Tautomeric Equilibria in 3‐Amino‐1‐(2‐aminoimidazol‐4‐yl)prop‐1‐ene, a Central Building Block of Marine Alkaloids

Abstract

Keywords: 2-Aminoimidazole / Tautomeric equilibria / ab initio calculationsThe tautomers I-IV of the marine metabolite 3-amino-1-(2-aminoimidazol-4-yl)prop-1-ene (1) were previously suggested to have rather similar stabilities.[1,2] Through a series of DFT and ab initio calculations, their relative stabilities were investigated in both the gas phase and in water, and also compared to their Z-isomers V-VIII. The tautomers I and III have almost identical stability in the gas phase and in water. The Z-isomer VII is more stable than other tautomers in the gas phase and quite competitive with I and III in

Biomimetic Synthesis of Marine Pyrrole‐2‐Aminoimidazole and Guanidinium Alkaloids

“…The pyridine containing pyraxinine 14 could be obtained from the vinylaminoimidazole (13) isolated from sponges. The proximity between oroidin (3) and dibromoagelaspongin (6) led to the formulation of a common chemical approach using the oxidative addition of guanidine derivatives to 1,2-dihydropyridine (44) (Scheme 7.9). The mechanistic rationalization for the formation of the pyridine derivative 14 from the 2-aminoimidazole (13) was based on the cyclization of tautomer 43 into 42 followed by the its ring opening when exposed to acidic or basic conditions.…”

“…Al-Mourabit and Potier proposed a biomimetic pathway from oroidin (3) through the dual tautomer of type III. After cyclization into aminals 50 (path a) [1] or 51 (path b) [27] and tautomerization into more stable intermediates 52 and 53, respectively, similar N9-C4 oxidative cyclizations could yield the hemiaminal dibromoagelaspongin (6). The notable macrocycle 50 (path a) was proposed as the common intermediate for both dibromophakellin and dibromoagelaspongin [1].…”

“…Nucleophilic and electrophilic reacting positions can vary depending on the tautomer engaged in the dimerization step [1]. The investigations, including molecular calculations, concluded that the tautomeric interconversion of I-V is more easily explained in acidic conditions [6]. The process leading to the selection of the reacting tautomers in Nature raises an interesting question.…”

“…Al-Mourabit's synthesis of oroidin (3) based on the biogenetic proposal for pyraxinine(14) and dibromoagelaspongine(6). Reagents and conditions: Scheme 7 10.…”

“…A pivotal step in the approach was the diastereoselective desymmetrization of the optically active cyclo (l-Pro-l-Pro) diketopiperazine 70 (Scheme 7.15). Stereocontrolled C-acylation of the corresponding monoenolate with benzyl chloroformate produced 71 as the 7.13 Al-Mourabit's approach to rac-dibromoagelaspongin(6). Reagents and conditions:…”