A Simple and Versatile Method for the Formation of Acetals/Ketals Using Trace Conventional Acids.

An important and surprising finding that the acetalization and ketalization of aldehydes and ketones with alcohols, respectively, proceed smoothly in the presence of 0.1 mol % acid, without removing water, has been presented. This process has many merits, such as commercial available catalysts with low cost and low loadings (as low as 0.03 mol %), quite a broad substrate scope (including various aldehydes, ketones, acid-sensitive substrates, and diols), a wide range of reaction temperature (-60 to 50 °C), high yields, large-scale preparation, environmental friendliness, and simple work-up procedure. This new protocol has also been successfully applied to protect the important organic compounds, such as 1,3-diols, 1,2-diols, acid-sensitive substrates, glucose, and 1,3-dicarbonyl compounds.

Introduction

A protective group is introduced into a molecule by chemical modification of a functional group to obtain chemoselectivity in the course of the preparation of multifunctional complex organic molecules. It plays an important role in organic, medicinal, carbohydrate, and drug design chemistry. As Wuts pointed out, organic synthesis has not yet matured to the point where protective groups are not needed for the synthesis of natural and unnatural products; thus, the development of new methods for functional group protection and deprotection continues.[1] Acetalization/ketalization is one of the most useful methods for the protection of aldehydes/ketones, which are extensively encountered in organic synthesis.[2,3] The acetals/ketals are traditionally generated by treating aldehydes/ketones with alcohols in the presence of typical acid catalysts (such as dry HCl, H2SO4, trifluoroacetic acid, and p-toluenesulfonic acid), which are often corrosive.[4−7] In addition, the acidic environment is incompatible with the acid-sensitive groups of the substrates, such as N-Boc-protected amines, silyl-protected alcohols, alkenes, and alkynes.[8] Especially, the acetals/ketals are also highly unstable because of the reversible reactions to the hemiacetals/hemiketals and the starting carbonyls; as a result, to avoid shifting the equilibrium back to the reactants, the byproduct water has to be removed by additional physical and chemical means.[8] A series of Lewis acids,[9−17] other homogeneous catalysts,[18−23] different metal complexes of Pt(II), Pd(II), and Rh(II),[24−28] and various heterogeneous catalysts[29−36] have been reported to catalyze the acetal/ketal protection of carbonyl compounds. Recently, a few more successful and efficient acetalization/ketalization reactions have also been reported: (1) Connon designed a series of aprotic salts (pyridinium salt derivatives, dialkyltriazolium salts, and imidazolium salts) capable of behaving as Brønsted acids for chemoselective acetalization of aldehydes;[37−40] (2) Azzena developed a novel acetalization/ketalization formation process of aliphatic and aromatic aldehydes/ketones with diols employing ammonium salts as acidic catalysts, using cyclopentyl methyl ether as a solvent under Dean–Stark conditions;[41] (3) Ying’s group developed a mesoporous polymelamine–formaldehyde polymer as an efficient catalyst for chemoselective acetalization of aldehydes;[42] and (4) very recently, graphitic-C3N4 has been shown to catalyze photoacetalization of aldehydes/ketones with alcohols to acetals/ketals in high yields using visible light under ambient conditions.[43] Although acetalization has been widely reported and well-investigated, the recent reports also suffered from lack of generality because of the following drawbacks: limited substrate scope, use of toxic or expensive reagents, high reaction temperature, stoichiometric amounts of catalysts, poor chemoselectivity and atom-economy, the need for excess amounts of drying agents, long reaction times, low yields, high costs, tedious workup, and so forth. Consequently, most of the reported methods had inevitably lowered the overall synthetic efficiency. Up to now, the application of acetalization/ketalizationin chemical industries, including pharmaceuticals, agrochemicals, and intermediates, has been hampered by the lack of general methods. Therefore, the development of alternative technologies that are greener, safer, and more environmentally friendly is highly desirable.

Can we find a simple molecule which acts like an enzyme for the acetalization of aldehydes/ketones? If this was possible, it would represent a remarkable synthetic alternative to many established acetalizations. In particular, such processes would overcome the abovementioned catalyst limitations/problems and allow the cost-effective manufacture of acetals or ketals in chemicals and pharmaceutical industries on an industrial scale. As we know, to increase the electrophilicity of the carbonyl carbon in the acetalization/ketalization process, acids are required to promote the reactions by forming oxonium ions. However, the alcohols would also be protonated in the presence of excess acids, dramatically decreasing the nucleophilicity of the alcohols. In addition, the excess acids may also cause the hydrolysis of acetals/ketals to aldehydes/ketones. Therefore, we envisioned that the acid loadings would be crucial for the generation of acetal/ketal products.

Results and Discussion

To verify the feasibility of our proposal, acetalization of trans-cinnamaldehyde with methanol was chosen as the model reaction using different loadings of concentrated hydrochloric acid (12 mol/L) at ambient temperature. The results are summarized in Table and Figure . Surprisingly, the formation of dimethyl acetal proceeded quite well in 20 min even with 0.005 mol % hydrochloric acid without removing the water (Table , entry 1). The conversion was significantly improved by increasing the acid loadings (entry 2), and excellent data were observed by using a range of acid loadings from 0.03 to 30 mol % (entries 3–14). However, the formation of acetal decreased as the acid loadings continued to increase (entries 15–18), and only 6% conversion was observed by using 600 mol % acid, probably because the alcohols were also protonated resulting in decreased nucleophilicity in the presence of excess acid (entry 19). In addition, the hydrolysis of acetals might also become a serious problem and the acetalization conversions decreased. It should be noted that no cis–transisomerization was observed during our investigation.[44] We further explored the reaction with 0.1 mol % hydrochloric acid at different temperatures. It was found that the temperature has little effect on the reactions, and excellent conversions were also obtained ranging from 50 to −60 °C (entries 20–24). For example, hydrochloric acid exhibited a very high catalytic activity when the domino reaction was carried out at 50 °C, and excellent conversion was obtained in 3 min with 0.1 mol % hydrochloric acid (entry 20); the reaction time should be extended at lower temperature (entry 24). Although the reported methods pointed out that the acetalization reaction was sensitive to water, the reaction proceeded smoothly without any effect on the reaction rate and conversion even when the concentration of HCl decreased from 12 to 4 mol/L (entries 25 and 26), indicating that the loadings of the acid catalyst should be more crucial for the formation of acetal products.

Table 1

Screenings of the Acid Loadingsa

entry	loading (mol %)	conv. (%)b	entry	loading (mol %)	conv. (%)b
1	0.005	68	14	30	91
2	0.01	76	15	60	89
3	0.03	91	16	100	84
4	0.05	93	17	120	78
5	0.10	95	18	300	59
6	0.20	94	19	600	6
7	0.25	94	20c	0.10	93
8	0.30	95	21d	0.10	93
9	0.5	92	22e	0.10	96
10	1.0	92	23f	0.10	93
11	3.0	92	24g	0.10	91
12	6.0	92	25h	0.10	92
13	12	91	26i	0.10	91

Unless otherwise specified, a mixture of aldehydes 1 (2 mmol) and hydrochloric acid in methanol was stirred at an ambient temperature for 20 min.

Determined by gas chromatography (GC) analyses of the crude reaction mixture.

At 50 °C for 3 min.

At 30 °C for 10 min.

At 0 °C for 2 h.

At −20 °C for 2 h.

At −60 °C for 24 h.

8 mol/L HCl was used.

4 mol/L HCl was used.

Figure 1

Conversion plot for acetalization of trans-cinnamaldehyde with different acid loadings. See Table for reaction conditions.

Subsequently, our attention turned to evaluate its general utility in the acetalization of other aldehydes. A wide range of aldehydes 1 were treated with methanol at an ambient temperature, catalyzed by 0.1 mol % hydrochloric acid. As shown in Table , several points are noteworthy: (1) All acetalization reactions proceeded smoothly with excellent conversions and yields (entries 1–18). (2) The scope of the acetalization reactions was proven to be quite broad. Various aldehydes, such as substituted cinnamaldehydes (entries 1–5), aliphatic α β-unsaturated aldehydes (entry 6), aryl aldehydes (entries 7–12), and aliphatic aldehydes (entries 15 and 16), could be transformed into the corresponding acetals with excellent conversions and isolated yields. In addition, the heteroaromatic aldehydes, such as 2-furaldehyde and 2-thiophenecarboxaldehyde (entries 13 and 14) as well as the cyclic aldehyde (entry 17), could be successfully employed to afford excellent yields. Especially, bisaldehyde 1r also furnished bisacetal 2r in an excellent yield. (3) In general, the substituents on the aryl ring of aromatic aldehydes had little effect on the efficiency of the acetalization reaction. However, cinnamaldehydes with electron-donating groups showed a relatively low activity, and trimethylorthoformate (TMOF) had to be added to improve the conversion (entry 3). (4) Most of the aldehydes underwent nearly quantitative acetalization at 0.1 mol % catalyst loading in a short reaction time, and the products could be purified by a simple extraction. (5) The reaction is not confined to methanolysis. The protection of benzaldehyde as a stable five-membered cyclic 1,3-dioxolane also proceeded smoothly under identical reaction conditions to afford products with excellent conversions (entries 19 and 20). (6) Other alcohols, such as ethanol, n-propanol, and 1,3-diols, were also effective for the acetalization (entries 21–23). (7) To investigate the scalability of this protocol, the acetalization reaction of cinnamaldehyde with methanol was scaled up by 200 times in comparison with the reaction shown in Table , entry 1. The reaction was also effective and could conveniently prepare the acetal on a gram scale (>5 g) with slightly inferior conversion.

Table 2

Acetalization of Various Aldehydes with Methanol Catalyzed by 0.1 mol % Hydrochloric Acida

Unless otherwise specified, a mixture of aldehydes 1 (2 mmol) and 0.1 mol % hydrochloric acid in methanol was stirred at an ambient temperature for 30 min.

Determined by GC analyses of the crude reaction mixture.

Isolated yield.

For 30 min.

Scaled up by 200 times.

1.2 equiv of TMOF was added.

For 24 h.

Ethylene glycol was used as the solvent.

Ethanol was used as the solvent.

n-Propanol was used as the solvent.

1,3-Propanediol was used as the solvent.

Ketalization usually requires an elevated temperature or water needs to be removed under Dean–Stark conditions to avoid shifting the equilibrium back to the reactants. Therefore, the development of a greener, safer, and environmentally friendly method for ketalization still represents a major challenge in organic synthesis. Encouraged by the successful results described above, we turned our attention to the possible ketalization reactions of ketones, taking acetophenone 3a and methanol as the model compounds under the optimized conditions; a low conversion (7%) was observed after 30 min, even when the reaction time was extended to 24 h. Fortunately, a quantitative conversion of the ketone was achieved when 1.2 equiv of TMOF was added in 12 h (Table , entry 1). Subsequently, a series of ketones were applied to establish the scope and generality of the protocol, and the corresponding ketals 4 were produced in high conversions/yields (entries 1–15). The aromatic ketones 3 with electron-donating groups such as a methyl group showed a relatively low activity, and the reaction time was extended to 24 h (entries 5 and 7). On the other hand, the aromatic ketones 3 with electron-withdrawing groups such as F, Cl, and Br ones showed a higher activity, giving almost quantitative ketal products (entries 2–4 and 8). The cyclic and acyclic aliphatic ketones also exhibited a high reactivity (entries 9–13). Interestingly, when methanol was replaced with ethylene glycol, 1,3-dioxolanes 4j′, 4k′ were also obtained with excellent conversions/yields (entries 11 and 13). We also investigated the reactions of conjugated enone systems, and excellent conversions/yields were obtained without forming oxy-Michael/ketal derivatives (entries 14 and 15).[45−49]

Table 3

Ketalization of Various Ketones with Alcohol Catalyzed by 0.1 mol % Hydrochloric Acida

Unless otherwise specified, a mixture of ketones 1 (2 mmol), CH(OCH3)3 (2.4 mmol) and 0.1 mol % hydrochloric acid in methanol was stirred at an ambient temperature for 12 h.

Determined by GC analyses of the crude reaction mixture.

Isolated yield.

12 h.

For 24 h.

For 30 min.

At 40 °C.

Ethylene glycol was used as the solvent.

THF was used as the solvent.

The diols or polyols are widely encountered in carbohydrates, macrolides, pharmaceutical agents, and nucleosides, leading to the development of a number of new methods for these functional groups protection. Dioxolanes and dioxanes are the most common protective groups for diols. Gratifyingly, 0.1 mol % hydrochloric acid readily promoted the ketalization of a range of diols with acetone (Scheme ). 1,3-Diols, such as glycol 5a, was transformed to the dioxanes in a high yield, as well as the 1,2-diols (e.g. 1,2-octanediol 5b) and R-(−)-3-chloro-1,2-propanediol 5c were transformed to the dioxolanes in high yields, usually without further purification. These studies demonstrate the high efficiency and the wide synthetic utility of the current catalytic protocol.

Scheme 1

Ketalization of Acetone with 1,3-Diols and 1,2-Diols

The compatibility of this method with other acid-sensitive substrates, such as N-Boc-protected amines 7a, silyl-protected alcohols 7c, and tetrahydropyranyl-protected alcohols 7b, was also established. As exemplified in Scheme , the acetalization/ketalization of aldehydes/ketones with methanol to form dimethyl acetals/ketals proceeded well with 0.1 mol % hydrochloric acid, also without affecting the abovementioned groups.

Scheme 2

Acetalization/Ketalization of Acid-Sensitive Substrates with Methanol

To further demonstrate its potential applications, this method was applied for the protection of other important carbonyl compounds (Scheme ). Ethyl acetoacetate 7d, which is used as one of the most important chemical intermediates in the production of a wide variety of compounds, was transformed to dimethyl ketal 8d in a good yield under mild conditions, while TMOF should be added (eq 1). 5-norbornene-2-carboxaldehyde 7e, which contained a double bond, could be successfully employed to afford the corresponding acetal 8e in an excellent yield, while the double bond remained intact (eq 2). The acetalization/ketalization of glucose is very important in carbohydrate chemistry. However, the methylation of d-glucose was usually treated with SOCl2,[50] AcCl,[51] Ac2O/BF3,[52] and Amberlyst-15.[53] To our delight, direct treatment of d-glucose 7f with 0.5 mol % hydrochloric acid in methanol at 60 °C for 72 h gave methyl d-glucopyranoside 8f in 91% yield (eq 3).

Scheme 3

Acetalization/Ketalization of Other Important Carbonyl Compounds with Alcohol

To further assess the usefulness of our protocol, the applications of this acetalization protocol in heterocyclic compounds synthesis were also investigated. As described in Scheme , two possible structural isomers (10 and 10′) would be obtained by treatment of o-isothiocyanato-(E)-cinnamaldehydes 9 with primary amines. Interestingly, only 3,4-dihydroquinazolines 10 were exclusively obtained when the reaction was performed in methanol under an ambient temperature. However, when the o-isothiocyanato-(E)-cinnamaldehydes were treated with hydrochloric acid (0.1 mol %) in methanol, they could be transformed into their corresponding acetals 11 in excellent isolated yields. Subsequently, the S-terminus of thiourea, not the N-terminus of thiourea, served as a nucleophilic site to react with the γ-position of β,γ-unsaturated dimethylacetal, following elimination a methoxyl group to afford the cyclizing product 4H-3,1-benzothiazine derivative 12 with excellent selectivity in refluxing toluene.

Scheme 4

Applications of This Acetalization Protocol in Heterocyclic Compound Synthesis

In addition, other strong inorganic acids (e.g., H2SO4 and HNO3) as well as the strong organic acids (e.g., CF3COOH and CF3SO3H) also exhibited a high catalytic activity for the acetalization of cinnamaldehyde with methanol at a low acid loading under an ambient temperature, and excellent conversions were obtained in 20 min (Table , entries 1–4). As expected, the weak acids, such as acetic acid and p-nitrobenzoic acid (PNBA), showed poor catalytic activity (entries 5 and 6).

Table 4

Other Acids for the Acetalization

entry	acid	conv (%)b	entry	acid	conv (%)b
1	H2SO4	93	4	CF3SO3H	93
2	HNO3	92	5	HAc	24
3	CF3COOH	93	6	PNBA	4

Conclusions

In conclusion, we have presented a greener, safer, more efficient and environmentally friendly procedure for the acetalization/ketalization of aldehydes and ketones. Most of the catalyst problems for acetalization/ketalization have been overcome, and these are the important points: (1) The main advantage of this process is the use of very low catalyst loadings (0.1 mol %), and the acid loadings varying from 0.03 to 30 mol % have marginal effect on the catalytic activity and conversion; (2) The acetalization/ketalization protocol is insensitive to water, and there is no need to remove water by additional physical and chemical means; (3) The acetalization/ketalization process is relatively insensitive to temperature. (4) The reaction scope is much broader than previous reports. Whether aldehydes (such as α,β-unsaturated aldehydes, aryl aldehydes, and aliphatic aldehydes) or ketones (such as aromatic ketones, cyclic ketones, conjugated enones, and aliphatic ketones) or even the diols (such as 1,2-diols and 1,3-diols), excellent results were generally obtained. (5) The acetalization/ketalization protocol is compatible with acid-sensitive substrates, such as N-Boc-protected amines, silyl-protected alcohols, and alkenes; (6) The reaction was proven effectively and conveniently to prepare the acetals on a gram scale (>5 g) with slightly inferior conversion; (7) The work-up procedure is remarkably simple, and the products do not require further purification in most cases; (8) The strong inorganic acids, as well as the strong organic acids, also exhibited high catalytic activity for the acetalization; (9) The acetalization/ketalization protocol has been successfully applied to construct heterocyclic compounds and protect the complex organic molecules. The broad reaction scope and remarkable versatility will enable us to tackle a broad spectrum of aldehydes/ketones/diols for the production of acetals/ketals with satisfied results. These new and important findings will have a promising prospective in industrial applications because of its considerably low catalyst loading, cost effectiveness, ease of handling, environmental friendliness, high conversions, and compatibility with acid-sensitive substrates. We anticipate that this protocol would arouse more interest in the chemistry of acetalization/ketalization and their further applications.

Experimental Section

General Methods

Nuclear magnetic resonance (NMR) spectra were recorded with tetramethylsilane as the internal standard. Thin-layer chromatography was performed on glass-backed silica plates. Column chromatography was performed using silica gel (150–200 mesh) eluting with ethyl acetate and petroleum ether. 1H NMR spectra were recorded at 600 MHz, and 13C NMR spectra were recorded at 150 MHz. Chemical shifts (δ) are reported in ppm downfield from CDCl3 (δ = 7.26 ppm) or dimethyl sulfoxide (DMSO) (δ = 2.50 ppm) for 1H NMR and relative to the central CDCl3 resonance (δ = 77.0 ppm) or DMSO resonance (δ = 39.5 ppm) for 13C NMR spectroscopy. Coupling constants (J) are given in Hz. Electrospray ionization high-resolution mass spectra (ESI-HRMS) were measured with an ion trap mass spectrometer.

General Procedure for the Acetalization

A mixture of aldehyde 1 (0.3 mmol) and 0.1 mol % hydrochloric acid in methanol (4 mL) was stirred at an ambient temperature for 30 min. Then 0.15 mol % NaHCO3 was added and stirred for a few minutes. After that, the organic layer was concentrated in vacuo, and column purification on silica gel was performed using hexane–ethyl acetate in 1% triethylamine to obtain the product and its isolated yield.

(E)-(3,3-Dimethoxyprop-1-en-1-yl)benzene (2a)

50 mg, 93% yield, colorless oily liquid. The NMR spectra of 2a were consistent with those previously reported.[5]1H NMR (600 MHz, CDCl3) δ: 7.42–7.39 (m, 2H), 7.32 (m, 2H), 7.28–7.24 (m, 1H), 6.72 (d, J = 16.2 Hz, 1H), 6.16 (dd, J = 16.2, 4.9 Hz, 1H), 4.96 (dd, J = 4.9, 1.2 Hz, 1H), 3.38 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 136.1, 133.6, 128.6, 128.6, 128.2, 126.8, 126.8, 125.7, 103.0, 52.8, 52.8.

(E)-1-Bromo-4-(3,3-dimethoxyprop-1-en-1-yl)benzene (2b)

69 mg, 90% yield, colorless oily liquid. The NMR spectra of 2b were consistent with those previously reported.[5]1H NMR (600 MHz, CDCl3) δ: 7.46–7.43 (m, 2H), 7.28–7.25 (m, 2H), 6.66 (d, J = 16.2 Hz, 1H), 6.14 (dd, J = 16.2, 4.8 Hz, 1H), 4.95 (dd, J = 4.8, 1.2 Hz, 1H), 3.37 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 135.1, 132.4, 132.4, 131.8, 129.8, 128.3, 126.6, 122.0, 102.7, 52.8, 52.8. ESI-HRMS: calcd for C11H13BrO2 + H, 257.0177; found, 257.0182.

(E)-1-(3,3-Dimethoxyprop-1-en-1-yl)-4-methoxybenzene (2c).[12]

52 mg, 83% yield, colorless oily liquid. 1H NMR (600 MHz, CDCl3) δ: 7.35 (m, 2H), 6.86 (m, 2H), 6.66 (d, J = 16.1 Hz, 1H), 6.02 (dd, J = 16.1, 5.0 Hz, 1H), 4.93 (dd, J = 5.0, 1.1 Hz, 1H), 3.80 (s, 3H), 3.37 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 159.6, 133.1, 128.9, 128.0, 128.0, 123.4, 114.0, 114.0, 103.3, 55.3, 52.8, 52.8.

(E)-1-Chloro-2-(3,3-dimethoxyprop-1-en-1-yl)benzene (2d)

60 mg, 94% yield, colorless oily liquid. 1H NMR (600 MHz, CDCl3) δ: 7.54 (m, 1H), 7.37–7.35 (m, 1H), 7.25–7.18 (m, 2H), 7.11 (d, J = 16.1 Hz, 1H), 6.13 (dd, J = 16.1, 5.0 Hz, 1H), 4.98 (d, J = 4.9 Hz, 1H), 3.40 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 134.4, 133.5, 129.9, 129.8, 129.1, 128.5, 127.1, 126.9, 102.9, 52.9, 52.9. ESI-HRMS: calcd for C11H13ClO2 + H, 213.0682; found, 213.0689.

(E)-1-Bromo-2-(3,3-dimethoxyprop-1-en-1-yl)benzene (2e)

72 mg, 94% yield, colorless oily liquid. 1H NMR (600 MHz, CDCl3) δ: 7.54 (m, 2H), 7.29–7.25 (m, 1H), 7.14–7.10 (m, 1H), 7.07 (d, J = 16.0 Hz, 1H), 6.09 (dd, J = 16.1, 5.0 Hz, 1H), 4.97 (dd, J = 5.0, 1.2 Hz, 1H), 3.40 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 136.2, 133.0, 132.5, 129.4, 128.7, 127.6, 127.3, 124.0, 102.8, 52.9, 52.9. ESI-HRMS: calcd for C11H13BrO2 + H, 257.0177; found, 257.0190.

(E)-1,1-Dimethoxyhex-2-ene (2f)

39 mg, 90% yield, colorless oily liquid. The NMR spectra of 2f were consistent with those previously reported.[57]1H NMR (600 MHz, CDCl3) δ: 5.83 (m, 1H), 5.46 (m, 1H), 4.73 (d, J = 5.4 Hz, 1H), 3.32 (s, 6H), 2.09–2.04 (m, 2H), 1.48–1.40 (m, 2H), 0.92 (t, J = 7.4 Hz, 3H). 13C NMR (151 MHz, CDCl3) δ: 135.6, 126.5, 103.5, 52.6, 52.6, 34.2, 22.0, 13.7.

(Dimethoxymethyl)benzene (2g)

42 mg, 93% yield, colorless oily liquid. The NMR spectra of 2g were consistent with those previously reported.[12,34]1H NMR (600 MHz, CDCl3) δ: 7.45 (m, 2H), 7.37 (m, 2H), 7.34–7.30 (m, 1H), 5.39 (s, 1H), 3.33 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 138.1, 128.5, 128.5, 128.2, 126.7, 126.7, 103.2, 52.7, 52.7.

1-(Dimethoxymethyl)-4-methoxybenzene (2h)

54 mg, 99% yield, colorless oily liquid. The NMR spectra of 2h were consistent with those previously reported.[22,34]1H NMR (600 MHz, CDCl3) δ: 7.37 (m, 2H), 6.89 (m, 2H), 5.35 (s, 1H), 3.81 (s, 3H), 3.31 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 159.7, 130.4, 128.0, 113.5, 103.1, 55.3, 52.6, 52.6.

1-(Dimethoxymethyl)-2-methoxybenzene (2i)

51 mg, 97% yield, colorless oily liquid. The NMR spectra of 2i were consistent with those previously reported.[58]1H NMR (600 MHz, CDCl3) δ: 7.52 (m, 1H), 7.30 (m 1H), 6.97 (m, 1H), 6.89 (m, 1H), 5.68 (s, 1H), 3.85 (s, 3H), 3.36 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 157.1, 129.7, 127.3, 126.0, 120.2, 110.6, 99.0, 55.6, 53.5, 53.5.

1-Chloro-4-(dimethoxymethyl)benzene (2j)

55 mg, 99% yield, colorless oily liquid. The NMR spectra of 2j were consistent with those previously reported.[12]1H NMR (600 MHz, CDCl3) δ: 7.40–7.37 (m, 2H), 7.35–7.33 (m, 2H), 5.37 (s, 1H), 3.31 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 136.6, 134.3, 128.4, 128.4, 128.2, 128.2, 102.3, 52.6, 52.6.

1-Chloro-3-(dimethoxymethyl)benzene (2k)

54 mg, 97% yield, colorless oily liquid. The NMR spectra of 2k were consistent with those previously reported.[59]1H NMR (600 MHz, CDCl3) δ: 7.48–7.45 (m, 1H), 7.34–7.31 (m, 1H), 7.31–7.28 (m, 2H), 5.36 (s, 1H), 3.32 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 140.2, 134.3, 129.5, 128.6, 127.0, 124.9, 102.1, 52.6, 52.6.

1-Chloro-2-(dimethoxymethyl)benzene (2l)

55 mg, 98% yield, colorless oily liquid. The NMR spectra of 2l were consistent with those previously reported.[59]1H NMR (600 MHz, CDCl3) δ: 7.62 (m, 1H), 7.38–7.35 (m, 1H), 7.30–7.25 (m, 2H), 5.63 (s, 1H), 3.39 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 135.3, 133.2, 129.8, 129.6, 128.1, 126.6, 101.0, 53.8, 53.8.

2-(Dimethoxymethyl)furan (2m)

41 mg, 97% yield, colorless oily liquid. The NMR spectra of 2m were consistent with those previously reported.[12,34]1H NMR (600 MHz, CDCl3) δ: 7.41 (dd, J = 1.7, 0.8 Hz, 1H), 6.42 (d, J = 3.3 Hz, 1H), 6.37 (dd, J = 3.2, 1.8 Hz, 1H), 5.44 (s, 1H), 3.35 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 150.9, 142.5, 110.1, 108.5, 98.0, 52.8, 52.8.

2-(Dimethoxymethyl)thiophene (2n)

46 mg, 96% yield, colorless oily liquid. The NMR spectra of 2n were consistent with those previously reported.[60]1H NMR (600 MHz, CDCl3) δ: 7.30 (dd, J = 5.0, 1.2 Hz, 1H), 7.08 (dt, J = 3.5, 1.0 Hz, 1H), 7.01 (dd, J = 5.0, 3.5 Hz, 1H), 5.64 (d, J = 0.7 Hz, 1H), 3.37 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 141.5, 126.7, 125.7, 125.5, 100.1, 52.6, 52.6.

1,1-Dimethoxyheptane (2o)

45 mg, 93% yield, colorless oily liquid. The NMR spectra of 2o were consistent with those previously reported.[12]1H NMR (600 MHz, CDCl3) δ: 4.37 (t, J = 5.8 Hz, 1H), 3.32 (s, 6H), 1.63–1.56 (m, 2H), 1.35–1.26 (m, 8H), 0.89 (t, J = 7.0 Hz, 3H). 13C NMR (151 MHz, CDCl3) δ: 104.6, 52.5, 52.5, 32.5, 31.8, 29.1, 24.6, 22.6, 14.0.

1,1-Dimethoxynonane (2p)

54 mg, 95% yield, colorless oily liquid. The NMR spectra of 2p were consistent with those previously reported.[6]1H NMR (600 MHz, CDCl3) δ: 4.36 (t, J = 5.8 Hz, 1H), 3.31 (s, 6H), 1.62–1.56 (m, 2H), 1.34–1.23 (m, 12H), 0.88 (t, J = 7.0 Hz, 3H). 13C NMR (151 MHz, CDCl3) δ: 104.6, 52.6, 52.6, 32.5, 31.9, 29.5, 29.5, 29.2, 24.6, 22.7, 14.1.

(Dimethoxymethyl)cyclohexane (2q)

46 mg, 96% yield, colorless oily liquid. The NMR spectra of 2q were consistent with those previously reported.[6]1H NMR (600 MHz, CDCl3) δ: 3.99 (d, J = 7.2 Hz, 1H), 3.33 (s, 6H), 1.75 (m, 4H), 1.67–1.56 (m, 2H), 1.26–1.17 (m, 2H), 1.17–1.11 (m, 1H), 0.99 (tt, J = 12.6, 6.4 Hz, 2H). 13C NMR (151 MHz, CDCl3) δ: 108.5, 53.5, 53.5, 40.0, 28.0, 28.0, 26.4, 25.8, 25.8.

1,4-Bis(dimethoxymethyl)benzene (2r)

61 mg, 90% yield, colorless oily liquid. The NMR spectra of 2r were consistent with those previously reported.[6]1H NMR (600 MHz, CDCl3) δ: 7.46 (s, 4H), 5.40 (s, 2H), 3.32 (s, 12H). 13C NMR (151 MHz, CDCl3) δ: 138.3, 138.3, 126.6, 126.6, 126.6, 126.6, 102.9, 102.9, 52.7, 52.7, 52.7, 52.7.

2-Phenyl-1,3-dioxolane (2s)

44 mg, 97% yield, colorless oily liquid. The NMR spectra of 2s were consistent with those previously reported.[12]1H NMR (600 MHz, CDCl3) δ: 7.54–7.50 (m, 2H), 7.44–7.39 (m, 3H), 5.85 (s, 1H), 4.17 (dd, J = 8.7, 5.1 Hz, 2H), 4.07 (dd, J = 8.7, 5.2 Hz, 2H). 13C NMR (151 MHz, CDCl3) δ: 137.9, 129.2, 128.4, 128.4, 126.5, 126.5, 103.8, 65.3, 65.3.

2-Octyl-1,3-dioxolane (2t)

53 mg, 95% yield, colorless oily liquid. The NMR spectra of 2t were consistent with those previously reported.[12]1H NMR (600 MHz, CDCl3) δ: 4.83 (t, J = 4.9 Hz, 1H), 3.98–3.93 (m, 2H), 3.86–3.82 (m, 2H), 1.67–1.62 (m, 2H), 1.41 (m, 2H), 1.29 (m, 12H), 0.87 (t, J = 7.1 Hz, 3H). 13C NMR (151 MHz, CDCl3) δ: 104.7, 64.8, 64.8, 33.9, 31.9, 29.6, 29.5, 29.2, 24.1, 22.7, 14.1.

(Diethoxymethyl)benzene (2u)

53 mg, 98% yield, colorless oily liquid. 1H NMR (600 MHz, CDCl3) δ: 7.48 (d, J = 7.8 Hz, 2H), 7.36 (dd, J = 11.3, 4.2 Hz, 2H), 7.33–7.29 (m, 1H), 5.51 (s, 1H), 3.62 (ddt, J = 8.0, 7.1, 4.0 Hz, 2H), 3.57–3.51 (m, 2H), 1.24 (td, J = 7.0, 1.0 Hz, 6H). 13C NMR (151 MHz, CDCl3) δ: 139.1, 128.3, 128.2, 128.2, 126.7, 126.7, 101.6, 61.0, 61.0, 15.2, 15.2.

(Dipropoxymethyl)benzene (2v)

62 mg, 99% yield, colorless oily liquid. 1H NMR (600 MHz, CDCl3) δ: 7.51 (d, J = 7.4 Hz, 2H), 7.39 (t, J = 7.4 Hz, 2H), 7.36–7.32 (m, 1H), 5.55 (s, 1H), 3.54 (dt, J = 9.3, 6.7 Hz, 2H), 3.46 (dt, J = 9.3, 6.7 Hz, 2H), 1.70–1.64 (m, 4H), 0.98 (t, J = 7.4 Hz, 6H). 13C NMR (151 MHz, CDCl3) δ: 139.1, 128.2, 128.1, 128.1, 126.7, 126.7, 101.5, 67.1, 67.1, 23.0, 23.0, 10.8, 10.8.

2-Phenyl-1,3-dioxane (2w)

48 mg, 97% yield, colorless oily liquid. 1H NMR (600 MHz, CDCl3) δ: 7.48 (dd, J = 8.1, 1.2 Hz, 2H), 7.38–7.31 (m, 3H), 5.50 (s, 1H), 4.29–4.24 (m, 2H), 4.02–3.96 (m, 2H), 2.23 (dtt, J = 13.4, 12.5, 5.0 Hz, 1H), 1.44 (dtt, J = 13.5, 2.6, 1.4 Hz, 1H). 13C NMR (151 MHz, CDCl3) δ: 138.7, 128.9, 128.3, 128.3, 126.0, 126.0, 101.7, 67.4, 61.9, 25.8.

General Procedure for the Ketalization

A mixture of ketone 3 (0.3 mmol), 1.2 equiv of TMOF, and 0.1 mol % hydrochloric acid in methanol (4 mL) was stirred at an ambient temperature for 12 h. Then, 0.15 mol % NaHCO3 was added and stirred for a few minutes. After that, the organic layer was concentrated in vacuo, and column purification on silica gel was performed using hexane–ethyl acetate in 1% triethylamine to obtain the product and its isolated yield.

(1,1-Dimethoxyethyl)benzene (4a)

49 mg, 99% yield, colorless oily liquid. The NMR spectra of 4a were consistent with those previously reported.[8]1H NMR (600 MHz, CDCl3) δ: 7.49 (m, 2H), 7.36–7.31 (m, 2H), 7.28–7.24 (m, 1H), 3.18 (s, 6H), 1.53 (s, 3H). 13C NMR (151 MHz, CDCl3) δ: 142.9, 128.1, 128.1, 127.5, 126.3, 126.3, 101.7, 48.9, 48.9, 26.1.

1-(1,1-Dimethoxyethyl)-4-fluorobenzene (4b)

55 mg, 99% yield, colorless oily liquid. The NMR spectra of 4b were consistent with those previously reported.[61]1H NMR (600 MHz, CDCl3) δ: 7.49–7.44 (m, 2H), 7.02 (m, 2H), 3.17 (s, 6H), 1.52 (s, 3H). 13C NMR (151 MHz, CDCl3) δ: 163.06 (s), 161.43 (s), 128.1, 128.0, 114.9, 114.7, 101.4, 48.9, 48.9, 26.1.

1-Bromo-4-(1,1-dimethoxyethyl)benzene (4c)

72 mg, 98% yield, colorless oily liquid. The NMR spectra of 4c were consistent with those previously reported.[61]1H NMR (600 MHz, CDCl3) δ: 7.48–7.45 (m, 2H), 7.39–7.35 (m, 2H), 3.16 (s, 6H), 1.50 (s, 3H). 13C NMR (151 MHz, CDCl3) δ: 142.0, 131.2, 131.2, 128.2, 128.2, 121.7, 101.3, 49.0, 49.0, 26.0.

1-Chloro-4-(1,1-dimethoxyethyl)benzene (4d)

58 mg, 96% yield, colorless oily liquid. The NMR spectra of 4d were consistent with those previously reported.[62]1H NMR (600 MHz, CDCl3) δ: 7.46–7.41 (m, 2H), 7.34–7.29 (m, 2H), 3.17 (s, 6H), 1.51 (s, 3H). 13C NMR (151 MHz, CDCl3) δ: 141.5, 133.4, 128.2, 128.2, 127.8, 127.8, 101.3, 49.0, 49.0, 26.0.

1-(1,1-Dimethoxyethyl)-4-methylbenzene (4e)

49 mg, 91% yield, colorless oily liquid. The NMR spectra of 4e were consistent with those previously reported.[61]1H NMR (600 MHz, CDCl3) δ: 7.40–7.35 (m, 2H), 7.15 (m, 2H), 3.17 (s, 6H), 2.34 (s, 3H), 1.52 (s, 3H). 13C NMR (151 MHz, CDCl3) δ: 140.0, 137.2, 128.8, 128.8, 126.2, 126.2, 101.7, 48.9, 48.9, 26.2, 21.1.

(1,1-Dimethoxypropyl)benzene (4f)

52 mg, 97% yield, colorless oily liquid. The NMR spectra of 4f were consistent with those previously reported.[62]1H NMR (600 MHz, CDCl3) δ: 7.49 (m, 2H), 7.40–7.36 (m, 2H), 7.33–7.29 (m, 1H), 3.19 (s, 6H), 1.95 (q, J = 7.5 Hz, 2H), 0.62 (t, J = 7.5 Hz, 3H). 13C NMR (151 MHz, CDCl3) δ: 140.7, 128.6, 128.0, 127.8, 127.5, 127.1, 104.2, 48.6, 48.6, 29.9, 7.7.

1-(1,1-Dimethoxypropyl)-4-methylbenzene (4g)

54 mg, 92% yield, colorless oily liquid. The NMR spectra of 4g were consistent with those previously reported.[62]1H NMR (600 MHz, CDCl3) δ: 7.33 (m, 2H), 7.15 (m, 2H), 3.15 (d, J = 5.9 Hz, 6H), 2.35 (s, 3H), 1.89 (q, J = 7.5 Hz, 2H), 0.59 (t, J = 7.5 Hz, 3H). 13C NMR (151 MHz, CDCl3) δ: 137.7, 137.1, 128.6, 128.6, 127.0, 127.0, 104.2, 48.5, 48.5, 29.9, 21.1, 7.8.

1-(1,1-Dimethoxypropyl)-4-fluorobenzene (4h)

59 mg, 99% yield, colorless oily liquid. 1H NMR (600 MHz, CDCl3) δ: 7.47–7.42 (m, 2H), 7.04 (m, 2H), 3.16 (s, 6H), 1.91 (q, J = 7.5 Hz, 2H), 0.60 (t, J = 7.5 Hz, 3H). 13C NMR (151 MHz, CDCl3) δ: 163.1, 161.4, 136.5, 128.8, 114.7, 114.6, 103.9, 48.5, 48.5, 29.9, 7.7. ESI-HRMS: calcd for C11H15FO2 + H, 199.1134; found, 199.1141.

1,1-Dimethoxycyclohexane (4i)

40 mg, 93% yield, colorless oily liquid. The NMR spectra of 4i were consistent with those previously reported.[13,28]1H NMR (600 MHz, CDCl3) δ: 3.18 (s, 6H), 1.67–1.60 (m, 4H), 1.50 (dt, J = 11.9, 6.0 Hz, 4H), 1.40 (dd, J = 11.3, 5.9 Hz, 2H). 13C NMR (151 MHz, CDCl3) δ: 100.0, 47.3, 47.3, 32.7, 32.7, 25.6, 22.8, 22.8.

1,1-Dimethoxycyclopentane (4j)

37 mg, 95% yield, colorless oily liquid. The NMR spectra of 4j were consistent with those previously reported.[22,28]1H NMR (600 MHz, CDCl3) δ: 3.21 (s, 6H), 1.79–1.74 (m, 4H), 1.67–1.64 (m, 4H). 13C NMR (151 MHz, CDCl3) δ: 112.2, 49.3, 49.3, 34.2, 34.2, 23.2, 23.2.

1,4-Dioxaspiro[4.4]nonane (4j′)

36 mg, 94% yield, colorless oily liquid. The NMR spectra of 4j′ were consistent with those previously reported.[19]1H NMR (600 MHz, CDCl3) δ: 3.90 (s, 4H), 1.81–1.75 (m, 4H), 1.72–1.66 (m, 4H). 13C NMR (151 MHz, CDCl3) δ: 118.5, 64.2, 64.2, 35.9, 35.9, 23.5, 23.5.

3,3-Dimethoxypentane (4k)

37 mg, 94% yield, colorless oily liquid. The NMR spectra of 4k were consistent with those previously reported.[61]1H NMR (600 MHz, CDCl3) δ: 3.16 (s, 6H), 1.60 (q, J = 7.5 Hz, 4H), 0.82 (t, J = 7.5 Hz, 6H). 13C NMR (151 MHz, CDCl3) δ: 104.1, 47.6, 47.6, 24.2, 24.2, 7.9, 7.9.

2,2-Diethyl-1,3-dioxolane (4k′)

37 mg, 94% yield, colorless oily liquid. The NMR spectra of 4k′ were consistent with those previously reported.[63]1H NMR (600 MHz, CDCl3) δ: 3.94 (s, 4H), 1.63 (q, J = 7.5 Hz, 4H), 0.90 (t, J = 7.5 Hz, 6H). 13C NMR (151 MHz, CDCl3) δ: 112.3, 65.0, 65.0, 29.4, 29.4, 8.0, 8.0.

(E)-2-Methyl-2-styryl-1,3-dioxolane (4l)

54 mg, 95% yield, colorless oily liquid. The NMR spectra of 4l were consistent with those previously reported.[61]1H NMR (600 MHz, CDCl3) δ: 7.39 (m, 2H), 7.31 (m, 2H), 7.26–7.22 (m, 1H), 6.70 (d, J = 16.0 Hz, 1H), 6.15 (d, J = 16.0 Hz, 1H), 4.02–3.91 (m, 4H), 1.56 (s, 3H). 13C NMR (151 MHz, CDCl3) δ: 129.8, 129.7, 129.7, 128.6, 128.6, 127.9, 126.8, 126.8, 107.7, 64.7, 64.7, 25.3.

(E)-2-(4-Chlorostyryl)-2-methyl-1,3-dioxolane (4m)

65 mg, 96% yield, colorless oily liquid. The NMR spectra of 4m were consistent with those previously reported.[64]1H NMR (600 MHz, CDCl3) δ: 7.33–7.30 (m, 2H), 7.29–7.26 (m, 2H), 6.65 (d, J = 16.0 Hz, 1H), 6.13 (d, J = 16.0 Hz, 1H), 4.03–3.98 (m, 2H), 3.97–3.92 (m, 2H), 1.55 (s, 3H). 13C NMR (151 MHz, CDCl3) δ: 134.7, 133.5, 130.5, 128.8, 128.8, 128.5, 128.0, 128.0, 107.5, 64.7, 64.7, 25.2.

General Procedure for the Ketalization of Acetone with Diols

A mixture of diol 5 (0.3 mmol), acetone (4 mL), and 0.1 mol % hydrochloric acid was stirred at an ambient temperature for 24 h. Then, 0.15 mol % NaHCO3 was added and stirred for a few minutes. After that, the organic layer was concentrated in vacuo, and column purification on silica gel was performed using hexane–ethyl acetate in 1% triethylamine to obtain the product and its isolated yield.

2,2,5,5-Tetramethyl-1,3-dioxane (6a)

39 mg, 91% yield, colorless oily liquid. The NMR spectra of 6a were consistent with those previously reported.[13]1H NMR (600 MHz, CDCl3) δ: 3.50 (s, 4H), 1.42 (s, 6H), 0.96 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 97.6, 70.6, 70.6, 30.0, 23.8, 23.8, 22.5, 22.5.

4-Hexyl-2,2-dimethyl-1,3-dioxolane (6b)

52 mg, 93% yield, colorless oily liquid. The NMR spectra of 6b were consistent with those previously reported.[65]1H NMR (600 MHz, CDCl3) δ: 4.04–3.99 (m, 1H), 3.97–3.44 (m, 2H), 1.62–1.40 (m, 2H), 1.36 (m, 3H), 1.30 (s, 3H), 1.28–1.17 (m, 8H), 0.83 (t, J = 7.0 Hz, 3H). 13C NMR (151 MHz, CDCl3) δ: 108.5, 76.1, 69.4, 33.6, 31.7, 29.3, 26.9, 25.7, 25.7, 22.5, 14.0.

(R)-4-(Chloromethyl)-2,2-dimethyl-1,3-dioxolane (6c)

41 mg, 90% yield, colorless oily liquid. The NMR spectra of 6c were consistent with those previously reported.[65]1H NMR (600 MHz, CDCl3) δ: 4.31 (m, 1H), 4.12–3.88 (m, 2H), 3.58–3.47 (m, 2H), 1.46–1.43 (m, 3H), 1.36 (m, 3H). 13C NMR (151 MHz, CDCl3) δ: 110.1, 75.4, 67.5, 44.5, 26.9, 25.3.

Synthesis of Tert-Butyl 4,4-Dimethoxypiperidine-1-carboxylate (8a)

A mixture of aldehyde 7a (0.3 mmol), 1.2 equiv of TMOF, and 0.1 mol % hydrochloric acid in methanol (4 mL) was stirred at an ambient temperature for 30 min. Then 0.15 mol % NaHCO3 was added and stirred for a few minutes. After that, the organic layer was concentrated in vacuo, and column purification on silica gel was performed using hexane–ethyl acetate in 1% triethylamine to obtain the product and its isolated yield.

tert-Butyl 4,4-dimethoxypiperidine-1-carboxylate (8a)

67 mg, 91% yield, colorless oily liquid. The NMR spectra of 8a were consistent with those previously reported.[54]1H NMR (600 MHz, CDCl3) δ: 3.45–3.38 (m, 4H), 3.19 (s, 6H), 1.70 (s, 4H), 1.45 (s, 9H). 13C NMR (151 MHz, CDCl3) δ: 154.6, 98.4, 79.3, 47.4, 47.4, 32.2, 32.2, 32.2, 32.2, 28.3, 28.3, 28.3.

Synthesis of 2-(4-(Dimethoxymethyl)phenoxy)tetrahydro-2H-pyran (8b)

A mixture of aldehyde 7b (0.3 mmol), 1.2 equiv of TMOF, and 0.1 mol % hydrochloric acid in methanol (4 mL) was stirred at an ambient temperature for 30 min. Then, 0.15 mol % NaHCO3 was added and stirred for a few minutes. After that, the organic layer was concentrated in vacuo, and column purification on silica gel was performed using hexane–ethyl acetate in 1% triethylamine to obtain the product and its isolated yield.

2-(4-(Dimethoxymethyl)phenoxy)tetrahydro-2H-pyran (8b)

68 mg, 90% yield, colorless oily liquid. The NMR spectra of 8b were consistent with those previously reported.[55]1H NMR (600 MHz, CDCl3) δ: 7.35 (m, 2H), 7.07–7.01 (m, 2H), 5.42 (t, J = 3.2 Hz, 1H), 5.34 (s, 1H), 3.89–3.56 (m, 2H), 3.30 (s, 6H), 2.04–1.56 (m, 2H), 1.87–1.83 (m, 2H), 1.71–1.62 (m, 2H). 13C NMR (151 MHz, CDCl3) δ: 157.2, 131.2, 127.8, 127.8, 116.0, 116.0, 103.0, 96.3, 62.0, 52.6, 52.6, 30.4, 25.2, 18.8.

A Mixture of Aldehyde 7c

TMOF (0.3 mmol, 1.2 equiv) and 0.1 mol % hydrochloric acid in methanol (4 mL) was stirred at an ambient temperature for 30 min. Then, 0.15 mol % NaHCO3 was added and stirred for a few minutes. After that, the organic layer was concentrated in vacuo, and column purification on silica gel was performed using hexane–ethyl acetate in 1% triethylamine to obtain the product and its isolated yield.

tert-Butyl(4-(dimethoxymethyl)phenoxy)dimethylsilane (8c)

79 mg, 93% yield, colorless oily liquid. The NMR spectra of 8c were consistent with those previously reported.[56] H NMR (600 MHz, CDCl3) δ: 7.33 (m, 2H), 6.86 (m, 2H), 5.37 (s, 1H), 3.34 (s, 6H), 1.01 (s, 9H), 0.22 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 155.8, 130.9, 127.9, 127.9, 119.8, 119.8, 103.2, 52.7, 52.7, 25.7, 25.7, 25.7, 18.2, −4.4, −4.4.

Synthesis of Ethyl 3,3-Dimethoxybutanoate (8d)

A mixture of aldehyde 7d (0.3 mmol), 1.2 equiv of TMOF, and 0.1 mol % hydrochloric acid in methanol (4 mL) was stirred at an ambient temperature for 30 min. Then, 0.15 mol % NaHCO3 was added and stirred for a few minutes. After that, the organic layer was concentrated in vacuo, and column purification on silica gel was performed using hexane–ethyl acetate in 1% triethylamine to obtain the product and its isolated yield.

Ethyl 3,3-Dimethoxybutanoate (8d)

50 mg, 95% yield, colorless oily liquid. The NMR spectra of 8d were consistent with those previously reported.[28]1H NMR (600 MHz, CDCl3) δ: 4.16 (q, J = 7.1 Hz, 2H), 3.23 (s, 6H), 2.66 (s, 2H), 1.47 (s, 3H), 1.27 (t, J = 7.1 Hz, 3H). 13C NMR (151 MHz, CDCl3) δ: 169.6, 99.8, 60.5, 48.3, 48.3, 42.3, 21.8, 14.1.

Synthesis of (1R,4R)-5-(Dimethoxymethyl)bicyclo[2.2.1]hept-2-ene (8e)

A mixture of aldehyde 7e (0.3 mmol) and 0.1 mol % hydrochloric acid in methanol (4 mL) was stirred at ambient temperature for 30 min. Then, 0.15 mol % NaHCO3 was added and stirred for a few minutes. After that, the organic layer was concentrated in vacuo, and column purification on silica gel was performed using hexane–ethyl acetate in 1% triethylamine to obtain the product and its isolated yield.

(1R,4R)-5-(Dimethoxymethyl)bicyclo[2.2.1]hept-2-ene (8e)

48 mg, 96% yield, colorless oily liquid. The NMR spectra of 8e were consistent with those previously reported.[50]1H NMR (600 MHz, CDCl3) δ: 6.17–6.13 (m, 1H), 6.09–5.97 (m, 1H), 4.20–3.76 (m, 1H), 3.36 (d, J = 17.5 Hz, 3H), 3.30 (d, J = 16.4 Hz, 3H), 2.90–2.77 (m, 2H), 2.42–1.83 (m, 1H), 1.71–0.80 (m, 1H), 1.42 (m, 1H), 1.33 (s, 1H), 1.28 (m, 1H). 13C NMR (151 MHz, CDCl3) δ: 137.6, 136.4, 107.9, 52.9, 52.9, 49.4, 43.9, 42.2, 41.3, 28.6.

Synthesis of 8f

A mixture of d-glucose 7f (3 mmol) and 0.5 mol % hydrochloric acid in methanol (40 mL) was stirred at 60 °C for 72 h, after which 1.0 mol % NaHCO3 was added, and the solution was stirred for 20 min. The solution was filtered and then concentrated in vacuo. After that, the organic layer was concentrated in vacuo, and column purification on silica gel was performed using CH2Cl2–MeOH (7:1) in 1% triethylamine to obtain the product as a white solid. The NMR spectra of 8f were consistent with those previously reported.[50]

53 mg, 91% yield, colorless oily liquid. 1H NMR (400 MHz, D2O) δ: 4.69 (d, J = 27.4 Hz, 5H), 3.80 (d, J = 16.9 Hz, 2H), 3.65 (d, J = 7.8 Hz, 2H), 3.57–3.44 (m, 2H), 3.30 (s, 3H). 13C NMR (100 MHz, D2O) δ: 100.7, 72.4, 70.4, 69.7, 66.6, 60.8, 54.5.

Synthesis of 2-(3-Benzyl-2-thioxo-1,2,3,4-tetrahydroquinazolin-4-yl)acetaldehydes (10)

A mixture of o-isothiocyanato-(E)-cinnamaldehyde 9 (0.3 mmol) and benzylamine (0.36 mmol) was stirred in methanol (2.0 mL) at room temperature for 5 min; then, flash chromatography on silica gel (25% ethyl acetate/petroleum ether) gave 10 as a white solid.

2-(3-Benzyl-2-thioxo-1,2,3,4-tetrahydroquinazolin-4-yl)acetaldehyde (10a)

81 mg, 91% yield, white solid, mp 66–69 °C. 1H NMR (600 MHz, CDCl3) δ: 9.59 (s, 1H), 9.31 (s, 1H), 7.36 (m, 2H), 7.30 (m, 3H), 7.24–7.19 (m, 1H), 7.01 (m, 2H), 6.94 (d, J = 8.0 Hz, 1H), 5.92 (d, J = 15.2 Hz, 1H), 5.08 (dd, J = 8.2, 4.0 Hz, 1H), 4.70 (d, J = 15.2 Hz, 1H), 2.93 (dd, J = 17.8, 8.2 Hz, 1H), 2.78 (dd, J = 17.8, 3.9 Hz, 1H). 13C NMR (151 MHz, CDCl3) δ: 198.5, 177.8, 135.8, 134.2, 129.1, 128.9, 128.9, 128.1, 127.9, 127.9, 125.7, 124.1, 121.3, 113.8, 55.2, 53.2, 48.3. ESI-HRMS: calcd for C17H16N2OS + H, 297.1058; found, 297.1054.

2-(3-Benzyl-6-bromo-2-thioxo-1,2,3,4-tetrahydroquinazolin-4-yl)acetaldehyde (10b)

101 mg, 91% yield, white solid, mp 77–80 °C. 1H NMR (600 MHz, CDCl3) δ: 9.61 (s, 1H), 9.27 (s, 1H), 7.37–7.29 (m, 6H), 7.18 (d, J = 2.0 Hz, 1H), 6.82 (d, J = 8.5 Hz, 1H), 5.88 (d, J = 15.1 Hz, 1H), 5.05 (dd, J = 8.1, 4.0 Hz, 1H), 4.67 (d, J = 15.1 Hz, 1H), 2.93 (dd, J = 18.2, 8.2 Hz, 1H), 2.80 (dd, J = 18.2, 4.0 Hz, 1H). 13C NMR (151 MHz, CDCl3) δ: 198.0, 177.7, 135.6, 133.3, 132.1, 129.0, 129.0, 128.6, 128.3, 128.0, 123.2, 116.2, 115.4, 55.2, 52.5, 48.2, 29.7. ESI-HRMS: calcd for C17H15BrN2OS + H, 375.0161; found, 375.0155.

Synthesis of (E)-1-(3,3-Dimethoxyprop-1-en-1-yl)-2-isothiocyanatobenzenes (11)

A mixture of o-isothiocyanato-(E)-cinnamaldehyde 9 (0.3 mmol) and 0.1 mol % hydrochloric acid in methanol (2 mL) was stirred at an ambient temperature for 20 min. Then, 0.15 mol % NaHCO3 was added and stirred for a few minutes. After that, the organic layer was concentrated in vacuo, and column purification on silica gel was performed using hexane–ethyl acetate in 1% triethylamine to obtain the product and its isolated yield.

(E)-1-(3,3-Dimethoxyprop-1-en-1-yl)-2-isothiocyanatobenzene (11a)

67 mg, 95% yield, yellow oily liquid. 1H NMR (600 MHz, CDCl3) δ: 7.55–7.50 (m, 1H), 7.27–7.23 (m, 3H), 7.00–6.93 (m, 1H), 6.21 (dd, J = 16.1, 4.9 Hz, 1H), 4.98 (dd, J = 4.9, 1.2 Hz, 1H), 3.41 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 132.4, 129.4, 129.3, 129.0, 129.0, 128.0, 128.0, 127.4, 126.6, 102.8, 53.1, 53.1. ESI-HRMS: calcd. for C12H13NO2S + H, 236.0740; found, 236.0738.

(E)-4-Bromo-2-(3,3-dimethoxyprop-1-en-1-yl)-1-isothiocyanatobenzene (11b)

91 mg, 97% yield, yellow oily liquid. 1H NMR (600 MHz, CDCl3) δ: 7.64 (d, J = 2.2 Hz, 1H), 7.36 (dd, J = 8.5, 2.2 Hz, 1H), 7.12 (d, J = 8.5 Hz, 1H), 6.89 (d, J = 16.1 Hz, 1H), 6.20 (dd, J = 16.1, 4.7 Hz, 1H), 4.99 (dd, J = 4.7, 1.1 Hz, 1H), 3.40 (s, 6H). 13C NMR (151 MHz, CDCl3) δ: 138.2, 134.1, 131.9, 130.7, 129.5, 128.6, 126.8, 126.8, 121.0, 102.3, 53.0, 53.0. ESI-HRMS: calcd. for C12H12BrNO2S + H, 313.9845; found, 313.9845.

Synthesis of (E)-N-Benzyl-4-(2-methoxyvinyl)-4H-Benzo[d][1,3]thiazin-2-amines (12)

The above compound 11 was dissolved in toluene (1 mL). The mixture was refluxed for 2 h; then, flash chromatography on silica gel (25% ethyl acetate/petroleum ether) gave 12 as a white solid.

(E)-N-Benzyl-4-(2-methoxyvinyl)-4H-benzo[d][1,3]thiazin-2-amine (12a)

80 mg, 86% yield, yellow oily liquid. 1H NMR (600 MHz, CDCl3) δ: 7.37–7.31 (m, 4H), 7.29–7.23 (m, 2H), 7.17 (dd, J = 7.8, 1.0 Hz, 1H), 7.14–7.11 (m, 1H), 7.05 (t, J = 7.4, 1.3 Hz, 1H), 6.37 (d, J = 12.6 Hz, 1H), 4.94 (dd, J = 12.6, 8.7 Hz, 1H), 4.73 (s, 2H), 4.62 (d, J = 8.7 Hz, 1H), 3.51 (s, 3H). 13C NMR (151 MHz, CDCl3) δ: 150.1, 145.2, 138.6, 128.7, 128.7, 128.3, 128.3, 128.0, 128.0, 127.6, 127.6, 125.8, 125.0, 123.6, 101.8, 56.3, 46.5, 43.2. ESI-HRMS: calcd for C18H18N2OS + H, 311.1213; found, 311.1208. E/Z = 98:2.

(E)-N-Benzyl-6-bromo-4-(2-methoxyvinyl)-4H-benzo[d][1,3]thiazin-2-amine (12b)

93 mg, 80% yield, yellow oily liquid. 1H NMR (400 MHz, CDCl3) δ: 7.38–7.31 (m, 5H), 7.30–7.23 (m, 2H), 7.03 (d, J = 8.4 Hz, 1H), 6.40 (d, J = 12.5 Hz, 1H), 4.88 (dd, J = 12.5, 8.9 Hz, 1H), 4.71 (s, 2H), 4.58 (d, J = 8.8 Hz, 1H), 3.54 (s, 3H). 13C NMR (101 MHz, CDCl3) δ: 150.7, 138.3, 131.2, 128.7, 128.7, 128.5, 128.5, 127.9, 127.9, 127.6, 127.6, 126.5, 125.6, 115.9, 100.8, 56.5, 46.5, 42.8. ESI-HRMS: calcd for C18H17N2OS + H, 389.0318; found, 389.0310. E/Z = 99:1.