Phosphoramidite activator for oligonucleotide synthesis

1. A phosphoramidite activator comprising a solution of 5-benzylmercaptotetrazole in acetonitrile and an N-alkylimidazole.

2. The activator of claim 1 comprising at least about 0.5 percent of an N-alkylimidazole by weight.

3. The activator of claim 2 comprising at least about 1.0 percent of an N-alkylimidazole by weight.

4. The activator of claim 3 comprising at least about 1.5 percent of an N-alkylimidazole by weight.

5. The activator of claims 1 wherein the N-alkylimidazole is N-methylimidazole.

6. The activator of claim 1 comprising at least about a 0.25 molar solution of benzylmercaptotetrazole.

7. The activator of claim 1 comprising at least about a 0.30 molar solution of benzylmercaptotetrazole.

8. The activator of claim 1 comprising at least about a 0.35 molar solution of benzylmercaptotetrazole.

9. The activator of claim 1 comprising at least about a 0.40 molar solution of benzylmercaptotetrazole.

10. A method of phosphoramidite activation comprising the steps of:

(a) providing a phosphoramidite; and

(b) reacting the phosphoramidite in step (a) with the phosphoramidite activator of claim 1 to produce an intermediate product capable of reacting with a 5′-O site of a nucleoside or oligonucleotide.

11. The method of claim 10 , wherein said phosphoramidite has the formula:

wherein Pg is a phosphorous protecting group selected from the group consisting of —CH 2 CH 2 CN, —CH 2 CH═CHCH 2 CN, para-CH 2 C 6 H 4 CH 2 CN, —(CH 2 ) 2-5 N(H)COCF 3 , —CH 2 CH 2 Si(C 6 H 5 ) 2 CH 3 , and CH 2 CH 2 N(CH 3 )COCF 3 ;

R 1 is a nucleoside or an oligonucleotide; and

R 2 is morpholino, dialkylamino, or —N(R 3 ) 2 , wherein R 3 is independently C 1 -C 6 alkyl or 4-7 member heterocycloalkyl or heterocycloalkyenyl ring having up to 3 heteroatoms selected from the group consisting of nitrogen, sulfur, and oxygen.

12. A method of preparing an oligonucleotide comprising the steps of:

(a) providing a 3′-nucleoside phosphoramidite or 3′-oligonucleotide phosphoramidite; and

(b) reacting said 3′-nucleoside phosphoramidite or 3′-oligonucleotide phosphoramidite with the 5′-hydroxyl of a nucleoside, or oligonucleotide in the presence of a phosphoramidite activator;

wherein said phosphoramidite activator comprises a solution of 5-benzylmercaptotetrazole in acetonitrile and N-alkylimidazole, said solution having a 5-benzylmercaptotetrazole concentration greater than about 0.25M.

13. The method of claim 12 wherein said nucleoside or oligonucleotide is bound to a solid support.

14. A method for preparing covalent intersugar linkages in an oligonucleotide synthesis comprising the steps of:

(a) providing a phosphoramidite; and

(b) reacting said phosphoramidite with a hydroxyl group of the sugar moiety of a nucleoside or an oligonucleotide in the presence of a phosphoramidite activator to produce a compound having the formula:

wherein Pg is a phosphorous protecting group selected from the group consisting of —CH 2 CH 2 CN, —CH 2 CH═CHCH 2 CN, para-CH 2 C 6 H 4 CH 2 CN, —(CH 2 ) 2-5 N(H)COCF 3 , —H 2 CH 2 Si(C 6 H 5 ) 2 CH 3 , and CH 2 CH 2 N(CH 3 )COCF 3 ;

R 1 is a nucleoside or an oligonucleotide; and

R 4 is a nucleoside or a oligonucleotide;

said phosphoramidite activator comprising a solution of at least 0.25M 5-benzylmercaptotetrazole in acetonitrile and N-alkylimidazole.

15. The method of claim 14 wherein said phosphoramidite has the formula:

wherein Pg is a phosphorous protecting group selected from the group consisting of —CH 2 CH 2 CN, —CH 2 CH═CHCH 2 CN, para-CH 2 C 6 H 4 CH 2 CN, —(CH 2 ) 2-5 N(H)COCF 3 , —CH 2 CH 2 Si(C 6 H 5 ) 2 CH 3 , and CH 2 CH 2 N(CH 3 )COCF 3 ;

R 1 is a nucleoside or an oligonucleotide; and

R 2 is morpholino, dialkylamino, or —N(R 3 ) 2 , wherein R 3 is independently C 1 -C 6 alkyl or 4-7 member heterocycloalkyl or heterocycloalkyenyl ring having up to 3 heteroatoms selected from the group consisting of nitrogen, sulfur, and oxygen.

16. The method of claim 14 , further comprising wherein the steps (a) and (b) are performed iteratively in appropriate sequence with standard process steps including deprotection, oxidation of phosphorus (III) to phosphorus (V), and capping to produce an oligonucleotide according to a preselected nucleotide base sequence.