Synthesis of methylated phenylalanines via hydrogenolysis of corresponding 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acids. Synthesis and biological activity of oxytocin analogs with methylated phenylalanines in position 2.

A new method of synthesizing ortho-methylated phenylalanines has been developed. Phenylalanines with at least one free ortho-position undergo a Pictet-Spengler cyclization with formaldehyde followed by hydrogenolytic splitting of the endocyclic benzylic C--N bond of 1,2,3,4-tetrahydroisoquinolines and afford corresponding ortho-methyl derivatives. Repeating this reaction sequence on the ortho-substituted phenylalanines yielded ortho,ortho-disubstituted derivatives, and para-substituted phenylalanines yielded ortho,para-disubstituted analogs. Our modified method of cyclization preserved the configuration at the chiral center: hydrogenolysis, on the other hand, led to racemization. Incorporation of the methylated phenylalanines into position 2 of oxytocin led to, in the case of the D-isomers, potent uterotonic inhibitors.