将甲酰基以适当形式保护后，变易产生负离子，与亲电试剂反应后可重新产生甲酰基。如用 1，3-二噻烷合成醛的反应：

常用的甲酰负离子的等价物有：

由合成子合成醛反应示例 1

    A. 2-Tetradecyl-sym-trithiane. A 1-l., round-bottomed, side-armed flask containing a magnetic stirring bar is charged with 25.0 g. (0.180 mole) of finely ground, pure, sym-trithiane. The flask is equipped with a three-way stopcock and a rubber septum on the side arm. The air in the flask is replaced with dry nitrogen. Tetrahydrofuran (350 ml.) is added by syringe, and the resulting slurry is stirred vigorously in a cooling bath at −30°. After the addition of 0.190 mole of n-butyllithium (1.5–2.5 molar in n-hexane), the mixture is stirred for 1.5–2.5 hours, keeping the bath temperature between −25° and −15°. After this period of time the trithiane is dissolved, and dry ice is added (no excess!) to the bath until the temperature is about −70°. To this cooled solution is rapidly added 50.0 g. (49.5 ml., 0.180mole) of 1-bromotetradecane (myristyl bromide) by syringe, and the resulting mixture is stirred overnight, during which time the bath temperature rises to 0–25° and a heavy, colorless  precipitate separates. Stirring is continued for 1 hour at room temperature before the mixture is poured into a 2-l. separatory funnel containing 800 ml. of water and 500 ml. of carbon tetrachloride. After shaking, the layers are separated and the aqueous layer is shaken with two additional 500-ml. portions of carbon tetrachloride. Some undissolved trithiane is filtered from the combined organic layers, which are washed with water and dried over anhydrous potassium carbonate. The solvent is removed by evaporation, yielding 54–59 g. of crude, solid 2-tetradecyl-sym-trithiane, after drying under reduced pressure.

    B. Pentadecanal dimethyl acetal. The crude material obtained from Part A is placed in a 2-l., three-necked flask fitted with an overhead stirrer, a reflux condenser with drying tube, and a stopper. Methanol (1 l., reagent grade) is added, the stirrer is started, and 40 g. (0.18mole) of mercury (II) oxide and 100 g. (0.368 mole) of mercury(II) chloride are introduced.The mixture is heated under reflux for 4.5 hours and filtered through a Büchner funnel after cooling. The residue is washed with 300 ml. of pentane, and the combined organic solutions are poured into 1 l. of water. The layers are separated, and the lower aqueous layer is shaken with two 500-ml. portions of pentane. The combined organic layers are quickly washed with 10% ammonium acetate solution and water and dried over sodium sulfate. The pentane is evaporated under reduced pressure, giving 30.0–32.5 g. of the crude acetal as a mobile, slightly yellow oil.

     C. n-Pentadecanal. The crude acetal from Part B is dissolved in 600 ml. of tetrahydrofuran,and 150 ml. of water containing 2 g. of p-toluenesulfonic acid monohydrate is added. The resulting pale mixture is heated at reflux for 1 hour and cooled. The hydrolysate is poured into 600 ml. of water and extracted with three 300-ml. portions of pentane. The colorless pentane extracts are combined, washed three times with saturated sodium hydrogen carbonate solution and once with water, and dried over sodium sulfate. Evaporation of the solvent furnishes an oil which upon distillation under reduced pressure yields 18.7–22.5 g. of n-pentadecanal, b.p. 103–106° (0.2 mm.). The overall yield from 1-bromotetradecane is 47–55%. The product solidifies eventually and should be kept under an inert atmosphere in the refrigerator.

由合成子合成酮反应示例 2 ：

   A. 5,9-Dithiaspiro[3.5]nonane. A dry, 2-l., one-necked, round-bottomed flask containing a magnetic stirring bar is flushed with dry nitrogen, and 1.25 l. of dry tetrahydrofuran and 50g. (0.42 mole) of 1,3-dithiane are added. The flask is quickly equipped with a three-way stopcock bearing a standard tapered joint, a rubber septum, and a nitrogen inlet. The solution is stirred with an efficient magnetic stirrer and cooled to an external temperature of −20° with a methanol–dry ice bath. A 3% excess (total of 0.43 mole) of 1.5–2.5 M n-butyllithium in n-hexane is added through the rubber septum with a syringe. The bath temperature is kept between −10° and −20° for 2 hours before the temperature of the bath is reduced to −75°and 65.5 g. (44.5 ml., 0.417 mole) of 1-bromo-3-chloropropane is added with a syringe over 10 minutes. The temperature of the bath is raised to −30° over a 2-hour period by gradually replacing the cold methanol with warm methanol. The bath is removed and stirring is continued until the reaction flask is at room temperature. The flask is again cooled to −75°and 0.44 mole of n-butyllithium in n-hexane is added with a syringe over 10 minutes. After the addition is complete, the temperature of the reaction flask is allowed to rise to room temperature overnight. The solvent is removed from the product with a rotary evaporator at 50° and aspirator pressure. Water (300 ml.) and diethyl ether (500 ml.) are added to the product in the flask, the ether layer is separated, and the aqueous layer is washed again with 500 ml. of ether. The organic layers are combined, washed with 200 ml. of water, and dried over 10 g. of anhydrous potassium carbonate. The ether is removed by distillation to yield about 75 g. of crude product. Distillation through a packed column gives 44–57 g. (65–84%) of 5,9-dithiaspiro[3.5]nonane, b.p. 65–75° (1 mm.)

     B. Cyclobutanone. A 2-l., three-necked flask is fitted with an efficient mechanical stirrer and a water-cooled condenser assembled for downward distillation to which is attached a 250-ml. receiver with a side arm. Two cold traps are attached consecutively to the distillation apparatus. The receiver is immersed in an ice-water bath, and the traps are immersed in dry ice and acetone. To the flask is added 45 g. (0.28 mole) of 5,9-dithiaspiro[3.5]nonane, 900 ml. of triethylene glycol, and 150 ml. of water. Stirring is begun, and 163 g. (0.600 mole) of  mercury(II) chloride and 51.5 g. (0.300 mole) of cadmium carbonate are added. A nitrogen-inlet tube reaching to the bottom of the flask is inserted into the third neck of the flask, and nitrogen is introduced at approximately 50 ml. per minute. The reaction flask is heated to 90° in an oil bath, and the temperature is slowly increased to 110° over a 2–3 hour period. Water and cyclobutanone are carried into the receivers. The water in the receiving flask is saturated with sodium chloride, and the resulting solution is transferred to a separatory funnel. The flask is rinsed with 25 ml. of dichloromethane, and this solution is used for an initial washing of the aqueous solution. The aqueous solution is shaken three additional times with 25-ml. portions of 12chloromethane. The dichloromethane solutions are combined and added to the traps to dissolve cyclobutanone. The resulting solution is transferred to a 250-ml. flask, the traps are rinsed with a small amount of dichloromethane, and the rinse is combined with the original solution. The organic solution is dried over 5 g. of anhydrous sodium sulfate, filtered, and the solvent is removed by distillation through a 20-cm., helix-packed, vacuum-insulated column. The product is transferred to a 25-ml. flask, 5 ml. of mesitylene is added, and the product is distilled through a spinning-band column. The fraction boiling at 95–100° is collected, yielding 12–15.8 g. (60–81%) of cyclobutanone.