Ascidiacyclamide [cyclo(-Ile 1,5 -oxazoline 2,6 -D-Val 3,7 -thiazole 4,8 -) 2 ] is a cytotoxic cyclic peptide from ascidian. Through structural analyses using monosubstituted analogues (Xaa 1 : Ala, 2-aminobutyric acid, Val, cyclohexylglycine, and phenylglycine), we previously demonstrated the conformational equilibrium between its square and folded forms. As the bulkiness of the Xaa 1 residue side chain was reduced, spontaneous folding was promoted, and the cytotoxicity decreased accordingly. In the present study, five disubstituted analogues in which a tert-leucine residue (Tle) was incorporated at the 5-position of the abovementioned monosubstituted analogues were synthesized, after which their structures were analyzed using X-ray diffraction, circular dichroism (CD) spectral measurements, and 1 H NMR-based quantitative analysis. The side chains of the Tle and Ile residues are structural isomers of one another, and the Tle residue bearing the tert-butyl group can be expected to play a role as a building block. In fact, peptides incorporating Tle 5 exhibited much less spontaneous folding than their Ile 5 counterparts in both crystal and solution. Increases in enthalpy and entropy due to the tert-butyl group during the folding process resulted in increased conformational free energy (ΔG ). The powerful plasticity of the tert-butyl group would stabilize the square form relating with cytotoxicity.

Microwave irradiation of 2-alkynyl-5-(phenyl or alkyl)tetrazoles afforded 2-(phenyl or alkyl)-5,6-dihydro-4H-pyrrolo[1,2-b]pyrazoles via intramolecular [3+2] cyclization of nitrile-imine intermediates. In the present method, use of 5-alkyltetrazoles as the starting materials was more advantageous, because it was difficult with conventional photoreactions. From 2-phenylalkynyl-5-methylthio-1H-tetrazoles, the reaction efficiently produced 2-methylthio-3-phenyl-5,6-dihydro-4H-pyrrolo[1,2-b]pyrazoles. The procedure using the methylthio group was applied to the total synthesis of three naturally-occurring withasomnines. The method was extended to the construction of molecules in which bicyclic pyrazoles were fused to six- to eight- membered rings.