Synthesis and reactivity of β-stannylated phenylalanines

Abstract

Abstract The free radical hydrostannation of a series of N-benzoyl and N-acetyl dehydrophenylalanine esters 2a – h yields β-stannylated phenylalanine derivatives 3 and 4. This addition of tin hydride to such unsaturated compounds simultaneously creates two new chiral centers leading to mixtures of two diastereomeric pairs of enantiomers. The reaction of 3-stannylated phenylalanine 3 with methanolic HCl yields chlorostannyl-substituted compounds 5 and 6 and, with one equivalent of bromine, the bromostannylated compounds 7 and 8 are formed. The bromostannylated phenylalanine derivative 7 reacts with one further equivalent of bromine to produce the dibromostannylated compound 9. Even the chlorostannylated phenylalanine derivative 5 reacts with one further equivalent of HCl to give the dichlorostannylated compound 10. The products were characterized by elemental analysis, infrared (IR), and multinuclear (1H, 13C, 119Sn) nuclear magnetic resonance (NMR) spectroscopy. Attempts were made to assign the preferred conformation of the stannylated phenylalanine derivatives using Karplus-type relationship of coupling constants 3 J(H,H), 3 J(Sn,H), and 3 J(Sn,C=O). The results of these analyses have been confirmed by three crystal structure determinations.