Paternò–Büchi reaction between aromatic carbonyl compounds and 1-(3-furyl)alkanols

Abstract

The photochemical reaction between aromatic carbonyl compounds and 3-furylmethanol derivatives occurs with high regioselectivity. In most of the experiments formation of oxetanes occurs at the hydroxyalkylated double bond. With chiral 1-(3-furyl)alkanols the reaction occurs with good-high stereoselectivity. In the case of 1-(3-furyl)-benzyl alcohol the stereoselectivity can be explained on the basis of the conformers of the reagent, assuming the formation of a complex between the carbonyl compound and the hydroxyl group.

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Introduction

The stereoselectivity of the Paternò–Büchi reaction on allylic alcohols has been studied in the last years.¹ Adam and co-workers showed that the allylic alcohol 1 reacted with benzophenone to give the corresponding adducts 2 and 3 with high regio- and stereoselectivity (Scheme 1).²

Scheme 1 Paternò–Büchi reaction on allylic alcohols.

The reaction of 1-(2-furyl)ethanol 4 with benzophenone gave a 1 : 1 mixture of two diastereoisomers (Scheme 2).³ On the contrary, enantiopure 1-(2-furyl)-benzyl alcohol 6 gave the corresponding adduct 7 with d.e. > 98% (Scheme 2). The reaction of 1-(2-furyl)alkanols with aliphatic aldehydes and ketones gave the corresponding adducts 8 with high regioselectivity but no diastereoselectivity (Scheme 2).⁴ Finally, the reaction of 1-(2-furyl)-1-phenylethanol 9 with benzophenone gave the adduct 10 with d.e. = 48% (Scheme 2).⁵

Scheme 2 Paternò–Büchi reaction on 1-(2-furyl)alkanols.

These results were explained assuming the attack of the excited carbonyl compound on the C2/C3 double bond, through the formation of a hydrogen bond or of a complex. This type of attack gave the corresponding biradical intermediate in preferential conformations. The relative energies of these conformers account for the observed stereoselectivity.^5,6

On the basis of these results we were not able to predict whether the presence of a hydroxyl group in the β position on the furan ring could induce regio- and stereoselectivity in the reaction with aromatic carbonyl compounds. Furthermore, recently no hydroxy directing effect was found in 2,3-dihydrofuran-3-ol derivatives.⁷ In this work we tested the reactivity of some 1-(3-furyl)alkanol derivatives.

Materials and methods

Photochemical synthesis of oxetanes—general procedure

3-Furylmethanol derivative (3 mmol) was dissolved in benzene (70 mL) in the presence of the carbonyl compound (4.5 mmol). The mixture was purged with nitrogen for 1 h and then irradiated with a 125 W high pressure mercury arc (Helios-Italquartz, Milan, Italy). At the end of the reaction the solvent was evaporated and the crude product was chromatographed on silica gel. Elution with ethyl acetate–hexanes 1 : 9 gave pure products. All the yields reported refer to isolated chromatographically pure products.

5-Hydroxymethyl-6,6-diphenyl-2,7-dioxabicyclo[3.2.0]-3-heptene 12

δ _H (300 MHz; CDCl₃; Me₄Si) 7.4–7.2 (10 H, m, aromatic protons), 6.44 (1 H, d, J 2.6, 3-H), 6.26 (1 H, s, 1-H), 5.37 (1 H, d, J 2.6, 4-H), 3.71 (1 H, d, J 11, CH_aH_bOH), 3.54 (1 H, d, J 11, CH_aH_bOH), 1.42 (1 H, br s, OH); δ_C (CDCl₃; Me₄Si) 147.7 (C-3), 128.9, 128.0, 127.8, 127.1, 125.9, 105.9 (C-4), 105.6 (C-1), 96.9 (C-6), 66.6 (C-5), 62.1 (CH₂OH). Analysis Found: C, 77.20; H, 5.62. C₁₈H₁₆O₃ requires: C, 77.12; H, 5.75%.

5-(1-Hydroxy-n-heptyl)-6,6-diphenyl-2,7-dioxabicyclo-[3.2.0]-3-heptene 14

δ _H (300 MHz; CDCl₃; Me₄Si) 7.7–7.2 (10 H, m, aromatic protons), 6.41 (1 H, d, J 2.4, 3-H), 6.20 (1 H, s, 1-H), 5.35 (1 H, d, J 2.4, 4-H), 3.58 (1 H, m, CHOH), 1.55 (1 H, d, J 4.2, OH), 1.28 (10 H, m, CH₂), 0.88 (3 H, m, CH₃); δ_C (CDCl₃, Me₄Si) 146.9 (C-3), 128.9, 128.7, 128.6, 128.3, 128.1, 127.9, 127.8, 127.6, 127.5, 127.2, 127.0, 126.2, 109.5 (C-4), 105.3 (C-1), 98.1 (C-6), 70.3 (CHOH), 69.6 (C-5), 32.9, 31.9, 30.0, 25.9, 22.8, 14.4. Analysis Found: C, 78.89; H, 7.85. C₂₄H₂₈O₃ requires: C, 79.09; H, 7.74%.

5-(1-Hydroxy-n-heptyl)-6-phenyl-2,7-dioxabicyclo[3.2.0]-3-heptene 15

δ _H (300 MHz; CDCl₃; Me₄Si) 7.5–7.4 (5 H, m, aromatic protons), 6.75 (1 H, s, 1-H), 6.20 (1 H, s, 3-H), 5.77 (1 H, s, 6-H), 5.63 (1 H, d, J 1, 4-H), 3.60 (1 H, m, CHOH), 3.50 (1 H, m, OH), 1.20 (10 H, m, CH₂), 0.86 (3 H, m, CH₃); δ_C (CDCl₃; Me₄Si) 148.3 (C-3), 138.5, 130.7, 129.9, 128.8, 128.5, 126.2, 108.2 (C-4), 105.9 (C-1), 68.8 (C-6), 65.1 (CHOH), 31.6, 31.4, 29.2, 29.0, 22.4, 14.0. Analysis Found: C, 74.81; H, 8.30. C₁₈H₂₄O₃ requires: C, 74.97; H, 8.39%.

4-(1-Hydroxy-n-heptyl)-6-phenyl-2,7-dioxabicyclo[3.2.0]-3-heptene 16

δ _H (500 MHz; CDCl₃; Me₄Si) 7.5–7.2 (5 H, m, aromatic protons), 6.74 (1 H, d, J 2.5, 1-H), 6.38 (1 H, s, 3-H), 5.61 (1 H, s, 6-H), 5.32 (1 H, d, J 2.5, 5-H), 3.47 (1 H, br s, CHOH), 1.25 (1 H, s, OH) 1.2-0.65 (13 H, m); δ_C (CDCl₃; Me₄Si) 150.3 (C-3), 138.2, 129.2, 126.3, 109.7 (C-4), 104.1 (C-1), 94.2 (C-6) 69.0 (C-5), 64.8 (CHOH), 32.3, 31.8, 30.0, 28.5, 25.5, 14.3. Analysis Found: C, 74.78; H, 8.54. C₁₈H₂₄O₃ requires: C, 74.97; H, 8.39%.

5-(α-Hydroxybenzyl)-6,6-diphenyl-2,7-dioxabicyclo[3.2.0]-3-heptene 18

δ _H (500 MHz; CDCl₃; Me₄Si) 7.7–7.1 (15 H, m, aromatic protons), 6.05 (1 H, d, J 3, 3-H), 6.04 (1 H, s, 1-H), 5.39 (1 H, d, J 3, 4-H), 4.62 (1 H, s, CHOH), 1.81 (1 H, br s, OH); δ_C (CDCl₃; Me₄Si) 144.6 (C-3), 140.7, 140.3, 137.3, 126.2, 125.8, 125.7, 125.3, 124.5, 124.2, 124.0, 123.7, 102.1 (C-4), 101.9 (C-1), 95.4 (C-6), 69.5 (CHOH), 67.8 (C-5). Analysis Found: C, 80.97; H, 5.54. C₂₄H₂₀O₃ requires: C, 80.88; H, 5.66%.

Preparation of enantiomerically enriched 1-(3-furyl)alkanols by kinetic resolution

L-(+)-DIPT (diisopropyl L-tartrate) (0.54 mL, 5.15 mmol) was added to a solution of Ti(O-i-Pr)₄ (0.64 mL, 4.30 mmol) in anhydrous CH₂Cl₂ (10.5 mL) at –20 °C. The temperature was lowered to –30 °C and maintained for 10 min. Then, (±)-3-furylmethanol derivative (0.750 g, 4.3 mmol), dissolved in anhydrous CH₂Cl₂ (0.51 mL), and t-butyl hydroperoxide (0.43 mL) were added. The mixture was maintained at –21 °C for 40 h under stirring. The mixture was poured into a mixture of 10% tartaric acid (0.09 mL), diethyl ether (3.51 mL), and NaF (0.54 g). The mixture was stirred at room temperature for 3 h and then filtered over Celite. The solvent was evaporated and the residue was dissolved in diethyl ether (18 mL) and treated with 1 N NaOH (8.8 mL) for 30 min under stirring at 0 °C. The mixture was washed with brine and dried over anhydrous Na₂SO₄. Evaporation of the solvent gave a crude product that was chromatographed on silica gel. The elution with 8 : 2 hexanes–diethyl ether gave pure products.

Acetylation of 3-furylphenylmethyl alcohol

A solution of 17 (610 mg), Et₃N (5.6 mL), acetic anhydride (2.7 mL) and a trace of N,N-dimethylaminopyridine (DMAP) was stirred at room temperature. The reaction was checked by TLC (9 : 1 hexanes–Et₂O) and quenched after two hours with methanol (3–4 mL) by putting the flask in an ice bath. The solvents were evaporated and the residue was treated with n-hexane until the acetic acid was totally removed. The crude product (∼6 g) was chromatographed on a silica gel column (9 : 1 hexanes–Et₂O) giving 189 mg of the acetylated product 19: δ_H (500 MHz; CDCl₃; Me₄Si) 7.45–7.30 (7 H, m), 6.88 (1 H, s), 6.36 (1 H, s), 2.15 (3 H, s); δ_C (CDCl₃; Me₄Si) 170.3, 143.7, 141.1, 139.6, 128.8, 128.4, 127.2, 125.8, 109.9, 70.6, 21.5; m/z, 216 (28%), 175 (15), 174 (82), 157 (30), 156 (32), 155 (24), 145 (25), 129 (30), 128 (100), 127 (22), 115 (10), 102 (14), 77 (20), 43 (38).

Photocycloaddition of acetylated 3-furylphenylmethyl alcohol and benzophenone

A solution of acetylated 3-furylphenylmethyl alcohol (100 mg) and benzophenone (1 g) in benzene (70 mL) was irradiated with a high pressure mercury lamp (125 W) for 27 h. Benzene was evaporated and the residue was chromatographed on a silica gel column (9 : 1hexanes–Et₂O) giving two products (20: 49 mg; 21: 45 mg). 20: δ_H (500 MHz; CDCl₃; Me₄Si) 7.5–6.8 (15 H, m, aromatic protons), 6.71 (1 H, s, CHOAc), 6.27 (1 H, d, J 3, 3-H), 6.08 (1 H, s, 1-H), 5.01 (1 H, d, J 3, 4-H), 1.78 (3 H, s, OAc); δ_C (CDCl₃; Me₄Si) 169.7 (CO), 146.6 (C-3), 105.5 (C-1), 96.9 (C-6), 73.4 (CHOAc), 68.3 (C-5), 20.6 (CH₃). 21: δ_H (500 MHz; CDCl₃; Me₄Si) 7.6–7 (15 H, m, aromatic protons), 6.24 (1 H, s, CHOAc), 6.17 (1 H, d, J 3, 3-H), 5.50 (1 H, s, 1-H), 5.44 (1 H, d, J 3, 4-H), 2.16 (3 H, s, OAc); δ_C (CDCl₃; Me₄Si) 168.2 (CO), 145.8 (C-3), 104.8 (C-1), 97.2 (C-6), 74.2 (CHOAc), 68.3 (C-5), 21.2 (CH₃).

Results and discussion

The irradiation of 3-furylmethanol 11 in the presence of benzophenone with a 125 W high pressure mercury arc gave the corresponding adduct 12 (Scheme 3, Table 1): the reaction occurs only at the C2/C3 double bond. This behaviour is in agreement with our previous reported results in this field. The regioselectivity of the reaction depended on the relative stability of the biradical intermediate.¹ In this case, addition of an excited carbonyl compound to C2 forms a tertiary radical site at C3, while such addition to C5 would generate only a secondary radical at C4. Unfortunately, the reaction of 11 in the presence of benzaldehyde did not occur (Scheme 3, Table 1). Probably, considering the mechanism of this reaction (see below), in this case, the retrocleavage reaction of the biradical intermediate does not allow the isolation of an oxetane.

Table 1 Paternò–Büchi reaction on 3-furylmethanol derivatives

Furan	Carbonyl compound	Irradiation time/h	Product	Yield (%)^a
a All the yields refer to isolated chromatographically pure products.
11	Ph2CO	44	12	42
11	PhCHO	27	—	—
13	Ph2CO	18	14	73
(R)-13	Ph2CO	18	(1S,5S,5′R)-14	56
13	PhCHO	24	15	42
			16	27
(R)-13	PhCHO	24	(1S,5S,6S,5′R)-15	54
			16	36
17	Ph2CO	24	18	58
(R)-17	Ph2CO	24	(1S,5S,5′S)-18	60
17	PhCHO	18	—	—

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Scheme 3 Paternò–Büchi reactions on 3-furylmethanol.

The irradiation of 1-(3-furyl)-n-heptanol 13 in the presence of benzophenone gave the adduct 14 (Scheme 4, Table 1). In this case the reaction could be performed on enantiomerically enriched 13. The kinetic resolution of 13 by using Sharpless oxidation gave (R)-13 in 30% yield with an e.e. = 60% ([α]_D = –20.4 (c 1, CHCl₃)).⁸

Scheme 4 Paternò–Büchi reactions on 1-(3-furyl)-n-heptanol.

The irradiation of (R)-13 in the presence of benzophenone gave the (1S,5S,5′R)-14 (Scheme 4, Table 1). For the stereochemical assignment see below. Chiral HPLC (Chiralcel OD) gave a d.e. = 23%. The irradiation of 13 in the presence of benzaldehyde gave a mixture of regioisomeric products 15 and 16 (Scheme 4, Table 1). The formation of regioisomers is due to the relative stability of the regioisomeric biradical intermediates.¹ The same reaction was performed on (R)-13 showing that (1S,5S,6S,5′R)-15 was obtained with a d.e. = 43% while 16 was obtained as a mixture of diastereoisomers (Scheme 4, Table 1).

The stereochemical assignment of the products could be obtained on the basis of NOE (nuclear Overhauser effect) experiments on (1S,5S,6S,5′R)-15. In fact, the irradiation on the singlet at δ 5.77 ppm (proton at C-6) gave a positive NOE (7%) on the ortho aromatic protons at δ 7.48 ppm. Furthermore, we observed a negative NOE on the CHOH proton at 3.60 ppm. PM3 optimization of the structures showed that the most stable conformation of 15 with 1R,5R,6R,1′R configuration should not present this negative NOE (the distance between the ortho aromatic protons and CHOH was 4.76 Å) while this effect should be observable in the 1S,5S,6S,5′R diastereoisomer (the distance between the ortho aromatic protons and CHOH was 2.48 Å).

The irradiation of the 3-furylmethanol derivative 17 in the presence of benzophenone gave the adduct 18 (Scheme 5, Table 1). Kinetic resolution of 17 gave (R)-17 in 20% yield and e.e = 90% ([α]_D = –9.6 (c 1, CHCl₃)). The reaction of (R)-17 with benzophenone gave the (1S,5S,5′S)-18 diastereoisomer with a d.e. > 99% (Chiralcel OD) ([α]_D = 67.0 (c 1, CHCl₃)) (Scheme 5, Table 1). The relative configuration was assigned on the basis of the NOE experiments on 15. The irradiation of 17 in the presence of benzaldehyde gave only decomposition products (Scheme 5, Table 1). The reaction occurs but we were not able to isolate the product after chromatography. Probably, it decomposed during the purification process.

Scheme 5 Paternò–Büchi reactions on 1-(3-furyl)-benzyl alcohol.

To confirm the presence of a hydroxyl directing effect in this reaction, we performed the reaction of the acetyl derivative of the compound 17 with benzophenone (Scheme 6). The reaction of 19 with benzophenone gave the corresponding adducts 20 and 21 with high regioselectivity. However, it was a 1 : 1 mixture of two diastereoisomeric compounds. This result is in agreement with the presence of a hydroxyl directing effect: if the hydroxy group is not present no diastereoselectivity is observed.

Scheme 6 Formation of oxetanes of 1-(3-furyl)benzyl acetate (19).

As reported above the observed stereoselectivity in the reaction of 2-furylmethanol derivatives was explained on the basis of the possible conformers of the biradical intermediates obtainable admitting a directing effect of the hydroxyl group through coordination or hydrogen bond with the carbonyl compound.⁵ Recently, a transient absorption with λ_max at 531 nm has been observed in the reaction between furan and benzophenone in agreement with the presence of a biradical intermediate.⁹ The presence of the biradical intermediate is in agreement with the mechanism depicted in Scheme 7. We followed the reaction kinetics in order to unravel whether the kinetics of the reactions of 3-furylmethanol derivatives should be compatible with this mechanism. The reaction of 13 with benzophenone follows a second order kinetics (k = 1.6 × 10^–3 M^–1 s^–1, Fig. 1) in agreement with a mechanism depicted in Scheme 7, where k = k₁k₂/k_–1 (see appendix).¹⁰ These results are in agreement with the presence of a biradical intermediate.

Fig. 1 Kinetics of the reaction of 13 with benzophenone.

Scheme 7 Proposed mechanism for the reaction 13 → 14.

To explain the stereochemistry, we performed some calculations (DFT/B3LYP/6-31G+(d,p)).¹¹ We have only two conformers, 17A and 17B. 17A is more stable than the other one by 1.18 kcal mol^–1 (Fig. 2). Then, there is not a preferential conformation.

Fig. 2 Conformers of 17.

We used the approach described in another paper.⁵ The attack of photoexcited benzophenone on 17 affects the formation of diastereoisomeric biradical intermediates. The directing effect exerted by the hydroxyl group is either due to the formation of a hydrogen bond between the hydroxyl group and the oxygen of the excited carbonyl compound, or to the formation of a complex. This type of interaction could favour the formation of a preferential conformation in the biradical intermediate where the hydroxyl group and the oxygen of the carbonyl compound are near. These conformations could have different energies for different diastereoisomeric biradicals, giving an explanation of the observed behaviour. The above described hypothesis requires that the biradical intermediates have a very short lifetime enabling them to equilibrate to the most stable one. In Fig. 3 we reported the calculated conformations of the biradical intermediates.

Fig. 3 Calculated conformations of the diastereomeric biradical intermediates in the reaction between 17 and photoexcited benzophenone.

The above described conformations of 17, when attacked by benzophenone and assuming the formation of a hydrogen bond between the hydroxy group and the oxygen atom of the excited carbonyl compound, can give the biradical intermediate conformation described in Fig. 4. The conformation of the biradical intermediate is able to induce the observed stereoselectivity. In fact, it induces the formation of the 1S,5S stereoisomer, if we start from (R)-17, and the 1R,5R stereoisomer starting from (S)-17.

Fig. 4 The biradical intermediate obtained from the calculated conformations of 17.

In the case of the compound (R)-13, calculations revealed two relevant conformers 13A and 13B (Fig. 5). Also in this case, there is not a preferential conformation (13A is more stable than 13B by 0.7 kcal mol^–1).

Fig. 5 Relevant conformers of 13.

In the reaction of (R)-13 with benzophenone six conformers of the biradical intermediates could be obtained. These conformers are reported in Fig. 6(A).

Fig. 6 (A) Calculated conformations of the two diastereoisomeric biradical intermediates in the reaction between 13 and photoexcited benzophenone. (B) Relevant conformers of the possible biradical intermediates between 13 and photoexcited benzophenone.

Assuming the presence of a hydroxy directing effect, benzophenone can give only four conformers of the biradical intermediates, two of them are RS conformers (22, 24) while the other two are RR ones (25, 27) (Fig. 6(B)).

The energies of the conformers 22, 24, 25, and 27 are reported in Table 2. We can see that RS conformers are more stable than the RR ones. Furthermore, the energy difference between the most stable RS conformer (22) and the most stable RR conformer (25) is 10.88 kcal mol^–1. This energy difference is in agreement with the observed diastereoisomeric excess.⁵

Table 2 Energy (in hartree, E_h) of the relevant conformers obtained in the reaction between 13 and benzophenone

Conformer	Energy/Eh
22	–1157.07698862
24	–1157.07585758
25	–1157.07284048
27	–1157.07134174

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In conclusion, we showed that the Paternò–Büchi reaction on 3-furylmethanols can occur with high regioselectivity and with a good stereoselectivity, depending on the hydroxyl directing effect.

Appendix

For a sequenceAssuming that the reaction of A with B follows a second order kinetics, we haveApplying the steady state approximationThenAssuming k_–1 ≫ k₂, we obtain

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