Jidong Shaoa,
Liqi Li*a,
Jie Zhangb,
Jingping Hua,
Jijun Xuea and
Ying Li*a
aState Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, P. R. China. E-mail: liying@lzu.edu.cn; lilq@lzu.edu.cn
bState Key Laboratory of Fluorine & Nitrogen Chemicals, Xi'an Modern Chemistry Research Institute, Xi'an, 710065, P. R. China
First published on 22nd March 2016
A new access to spiro[5.5]undecane frameworks was reported through the ZnMe2-promoted alkynylation of salicylaldehyde and HCO2H-mediated dearomatizative cyclization which can be used to construct an all-carbon quaternary spirocenter. This method can be applied to the rapid synthesis towards a series of useful motifs of natural products, such as the core of elatol and aphidicolin.
One commonly exploited mechanistic approach to construct spirocycle skeleton is the dearomatizative spirocyclization of functionalized phenols and naphthols. For example, chiral hypervalent iodine catalyst using m-CPBA as terminal oxidant was developed for intramolecular spirolactonization.5,6 Palladium- or iridium-catalyzed intramolecular allylic dearomatization of phenols was also described.7 Feringa et al. developed one-pot spirocyclization involving asymmetric conjugate addition and subsequent oxidative dearomatization of 2-naphthols.8 Katsuki et al. reported iron-catalyzed asymmetric tandem spirocyclization from 1-methyl-2-naphthols and phenols.9 Luan et al. and You et al. described Ru or Rh-catalyzed vinylative dearomatization of naphthols or phenols via a C(sp2)–H bond activation approach.10 Gulías et al. and Lam et al. independently reported Rh-catalyzed spiroannulation of ortho-vinylphenols triggered by terminal C–H functionalization of the alkenyl moiety.11 Luan et al. developed a Pd-catalyzed [2 + 2 + 1] annulative dearomatization between β-naphthols and two alkyne units,12 and Pd-catalyzed alkyne insertion/β-naphthol dearomatization cascade to construct spirocycles.13 Other methods were also described by various groups.14–18 However, the reported approaches are either transition-metal catalysis or their substrates are mostly limited to structurally similar cyclic lactones or lactams. Thus, new strategies for intermolecular spirocyclization using different classes of starting materials in one-pot process are highly desirable but challenging.
In 2015, we reported an approach to synthesize 2,4-difunctionalized benzopyrans in one-pot process (Scheme 1, top).19 As a continuation of our focus on this research program, the present paper describes a new access to spiro[5.5]undecane frameworks through the ZnMe2-promoted alkynylation of salicylaldehyde and HCO2H-mediated dearomatizative cyclization that can be applied to construct an all-carbon quaternary spirocenter (Scheme 1, bottom).
The present investigation began by reacting different diene substrate (3a) with 5-methoxysalicylaldehyde (1a) and 3-methoxyprop-1-yne (2a) under standard reaction conditions, which were developed in our previous research.19 We unexpectedly obtained a spirocycle product (4a) in 7% yield instead of benzopyran compound (Table 1, entry 2). The structure of 4a was identified by X-ray crystallography (Fig. 2).
Entryb | Acid | Solvent | Temp (°C) | Yieldc (%) |
---|---|---|---|---|
a Unless otherwise specified, all reactions were performed using Cu(OTf)2 (8 mol%), ZnMe2 (5 equiv.), 1a (1 equiv.), 2a (4 equiv.), 3a (5 equiv.), and acid (1.5 equiv.).b For all entries, dr is >20![]() ![]() ![]() ![]() |
||||
1 | BiCl3 | Toluene | 40 | Trace |
2 | ZnCl2·Et2O | Toluene | 40 | 7 |
3 | CF3CO2H | Toluene | 40 | Trace |
4 | MSA | Toluene | 40 | 7 |
5 | (R)-(−)-BPAd | Toluene | 40 | 11 |
6 | p-TsOH | Toluene | 40 | 18 |
7 | CSA | Toluene | 40 | 32 |
8 | CH3CO2H | Toluene | 40 | 41 |
9 | HCO2H | Toluene | 40 | 43 |
10e | HCO2H | Toluene | 40 | 39 |
11f | HCO2H | Toluene | 40 | 32 |
12 | HCO2H | THF | 40 | Trace |
13 | HCO2H | Hexane | 40 | 30 |
14 | HCO2H | CH2Cl2 | 40 | 25 |
15 | HCO2H | CH2Cl2 | r.t. | 46 |
On the basis of this initial observation, we used 5-methoxysalicylaldehyde (1a), 3-methoxyprop-1-yne (2a), and 6,6-dimethyl-1-vinylcyclohex-1-ene (3a) as model substrates for further optimization (Table 1). Various acids were initially evaluated in toluene at 40 °C (entries 1–9). The experimental results indicated that HCO2H is a beneficial acid. By using the ideal acid, we discovered that the stoichiometric ratio of 2a and ZnMe2 in alkynylation sequence effectively facilitated the desired transformation (entries 9–11). We then screened several organic solvents, including THF, hexane and CH2Cl2 (entries 12–15). As a result, CH2Cl2 was found to be the best choice at room temperature, and the anticipated spirocycle containing product 4a can be produced in 46% yield. We also performed a stepwise investigation under the optimal conditions but leading to a lower yield (22%, for details see ESI†). So we proposed that the existence of ZnMe2 may play a very significant role in the one-pot process.
Having identified optimal reaction conditions (Table 1, entry 15), we first examined the substrate scope of this transformation with respect to the substituted salicylaldehydes partner. Table 2 shows that the electronic effect of the substrates is very significant. Electron-donating groups, such as dimethoxy (4g), methoxy (4a, h), and methyl (4b) are more favorable for the envisioned spirocycle products than the electron-withdrawing groups, such as bromo (4d), chloro (4e), and ester (4f). The reaction is also restricted to the terminal alkyne (4a, 4h–k). The steric effect of these electron-rich alkynes exerts a great impact on the yields (23–46%). Functionalized 1,3-butadienes were also investigated. We obtained both spirocycle products (4l-1, 4m-1, 4n-1) and benzopyran compounds (4l-2, 4m-2, 4n-2) when the reaction was treated with isoprene or 2,3-dimethyl-1,3-butadiene. Benzopyran compounds were not observed when using 6,6-dimethyl-1-vinylcyclohex-1-ene (3a) as diene (Table 2, 4a–k). We then speculated that the benzopyran products corresponding to 3a are unstable under acidic conditions.
a For all compounds, dr was determined by 1H NMR spectroscopy, if diastereoisomers could not be detected in 1H NMR spectra, we identified the dr as >20![]() ![]() |
---|
![]() |
Based on these results and our previous findings, a plausible reaction mechanism for this spirocyclization is illustrated in Scheme 2. The reaction of ZnMe2 with terminal alkyne (2) generated an active alkynylzinc,20 which attacked the substituted salicylaldehyde (1) to form alcohol (5). Under acidic conditions, 5 was converted into carbocation TS-A, which was difunctionally stabilized by a phenyl group and an acetylenyl group through conjugation effects. The formed carbocation TS-A underwent electrophilic addition with diene 3 to deliver the resonance hybrids TS-B and TS-C. TS-C followed by enol–keto tautomerization of the phenol ring to produce spirocycle 4-1 (path a). At the same time, TS-B underwent intramolecular nucleophilic addition to form the benzopyran compound 4-2 through path b.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 1447716. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra02463g |
This journal is © The Royal Society of Chemistry 2016 |