5'-MMT protected RNA Amidites

Advantages and Application of 5′ -MMTr in Purine Bases for Oligo Synthesis:

• Use of more potent activator such as 5-(Benzylmercapto)-1H-tetrazole (BTT, pK_a=4.08),¹ 5-Ethylthiotetrazole (ETT,  pK_a= 4.28),¹ which are more acidic in nature compared to 1-H-Tetrazole (pK_a= 4.89)¹, might lead to the removal of the DMTr group from purine bases during the coupling step of oligo synthesis, thereby causing undesired M+1 oligo growth.
• Since purine bases, A & G, cleave at the 5′-DMTr at a much higher rate than 5′-MMTr, the 5′-MMTr protected nucleoside phosphoramidites is pre­ferred; thus offering a minimal level of M+1 impurities.

References:

1) Welz, R.;  Muller, S. Tet. Lett. 2002, 43, 795-797.