N-Desmethyl derivatives of deoxybouvardin and RA-VII: Synthesis and evaluation

Article abstract of DOI:10.1021/ja00133a010

The synthesis of the complete set of seven N-desmethyl derivatives of RA-VII (8) are described. Thus, the synthesis of the four 14-membered cycloisodityrosine derivatives 21-24 and their coupling with the two tetrapeptides 32 and 33 followed by formation of the 18-membered ring with macrocyclization provided the full set of seven desmethyl derivatives 14-20 of RA-VII (8). The solution phase conformational properties of 8 and 14-20 were examined by 1D and 2D ¹H NMR to reveal the role of N-methylation on the key conformational aspects of the natural agents. In contrast to each of the simple cycloisodityrosine derivatives 21-24 which adopt a single, rigid solution conformation possessing a secondary or tertiary trans amide central to the 14-membered ring, the natural agents including 8 adopt a single predominant solution conformation (83-88%) that corresponds closely to the X-ray structure conformation which possesses an inherently disfavored cis C³⁰-N²⁹ tertiary amide central to the 14-membered cycloisodityrosine subunit. Moreover, this cis amide is the predominant conformation (85-95%) observed with N²⁹-desmethyl RA-VII (14) indicating that even a secondary C³⁰-N²⁹ amide adopts this inherently disfavored cis amide stereochemistry. The minor conformation of 8 observed in solution (12-17%) is shown to be derived from a minor cis C⁸-N⁹ tertiary amide which was not observed with its conversion to a secondary amide. Both N⁹-desmethyl RA-VII (15) and N⁹,N²⁹-desmethyl RA-VII (18) adopt exclusively a single solution conformation that corresponds to the major solution conformations of 8 and 14. This conformation contains a characteristic cis C³⁰-N²⁹ amide central to a type VI β-turn and the cycloisodityrosine subunit, a trans C⁸-N⁹ amide central to a typical type II β-turn capped with a tight Ala⁴-NH-O=C-Ala¹ hydrogen bond, and a trans C¹⁴-N¹⁵ N-methyl amide. In sharp contrast, removal of the N¹⁵ methyl group within 16, 17, 19, and 20 results in the adoption of solution conformations possessing the inherently favored trans C³⁰-N²⁹ amide central to the cycloisodityrosine 14-membered subunit. Thus, the N¹⁵-methyl group within 8 is responsible for the agents adoption of the disfavored cis C³⁰-N²⁹ amide central to the cycloisodityrosine subunit. Importantly, preceding studies have defined the cycloisodityrosine subunit of 8 as the pharmacophore and, in a reversal of the initially assigned roles, revealed that it is the tetrapeptide housed in the 18-membered ring that induces and maintains the rigid, normally inaccessible cis C³⁰-N²⁹ amide conformation within the 14-membered cycloisodityrosine subunit. The studies detailed herein reveal that it is the N¹⁵-methyl group that induces this conformational preference for the disfavored cis C³⁰-N²⁹ amide and that its removal results in a major conformational change with adoption of the trans C³⁰-N²⁹ amide and a loss of biological activity. Thus, the N¹⁵-methyl group is essential for maintenance of the conformational and biological properties of 8; the N⁹-methyl group is not essential, and its removal leads to exclusive population of a single biologically active conformation; and the N²⁹-methyl group once thought essential to the adoption of the C³⁰-N²⁹ cis amide is not essential, and its removal does not alter the conformational or biological properties of 8.