Abstract
Regioselective reactions can play pivotal roles in synthetic organic chemistry. The reduction of several 1-substituted 1,2,3-triazole 4,5-diesters by sodium borohydride has been found to be regioselective, with the C(5) ester groups being more reactive towards reduction than the C(4) ester groups. The amount of sodium borohydride and reaction time required for reduction varied greatly depending on the N(1)-substituent. The presence of a β-hydroxyl group on the N(1)-substituent was seen to have a rate enhancing effect on the reduction of the C(5) ester group. The regioselective reduction was attributed to the lower electron densities of the C(5) and the C(5) ester carbonyl carbon of the 1,2,3-triazole, which were further lowered in cases involving intramolecular hydrogen bonding.
Highlights
Sodium borohydride (NaBH4) is a mild reducing agent used widely for converting aldehydes and ketones to alcohols [1]
The 1,4-disubstituted triazoles made from terminal alkynes are available by the regioselective method known as CuAAC (copper(I) azide-alkyne cycloaddition) [6,7], whereas regioselective synthesis of 1,5-disubstituted triazoles employs other catalysts [8,9,10]
The regioselective NaBH4 reduction of the C(5) 1H-1,2,3-triazole esters was found to be a dependable reaction that occurred with all 1H-1,2,3-triazole 4,5-diesters examined in this study
Summary
Sodium borohydride (NaBH4) is a mild reducing agent used widely for converting aldehydes and ketones to alcohols [1]. It has been reported that some esters can be reduced by NaBH4 in alcohols, higher proportions of the reagent and longer reaction times are required than for reduction of aldehydes and ketones [2,3] Based on these reports, we developed an interest in the opportunity to prepare and selectively reduce heterocyclic diesters, which became our goal. The 1,4-disubstituted triazoles made from terminal alkynes are available by the regioselective method known as CuAAC (copper(I) azide-alkyne cycloaddition) [6,7], whereas regioselective synthesis of 1,5-disubstituted triazoles employs other catalysts [8,9,10] Reduction of both ester groups of oxazole 2,4-diesters has been reported using NaBH4 in ethanol at room temperature [11].
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