112. Novel heterospirocyclic 3-amino-2H-azirines as synthons for heterocyclic α-amino acids)

Article abstract of DOI:10.1002/hlca.19970800515

The heterospirocyclic N-methyl-N-phenyl-2H-azirin-3-amines (3-(N- methyl-N-phenylamino)-2H-azirines) 1a-d with a tetrahydro-2H-pyran, tetrahydro-2H-thiopyran, and a N-protected piperidine ring respectively, were synthesized from the corresponding heterocyclic 4-carboxamides 2 by consecutive treatment with lithium diisopropylamide (LDA), diphenyl phosphorochloridate (DPPCl), and sodium azide (Scheme 4). The reaction of these aminoazirines with thiobenzoic acid in CH₂Cl₂ at room temperature gave the thiocarbamoyl-substituted benzamides 13a-d in high yield. The azirines 1a-d were used as synthons for heterocyclic α-amino acids in the preparation of tripeptides of the type Z-Aib-Xaa-Aib-N(Ph)Me (18) by following the protocol of the 'azirine/ox-azolone method': treatment of Z- Aib with 1 to give the dipeptide amide 15, followed by selective hydrolysis to the corresponding acid 16 and coupling with the 2,2-dimethyl-2H-azirin-3- amine 17 gave 18, again in high yield (Scheme 5). With some selected examples of 18, the selective deprotection of the amino and the carboxy group, respectively, was demonstrated (Scheme 6). The solid-state conformations of the protected tripeptides 18a-d, as well as that of the corresponding carbocyclic analogue 18e, were determined by X-ray crystallography (Figs. 1- 3 and Tables 1-3). All five tripeprides adopt a β-turn conformation of type III or III. The solvent dependence of the chemical shifts of the NH resonances (Fig. 6) suggests that there is an intramolecular H-bond between H-N(4) and O(11) in all cases, which is an indication that a relatively rigid β-turn structure also persists in solution. Surprisingly, the tripeptide acid 20a shows no intramolecular H-bond in the crystalline state (Fig. 7); O(11) is involved in an intermolecular H-bond with the OH group of the carboxy function.