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Synlett 2023; 34(07): 841-845
DOI: 10.1055/s-0042-1751365
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Chemical Synthesis and Catalysis in India

Synthesis of Linear Tetraquinanes by [3+2] Cycloaddition, Chemoselective Allylation of 7-Ketonorbornene, and Ring-Rearrangement Metathesis as Key Steps

Sambasivarao Kotha∗
,
Arpit Agrawal

Funded by Council of Scientific and Industrial Research (CSIR), New Delhi, [02(0272)/16/EMR-II].
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Abstract

A nine-step synthetic sequence to linear tetraquinanes involving [3+2] cycloaddition, chemoselective allylation, and ring-rearrangement metathesis as key steps is reported. A chemoselective allylation of 7-ketonorbornene was realized for the first time by using indium powder and allyl bromide. By this method, norbornene ketones can be selectively allylated in the presence of a cyclopentanone moiety to give good yields of monoallylated Barbier-type products.


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Key words

[3+2]-cycloaddition - allylation - polyquinanes - tetraquinane - ring-rearrangement metathesis

Recently, there has been growing interest in the synthesis of polyquinanes through olefin metathesis and, more specifically, by ring-rearrangement metathesis (RRM).[1] Syntheses of many natural products containing cyclopentane ring skeletons have been developed, and polycyclic compounds obtained by Diels–Alder reaction[2] appear to be useful precursors for the metathesis strategy. Linear arrays of cyclopentane rings with well-defined stereochemistries have been proposed as possible steroid analogues, and interest in such nonnatural products has grown.[3] Natural compounds based on tetraquinane are a significant class of diterpenoids that contain both linear and angular triquinane moieties.[4] Many groups have attempted syntheses of various tetraquinanes with both linear and angular architectures. As our interest is aligned with tetraquinanes, selected examples of linear tetraquinanes 1–6 reported in the literature are shown in Figure [1].

Zoom Image
Figure 1 Representative examples of linear tetraquinanes made by various groups

In 1984, Eaton and his colleagues modified their earlier approach to tetraquinanes and synthesized the dienone 1 by using a readily available diketone as a starting material.[5] In 1986, Mehta and Krishnamurthy reported a concise method for building the basic 5–8–5 ring system of biologically active fusicoccane (10) and related natural diterpenes by using the tetraquinane intermediate 2, generated by a Nazarov cyclization sequence (Figure [2]).[6] Cook and colleagues synthesized the linear tetraquinane 3 by using a Weiss-Cook reaction and a SmI2-mediated reductive coupling as key steps.[7] In 1985, Kotha and Mehta reported that the readily available Cookson’s cage dione can be used as a precursor to the cis–syn–cis–anti tetraquinane framework 4.[8] Keese and co-workers investigated a [2+2] photocycloaddition process involving enone and indane derivatives that occurs preferentially through an endo-face attack. The primary product rearranges into the benzannulated tetraquinane 5 in the presence of trimethylsilyl iodide. The linear tetraquinane skeleton is predicted to be a structural homologue of the steroid estrone.[9] In 1997, Kotha and colleagues employed a cyclopentane annulation sequence to assemble tetracyclic diones such as 6, useful as precursors to dodecahedrane.[10] These results demonstrate the importance of various tetraquinane frameworks 1–7 present in natural (9 and 10) and nonnatural products.

Zoom Image
Figure 2 Example of synthesis of core skeleton of diterpenoids 9 and 10 using linear tetraquinane as precursors

Our journey began with the preparation of the key building block 15, starting from cyclopentanone (11) and following a four-step synthetic sequence via compounds 12–14 (Scheme [1]).[11] Enone 15 was then subjected to Hosomi–Sakurai-type [3+2]-cycloaddition reaction using allyl(triisopropyl)silane in the presence of titanium tetrachloride to produce the tetracyclic compound 16. This annulation is based on the work of the Knölker group.[12] Compound 16 was characterized by spectral analyses (1H and 13C NMR) and, later, its stereo structure was unambiguously established by single-crystal X-ray diffraction (XRD) studies.[13] [12d] When we attempted this annulation with compounds that did not contain bromine atoms and lacked a ketal functional group, the [3+2]-annulation failed.[14] We then attempted an allylation of compound 16 with allyl bromide in the presence of NaHMDS but, unfortunately, we obtained a complex mixture and were unable to isolate any pure compound. We then subjected keto derivative 16 to an attempted 1,2-addition with an allyl Grignard reagent, but this transformation was also unsuccessful and the starting material was recovered. This might have been due to the steric hindrance present in compound 16. To proceed further, we removed the bromine atoms in the hope that the steric factor might be reduced and the resulting compound 17 could be allylated. When we reduced compound 16 with tributyltin hydride in the presence of AIBN, we obtained the bromine-free compound 17.[15]

Zoom Image
Scheme 1 Synthesis of compound 17 starting with cyclopentanone
Zoom Image
Scheme 2 Allylation of diketo compound 19
Zoom Image
Figure 3 Structure of catalysts used in the RRM optimization.

We then attempted allylations of compound 17 with allyl bromide/NaHMDS or allyl Grignard reagent, but these attempts were also unsuccessful. Our previous experience suggested that the presence of a keto or ketal group at the 7-position of norbornene prevents an RRM sequence. We therefore decided to deprotect the norbornene ketal and to proceed with an allylation of the 7-ketonorbornene derivative. Compound 17 was refluxed with 1 N HCl in 1,4-dioxane, and the resulting keto derivative 18 was then treated with allylmagnesium bromide to give two products: the monoallyl compound 19 and the diallyl compound 20 (Scheme [2]).

Table 1 Optimization of the RRM Reactiona

Catalyst

Temp (°C)

Yield (%)

22a

22b

G-I

Rt

NRb

NR

G-I

50

NR

NR

G-II

Rt

40

5

G-II

50

65

20

HG-I

Rt

NR

NR

HG-I

50

NR

NR

a All reactions were performed for 10 h in CH2Cl2 as the solvent.

b NR = no reaction.

To obtain the monoallyl product 19 selectively, we tried performing the Grignard reaction at –40 °C, but we obtained a mixture of compounds 19 and 20, along with some unreacted starting material 18. To improve the yield of the monoallyl product 19, we attempted to remove the carbonyl group present on cyclopentane ring by a Wolff–Kishner reduction of compound 17. To our surprise, instead of a reduction of the ketone to give compound 21a, the reduction of the double bond in the norbornene occurred, giving compound 21b (Scheme [3]). Compound 21b was identified by spectral analyses (1H and 13C NMR), and its structure was later confirmed by a single-crystal X-ray diffraction analysis.[12d] Because the norbornene double bond is highly strained, it was reduced instead of the targeted keto functionality. To increase the yield of the monoallyl product 19, we took advantage of the reactivity of the 7-ketonorbornene portion, which is highly strained compared with the simple carbonyl group present in cyclopentanone moiety. Consequently, we surmised that allylindium bromide might be a better reagent than allylmagnesium bromide, because of the less-ionic character of indium.[16]

Zoom Image
Scheme 3 Attempted reduction of the carbonyl group of compound 17, and capped-sticks structure of compound 21b (CCDC 2176327)
Zoom Image
Scheme 4 Synthesis of tetraquinanes 22a and 22b by RRM.

With this idea in mind, we treated compound 18 with allyl bromide in the presence of indium powder, and we obtained compound 19 as the sole product.[17] This was really interesting, because the norbornene ketone was allylated chemoselectively in the presence of a cyclopentanone group with the aid of indium powder and, hence, for the first time, we report a highly selective allylation of a norbornene ketone in the presence of a cyclopentanone carbonyl group. Now, to synthesize the targeted tetraquinanes 22a and 22b, compound 19 was treated with various catalysts (Figure [3]) in a range of reaction conditions (Table [1]), but only the G-II catalyst was found to induce the RRM sequence to produce the linear tetraquinanes 22a and 22b (Scheme [4]). These tetraquinanes were separated by column chromatography, and their structures were confirmed by 1H and 13C NMR spectral analyses and HRMS data. A preliminary single-crystal X-ray diffraction study of compound 22b confirmed its structure.[18] Note that during the RRM, no new stereochemical information is generated; the stereochemistry present in the starting material is retained.

We have successfully synthesized the highly functionalized linear tetraquinanes 22a and 22b containing eight stereocenters by a nine-step sequence involving a simple strategy. An interesting chemoselective allylation of the 7-ketonorbornene derivative 18 by using indium powder and allyl bromide is reported; this might be useful in total syntheses of natural products and in increasing the overall yields of allylation sequences. All the stereocenters in tetraquinanes 22a and 22b were defined with the help of single-crystal X-ray diffraction analyses of the related intermediates 16 and 21b, and a preliminary X-ray crystal structure analysis of compound 22b.


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Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

A.A. thanks Mr. Ramakrishna Keesari Reddy for his valuable suggestions. Special thanks to Ms. Saima Ansari, Ms. Kunkumita Jena, and Ms. Rajashi Haldar for their help in the analysis of the X-ray diffraction data.

Supporting Information

    Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/s-0042-1751365.
  • Supporting Information
  • CIF File

  • References and Notes

  • 6 Mehta G, Krishnamurthy N. J. Chem. Soc., Chem. Commun. 1986; 1319
    CrossrefPubMed
  • 7 Lannoye G, Sambasivarao K, Wehrli S, Cook JM, Weiss U. J. Org. Chem. 1988; 53: 2327
    CrossrefPubMedSearch in Google Scholar
  • 8 Mehta G, Rao KS. J. Org. Chem. 1985; 50: 5537
    CrossrefSearch in Google Scholar
  • 9 Aebi R, Luef W, Keese R. J. Org. Chem. 1994; 59: 1199
    CrossrefSearch in Google Scholar
  • 10 Kotha S, Brahmachary E, Sivakumar R, Joseph A, Sreenivasachary N. Tetrahedron Lett. 1997; 38: 4497
    CrossrefPubMedSearch in Google Scholar
  • 11 Chapman NB, Key JM, Toyne KJ. J. Org. Chem. 1970; 35: 3860
    CrossrefPubMedSearch in Google Scholar
    • 12a Knölker H.-J, Jones PG, Wanzl G. Synlett 1998; 613
      Thieme Connect
    • 12b Knölker H.-J. J. Prakt. Chem./Chem.-Ztg. 1997; 339: 304
      CrossrefSearch in Google Scholar
    • 12c Schmidt A, Knölker H.-J. Synlett 2010; 2207
    • 12d CCDC 2176916 and 2176327 contains the supplementary crystallographic data for compounds 16 and 21b. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures
  • 13 (2′S,3a′R,3b′S,4′S,7′S,7a′R,8a′S)-4′,8a′-Dibromo-2′-(triisopropylsilyl)-2′,3′,3a′,3b′,4′,7′,7a′,8a′-octahydrospiro{[1,3]dioxolane-2,9′-[4,7]methanocyclopenta[a]inden}-8′(1′H)-one (16)Compound 15 (3 g, 8.2 mmol) was stirred in anhyd CH2Cl2 (50 mL). TiCl4 (4.7 g, 24.8 mmol) and allyl(triisopropyl)silane (2.4 g, 12.4 mmol) were added sequentially at rt under an inert atmosphere and the mixture was stirred for 12 h. The reaction was quenched with aq NH4Cl, and the mixture was extracted with CH2Cl2 (×3). The combined organic layer was washed with H2O, dried (Na2SO4), and concentrated. The crude product was purified by column chromatography (silica gel, 10% EtOAc–PE) to give a white solid; yield: 2.5 g (54%); mp 108–110 °C. 1H NMR (400 MHz, CDCl3): δ = 6.25–6.21 (m, 2 H), 4.25–4.15 (m, 2 H), 4.03–3.98 (m, 1 H), 3.94–3.90 (m, 1 H), 3.28–3.25 (m, 1 H), 2.99–2.96 (m, 1 H), 2.75 (dd, J = 5.51, 3.87 Hz, 1 H), 2.68–2.65 (m, 1 H), 2.54–2.49 (m, 1 H), 2.36–2.27 (m, 2 H), 1.97 (dd, J = 7.73, 5.15 Hz, 1 H), 1.584–1.581 (m, 1 H), 1.08–1.05 (m, 21 H). 13C NMR (100 MHz, CDCl3): δ = 215.4, 137.5, 134.6, 125.8, 71.4, 69.3, 66.6, 65.8, 54.6, 54.3, 53.3, 47.8, 47.4, 39.2, 24.8, 19.28, 19.25, 11.46. DEPT135 (100 MHz, CDCl3): CH, CH3 (δ = 137.5, 134.6, 54.6, 54.3, 53.2, 47.4, 24.8, 19.28, 19.25, 11.4); CH2 (δ = 66.6, 65.8, 47.8, 39.2). HRMS (ESI): m/z [M + Na]+ calcd for C24H36Br2NaO3Si: 581.0693; found: 581.0691.
  • 14 Kotha, S.; Jena, K. 2022, unpublished results.
  • 15 (2′S,3a′R,3b′R,4′R,7′S,7a′S,8a′R)-2′-(Triisopropylsilyl)-2′,3′,3a′,3b′,4′,7′,7a′,8a′-octahydrospiro{[1,3]dioxolane-2,9′-[4,7]methanocyclopenta[a]inden}-8′(1′H)-one (17) A solution of 16 (2 g, 3.5 mmol) in anhyd toluene was treated with AIBN (10 mol%). The reaction vessel was covered with aluminum foil to maintain dark conditions. Bu3SnH (3 g, 10.7 mmol) was added dropwise, and the mixture was refluxed at 110 °C for 4 h until the reaction was complete (TLC). The mixture was then cooled and the solvent was evaporated under reduced pressure. The resulting crude mixture was purified by column chromatography (silica gel, 10% EtOAc–PE) to give a pale-green liquid; yield: 962 mg (67%). 1H NMR (400 MHz, CDCl3): δ = 6.29–6.27 (m, 1 H), 6.17–6.14 (m, 1 H), 3.94–3.87 (m, 2 H), 3.84–3.80 (m, 2 H), 3.02–2.99 (m, 1 H), 2.96–2.93 (m, 1 H), 2.91–2.89 (m, 1 H), 2.68–2.64 (m, 1 H), 2.36–2.29 (m, 2 H), 2.08–2.03 (m, 1 H), 1.88–1.85 (m, 2 H), 1.71–1.67 (m, 1 H), 1.60 (s, 1 H), 1.05–1.03 (m, 21 H). 13C (100 MHz, CDCl3): δ = 226.1, 133.9, 133.7, 127.0, 65.1, 64.6, 58.6, 55.0, 51.5, 50.0, 47.0, 42.9, 40.5, 36.2, 24.4, 19.3, 11.5. DEPT135 (100 MHz, CDCl3): CH, CH3 ( δ = 133.9, 133.7, 58.6, 55.0, 51.5, 49.9, 47.0, 42.9, 24.4, 19.3, 11.5); CH2 (δ = 65.1, 64.6, 40.5, 36.2). HRMS (ESI) m/z [M + H]+ calcd for C24H39O3Si: 403.2663; found: 403.2660.
  • 17 (2S,3aR,3bS,4R,7S,7aS,8aR,9S)-9-Allyl-9-hydroxy-2-(triisopropylsilyl)-2,3,3a,3b,4,7,7a,8a-octahydro-4,7-methanocyclopenta[a]inden-8(1H)-one (19) A solution of compound 18 (100 mg, 0.27 mmol) in 3:1 THF–H2O (20 mL) was treated with allyl bromide (267 mg, 2.2 mmol) and In powder (96 mg, 0.83 mmol), and the mixture was stirred at rt for 6 h. When the reaction was complete, the mixture was filtered through Celite and extracted with EtOAc (×3). The combined organic layer was washed with H2O, dried (Na2SO4), and concentrated. The crude product was purified by column chromatography (silica gel, 10% EtOAc–PE) to give a pale-yellow liquid; yield: 104 mg (93%). 1H NMR (500 MHz, CDCl3): δ = 6.16–6.14 (m, 1 H), 6.04–6.02 (m, 1 H), 5.76–5.67 (m, 1 H), 5.16–5.09 (m, 2 H), 3.16–3.13 (m, 1 H), 2.83–2.78 (m, 3 H), 2.46–2.34 (m, 5 H), 2.08–2.04 (m, 1 H), 1.97 (br s, 1 H), 1.86–1.85 (m, 2 H), 1.71–1.67 (m, 1 H), 1.03 (s, 21 H). 13C (125 MHz, CDCl3): δ = 227.6, 134.94, 134.91, 134.7, 119.5, 95.0, 59.1, 56.4, 54.1, 53.5, 48.2, 42.7, 40.6, 37.0, 36.2, 24.4, 19.3, 11.5. DEPT135 (125 MHz, CDCl3): CH, CH3 (δ = 134.94, 134.91, 134.7, 59.1, 56.4, 54.1, 53.5, 48.2, 42.7, 24.4, 19.3, 11.5); CH2 (δ = 119.5, 40.6, 37.0, 36.2). HRMS (ESI) m/z [M + H]+ calcd for C25H41O2Si: 401.2877; found: 401.2876.
  • 18 Preliminary single-crystal X-ray diffraction data for compound 22b are given in the Supporting Information.

Corresponding Author

Sambasivarao Kotha
Indian Institute of Technology
Bombay, Powai, Mumbai, Maharashtra–400076
India   

Publication History

Received: 09 June 2022

Accepted after revision: 28 July 2022

Article published online:
09 September 2022

© 2022. Thieme. All rights reserved

Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany

  • References and Notes

  • 6 Mehta G, Krishnamurthy N. J. Chem. Soc., Chem. Commun. 1986; 1319
    CrossrefPubMed
  • 7 Lannoye G, Sambasivarao K, Wehrli S, Cook JM, Weiss U. J. Org. Chem. 1988; 53: 2327
    CrossrefPubMedSearch in Google Scholar
  • 8 Mehta G, Rao KS. J. Org. Chem. 1985; 50: 5537
    CrossrefSearch in Google Scholar
  • 9 Aebi R, Luef W, Keese R. J. Org. Chem. 1994; 59: 1199
    CrossrefSearch in Google Scholar
  • 10 Kotha S, Brahmachary E, Sivakumar R, Joseph A, Sreenivasachary N. Tetrahedron Lett. 1997; 38: 4497
    CrossrefPubMedSearch in Google Scholar
  • 11 Chapman NB, Key JM, Toyne KJ. J. Org. Chem. 1970; 35: 3860
    CrossrefPubMedSearch in Google Scholar
    • 12a Knölker H.-J, Jones PG, Wanzl G. Synlett 1998; 613
      Thieme Connect
    • 12b Knölker H.-J. J. Prakt. Chem./Chem.-Ztg. 1997; 339: 304
      CrossrefSearch in Google Scholar
    • 12c Schmidt A, Knölker H.-J. Synlett 2010; 2207
    • 12d CCDC 2176916 and 2176327 contains the supplementary crystallographic data for compounds 16 and 21b. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures
  • 13 (2′S,3a′R,3b′S,4′S,7′S,7a′R,8a′S)-4′,8a′-Dibromo-2′-(triisopropylsilyl)-2′,3′,3a′,3b′,4′,7′,7a′,8a′-octahydrospiro{[1,3]dioxolane-2,9′-[4,7]methanocyclopenta[a]inden}-8′(1′H)-one (16)Compound 15 (3 g, 8.2 mmol) was stirred in anhyd CH2Cl2 (50 mL). TiCl4 (4.7 g, 24.8 mmol) and allyl(triisopropyl)silane (2.4 g, 12.4 mmol) were added sequentially at rt under an inert atmosphere and the mixture was stirred for 12 h. The reaction was quenched with aq NH4Cl, and the mixture was extracted with CH2Cl2 (×3). The combined organic layer was washed with H2O, dried (Na2SO4), and concentrated. The crude product was purified by column chromatography (silica gel, 10% EtOAc–PE) to give a white solid; yield: 2.5 g (54%); mp 108–110 °C. 1H NMR (400 MHz, CDCl3): δ = 6.25–6.21 (m, 2 H), 4.25–4.15 (m, 2 H), 4.03–3.98 (m, 1 H), 3.94–3.90 (m, 1 H), 3.28–3.25 (m, 1 H), 2.99–2.96 (m, 1 H), 2.75 (dd, J = 5.51, 3.87 Hz, 1 H), 2.68–2.65 (m, 1 H), 2.54–2.49 (m, 1 H), 2.36–2.27 (m, 2 H), 1.97 (dd, J = 7.73, 5.15 Hz, 1 H), 1.584–1.581 (m, 1 H), 1.08–1.05 (m, 21 H). 13C NMR (100 MHz, CDCl3): δ = 215.4, 137.5, 134.6, 125.8, 71.4, 69.3, 66.6, 65.8, 54.6, 54.3, 53.3, 47.8, 47.4, 39.2, 24.8, 19.28, 19.25, 11.46. DEPT135 (100 MHz, CDCl3): CH, CH3 (δ = 137.5, 134.6, 54.6, 54.3, 53.2, 47.4, 24.8, 19.28, 19.25, 11.4); CH2 (δ = 66.6, 65.8, 47.8, 39.2). HRMS (ESI): m/z [M + Na]+ calcd for C24H36Br2NaO3Si: 581.0693; found: 581.0691.
  • 14 Kotha, S.; Jena, K. 2022, unpublished results.
  • 15 (2′S,3a′R,3b′R,4′R,7′S,7a′S,8a′R)-2′-(Triisopropylsilyl)-2′,3′,3a′,3b′,4′,7′,7a′,8a′-octahydrospiro{[1,3]dioxolane-2,9′-[4,7]methanocyclopenta[a]inden}-8′(1′H)-one (17) A solution of 16 (2 g, 3.5 mmol) in anhyd toluene was treated with AIBN (10 mol%). The reaction vessel was covered with aluminum foil to maintain dark conditions. Bu3SnH (3 g, 10.7 mmol) was added dropwise, and the mixture was refluxed at 110 °C for 4 h until the reaction was complete (TLC). The mixture was then cooled and the solvent was evaporated under reduced pressure. The resulting crude mixture was purified by column chromatography (silica gel, 10% EtOAc–PE) to give a pale-green liquid; yield: 962 mg (67%). 1H NMR (400 MHz, CDCl3): δ = 6.29–6.27 (m, 1 H), 6.17–6.14 (m, 1 H), 3.94–3.87 (m, 2 H), 3.84–3.80 (m, 2 H), 3.02–2.99 (m, 1 H), 2.96–2.93 (m, 1 H), 2.91–2.89 (m, 1 H), 2.68–2.64 (m, 1 H), 2.36–2.29 (m, 2 H), 2.08–2.03 (m, 1 H), 1.88–1.85 (m, 2 H), 1.71–1.67 (m, 1 H), 1.60 (s, 1 H), 1.05–1.03 (m, 21 H). 13C (100 MHz, CDCl3): δ = 226.1, 133.9, 133.7, 127.0, 65.1, 64.6, 58.6, 55.0, 51.5, 50.0, 47.0, 42.9, 40.5, 36.2, 24.4, 19.3, 11.5. DEPT135 (100 MHz, CDCl3): CH, CH3 ( δ = 133.9, 133.7, 58.6, 55.0, 51.5, 49.9, 47.0, 42.9, 24.4, 19.3, 11.5); CH2 (δ = 65.1, 64.6, 40.5, 36.2). HRMS (ESI) m/z [M + H]+ calcd for C24H39O3Si: 403.2663; found: 403.2660.
  • 17 (2S,3aR,3bS,4R,7S,7aS,8aR,9S)-9-Allyl-9-hydroxy-2-(triisopropylsilyl)-2,3,3a,3b,4,7,7a,8a-octahydro-4,7-methanocyclopenta[a]inden-8(1H)-one (19) A solution of compound 18 (100 mg, 0.27 mmol) in 3:1 THF–H2O (20 mL) was treated with allyl bromide (267 mg, 2.2 mmol) and In powder (96 mg, 0.83 mmol), and the mixture was stirred at rt for 6 h. When the reaction was complete, the mixture was filtered through Celite and extracted with EtOAc (×3). The combined organic layer was washed with H2O, dried (Na2SO4), and concentrated. The crude product was purified by column chromatography (silica gel, 10% EtOAc–PE) to give a pale-yellow liquid; yield: 104 mg (93%). 1H NMR (500 MHz, CDCl3): δ = 6.16–6.14 (m, 1 H), 6.04–6.02 (m, 1 H), 5.76–5.67 (m, 1 H), 5.16–5.09 (m, 2 H), 3.16–3.13 (m, 1 H), 2.83–2.78 (m, 3 H), 2.46–2.34 (m, 5 H), 2.08–2.04 (m, 1 H), 1.97 (br s, 1 H), 1.86–1.85 (m, 2 H), 1.71–1.67 (m, 1 H), 1.03 (s, 21 H). 13C (125 MHz, CDCl3): δ = 227.6, 134.94, 134.91, 134.7, 119.5, 95.0, 59.1, 56.4, 54.1, 53.5, 48.2, 42.7, 40.6, 37.0, 36.2, 24.4, 19.3, 11.5. DEPT135 (125 MHz, CDCl3): CH, CH3 (δ = 134.94, 134.91, 134.7, 59.1, 56.4, 54.1, 53.5, 48.2, 42.7, 24.4, 19.3, 11.5); CH2 (δ = 119.5, 40.6, 37.0, 36.2). HRMS (ESI) m/z [M + H]+ calcd for C25H41O2Si: 401.2877; found: 401.2876.
  • 18 Preliminary single-crystal X-ray diffraction data for compound 22b are given in the Supporting Information.

   Permissions and Reprints All articles of this category
Zoom Image
Figure 1 Representative examples of linear tetraquinanes made by various groups
Zoom Image
Figure 2 Example of synthesis of core skeleton of diterpenoids 9 and 10 using linear tetraquinane as precursors
Zoom Image
Scheme 1 Synthesis of compound 17 starting with cyclopentanone
Zoom Image
Scheme 2 Allylation of diketo compound 19
Zoom Image
Figure 3 Structure of catalysts used in the RRM optimization.
Zoom Image
Scheme 3 Attempted reduction of the carbonyl group of compound 17, and capped-sticks structure of compound 21b (CCDC 2176327)
Zoom Image
Scheme 4 Synthesis of tetraquinanes 22a and 22b by RRM.