Darzen's reaction on oxoketene dithioacetals: a new versatile synthesis of substituted and annelated furan-2-carboxylates

Abstract

Darzen's condensation of lithiobromoacetate with α-oxoketene dithioacetals affords the title furans 4 in moderate to good yields. A few of the 5-methylthio furan-2-carboxylates were desulphurized to yield 5-unsubstituted derivatives 5 in good yields. The reaction of α-cinnamoylketene dithioacetals 6 with lithiobromoacetate under identical conditions yielded the corresponding 5-methylthio-3-styryl furan-2-carboxylates 7 along with 3-(2-carbethoxy-3-aryl cycloprop-1-yl)-5-methylthio furan-2-carboxylates 8 which are shown to be formed by conjugate addition elimination of lithiobromoacetate on 7. The corresponding α-(5-aryl-2, 4-pentadienoyl)-(9a- b ) and α-(7-aryl-2, 4, 6-heptatrienoyl)-(11a- b ) ketene dithioacetals gave the corresponding 3-(4-aryl-1, 3-butadienyl)-(10a-b ) and 3-(6-aryl-1, 3, 5-hexatrienyl)-(12a-b ) 5-methylthio furan-2-carboxylates exclusively in good yields.