Stereoselective Synthesis of CF2-Substituted Phosphothreonine Mimetics and Their Incorporation into Peptides Using Newly Developed Deprotection Procedures,

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Abstract
Stereoselective syntheses of all four stereoisomers of CF2-substituted nonhydrolyzable phosphothreonine derivatives (33, 39, and their enantiomers) and their incorporation into peptides are described herein. Key to the synthesis of these amino acids was construction of secondary phosphate-mimicking difluoromethylphosphonate units along with generation of two stereocenters. The former was achieved using a Cu(I)-mediated cross-coupling reaction of BrZnCF2P(O)(OEt)2 (8) and β-iodo-α,β-unsaturated ester 12, with stereochemistry of both α- and β-stereocenters being established using bornane-10,2-sultam as a chiral auxiliary. Diastereoselective hydrogenation of a chiral α,β-unsaturated acylsultam (for the β-center) (e.g., 16a) and subsequent stereoselective bromination (for the α-center of the threo derivative) or amination (for the α-center of erythro (allo) derivative) were utilized. Transesterification of the bromide to the benzyl ester followed by azide displacement of the halogen, then reduction of the resulting azide, followed by Boc-protection and finally removal of the benzyl group, afforded protected both l- and d-phosphothreonine mimetics (39 and its enantiomer). On the other hand, protected both l- and d-allo-phosphothreonine mimetics (33 and its enantiomer) were synthesized via transesterification of the above-mentioned amination product, followed by hydrogenolytic removal of the benzyl group. Key to utilization of these amino acid analogues in peptide synthesis was removal of ethyl protection from the difluoromethylphosphonate moiety. A two-step deprotection methodology, consisting of a combination of a first-step reagent [0.3 M BSTFA−TBAI in CH2Cl2, BF3·Et2O] followed by a second-step reagent [1 M TMSOTf−thioanisole in TFA, m-cresol, EDT] was developed for use in solid-phase protocols. A 12-residue Cdc (cell division cycle) 2-peptide 41, possessing two nonhydrolyzable phosphoamino acid mimetics (F2Pmab 6 and F2Pmp 4), was subjected to this deprotection procedure and was obtained in 25% yield based on the protected resin. The present synthetic method affords nonhydrolyzable phosphoamino acid mimetics-containing peptides in high yield without accompanying side reactions.