Metal triflate promoted synthesis of naphthalenes

Abstract

A synthetic route to derive the skeleton of naphthalenes starting with isovanillin is described with modest total yields via the key transformation of metal triflate-mediated intramolecular benzannulation of o-formyl or o-benzoyl allylbenzenes in MeNO₂ at rt.

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Introduction

A naphthalene ring system¹ is a key core skeleton for bioactive molecules,² natural products³ and functionalized materials.⁴ A considerable number of attempts have been developed to determine the synthetic routes of the bicyclic skeleton. Two major key transformations for the formation of a functionalized naphthalene skeleton include: (1) acids (or Lewis acids) promote intramolecular Friedel–Crafts type annulations,⁵ and (2) bases promote intramolecular anionic annulations.⁶ Transition metal (e.g., Cr, Mn, Pd, W, Rh, Co, Ru, Ni, Co, Zn, Ti and Au) catalyzed benzannulations provide another approach for preparing the core structure.⁷ Therefore, a new synthetic route for diverse naphthalenes via intramolecular annulation of the readily available starting materials is still a continuing need in the synthetic field.

As part of our efforts in the synthetic applications of dialkoxy o-formyl allylbenzenes 1⁸ and the development of metal triflate-promoted robust and efficient reactions,⁹ we have now streamlined the facile synthesis of substituted naphthalenes via metal triflate-mediated intramolecular benzannulation of o-carbonyl synthon allylbenzenes, such as o-formyl or o-benzoyl allylbenzenes, in MeNO₂ at rt. To the best of our knowledge, few reports on the metal triflate-mediated intramolecular annulation of o-carbonyl synthon allylbenzenes have been documented.¹⁰ In 2011, Kuninobu and Takai described that metal triflates-catalyzed the dehydrative cycloaromatization of o-benzylbenzaldehyde 2 provided tricyclic anthracene in a 97% yield.^10a Among the metal triflates-mediated syntheses, In(OTf)₃ and Sc(OTf)₃ demonstrated excellent transformation. In 2014, Luo demonstrated that Cu(OTf)₂ provided a tetralin skeleton via the conjugate addition of 2′-allylchalcone 3 in a higher (90%) yield than the other scanned catalysts.^10b The adopted synthetic routes are described in Scheme 1. Our aim was to find an optimal metal triflate as the catalyst for synthesizing naphthalenes 5 via the intramolecular benzannulation of o-formyl allylbenzene 4 (a hybrid combining the formyl motif of 2 and the allyl group of 3).

Scheme 1 Metal triflates-catalyzed annulations.

Results and discussion

Initially, six metal triflates were chosen as the catalysts for examining the benzannulation of o-formyl allylbenzene 4a.^11,12 In line with two preliminary studies, 10 mol% catalytic amounts of In(OTf)₃, Sc(OTf)₃, Cu(OTf)₂, Bi(OTf)₃, and AgOTf were first scanned for the formation of 1,2-dimethoxynaphthalene (5a) in MeNO₂ at rt for 8 h, as shown in Table 1 and entries 1–5. However, 5a was isolated in a range of poor to moderate yields (18–72%). When Sn(OTf)₂ or Fe(OTf)₃ was applied to the reaction conditions, yields were increased to 95% or 88% (entries 6 and 7). Under the same conditions, treatment of 4a with Ga(OTf)₃ provided 5a in a 44% yield along with a 38% yield of 6a (entry 8). In addition, 6a was generated via Ga(OTf)₃-mediated by the addition of 4a with MeNO₂. With these results in mind, a series of commercially available lanthanide triflates (Ln(OTf)₃) were examined next, including: La(OTf)₃, Ce(OTf)₃, Pr(OTf)₃, Nd(OTf)₃, Sm(OTf)₃, Eu(OTf)₃, Gd(OTf)₃, Tb(OTf)₃, Dy(OTf)₃, Ho(OTf)₃, Er(OTf)₃, Yb(OTf)₃ and Tm(OTf)₃. However, no isolation of the desired 5a was observed and only 4a was recovered under Ln(OTf)₃-mediated reactions. With the results in hand, we found that Sn(OTf)₂ was among the most reactive and used catalyst compared to other metal triflate derivatives. Along this line, we planned to take advantage of the intrinsically high catalytic ability of Sn(OTf)₂ to construct the naphthalene system. After decreasing the catalytic amounts (10 → 5 mol%), a poor yield (72%) was observed (entry 9). Furthermore, we studied the factors of solvent and temperature. After changing the solvents (from MeNO₂ to CH₂Cl₂ and toluene), different results were observed (entries 10 and 11). In entry 12, the complex mixture occurred (35%) when elevating the temperature (25 → 101).

Table 1 Metal triflates mediated reaction conditions^a

Entry	Catalyst (mol%)	Solvent	Temp (°C)	Time (h)	5a^b (%)
a Reactions were run on 4a (0.5 mmol), solvent (5 mL).b Isolated yields.c 4a was recovered (entry 5, 26%; entry 6, 54%; entry 14, 85%; entry 16, 80%).d 6a (25%) was isolated.e Unknown product (35%) was formed.f Proton scavenger (2,6-di-tert-butylpyridine, 10 mol%) was added.g No reaction.h Water (10 mg) was added.
1	Bi(OTf)3 (10)	MeNO2	25	8	72
2	In(OTf)3 (10)	MeNO2	25	8	70
3	AgOTf (10)	MeNO2	25	8	64
4	Sc(OTf)3 (10)	MeNO2	25	8	40^c
5	Cu(OTf)2 (10)	MeNO2	25	8	18^c
6	Sn(OTf)2 (10)	MeNO2	25	8	95
7	Fe(OTf)3 (10)	MeNO2	25	8	88
8	Ga(OTf)3 (10)	MeNO2	25	8	44 (38)^d
9	Sn(OTf)2 (5)	MeNO2	25	8	72
10	Sn(OTf)2 (10)	CH2Cl2	25	8	79
11	Sn(OTf)2 (10)	Toluene	25	8	69
12	Sn(OTf)2 (10)	MeNO2	101	8	42^e
13	Sn(OTf)2 (10)	MeNO2	25	20	83
14	SnCl2 (10)	MeNO2	25	8	<5
15	TfOH (10)	MeNO2	25	8	60
16	Sn(OTf)2 (10)^f	MeNO2	25	8	—^c^,^g
17	Sn(OTf)2 (10)^h	MeNO2	25	8	90

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Next, Sn(OTf)₂ exhibited a low yield (83%) under an elongated reaction time (8 → 20 h), as shown in entry 13. Another type of tin(II) salt (e.g. SnCl₂) was examined. In entry 14, 5a was isolated in trace amounts only. By the involvement of 10 mol% TfOH (entry 15), 60% of 5a was generated. The control experiment revealed that the real catalyst was the trace amount of TfOH resulting from the hydrolysis of the metal triflates. In the other hand, proton scavenger (2,6-di-tert-butylpyridine, 10 mol%) was involved in the reaction system (entry 16). The resulting mixture failed to promote dehydrative benzannulation in the presence of 4a due to the proton scavenger could block in situ generated TfOH. To examine the stability and reactivity of Sn(OTf)₂ in the presence of water, H₂O (10 mg) was added to the reaction condition and a 90% yield of 5a was generated (entry 17). According to the results, a 10 mol% of Sn(OTf)₂/MeNO₂/rt condition would be an optimal combination for yielding 5a.

Aside from the present benzannulation of o-formyl allylbenzene 4a, Sn(OTf)₂ has also been reported as a catalyst for aldol reactions,^13a Mannish-type reactions,^13b,c rearrangements,^13d,e (3 + 2) annulations,^13f–h and other routes.^13i–k For branched, linear and cyclic alkyl substituents of o-formyl allylbenzenes 4b–d (R = Me, iPr, nBu, and c-C₅H₉), 5b–d provided good yields (90%, 91% or 88%) under the above-mentioned conditions (Scheme 2).

Scheme 2 Synthesis of 5a–d.

On the basis of the results, a plausible reaction mechanism is shown in Scheme 3. Mechanistically, two general distinctive activation modes of the o-allylbenzaldehydes can be proposed for intramolecular dehydrative benzannulations. The first one is a purely metallic-based Lewis acid pathway. The sequence initiates the formation of intermediate A by complexation of a carbonyl motif of 3a with Sn(OTf)₂. The in situ generated triflate anion deprotonates the H_a (blue) of A to give B as the result of a tandem intramolecular electrophilic annulation. Following the protonation of B with the TfOH, C is obtained. On the basis of the triflate anion-mediated deprotonation of C (for H_b, pink), 5a is provided via the removal of Sn(OH)OTf and TfOH. Subsequently, Sn(OTf)₂ is regenerated for the next catalytic cycle by the complexation of Sn(OH)OTf and TfOH. The second possibility would involve a Brønsted acid catalysis. By Sn(OTf)₂ assisted in situ formed TfOH, D is generated.¹⁷ Following the above similar route, 5a is also produced via (i) the triflate anion deprotonates the H_a of D, (ii) protonation of E with the TfOH, (iii) triflate anion-mediated deprotonation of F (for H_b), (iv) the removal of water and the regeneration of TfOH. Although metal triflate-mediated transformations have been described proceeding via proton transfer, we think that the nature of the catalytic species may be still under debate.¹⁸ For the overall benzannulation procedure, water is the only by-product. Therefore, it is important to use Sn(OTf)₂ that are stable and show high reactivity even in the presence of water.^9a,b,g,10a Metal cations (e.g. Bi³⁺, Al³⁺ or Sn²⁺) in water exhibit strong acidic properties because of the acidification through coordination of water molecules present in the “inner-sphere” of the cation.¹⁹ According to Duñach report,^18d metal triflates such as Bi(OTf)₃ are generally obtained and used as its hydrate form and therefore presents indubitably a strong induced Brønsted acidity.

Scheme 3 Proposed mechanism.

Furthermore, Sn(OTf)₂-mediated conversion of o-aroyl allylbenzenes 4e–l into arylnaphthalenes 5e–l was examined, as shown in Scheme 4. The starting materials 4e–l (R = H, allyl; Ar = Ph, 4-FC₆H₄, 2-MeC₆H₄, 2-MeOC₆H₄, 3-MeOC₆H₄, 4-MeOC₆H₄, 3,4-CH₂O₂C₆H₃) were prepared by our previous reports.^8p The substituents of R and Ar on 4e–l did not affect the yield outcome for the benzannulation procedure and no obvious yield changes were observed for the generation of 5e–l. The isolated yields were provided in the range of 88–95%. The structures of 5e and 5f were determined by single-crystal X-ray crystallography.¹⁴

Scheme 4 Synthesis of 5e–l.

As an extension of the benzannulation, the skeleton of aroylbenzene having two allyl arms (ortho and para positions) was studied (Scheme 5). Treatment of 4m–o (R = H, Me; Ar = 4-MeOC₆H₄, 3,4-CH₂O₂C₆H₃)^8p with Sn(OTf)₂ provided 5m–o in 95%, 90% and 91% yields, respectively. Notably, the p-allyl substituent was unaffected. With the results in hands, 10 mol% of Sn(OTf)₂ was applied to the annulation of two substrates 4j (Ar = 4-MeOC₆H₄) or 4l (Ar = 3,4-CH₂O₂C₆H₃) having two allyl arms: one o-allyl group (blue) and one C-allyl group (red). Under a two-step procedure of Sn(OTf)₂-mediated reaction and then O-methylation (K₂CO₃, MeI), we observed C-allyl isomers 7a or 7b were produced in only 21% and 15% yields, respectively via a one-pot tandem intramolecular dehydrative benzannulation and Claisen rearrangement (Scheme 6). By increasing the use of Sn(OTf)₂ from catalytic amounts (10 mol%) to stoichiometric amounts (1.0 equivalent),¹⁵ the yields of 7a or 7b were enhanced to 85% and 80% yields, respectively via the two-step route. The non-catalytic amounts (1.0 equivalent) of Sn(OTf)₂ which mediated the tandem intramolecular procedure included: (1) benzannulation of o-allyl and aroyl groups and (2) double Claisen rearrangement of the o-allyl group. In particular, the present double Claisen rearrangement occurred at room temperature. In comparison with the literature reports (150–200 °C),¹⁶ a few examples were described.

Scheme 5 Synthesis of 5m–o.

Scheme 6 Synthesis of 7a–b.

To change the position of the aryl group on the naphthalene skeleton, eight 4p–v (Ar = Ph, 4-FC₆H₄, 4-MeC₆H₄, 4-MeOC₆H₄, 4-CF₃C₆H₄, 2-MeOC₆H₄, 4-PhC₆H₄) were investigated next. By this synthetic protocol, 8a–g were synthesized in 74–81% yields, as shown in Scheme 7. By the removal of the m-oxygenated group, 4w–x having the o-allyl side arms (Y = H, MeO) were then examined. When benzannulation of 4w–x was treated with Sn(OTf)₂, the yields of 9a (61%) and 9b (40%) provide good results (Scheme 8). The structures of 8d, 8g, 9a and 9b were determined by single-crystal X-ray crystallography.¹⁴ By a one-pot two-step route (Suzuki–Miyaura coupling and our method),^8p a simple naphthalene 10a could be obtained from o-formyl phenyl boronic acids in a 70% yield via the formation of o-allylbenzaldehyde 4y (Scheme 9). Although the isolated yield of 10a was low, it still provided a novel and efficient transformation from o-formyl phenylboronic acid to naphthalene.

Scheme 7 Synthesis of 8a–g.

Scheme 8 Synthesis of 9a–b.

Scheme 9 Synthesis of 10a.

Conclusion

In summary, we have successfully presented a synthetic route for the synthesis of substituted naphthalenes in good yields via Sn(OTf)₂-mediated benzannulation of o-formyl or o-aroyl allylbenzenes. The use of various metal triflates was investigated for the one-pot facile approach and efficient transformation. Further investigations regarding the synthetic application of metal triflates will be conducted and published in due course.

Experimental

General

All reagents and solvents were obtained from commercial sources and used without further purification. Reactions were routinely carried out under an atmosphere of dry air with magnetic stirring. Products in organic solvents were dried with anhydrous magnesium sulfate before concentration in vacuo. Melting points were determined with a SMP3 melting apparatus. ¹H and ¹³C NMR spectra were recorded on a Varian INOVA-400 spectrometer operating at 400 and at 100 MHz, respectively. Chemical shifts (δ) are reported in parts per million (ppm) and the coupling constants (J) are given in Hertz. High resolution mass spectra (HRMS) were measured with a mass spectrometer Finnigan/Thermo Quest MAT 95XL. X-ray crystal structures were obtained with an Enraf-Nonius FR-590 diffractometer (CAD4, Kappa CCD).

A representative procedure of skeleton 5 and compound 6a is as follows

Sn(OTf)₂ (42 mg, 0.1 mmol) was added to a solution of 4a–o (1.0 mmol) in MeNO₂ (5 mL) at 25 °C. The reaction mixture was stirred for 8 h at 25 °C and the solvent was concentrated. The residue was diluted with water (10 mL) and the mixture was extracted with CH₂Cl₂ (3 × 20 mL). The combined organic layers were washed with brine, dried, filtered and evaporated to afford crude product. Purification on silica gel (hexanes/EtOAc = 100/1–10/1) afforded 5 and 6a.

1,2-Dimethoxynaphthalene (5a). Yield = 95% (179 mg); colorless oil; HRMS (ESI, M⁺ + 1) calcd for C₁₂H₁₃O₂ 189.0916, found 189.0919; ¹H NMR (400 MHz, CDCl₃): δ 8.15 (d, J = 8.4 Hz, 1H), 7.80 (d, J = 8.4 Hz, 1H), 7.62 (d, J = 8.8 Hz, 1H), 7.52–7.48 (m, 1H), 7.40–7.36 (m, 1H), 7.31 (d, J = 8.8 Hz, 1H), 4.03 (s, 3H), 4.01 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 148.28, 142.84, 129.63, 128.98, 127.61, 126.05, 124.17, 124.01, 121.23, 115.15, 61.06, 56.83.

1-(2-Allyl-3,4-dimethoxyphenyl)-2-nitroethanol (6a). For Ga(OTf)₃ promoted reaction, 6a was generated in 38% (101 mg); colorless solid; mp = 78–80 °C (recrystallized from hexanes and EtOAc); HRMS (ESI, M⁺ + 1) calcd for C₁₃H₁₈NO₅ 268.1185, found 268.1188; ¹H NMR (400 MHz, CDCl₃): δ 7.24 (d, J = 8.4 Hz, 1H), 6.87 (d, J = 8.4 Hz, 1H), 6.02–5.92 (m, 1H), 5.60 (dd, J = 2.8, 10.0 Hz, 1H), 5.60 (dq, J = 1.6, 10.0 Hz, 1H), 4.93 (dq, J = 1.6, 17.2 Hz, 1H), 4.49 (q, J = 13.2 Hz, 1H), 4.48 (q, J = 13.2 Hz, 1H), 3.87 (s, 3H), 3.80 (s, 3H), 3.53 (t, J = 5.6 Hz, 2H), 2.41 (br s, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 152.87, 147.13, 136.98, 130.59, 129.66, 121.95, 115.74, 110.93, 80.88, 67.28, 60.89, 55.66, 29.86.

1-Isopropoxy-2-methoxynaphthalene (5b). Yield = 90% (194 mg); colorless oil; HRMS (ESI, M⁺ + 1) calcd for C₁₄H₁₇O₂ 217.1229, found 217.1230; ¹H NMR (400 MHz, CDCl₃): δ 8.19 (d, J = 8.4 Hz, 1H), 7.78 (d, J = 8.4 Hz, 1H), 7.59 (d, J = 9.2 Hz, 1H), 7.49–7.45 (m, 1H), 7.38–7.34 (m, 1H), 7.30 (d, J = 8.8 Hz, 1H), 4.72–4.65 (m, 1H), 3.98 (s, 3H), 1.40 (d, J = 6.0 Hz, 6H); ¹³C NMR (100 MHz, CDCl₃): δ 148.53, 140.93, 130.40, 129.72, 127.43, 125.67, 123.88, 123.67, 122.17, 115.36, 75.30, 56.84, 22.71 (2x).

1-Butoxy-2-methoxynaphthalene (5c). Yield = 91% (209 mg); colorless oil; HRMS (ESI, M⁺ + 1) calcd for C₁₅H₁₉O₂ 231.1385, found 231.1388; ¹H NMR (400 MHz, CDCl₃): δ 8.17 (d, J = 8.4 Hz, 1H), 7.79 (d, J = 8.4 Hz, 1H), 7.60 (d, J = 8.8 Hz, 1H), 7.50–7.46 (m, 1H), 7.39–7.35 (m, 1H), 7.30 (d, J = 8.8 Hz, 1H), 4.16 (t, J = 6.4 Hz, 2H), 3.99 (s, 3H), 1.94–1.87 (m, 2H), 1.67–1.58 (m, 2H), 1.04 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 148.36, 142.34, 129.77, 129.45, 127.55, 125.88, 123.98, 123.86, 121.52, 115.53, 73.43, 57.00, 32.50, 19.34, 13.94.

1-Cyclopentyloxy-2-methoxynaphthalene (5d). Yield = 88% (213 mg); colorless oil; HRMS (ESI, M⁺ + 1) calcd for C₁₆H₁₉O₂ 243.1385, found 243.1382; ¹H NMR (400 MHz, CDCl₃): δ 8.13 (d, J = 8.0 Hz, 1H), 7.76 (d, J = 8.0 Hz, 1H), 7.57 (d, J = 8.8 Hz, 1H), 7.47–7.43 (m, 1H), 7.37–7.33 (m, 1H), 7.29 (d, J = 8.8 Hz, 1H), 5.09–5.05 (m, 1H), 3.97 (s, 3H), 2.04–1.92 (m, 4H), 1.80–1.72 (m, 2H), 1.70–1.61 (m, 2H); ¹³C NMR (100 MHz, CDCl₃): δ 148.33, 141.15, 130.34, 129.81, 127.48, 125.70, 123.95, 123.55, 121.98, 115.71, 84.79, 57.00, 32.91 (2x), 23.78 (2x).

2-Methoxy-5-phenylnaphthalen-1-ol (5e). Yield = 95% (238 mg); colorless solid; HRMS (ESI, M⁺ + 1) calcd for C₁₇H₁₅O₂ 251.1072, found 251.1078; ¹H NMR (400 MHz, CDCl₃): δ 8.22 (d, J = 8.4 Hz, 1H), 7.56–7.41 (m, 7H), 7.32 (d, J = 6.8 Hz, 1H), 7.19 (d, J = 9.2 Hz, 1H), 6.13 (br s, 1H), 3.98 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 141.02, 139.83, 139.69, 129.98 (2x), 128.90, 128.12 (2x), 127.70, 127.11, 125.33, 124.92, 124.34, 120.72, 117.93, 112.93, 57.00. Single-crystal X-ray diagram: crystal of compound 5e was grown by slow diffusion of EtOAc into a solution of compound 5e in CH₂Cl₂ to yield colorless prisms. The compound crystallizes in the orthorhombic crystal system, space group Fdd2, a = 13.3885(3) Å, b = 66.3135(18) Å, c = 5.5816(2) Å, V = 4955.6(2) ų, Z = 16, d_calcd = 1.342 g cm⁻³, F(000) = 2112, 2θ range 1.23–26.39°, R indices (all data) R₁ = 0.0428, wR₂ = 0.1154.

5-(4-Fluorophenyl)-2-methoxynaphthalen-1-ol (5f). Yield = 90% (241 mg); colorless solid; HRMS (ESI, M⁺ + 1) calcd for C₁₇H₁₄FO₂ 269.0978, found 269.0983; ¹H NMR (400 MHz, CDCl₃): δ 8.26 (d, J = 8.4 Hz, 1H), 7.53–7.39 (m, 4H), 7.7.29 (dd, J = 1.2, 6.8 Hz, 1H), 7.22–7.17 (m, 3H), 6.18 (br s, 1H), 3.99 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 162.16 (d, J = 244.1 Hz), 141.08, 139.74, 138.71, 136.87 (d, J = 3.0 Hz), 131.46 (d, J = 17.5 Hz, 2x), 127.71, 125.41, 124.87, 124.35, 120.92, 117.65, 115.02 (d, J = 21.2 Hz, 2x), 113.01, 56.94. Single-crystal X-ray diagram: crystal of compound 5f was grown by slow diffusion of EtOAc into a solution of compound 5f in CH₂Cl₂ to yield colorless prisms. The compound crystallizes in the triclinic crystal system, space group P1̄, a = 6.7021(11) Å, b = 7.9371(12) Å, c = 12.170(2) Å, V = 631.92(18) ų, Z = 2, d_calcd = 1.410 g cm⁻³, F(000) = 280, 2θ range 1.70–26.38°, R indices (all data) R₁ = 0.0431, wR₂ = 0.1097.

1-Allyloxy-2-methoxy-5-o-tolylnaphthalene (5g). Yield = 91% (277 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₂₁H₂₁O₂ 305.1542, found 305.1549; ¹H NMR (400 MHz, CDCl₃): δ 8.19 (dt, J = 0.4, 8.4 Hz, 1H), 7.51 (dd, J = 6.8, 8.4 Hz, 1H), 7.38–7.16 (m, 7H), 6.30–6.20 (m, 1H), 5.49 (dq, J = 1.6, 17.2 Hz, 1H), 5.30 (dq, J = 1.6, 10.4 Hz, 1H), 4.71 (dt, J = 1.6, 6.0 Hz, 2H), 3.97 (s, 3H), 2.03 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 148.20, 141.77, 140.33, 139.58, 136.76, 134.49, 130.30, 129.81, 129.61, 128.23, 127.50, 125.68, 125.48, 124.89, 122.60, 121.00, 117.39, 114.90, 74.33, 56.80, 20.01.

1-Allyloxy-2-methoxy-5-(2-methoxyphenyl)naphthalene (5h). Yield = 88% (282 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₂₁H₂₁O₃ 321.1491, found 321.1496; ¹H NMR (400 MHz, CDCl₃): δ 8.19 (dt, J = 1.2, 8.4 Hz, 1H), 7.52 (dd, J = 7.2, 8.8 Hz, 1H), 7.43 (ddd, J = 1.6, 7.2, 9.2 Hz, 1H), 7.34 (dd, J = 0.8, 9.2 Hz, 1H), 7.30–7.27 (m, 2H), 7.18 (d, J = 9.2 Hz, 1H), 7.09 (dd, J = 0.8, 7.2 Hz, 1H), 7.05 (d, J = 7.2 Hz, 1H), 6.30–6.20 (m, 1H), 5.49 (dq, J = 1.6, 17.2 Hz, 1H), 5.29 (dq, J = 1.6, 10.4 Hz, 1H), 4.70 (dt, J = 1.6, 6.0 Hz, 2H), 3.97 (s, 3H), 3.71 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 157.19, 148.08, 141.68, 136.74, 134.56, 131.91, 129.64, 129.57, 128.92, 128.36, 125.67, 125.60, 122.94, 121.22, 120.50, 117.34, 114.76, 111.01, 74.33, 56.82, 55.59.

1-Allyloxy-2-methoxy-5-(3-methoxyphenyl)naphthalene (5i). Yield = 95% (304 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₂₁H₂₁O₃ 321.1491, found 321.1499; ¹H NMR (400 MHz, CDCl₃): δ 8.18 (d, J = 8.4 Hz, 1H), 7.68 (d, J = 9.2 Hz, 1H), 7.51 (dd, J = 7.2, 8.4 Hz, 1H), 7.40 (t, J = 8.0 Hz, 1H), 7.31 (dd, J = 0.8, 6.8 Hz, 1H), 7.23 (d, J = 9.2 Hz, 1H), 7.09–7.06 (m, 1H), 7.04 (t, J = 2.4 Hz, 1H), 6.98 (ddd, J = 0.8, 2.8, 8.4 Hz, 1H), 6.29–6.19 (m, 1H), 5.48 (dq, J = 1.6, 17.2 Hz, 1H), 5.29 (dq, J = 1.6, 10.0 Hz, 1H), 4.70 (dt, J = 1.6, 6.0 Hz, 2H), 3.98 (s, 3H), 3.86 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 159.41, 148.23, 142.27, 141.63, 139.95, 134.41, 129.87, 129.15, 127.76, 125.65, 125.03, 122.65, 122.51, 121.21, 117.52, 115.52, 114.87, 112.86, 74.33, 56.78, 55.28.

1-Allyloxy-2-methoxy-5-(4-methoxyphenyl)naphthalene (5j). Yield = 90% (288 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₂₁H₂₁O₃ 321.1491, found 321.1493; ¹H NMR (400 MHz, CDCl₃): δ 8.15 (dt, J = 0.8, 8.4 Hz, 1H), 7.66 (dd, J = 0.8, 9.2 Hz, 1H), 7.48 (dd, J = 7.2, 8.8 Hz, 1H), 7.40 (d, J = 8.8 Hz, 2H), 7.27 (dd, J = 0.8, 7.2 Hz, 1H), 7.21 (d, J = 9.2 Hz, 1H), 7.01 (d, J = 8.8 Hz, 2H), 6.27–6.17 (m, 1H), 5.46 (dq, J = 1.6, 17.2 Hz, 1H), 5.27 (dq, J = 1.6, 10.4 Hz, 1H), 4.68 (dt, J = 1.6, 6.0 Hz, 2H), 3.96 (s, 3H), 3.88 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 158.90, 148.19, 141.66, 139.76, 134.43, 133.24, 131.03 (2x), 129.93, 128.05, 125.73, 125.18, 122.67, 120.84, 117.49, 114.76, 113.64 (2x), 74.32, 56.79, 55.32.

1-Allyloxy-5-(4-fluorophenyl)-2-methoxynaphthalene (5k). Yield = 91% (280 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₂₀H₁₈FO₂ 309.1291, found 309.1293; ¹H NMR (400 MHz, CDCl₃): δ 8.22 (dt, J = 0.8, 8.4 Hz, 1H), 7.60 (dd, J = 0.8, 9.2 Hz, 1H), 7.51 (dd, J = 6.8, 8.4 Hz, 1H), 7.47–7.42 (m, 2H), 7.27 (dd, J = 1.2, 7.2 Hz, 1H), 7.24 (d, J = 9.2 Hz, 1H), 7.21–7.15 (m, 2H), 6.30–6.20 (m, 1H), 5.49 (dq, J = 1.6, 17.2 Hz, 1H), 5.30 (dq, J = 1.6, 10.4 Hz, 1H), 4.72 (dt, J = 1.6, 6.0 Hz, 2H), 3.99 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 162.21 (d, J = 244.1 Hz), 148.26, 141.69, 138.97, 136.76 (d, J = 3.0 Hz), 134.36, 131.46 (d, J = 7.5 Hz, 2x), 129.90, 127.82, 125.64, 125.25, 122.31, 121.32, 117.52, 115.09 (d, J = 21.2 Hz, 2x), 114.99, 74.31, 56.75.

5-(5-Allyloxy-6-methoxynaphthalen-1-yl)benzo[1,3]dioxole (5l). Yield = 88% (294 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₂₁H₁₉O₄ 335.1283, found 335.1288; ¹H NMR (400 MHz, CDCl₃): δ 8.16 (dt, J = 1.2, 8.4 Hz, 1H), 7.68 (dd, J = 1.2, 9.2 Hz, 1H), 7.48 (dd, J = 6.8, 8.4 Hz, 1H), 7.26 (dd, J = 1.2, 7.2 Hz, 1H), 7.23 (d, J = 9.2 Hz, 1H), 6.96 (br s, 1H), 6.93 (br s, 2H), 6.27–6.17 (m, 1H), 6.04 (s, 2H), 5.47 (dq, J = 1.6, 16.8 Hz, 1H), 5.28 (dq, J = 1.6, 10.4 Hz, 1H), 4.68 (dt, J = 1.6, 6.0 Hz, 2H), 3.98 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 148.23, 147.42, 141.66, 139.66, 134.74, 134.41, 129.92, 127.96, 126.01, 125.67, 125.16, 123.35, 122.58, 121.06, 117.52, 114.85, 110.57, 108.17, 101.09, 74.33, 56.81.

2-Allyl-5-(4-methoxyphenyl)naphthalen-1-ol (5m). Yield = 95% (276 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₂₀H₁₉O₂ 291.1385, found 291.1386; ¹H NMR (400 MHz, CDCl₃): δ 8.20 (dt, J = 1.2, 8.4 Hz, 1H), 7.51 (dd, J = 6.8, 8.4 Hz, 1H), 7.46 (dd, J = 0.8, 8.4 Hz, 1H), 7.42 (d, J = 8.8 Hz, 2H), 7.38 (dd, J = 1.2, 6.8 Hz, 1H), 7.17 (d, J = 8.4 Hz, 1H), 7.03 (d, J = 8.8 Hz, 2H), 6.14–6.04 (m, 1H), 5.60 (br s, 1H), 5.30–5.23 (m, 2H), 3.90 (s, 3H), 3.58 (dt, J = 1.6, 6.4 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃): δ 158.88, 149.77, 139.59, 136.05, 133.35, 132.19, 131.07 (2x), 128.27, 126.89, 125.24, 124.86, 120.60, 118.65, 117.57, 117.05, 113.63 (2x), 55.33, 35.73.

2-Allyl-1-methoxy-5-(4-methoxyphenyl)naphthalene (5n). Yield = 90% (274 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₂₁H₂₁O₂ 305.1542, found 305.1548; ¹H NMR (400 MHz, CDCl₃): δ 8.16 (dt, J = 1.2, 8.4 Hz, 1H), 7.69 (dd, J = 0.8, 8.4 Hz, 1H), 7.56 (dd, J = 7.2, 8.4 Hz, 1H), 7.43 (d, J = 8.8 Hz, 2H), 7.40 (dd, J = 1.6, 6.8 Hz, 1H), 7.29 (d, J = 8.8 Hz, 1H), 7.05 (d, J = 8.8 Hz, 2H), 6.12–6.02 (m, 1H), 5.16–5.11 (m, 2H), 3.99 (s, 3H), 3.91 (s, 3H), 3.64 (dt, J = 1.6, 6.4 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃): δ 158.89, 153.47, 140.10, 137.11, 133.18, 132.25, 131.03 (2x), 128.55, 128.18, 127.76, 126.64, 125.45, 122.25, 121.25, 115.85, 113.64 (2x), 62.14, 55.29, 33.82.

5-(6-Allyl-5-methoxynaphthalen-1-yl)benzo[1,3]dioxole (5o). Yield = 91% (289 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₂₁H₁₉O₃ 319.1334, found 319.1336; ¹H NMR (400 MHz, CDCl₃): δ 8.13 (dt, J = 1.2, 8.4 Hz, 1H), 7.67 (d, J = 8.8 Hz, 1H), 7.52 (dd, J = 7.2, 8.4 Hz, 1H), 7.37 (dd, J = 1.2, 6.8 Hz, 1H), 7.27 (d, J = 8.8 Hz, 1H), 6.97 (t, J = 1.6 Hz, 1H), 6.93 (br s, 2H), 6.04 (s, 2H), 6.10–6.00 (m, 1H), 5.14–5.08 (m, 2H), 3.96 (s, 3H), 3.61 (dt, J = 1.6, 6.4 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃): δ 153.50, 147.44, 146.86, 140.04, 137.10, 134.70, 132.21, 128.56, 128.30, 127.86, 126.64, 125.41, 123.37, 122.16, 121.48, 115.90, 110.58, 108.17, 101.10, 62.18, 33.82.

A representative procedure of skeleton 7 is as follows

Sn(OTf)₂ (420 mg, 1.0 mmol) was added to a solution of 4j or 4l (1.0 mmol) in MeNO₂ (5 mL) at 25 °C. The reaction mixture was stirred for 8 h at 25 °C and the solvent was concentrated. The residue was diluted with water (10 mL) and the mixture was extracted with CH₂Cl₂ (3 × 20 mL). The combined organic layers were washed with brine, dried, filtered and evaporated to afford crude product. Without further purification, K₂CO₃ (276 mg, 2.0 mmol) was added to a solution of the resulting products in acetone (10 mL) at 25 °C for 10 min. MeI (426 mg, 3.0 mmol) was added to the reaction mixture. The reaction mixture was stirred for 8 h at reflux, cooled to 25 °C and the solvent was concentrated. The residue was diluted with water (10 mL) and the mixture was extracted with CH₂Cl₂ (3 × 20 mL). The combined organic layers were washed with brine, dried, filtered and evaporated to afford crude product. Purification on silica gel (hexanes/EtOAc = 100/1–10/1) afforded 7.

4-Allyl-1,2-dimethoxy-5-(4-methoxyphenyl)naphthalene (7a). Yield = 85% (284 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₂₂H₂₃O₃ 335.1647, found 335.1652; ¹H NMR (400 MHz, CDCl₃): δ 8.21 (dt, J = 1.2, 8.4 Hz, 1H), 7.44 (dd, J = 6.8, 8.4 Hz, 1H), 7.24 (d, J = 8.8 Hz, 2H), 7.17 (dd, J = 1.2, 6.8 Hz, 1H), 7.13 (s, 1H), 6.93 (d, J = 8.8 Hz, 2H), 5.70–5.60 (m, 1H), 4.92 (dq, J = 1.6, 10.0 Hz, 1H), 4.73 (dq, J = 1.6, 17.2 Hz, 1H), 4.01 (s, 3H), 3.98 (s, 3H), 3.88 (s, 3H), 3.13 (dt, J = 1.6, 6.4 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃): δ 158.64, 147.29, 141.43, 139.61, 138.20, 137.16, 134.02, 131.03, 130.21 (2x), 128.53, 124.70, 121.42, 117.95, 115.56, 113.64, 113.17 (2x), 61.08, 56.69, 55.28, 39.75.

5-(8-Allyl-5,6-dimethoxynaphthalen-1-yl)benzo[1,3]dioxole (7b). Yield = 80% (278 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₂₂H₂₁O₄ 349.1440, found 349.1450; ¹H NMR (400 MHz, CDCl₃): δ 8.21 (dd, J = 1.2, 8.4 Hz, 1H), 7.43 (dd, J = 7.2, 8.4 Hz, 1H), 7.17 (dd, J = 1.6, 7.2 Hz, 1H), 7.14 (s, 1H), 6.84 (d, J = 8.0 Hz, 1H), 6.81 (d, J = 1.6 Hz, 1H), 6.77 (dd, J = 1.6, 8.0 Hz, 1H), 6.05 (d, J = 1.2 Hz, 1H), 6.01 (d, J = 1.2 Hz, 1H), 5.74–5.64 (m, 1H), 4.96 (dq, J = 1.6, 10.4 Hz, 1H), 4.78 (dq, J = 1.6, 17.2 Hz, 1H), 4.00 (s, 3H), 3.98 (s, 3H), 3.19 (d, J = 6.0 Hz, 2H); ¹³C NMR (100 MHz, CDCl₃): δ 147.35, 147.02, 146.57, 141.44, 139.42, 138.54, 138.18, 133.88, 130.75, 128.40, 126.69, 124.66, 122.54, 121.63, 118.03, 115.65, 110.02, 107.77, 101.03, 61.09, 56.69, 39.61.

A representative procedure of skeleton 8 is as follows

Sn(OTf)₂ (42 mg, 0.1 mmol) was added to a solution of 4p–v (1.0 mmol) in MeNO₂ (5 mL) at 25 °C. The reaction mixture was stirred for 8 h at 25 °C and the solvent was concentrated. The residue was diluted with water (10 mL) and the mixture was extracted with CH₂Cl₂ (3 × 20 mL). The combined organic layers were washed with brine, dried, filtered and evaporated to afford crude product. Purification on silica gel (hexanes/EtOAc = 100/1–10/1) afforded 8.

1,2-Dimethoxy-4-phenylnaphthalene (8a). Yield = 81% (214 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₁₈H₁₇O₂ 265.1229, found 265.1230; ¹H NMR (400 MHz, CDCl₃): δ 8.22 (d, J = 8.8 Hz, 1H), 7.81 (d, J = 8.8 Hz, 1H), 7.53–7.43 (m, 6H), 7.34–7.30 (m, 1H), 7.27 (s, 1H), 4.06 (s, 3H), 4.02 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 147.55, 142.32, 140.49, 136.89, 130.07 (2x), 129.32, 128.26 (2x), 127.74, 127.28, 126.05 (2x), 124.14, 121.54, 116.40, 61.14, 56.91.

4-(4-Fluorophenyl)-1,2-dimethoxynaphthalene (8b). Yield = 74% (209 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₁₈H₁₆FO₂ 283.1134, found 283.1139; ¹H NMR (400 MHz, CDCl₃): δ 8.20 (d, J = 8.8 Hz, 1H), 7.74 (d, J = 8.8 Hz, 1H), 7.52–7.48 (m, 1H), 7.47–7.42 (m, 2H), 7.34–7.30 (m, 1H), 7.21 (s, 1H), 7.20–7.16 (m, 2H), 4.05 (s, 3H), 4.01 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 162.28 (d, J = 244.9 Hz), 147.54, 142.51, 136.39 (d, J = 3.1 Hz), 135.75, 131.59 (d, J = 7.6 Hz, 2x), 129.35, 127.83, 126.12, 125.81, 124.31, 121.63, 116.57, 115.21 (d, J = 21.2 Hz, 2x), 61.16, 56.99.

1,2-Dimethoxy-4-p-tolylnaphthalene (8c). Yield = 77% (214 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₁₉H₁₉O₂ 279.1385, found 279.1388; ¹H NMR (400 MHz, CDCl₃): δ 8.23–8.20 (m, 1H), 7.85–7.82 (m, 1H), 7.50 (ddd, J = 1.2, 6.8, 8.4 Hz, 1H), 7.40 (d, J = 8.0 Hz, 2H), 7.34–7.29 (m, 3H), 7.25 (s, 1H), 4.06 (s, 3H), 4.02 (s, 3H), 2.48 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 147.57, 142.17, 137.55, 137.01, 136.91, 129.94 (2x), 129.32, 128.98 (2x), 127.83, 126.13, 125.99, 124.05, 121.51, 116.33, 61.13, 56.88, 21.19.

1,2-Dimethoxy-4-(4-methoxyphenyl)naphthalene (8d). Yield = 76% (223 mg); colorless solid; mp = 118–120 °C (recrystallized from hexanes and EtOAc); HRMS (ESI, M⁺ + 1) calcd for C₁₉H₁₉O₃ 295.1334, found 295.1338; ¹H NMR (400 MHz, CDCl₃): δ 8.20 (d, J = 8.4 Hz, 1H), 7.82 (d, J = 8.4 Hz, 1H), 7.51–7.47 (m, 1H), 7.35 (d, J = 8.4 Hz, 2H), 7.33–7.29 (m, 1H), 7.23 (s, 1H), 7.41 (d, J = 8.8 Hz, 2H), 4.05 (s, 3H), 4.01 (s, 3H), 3.90 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 159.00, 147.57, 142.10, 136.60, 132.85, 131.11 (2x), 129.34, 127.96, 126.12, 125.99, 124.05, 121.52, 116.36, 113.74 (2x), 61.14, 56.90, 55.36. Single-crystal X-ray diagram: crystal of compound 8d was grown by slow diffusion of EtOAc into a solution of compound 8d in CH₂Cl₂ to yield colorless prisms. The compound crystallizes in the monoclinic crystal system, space group P2₁/c, a = 13.3450(12) Å, b = 15.2538(14) Å, c = 7.5723(7) Å, V = 1518.9(2) ų, Z = 4, d_calcd = 1.287 g cm⁻³, F(000) = 624, 2θ range 2.045–26.421°, R indices (all data) R₁ = 0.1172, wR₂ = 0.1285.

1,2-Dimethoxy-4-(4-trifluoromethylphenyl)naphthalene (8e). Yield = 80% (266 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₁₉H₁₆F₃O₂ 333.1102, found 333.1108; ¹H NMR (400 MHz, CDCl₃): δ 8.26 (d, J = 8.4 Hz, 1H), 7.78 (d, J = 8.0 Hz, 2H), 7.74 (d, J = 8.4 Hz, 1H), 7.63 (d, J = 8.0 Hz, 2H), 7.55–7.51 (m, 1H), 7.37–7.33 (m, 1H), 4.09 (s, 3H), 4.04 (s, 3H), 7.27 (s, 1H); ¹³C NMR (100 MHz, CDCl₃): δ 147.54, 144.19, 142.97, 135.19, 130.41 (2x), 129.51 (d, J = 31.9 Hz), 129.40, 127.44, 126.26, 125.48, 125.23 (q, J = 3.8 Hz, 2x), 124.58, 124.28 (d, J = 270.6 Hz), 121.74, 116.60, 61.14, 56.97.

1,2-Dimethoxy-4-(2-methoxyphenyl)naphthalene (8f). Yield = 80% (235 mg); colorless gum; HRMS (ESI, M⁺ + 1) calcd for C₁₉H₁₉O₃ 295.1334, found 295.1336; ¹H NMR (400 MHz, CDCl₃): δ 8.20–8.17 (m, 1H), 7.51–7.46 (m, 2H), 7.44 (dd, J = 1.6, 8.0 Hz, 1H), 7.31–7.264 (m, 2H), 7.24 (s, 1H), 7.10 (dd, J = 1.2, 7.6 Hz, 1H), 7.07 (d, J = 8.0 Hz, 1H), 4.06 (s, 3H), 4.00 (s, 3H), 3.73 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 157.22, 147.56, 142.21, 133.41, 132.04, 129.18, 129.06, 129.02, 128.23, 126.42, 125.80, 123.76, 121.38, 120.53, 116.74, 111.03, 61.11, 56.81, 55.56.

4-Biphenyl-4-yl-1,2-dimethoxynaphthalene (8g). Yield = 75% (255 mg); colorless solid; mp = 118–120 °C (recrystallized from hexanes and EtOAc); HRMS (ESI, M⁺ + 1) calcd for C₂₄H₂₁O₂ 341.1542, found 341.1548; ¹H NMR (400 MHz, CDCl₃): δ 8.24–8.21 (m, 1H), 7.90–7.88 (m, 1H), 7.74 (d, J = 8.4 Hz, 2H), 7.71–7.69 (m, 2H), 7.58 (d, J = 8.4 Hz, 2H), 7.54–7.48 (m, 3H), 7.42–7.38 (m, 1H), 7.36–7.32 (m, 1H), 7.30 (s, 1H), 4.07 (s, 3H), 4.03 (s, 3H); ¹³C NMR (100 MHz, CDCl₃): δ 147.62, 142.42, 140.77, 140.21, 139.48, 136.47, 130.51 (2x), 129.39, 128.85 (2x), 127.76, 127.40, 127.12 (2x), 127.02 (2x), 126.11, 126.06, 124.23, 121.60, 116.42, 61.18, 56.95. Single-crystal X-ray diagram: crystal of compound 8g was grown by slow diffusion of EtOAc into a solution of compound 8g in CH₂Cl₂ to yield colorless prisms. The compound crystallizes in the triclinic crystal system, space group P1̄, a = 6.6111(3) Å, b = 9.8830(5) Å, c = 14.4700(8) Å, V = 867.14(8) ų, Z = 2, d_calcd = 1.304 g cm⁻³, F(000) = 360, 2θ range 1.488–26.356°, R indices (all data) R₁ = 0.0460, wR₂ = 0.1231.

A representative procedure of skeleton 9 is as follows

Sn(OTf)₂ (42 mg, 0.1 mmol) was added to a solution of 4w–x (1.0 mmol) in MeNO₂ (5 mL) at 25 °C. The reaction mixture was stirred for 8 h at 25 °C and the solvent was concentrated. The residue was diluted with water (10 mL) and the mixture was extracted with CH₂Cl₂ (3 × 20 mL). The combined organic layers were washed with brine, dried, filtered and evaporated to afford crude product. Purification on silica gel (hexanes/EtOAc = 100/1–10/1) afforded 9.

6-Methoxy-1-(4-methoxyphenyl)naphthalene (9a). Yield = 61% (161 mg); colorless solid; mp = 83–85 °C (recrystallized from hexanes and EtOAc); HRMS (ESI, M⁺ + 1) calcd for C₁₈H₁₇O₂ 265.1229, found 265.1229; ¹H NMR (400 MHz): δ 7.84 (d, J = 9.2 Hz, 1H), 7.74 (d, J = 8.4 Hz, 1H), 7.50–7.40 (m, 3H), 7.27 (dd, J = 1.2, 6.8 Hz, 1H), 7.20 (d, J = 2.8 Hz, 1H), 7.10 (dd, J = 2.8, 9.2 Hz, 1H), 7.05–7.01 (m, 2H), 3.95 (s, 3H), 3.90 (s, 3H); ¹³C NMR (100 MHz): δ 158.89, 157.39, 139.89, 135.10, 133.23, 131.02 (2x), 127.70, 127.23, 126.16, 126.03, 124.75, 118.52, 114.14, 113.67 (2x), 55.34, 55.30. Single-crystal X-ray diagram: crystal of compound 9a was grown by slow diffusion of EtOAc into a solution of compound 9a in CH₂Cl₂ to yield colorless prisms. The compound crystallizes in the monoclinic crystal system, space group P12₁/c1, a = 7.654(4) Å, b = 13.820(8) Å, c = 13.657(8) Å, V = 1429.0(14) ų, Z = 4, d_calcd = 1.229 g cm⁻³, F(000) = 560, 2θ range 2.11–26.46°, R indices (all data) R₁ = 0.0711, wR₂ = 0.1581.

2,6-Dimethoxy-1-(4-methoxyphenyl)naphthalene (9b). Yield = 40% (118 mg); colorless solid; mp = 65–67 °C (recrystallized from hexanes and EtOAc); HRMS (ESI, M⁺ + 1) calcd for C₁₉H₁₉O₃ 295.1334, found 295.1335; ¹H NMR (400 MHz, CDCl₃): δ 7.82 (d, J = 8.8 Hz, 1H), 7.76 (d, J = 8.8 Hz, 1H), 7.51–7.41 (m, 2H), 7.27 (dd, J = 1.2, 6.8 Hz, 1H), 7.21 (d, J = 2.4 Hz, 1H), 7.11 (dd, J = 2.8, 9.2 Hz, 1H), 7.02–6.96 (m, 2H), 3.98 (s, 3H), 3.97 (s, 3H), 3.90 (s, 3H). Single-crystal X-ray diagram: crystal of compound 9b was grown by slow diffusion of EtOAc into a solution of compound 9b in CH₂Cl₂ to yield colorless prisms. The compound crystallizes in the monoclinic crystal system, space group C1c1, a = 8.6905(3) Å, b = 21.9250(7) Å, c = 8.2652(3) Å, V = 1494.97(9) ų, Z = 4, d_calcd = 1.308 g cm⁻³, F(000) = 624, 2θ range 1.86–26.52°, R indices (all data) R₁ = 0.0291, wR₂ = 0.0743.

Naphthalene (10a). Na₂CO₃ (106 mg, 1.0 mmol) was added to a solution of allyl bromide (240 mg, 2.0 mmol) in dimethoxyethane (DME, 5 mL) at 25 °C. The reaction mixture was stirred at 25 °C for 5 min. Then, o-formylphenylboronic acid (150 mg, 1.0 mmol) was added to the reaction mixture at 25 °C. The reaction mixture was stirred at 25 °C for 5 min. Pd(OAc)₂ (11 mg, 0.05 mmol) was added to the reaction mixture at 25 °C. The reaction mixture was stirred at 80 °C for 5 h. The reaction mixture (in situ formed 4y) was cooled to 25 °C and Sn(OTf)₂ (42 mg, 0.1 mmol) was added to the reaction mixture. The reaction mixture was stirred for 8 h at 25 °C and the solvent was concentrated. The residue was diluted with water (10 mL) and the mixture was extracted with CH₂Cl₂ (3 × 20 mL). The combined organic layers were washed with brine, dried, filtered and evaporated to afford crude product. Purification on silica gel (hexanes/EtOAc = 100/1–10/1) afforded 10a (51%, 65 mg).

Acknowledgements

The authors would like to thank the Ministry of Science and Technology of the Republic of China for its financial support (MOST 105-2113-M-037-001).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 790259, 923282–923284, 1519809 and 1523594. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7ra04960a

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