Although esters are normally stable to sodium borohydride, the reduction of ketones and esters by sodium borohydride is dramatically accelerated by U.V. irradiation. However, this does not generally apply to all ketones and esters, and also the photochemical reduction is dependent on solvent polarity. The quantum yield of photocatalyzed reduction of methyl benzoate with sodium borohydride are 0.5 in diglyme, 0.08 in ethanol, 0.01 in isopropyl alcohol, 0.03 in dimethyl formamide, and 0.04 in dimethyl sulfoxide solution. In diglyme solution, the photoreduction quantum yields are 0.5 for methyl benzoate, 0.03 for methyl 4-methoxy benzoate, 0.5 for methyl 2-chlorobenzoate, 0.05 for methyl 4-nitrobenzoate, 0.01 for methyl 2-Furancarboxylate, and 0.001 for 5-methyl-4-imidazole carboxylic acid ethyl ester. The corresponding benzoic acids and their sodium salts in diglyme solution and in aq. solution are inert to sodium borohydride upon U.V. irradiation.
The reduction of aliphatic carboxylic acid esters, particularly phenyl esters, with sodium borohydride is also accelerated by photoexcitation. The quantum yields of photocatalyzed reduction of cyclohexanecarboxylic acid esters with sodium borohydride are 1.0 for phenyl ester, 3 for 2-chlorophenyl ester, 4 for 4-chlorophenyl ester, 0.4 for 2-methylphenyl ester, 0.2 for 3-methylphenyl ester, 0.7 for 4-methylphenyl ester, and 0.05 for 4-methoxyphenyl ester.
The reduction of ketones by sodium borohydride is generally accelerated by U.V. irradiation.
When the (n,$π^*$) state is the lowest excited state in the molecule, the photocatalyzed reduction quantum yields of esters or ketones are enhanced, and when the (π, $π^*$) state is the lowest excited state or the charge transfer character is present in the molecule, the acceleration of reduction with sodium borohydride is negligible.
β-아미노크로톤산 메틸 에스테르(14)를 200℃에서 그리고 β-아미노크로톤아미드(17)을 180℃에서 가열하면 각각 20%와 80%의 수율로 2,6-디메틸-피리미돈(15)가 생성된다.
이러한 열축합의 반응기전을 알기 위해서 17을 120℃에서 가열하니 두가지 중간체로서 1,2-디히드로-2,6-디메틸-4-피리돈-2-일 아세트아미드(23)과 1,5-디메틸-3,7-디옥소-4,8,9-트리아자바이사이클로[3,3,1] 노난(24)의 혼합물이 70%의 수율로 얻어졌다.
23과 24는 각각 DMSO 용매에서 가열하거나 실온에서 방치하면 서로 이성질체화 하며 180℃로 가열하면 정량적으로 15가 생성된다. 이로 미루어 보아 17로부터 15의 생성반응은 23과 24를 반응중간체로함이 틀림없다. 이러한 열축합기전의 원리를 이용하여 o-아미노벤즈아미드(18) 또는 o-페닐렌디아민(19)를 1,3-디카보닐화합물과 반응시켜서 그들의 2-위치에 에스테르 혹은 케톤기를 가진 2-치환 -4-퀴나졸리논 또는 2-치환-벤즈이미다졸을 용이하게 합성하였다.
