Synthesis and reactivity of β-stannylated phenylalanines

The free radical hydrostannation of a series of N-benzoyl and N-acetyl dehydrophenylalanine esters 2a-h yields beta-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 (1(H), C-13, Sn-119) nuclear magnetic resonance (NMR) spectroscopy. Attempts were made to assign the preferred conformation of the stannylated phenyl-alanine 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.