Disila-Okoumal: A Silicon Analogue of the Ambergris Odorant Okoumal

Synthesis and Olfactory Properties of a 6'-Silasubstituted "Spiro[4.5]-δ-Damascone"

The silicon analogue of the potent spirocyclic δ-damascone odorant 6 was synthesized from allyltrichlorosilane (15) and but-2-en-1-ol (16). The latter was transformed to 3-methylpen-4-enenitrile (11) by Saucy-Marbet reaction with ethoxyethane and subsequent treatment with HONH₂ ⋅HCl. The resulting γ,δ-unsaturated nitrile 11 was silylated with 1-allyl-1-chlorosilolane (14), which was prepared from allyltrichlorosilane (15) and the bis-Grignard reagent of 1,4-dichlorobutane. Metathetic ring closure employing the Grubbs I catalyst, followed by DIBAL reduction with non-aqueous work up, Grignard reaction with prop-1-en-1-ylmagnesium bromide, and Attenburrow MnO₂ oxidation concluded the synthesis. The target compound was found to be olfactorily related to the spiro[4.5]-δ-damascone lead, but approximately 900 times weaker. In a type of enol Brook rearrangement, it thermally decomposes however to 3,6-dihydro-2H-1,2-oxasilocine (20), which surprisingly is a damascone odorant as well; although, 12 000 times weaker.

One-pot enyne metathesis/Diels–Alder/oxidation to six-membered silacycles with a multi-ring core: discovery of novel fluorophores

One-pot enyne metathesis/Diels–Alder/oxidation methodology was developed to give fluorescent six-membered silacycle with multi-ring core.

Disila-galaxolide and derivatives: synthesis and olfactory characterization of silicon-containing derivatives of the musk odorant galaxolide

A series of silicon-containing derivatives of the polycyclic musk odorant galaxolide (4 a) was synthesized, that is, disila-galaxolide ((4RS,7SR)-4 b/(4RS,7RS)-4 b), its methylene derivative rac-9, and its nor analogue rac-10. The tricyclic title compounds with their 7,8-dihydro-6,8-disila-6 H-cyclopenta[g]isochromane skeleton were prepared in multistep syntheses by using a cobalt-catalyzed [2+2+2] cycloaddition of the mono- yne H2C=CHCH2 OCH2 C≡CB(pin) (B(pin)=4,4,5,5-tetramethyl-1,3,2-di- oxaborolan-2-yl) with the diynes H2C=C[Si(CH3 )2 C≡CH]2 or H2C- [Si(CH3)2 C≡CH]2 as the key step. Employing [Cr(CO)3 (MeCN)3 ] as an auxiliary, the disila-galaxolide diastereomers (4RS,7SR)-4 b and (4RS,7RS)-4 b could be chromatographically separated through their tricarbonylchromium(0) complexes, followed by oxidative decomplexation. The identity of the title compounds and their precursors was established by elemental analyses and multinuclear NMR spectroscopic studies and in some cases additionally by crystal structure analyses. Compounds (4RS,7SR)-4 b, (4RS,7RS)-4 b, rac-9, and rac-10 were characterized for their olfactory properties, including GC-olfactory studies of the racemic compounds on a chiral stationary phase. As for the parent galaxolide stereoisomers 4 a, only one enantiomer of the silicon compounds (4RS,7SR)-4 b, (4RS,7RS)-4 b, rac-9, and rac-10, smelt upon enantioselective GC-olfactometry, which according to the elution sequence is assumed to be also (4S)-configured as in the case of the galaxolide stereoisomers. The disila-analogues (4S,7R)-4 b and (4S,7S)-4 b were, however, about one order of magnitude less intense in terms of their odor threshold than their parent carbon compounds (4S,7R)-4 a and (4S,7S)-4 a. The introduction of a 7-methylene group in disila-galaxolide (4 b→rac-9) improved the odor threshold by a factor of two. With the novel silicon-containing galaxolide derivatives, the presumed hydrophobic bulk binding pocket of the corresponding musk receptor(s) could be characterized in more detail, which could be useful for the design of novel musk odorants with an improved environmental profile.

Disila-analogues of the synthetic retinoids EC23 and TTNN: synthesis, structure and biological evaluation

Silicon chemistry offers the potential to tune the effects of biologically active organic molecules. Subtle changes in the molecular backbone caused by the exchange of a carbon atom for a silicon atom (sila-substitution) can significantly alter the biological properties. In this study, the biological effects of a two-fold sila-substitution in the synthetic retinoids EC23 (4-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydronaphthalen-2-ylethynyl)benzoic acid (4a)) and TTNN (6-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydronaphthalen-2-yl)-2-naphthoic acid (7a)) as well as their corresponding analogues with an indane instead of a 1,2,3,4-tetrahydronaphthalene skeleton (compounds 5a and 8a) were investigated. Two-fold C/Si exchange in 4a, 5a, 7a and 8a leads to the silicon-analogues disila-EC23 (4b), 5b, disila-TTNN (7b) and 8b, which contain a 1,2,3,4-tetrahydro-1,4-disilanaphthalene (4b, 7b) or 1,3-disilaindane skeleton (5b, 8b). Exchange of the SiCH(2)Si moiety of 5b for an SiOSi fragment leads to the disiloxane 6 (2-oxa-1,3-disilaindane skeleton). The EC23 derivative 5a, the TTNN derivative 8a and the silicon-containing analogues 4b, 5b, 6, 7b and 8b were synthesised, and the biological properties of the C/Si pairs 4a/4b, 5a/5b, 7a/7b and 8a/8b and compound 6 were evaluated in vivo using RAR isotype-selective reporter cells. EC23 (4a) and its derivatives disila-EC23 (4b), 5a, 5b and 6 are very potent RAR agonists, which are even more potent than the powerful reference compound TTNPB. Disila-substitution of EC23 (4a) and 5a leads to a moderate decrease in RARα activation, whereas the RARβ,γ activation is almost not affected. In contrast, two-fold C/Si exchange in the weak retinoid agonist TTNN (7a) and 8a resulted in considerably different effects: a significant increase (7a→7b) and almost no change (8a→8b) in transcription activation potential for all three RAR isotypes. Disila-TTNN (7b) can be regarded as a powerful RARβ,γ-selective retinoid.

Odorant design based on the carbon/silicon switch strategy

Silicon chemistry has been demonstrated to be a novel source of chemical diversity in odorant design. The carbon/silicon switch strategy, i.e., sila-replacement in known odorants, is one of the methods currently used for the development of silicon-based odorants. Examples resulting from this strategy are sila-coranol, sila-dimetol, sila-linalool, sila-muguetalcohol, sila-majantol, sila-hydratropyl acetate, sila-bourgeonal, sila-lilial, disila-versalide, and disila-okoumal.