Platinum-Catalyzed Tandem Cyclization-Nazarov Cylization Reactions: An Easy Access to Complex Carbocyclic Molecules from 2-Alkenyl-(1′-hydroxyl-4-en-2-ynyl)benzenes

Abstract

In the presence of PtCl₂/CO catalyst, 2-alkenyl-(1′-hydroxyl-4-en-2-ynyl)benzenes undergo a tandem electrophilic 6-exo-dig cyclization-Nazarov cyclization sequence to give 1H-cyclopenta[a]naphthalenes efficiently and regioselectively.

Key words

alcohols - alkynes - carbenoids - catalysis - tandem reactions

References and Notes

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13

Magnesium oxide was added to remove Brønsted acid given from PtCl₂ and water in this system. We envisage that Brønsted may enhance the transformation of alcohol substrate 13 into indene derivative 24 through a simple Nazarov cyclization as depicted in Scheme [6] .

14

See ref. 7d for the formation mechanism of the ketone product 36.

15

Typical Procedure for the Synthesis of 4-Methyl-1-[2-(prop-1-en-2-yl)phenyl]pent-4-en-2-yn-1-ol (3)
To a THF (15 mL) solution of 2-bromobenzaldehyde (1.00 g, 5.40 mmol) was added tributyl(prop-1-en-2-yl)stannane (1.87 g, 1.05 mmol), Pd(PPh₃)₄ (311 mg, 0.27 mmol), CuCl (0.80 g, 8.10 mmol) and LiBr (0.7 g, 8.10 mmol); the mixture was heated in a sealed tube at 60 °C for 8 h. The resulting solution was concentrated under reduced pressure, and the residues were eluted through a silica column (hexane-EtOAc, 98:2) to afford 2-(prop-1-en-2-yl)benz-aldehyde (639 mg, 4.37 mmol, 81%) as colorless oil. To a THF (8 mL) solution of 2-methylbut-1-en-3-yne (0.29 g, 4.45 mmol) was slowly added n-BuLi (1.6 mL, 2.5 M in hexane) at -78 °C; the solution was stirred for 30 min at -78 °C before addition of a THF solution (2 mL) of 2-(prop-1-en-2-yl)benzaldehyde (0.50 g, 3.42 mmol). After 30 min, to this solution was added H₂O (3 mL); the solution was extracted with EtOAc, and concentrated to afford crude alcohol. The crude product was eluted through a silica column (hexane-EtOAc, 5:1) to afford substrate 3 (640 mg, 3.01 mmol, 88%) as a light yellow oil.

16

Typical Procedure for the 6- exo - dig -Nazarov tandem cyclization of 4-Methyl-1-(2-prop-1-en-2-yl)phenylpent-4-en-2-yn-1-ol(1) to 1,5-Dimethyl-1 H -cyclopenta[ a ]naphthalene (4)
A long tube containing PtCl₂ (6.3 mg, 0.023 mmol) was evacuated and backfilled with CO. After repeating this procedure twice, the tube was charged with alcohol substrate 3 (100 mg, 0.472 mmol) and dry THF (2 mL). The resulting mixture was stirred at 23 °C for 20 h. The solution was concentrated and eluted through a silica column (hexane) to give compound 4 (81 mg, 0.417 mmol, 89%) as viscous oil.

17

4-Methyl-1-[2-(prop-1-en-2-yl)phenyl])pent-4-en-2-yn-1-ol (3)
IR (neat): 3654 (m), 1624 (m), 1610 (m) cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 7.76 (d, J = 7.0 Hz, 1 H), 7.33-7.29 (m, 2 H), 7.17 (d, J = 7.0 Hz, 1 H), 5.79 (s, 1 H), 5.32 (s, 1 H), 5.25 (s, 1 H), 5.24 (s, 1 H), 4.99 (s, 1 H), 2.12 (s, 3 H), 1.89 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 144.0, 142.4, 137.5, 128.0, 127.8, 127.4, 127.1, 126.1, 122.1, 116.0, 88.7, 87.1, 61.8, 25.3, 23.1. HRMS: m/z calcd for C₁₅H₁₆O: 212.1201; found: 212.1204.

18

1,5-Dimethyl-1 H -cyclopenta[ a ]naphthalene (4)
IR (neat): 1598 (m) cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 7.96 (d, J = 8.0 Hz, 1 H), 7.68 (d, J = 8.0 Hz, 1 H), 7.44 (t, J = 8.0 Hz, 1 H), 7.36 (t, J = 8.0 Hz, 1 H), 7.31 (s, 1 H), 6.53 (s, 1 H), 3.58 (s, 2 H), 2.69 (s, 3 H), 2.22 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 145.6, 142.9, 137.2, 133.0, 129.9, 129.8, 127.6, 125.6, 125.1, 123.7, 123.6, 120.8, 41.5, 19.7, 16.8. HRMS: m/z calcd for C₁₅H₁₄: 194.1096; found: 194.1092.