Efficient syntheses of benzyl and n ‐octyl [1,3‐ 13 C 2 ]acetoacetates, and their application to syntheses of 13 C‐labelled pyrrole and 13 C‐labelled hymecromone

Benzyl [1‐ 13 C]acetate (2a) was prepared via esterification of sodium [1‐ 13 C]acetate (1) with benzyl bromide in the presence of 18‐crown‐6‐ether in 97% yield. n ‐Octyl [1‐ 13 C]acetate (2b) was rapidly obtained by microwave irradiation of 1‐bromooctane and potassium [1‐ 13 C]acetate (obtained by salt exchange of 1) absorbed on Al 2 O 3 in 82% yield. Solvent‐free Claisen condensation of benzyl or n ‐octyl [1‐ 13 C]acetate (2a or 2b) in the presence of potassium tert ‐butoxide efficiently gave benzyl or n ‐octyl [1,3‐ 13 C 2 ]acetoacetate (3a or 3b) in 51 or 68% yield, respectively. Dibenzyl 2,4‐dimethyl[2,4‐ 13 C 2 ]pyrrole‐3,5‐di[ 13 C]carboxylate (4) was synthesized from benzyl [1,3‐ 13 C 2 ]acetoacetate (3a) in 54% yield. [2,4‐ 13 C 2 ]Hymecromone (6) (7‐hydroxy‐4‐methyl[2,4‐ 13 C 2 ]coumarin) was obtained from n ‐octyl [1,3‐ 13 C 2 ]acetoacetate (3b) and 1,3‐benzenediol (5) in 73% yield. Copyright © 2008 John Wiley & Sons, Ltd.