用金属氢化物还原
一般不用金属氢化物直接部分还原羧酸得醛，而是先将羧酸还原至醇后再氧化比较可靠。但化学家也开发出了一些比较有用的直接还原方法，如用有机硼氢化物 2，3-二甲基-2-丁基甲硼烷、1，1，2-三甲基丙基卤代甲硼烷、9-BBN 及叔丁基锂等还原羧酸为醛。这些方法用于脂肪羧酸还原时非常有效。对于含有氨基、硝基及卤素等取代基的芳族羧酸 ，也可得到很高的收率。 虽然有报道用氢化二异丁基铝（DIBAL）在低温下还原羧酸可得到醛，但并不适用于合成。但氢化氨基铝是一种很好的羧酸还原试剂，可高收率地得到醛。氢化氨基铝可由 LAH 与 N-甲基哌嗪制取。 用金属氢化物还原反应示例
A. N,N-Dimethylchloromethylenammonium chloride. A 500-mL, three-necked, round-bottomed flask is equipped with a magnetic stirring bar, a thermometer, and a three-way stopcock fitted with a drying tube containing anhydrous calcium chloride and a rubber septum. The flask is charged with 50 mL of dichloromethane and 3.07 (0.042 mol) of N,N-dimethylformamide added through the septum from a syringe, and cooled in an ice bath. To the cooled mixture is slowly added 5.23 mL (0.06 mol) of oxalyl chloride by means of a syringe. The addition is accompanied by gas evolution and formation of a white precipitate. The reaction mixture is stirred for an additional hour at 0°C. Excess oxalyl chloride and solvent are removed under reduced pressure by first using a water aspirator and then a rotary pump at room temperature through the drying tube. The white solid remaining in the flask is N,N-dimethylchloromethylenammonium chloride, which is used directly in Part B.
B. 6-Oxodecanal. The drying tube is removed and the flask is flushed with nitrogen. A nitrogen atmosphere is maintained throughout the subsequent reaction. A dropping funnel is attached and charged with 7.45 g (0.04 mol) of 6-oxodecanoic acid, 3.32 g of pyridine, and 80mL of tetrahydrofuran, which are mixed well by shaking. The flask is charged with 45 mL of acetonitrile and 80 mL of tetrahydrofuran and cooled (methanol–liquid nitrogen) to −30°C. The contents of the funnel are added to the flask at −30°C over 30 min. The reaction mixture is stirred at −30°C for an additional hour and at −20°C for 30 min. After the mixture is cooled to −90°C, 34 mL (0.046 mol) of a 1.35 M solution of lithium tri(tert-butoxy)aluminum hydride in tetrahydrofuran is injected through the septum by means of a syringe over 30 min, while the internal temperature is kept below −85°C. Stirring is continued for an additional 30 min at −90°C. To the flask is added 50 mL of 2 M hydrochloric acid solution, and the cooling
bath is immediately removed. The organic layer is separated and the aqueous layer is extracted with three 50-mL portions of ether. The combined organic extracts are washed with two 50-mL portions of saturated sodium hydrogen carbonate solution and 50 mL of brine, dried over anhydrous sodium sulfate, and filtered. The solvent is removed with a rotary evaporator and the residual liquid is distilled under reduced pressure to yield 5.78–6.35 g (85–93%) of 6-oxodecanal as a fragrant liquid, bp 85–90°C (1.4 mm).
由脱 CO2合成醛 α-酮酸及α，β-环氧酸（缩水甘油酸）加热时易脱羧成醛。如为α-酮酸，则加入苯胺或吗啉，脱羧后以亚胺或烯胺形式存在，难发生副反应。反应停止后用酸处理即可高收率的得到醛。 由羧酸合成酮 A 2-L, three-necked, round-bottomed flask equipped with a magnetic stirring bar, inert gasinlet, and two stoppers is flame dried and cooled to room temperature under an atmosphere of argon. The flask is charged with 42.5 g (0.20 mol) of 3-Oxo-butyric acid methyl ester in 480 mL of anhydrous toluene. To the resulting suspension is added 15.3 mL (0.21 mol) of thionyl chloride followed by 2 mL of anhydrous N,N- imethylformamide (DMF). As soon as the DMF is added the color of the mixture turns light red. The mixture is stirred at room temperature overnight. A 1-L, three-necked, round-bottomed flask equipped with a magnetic stirring bar, 250-mL pressure-equalizing funnel, and an inert gas inlet is flame dried and allowed to cool to room temperature under argon. The flask is charged with 10.1 g (0.49 mol) of 97% sodium hydride in 280 mL of dry tetrahydrofuran (THF) and then cooled to 0°C in an ice bath. A solution of 45.3 mL (0.42 mol) of methyl acetoacetate in 160 mL of dry THF is placed in the addition funnel and added dropwise to the reaction flask over a period of 45–50 min. After the addition is complete the light gray-colored solution is quickly transferred to a dried, 500-mL addition funnel. The funnel is mounted on the flask containing the acid chloride solution. The solution of methyl acetoacetate anion is added to the crude acid chloride solution over 5 min and the resulting mixture is stirred overnight at room temperature. Water (350 mL) is added to the reaction mixture and the mixture is stirred for 15 min before being poured into a 2-L separatory funnel. The layers are separated and the aqueous layer is extracted with ether (2 × 150 mL). The combined organic layers are washed with brine and dried over sodium sulfate. The filtered organic layer is concentrated under reduced pressure and the residue is dissolved in 100 mL of methanol. After cooling in the freezer overnight, the yellow crystals that separate are collected and dried to give 34.8–37.3 g (56–60%) of methyl (5-methoxy-2-nitrophenylacetyl)acetoacetate.
