甲基可被许多氧化剂如SeO2、铬酸、次卤酸等氧化为醛，特别是与羰基及芳香环相邻的活性甲基更易氧化。其中SeO2的选择性较好，是常用的氧化方法之一。相比之下，由亚甲基氧化合成酮的方法较多。主要有用SeO2氧化、用空气氧化、用铬酸氧化、用高锰酸盐氧化、用醌氧化等方法。
用SeO2氧化合成醛酮
羰基及与芳香环相邻的活泼甲基、亚甲基很容易被SeO2氧化为相应的醛酮。反应操作简单，选择性、重复性良好。
用SeO2氧化合成醛酮示例1
A-mL,three-necked,round-bottomedflask,equippedformechanicalstirringandoutfittedwitharefluxcondenser,ischargedwith20.0g(0.13mol)of(+)-camphor(1),8.0g(0.07mol)ofseleniumdioxide,and14.0mLofreagentgradeaceticanhydride.Thegreenmixtureisstirredatrefluxfor1hr,cooledtoambienttemperature,andanadditional8.0g(0.07mol)ofseleniumdioxideisadded.Themixtureisagainheatedtoreflux,andtwofurtherbatchesof8.0g(0.07mol)ofseleniumdioxideareaddedat2.5-hrand6-hrintervals.Afterthereactionisheatedatrefluxforanadditional8hr,duringwhichtimeprecipitationofseleniummetalisobserved,itiscooledtoambienttemperatureandtransferredtoa-mLbeakerwiththeaidof50mLofethylacetate.Theblackprecipitateisremovedbyfiltration,andthefiltrateisdilutedwithmLoftoluene.Concentrationofthefiltratebyrotaryevaporationgivescrude2asanorangesolid.ThisisdissolvedinmLofethylacetate,andthesolutionisfilteredbyvacuumfiltrationthroughCelite.Thefiltrateistransferredtoa1-LseparatoryfunnelandiswashedsuccessivelywithmLof10%aqueoussodiumhydroxidesolutionandmLofsaturatedaqueoussodiumchloridesolution.Theorganicsolutionisdriedovermagnesiumsulfate,filtered,andconcentratedbyrotaryevaporationtoafford20.89gofcrude(1R,4S)-(?)-camphorquinone(2)asyellowcrystals,mp-°C
用SeO2氧化合成醛酮示例2
A5-L,three-necked,Mortonflask,equippedwitharefluxcondenser,thermometer,nitrogeninlet,andmechanicalstirrerischargedwith81g(0.59mol)ofpotassiumdihydrogenphosphate,mLofwater,and2Lofp-dioxane.Withvigorousmechanicalstirring,g(2.80mol)ofcycloheptatrieneisadded,followedbyg(2.87mol)ofseleniumdioxide.Themixtureisheatedto90°Candmaintainedwithvigorousstirringfor20hr.Aftertheblacksuspensioniscooledtoroomtemperature,1LofwaterisaddedandthemixtureisfilteredoverCelite.Thesolutionisdividedintotwohalvesandeachisextractedwithdichloromethane(2×1.5L).Eachorganicextractiswashedwithsaturatedsodiumbicarbonatesolution(mL)andthesolventisremovedunderreducedpressure.The