Condensation of piperonal (I) with nitromethane in the presence of ammonium acetate in AcOH provided nitrostyrene (II). Ketoester (IV) was prepared by carbethoxylation of 4'-propoxyacetophenone (III) with diethyl carbonate. Subsequent conjugate addition of ketoester (IV) to nitrostyrene (II) using DBU provided adduct (V). Hydrogenation of the nitro group of (V) over Raney Nickel, with concomitant ring closure formed the cyclic imine (VI), and further reduction of (VI) with NaBH3CN yielded the corresponding pyrrolidine as a diastereomeric mixture. Chromatographic separation removed the cis,cis isomer, affording a mixture of trans,trans and cis,trans pyrrolidines (VIIa, VIIb). 2,6-Diethylbromoacetanilide (IX) was prepared by acylation of 2,6-diethylaniline (VIII) with bromoacetyl bromide. N-Alkylation of the mixture of pyrrolidines (VIIa, VIIb) with bromoacetanilide (IX) furnished (Xa, Xb).

Then, hydrolysis of the ester group of (Xa, Xb) with NaOH gave rise to the title trans,trans pyrrolidine-3-carboxylic acid. The undesired cis,trans isomeric ester (XI) was not hydrolyzed under these conditions, allowing its removal from the product by means of differential extraction

Condensation of piperonal (I) with nitromethane in the presence of ammonium acetate in AcOH provided nitrostyrene (II). Ketoester (IV) was prepared by carbethoxylation of 4'-propoxyacetophenone (III) with diethyl carbonate. Subsequent conjugate addition of ketoester (IV) to nitrostyrene (II) using DBU provided adduct (V). Hydrogenation of the nitro group of (V) over Raney Nickel, with concomitant ring closure formed the cyclic imine (VI), and further reduction of (VI) with NaBH3CN yielded the corresponding pyrrolidine as a diastereomeric mixture. Chromatographic separation removed the cis,cis isomer, affording a mixture of trans,trans and cis,trans pyrrolidines (VIIa, VIIb). 2,6-Diethylbromoacetanilide (IX) was prepared by acylation of 2,6-diethylaniline (VIII) with bromoacetyl bromide. N-Alkylation of the mixture of pyrrolidines (VIIa, VIIb) with bromoacetanilide (IX) furnished (Xa, Xb).

Then, hydrolysis of the ester group of (Xa, Xb) with NaOH gave rise to the title trans,trans pyrrolidine-3-carboxylic acid. The undesired cis,trans isomeric ester (XI) was not hydrolyzed under these conditions, allowing its removal from the product by means of differential extraction

Condensation of piperonal (I) with nitromethane in the presence of ammonium acetate in AcOH provided nitrostyrene (II). Ketoester (IV) was prepared by carbethoxylation of 4'-propoxyacetophenone (III) with diethyl carbonate. Subsequent conjugate addition of ketoester (IV) to nitrostyrene (II) using DBU provided adduct (V). Hydrogenation of the nitro group of (V) over Raney Nickel, with concomitant ring closure formed the cyclic imine (VI), and further reduction of (VI) with NaBH3CN yielded the corresponding pyrrolidine as a diastereomeric mixture. Chromatographic separation removed the cis,cis isomer, affording a mixture of trans,trans and cis,trans pyrrolidines (VIIa, VIIb). 2,6-Diethylbromoacetanilide (IX) was prepared by acylation of 2,6-diethylaniline (VIII) with bromoacetyl bromide. N-Alkylation of the mixture of pyrrolidines (VIIa, VIIb) with bromoacetanilide (IX) furnished (Xa, Xb).

Then, hydrolysis of the ester group of (Xa, Xb) with NaOH gave rise to the title trans,trans pyrrolidine-3-carboxylic acid. The undesired cis,trans isomeric ester (XI) was not hydrolyzed under these conditions, allowing its removal from the product by means of differential extraction