methoxy-substituted substrates furnished the desired indanes in low yield and additionally the main product were those related to the addition of solvent, as exemplified by the behavior of compound 1b. Motivated by the importance of oxygen- and nitrogen-substituted indanes, we herein show the ring contraction of additional 1,2-dihydronaphthalenes with iodine(III).

Scheme 1
Oxidation of 1,2-dihydronaphthalenes 1a-b with iodine(III).

Results and Discussion

The preparation of the required substrates was performed as described in the following paragraphs. The protected amine tetralones (7c-f) were prepared by classical transformations (Table 1). Amine tetralone 5 in the presence of benzoyl chloride (BzCl), triethylamine (Et₃N) and dichloromethane (DCM) as solvent gave benzoyl protected amine 7cin 89% yield (entry 1).³² The tosyl protected amine tetralone 7d was obtained in 96% yield in pyridine with tosyl chloride (p-TsCl) (entry 2).³³ In a similar manner, base labile Fmoc protected amine 7e was formed in 98% yield with 9-fluorenylmethyloxycarbonyl chloride (Fmoc-Cl) and pyridine in DCM (entry 3).³⁴ The acetylation of tetralone 6 was achieved using 4-dimethylaminopyridine (DMAP), acetic anhydride (Ac₂O) in Et₃N giving acetyl protected ketone 7f in 97% yield (entry 4).^13,35

Table 1
Synthesis of protected amines tetralones 7c-f

Several cyclic olefinic substrates were synthesized via reduction/dehydration protocol. The commercially available hydroxy ketone 8g was reduced with NaBH₄ in MeOH to the corresponding alcohol as a white solid. Dehydration of alcohol at 130 ºC in the presence of phosphoric acid (H₃PO₄) gave alkene product 1g in 76% yield (Table 2, entry 1).³⁵ The methoxy functionalized ketones 8h and 8bwere also transformed into cyclic alkenes 1h and 1b in 93 and 76% yields, respectively (entries 2 and 3).^13,36,37 The benzoyl, tosyl, Fmoc and acetyl protected amino ketones (7c-f) were smoothly converted into desired olefinic substrates (1c-f) in good yields (entries 4-7).^13,38

Table 2
Preparation of dihydronaphthalenes 1b-h via reduction/dehydration

The acetylation of phenol 1g was accomplished with DMAP, Ac₂O in Et₃N as solvent,¹³ leading to protected alkene 9g in 96% yields (Table 3, entry 1). Similarly, phenol 1g was also protected with benzoyl group in 94% yield using BzCl and Et₃N as a base (entry 2).³² Methyl ethers (1h and 1b)were first demethylated using sodium ethanethiolate (generated in situ) in dimethylformamide (DMF) at 140 ºC giving the corresponding phenol,^39,40 which were acetylated under standard conditions (entries 3 and 4).

Table 3
Preparation of O-substituted dihydronaphthalenes

Iodine(III) is known to act as single-electron-transfer (SET) for methoxy-substituted aromatic compounds forming reactive cation radical intermediates.^41,42 The generation of these cation radicals could be a possible reason of rearrangement failure in substrates like 1b, due to their susceptibility towards nucleophilic attack and other side reactions. Thus, we consider that the ring contraction could take place with an acetyl group instead of a methoxy making it useful in the synthesis of oxygen-substituted indanes. Several reaction conditions were tested for the oxidation of alkene 9g with PhI(OH)OTs (HTIB) (Table 4). The desired ring contraction product 2g was successfully obtained in 47% yield using trifluoroethanol (TFE)/DCM (1:4) as solvent (entry 1). Using pure TFE, the yield of acetal 2g increased to 66% (entry 2). The deprotection of substrate 9g into phenol 1g was observed when MeOH was used as solvent (entry 3).

Table 4
Reactions of acetyl protected alkene 9g with HTIB

With the optimized conditions, ring contraction was successfully carried out in other oxygenated substrates (Table 5). Benzoyl protected alkene 9gg gave acetal 2gg in 71% yield (entry 1). Acetyl protected alkene 9h smoothly afforded product 2h in 60% yield (entry 2). Acetyl alkene 9b gave acetal 2b in 65% yield (entry 3).

Table 5
Reactions of oxygenated alkenes 9b, gg and 9h with 1.2 equiv of HTIB

Oxidation with HTIB was also studied in amine protected alkenes (Table 6). Benzoyl protected alkene 1cexperienced ring contraction giving acetal product 3c in 77% yield in MeOH/DCM (8:1) as solvent (entry 1). DCM was added to solubilize substrate 1c and to increase the rate of reaction. When the reaction was tried in pure MeOH, indane 3c was obtained in 51% yield and took 1 h to consume all starting material (entry 2). The protecting group tolerance was further investigated with Fmoc protected alkene 1e and the anticipated ring contraction product 3e was isolated in 64% yield (entry 3).

Table 6
Reaction of 7-amine alkenes with 1.2 equiv HTIB at 0 oC

The reaction of tosyl protected 7-amine alkene 1d with HTIB was also investigated under different reaction conditions (Table 7). Tosyl amide could have a facilitating effect on ring contraction by increasing electronic density on migrating carbon 4a.¹³ However, only the formation of addition products trans-4d and cis-4d were observed in MeOH and in DCM/MeOH as solvents (entries 1 and 2). Fluorinated solvents like TFE and hexafluoroisopropanol (HFIP) proved to be ineffective under the conditions tested (entries 3 and 4).

Table 7
Reaction of tosylamide 1d with HTIB

Oxidation in acetyl protected 6-amine alkene 1f was explored. In this case, amide substituent meta to migrating carbon would decrease the electronic density by inductive effect of nitrogen atom and does not increase directly the electronic density at migrating carbon by mesomeric effect.¹³ Several conditions were tested for ring contraction of alkene 1f (Table 8). Slow reaction and complex mixtures were observed for substrate 1f using HTIB either in MeOH or in DCM/MeOH at different temperatures (entries 1-3). Fluorinated solvents, such as TFE and HFIP/DCM, also did not provide ring contraction product (entries 4 and 5).

Table 8
Reaction of 6-acetamide 1f with HTIB

Conclusions

In conclusion, the ring contraction of 1,2-dihydronaphthalenes using HTIB was expanded to substrates bearing oxygen and nitrogen substituents in the aromatic ring. Oxidative rearrangement was successfully carried out in oxygenated substrates independent on their position on aromatic ring. Acetoxy and benzoyloxy alkenes afforded indanes in 60-71% yield. The N-protecting groups Fmoc and Bz are stable under the reaction conditions giving indanes in 64-77% yield. The Ts-protected substrate gave only addition products. The results showed the tolerance of protecting groups in ring contraction reaction mediated by HTIB.

Experimental

All commercially available reagents were used without further purification unless otherwise noted. All solvents used for reactions and chromatography were dried and purified by standard methods.⁴³ Thin-layer chromatography (TLC) analyses were performed using silica gel 60F 254 precoated plates, with detection by UV-absorption (254 nm) and by spraying with p-anisaldehyde and phosphomolybdic acid solutions followed by charring at ca. 150 °C for visualization. Flash column chromatography was performed using silica gel 200-400 Mesh. All nuclear magnetic resonance (NMR) analyses were recorded using CDCl₃ as solvent and tetramethylsilane (TMS) as internal standard. Chemical shifts are reported in ppm downfield from TMS with reference to internal solvent.

Preparation of substrates

N-(7,8-Dihydronaphthalen-2-yl)benzamide (1c)

To a stirred solution of ketone 5 (0.677 g, 4.20 mmol) and Et₃N (0.65 mL, 0.467 g, 4.62 mmol) in DCM (35 mL) was added BzCl (0.683 mL, 0.826 g, 5.88 mmol). After 3 h, the reaction mixture was washed with 10% HCl, saturated solution of NaHCO₃ and dried over anhydrous MgSO₄. Purification by flash column chromatography (40-60% EtOAc in hexane) gave benzoyl protected ketone 7c.⁴⁴ Yield: 0.995 g (89%); light yellow solid; mp: 193.5-195.2 ºC; ¹H NMR (300 MHz, CDCl₃) δ 2.13 (quin, 2H, J 6.3 Hz), 2.64 (t, 2H, J 6.3 Hz), 2.97 (t, 2H, J 6 Hz), 7.37 (dd, 1H, J 8.4, 1.8 Hz), 7.45-7.52 (m, 2H), 7.55-7.61 (m, 1H), 7.86-7.89 (m, 3H), 8.04 (d, 1H, J 8.7 Hz), 8.11 (d, 1H, J 6.9 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 23.2, 30.0, 117.8, 118.8, 127.1, 128.6, 128.8, 130.1, 132.2, 134.4, 142.3, 146.3, 165.8, 197.4. To a two neck round bottom flask with magnetic stirrer under nitrogen atmosphere was added NaBH₄ (0.170 g, 4.50 mmol) to the solution of ketone 7c (0.995 g, 3.75 mmol) in MeOH (40 mL). The mixture was stirred for 30 minutes at 0 ºC. The temperature was raised to room temperature (rt) for another 1 h. The reaction was quenched by addition of distilled H₂O (10 mL) and the mixture was extracted with EtOAc (3 × 30 mL). The combined organic extracts were washed with brine (2 × 10 mL) and dried over anhydrous MgSO₄. The solvent was removed under reduced pressure and N-(5-hydroxy-5,6,7,8-tetrahydronaphthalen-2-yl)benzamide was obtained as white solid (0.980 g, 3.67 mmol, 98.0%) and used in the next step without purification. The N-(5-hydroxy-5,6,7,8-tetrahydronaphthalen-2-yl)benzamide(0.980 g, 3.67 mmol) was dissolved in toluene (30 mL) with the addition of few crystals of TsOH.H₂O in round bottom flask, fitted with Dean-Stark apparatus. The system was maintained at 130 ºC and the reaction was monitored by TLC. The reaction was quenched by addition of saturated solution of NaHCO₃ and extracted with hexane (3 × 30 mL). The combined organic extracts were washed with brine (2 × 10 mL) and dried over anhydrous MgSO₄. The solvent was removed at reduced pressure and the residue was purified by flash chromatography (15-20% EtOAc in hexane) giving compound 1c.³⁸ Yield: 0.735 g (80%); white solid; mp: 170.2-172 ºC; ¹H NMR (300 MHz, CDCl₃) δ 2.30-2.36 (m, 2H), 2.83 (t, 2H, J 8.2 Hz), 5.99 (dt, 1H, J 9.2, 4.5 Hz), 6.45 (dt, 1H, J 9.6, 1.8 Hz), 7.02 (d, 1H, J 8.1 Hz), 7.35 (dd, 1H, J 8.1, 2.4 Hz), 7.46-7.58 (m, 4H), 7.75 (bs, 1H), 7.84-7.88 (m, 2H); ¹³C NMR (75 MHz, CDCl₃) δ 23.0, 27.7, 117.8, 119.4, 126.3, 126.9, 127.1, 127.9, 128.8, 130.8, 131.8, 135.1, 136.4, 136.6, 165.4.

N-(7,8-Dihydronaphthalen-2-yl)-4-methylbenzenesulfonamide (1d)

To a stirred solution of ketone 5 (0.161 g, 1.00 mmol) in pyridine (5 mL) was added TsCl (0.191 g, 1.35 mmol). The reaction mixture was heated at reflux for 16 h. The solvent was removed under reduced pressure and the crude residue was extracted with EtOAc (3 × 10 mL) and washed with 1 mol L^-1 HCl (10 mL), water (10 mL) and brine (10 mL) and dried over anhydrous MgSO₄. Purification by flash column chromatography (40-50% EtOAc in hexane) gave tosyl protected ketone 7d.⁴⁵ Yield: 0.304 g (96%); white solid; mp: 214.2-215.6 ºC; ¹H NMR (300 MHz, CDCl₃) δ 2.03 (m, 2H), 2.39 (s, 3H), 2.59 (t, 2H, J 6.4 Hz), 2.87 (t, 2H, J 6 Hz), 6.94 (dd, 1H, J 8.7, 2.4 Hz), 6.98-6.99 (m, 1H), 7.15 (bs, 1H), 7.25-7.33 (m, 2H), 7.74 (d, 1H, J 8.1 Hz), 7.90 (d, 1H, J 8.4 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 21.6, 23.1, 29.8, 38.8, 117.2, 118.1, 124.8, 127.2, 129.1, 129.9, 135.9, 141.1, 144.4, 146.3, 197.1. The reaction was performed using ketone 7d(0.788 g, 2.50 mmol), MeOH (30 mL) and NaBH₄ (0.283 g, 7.50 mmol). After workup, solvent was removed under reduced pressure and N-(5-hydroxy-5,6,7,8-tetrahydronaphthalen-2-yl)-4-methylbenzenesulfonamide (0.761 g, 2.39 mmol, 96.0%) was obtained as yellowish white solid and used in the next step without purification. The reaction was performed using N-(5-hydroxy-5,6,7,8-tetrahydronaphthalen-2-yl)-4-methylbenzenesulfonamide (0.761 g, 2.39 mmol), toluene (30 mL) and TsOH.H₂O (cat. few crystals) at 130 ºC. The crude product was purified by flash column chromatography (15-20% EtOAc in hexane) giving alkene 1d. Yield: 0.542 g (76%); white solid; mp: 126.7-127.8 ºC; IR (film) v_max / cm^-1 3524, 3257, 3033, 2930, 2883, 2830, 1916, 1735, 1609, 1599, 1575, 1497, 1465, 1397, 1341, 1329, 1314, 1291, 1241, 1209, 1185, 1163, 1124; ¹H NMR (300 MHz, CDCl₃) δ 2.21-2.29 (m, 2H), 2.37 (s, 3H), 2.69 (t, 2H, J 8.4 Hz), 5.96 (dt, 1H, J 9.6, 4.4 Hz), 6.36 (dt, 1H, J 9.6 Hz), 6.78-6.86 (m, 4H), 7.22 (d, 2H, J 8.1 Hz), 7.67 (d, 2H, J 8.4 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 21.5 (CH₃), 22.8 (CH₂), 27.4 (CH₂), 119.3 (CH), 121.0 (CH), 126.4 (CH), 126.8 (CH), 127.2 (CH), 128.3 (CH), 129.6 (CH), 131.5 (C), 134.7 (C), 136.1 (C), 136.7 (C), 143.7 (C); HRMS [ESI(+)-TOF] calcd. for [C₁₇H₁₇NO₂S + H]⁺: 300.1053; found: 300.1060.

(9H-Fluoren-9-yl)methyl (7,8-dihydronaphthalen-2-yl)carbamate (1e)

To a stirred solution of amino ketone 5 (0.484 g, 3.00 mmol) and pyridine (0.29 mL, 3.60 mmol) in anhydrous DCM (25 mL) at 0 ºC was added solution of Fmoc-Cl (0.854 g, 3.30 mmol) in anhydrous DCM and the resulting reaction mixture was allowed to stir at rt. After 1 h the solution was acidified with 1 mol L^-1 HCl. The product was extracted with DCM (3 × 10 mL) and dried over anhydrous Mg₂SO₄. After workup solvent was removed under reduced pressure. The crude product was purified by flash column chromatography (15-25% EtOAc in hexane) giving Fmoc protected amino ketone 7e. Yield: 1.13 g (98%); white solid; mp: 160.6-161.5 ºC; IR (film) v_max / cm^-1 3305, 3065, 2946, 2890, 1737, 1665, 1602, 1585, 1537, 1495, 1478, 1450, 1427, 1412, 1350, 1336, 1323, 1287, 1219, 1185, 1164, 1129, 1105; ¹H NMR (300 MHz, CDCl₃) δ 2.06-2.15 (m, 2H), 2.62 (t, 2H, J 6.3 Hz), 2.91 (t, 2H, J 6.0 Hz), 4.27 (t, 1H, J 6.4 Hz, 1H), 4.57 (d, 2H, J 6.6 Hz), 6.95 (bs, 1H), 7.14 (dd, 1H, J 8.4, 1.5 Hz), 7.32 (td, 1H, J 7.6, 1.2 Hz, 1H), 7.37-7.44 (m, 3H), 7.56-7.62 (m, 2H), 7.78 (dt, 2H, J 7.5, 0.9 Hz), 7.98 (d, 1H, J 8.7 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 23.2 (CH₂), 30.0 (CH₂), 38.9 (CH₂), 47.0 (CH), 67.0 (CH₂), 116.4 (CH), 117.0 (CH), 120.1 (CH), 124.8 (CH), 127.1 (CH), 127.8 (CH), 128.1 (C), 128.7 (CH), 141.3 (C), 142.2 (C), 143.5 (C), 146.3 (C), 152.9 (C), 197.3 (C); HRMS [ESI(+)-TOF] calcd. for [C₂₅H₂₁NO₃ + H]⁺: 384.1600; found: 384.1600. The reaction was performed using ketone 7e (1.15 g, 3.00 mmol), MeOH (50 mL) and NaBH₄ (0.227 g, 6.00 mmol). After workup, solvent was removed under reduced pressure and crude alcohol(1.12 g, 2.91 mmol, 97.0%) was obtained as white solid and used in the next step without purification. The reaction was performed using (9H-fluoren-9-yl)methyl (5-hydroxy-5,6,7,8-tetrahydronaphthalen-2-yl)carbamate (1.12 g, 2.91 mmol), toluene (50 mL) and TsOH.H₂O (cat. few crystals) at 130 ºC. The crude product was purified by flash column chromatography (10-30% EtOAc in hexane) giving alkene 1e. Yield: 0.890 g (83%);white solid; mp: 141-142 ºC; IR (film) v_max / cm^-1 3307, 3030, 2931, 2882, 2826, 1703, 1612, 1585, 1527, 1478, 1465, 1450, 1425, 1326, 1308, 1278, 1219, 1170, 1104; ¹H NMR (300 MHz, CDCl₃) δ 2.25-2.33 (m 2H), 2.77 (t, 2H, J 8.2 Hz), 4.27 (t, 2H, J 6.6 Hz), 4.53 (d, 2H, J 6.9 Hz), 5.95 (dt, 1H, J 9.6, 4.4 Hz), 6.41 (dt, 1H, J 9.6, 1.6 Hz), 6.57 (bs, 1H), 6.94 (d, 1H, J 8.1 Hz), 7.08-7.18 (m, 2H), 7.32 (td, 2H, J 7.4, 1.2 Hz), 7.38-7.44 (m, 2H), 7.61 (d, 2H, J 7.2 Hz), 7.61 (d, 2H, J 7.2 Hz), 7.78 (d, 2H, J 7.5 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 23.0 (CH₂), 27.7 (CH₂), 47.1 (CH), 66.8 (CH₂), 116.5 (CH), 118.2 (CH), 120.0 (CH), 124.9 (CH), 126.4 (CH), 127.0 (CH), 127.1 (CH), 127.5 (CH), 127.7 (CH), 130.0 (C), 136.1 (C), 136.6 (C), 141.3 (C), 143.7 (C), 153.3 (C); HRMS [ESI(+)] calcd. for [C₂₅H₂₁NO₂ + H]⁺: 368.1651; found: 368.1648.

N-(5,6-Dihydronaphthalen-2-yl)acetamide (1f)

Compound 1f was prepared according to reported protocol in literature.³⁵ Yield: 0.184 g (98%); white solid; mp: 52.1-53.9 °C (lit. 51.2-54.6 ºC).

7,8-Dihydronaphthalen-1-yl acetate (9g)

The reaction was performed using ketone 8g (0.892 g, 5.50 mmol), MeOH (40 mL) and NaBH₄ (0.208 g, 5.50 mmol). After workup, solvent was removed under reduced pressure and 1,2,3,4-tetrahydronaphthalene-1,5-diol(0.885 g, 5.39 mmol, 98.0%) was obtained as white solid and used in the next step without purification. The crude 1,2,3,4-tetrahydronaphthalene-1,5-diol(0.885 g, 5.39 mmol), dissolved in anhydrous tetrahydrofuran (THF) (10 mL) was added with H₃PO₄ (6 mL). The system was maintained at 80 ºC and the reaction was monitored by TLC. The reaction was quenched by addition of saturated solution of NaHCO₃ and the mixture was extracted with EtOAc (3 × 10 mL). The combined organic extracts were washed with brine and dried over anhydrous MgSO₄. The solvent was removed at reduced pressure and the residue was purified by flash chromatography (10-15% ethyl acetate in hexane) giving alkene 1g.³⁵ Yield: 0.614 g (78%); white solid; mp: 66-67 ºC (lit. 53.3-54.6 ºC⁴⁶ and 73.5-74.5 ºC).³⁵ To a stirred solution of alkene 1g (0.614 g, 4.20 mmol) and DMAP (0.015 g, 0.12 mmol) in Et₃N (16 mL) was added Ac₂O (1.50 mL, 15.90 mmol) at rt. After 1 h, the reaction was quenched with MeOH (5 mL) and H₂O (8 mL). The reaction was extracted with EtOAc (3 × 10 mL), washed with brine (2 × 10 mL) and dried over anhydrous MgSO₄. Purification by flash column chromatography (3-5% EtOAc in hexane) gave acetyl protected alkene 9g.⁴⁷ Yield: 0.760 g (96%); colorless liquid.

7,8-Dihydronaphthalen-1-yl benzoate (9gg)

The reaction was performed using phenol 1g (0.248 g, 1.70 mmol), BzCl (0.22 mL, 0.267 g, 1.90 mmol) and Et₃N (0.51 mL, 0.374 g, 3.70 mmol) in DCM (10 mL). Purification by flash column chromatography (3-4% EtOAc in hexane) gave benzoyl protected alkene 9gg. Yield: 0.398 g (94%); white solid; mp: 58.1-58.7 ºC; IR (film) v_max / cm^-1 3035, 2935, 2887, 2834, 2127, 1735, 1651, 1601, 1583, 1569, 1491, 1451, 1395, 1342, 1314, 1296, 1266, 1247, 1229, 1212; ¹H NMR (300 MHz, CDCl₃) δ 2.24-2.32 (m, 2H), 2.71 (t, 2H, J 8.4 Hz), 6.04 (dt, 1H, J 9.6, 4.5 Hz), 6.49 (dt, 1H, J 9.6, 1.8 Hz), 6.96 (d, 1H, J 7.5 Hz), 7.00 (dd, 1H, J 8.1, 1.2 Hz), 7.17-7.22 (m, 1H), 7.48-7.54 (m, 2H), 7.61-7.66 (m, 1H), 8.21-8.25 (m, 2H); ¹³C NMR (75 MHz, CDCl₃) δ 20.6 (CH₂), 22.3 (CH₂), 120.8 (CH), 123.8 (CH), 126.8 (CH), 127.2 (C), 127.3 (CH), 128.6 (CH), 129.0 (CH), 129.4 (C), 130.1 (CH), 133.5 (CH), 135.7 (C), 148.0 (C), 164.8 (C); LRMS m/z (rel. int.): 250 (M^+●, 9), 144 (3), 128(11), 115 (10), 105 (100), 91 (2), 77 (36), 63 (3), 51 (10), 39 (2); HRMS [ESI(+)-TOF] calcd. for [C₁₇H₁₄O₂ + Na]⁺: 273.0891; found: 273.0890.

7,8-Dihydronaphthalen-2-yl acetate (9h)

The reaction was performed using ketone 8h (2.000 g, 11.35 mmol), MeOH (80 mL) and NaBH₄ (0.4290 g, 11.35 mmol). After workup, solvent was removed under reduced pressure and crude alcohol (1.986 g, 11.14 mmol, 98.0%) was obtained as white solid and used in the next step without purification. The reaction was performed following using 6-methoxy-1,2,3,4-tetrahydronaphthalen-1-ol(1.986 g, 11.14 mmol), toluene (60 mL) and TsOH.H₂O (cat. few crystals) at 130 ºC. The crude product was purified by flash column chromatography (20-40% EtOAc in hexane) giving alkene 1h.³⁶ Yield: 1.702 g (95%); colorless oil. Under inert atmosphere N₂, NaH (6.24 g, 260 mmol, 60% in mineral oil) was washed with anhydrous hexanes (3 × 20 mL). After a few minutes, anhydrous DMF (75 mL) was added. To this mixture was slowly added a solution of EtSH (13.0 mL, 180 mmol, 30 equiv based on olefin 1h) in anhydrous DMF (14 mL) at 0 °C and the resulting yellow solution was stirred for 20 min at rt. A solution of alkene 1h (0.961 g, 6.0 mmol) in DMF (15 mL) was then added dropwise and the resulting mixture was stirred for 5 h at 140 °C. When the reaction was becoming slightly brown, the mixture was cooled to the rt and a saturated solution of NH₄Cl was added. The mixture was extracted with Et₂O. The organic phase was washed with water, brine and dried over anhydrous MgSO₄. The solvent was removed under reduced pressure. The crude product was purified by flash column chromatography (20-40% EtOAc in hexane) giving 7,8-dihydronaphthalen-2-ol.³⁹ Yield: 0.851 g (97%); viscous colorless liquid. The reaction was performed using 7,8-dihydronaphthalen-2-ol (0.365 g, 2.50 mmol), DMAP (0.09 g, 0.07 mmol) and Ac₂O (0.9 mL, 9.5 mmol) in Et₃N (10 mL). Purification by flash column chromatography (3-5% EtOAc in hexane) gave acetyl protected alkene 9h.⁴⁸ Yield: 0.398 g (85%); colorless liquid.

5,6-Dihydronaphthalen-2-yl acetate (9b)

The reaction was performed using ketone 8b (0.705 g, 4.00 mmol), MeOH (40 mL) and NaBH₄ (0.152 g, 4.00 mmol). After workup, solvent was removed under reduced pressure and crude alcohol(0.691 g, 3.88 mmol, 97%) was obtained as white solid and used in the next step without purification. The reaction was performed using 7-methoxy-1,2,3,4-tetrahydronaphthalen-1-ol (0.691 g, 3.88 mmol), toluene (40 mL) and TsOH.H₂O (cat. few crystals) at 130 ºC. After workup solvent was removed under reduced pressure. The crude product was purified by flash column chromatography (1-2% EtOAc in hexane) giving alkene 1b.^13,37 Yield: 0.488 g (78%); colorless liquid. The reaction was performed using alkene 1b(0.481 g, 3.00 mmol), NaH (3.12 g, 130 mmol, 60% in mineral oil), EtSH (6.5 mL, 90 mmol, 30 equiv. based on olefin 1b) in DMF (35 mL) at 140 ºC. The crude product was purified by flash column chromatography (10-15% ethyl acetate in hexane) giving 5,6-dihydronaphthalen-2-ol. Yield: 0.351 g (80%); white solid; mp: 98-99 ºC; IR (film) v_max / cm^-1 3247, 3027, 2936, 2880, 2851, 2818, 1629, 1614, 1574, 1492, 1477, 1465, 1435, 1426, 1395, 1349, 1327, 1282, 1265, 1215; ¹H NMR (300 MHz, CDCl₃) δ 2.25-2.32 (m, 2H), 2.71 (t, 2H, J 8.1 Hz), 4.59 (s, 1H), 6.04 (dt, 1H, J 9.6, 4.4 Hz), 6.38 (dt, 1H, J 9.6, 1.8 Hz), 6.52 (d, 1H, J 2.7 Hz), 6.59 (dd, 1H, J 7.8, 2.7 Hz), 6.96 (d, 1H, J 8.1, Hz); ¹³C NMR (75 MHz, CDCl₃) δ 23.5 (CH₂), 26.6 (CH₂), 112.9 (CH), 113.1 (CH), 127.5 (CH), 127.7 (C), 128.3 (CH), 129.5 (CH), 135.3 (C), 154.0 (C); LRMS m/z (rel. int.): 146 (M^+●, 100), 145 (71), 131 (45), 127 (43), 117 (36), 115 (55), 103 (5), 91 (12), 77 (7), 63 (14), 51 (13), 39 (10); HRMS [ESI(+)-TOF] calcd. for [C₁₀H₁₀O + K]⁺: 185.0369; found: 185.0361. The reaction was performed using 5,6-dihydronaphthalen-2-ol (0.351 g, 2.40 mmol), DMAP (0.08 g, 0.065 mmol) and Ac₂O (0.9 mL, 9.0 mmol) in Et₃N (10 mL). Purification by flash column chromatography (3-5% EtOAc in hexane) gave acetyl protected alkene 9b. Yield: 0.359 g (85%); colorless liquid; IR (film) v_max / cm^-1 3034, 2935, 2885, 2831, 1761, 1609, 1575, 1491, 1432, 1369, 1328, 1271, 1210; ¹H NMR (300 MHz, CDCl₃) δ 2.27-2.36 (m, 2H), 2.28 (s, 3H), 2.77 (t, 2H, J 8.0 Hz), 6.06 (m, 1H), 6.41 (dt, 1H, J 9.6, 1.8 Hz), 6.74 (d, 1H, J 2.4 Hz), 6.81 (dd, 1H, J 8.1, 2.4 Hz), 7.08 (d, 1H, J 8.1 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 21.1 (CH₃), 23.2 (CH₂), 26.8 (CH₂), 118.8 (CH), 119.4 (CH), 127.3 (CH), 128.2 (CH), 129.6 (CH), 132.9 (C), 135.3 (C), 149.2 (C), 169.7 (C); LRMS m/z (rel. int.): 188 (M^+●, 20), 146 (100), 145 (52), 131 (30), 127 (19), 117 (24), 115 (42), 91 (15), 77 (5), 63 (9), 43 (30), 39 (10); HRMS [ESI(+)] calcd. for [C₁₂H₁₂O₂ + Na]⁺: 211.0735; found: 211.0724.

Ring contraction reactions mediated by HTIB: general procedure for 2b, 2g, 2gg and 2h

1-(Bis(2,2,2-trifluoroethoxy)methyl)-2,3-dihydro-1H-inden-4-yl acetate (2g)

To a solution of alkene 9g (0.094 g, 0.50 mmol) in TFE (10 mL) was added HTIB (0.235 g, 0.60 mmol) at 0 ºC. The reaction mixture was stirred at 0 °C for 5 min. The reaction was quenched with a saturated solution of NaHCO₃. The reaction mixture was extracted with EtOAc (3 × 15 mL), washed with brine (2 × 10 mL) and dried over anhydrous MgSO₄. The solvent was evaporated under reduced pressure. The crude product was purified by flash column chromatography (4-5% EtOAc in hexanes) giving acetal 2g. Yield: 0.128 g (66%); white solid; mp: 61.6-63.4 ºC; IR (film) v_max / cm^-1 2947, 1764, 1615, 1587, 1468, 1433, 1372, 1281, 1213; ¹H NMR (300 MHz, CDCl₃) δ 1.98-2.10 (m, 1H), 2.22-2.32 (m, 1H), 2.30 (s, 3H), 2.70-2.91 (m, 2H), 3.47-3.55 (m, 1H), 3.84-4.07 (m, 4H), 4.71 (d, 1H, J 8.1 Hz), 6.94 (dt, 1H, J 7.8 Hz), 7.21 (t, 1H, J 7.8 Hz), 7.28 (d, 1H, J 7.5 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 20.8 (CH₃), 26.8 (CH₂), 28.1 (CH₂), 47.5 (CH), 61.8 (q, J 34.8 Hz) (CH₂), 63.3 (q, J 34.8 Hz) (CH₂), 105.0 (CH), 120.6 (CH), 123.2 (CH), 123.6 (q, J 276 Hz) (CF₃), 123.7 (q, J 276 Hz) (CF₃), 128.0 (CH), 136.6 (C), 143.3 (C), 147.1 (C), 168.9 (C); HRMS [ESI(+)-TOF] calcd. for [C₁₆H₁₆F₆O₄ + Na]⁺: 409.0850; found: 409.0851.

1-(Bis(2,2,2-trifluoroethoxy)methyl)-2,3-dihydro-1H-inden-4-yl benzoate (2gg)

Yield: 0.159 g (71%); white solid; mp: 79-79.8 ºC; IR (film) v_max / cm^-1 3067, 2948, 1738, 1602, 1585, 1469, 1453, 1423, 1383, 1279, 1230, 1168, 1134; ¹H NMR (300 MHz, CDCl₃) δ 1.99-2.11 (m, 1H), 2.20-2.32 (m, 1H), 2.76-2.97 (m, 2H), 3.54 (q, 1H, J 8.1 Hz), 3.85-4.09 (m, 4H), 4.74 (d, 1H, J 8.1 Hz), 7.10 (dt, 1H, J 7.8, 1.2 Hz), 7.26 (t, 1H, J 7.8 Hz), 7.33 (d, 1H, J 7.8 Hz), 7.48-7.54 (m, 2H), 7.64 (tt, 1H, J 7.5, 1.5 Hz), 8.18-8.22 (m, 2H); ¹³C NMR (75 MHz, CDCl₃) δ 26.8 (CH₂), 28.2 (CH₂), 47.6 (CH), 61.9 (q, J 34.8 Hz) (CH₂), 63.4 (q, J 34.8 Hz) (CH₂), 105.1 (CH), 120.7 (CH), 123.2 (CH), 123.69 (q, J 276 Hz) (CF₃), 123.74 (q, J 276 Hz) (CF₃), 128.1 (CH), 128.6 (CH), 129.4 (C), 130.2 (CH), 133.6 (CH), 136.8 (C), 143.4 (C), 147.4 (C), 164.5 (C); HRMS [ESI(+)-TOF]: calcd. for [C₂₁H₁₈F₆O₄ + Na]⁺: 471.1007; found: 471.1010.

1-(Bis(2,2,2-trifluoroethoxy)methyl)-2,3-dihydro-1H-inden-5-yl acetate (2h)

Yield:0.117 g (60%); viscous colorless liquid; IR (film) v_max / cm^-1 2953, 1761, 1610, 1592, 1485, 1460, 1431, 1372, 1282, 1216, 1163, 1134, 1103; ¹H NMR (300 MHz, CDCl₃) δ 1.97-2.09 (m, 1H), 2.21-2.33 (m, 1H), 2.28 (s, 3H), 2.82-3.01 (m, 2H), 3.43 (q, 1H, J 7.8 Hz), 3.86-4.08 (m, 4H), 4.68 (d, 1H, J 8.4 Hz), 6.87 (dd, 1H, J 8.4, 2.2 Hz), 6.94 (s, 1H), 7.38 (d, 1H, J 8.1 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 21.0 (CH₃), 27.4 (CH₂), 31.1 (CH₂), 46.4 (CH), 61.6 (q, J 34.8 Hz) (CH₂), 63.3 (q, J 34.8 Hz) (CH₂), 105.1 (CH), 117.8 (CH), 119.7 (CH), 123.6 (q, J 276 Hz) (CF₃), 123.7 (q, J 276 Hz) (CF₃), 126.1 (CH), 138.3 (C), 146.1 (C), 150.3 (C), 169.7 (C); LRMS m/z (rel. int.): 386 (M^+●, 4), 344 (7), 245 (7), 211 (14), 175 (8), 145 (6), 133 (100), 115 (7), 105 (11), 83 (11), 77 (6), 43 (19); HRMS [ESI(+)-TOF]: calcd. for [C₁₆H₁₆F₆O₄ + Na]⁺: 409.0850; found: 409.0851.

3-(Bis(2,2,2-trifluoroethoxy)methyl)-2,3-dihydro-1H-inden-5-yl acetate (2b)

Yield: 0.126 g (65%); white solid; mp: 63.6-65.2 ºC; IR (film) v_max / cm^-1 2919, 2850, 1760, 1610, 1591, 1540, 1484, 1459, 1429, 1372, 1281, 1214; ¹H NMR (300 MHz, CDCl₃) δ 1.98-2.09 (m, 1H), 2.21-2.33 (m, 1H), 2.28 (s, 3H), 2.79-2.99 (m, 2H), 3.46 (q, 1H, J 7.8 Hz), 3.85-4.06 (m, 2H), 4.69 (d, 1H, J 8.4 Hz), 6.91 (ddd, 1H, J 8.1, 2.1, 0.3 Hz), 7.10 (d, 1H, J 1.8 Hz), 7.20 (d, 1H, J 8.1 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 21.1 (CH₃), 27.5 (CH₂), 30.6 (CH₂), 61.7 (q, J 34.8 Hz) (CH₂), 63.5 (q, J 34.8 Hz) (CH₂), 105.1 (CH), 118.9 (CH), 120.9 (CH), 123.6 (q, J 276 Hz) (CF₃), 123.7 (q, J 276 Hz) (CF₃), 125.1 (CH), 142.0 (C), 142.2 (C), 149.5 (C), 169.8 (C); LRMS m/z (rel. int.): 386 (M^+●, 2), 344 (25), 287 (7), 244 (8), 211 (100), 145 (17), 133 (92), 115 (15), 105 (17), 83 (29), 77 (12), 43 (43); HRMS [ESI(+)-TOF] calcd. for [C₁₆H₁₆F₆O₄ + Na]⁺: 409.0850; found: 409.0851.

N-(1-(Dimethoxymethyl)-2,3-dihydro-1H-inden-5-yl)benzamide (3c)

To a solution of alkene 1l (0.125 g, 0.500 mmol) in MeOH/DCM (8:1) (10 mL) was added HTIB (0.235 g, 0.60 mmol) at 0 °C for 10 min. The reaction was quenched with a saturated solution of NaHCO₃. The reaction mixture was extracted with EtOAc (3 × 15 mL), washed with brine (2 × 10 mL) and dried over anhydrous MgSO₄. The solvent was evaporated under reduced pressure. The crude product was purified by flash column chromatography (40-50% EtOAc in hexane), giving acetal 3c. Yield: 0.12 g (77%); yellowish white solid; mp: 96-97.5 ºC; IR (film) v_max / cm^-1 3307, 3060, 2937, 2830, 1737, 1650, 1601, 1580, 1532, 1493, 1447, 1424, 1374, 1328, 1283, 1248, 1210, 1187, 1154, 1116; ¹H NMR (300 MHz, CDCl₃) δ 1.92-2.04 (m, 1H), 2.15-2.27 (m, 1H), 2.78-2.99 (m, 2H), 3.37-3.47 (m, 1H), 3.38 (s, 3H), 3.42 (s, 3H), 4.29 (d, 1H, J 7.5 Hz), 7.29 (dd, 1H, J 8.1, 2.1 Hz), 7.38 (d, 1H, J 8.4 Hz), 7.42-7.55 (m, 3H), 7.61 (s, 1H), 7.83-7.86 (m, 2H), 7.93 (bs, 1H); ¹³C NMR (75 MHz, CDCl₃) δ 27.5 (CH₂), 31.4 (CH₂), 46.9 (CH), 52.8 (CH₃), 54.2 (CH₃), 107.1 (CH), 116.6 (CH), 118.5 (CH), 125.6 (CH), 126.9 (CH), 128.5 (CH), 131.5 (CH), 134.9 (C), 136.8 (C), 139.1 (C), 145.7 (C), 165.8 (C); HRMS [ESI(+)]: calcd. for [C₁₉H₂₁NO₃ + H]⁺: 312.1600; found: 312.1600.

(9H-Fluoren-9-yl)methyl (1-(dimethoxymethyl)-2,3-dihydro-1H-inden-5-yl)carbamate (3e)

The reaction was performed using alkene 1e (0.183 g, 0.50 mmol), HTIB (0.235 g, 0.60 mmol) and MeOH/DCM (4:1) (10 mL) at 0 °C for 10 min.¹³ The crude product was purified by flash column chromatography (15-25% EtOAc in hexane) giving acetal 3e. Yield: 0.138 g (64%);white solid; mp: 115-117 ºC; IR (film) v_max / cm^-1 3307, 3066, 2942, 2849, 2830, 1730, 1708, 1598, 1538, 1493, 1478, 1450, 1431, 1375, 1326, 1297, 1220; ¹H NMR (300 MHz, CDCl₃) δ 1.89-2.01 (m, 1H), 2.12-2.24 (m, 1H), 2.73-2.94 (m, 2H), 3.34-3.43 (m, 1H), 3.35 (s, 3H), 3.40 (s, 3H), 4.22-4.26 (m, 1H), 4.26 (d, 1H, J 7.5 Hz), 4.51 (d, 2H, J 6.6 Hz), 6.71 (bs, 1H), 7.04 (d, 1H, J 7.2 Hz), 7.27-7.32 (m, 4H), 7.36-7.41 (m, 2H), 7.59 (d, 2H, J 7.5 Hz), 7.76 (d, 2H, J 7.5 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 27.5 (CH₂), 31.4 (CH₂), 46.8 (CH), 47.0 (CH), 52.8 (CH₃), 54.1 (CH₃), 66.6 (CH₂), 107.1 (CH), 115.0 (CH), 117.0 (CH), 119.9 (CH), 124.8 (CH), 125.6 (CH), 127.0 (CH), 127.6 (CH), 136.6 (C), 138.0 (C), 141.2 (C), 143.7 (C), 145.7 (C), 153.5 (C); HRMS [ESI(+)-TOF]: calcd. for [C₂₇H₂₇NO₄ + Na]⁺: 452.1832; found: 452.1836.

N-((5R,6R)-5,6-Dimethoxy-5,6,7,8-tetrahydronaphthalen-2-yl)-4-methylbenzenesulfonamide (trans-4d)and N-((5R,6S)-5,6-dimethoxy-5,6,7,8-tetrahydronaphthalen-2-yl)-4-methylbenzenesulfonamide (cis-4d)

The reaction was performed using alkene 1d(0.150 g, 0.50 mmol), HTIB (0.235 g, 0.60 mmol) and MeOH (10 mL) at 0 °C.¹³ The crude product was purified by flash column chromatography (20-50% EtOAc in hexane) giving trans-4d, cis-4d and starting material 1d (0.021 g, 0.070 mmol, 14%) was recovered.

N-((5R,6R)-5,6-Dimethoxy-5,6,7,8-tetrahydronaphthalen-2-yl)-4-methylbenzenesulfonamide (trans-4d)

Yield: 0.019 g (10%); viscous colorless oil; IR (film) v_max / cm^-1 3257, 2932, 2825, 1919, 1614, 1599, 1502, 1463, 1399, 1341, 1320, 1292, 1270, 1185, 1162; ¹H NMR (300 MHz, CDCl₃) δ 1.82-1.92 (m, 1H), 1.99-2.09 (m, 1H), 2.37 (s, 3H), 2.57-2.80 (m, 2H), 3.42 (s, 3H), 3.48 (s, 3H), 3.65-3.70 (m, 1H), 4.14 (d, 1H, J 4.8 Hz), 6.82-6.88 (m, 3H), 7.17-7.23 (m, 3H), 7.67 (d, 2H, J 8.7 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 21.5 (CH₃), 22.9 (CH₂), 25.1 (CH₂), 56.5 (CH₃), 57.6 (CH₃), 77.5 (CH), 79.1 (CH), 118.5 (CH), 120.5 (CH), 127.2 (CH), 129.6 (CH), 130.9 (CH), 131.4 (C), 135.9 (C), 136.2 (C), 138.2 (C), 143.7 (C); HRMS [ESI(+)-TOF]: calcd. for [C₁₉H₂₃NO₄S + K]⁺: 400.0985; found: 400.0988.

N-((5R,6S)-5,6-Dimethoxy-5,6,7,8-tetrahydronaphthalen-2-yl)-4-methylbenzenesulfonamide (cis-4d)

Yield: 0.068 g (37%); viscous colorless oil; IR (film) v_max / cm^-1 3249, 3065, 2932, 2827, 1919, 1674, 1647, 1612, 1500, 1463, 1399, 1340, 1320, 1292, 1230; ¹H NMR (300 MHz, CDCl₃) δ 1.82-1.90 (m, 1H), 2.08-2.18 (m, 1H), 2.36 (s, 1H), 2.60-2.71 (m, 1H), 2.84-2.91 (m, 1H), 3.42 (s, 3H), 3.44 (s, 3H), 3.59 (dt, 1H, J 9.9, 3.0 Hz), 4.26 (d, 1H, J 2.7 Hz), 6.86 (d, 1H, J 2.1 Hz), 6.90 (dd, 1H, J 8.1, 2.4 Hz), 7.15 (d, 1H, J 8.4 Hz), 7.21 (dd, 2H, J 8.7, 0.6 Hz), 7.50 (bs, 1H), 7.69 (dt, 2H, J 8.4, 1.8 Hz); ¹³C NMR (75 MHz, CDCl₃) δ 21.4 (CH₃), 22.1 (CH₂), 26.8 (CH₂), 56.3 (CH₃), 57.0 (CH₃), 77.3 (CH), 77.5 (CH), 117.9 (CH), 120.6 (CH), 127.1 (CH), 129.5 (CH), 130.5 (CH), 131.1 (C), 136.1 (C), 136.4 (C), 137.7 (C), 143.7 (C); HRMS [ESI(+)-TOF]: calcd. for [C₁₉H₂₃NO₄S + Na]⁺: 384.1245; found: 384.1248.

Supplementary Information

Supplementary Information (¹H NMR and ¹³C NMR spectra) is available free of charge at http://jbcs.sbq.org.br as PDF file.

https://minio.scielo.br/documentstore/1678-4790/LPymp8m9LqpsC5jfxRjS34t/b4df6c9c490f6f269ac619c1f25bb2073d08555d.pdf

• FAPESP has sponsored the publication of this article.

Acknowledgments

The authors thank CAPES, FAPESP and CNPq for financial support.

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Publication Dates

• Publication in this collection
  Oct 2016

History

• Received
  08 Jan 2016
• Accepted
  01 Mar 2016