STUDIES OF N-TERMINAL TEMPLATES FOR ALPHA-HELIX FORMATION - SYNTHESIS AND CONFORMATIONAL-ANALYSIS OF (2S,5S,8S,11S)-1-ACETYL-1,4-DIAZA-3-KETO-5-CARBOXY-10-THIATRICYCLO[2.8.1.0(4,8)]TRIDECANE (AC-HEL1-OH)

A convergent synthesis of the title compound, a conformationally restricted analogue of acetyl-L-prolyl-L-proline, from 1-acetyl-2(S)-carboxy-4(S)-mercaptopyrrolidine and trans-1-(tert-butoxycarbonyl)-2(S)-(methoxy-carbonyl)-5(S)-[(tosyloxy)methyl]pyrrolidine (Ac-Hel1-OH) is reported, along with a synthesis of (2S,8S,11S)-1-acetyl-1,4-diaza-3-keto-10-thiatricyclo[2.8.0(4,8)]tridecane. In the crystal and in CDCl3 solution the conformation of Ac-Hel1-OH is shown to approximate a staggered orientation at the 8,9-CC bond and an s-cis orientation at the acetyl amide bond. In DMF, DMSO, MeCN, and D2O significant amounts of the s-trans conformer are also present.