经典合成反应标准操作— 醛酮的合成
OHOOEt 药明康德新药开发有限公司
OOOEtJones reagentsacetone, 0oC to rt A 1-L, round-bottomed flask equipped with a magnetic stirring bar and pressure-equalizing dropping funnel is charged with ethyl 3-hydroxy-4-pentenoate and 400 mL of acetone. The mixture is cooled in an ice bath and Jones reagent (175 mL) is added dropwise via the dropping funnel (addition time is approximately 30–40 min). When addition of the Jones reagent is complete, the reaction mixture is allowed to warm slowly to room temperature and is stirred overnight (10–20 hr). Methanol (20 mL) is added to quench excess Jones reagent and the reaction mixture is poured into a 2-L separatory funnel containing diethyl ether (800 mL). After thorough mixing, the layers are separated and the aqueous layer is extracted with diethyl ether (three 200-mL portions). The combined organic layers are washed with brine (two 200-mL portions), dried over MgSO4, filtered, and the solvent is removed by simple distillation. Final purification is accomplished by Kugelrohr distillation at 0.60 mm (oven temp 45°C) with a 250-mL receiving bulb cooled to ?78°C using a dry ice/isopropyl alcohol cold bath. The purified product (14.9 g, 52%) can be stored at ?20°C for several months without decomposition.
Notes： Jones reagent is prepared by dissolving chromium oxide (CrO3) (23.5 g) in con. sulfuric acid (21 mL) with cooling and then diluting with distilled water to give a total volume of 175 mL.
Reference: Organic Syntheses, Coll. Vol. 9, p.432; Vol. 71, p.236
2.1.2 Collins氧化（Cr2O3·2Py）
Collins氧化法是利用CrO3-pyridine配合物将伯醇和仲醇依次氧化成醛（和/或酸）和酮的方法。（G. I. Poos, G. E. Arth, R. E. Beyler, L. H. Sarett, J. Am. Chem. Soc. 75, 422 (1953).）
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Collins氧化法是在Sarett氧化法（以吡啶为溶剂）基础上的改进，以二氯甲烷为溶剂氧化伯醇为醛（J. C. Collins, Tetrahedron Letters 1968, 3363; J. C. Collins, W. W. Hess, Org. Syn. 52, 5 (1972); R. W. Ratcliffe, ibid. 55, 84 (1976).）.
2.1.2.1 Dipyridine chromium(VI) oxide的制备方法
A dry, 1-l., three-necked flask fitted with a sealed mechanical stirrer, a thermometer, and a drying tube, is charged with 500 ml. of anhydrous pyridine, which is stirred and cooled to approximately 15 °C with an ice bath. The drying tube is periodically removed and 68 g. (0.68 mole) of anhydrous chromium (VI) oxide is added in portions through the neck of the flask over a 30-minute period. The chromium trioxide should be added at such a rate that the temperature does not exceed 20 °C and in such a manner that the oxide mixes rapidly with the pyridine and does not adhere to the side of the flask. As the chromium trioxide is added, an intensely yellow, flocculent precipitate separates from the pyridine and the viscosity of the mixture increases. When the addition is complete, the mixture is allowed to warm slowly to room temperature with stirring. Within one hour the viscosity of the mixture decreases and the initially yellow product changes to a deep red, macrocrystalline form that settles to the bottom of the flask when stirring is discontinued. The supernatant pyridine is decanted from the complex and the crystals are washed several times by decantation with 250-ml. portions of anhydrous petroleum ether. The product is collected by filtration on a sintered glass funnel and washed with anhydrous petroleum ether, avoiding contact with the atmosphere as much as possible. The complex is dried at 10 mm. until it is free-flowing, leaving 150–160 g. (85–91%) of dipyridine chromium (VI) oxide as red crystals. The product is extremely hygroscopic; contact with moisture converts it rapidly to the yellow dipyridinium dichromate. It is stored at 0° in a brown bottle.
Reference: Organic Syntheses, Coll. Vol. 6, p.644; Vol. 52, p.5
2.1.2.2 Collins氧化标准操作
A sufficient quantity of a 5% solution of dipyridine chromium (VI) oxide in anhydrous dichloromethane is prepared to provide a sixfold molar ratio of complex to alcohol, an excess
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usually required for complete oxidation to the aldehyde. The freshly prepared, pure complex dissolves completely in dichloromethane at 25 °C at 5% concentration, giving a deep red solution, but solutions usually contain small amounts of brown, insoluble material when prepared from crude complex. The alcohol, either pure or as a solution in anhydrous dichloromethane, is added to the red solution in one portion with stirring at room temperature or lower. The oxidation of unhindered primary (and secondary) alcohols proceeds to completion within 5 to 15 minutes at 25 °C with deposition of brownish-black, polymeric, reduced chromium–pyridine products. When deposition of reduced chromium compounds is complete (monitoring the reaction by GC or TLC is helpful), the supernatant liquid is decanted from the (usually tarry) precipitate, which is rinsed thoroughly with dichloromethane.
The combined dichloromethane solutions may be washed with dilute hydrochloric acid, sodium hydrogen carbonate solution, and water, or filtered directly through a filter aid, or passed through a chromatographic column to remove traces of pyridine and chromium salts. The product is obtained by removal of dichloromethane; any pyridine that remains can often be removed under reduced pressure.
2.1.3 PCC（Pyrindium Chlorochromate）氧化
PCC易于合成和保存，操作简单，是将伯醇和仲醇氧化成醛和酮的应用最广的氧化方法。PCC中所用的碱除吡啶外，也可以是其它碱，且随着碱性部分碱性的增强，氧化的选择性也提高。其中，DMAP·HCrO3Cl为适用于烯丙醇类及苄醇类的选择性氧化试剂。
OHHODMAP, HCrO3ClOHO
PCC的氧化以均相反应为主，但有的方法是将催化剂吸附于硅胶、氧化铝等无机载体或离子交换树脂等有机高分子载体上，对醇作非均相催化氧化。后处理简单并可控制反应的选择性。
2.1.3.1 PCC氧化合成方法示例:
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Preparation of 1,2:4,5-Di-O-isopropylidene-D-erythro-2,3-hexodiulo-2,6-pyranose
A 500-mL, round-bottomed flask equipped with a 4.5-cm, egg-shaped Teflon-coated magnetic stir bar is charged with 130 mL of CH2Cl2, the alcohol prepared in Step A (10.4 g, 40.0 mmol), and 15 g of freshly powdered 3 ? molecular sieves. Pyridinium chlorochromate (21.5 g, 100 mmol) is added portionwise over 10 min and the resulting mixture is stirred at room temperature for 15 hr. Ether (200 mL) is added slowly with vigorous stirring and the solution is filtered under vacuum through a pad of 35 g of Celite. The solids remaining in the reaction flask are transferred to the Celite pad by scraping with a spatula and washing with three 50-mL portions of ether. The resulting cloudy brown filtrate is concentrated by rotary evaporation at room temperature to give a brown solid. To this solid is added 25 mL of 1:1 ether:hexane and the solids are scraped with a spatula. The mixture is then poured onto 60 g of Whatman 60 ? (230-400 mesh) silica gel packed in a 4-cm diameter chromatography column and the liquid is adsorbed onto the silica gel by gravity. The material remaining in the flask is further washed with 1:1 ether:hexane and transferred onto the silica gel; this process is repeated until all the material has been loaded onto the silica gel. The ketone is eluted using 500 mL of 1:1 ether:hexane and the eluent is concentrated by rotary evaporation to afford the crude ketone as a white solid. This material is dissolved in 40-45 mL of boiling hexane. Upon cooling the solution to room temperature, the ketone begins to crystallize. The flask is then cooled to ?25 °C for 2 hr. The resulting solids are collected by filtration, washed with three 25-mL portions of cold (?25 °C) hexane, and dried to afford 8.84-9.08 g, (86-88%) of the ketone as a white solid.
Notes： PCC is prepared by addition of pyridine to a solution of chromium trioxide (CrO3) in aqueous HCl and crystallization. Reference:Organic Syntheses, Vol. 80, p.1
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PCC的合成方法
100 g of Chromium trioxide (CrO3) was dissolved in 100 ml of water and 80.6 ml of pyridine was added (keep the temperature under 30 oC), followed by 400 ml of acetone. Reaction mixture was cooled to -20 oC and yellow crystall was collected and washed with acetone and dried to give 127.2 g of PCC. Yield: 68%.
2.1.4 PDC（Pyrindium Dichromate）氧化
PDC的氧化能力较PCC强，其氧化作用一般在中性条件下进行，而PCC则需在酸性中进行。因此，对酸不稳定的化合物用PCC氧化时，必须在醋酸钠存在下进行。PDC的氧化一般在二氯甲烷中进行，如在DMF中进行时，氧化性增强，能将伯醇最终氧化成酸。PDC的氧化操作基本和PCC相同，这里不再举例说明。
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