Dicyclohexylcarbodiimide → ethyl-N-[(benzyloxy)carbonyl]-L-isoleucinate

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A mixture of acyclovir (1.0 g, 4.4. mmol; Burroughs Wellcome Co.), 4-dimethylaminopyridine (74 mg, 0.6 mmol; Aldrich Chemical Co. and Chem. Ber. 89 2921-33 [1956]), 1,3-dicyclohexylcarbodiimidide (1.6 g, 8.0 mmol; Aldrich Chemical Co. and U.S. Pat. No. 2,656,383), N-carbobenzoxy-L-isoleucine (1.8 g, 6.6 mmol; Sigma Chemical Co. and Bull. Chem. Soc. Jap (1966) 39 947 or Tetrahedron 40 (24) 5207-11 [1984]), and molecular sieves (0.3 g, Davison type 3A; Fisher Scientific, Co.) in dry N,N-dimethylformamide (80 ml) was stirred at room temperature under nitrogen. After 4 days, additional 1,3-dicyclohexylcarbodiimide (1.6 g, 8.0 mmol) and N-carbobenzoxy-L-isoleucine (1.8 g, 6.6 mmol) were added. Stirring at room temperature was continued for 7 days. The resulting mixture was filtered and the clear filtrate was concentrated in vacuo to a semi-solid residue. Elution of the residue from silica gel 60 (EM, 230-400 mesh; 8.5×14 cm) with 2.5-5% methanol/methylene chloride gave 2-[2-Amino-1,6-dihydro-6-oxo-9H-purin-9-yl)methoxy]ethyl-N-[(benzyloxy)carbonyl]-L-isoleucinate 0.8 g, 45%) as a white solid; mp 155°-157° C.

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