A New Family of Rigid Dienone Musks Challenges the Perceptive Range of the Human Olfactory Receptor OR5AN1The financial support from the National Natural Science Foundation of China (NNSFC; Grant 21971042) is as well acknowledged with gratitude.
Received: 13 February 2020
Accepted after revision: 08 March 2020
24 March 2020 (online)
Dedicated to the memory of Professor Georg Fráter (* 27 September 1941; † 25 June 2019)
A new family of dienone musks was discovered by alkylation of different aldehydes with but-3-en-1-yn-1-yllithium and subsequent domino reaction of a Saucy–Marbet transfer vinylation–Claisen rearrangement with an intramolecular Diels–Alder reaction, and concluding Lewis acid catalyzed double-bond isomerization. The newly synthesized dienone structures possess pleasant musk odors displaying fatty, slightly fruity and green facets. Although the dienone musks were predicted in silico to bind to the OR5AN1 receptor based on QM/MM calculations, they were found to be inactive in the in vitro assay. The latter results suggest that the OR5AN1 receptor is not the prime musk receptor but primarily responsible for the animalic character of certain macrocyclic ketones and nitro musks.
Key wordsfragrances - musk odorants - odorant receptors - olfactory properties - structure–activity relationships
References and Notes
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- 16 Saucy–Marbet/IMDA Reaction to Ketone 19: A 100-mL autoclave was charged with a mixture of vinyl propargyl alcohol 15 (2.69 g, 14.0 mmol), isopropenyl methyl ether (16, 4 equiv, 4.04 g, 56.0 mmol), p-toluenesulfonic acid monohydrate (2 mg) and toluene (40 mL). The reaction mixture was heated to 150 °C for 24 h, prior to removal of the solvent and purification by flash chromatography (PE/MTBE = 95:5, Rf 0.44) and bulb-to-bulb distillation, which furnished the tetrahydroindenyl ketone 19 (2.8 g, 85%, b.p. 152 °C/0.18 mbar) as a colorless odoriferous liquid.
- 17 Isomerization of 19 to the Dienone Musk 12: At 0 °C, AlCl3 (0.1 equiv, 58 mg, 0.43 mmol) was added to a solution of ketone 19 (1.0 g, 4.3 mmol) in dichloromethane (20 mL). The resulting mixture was stirred for 2 h at this temperature, and then allowed to warm to room temp. After stirring overnight, quenching with 1m aq. HCl, extraction with dichloromethane (3 × 30 mL), and purification by flash chromatography (PE/MTBE = 95:5, Rf 0.43) with subsequent bulb-to-bulb distillation provided the (E)-configured dienone musk 12 (510 mg, 51%, b.p. 140 °C/0.14 mbar) as pale-yellow liquid. 1H NMR (400 MHz, CDCl3): δ = 5.82 (s, 1 H, 3-H), 2.98 (td, J = 6.5, 2.0 Hz, 2 H, 5′-H2), 2.30 (s, 2 H, 3′-H2), 2.27–2.25 (m, 2 H, 1′-H2), 2.19 (s, 3 H, 1-H3), 1.55 (t, J = 6.5 Hz, 2 H, 6′-H2), 1.09 (s, 6 H, Me2C-2′), 1.02 (s, 6 H, Me2C-7′) ppm. 1H,1H NOESY (500 MHz, CDCl3): 1-H3 × 5′-H2, 3-H × 3′-H2. 13C NMR (100 MHz, CDCl3): δ = 199.2 (s, C-2), 159.8 (s, C-7a′), 151.7 (s, C-4′), 132.9 (s, C-3a′), 118.1 (d, C-3), 48.0 (t, C-1′), 46.7 (t, C-3′), 37.6 (t, C-6′), 36.9 (s, C-2′), 32.7 (s, C-7′), 32.0 (q, C-1), 29.7 (2q, Me2C-2′), 26.3 (2q, Me2C-7′), 24.4 (t, C-5′) ppm. Odor description: musky, slightly fatty, slightly green. Odor threshold (th): 2.9 ng/L air.
- 18 In vitro OR5AN1 Assay: HEK293T cells were grown in DMEM medium containing FBS (9%). The cells were seeded in 96-well plates (10,000 cells/well). After 24 h, the cells were transfected with plasmids for the expression of human OR5AN1, a human RTP1S variant (V227I), and a cAMP-responsive element luciferase reporter using Lipofectamine 2000 according to the manufacturer’s instructions. For each plate pcDNA3.1(+)-Lucy-FLAG-rho-OR5AN1 (0.625 μg), pcDNA3.1(+)-RTP1S-V227I (1 μg) and pGL4.29 (1 μg) were transfected. The cells were stimulated with the musks (dissolved in growth medium) 18 h post transfection. After 4 h, the cells were lysed, and the luciferase activity was measured. Efficacy was determined as maximal activation over the full dose-response curve. EC50 values (conc. for 50% efficacy) were determined by the Graph pad prism program v 6.09 with four-parameter curve fit. Relative efficacy was calculated in relation to the most active compound (musk tibetene = 100%). Compounds with maximal efficacy <10% were considered inactive.
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