# A Michaelis‐Arbuzov‐Type Pathway to a Protected 2ʹ‐Deoxy‐2ʹ‐Selenomethyl‐Adenosine‐3ʹ,5ʹ‐Phosphoroselenolate Guanosine Dinucleotide for Use in Modified m7G Cap Synthesis

## TL;DR

This paper describes a new chemical synthesis method for a modified RNA building block that could be useful in making therapeutic mRNA.

## Contribution

A novel Michaelis-Arbuzov coupling and selenomethylation strategy for synthesizing a modified dinucleotide with potential mRNA therapeutic applications.

## Key findings

- A 3ʹ-H-phosphonate/5ʹ-selenocyanate Michaelis-Arbuzov coupling successfully forms the phosphoroselenolate linkage.
- Amosova reductive selenomethylation effectively installs the 2ʹ-selenomethyl group without oxidation.
- A solid CPR II-mediated phosphitylation and t-BuO2H oxidation yield the phosphorylated dinucleotide without damaging the selenomethyl group.

## Abstract

In this paper, a solution‐phase total synthesis is described of the 5ʹ‐O‐phosphorylated 2ʹ‐deoxy‐2ʹ‐selenomethyl‐adenosine 3ʹ,5ʹ‐phosphoroselenolate guanosine dinucleotide (4) using a rt, 2,6‐lutidine‐mediated, nucleoside 3ʹ‐H‐phosphonate/5ʹ‐selenocyanate Michaelis‐Arbuzov ligative coupling, whose potential mechanism is discussed herein. Our route to 4 is predicated upon the highly efficient new 2ʹ‐O‐triflate selenocyanate anion displacement of 10 in MeCN, to obtain the 2ʹ‐selenocyanate 9, which was allied with an Amosova NaBH4/MeI mediated reductive selenomethylation in MeOH. A novel solid CPR II‐mediated 5ʹ‐O‐phosphitylation of the dinucleotide alcohol 6 and a 70% aqueous t‐BuO2H oxidation successfully installed the 5ʹ‐O‐phosphate moiety within 4 without oxidizing the 2ʹ‐selenomethyl group. It is envisaged that 4 will be of value for a future total synthesis of the m7G cap 1; a molecule of potential utility for therapeutic mRNA manufacture.

Here we report a synthesis of the dinucleotide 4 for a projected future total synthesis of the m7G cap 1. Highlights of the route include: 1) a 3ʹ‐H‐phosphonate/5ʹ‐selenocyanate Michaelis‐Arbuzov coupling to forge the 3ʹ,5ʹ‐phosphoroselenolate; 2) an Amosova reductive selenomethylation to install the 2ʹ‐SeMe; 3) a solid CPR II 5ʹ‐O‐phosphitylation; and 4) an aq. tBuO2H 5ʹ‐O‐phosphite oxidation without accompanying selenoxide formation. Also, MeCN has been found to be especially beneficial for the S
N
2 reactions of 10 and the iodide 12.