摩熵化学
数据库官网
小程序
打开微信扫一扫
首页 分子通 化学资讯 化学百科 反应查询 关于我们
请输入关键词
历史搜索
    热门化合物HOT
    • 喹啉
    • 水杨醛
    • 二溴甲烷
    • 谷胱甘肽
    • L-乳酸
    • 苯巴比妥
    • 辣椒碱
    • 非那明
    • 百草枯
    • 联苯烯
    首页 分子通 按分子结构分类 有机化合物 有机杂环化合物 内酰胺类

    内酰胺类

    中文名称 英文名称 CAS号 化学式 结构式图片
    —— (6<i>R</i>)-3-methyl-8-oxo-7<i>t</i>-(2-phenyl-acetylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid (Ξ)-1-isobutyryloxy-butyl ester 40260-06-8 C24H30N2O6S
    (6<i>R</i>)-3-methyl-8-oxo-7<i>t</i>-(2-phenyl-acetylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid (Ξ)-1-isobutyryloxy-butyl ester
    —— (6R,7R)-7-[(Z)-2-(2-amino-4-thiazolyl)-2-[[(t-butoxycarbonyl)methoxy]imino]acetamido]-3-[[[2-carbamoyl-5-methyl-s-triazolo [1,5-a]pyrimidin-7-yl]thio]methyl]-8-oxo-5-thia-1-azabicyclo [4.2.0]oct-2-ene-2-carboxylic acid 120490-91-7 C26H28N10O8S3
    (6R,7R)-7-[(Z)-2-(2-amino-4-thiazolyl)-2-[[(t-butoxycarbonyl)methoxy]imino]acetamido]-3-[[[2-carbamoyl-5-methyl-s-triazolo [1,5-a]pyrimidin-7-yl]thio]methyl]-8-oxo-5-thia-1-azabicyclo [4.2.0]oct-2-ene-2-carboxylic acid
    —— benzhydryl 7β-(2-phenylacetamido)-3-(1,2,4-triazol-3-yl)thio-3-cephem-4-carboxylate 179034-80-1 C30H25N5O4S2
    benzhydryl 7β-(2-phenylacetamido)-3-(1,2,4-triazol-3-yl)thio-3-cephem-4-carboxylate
    —— (4R,5R,6R)-4-Methyl-7-oxo-6-(2-phenoxy-acetylamino)-3-phenylsulfanyl-1-aza-bicyclo[3.2.0]hept-2-ene-2-carboxylic acid allyl ester 123201-28-5 C25H24N2O5S
    (4R,5R,6R)-4-Methyl-7-oxo-6-(2-phenoxy-acetylamino)-3-phenylsulfanyl-1-aza-bicyclo[3.2.0]hept-2-ene-2-carboxylic acid allyl ester
    —— diphenylmethyl 7-(2-thienylacetamido)-2-methyl-3-formylceph-2-em-4-carboxylate 80366-50-3 C28H24N2O5S2
    diphenylmethyl 7-(2-thienylacetamido)-2-methyl-3-formylceph-2-em-4-carboxylate
    —— 1-[2-Carboxy-8-oxo-7-(2-phenyl-2-sulfo-acetylamino)-5-thia-1-aza-bicyclo[4.2.0]oct-2-en-3-ylmethyl]-pyridinium 69682-49-1 C21H20N3O7S2
    1-[2-Carboxy-8-oxo-7-(2-phenyl-2-sulfo-acetylamino)-5-thia-1-aza-bicyclo[4.2.0]oct-2-en-3-ylmethyl]-pyridinium
    —— N,N'-Diphenyl-2,5-diaza-3,6-dioxobicyclo<2.2.2>octan 27062-59-5 C18H16N2O2
    N,N'-Diphenyl-2,5-diaza-3,6-dioxobicyclo<2.2.2>octan
    —— (1R,6S)-3,4-Dibromo-1,6-dimethyl-7-aza-bicyclo[4.2.0]octan-8-one 27062-83-5 C9H13Br2NO
    (1R,6S)-3,4-Dibromo-1,6-dimethyl-7-aza-bicyclo[4.2.0]octan-8-one
    —— Bicyclo<4.2.2>decen-(3)-dicarbonsaeure-(3,4)-N-methyl-imid 23070-64-6 C13H17NO2
    Bicyclo<4.2.2>decen-(3)-dicarbonsaeure-(3,4)-N-methyl-imid
    —— 3-[α-(methoxycarbonyl)benzyl]-3-[2-(methoxycarbonyl)ethylthio]-1-(4-methoxyphenyl)-4-phenylazetidin-2-one 1125691-80-6 C29H29NO6S
    3-[α-(methoxycarbonyl)benzyl]-3-[2-(methoxycarbonyl)ethylthio]-1-(4-methoxyphenyl)-4-phenylazetidin-2-one
    —— 2-<3-phenylacetamido-4-(aminocarbonyl-methyl)sulfonyl-(3R,4R)-azetidin-2-on-1-yl>-phthalidyl acetate 122692-60-8 C23H21N3O9S
    2-<3-phenylacetamido-4-(aminocarbonyl-methyl)sulfonyl-(3R,4R)-azetidin-2-on-1-yl>-phthalidyl acetate
    —— 2-<3-phenylacetamido-4-(p-methoxybenzyl)dithiocarbonate-(3R,4R)-azetidin-2-on-1-yl>-phthalidyl acetate 122692-02-8 C30H26N2O8S2
    2-<3-phenylacetamido-4-(p-methoxybenzyl)dithiocarbonate-(3R,4R)-azetidin-2-on-1-yl>-phthalidyl acetate
    —— (6<i>R</i>)-3-(6-methoxy-2-oxy-pyridazin-3-ylsulfanylmethyl)-8-oxo-7<i>t</i>-(2-thiophen-2-yl-acetylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid 36996-09-5 C19H18N4O6S3
    (6<i>R</i>)-3-(6-methoxy-2-oxy-pyridazin-3-ylsulfanylmethyl)-8-oxo-7<i>t</i>-(2-thiophen-2-yl-acetylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid
    —— (6<i>R</i>)-3-methyl-7<i>t</i>-(2-methylsulfanylmethylsulfanyl-acetylamino)-8-oxo-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid 55987-65-0 C12H16N2O4S3
    (6<i>R</i>)-3-methyl-7<i>t</i>-(2-methylsulfanylmethylsulfanyl-acetylamino)-8-oxo-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid
    —— 159492-55-4 C18H19N5O7S2
    —— (6R,7R)-7-[(Z)-2-(2-Amino-4-thiazolyl)-2-(methoxyimino)acetamido]-3-[[[2-carbamoyl-5-methyl-s-triazolo[1,5-a]-pyrimidin-7-yl]thio]methyl]-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid 120490-90-6 C21H20N10O6S3
    (6R,7R)-7-[(Z)-2-(2-Amino-4-thiazolyl)-2-(methoxyimino)acetamido]-3-[[[2-carbamoyl-5-methyl-s-triazolo[1,5-a]-pyrimidin-7-yl]thio]methyl]-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid
    —— 2,3-Dioxo-2,3-dihydro-benzo[1,4]thiazine-4-carboxylic acid [3-chloro-2-(4-dimethylamino-phenyl)-4-oxo-azetidin-1-yl]-amide 129455-31-8 C20H17ClN4O4S
    2,3-Dioxo-2,3-dihydro-benzo[1,4]thiazine-4-carboxylic acid [3-chloro-2-(4-dimethylamino-phenyl)-4-oxo-azetidin-1-yl]-amide
    —— 56204-15-0 BrH*C19H22N6O6S
    —— (6R,7R)-3-Fluorosulfonyloxy-8-oxo-7-phenylacetylamino-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid benzhydryl ester 137167-58-9 C28H23FN2O7S2
    (6R,7R)-3-Fluorosulfonyloxy-8-oxo-7-phenylacetylamino-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid benzhydryl ester
    —— (6R,7S)-3-Chloro-8-oxo-7-(2-phenoxy-acetylamino)-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid 127132-94-9 C16H15ClN2O5
    (6R,7S)-3-Chloro-8-oxo-7-(2-phenoxy-acetylamino)-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid
    —— trans-1-(p-anisyl)-3-phthalimido-4-[(S)2,2-dimethyl-1,3-dioxalan-4-yl]azetidin-2-one 301532-83-2 C23H22N2O6
    trans-1-(p-anisyl)-3-phthalimido-4-[(S)2,2-dimethyl-1,3-dioxalan-4-yl]azetidin-2-one
    —— 3-[Hydroxy-(2-nitro-phenyl)-methyl]-1,4-diphenyl-azetidin-2-one 74017-83-7 C22H18N2O4
    3-[Hydroxy-(2-nitro-phenyl)-methyl]-1,4-diphenyl-azetidin-2-one
    —— (6<i>R</i>)-3-[3-(4-acetoxy-phenyl)-2-methoxy-acryloyloxymethyl]-7<i>t</i>-((<i>R</i>)-5-acetylamino-5-methoxycarbonyl-pentanoylamino)-7<i>c</i>-methoxy-8-oxo-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid methyl ester 38291-51-9 C31H37N3O13S
    (6<i>R</i>)-3-[3-(4-acetoxy-phenyl)-2-methoxy-acryloyloxymethyl]-7<i>t</i>-((<i>R</i>)-5-acetylamino-5-methoxycarbonyl-pentanoylamino)-7<i>c</i>-methoxy-8-oxo-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid methyl ester
    —— (6<i>R</i>)-3-acetoxymethyl-7<i>t</i>-(4-chloro-2-cyclohexylcarbamoyloxyimino-3-oxo-butyrylamino)-8-oxo-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid 70343-79-2 C21H25ClN4O9S
    (6<i>R</i>)-3-acetoxymethyl-7<i>t</i>-(4-chloro-2-cyclohexylcarbamoyloxyimino-3-oxo-butyrylamino)-8-oxo-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid
    —— (6<i>R</i>)-3-acetoxymethyl-7<i>t</i>-(4-chloro-2-isopropylcarbamoyloxyimino-3-oxo-butyrylamino)-8-oxo-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid 70343-94-1 C18H21ClN4O9S
    (6<i>R</i>)-3-acetoxymethyl-7<i>t</i>-(4-chloro-2-isopropylcarbamoyloxyimino-3-oxo-butyrylamino)-8-oxo-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid
    —— 2-<3-phenylacetamido-4-(m-acetamido)phenylthiocarbonate-(3R,4R)-azetidin-2-on-1-yl>acetic acid 122691-96-7 C22H21N3O7S
    2-<3-phenylacetamido-4-(m-acetamido)phenylthiocarbonate-(3R,4R)-azetidin-2-on-1-yl>acetic acid
    —— (6<i>R</i>)-3-methyl-8-oxo-7<i>t</i>-(2-phenyl-acetylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid (Ξ)-1-acetoxy-3-phenyl-propyl ester 40260-11-5 C27H28N2O6S
    (6<i>R</i>)-3-methyl-8-oxo-7<i>t</i>-(2-phenyl-acetylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid (Ξ)-1-acetoxy-3-phenyl-propyl ester
    —— 2-<3-phenylacetamido-4-thiocyanomethyl-(3R,4R)-azetidin-2-on-1-yl>-phthalidyl acetate 122692-31-3 C23H19N3O6S
    2-<3-phenylacetamido-4-thiocyanomethyl-(3R,4R)-azetidin-2-on-1-yl>-phthalidyl acetate
    —— Sodium; 3-acetoxymethyl-7-(2-carbamoyloxy-2-phenyl-acetylamino)-8-oxo-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylate 41393-69-5 C19H18N3O8S*Na
    Sodium; 3-acetoxymethyl-7-(2-carbamoyloxy-2-phenyl-acetylamino)-8-oxo-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylate
    —— 4-Allyl-5-methyl-2-(1-phenyl-butyl)-2,4-dihydro-pyrazol-3-one 55294-52-5 C17H22N2O
    4-Allyl-5-methyl-2-(1-phenyl-butyl)-2,4-dihydro-pyrazol-3-one
    —— 6-<N'-cyclohexyl-ureido>penicillansaeure N-aethylpiperidin 26774-65-2 C7H15N*C15H23N3O4S
    6-<N'-cyclohexyl-ureido>penicillansaeure N-aethylpiperidin
    —— 1193318-44-3 C18H26N4O7S2
    —— (2S,5S,6R,7S)-7-Benzyloxy-5-methyl-8-oxo-2-phenyl-1,4-diaza-bicyclo[4.2.0]octane-4-carboxylic acid tert-butyl ester 192320-07-3 C25H30N2O4
    (2S,5S,6R,7S)-7-Benzyloxy-5-methyl-8-oxo-2-phenyl-1,4-diaza-bicyclo[4.2.0]octane-4-carboxylic acid tert-butyl ester
    —— 3-[(2R,3S)-1-[(4-methoxyphenyl)methyl]-3-(4-methylphenyl)sulfonyloxy-5-oxopyrrolidin-2-yl]propyl 4-methylbenzenesulfonate 254730-11-5 C29H33NO8S2
    3-[(2R,3S)-1-[(4-methoxyphenyl)methyl]-3-(4-methylphenyl)sulfonyloxy-5-oxopyrrolidin-2-yl]propyl 4-methylbenzenesulfonate
    —— (6R,7R)-3-(4-Methoxy-phenyl)-8-oxo-7-phenylacetylamino-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid benzhydryl ester 122553-62-2 C35H30N2O5S
    (6R,7R)-3-(4-Methoxy-phenyl)-8-oxo-7-phenylacetylamino-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid benzhydryl ester
    —— (6<i>R</i>)-8-oxo-3-pyrazin-2-ylsulfanylmethyl-7<i>t</i>-(2-tetrazol-1-yl-acetylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid 65640-15-5 C15H14N8O4S2
    (6<i>R</i>)-8-oxo-3-pyrazin-2-ylsulfanylmethyl-7<i>t</i>-(2-tetrazol-1-yl-acetylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid
    —— (6<i>R</i>)-3-acetoxymethyl-8-oxo-7<i>t</i>-(3-thiophen-2-ylsulfanyl-but-2-enoylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid 14834-54-9 C18H18N2O6S3
    (6<i>R</i>)-3-acetoxymethyl-8-oxo-7<i>t</i>-(3-thiophen-2-ylsulfanyl-but-2-enoylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid
    —— (6<i>R</i>)-7<i>t</i>-(4-(Ξ<sub>a</sub>)-carboxymethylene-[1,3]dithietane-2-carbonylamino)-7<i>c</i>-methoxy-3-(1-methyl-1<i>H</i>-tetrazol-5-ylsulfanylmethyl)-8-oxo-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid 69712-43-2 C16H16N6O7S4
    (6<i>R</i>)-7<i>t</i>-(4-(Ξ<sub>a</sub>)-carboxymethylene-[1,3]dithietane-2-carbonylamino)-7<i>c</i>-methoxy-3-(1-methyl-1<i>H</i>-tetrazol-5-ylsulfanylmethyl)-8-oxo-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid
    —— (6<i>R</i>)-7<i>t</i>-[4-((Ξ<sub>a</sub>)-benzhydryloxycarbonyl-carbamoyl-methylene)-[1,3]dithietane-2-carbonylamino]-7<i>c</i>-methoxy-3-(1-methyl-1<i>H</i>-tetrazol-5-ylsulfanylmethyl)-8-oxo-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid benzhydryl ester 69713-33-3 C43H37N7O8S4
    (6<i>R</i>)-7<i>t</i>-[4-((Ξ<sub>a</sub>)-benzhydryloxycarbonyl-carbamoyl-methylene)-[1,3]dithietane-2-carbonylamino]-7<i>c</i>-methoxy-3-(1-methyl-1<i>H</i>-tetrazol-5-ylsulfanylmethyl)-8-oxo-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid benzhydryl ester
    —— (6R,7R)-7-{2-(2-Amino-thiazol-4-yl)-2-[(Z)-ethoxyimino]-acetylamino}-3-azidomethyl-8-oxo-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid 141613-27-6 C15H16N8O5S2
    (6R,7R)-7-{2-(2-Amino-thiazol-4-yl)-2-[(Z)-ethoxyimino]-acetylamino}-3-azidomethyl-8-oxo-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid
    —— 4-Phenyl-N-brom-azetidinon-(2) 34734-04-8 C9H8BrNO
    4-Phenyl-N-brom-azetidinon-(2)
    —— 9-Aza-bicyclo<8.4.0>tetradecan-dion-(3,8) 22766-42-3 C13H21NO2
    9-Aza-bicyclo<8.4.0>tetradecan-dion-(3,8)
    —— (2S,5R)-3,3-Dimethyl-6-{[4-(4-nitro-benzyloxycarbonylamino)-bicyclo[4.2.0]octa-1,3,5-triene-7-carbonyl]-amino}-7-oxo-4-thia-1-aza-bicyclo[3.2.0]heptane-2-carboxylic acid 37671-98-0 C25H24N4O8S
    (2S,5R)-3,3-Dimethyl-6-{[4-(4-nitro-benzyloxycarbonylamino)-bicyclo[4.2.0]octa-1,3,5-triene-7-carbonyl]-amino}-7-oxo-4-thia-1-aza-bicyclo[3.2.0]heptane-2-carboxylic acid
    —— (2-Methyl-4-oxo-azetidin-2-yl)-thioacetic acid S-(2-acetylamino-ethyl) ester 70997-81-8 C10H16N2O3S
    (2-Methyl-4-oxo-azetidin-2-yl)-thioacetic acid S-(2-acetylamino-ethyl) ester
    —— (6<i>R</i>)-3-acetoxymethyl-7<i>t</i>-[2-(2-amino-thiazol-4-yl)-2-(<i>Z</i>)-benzoyloxyimino-acetylamino]-8-oxo-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid 72697-16-6 C22H19N5O8S2
    (6<i>R</i>)-3-acetoxymethyl-7<i>t</i>-[2-(2-amino-thiazol-4-yl)-2-(<i>Z</i>)-benzoyloxyimino-acetylamino]-8-oxo-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid
    —— 2-<3-phenylacetamido-4-(N-diethyl)thiocarbamate-(3R,4R)-azetidin-2-on-1-yl>acetic acid 122692-08-4 C18H23N3O5S
    2-<3-phenylacetamido-4-(N-diethyl)thiocarbamate-(3R,4R)-azetidin-2-on-1-yl>acetic acid
    —— 3-benzoylamino-1,4-diphenylazetidin-2-one 34092-17-6 C22H18N2O2
    3-benzoylamino-1,4-diphenylazetidin-2-one
    —— (6<i>R</i>)-3-[3-(5-methyl-[1,3,4]thiadiazol-2-ylsulfanyl)-<i>trans</i>-propenyl]-8-oxo-7<i>t</i>-(2-thiophen-2-yl-acetylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid 69958-77-6 C19H18N4O4S4
    (6<i>R</i>)-3-[3-(5-methyl-[1,3,4]thiadiazol-2-ylsulfanyl)-<i>trans</i>-propenyl]-8-oxo-7<i>t</i>-(2-thiophen-2-yl-acetylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid
    —— (6<i>R</i>)-3-methyl-8-oxo-7<i>t</i>-(2-phenyl-acetylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid (Ξ)-phenyl-propionyloxy-methyl ester 40260-16-0 C26H26N2O6S
    (6<i>R</i>)-3-methyl-8-oxo-7<i>t</i>-(2-phenyl-acetylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid (Ξ)-phenyl-propionyloxy-methyl ester
    —— (6<i>R</i>)-3-methyl-8-oxo-7<i>t</i>-(2-phenyl-acetylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid (Ξ)-1-(2,2-dimethyl-propionyloxy)-butyl ester 40260-05-7 C25H32N2O6S
    (6<i>R</i>)-3-methyl-8-oxo-7<i>t</i>-(2-phenyl-acetylamino)-(6<i>r</i><i>H</i>)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid (Ξ)-1-(2,2-dimethyl-propionyloxy)-butyl ester