NHC–gold(I) catalysed [4 + 2] cycloaddition–acyclic addition of dialkyl substituted propargylic esters with 1,3-diphenylisobenzofuran: synthesis of novel benzo[c]fluorenols and substituted dienes

Abstract

A gold carbene complex [IPrAuCl/AgSbF₆] catalysed novel cycloaddition of propargylic esters with 1,3-diphenylisobenzofuran is reported. A [4 + 2] cycloaddition followed by sequential aromatic allylation leading to pentannulation with the expulsion of benzoic acid, then 1,2-phenyl migration coupled with ring opening and aromatisation leading to a new class of benzofluorenols, is discovered. This process involves facile multiple C–C bond formation. An accompanying second pathway involving the attack of benzoyloxy anion on the central allenic carbon affording substituted dienes is also observed. Key products are characterised by X-ray structure determination.

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Introduction

Gold catalysis has emerged as an important area in modern organic synthesis during the last decade.¹ The catalytic activity of gold salts finds many applications in recent methodologies for the construction of new C–C and C–X bonds^1c–f,2 leading to a wide range of heterocycles and carbocycles.³ Due to the alkynophilicity of gold catalysts, propargylic esters can undergo inter-/intra-molecular cycloaddition reactions.⁴ In addition to [4 + 2] cycloaddition,⁵ [4 + 3] cycloaddition of propargylic esters with α,β-unsaturated imines and furan leading to azepines^4a,e and trienes^4c,d respectively has been reported. Pioneering studies of Hashmi's group on gold catalysed intramolecular cyclisation of alkynes and furans has led to a new methodology for the synthesis of various phenol derivatives.⁶ Very recently, formation of phenols via cyclisation of acetylenes and furans has been reported by Echavarren's group.⁷ In continuation of our work on the reactions of propargylic alcohol/ester/and alkyne chemistry,⁸ we report herein NHC–gold(I) catalysed [4 + 2] cycloaddition and acyclic addition of dialkyl substituted propargylic esters with 1,3-diphenylisobenzofuran (IBF) leading to benzo[c]fluorenols and substituted dienes. Benzofluorene derivatives are organic electroluminescent compounds and used as key components in organic light emitting diodes.⁹ They are the core structures for kinamycins which are strongly active natural products against gram positive bacteria.¹⁰ They are also potent anticancer and antimicrobial agents.¹⁰ Natural products seongomycin,^11a cysfluretin,^11b shikometabolins^11c and fluostatins^11d also comprise benzofluorene core structure.

Results and discussion

The propargylic esters 1a–l used in the present study were synthesised from the corresponding silyl substituted propargylic esters (see ESI;† species I–V) or propargylic alcohols.

For optimisation, we chose cycloaddition of propargylic ester 1a with IBF 2 (Scheme 1). First it was confirmed that 1a did not react with 1,3-diphenylisobenzofuran (IBF, 1 : 1.5 molar ratio) in dichloromethane at rt under catalyst-free condition (Table 1, entry 1). Then, gold carbene complex was employed as the catalyst. To our delight, we found that 2 mol% each of IPrAuCl¹² and AgSbF₆ in CH₂Cl₂ worked well to lead to benzofluorenols 3a and substituted diene 4a in 36% and 54% yields respectively (Table 1, entry 2) based on 1a. The combined yield (3a + 4a; after isolation) is excellent (90%). An additional important point is that the R_f values of the two products are significantly different, and hence the benzofluorenols can be very easily isolated. Hence our trials were directed towards improving the yield of benzofluorenol derivatives rather than dienes. Thus when the quantity of IBF was reduced to 1.2 mmol equivalents, the yield of benzofluorenol was increased to 52% whereas yield of diene was 34% [combined yield 86%; Table 1, entry 3]. Equimolar ratio of substrates led to benzofluorenol and diene in 57% and 33% respectively (entry 4) whereas as 1.2 : 1 ratio of alkyne and IBF afforded improved yield of benzofluorenol (58%) with a combined yield of 91% (entry 5). Use of 2 : 1 ratio of alkyne and IBF led to decrease in the yield of benzofluorenol to 48% (entry 6). At 55 °C, the reaction afforded 32% of benzofluorenol only (entry 7). Use of 1-ethynylcyclohexyl acetate did not improve the yield of benzofluorenol (53%, entry 8). Silver-free IPrAu(NCMe)SbF₆ (ref. 13) led to 42% of benzofluorenol with overall yield of 82% (entry 9). Interestingly, use of 2 mol% IPrAuCl/AgOTf gave only the diene product in 48% yield (Table 1, entry 10). Changing the silver salt to AgNTf₂ or AgBF₄ led to benzofluorenol in lower yields (entries 11 and 12); 2 mol% IMesAuCl/AgSbF₆ could drive the reaction to obtain benzofluorenol in 50% of yield (entry 13). PicAuCl₂, IPrAuCl or AgSbF₆ individually were not effective to form benzofluorenol (entries 14, 15 and 16). Use of 2 mol% Ph₃PAuCl/AgSbF₆ led to diene product only (entry 17). Solvents like CH₃CN, THF and dioxane (entries 18, 19 and 20) in the presence of 2 mol% IPrAuCl/AgSbF₆ did not perform well in forming the benzofluorenol; it should be noted that the catalytic system in entry 18 can be treated as AgCl + [IPrAu(NCMe)SbF₆].¹³ Hence it is concluded that 2 mol% IPrAuCl/AgSbF₆ in dichloromethane at rt is the best choice for the formation of the benzofluorenol (entry 5). Details on the optimisation of the products using various screening conditions are presented in Table 1.

Scheme 1 Reaction of propargylic ester 1a with 1,3-diphenylisobenzofuran 2.

Table 1 Screening for the optimisation of the yields of benzofluorenol 3a and diene 4a

Entry^a	Alkyne/IBF	Catalyst	Solvent	Time (h)	Combined yield^b (%) (3a + 4a)
a All reactions were performed at room temperature.b Isolated yield.c Starting material remained.d A diketone^14 which is a dimeric form of IBF was also formed.e Oil bath temperature.f 1-Ethynylcyclohexyl acetate was used. The diene product, although present (ca. 20%; tlc), could not be isolated.
1	1/1.5	No catalyst	DCM	12	0^c
2	1/1.5	2% IPrAuCl/AgSbF6	DCM	4	90 (36 + 54)^d
3	1/1.2	2% IPrAuCl/AgSbF6	DCM	4	86 (52 + 34)
4	1/1	2% IPrAuCl/AgSbF6	DCM	4	90 (57 + 33)
5	1.2/1	2% IPrAuCl/AgSbF6	DCM	4	91 (58 + 33)
6	2/1	2% IPrAuCl/AgSbF6	DCM	4	84 (48 + 36)
7	1.2/1	2% IPrAuCl/AgSbF6	DCM	4 (at 55 °C)^e	74 (32 + 42)
8	1.2/1	2% IPrAuCl/AgSbF6	DCM	4	80 (60 + 20)^f
9	1.2/1	2% IPrAu(NCMe)SbF6	DCM	4	82 (42 + 40)
10	1.2/1	2% IPrAuCl/AgOTf	DCM	12	48 (0 + 48)^c
11	1.2/1	2% IPrAuCl/AgNTf2	DCM	12	62 (26 + 36)^c
12	1.2/1	2% IPrAuCl/AgBF4	DCM	4	85 (38 + 47)
13	1.2/1	2% IMesAuCl/AgSbF6	DCM	4	88 (50 + 38)
14	1.2/1	3% PicAuCl2	DCM	12	54 (0 + 54)^c
15	1.2/1	3% IPrAuCl	DCM	12	15 (0 + 15)^c
16	1.2/1	3% AgSbF6	DCM	12	68 (0 + 68)^c
17	1.2/1	2% Ph3PAuCl/AgSbF6	DCM	12	65 (0 + 65)^c
18	1.2/1	2% IPrAuCl/AgSbF6	CH3CN	12	∼32 (trace + 32)
19	1.2/1	2% IPrAuCl/AgSbF6	THF	12	38 (10 + 28)^c
20	1.2/1	2% IPrAuCl/AgSbF6	Dioxane	12	36 (6 + 30)^c

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To ascertain the efficacy and generality of the above catalytic system, various propargylic benzoates 1b–1k were treated with IBF (Scheme 1). These reactions afforded products 3(b–k)–4(b–k) (benzofluorenols and dienes) in combined yields of 90–96% with benzofluorenols as predominant and readily isolatable products (Table 2). The structure of benzofluorenol 3a was confirmed by X-ray crystallography (Fig. 1). The substituents were varied in terms of alkyl groups. The configuration at one of the double bonds in the diene product is Z and the other double bond exhibits (E + Z) isomeric mixture (∼1 : 1) if the alkyl groups are unsymmetrically substituted (Table 2, entries 4–8 and 10). The R_f values of the two diene isomers were very close to each other and hence they were not separated. However, in the case of mono-substituted propargyl benzoate 1l under similar conditions, isomeric dienes 4l and 4l′ were formed (via acyclic addition; Scheme 2). These were separated and the structure of 4l was confirmed by X-ray crystallography (Fig. 2). Use of the terminally substituted ester 1-(phenylethynyl)cyclohexyl benzoate led to a mixture with much of the starting material unreacted.

Fig. 1 ORTEP diagram for compound 3a. Selected bond lengths with esd's in parentheses: C2–C1 1.377(4), C3–C2 1.422(4), C4–C3 1.389(3), C17–C23 1.518(4), C11–C2 1.485(3).

Table 2 Synthesis of benzofluorenols 3a–k and dienes 4a–k

Entry	Alkyne	Benzofluorenol^a	Substituted diene^a
a Yields of the isolated products.b Diastereomeric mixture [dr ∼ 1 : 1].c E + Z (ca. 1 : 1) isomers.
1	1a
2	1b
3	1c
4	1d
5	1e
6	1f
7	1g
8	1h
9	1i
10	1j
11	1k

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Scheme 2 Formation of diene products 4l and 4l′.

Fig. 2 ORTEP diagram for compound 4l. Selected bond lengths with esd's in parentheses: C7–C8 1.320(3), C8–C9 1.445(3), C9–C10 1.328(3), O4–C23 1.218(2).

We have also performed the above reaction by using 4-nitrobenzoate (1a′) and 4-methoxybenzoate (1a′′)¹⁵ in place of unsubstituted benzoate 1a (Scheme 3). It was found that the yield of the isolated fluorenol product 3a was increased to 66% in the former while it decreased to 23% in the case of latter. The yield of the diene 5 (29%) or 6 (26%) was low in both the cases.

Scheme 3 Reaction of 4-nitro- and 4-methoxy-benzoate propargylic esters with 2.

The variation in the ester moiety having electron withdrawing –NO₂ and electron donating –OMe substrates in the cycloaddition favours a gold–alkyne pathway. The propargylic ester having the –NO₂ group resulted in higher yield. This suggests that 1,2- or 1,3-benzyloxy group migration^16,17 is less favourable. Based on these results, a plausible mechanism for the above reaction is shown in Scheme 4. Initially, alkyne coordinates to [Au], and undergoes [4 + 2] cycloaddition with 1,3-diphenylisobenzofuran to form VI. This intermediate undergoes elimination of benzoyl group to generate a five membered ring forming the polycycle VII with bridged oxygen between two phenyl rings. A new C–C bond is formed and benzoic acid is eliminated at this stage. Then the ring containing bridged oxygen is opened by the attack of the double bond to form allylic cationic intermediate VIII. The other phenyl group migrates to the adjacent carbocation followed by ketone formation to lead to the intermediate IX. Species IX aromatises to the benzofluorenol product. In the case of monoaryl substituted propargyl ester 1l, 1,2-benzoyloxy group migration is only observed which can be in line with the literature.^17d,18

Scheme 4 Proposed reaction pathway for the formation of benzofluorenols and substituted dienes.

For the diene product as shown on the right of the Scheme 4, the allenic intermediate X is formed first; attack of the benzoyloxy anion on the central allenic carbon of followed by reorganization of bonds leads to the diene product.

Conclusions

In summary, a new type of cycloaddition involving propargylic esters with the aid of N-heterocyclic carbene–gold complex under very mild conditions is discovered. The products are novel benzo[c]fluorenols and substituted dienes that are very conveniently isolated. While the former product involves sequential cycloaddition, carbocyclisation, 1,2-phenyl migration, ring opening and aromatisation, the latter involves attack of benzoyloxy anion on central allenic carbon followed by rearrangement. The structures of two such products have been unambiguously established by X-ray structure determination.

Experimental section

General procedure for the synthesis of benzofluorenols 3a–k, dienes 4a–l, 4l′ and 5–6

To a mixture of IPrAuCl [IPr = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene] (0.006 g, 0.01 mmol) and AgSbF₆ (0.003 g, 0.01) in DCM (2 mL) was added the corresponding propargyl benzoate 1a–l (0.6 mmol) and 1,3-diphenylisobenzofuran 2 (0.5 mmol). The contents were stirred at rt (25 °C) for 4 h. The solvent was removed under vacuum. Products 3a–k were separated from the reaction mixture by column chromatography by using acetone–hexane (1 : 100) mixture whereas 4a–l, 4l′ and 5–6 (1a′ and 1a′′ were used for these) were isolated by using acetone–hexane (1 : 50) mixture.

Compound 3a. White solid [R_f 0.5 in acetone–hexane (1 : 50) mixture]; yield 0.109 g (58%) using 1a, 0.124 g (66%) using 1a′ and 0.056 g (23%) using 1a′′; Mp 230–232 °C; IR ν_max(KBr): 3534, 3052, 2953, 2932, 2843, 1622, 1563, 1439, 1391, 1343, 1219, 1065, 1024, 752, 710, 666 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.75–0.79, 1.32–1.54, 1.76–2.00 and 2.21–2.28 (m, 10H, cyclohexyl-H), 5.10 (s, 1H, Ar-OH), 7.23–7.96, 8.36–8.42 and 8.86–8.88 (m, 13H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 22.3, 25.1, 32.6 and 52.4 (cyclohexyl-CH₂), 120.3, 122.2, 123.6, 123.7, 124.3, 124.8, 125.2, 126.7, 126.8, 127.4, 129.0, 129.2, 129.6, 130.5, 132.8, 134.2, 140.8, 149.4, 149.8 and 153.6 (Ar-C); HRMS (ESI): calcd for C₂₈H₂₅O [M⁺ + H]: m/z 377.1906. Found: 377.1904. X-ray structure has been determined for this compound after crystallisation from CH₂Cl₂–hexane mixture. CCDC no. 952109.

Compound 4a. Gummy liquid [R_f 0.3 in acetone–hexane (1 : 50) mixture]; yield 0.82 g (33%); IR ν_max(neat) 3063, 2926, 2854, 1726, 1671, 1599, 1452, 1276, 1068, 701 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 1.31–1.53 and 1.87–2.10 (m, 10H, cyclohexyl-H), 6.73 (s, 1H, PhC=CH), 7.10–7.76 (m, 19H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 26.2, 26.6, 27.0, 28.1, 29.4 (cyclohexyl-C), 121.1, 126.3, 127.5, 128.0₀, 128.0₂, 129.1, 129.3, 129.4, 129.7, 130.1, 130.2, 131.3, 132.6, 132.9, 134.1, 136.4, 137.2, 139.2, 140.4, 141.0, 142.6 (alkenyl-C + Ar-C), 164.3 (OCOPh), 196.3 (ArCOPh); HRMS (ESI): calcd for C₃₅H₃₀O₃ [M⁺ + H]: m/z 499.2274. Found: 499.2274.

Compound 3b. White solid; yield 0.104 g (48%); Mp 260–262 °C; IR ν_max(KBr) 3523, 3058, 3036, 2948, 2866, 1584, 1556, 1441, 1397, 1211, 1063, 751 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.73 and 0.89 (2 s, 18H, t-Bu-H), 1.28–2.30 (m, 18H, cyclohexyl-H), 5.10 and 5.11 (2 s, 2H, Ar-OH), 7.24–7.69 and 8.33–8.85 (m, 26H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 20.6, 23.3, 26.8, 27.7, 31.6, 32.6, 33.3, 33.6, 42.9 and 47.0 (t-Bu-C + cyclohexyl-C), 120.2, 120.2, 122.1, 122.2, 123.6, 124.4, 124.7, 124.8, 125.0, 126.2, 126.6, 126.8, 127.4, 128.8, 129.0, 129.1, 129.5, 129.6, 130.5, 132.5, 132.8, 134.1, 140.2, 140.8, 149.4 and 149.7 (Ar-C); HRMS (ESI): calcd for C₃₂H₃₃O [M⁺ + H]: m/z 433.2532. Found: 433.2530.

Compound 4b. Gummy liquid; yield 0.119 g (43%); IR ν_max(neat): 3057, 2953, 2860, 1731, 1665, 1599, 1457, 1260, 1101, 1024, 772, 745 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.80–1.03, 1.22–1.72 and 2.37–2.60 (m, 18H, cyclohexyl-H + t-Bu-H), 6.73 (s, 1H, PhC=CH), 6.94–7.75 (m, 19H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 27.6, 28.1, 29.3, 32.4 and 47.9 (cyclohexyl-C + t-Bu-C), 121.2, 126.3, 127.4, 127.4, 128.0, 129.3, 129.5, 129.8, 130.1, 130.2, 131.3, 132.5, 132.8, 133.8, 136.4, 137.3, 139.3, 140.4, 141.5 and 142.6 (alkenyl-C + Ar-C), 164.3 (OCOPh), 196.4 (ArCOPh); HRMS (ESI): calcd for C₃₉H₃₉O₃ [M⁺ + H]: m/z 555.2900. Found: 555.2903.

Compound 3c. White solid; yield 0.093 g (55%); Mp 150–152 °C; IR ν_max(KBr): 3507, 3057, 2981, 2959, 2860, 1578, 1441, 1391, 1216, 1057, 766 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 1.30 (s, 6H, C(CH₃)₂), 5.16 (s, 1H, Ar-OH), 7.28–7.60, 8.30–8.35 and 8.80–8.82 (m, 13H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 26.6 (C(CH₃)₂), 48.6 (C(CH₃)₂), 120.4, 121.9, 122.2, 123.5, 123.8, 124.8, 125.4, 126.5, 127.0, 127.2, 127.3, 127.9, 129.0, 129.9, 132.4, 134.0, 140.0, 149.1, 149.8 and 154.9 (Ar-C); HRMS (ESI): calcd for C₂₅H₂₀O [M⁺ + H]: m/z 337.1593. Found: 337.1592.

Compound 4c. Gummy liquid; yield: 0.094 g (41%); IR ν_max(neat): 3063, 2915, 2854, 1726, 1665, 1599, 1457, 1309, 1287, 1117, 701 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 1.41 and 1.62 (2 s, 6H, =C(CH₃)₂), 6.69 (s, 1H, PhC=CH), 7.10–7.74 (m, 19H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 18.5 and 19.2 (=C(CH₃)₂), 121.6, 126.4, 127.0, 127.5, 127.9, 128.5, 129.2, 129.3, 129.8, 130.0, 130.2, 130.6, 131.3, 132.6, 132.9, 137.1, 139.1, 139.3, 140.2, 140.7, 142.4 (alkenyl-C + Ar-C), 164.1 (OCOPh), 196.4 (ArCOPh); HRMS (ESI): calcd for C₃₂H₂₆O₃ [M⁺ + H]: m/z 459.1961. Found: 459.1964.

Compound 3d. White solid; yield 0.110 g (63%); Mp 118–120 °C; IR ν_max(KBr) 3436, 3063, 2970, 2920, 2860, 1660, 1594, 1452, 1277, 1069, 932 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.24 (t, ³J(H–H) = 7.2 Hz, 3H, CH₃CH₂), 1.31 (s, 3H, CH₃CAr), 1.69–1.72 and 1.78–1.82 (m, 2H, CH₂CH₃), 5.14 (s, 1H, Ar-OH), 7.28–7.71 and 8.29–8.80 (m, 13H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 8.7 (CH₃CH₂), 26.6 and 31.8 (CH₃CH₂ + CH₃CAr), 53.1 (ArC(Me)Et), 120.2, 121.7, 121.9, 123.5, 123.7, 124.8, 125.3, 126.9, 127.2, 128.0, 129.1, 129.8, 131.1, 132.3, 134.0, 141.3, 147.4, 149.0 and 152.8 (Ar-C); HRMS (ESI): calcd for C₂₆H₂₃O [M⁺ + H]: m/z 351.1750. Found: 351.1749.

Compound 4d. Gummy liquid; yield 0.071 g (30%); IR ν_max(neat): 3057, 2964, 2931, 1732, 1660, 1595, 1452, 1310, 1277, 1063, 773, 707 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.75 and 0.85 (2 t, ³J(H–H) = 6.4 Hz each, 6H, CH₃CH₂), 1.41 and 1.60 (2 s, 6H, CH₃C=C), 1.65 and 2.04 (2 br s, 4H, CH₂CH₃), 6.67 and 6.73 (2 s, 2H, PhC=CH), 7.07–7.77 (m, 38H, Ar-H). In the assignment, the proton numbers are doubled to show the presence of both the isomers; ¹³C NMR (100 MHz, CDCl₃): δ 11.7 and 12.0 (2 CH₃CH₂), 15.8, 16.5, 25.2 and 26.2 (2 CH₃CH₂ and 2 CH₃C=C), 120.6, 121.8, 126.3, 126.5, 127.5, 127.9, 128.1, 129.1, 129.4, 129.7, 129.8, 129.9, 130.1, 130.2, 131.1, 131.3, 132.5, 132.6, 132.9, 137.2, 137.3, 138.7, 139.0, 139.4, 140.0, 140.6, 140.9, 142.4, 142.5 (Ar-C), 163.9, 164.3 (OCOPh), 196.5 (ArCOPh); HRMS (ESI): calcd for C₃₃H₂₈NaO₃ [M⁺ + Na]: m/z 495.1936. Found: 495.1938.

Compound 3e. White solid; yield 0.097 g (53%); Mp 120–122 °C; IR ν_max(KBr) 3518, 2959, 2926, 2860, 1578, 1441, 1397, 1238, 1222, 1069, 795, 751, 707, 669 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.60 (t, ³J(H–H) = 6.4 Hz, 3H, CH₃CH₂), 1.31 (s, 3H, CH₃CAr), 1.62–1.75 (m, 4H, CH₂CH₂CH₃), 5.13 (s, 1H, Ar-OH), 7.27–7.70 and 8.28–8.81 (m, 13H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 14.1, 17.5, 27.0, 41.2 (CH₃ + propyl-C), 52.7 (ArC(Me)propyl), 120.2, 121.6, 121.9, 123.6, 124.8, 125.3, 126.9, 127.2, 127.7, 128.4, 129.1, 129.6, 129.8, 131.1, 132.3, 134.0, 141.0, 147.9, 149.0 and 153.3 (Ar-C); HRMS (ESI): calcd for C₂₇H₂₅O [M⁺ + H]: m/z 365.1905. Found: 365.1904.

Compound 4e. Gummy liquid; yield 0.102 g (42%); IR ν_max(neat): 3058, 2955, 2929, 2872, 1728, 1666, 1599, 1444, 1314, 1278, 1071, 931, 698 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.77 and 0.83 (2 t, ³J(H–H) = 7.4 Hz and 7.2 Hz respectively, 6H, CH₃CH₂), 0.86–0.97, 1.22–1.65 (m, 14H, CH₃ + CH₂CH₂CH₃), 6.68 and 6.73 (2 s, 2H, PhC=CH), 6.81–7.74 (m, 38H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 14.0, 14.2, 16.4, 17.1, 20.5, 21.0, 34.3 and 35.1 (propyl-C + CH₃), 121.1, 121.9, 126.3, 126.5, 127.5, 127.9, 128.1, 129.1, 129.5, 129.8, 129.9, 130.2, 130.3, 130.6, 131.2, 131.3, 132.4, 132.6, 132.8, 137.3₀, 137.3₄, 139.3, 139.7, 139.9, 140.1, 140.6, 140.9, 142.4 and 142.6 (Ar-C), 163.9, 164.3 (OCOPh), 196.4 (ArCOPh); HRMS (ESI): calcd for C₃₄H₃₁O₃ [M⁺ + H]: m/z 487.2274. Found: 487.2273.

Compound 3f. White solid; yield 0.105 g (55%); Mp 110–112 °C; IR ν_max(KBr): 3534, 3057, 2959, 2931, 2860, 1599, 1572, 1441, 1353, 1057, 1030 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.45 (br s, 1H, butyl-H), 0.66 (t, ³J(H–H) = 7.2 Hz, 3H, CH₃CH₂), 0.90–1.06 and 1.27–1.74 (m, 8H, CH₃ and butyl-H), 5.14 (s, 1H, Ar-OH), 7.27–7.69 and 8.30–8.82 (m, 13H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 13.9, 22.9, 26.3, 27.0 and 38.8 (CH₃ + butyl-C), 52.6 (C(Me)n-butyl), 120.2, 121.6, 122.0, 123.6, 124.8, 125.3, 126.9, 127.2, 127.7, 129.1, 129.9, 131.2, 132.3, 134.0, 141.1, 147.9, 149.0 and 153.3 (Ar-C); HRMS (ESI): calcd for C₂₈H₂₆NaO [M⁺ + Na]: m/z 401.1882. Found: 401.1883.

Compound 4f. Gummy liquid; yield 0.097 g (39%); IR ν_max(neat): 3057, 2953, 2926, 2860, 1731, 1660, 1599, 1446, 1320, 1282, 1172, 1095, 1030, 920 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.80–0.96, 1.15–1.24 and 1.35–2.04 (m, 24H, CH₃ + butyl-H), 6.67 and 6.73 (2 s, 2H, PhC=CH), 7.05–7.75 (m, 38H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 13.9, 14.9, 16.4, 17.0, 22.6₀, 22.6₂, 29.3, 29.9, 31.8 and 32.9 (CH₃ + butyl-C), 121.0, 121.8, 126.3, 126.4, 127.4, 127.9, 128.0₀, 128.0₂, 129.0, 129.3, 129.5, 129.7, 129.9, 130.1, 130.2, 130.7, 131.1, 131.3, 131.4, 132.4, 132.5, 132.8, 137.3, 139.1, 139.2, 140.0, 140.9, 142.4, 142.6, (alkenyl-C + Ar-C), 163.9 and 164.3 (OCOPh), 196.4 (ArCOPh); HRMS (ESI): calcd for C₃₅H₃₃ O₃ [M⁺ + H]: m/z 501.2430. Found: 501.2432.

Compound 3g. White solid; yield 0.102 g (52%); Mp 116–118 °C; IR ν_max(KBr): 3496, 2948, 2931, 2855, 1584, 1386, 1222, 1069, 751, 712 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.44 (br s, 1H, pentyl-H), 0.71 (t, ³J(H–H) = 6.0 Hz, 3H, CH₃CH₂), 0.95–1.04 (m, 5H, pentyl-H), 1.05 (s, 3H, CH₃), 1.59–1.75 (m, 2H, pentyl-H), 5.13 (s, 1H, Ar-OH), 7.29–7.71 and 8.28–8.82 (m, 13H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 14.2, 22.4, 23.8, 27.0, 32.1, 38.9 (CH₃ + pentyl-C), 52.5 (Ar-C(Me)n-pentyl), 120.3, 122.0, 123.5, 124.7, 125.3, 126.7, 127.1, 128.3, 129.1, 129.6, 131.1, 132.3, 133.9, 141.0, 147.9 and 153.2 (ArC); HRMS (ESI): calcd for C₂₉H₂₉O [M⁺ + H]: m/z 393.2219. Found: 393.2217.

Compound 4g. Gummy liquid; yield 0.098 g (38%); IR ν_max(neat): 3047, 2959, 2931, 2855, 1732, 1666, 1595, 1452, 1315, 1266, 1244, 1156, 767, 740, 712 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.81 and 0.86 (2 t, ³J(H–H) = 7.0 Hz and 6.8 Hz respectively, 6H, CH₃CH₂), 1.13–1.31 (m, 12H, pentyl-H), 1.43 and 1.66 (2 br s, 10H, pentyl-H + CH₃), 6.69 and 6.72 (2 s, 2H, PhC=CH), 6.74–7.79 (m, 38H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 14.0, 16.4, 17.1, 22.4, 22.6, 26.8, 27.0, 27.4, 31.8, 32.1 and 33.2 (n-pentyl-C + CH₃), 121.0, 121.9, 126.3, 126.5, 126.7, 127.5, 127.9, 128.1, 128.4, 129.1, 129.4, 129.8, 130.2, 130.8, 131.2, 131.4, 132.4, 132.6, 132.8, 137.4, 139.2, 139.3, 139.5, 139.8, 140.1, 140.7, 142.5 and 142.6 (alkenyl-C + Ar-C), 164.3 (OCOPh), 196.4 (ArCOPh); HRMS (ESI): calcd for C₃₆H₃₅O₃ [M⁺ + H]: m/z 515.2587. Found: 515.2589.

Compound 3h. White solid; yield 0.110 g (54%); Mp 114–116 °C; IR ν_max(KBr): 3534, 3063, 2955, 2924, 2851, 1625, 1583, 1459, 1438, 1392, 1350, 1273, 1221, 1061, 760, 703, 667 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.49 and 0.72 (2 br s, 2H, hexyl-H), 0.80 (t, ³J(H–H) = 7.4 Hz, 3H, CH₃CH₂), 1.03–1.78 (m, 11H, CH₃ + hexyl-H), 5.18 (s, 1H, Ar-OH), 7.31–7.73 and 8.32–8.86 (m, 13H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 14.1, 22.7, 24.1, 27.1, 29.5, 31.6, 39.0 (CH₃ + hexyl-C), 52.6 (ArC(Me)hexyl), 120.2, 121.6, 121.9, 123.5, 123.8, 124.7, 125.3, 126.9, 127.2, 127.7, 128.3, 129.1, 129.6, 129.8, 131.2, 132.3, 141.0, 147.9, 148.9 and 153.3 (Ar-C); HRMS (ESI): calcd for C₃₀H₃₁O [M⁺ + H]: m/z 407.2376. Found: 407.2373.

Compound 4h. Gummy liquid; yield 0.100 g (38%); IR ν_max(neat): 3053, 3022, 2924, 2856, 1733, 1661, 1599, 1449, 1268, 926 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.80 and 0.86 (2 t, ³J(H–H) = 7.0 Hz and 7.4 each, 6H, CH₃CH₂), 1.15–1.27 (m, 16H, hexyl-H), 1.43–1.66 (2 s, 6H, CH₃), 1.71–1.74 and 2.00–2.04 (m, 2H + 2H, hexyl-H), 6.69 and 6.74 (2 s, 2H, PhC=CH), 7.05–7.20, 7.27–7.37, 7.44–7.49, 7.54–7.76 (m, 38H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 14.1, 14.2, 16.4, 17.1, 22.6, 24.7, 27.1, 27.7, 29.2, 31.6, 31.8, 32.1, 33.2, 36.7 (n-C₆H₁₃ + CH₃), 121.0, 121.8, 126.3, 126.5, 127.1, 127.5, 127.9, 128.0₀, 128.0₄, 129.0, 129.4, 129.7, 129.9, 130.2, 130.9, 131.1, 131.3, 131.5, 132.5, 132.6, 132.8, 132.9, 137.2, 137.3, 139.1, 139.2, 139.4, 139.8, 140.0, 140.6, 140.9, 142.4, 142.6 (alkenyl-C + Ar-C), 163.9 and 164.2 (OCOPh), 196.5 (ArCOPh); HRMS (ESI): calcd for C₃₇H₃₇O₃ [M⁺ + H]: m/z 529.2743. Found: 529.2742.

Compound 3i. White solid; yield 0.120 g (66%); Mp 126–128 °C; IR ν_max(KBr): 3490, 2953, 2857, 1584, 1562, 1392, 1211, 1058, 756, 701 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.23 (t, ³J(H–H) = 7.2 Hz, 6H, CH₃CH₂), 1.67–1.82 (m, 4H, CH₃CH₂), 5.12 (s, 1H, Ar-OH), 7.27–7.69 and 8.27–8.82 (m, 13H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 8.3 (CH₃CH₂), 31.8 (CH₃CH₂), 58.3 (ArC(Et)₂), 120.1, 121.7, 123.7, 124.7, 125.3, 126.9, 127.1, 128.4, 129.3, 129.6, 131.0, 134.0, 142.7, 145.1, 148.8 and 150.9 (Ar-C); HRMS (ESI): calcd for C₂₇H₂₅O [M⁺ + H]: m/z 365.1906. Found: 365.1903.

Compound 4i. Gummy liquid; yield 0.063 g (26%); IR ν_max(neat): 3063, 2970, 2871, 1726, 1665, 1599, 1452, 1271, 1090, 706 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.75 and 0.83 (2 t, ³J(H–H) = 7.4 Hz each, 6H, CH₃CH₂), 1.98 and 2.13 (2 br s, 4H, CH₃CH₂), 6.72 (s, 1H, PhC=CH), 7.00–7.72 (m, 19H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 12.1 and 12.6 (CH₂CH₃), 22.8 and 23.6 (CH₂CH₃), 121.2, 126.4, 127.5, 127.9, 128.1, 128.4, 129.1, 129.4, 129.7, 129.9, 130.1, 130.2, 131.1, 132.6, 132.9, 137.3, 137.8, 139.0, 139.5, 139.8, 140.8, 142.6 (alkenyl-C + Ar-C), 164.1 (OCOPh), 196.4 (ArCOPh); HRMS (ESI): calcd for C₃₄H₃₁O₃ [M⁺ + H]: m/z 487.2274. Found: 487.2272.

Compound 3j. White solid; yield 0.113 g (60%); Mp 106–108 °C; IR ν_max(KBr): 3534, 3057, 3041, 2959, 2926, 2870, 1583, 1462, 1391, 1227, 1068, 876 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.25 (t, ³J(H–H) = 7.4 Hz, 3H, CH₃CH₂), 0.50–0.55 (m, 1H, alkyl-H), 0.62 (t, ³J(H–H) = 6.8 Hz, 3H, CH₃CH₂), 0.67–0.91 and 1.55–1.82 (m, 5H, alkyl-H), 5.14 (s, 1H, Ar-OH), 7.29–7.71 and 8.28–8.83 (m, 13H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 8.1, 14.2, 17.1, 32.1 and 41.4 (ethyl-C + propyl-C), 57.8 (C(Et)n-propyl), 120.1, 121.4, 121.7, 123.5, 123.7, 124.7, 125.3, 126.9, 127.2, 129.1, 129.3, 129.8, 131.1, 134.0, 142.4, 145.6, 148.9 and 151.4 (Ar-C); HRMS (ESI): calcd for C₂₈H₂₆NaO [M⁺ + Na]: m/z 401.1882. Found: 401.1883.

Compound 4j. Gummy liquid; yield 0.080 g (32%); IR ν_max(neat): 3063, 2965, 2932, 2871, 1731, 1671, 1595, 1452, 1321, 1271, 1069, 932, 762 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.76 and 0.93 (m, 12H, CH₃CH₂), 1.19–1.32 (m, 6H, alkyl-H), 1.64 and 2.09 (2 br s, 6H, alkyl-H), 6.73 (br s, 2H, PhC=CH), 7.02–7.72 (m, 38H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 12.1, 12.7, 14.2, 14.4, 20.9, 21.5, 23.4, 24.1, 32.0 and 32.7 (ethyl-C + propyl-C), 121.2, 121.3, 126.4, 127.4, 127.9₀, 127.9₆, 128.1, 128.6, 129.2, 129.6, 129.7, 130.0, 130.1, 130.4, 131.1, 132.5, 132.8, 136.5, 136.6, 137.4, 139.5, 139.7, 139.8, 140.8, 142.6 and 142.7 (alkenyl-C + Ar-C), 164.2 (OCOPh), 196.5 (ArCOPh); HRMS (ESI): calcd for C₃₅H₃₃ O₃ [M⁺ + H]: m/z 501.2430. Found: 501.2431.

Compound 3k. White solid; yield 0.110 g (54%); Mp 112–114 °C; IR ν_max(KBr): 3529, 3036, 2953, 2926, 2871, 1578, 1463, 1436, 1392, 1222, 1145, 767, 707 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.23 (t, ³J(H–H) = 7.2 Hz, 6H, CH₃CH₂), 1.60–1.74 (m, 8H, CH₃CH₂CH₂), 5.14 (s, 1H, Ar-OH), 7.29–7.71 and 8.27–8.82 (m, 13H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 14.2, 16.9, 41.6 (C₃H₇), 57.3 (Ar-C(n-propyl)₂), 120.0, 121.4, 121.7, 123.5, 123.7, 124.7, 125.2, 126.8, 127.2, 128.8, 129.3, 131.1, 134.0, 142.1, 146.1, 148.8 and 151.8 (ArC); HRMS (ESI): calcd for C₂₉H₂₉O [M⁺ + H]: m/z 393.2219. Found: 393.2217.

Compound 4k. Gummy liquid; yield 0.088 g (34%); IR ν_max(neat): 3063, 2959, 2931, 2866, 1732, 1666, 1600, 1452, 1315, 1266, 1063, 762, 712 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 0.79 and 0.86 (2 t, ³J(H–H) = 7.4 Hz and 7.2 Hz respectively, 6H, CH₃CH₂), 1.22–1.45 and 1.74–2.14 (m, 8H, CH₃CH₂CH₂), 6.76 (s, 1H, PhC=CH), 6.81–6.83, 6.98–7.54 and 7.60–7.71 (m, 19H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 14.2, 14.4, 20.9, 21.5, 32.4 and 33.0 (propyl-C), 126.3, 127.1, 127.4, 127.6, 127.8, 127.9, 128.0, 129.2, 129.5, 129.6, 130.0, 130.1, 131.1, 131.6, 131.8, 132.5, 132.7, 135.2, 137.3, 139.0, 139.3, 139.5, 140.0, 140.7, 142.6 and 144.7 (alkenyl-C + Ar-C), 164.1 (OCOPh), 196.5 (ArCOPh); HRMS (ESI): calcd for C₃₆H₃₅O₃ [M⁺ + H]: m/z 515.2587. Found: 515.2585.

Compound 4l. White solid; yield 0.118 g (44%); Mp 94–96 °C; IR ν_max(KBr): 3057, 3030, 2838, 1726, 1660, 1594, 1501, 1265, 1249, 1139, 1024, 706 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 3.79 (s, 3H, OCH₃), 6.22 (s, 1H, BzO–C=CH), 6.76–6.83 and 7.12–7.73 (m, 24H, PhC=CH + Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 55.3 (OCH₃), 113.8, 121.6, 124.1, 126.8, 127.3, 127.9, 128.1₀, 128.1₄, 129.3, 129.5, 129.8, 130.1, 130.3, 130.5, 131.2, 132.7, 133.0, 136.9, 139.3, 140.5, 142.0, 142.9, 143.9, 159.2 (alkenyl-C + Ar-C), 164.5 (OCOPh), 196.6 (ArCOPh); HRMS (ESI): calcd for C₃₇H₂₈NaO₄ [M⁺ + Na]: m/z 559.1886. Found: 559.1886. X-ray structure was determined for this compound after crystallisation from CH₂Cl₂–hexane mixture. CCDC no. 952110.

Compound 4l′. White solid; yield 0.107 g (40%); Mp 116–118 °C; IR ν_max (KBr): 3057, 3014, 2937, 2838, 1732, 1666, 1595, 1507, 1255, 1238, 1184, 1063, 773 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 3.74 (s, 3H, OCH₃), 6.22 (s, 1H, BzO–C=CH), 6.59–6.73 and 7.06–7.73 (m, 24H, PhC=CH + Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 55.2 (OCH₃), 114.0, 123.1, 124.8, 126.7, 127.1, 127.6, 127.7, 127.8, 128.0, 129.1, 129.3, 129.7, 130.1, 130.2, 131.4, 132.5, 133.2, 137.5, 139.4, 139.6, 140.0, 141.8, 143.9, 159.1 (alkenyl-C + Ar-C), 163.5 (OCOPh), 197.2 (ArCOPh); HRMS (ESI): calcd for C₃₇H₂₈NaO₄ [M⁺ + Na]: m/z 559.1886. Found: 559.1886.

Compound 5. Yellow solid; yield 0.080 g (29%); Mp 64–66 °C; IR ν_max(KBr): 3058, 2926, 2855, 1737, 1666, 1534, 1348, 1277, 1090, 712 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 1.35–1.55 (m, 6H, cyclohexyl-H), 1.90–1.94 (m, 2H, cyclohexyl-H), 2.20–2.24 (m, 2H, cyclohexyl-H), 6.74 (s, 1H, PhC=CH), 7.13–7.18 (m, 8H, Ar-H), 7.29–7.32 (m, 3H, Ar-H), 7.46 (t, J = 7.2 Hz, 1H, Ar-H), 7.64 (d, J = 7.6 Hz, 2H, Ar-H), 7.70 (d, J = 8.4 Hz, 2H, Ar-H), 8.10 (d, J = 8.4 Hz, 2H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 26.1, 26.7, 27.1, 28.2, 29.5 (cyclohexyl-C), 120.6, 123.0, 126.5, 127.6, 128.0, 129.2, 129.9, 130.1, 130.8, 131.3, 132.7, 134.6, 134.8, 136.3, 136.9, 139.4, 140.5, 141.0, 142.2, 150.3 (alkenyl-C + Ar-C), 162.3 (OCOPh), 196.2 (ArCOPh); HRMS (ESI): calcd for C₃₅H₂₉NO₅Na [M⁺ + Na]: m/z 566.1944. Found: 566.1947.

Compound 6 (other unidentified products were also present in the reaction mixture). Gummy liquid; yield 0.086 g (26%); IR ν_max(neat): 3063, 2926, 2855, 1721, 1671, 1606, 1507, 1441, 1321, 1249, 1162, 1080, 1025, 849, 767, 696 cm⁻¹; ¹H NMR (400 MHz, CDCl₃): δ 1.29–1.43 (m, 6H, cyclohexyl-H), 1.84–1.87 (m, 2H, cyclohexyl-H), 2.05–2.15 (m, 2H, cyclohexyl-H), 3.84 (s, 3H, OCH₃), 6.72 (s, 1H, PhC=CH), 6.75–6.78 (m, J ∼ 8.8 Hz, 2H, Ar-H), 7.13–7.19 (m, 8H, Ar-H), 7.33–7.39 (m, 3H, Ar-H), 7.47–7.51 (m, J ∼ 8.8 Hz, 3H, Ar-H), 7.75 (d, J = 8.4 Hz, 2H, Ar-H); ¹³C NMR (100 MHz, CDCl₃): δ 26.2, 26.6, 27.0, 28.1, 29.4 (cyclohexyl-C), 55.4 (OCH₃), 113.1, 121.4, 121.9, 126.3, 127.4, 127.9, 128.0, 129.2, 130.1, 130.2, 131.3, 131.8, 132.5, 134.0, 136.3, 137.2, 139.2, 140.2, 141.0, 142.6, 163.3 (alkenyl-C + Ar-C) 164.0 (OCOPh), 196.4 (ArCOPh); HRMS (ESI): calcd for C₃₆H₃₂O₄Na [M⁺ + Na]: m/z 551.2199. Found: 551.2199.

Single crystal X-ray data for compounds 3a, and 4l were collected on an OXFORD diffractometer using Mo-Kα (λ = 0.71073 Å) radiation. The structures were solved by direct methods and refined by full-matrix least squares method using standard procedures.¹⁹ Absorption corrections were done using SADABS program, where applicable. In general, all non-hydrogen atoms were refined anisotropically; hydrogen atoms were fixed by geometry or located by a Difference Fourier map and refined isotropically.

3a. Colourless block, C₂₈H₂₄O, M = 376.47, monoclinic, space group P2₁/c, a = 8.8110(8), b = 20.9480(13), c = 10.5926(7) Å, β = 90.386(7), V = 1955.1(3) ų, Z = 4, μ = 0.076 mm⁻¹, data/restraints/parameters: 3176/0/263, R indices (I > 2σ(I)): R1 = 0.0616, wR2 (all data) = 0.1025. CCDC no. 952109.

4l. Colourless block, C₄₀H₂₈O₂, M = 540.62, monoclinic, space group P2₁/n, a = 11.8178(5), b = 11.1901(4), c = 23.1887(9) Å, β = 103.940(4), V = 2976.2(2) ų, Z = 4, μ = 0.073 mm⁻¹, data/restraints/parameters: 4271/0/379, R indices (I > 2σ(I)): R1 = 0.0368, wR2 (all data) = 0.0841, CCDC no. 952110.

Acknowledgements

We thank Department of Science & Technology (DST, New Delhi) for financial support, single crystal X-ray diffractometer and HRMS facility. KCK thanks DST for a J. C. Bose fellowship. RK thanks CSIR (New Delhi) and ALSK thanks UGC for fellowships.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details on the synthesis of precursors 1b, 1f and 1h–j, ¹H and ¹³C NMR spectra and CIF files. CCDC 952109 and 952110. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra01105h

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