Unexpectedly convenient and stereoselective synthesis of 4α-acyloxy-2-chloropodophyllotoxins in the presence of BF3·Et2O

Twenty-one 4α-acyloxy-2-chloropodophyllotoxin derivatives (5a–u), whose C-4 spatial configuration was mainly stereocontrolled by the configuration of C-2 chlorine atom, were unexpectedly prepared by the reaction of 2-chloropodophyllotoxin with carboxylic acids in the presence of BF3·Et2O. Compared with ordinary esterifications of carboxylic acids mediated by the condensation agent, for example, N,N’-diisopropylcarbodiimide (DIC), the present method made the procedure for the preparation of 4α-acyloxy-2-chloropodophyllotoxins more convenient, practical and easy. Meanwhile, the insecticidal activity of compounds 5a–u was preliminarily evaluated against the pre-third-instar larvae of Mythimna separata Walker in vivo at the concentration of 1 mg/mL.

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