Eine neuartige 1,2-Methyl-Wanderung bei der Desaminierung Decarboxylierung von 3- Amino-Cyclopentancarbonsäuren

During deamination, γ-amino acids can undergo concomitant decarboxylation. Aiming at studying the course of the involved reaction, the following γ-amino acids were synthesized and characterized: (–)-3c-amino-1,2,2-trimethylcyclopentane-1r-carboxylic acid (= (–)-aminolauronic acid = (–)-3-aminocamphonanic acid; 39), cis-3-aminocyclopentanecarboxylic acid (41), (+)-3c-amino-2,2,3-trimethylcyclopentane-1r-carboxylic acid (= (–)-aminodihydrocampholytic acid; 42), and the novel (±)-cis-3-amino-2,2-dimethylcyclopentanecarboxylic acid (43). These γ-amino acids were deaminated by means of 4-diazoniobenzenesulfonate (= 'p-diazobenzenesulfonic acid'; generated from p-sulfanilic acid = 4-aminobenzenesulfonic acid), under moderately basic aqueous conditions (pH 8–9). The formed product mixtures were separated, the products identified, and their structures correlated with the starting γ-amino acids, thus establishing that decarboxylation occurs whenever the carbenium ion formed by loss of nitrogen can rearrange into a higher-substituted ion. This rearrangement accompanied by decarboxylation is the main reaction in the deamination of (–)-3c-amino-1,2,2-trimethylcyclopentane-1r-carboxylic acid (39). The known absolute configurations (1R,3S) of 39 and (3S) of the main product (–)-1,2,3-trimethylcyclopent-1-ene (= (–)-laurolene; 40) allow to elucidate the stereochemistry of the rearrangement. It consists of a new, selective 1,2-syn-alkyl shift, which is explained by an intermediate highly reactive unsolvated but conformationally defined carbenium ion. A further consequence of the high reactivity of the carbenium ion is the lack of participation of the carboxylate group in the formation of the substitution products. The fact that essentially inversion of configuration takes place at the reaction center (C(3)) suggests the involvement of steric hindrance by the counter ion.