Alicyclic β- and γ-Amino Acids: Useful Scaffolds for the Stereocontrolled Access to Amino Acid-Based Carbocyclic Nucleoside Analogs

Stereocontrolled synthesis of some amino acid-based carbocyclic nucleoside analogs containing ring C=C bond has been performed on β- and γ-lactam basis. Key steps were N-arylation of readily available β- or γ-lactam-derived amino ester isomers and amino alcohols with 5-amino-4,6-dichloropyrimidine; ring closure of the formed adduct with HC(OMe)3 and nucleophilic displacement of chlorine with various N-nucleophiles in the resulting 6-chloropurine moiety.


Scheme 3.
Examples of bioactive cyclic amino acid derivatives.

Results and Discussion
Taking into account the importance of carbocyclic nucleoside analogs and the bioactivity of peptidyl nucleoside antibiotics containing β-amino acids, our aim was the synthesis of new carbocyclic nucleoside analogs with an amino acid moiety on a β-and γ-lactam basis. This pathway is similar to the first synthesis of carbovir from unsaturated γ-lactam (±)-14 (also known as Vince lactam) [25][26][27]. The synthesis of some 6-membered carbocyclic nucleoside analogs containing γamino alcohol was also planned.
Our synthetic work started with the opening of the heteroring of racemic Vince lactam (±)-14 [28]. Construction of the nucleobase part on the resulting amino ester (±)-15 was accomplished in three steps. First, compound (±)-15 was subjected to N-arylation with 5-amino-4,6dichloropyrimidine to furnish (±)-16. This process was accompanied by C=C bond migration thanks to the basic conditions, enabling the formation of a more stable conjugated π-system. Then, reaction with trimethyl orthoformate generated the second heteroring. The remaining chlorine atom of the obtained nucleoside analog (±)-17 was then replaced with N-nucleophiles to obtain adenosine analogs (±)-18, (±)-19 and (±)-20 (Scheme 4). It is worth to note that compound (±)-19 contains a cyclopropylamino group similar to abacavir, while the azido group of compound (±)-20 enables many further transformations (e.g., triazole formation). We continued our synthetic work with ethyl cis β-amino ester hydrochloride (±)-22 obtained from β-lactam (±)-21 [29,30]. Lactam ring opening, construction of the nucleobase moiety, and aromatic nucleophilic substitution resulted in nucleoside analogs (±)-25 and (±)-26. From ethyl trans β-amino ester hydrochloride (±)-28 [31,32], azidonucleoside (±)-31 was prepared in a similar way (Schemes 5 and 6). Note that the synthetic protocol took place with stereocontrol in both cases. Since the configuration of the chiral centers are not affected during the syntheses, their integrity is conserved and therefore, the cis-amino acid starting material led to the corresponding carbanucleoside analog in which the relative configuration of the groups is cis, while the trans-amino acid provided the carbocyclic nucleobase analog with trans relative steric arrangement of the ester and the heterocycle.

Conclusions and Outlook
A stereocontrolled synthetic pathway was developed to prepare new carbocyclic nucleoside analogs containing a ring olefin bond with a β-amino acid, γ-amino acid or γ-amino alcohol moiety from readily available β-and γ-lactams (across the amino acid isomers). The structure of the starting cycloalkene amino acids determined the configuration of the stereogenic centers of the products. 6-Nucleoside analogs containing the chloropurine moiety proved to be useful intermediates in various reactions with nucleophiles to access substituted nucleobases. Taking into consideration our widespread experiences in selective and controlled functionalization of versatile unsaturated cyclic amino acid derivatives [35][36][37][38], further studies in order to investigate the possible functionalization of the ring olefin bond of product nucleoside analogs are currently being investigated in our laboratory. Furthermore, based on our experiences in enzymatic resolution of various bicyclic β-and γ-lactams [39,40], as well as on enzymatic ester hydrolysis methodologies [41], synthesis of enantiomerically pure substances will be performed.

General Procedure for N-Arylation of γ-Amino Alcohols with 5-Amino-2,6-Dichloropyrimidine
To a solution of the γ-amino alcohol (8 mmoles) in EtOH (25 mL), 5-amino-2,6dichloropyrimidine (8 mmoles) and Et3N (24 mmoles) were added, then the mixture was kept at boiling temperature for 20 h. After cooling to room temperature, the reaction mixture was concentrated under reduced pressure and the residue was taken up in EtOAc (100 mL). The organic layer was washed with water (3 × 40 mL), dried with Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel (eluent: n-hexane-EtOAc 1:2).

General Procedure for the Formation of the Purine Skeleton of Amino Ester Nucleoside Analogs
To a solution of amino ester (2 mmoles) in trimethyl orthoformate (5 mL), a catalytic amount of methanesulfonic acid or p-TsOH (30 mg) was added. After stirring at 20 °C for 6 h, the reaction mixture was diluted with EtOAc (25 mL) and washed with saturated aqueous NaCl solution (3 × 15 mL). The organic phase was dried with Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel (eluent: n-hexane-EtOAc 1:1).

General Procedure for the Formation of the Purine Skeleton of Amino Alcohol Nucleoside Analogs
To a solution of amino alcohol nucleoside analog (1 mmol) in trimethyl orthoformate (4 mL), a catalytic amount of p-TsOH (20 mg) was added. After stirring at 20 °C for 6 h, the reaction mixture was diluted with EtOAc (20 mL) and washed with saturated aqueous NaCl solution (3 × 15 mL). The organic phase was dried with Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel (eluent: n-hexane-EtOAc 1:2).

General Procedure for the Introduction of the Azido Group
To a solution of 6-chloropurinyl nucleoside analog (150 mg) in THF/H2O (10 mL, 4:1), sodium azide (4 eq.), acetic acid (3 drops), and Et3N (4 drops) were added. After heating at reflux temperature for 20 h, the reaction mixture was diluted with EtOAc (20 mL) and washed with water (2 × 15 mL). The organic phase was dried with Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel (eluent: n-hexane-EtOAc 1:2).

General Procedure for the Introduction of the Cyclopropylamino Group
To a solution of 6-chloropurinyl nucleoside analog (150 mg) in EtOH (10 mL), cyclopropylamine (4 eq.) was added. After the mixture was kept at boiling temperature for 12 h, the reaction mixture was concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel (eluent: n-hexane-EtOAc 1:1).