First Synthesis of 2′‐Deoxyfluoropuromycin Analogues: Experimental Insight into the Mechanism of the Staudinger Reaction

Abstract

The N(6),N(6)-dedimethyl-2'-deoxyfluoro analogue of puromycin (= 3'-deoxy-N(6),N(6)-dimethyl-3'-[O-methyltyrosylamido]adenosine), its 2',3'-regioisomer and a 3'-cytidyl-5'-(2'-deoxyfluoro)puromycyl dinucleotide analogue were synthesized following an approach involving i) the diastereospecific nitrite-assisted formation of a lyxo nucleosidic 2',3'-epoxide from an adenosine-2',3'-ditriflate derivative in a biphasic solvent mixture; ii) the regio- and stereoselective epoxide ring opening with sodium azide under mildly acidic aqueous conditions, iii) the stereospecific introduction of the fluor atom using DAST and iv) the reaction between the nucleosidyl or dinucleotidyl azide and an active ester of the N-protected amino acid using highly efficient solution conditions for the Staudinger-Vilarrasa coupling, to obtain the corresponding carboxamide directly from the in situ formed iminophosphorane. This coupling reaction furnished sterically quite demanding amides in 94 % isolated yields under very mild conditions and should therefore be of a more general value. Under certain reaction conditions we isolated (amino)acyltriazene derivatives from which dinitrogen was not eliminated. These secondary products are trapped and stabilized witnesses of the first intermediate of the Staudinger reaction, the phosphatriazenes (phosphazides, triazaphosphadienes) which usually eliminate dinitrogen in situ and rapidly rearrange into iminophosphoranes, unless they are derived from conjugated or sterically bulky azides and phosphines. The acyltriazenes could either be thermally decomposed or converted to the corresponding N-alkyl carboxamides through proton-assisted elimination of dinitrogen. All compounds were carefully characterized through MS spectrometry, (1)H, (19)F, (31)P and (13)C NMR spectroscopy.

A Phosphine‐Mediated Conversion of Azides into Diazo Compounds

“…[13] An obvious requirement for accessing triazenes would be the avoidance of N 2 (g)-extrusion. Although we were encouraged by numerous reports in the literature on the isolation and trapping of phosphazides, typically achieved through careful choice of both the phosphine and the azide components, [14] we were intrigued by doing so via an intramolecular acylation event reminiscent of the Staudinger ligation (Scheme 2). [15] …”

“…Acyl alkyl-triazenes and alkyl aryl-triazenes are known to exist in solution as a mixture of tautomers, wherein the acidic proton resides on either terminus of the triazene moiety. [14f,17] Unfortunately, we failed in our attempts to obtain crystals of the intermediate that were suitable for X-ray crystallography, due in part to its thermal instability. When, however, aryl azide 7 was allowed to react with phosphine 1c , acyl triazene 8 was isolated in excellent yield by column chromatography, and its structure was confirmed by X-ray crystallography (Scheme 4).…”

“…Subsequent investigation revealed the origin of the diminished conversion: phosphine 1e (and 1c ) reacted with diazo-compound 5a at a rate that was comparable to that of its reaction with azide 2a to give a compound whose spectroscopic data were consistent with acyl hydrazone 9 (Scheme 5). [14a,18] Under wet conditions and preceding basic workup, the thermal decomposition of acyl triazene 6 (the hydrolysis product of the acyl triazenophosphonium salt) is slow, allowing near complete consumption of azide and phosphine before accumulation of. appreciable concentrations of the diazo-compound.…”

Synthesis of Stable Aminoacyl‐tRNA Analogs

“…These ring systems are attractive for the synthesis of stable aminoacyl-tRNA analogs, since they can be obtained in optically active form from appropriately protected and activated amino acids. Alternative approaches for synthesis of stable aminoacyl-tRNA analogs include replacement of the ester bond by an amide linkage, as found in puromycin analogs (Charafeddine et al, 2007;Steger et al, 2010). The ester bond has also been replaced by a phosphate, phosphoramidate (Weinger et al, 2004;Schmeing et al, 2005;Huang et al, 2006;Cressina et al, 2007) that mimics tetrahedral transition states formed during the aminoacyl transfer reactions.…”

Non‐Hydrolyzable RNA–Peptide Conjugates: A Powerful Advance in the Synthesis of Mimics for 3′‐Peptidyl tRNA Termini

“…[8] Then, amidine protection of the adenine NH 2 group (5), tritylation of the arabinose 5'-OH (6), and inversion of the configuration at C2' (7 and 8) were achieved in straightforward manner; the latter transformation made use of triflate chemistry in analogy to a protocol that we originally developed for the synthesis of 2'-methylseleno-modified RNA. [9] Subsequent Staudinger-Vilarrasa [10] coupling furnished the amino acid Scheme 1. Synthesis of the 3'-amino-2',3'-dideoxyadenosine-functionalized support 4 (dA 3'-NH ) for the automated solid-phase synthesis of RNA-peptide conjugates.…”