Total synthesis and absolute configuration of minalemine A, a guanidine peptide from the marine tunicate Didemnum rodriguesi

The total synthesis of the 3S,2S and 3R,2S diastereomers (1a and 1b) of minalemine A and the identification of the natural compound as the 3R,2S isomer is described. The key step in the synthesis is the preparation of the two enantiomers of the beta -amino diacid 3-(N-carboxymethyl)-aminodecanoic acid (Ncma), which were obtained by stereoselective alkylation with allyl bromide of two nonanoic acid imides bearing chiral oxazolidinones as chiral auxiliaries. Natural minalemine A shows identical (1)H NMR and very similar (13)C NMR spectra compared to the two synthetic diastereomers. Sufficient differences in their chromatographic behavior to allow conclusive identification were not found. However, the corresponding N-2-naphthoyl amides presented quite distinct circular dichroism spectra (CD), and these confirmed the 3R,2S configuration for the natural minalemines and the R configuration for the constituent beta -amino diacid, Ncma.