A Novel Synthesis of Carboxylic Esters from 2-Methyl-4,6-pyrimidyl Dicarbonates and Grignard Reagents

Abstract

The carboxylic esters are mostly prepared by the reaction of carboxylic acids or their derivatives with alcohols and a large number of methods have been known. However, a few methods have been reported on the derivation of carboxylic esters from the alkoxycarbonylation of organometallics via σ bond formation between α-carbon and carbonyl carbon. Among them, the reaction of alkynyllithiums with methyl chloroformate or 2,2,2-trichloroethyl chloroformate affords the corresponding α,β-alkynoic esters at low temperature, but it requires an excess of chloroformate reagents to avoid competing side reactions. Although the treatment of ethyl chloroformate with organomanganates or organocopper reagents associated with magnesium salt under Pd catalysis produces the corresponding ethyl esters, these organometallics are prepared by an additional step and the scope is limited to primary and alkenyl group, respectively. The direct alkoxycarbonylation of Grignard reagents with dialkyl carbonates or alkyl chloroformates affords the corresponding esters, but the former requires 2 equiv of dialkyl carbonates and the latter is limited to the preparation of α-diazo esters at −78 C. Instead, the alkoxycarbonylation of Grignard reagents proceeds well with 1 equiv of 2-pyridyl carbonates, but the yield of 2,2,2-trichloroethyl ester depends on reaction temperature. Recently, alkyl cyanoformates have been utilized for the alkoxycarbonylation of organometallics. Thus, the alkoxycarbonylation of alkynyllithiums or allenyllithiums with 2 equiv of methyl or ethyl cyanoformate provides methyl akynoates or ethyl allenates, respectively. The reaction of heteroaromatic magnesium halides with an excess of alkyl cyanoformates also affords carboxylic esters in moderate to high yields. Furthermore, the treatment of methyl cyanoformate with magnesium metalated nitriles, derived from αbromonitriles and isopropylmagnesium chloride, at −78 C affords methyl α-cyanoates in moderate to high yields. Thus, the success of carboxylic esters synthesis by the alkoxycarbonylation of organometallics depends largely on the nature/amount of alkoxycarbonylating reagents and metals employed. However, there are no reports of alkoxycarbonylating reagents that can produce 2 equiv of carboxylic esters with organometallics. Inspired by our previous reports on ketones synthesis using pyrimidyl diesters, we investigated the synthesis of carboxylic esters from 2-methyl-4,6pyrimidyl dicarbonates and Grignard reagents. In choosing 2-methyl-4,6-pyrimidyl group as active moiety, we considered i) chelation effect between nitrogen atom and magnesium atom ii) thermal stability and storage of 2-methyl-4,6-pyrimidyl dicarbonates rather than other pyrimidyl dicarbonates. 2-Methyl-4,6-pyrimidyl dicarbonates 3 were newly prepared by the addition of 2 equiv of alkyl chloroformates 2 to a suspended solution of 4,6-dihydroxy-2-methylpyrimidine 1 and 2 equiv of triethylamine in THF at room temperature (Scheme 1). The nucleophilic acyl substitution of 2 with 1 was completed in 2-10 h at room temperature because 1 was slightly soluble in THF. After completion of the reaction, the resulting triethylamine hydrochloride precipitate was filtered off. After usual workup, the residue was subjected to silica gel (Davisil, pH = 7) or recystallized to afford 3 in 67-89% yields. During the synthesis of 2-methyl-4,6-pyrimidyl diisobutyl dicarbonate 3a, a small amount (~10%) of N,Ndiethyl isobutyl carbamate was produced by the nucleophilic acyl substitution of isobutyl chloroformate with triethylamine, which was distilled out under vacuum. However, 2-