Saponification of ester Boc-25a gave amino acid 44 with three functional groups. Methylation of the carboxylic group in 44, followed by extensive oxidation of the alcohol group provided valuable linker 45. The structure of the product was confirmed by X-ray analysis (Scheme 6). Crystallographic analysis. Next, we compared the geometric properties of 3-azabicyclo[3. 1.1]heptanes with those of pyridines. For that, we measured two C-C distances r and d to see the overall similarity of cores; and angle φ between two exit vectors to see the similarity of angular models (Figure ).