環(huán)狀的縮醛(酮)由醛酮在酸催化下與二醇脫水而成，有代表性的結(jié)構(gòu)如：
環(huán)狀縮醛(酮)較非環(huán)縮醛(酮)酸性條件下穩(wěn)定性更好，保護(hù)和脫保護(hù)速度不僅與羰基本身活性有關(guān)，且跟環(huán)的大小和取代基有關(guān)。如：對(duì)相同的底物縮醛(酮)的形成速度：
在酸性條件下(0.003M HCl in dioxane/H2O)水解相對(duì)速度：
環(huán)的大小不一樣，縮醛(酮)水解速度也不一樣，如酸催化縮酮的水解速度：
縮醛則正好相反。
取代的乙二醇或者丙三醇與羰基形成的縮醛(酮)的方法基本類似，根據(jù)取代基不同其酸性穩(wěn)定性也各異，脫保護(hù)條件也可以更溫和，如：
環(huán)狀縮醛(酮)中，由于五元環(huán)縮醛(酮)形成比較方便，穩(wěn)定性好，在多個(gè)羥基存在下，通過采用合適的條件也能具有一定的選擇性，所以五元環(huán)縮醛(酮)，可能是羰基保護(hù)中使用最多的方法。其形成和脫除的典型操作如下：
應(yīng)用舉例1：
4-Cyanobenz aldehyde(15 g, 114.4 mmol) was placed in a round flask together with ethyleneglycol (11.5 mL, 206 mmol, 1.8equiv.), dry toluene (300 mL), and p-toluenesulfonic acid as catalyst (1.5 g, 8.7 mmol). The solution was heated at reflux in a Dean–Stark apparatus with water trap for 18 h. It was then neutralized with saturated aqueous NaHCO3 solution (100 mL), and the phases were separated. The aqueous layer was extracted with toluene (3*50 mL), and the combined organic layers were collected, dried with MgSO4, and filtered. The toluene solution was concentrated under reduced pressure, and the desired compound was obtained as a pale yellow solid (20.04 g, yield 99 %). (N. Boyer,P. Gloanec, Euro. J. Org. Chem., 2008, 4277)
應(yīng)用舉例2
A solution of acetal (1.00 g,3.16 mmol) and p-toluenesulfonic acid monohydrate (59 mg, 0.31 mmol) in acetone(100 mL) was stirred at room temperature for 2 days. The reaction mixture was poured into saturated ammonium sulfate solution. The organic layer was concentrated in vacuum and the residue was dissolved in EtOAc. The aqueous layer was extracted with EtOAc. Combined organic layers were washed with saturated NaHCO3 solution and brine, dried over anhydrous magnesium sulfate and concentrated in vacuum. The residue was chromatographed over silica gel. Elution with hexane–ethyl acetate (1/1) gave desired ketone (0.86 g, quant.) as a colorless oil.(I. Ken; K. Ryo, W. Hidenori, Tetrahedron, 2006, 62, 2224)
應(yīng)用舉例3
A mixture of H2O (15 mL) and TFA (15 mL) was added to a stirred solution of pentacycle (427 mg, 0.78 mmol) in THF (3.5 mL). After 1 h, the reaction mixture was diluted with EtOAc (100 mL), the organic layer was washed with H2O(50 mL), cautiously basified by washing with saturated aqueous solution of Na2CO3(3*100 mL), washed with brine (200 mL), dried, concentrated in vacuum and purified by flash column chromatography (gradient elution: 0–50% EtOAc/petroleum ether) to give ketone (346 mg, 88%) as a colorless foam. (B.Aurelien, H. Shuji, Q-S. Beatrice, R. Louis, Tetrahedron, 2009, 65, 6730)
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