Letter pubs.acs.org/OrgLett
CuI Mediated One-Pot Cycloacetalization/Ketalization of o‑Carbonyl Allylbenzenes: Synthesis of Benzobicyclo[3.2.1]octane Core Chieh-Kai Chan, Yu-Lin Tsai, and Meng-Yang Chang* Department of Medicinal and Applied Chemistry, Kaohsiung Medical University Hospital, Kaohsiung Medical University, Kaohsiung 807, Taiwan S Supporting Information *
ABSTRACT: CuI/DMSO-mediated intramolecular cycloacetalization/ketalization of o-carbonyl allylbenzenes has been achieved for constructing [6,6,5]-tricycles having a ketal motif in good yields. The expeditious one-step route provides a three C−O bond formation. The key products with the structural framework of a benzofused dioxabicyclo[3.2.1]octane core have been confirmed by X-ray crystallographic analysis. Synthesis of dihydroisocoumarin has been studied.
M
Scheme 1. Retrosynthetic Route of Benzobicyclo[3.2.1]octane
any structures having cyclic acetals or ketals have been found in various sources such as bioactive natural products, functionalized materials, and diversified building blocks.1 Among the reported molecules in the family, a core structure having benzannulated acetals or ketals is relatively rare, especially those with a dioxacyclic scaffold. For naturally occurring products bearing this benzofused dioxabicycle core, the representative frameworks include integrastatin A,2a averufin,2b−d uroleuconaphins A2a,2e chloropreussomerin A,2f and cystophloroketals E.2g Attracted by their challenging frameworks, benzofused dioxabicycles have been the focus of considerable attention in synthetic communities.3,4 A great deal of effort has been devoted to benzofused acetals or ketals via dihydroxylation and sequential acetalization or ketalization. In general, dihydroxylative acetalization and ketalization have been involved as two practical steps,5 including (1) transition-metal or metal-free oxidant mediated dihydroxylation of functionalized olefins and (2) acidic catalyst-promoted acetalization or ketalization of the corresponding vicinal diols with carbonyl compounds such as aldehydes or ketones.6 As a consequence, numerous attempts have been devoted to the two-step protection of carbonyl synthons. Therefore, a new single-vessel route for simultaneous bond formation and ring construction is an important challenge because a one-pot process can reduce human effort, avoid chemical waste, and economize the cost and reaction time.7 Furthermore, it allows for rapid and efficient buildup of molecular complexity from relatively simple substrates. Continuing our research on synthetic applications of ocarbonyl allylbenzenes,8 herein, we present a one-pot coppercatalyzed synthesis of a benzofused bicyclo[3.2.1]octane core from o-carbonyl allylbenzenes, as shown in Scheme 1. This intramolecular one-pot cycloacetalization or -ketalization reaction proceeds under mild conditions and affords the products in high yields with high reaction efficiency. According to preliminary results,8 three kinds of o-carbonyl allylbenzenes 2, 4, and 5 were easily prepared from 3-hydroxybenzaldehyde (1a) © XXXX American Chemical Society
and isovanillin (1b) by a series of facile functional group transformations, as summarized in Scheme 2. Scheme 2. Synthesis of 2, 4, and 5
Received: March 1, 2017
A
DOI: 10.1021/acs.orglett.7b00630 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
temperatures (80 → 120 → 150 °C) and elongating the reaction times (10 → 15 h), the isolated yields of 6a were not improved (74%, 66%, and 84%; entries 12−14). By the involvement of TfOH (entry 15), no isolation of 6a was observed. Under a dry nitrogen atmosphere (entry 16), the yield of 6a was maintained (89%). From these observations, we concluded that 20 mol % of CuI and 2 mL of DMSO provided optimal reaction conditions (80 °C and 10 h) for one-pot cyclization. On the basis of the experimental results, a plausible mechanism for the formation of 6a is illustrated in Scheme 3.
Skeleton 2 shows synthesis via a three-step route which includes (1) O-allylation (Z = H, Me, Ph), (2) Claisen rearrangement (o-allyl and p-allyl), and (3) O-alkylation (R = Me, iPr, nBu, cC5H9, Bn). To increase the substrate variants, the o′-position of 1b was introduced with an aryl substituent to provide biphenyls 3a−3o via NBS-mediated bromination and a Suzuki−Miyaura cross-coupling. By use of similar three-step routes, skeleton 4 (Ar = Ph, 2-MeC6H4, 3-MeC6H4, 4-MeC6H4, 2-MeOC6H4, 3-MeOC6H4, 4-MeOC6H4, 2-FC6H4, 3-FC6H4, 4FC 6 H 4 , 3-PhC 6 H 4 , 4-PhC 6 H 4 , 3,4-(MeO) 2 C 6 H 3 , 3,4,5(MeO)3C6H2) was generated. Furthermore, Grignard addition of 2b and PCC-mediated oxidation of the resulting secondary alcohols afforded skeleton 5. With skeletons 2, 4, and 5 in hand, we commenced the study by treating 2a (1.0 mmol) with Cu(OTf)2 (10 mol %) in DMSO (2 mL) as the medium at room temperature for 10 h (Table 1, entry 1).9 Unfortunately, the
Scheme 3. Plausible Mechanism
Table 1. Reaction Conditionsa
entry
catalysts (mol %)
solvent
temp (°C)
time (h)
6a (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Cu(OTf)2 (10) Cu(OTf)2 (10) Cu(OTf)2 (10) Cu(OTf)2 (10) Cu(OAc)2 (10) CuSO4 (10) CuF2 (10) CuI (10) CuOTf (10) CuI (15) CuI (20) CuI (20) CuI (20) CuI (20) TfOH (20) CuI (20)
DMSO DMSO DMF MeNO2 DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO DMSO
25 80 80 80 80 80 80 80 80 80 80 120 150 80 80 80
10 10 10 10 10 10 10 10 10 10 10 10 10 15 10 10
−c 36 −c −c 28 17 10 80 54 88 90 74 66 84 −c 89d
Initially, complexation of CuI with a terminal olefin of 2a yields A. By intramolecular 6-exo-trig cyclization, B should be afforded via the first C−O bond formation. Following the addition of B by DMSO, C1 and C2 were generated via the second C−O bond formation. However, the copper arm and DMSO side of C1 should be orientated as a trans-configuration such that the occurrence of equilibrium between C1 and B is formed. Through a cis-configured conformation of C2, an in situ generated iodide group attacks the copper arm to lead to 6a by the intramolecular bridged ring annulation (the third C−O bond formation). Subsequently, CuI was regenerated and DMS was released. Finally, construction of the benzobicyclo[3.2.1]octane core was furnished via the cascade pathway of three C−O bond formations. With optimal conditions established (Table 1, entry 11) and the plausible mechanism proposed (Scheme 3), CuI mediated transformation from various o-carbonyl allylbenzenes 2, 4, and 5 to benzobicyclo[3.2.1]octanes 6−8 was examined next. As shown in Scheme 4, we found that this methodology allowed direct intramolecular cycloacetalization/ketalization of a broad range of substrates under mild and neutral conditions using a catalytic amount of CuI and a co-oxidant role of DMSO in good to excellent yields. The efficient formation of 6a−6h illustrated that the substituents (R and Z) on skeleton 2 did not affect the yield changes (82%−90%). However, when 2i (Z = Ph) was treated with CuI/DMSO, no detection of 6i was observed. Next, we explored the scope of biphenyls as the reaction substrates. A different aryl group of 4a−4o (for Ar, ortho-, meta-, paraposition; mono-, di-, trisubstituent) provided the desired 7a−o in good to excellent yields (70%−86%) under the above conditions. In particular, m-terphenyl 4k and p-terphenyl 4l could be suitable substrates, affording 7k and 7l in 80% and 84% yields, respectively. For the electronic nature of substituents, not only electron-neutral but also electron-withdrawing and electron-donating groups were appropriate. For the formation of 7a−o, the scope of aryl groups was very broad. Moreover, this reaction was found to not be limited to benzaldehydes as substrates, and benzoketones 5a−h (Y = alkyl and aromatic groups) also delivered 8a−8h in high yields (76%−86%). The
a
The reactions were run on a 1.0 mmol scale with 2a, solvent (2 mL), dry air atmosphere. bIsolated yields. cNo reaction. dDry nitrogen atmosphere.
reaction was unsuccessful, and therefore, it was repeated under heating at 80 °C (entry 2). Gratifyingly, the reaction at elevated temperatures was completed in 10 h furnishing a mixture of products from which the major product that was isolated in a 36% yield was identified as the expected 6a. The results prompted us to optimize the reaction for improving the yield of 6a. The reactions were performed under DMF and MeNO2 conditions (entries 3−4); however, no yields of 6a were provided. From the phenomenon, we understood that DMSO performed the dual solvent and co-oxidant roles under a dry air atmosphere.10 Next, some copper(II) complexes were studied, such as Cu(OAc)2, CuSO4, and CuF2. However, none of them provided higher yields of 6a than Cu(OTf)2 (entries 5−7). Changing the Cu(II) to Cu(I) salts, CuI and CuOTf provided 80% and 54% yields, respectively (entries 8−9). Furthermore, other catalytic amounts (15 and 20 mol %) of CuI were examined; however, the isolated yields only increased slightly to 88% and 90%, respectively (entries 10−11). After elevating the reaction B
DOI: 10.1021/acs.orglett.7b00630 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 4. Synthesis of 6−8a,b
a b
Scheme 6. Synthesis of 12
However, no further conversion was observed within 100 or 150 h due to the stereic hindrance of the ortho-aryl group. On the other hand, benzoketal 8a could not be converted to the dihydroisocoumarin since there was no occurrence of Habstraction. In summary, we have developed a CuI/DMSO promoted onepot cycloacetalizaition or -ketalization of o-carbonyl allylbenzenes 2, 4, and 5 at 80 °C for 10 h in good yields. The one-pot process provides benzobicyclo[3.2.1]octanes 6−8 via a cascade pathway of three C−O bond formations. The synthesis of dihydroisocoumarins 12 has been studied. The related plausible mechanisms have been proposed. The structures of the key products were confirmed by X-ray crystallography. Further investigations regarding the synthetic application of o-carbonyl allylbenzenes will be conducted and published in due course.
All reactions were performed at 1.0 mmol scale with 2, 4, and 5. Isolated yields.
structures of 7a, 8e, and 8g were determined by single-crystal Xray crystallography.11 Aside from the present one-pot cycloacetalization or cycloketalization reaction, CuI/DMSO has also been reported as a combination for cross-couplings,12a−k iodinations,12l aminations,12m−p and tandem cyclizations.12q−t By a one-pot two-step route (Suzuki−Miyaura coupling and our method), simple benzobicyclo[3.2.1]octane 11 could be obtained from o-formylphenyl boronic acids 9 in a 42% yield via the formation of intermediate 10 (Scheme 5). Although the isolated yield of 11 was low, it still provided a novel and efficient transformation from o-formyl arylboronic acid to cyclic acetal.
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ASSOCIATED CONTENT
S Supporting Information *
Scheme 5. One-Step Synthesis of 11
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b00630. Crystallographic data for 7a (CIF) Crystallographic data for 8e (CIF) Crystallographic data for 8g (CIF) Crystallographic data for 12b (CIF) Crystallographic data for 12d (CIF) Detailed experimental procedures and spectroscopic data for new compounds 6a−6h, 7a−7o, 8a−8h, 11, and 12a− 12h and X-ray analysis data of 7a, 8e, 8g, 12b, and 12d (PDF)
As an extension of one-pot cycloacetalization/ketalization, the reaction conditions were adjusted next (Scheme 6). To elongate a 10-fold reaction time, dihydroisocoumarin 12 was isolated.13 Isocoumarins and dihydroisocoumarins possess diverse biological activities, such as antifungal, antiallergic, antimicrobial, antiangiogenic, and antioxidant properities.13b,c,f Treatment of 6a−6h with CuI/DMSO provided 12a−12h in moderate yields. After a longer reaction time (10 → 100 h), benzoacetal 6 was slowly converted to dihydroisocoumarin 12. The structures of 12b and 12d were determined by single-crystal X-ray crystallography.11 The proposed mechanism is shown in Scheme 6. Complexation of CuI with an oxygen atom of 6 yielded I. By intramolecular C−O bond cleavage, II should be afforded. Following the addition of II by DMSO, III was generated via a new C−O bond formation. Finally, an in situ generated iodide group attacked the copper arm to lead to 12 by an oxidative process. Subsequently, CuI was regenerated and DMS was released. To investigate the CuI/DMSO-mediated conversion for biarylacetal skeleton 7, 7a was chosen as the model substrate.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Meng-Yang Chang: 0000-0002-1983-8570 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank the Ministry of Science and Technology of the Republic of China for financial support (MOST 105-2113-M-037-001). C
DOI: 10.1021/acs.orglett.7b00630 Org. Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.orglett.7b00630 Org. Lett. XXXX, XXX, XXX−XXX
