JP2003514766A - Oligonucleotide synthesis using Lewis acids as activators - Google Patents

Abstract

  (57) [Summary] A method for synthesizing oligonucleotides by phosphoramidite chemistry, the improvement being the use of Lewis acids as activators for the formation of phosphorus-oxygen bonds.

Description

Detailed Description of the Invention

TECHNICAL FIELD The present invention relates to the use of Lewis acids as activators in the preparation of oligonucleotides due to the chemical nature of phosphoramidites.

BACKGROUND OF THE INVENTION The present invention relates generally to the fields of organic chemistry and biology. In particular, the present invention is directed to compositions and methods for use in oligonucleotide synthesis.

Phosphoramidite chemistry (Beaucage, SL and Lye)
r, R. P. Tetrahedron (1992), 48, 2223-2311.
) Is the most widely used coupling chemistry for the synthesis of oligonucleotides. As is well known to those skilled in the art, phosphoramidite synthesis of oligonucleotides involves the reaction of an activator to form an activated intermediate, followed by an elongating oligonucleotide chain (typically one end immobilized on a suitable solid support). The activation of nucleoside phosphoramidite monomer precursors by sequentially adding activating intermediates to form oligonucleotide products. Generally tetrazole is used for activation of nucleoside phosphoramidite monomers. Its activation occurs by the mechanism shown in Scheme 1. Tetrazole has an acidic proton believed to protonate the basic nitrogen of the diisopropylaminophosphine group, thus making the diisopropylamino group a leaving group. The negatively charged tetrazolium ion then attacks the trivalent phosphorus, forming a transient phosphorus tetrazolide species. The 5'-OH group of the solid support attached to the nucleoside then attacks the active trivalent phosphorus species, resulting in the formation of an internal nucleotide bond. Finally, trivalent phosphorus is oxidized to pentavalent phosphorus.

[0004]

[Chemical 5]

The major drawbacks of tetrazole are its cost and instability, and tetrazole can explode (Material Safety Data Sheets).
That is, 1H-tetrazole is described as an important explosive dangerous substance in MSDS). Due to its inherent instability, the sublimated tetrazole generally needs to ensure the desired coupling yield. In addition, tetrazole (typically used near its saturation solubility of 0.5M) tends to precipitate from acetonitrile solutions at low temperatures. On some automated DNA synthesizers this can lead to valve blockage.

Other activators that work nearly as efficiently as tetrazole have drawbacks similar to those of tetrazole described above. These activators, both of which are proton donors, have the following members of the tetrazole class of activators. 5- (p-nitrophenyl) tetrazole (Froehler, BC and Mattcu)
cci, M .; D. , Tetrahedron Letters (1983), 2
4, 3171-3174); 5- (p-nitrophenyl) tetrazole and D
MAP (Pon, RT, Tetrahedron Letters (198
7), 28, 3643-3646; and 5- (ethylthio) -1H-tetrazole (Wright, P. et al., Tetrahedron Letters (19.
93), 34, 3373-3376). In addition to this tetrazole class of activators, the following activators have also been used. N-methylaniline trifluoroacetate (Fourrey, JL and Varenne, J., Tetrahed.
ron Letters (1984), 25, 4511-4514); N-methylaluminum trichloroacetate (Fourrey, JL et al., Tetr.
ahedron Letters (1987), 28, 1769-1772);
1-Methylimidazole trifluoromethanesulfonate (Arnold, L
． Others, Collect. Czech. Chem. Commun. (1989),
54,523-532); octanoic acid or triethylamine (Stec, W .;
J. And Zon, G .; , Tetrahedron Letters (1984)
), 25, 5279-5282); 1-methylimidazole HCl, 5-trifluoromethyl-1H-tetrazole, N, N-dimethylamine HCl, and N.
, N-dimethylaminopyridine HCl (Hering, G. et al., Nucleos
des and Nucleotides (1985), 4, 169-171.
). In general, these activators have poorer performance than tetrazole.

JP08301878 discloses the use of other proton activators. A 1: 1 mixture of benzimidazole and BF ₃ etherate is disclosed in which the BF ₃ component acts to increase the acidity of the benzimidazole proton required for activation of the phosphoramidite (intermediate). The benzimidazole BF ₃ complex acts in a similar manner to the tetrazole described in Scheme 1. The present invention uses Lewis acids for activation rather than activating the phosphoramidite intermediate with a proton donor. In particular, BF ₃ etherate is used in the present invention. B
An advantage of the F ₃ etherate benzimidazole BF ₃ complex is that it is commercially available and that removal of diethyl ether is easier than removal of benzimidazole.

Furthermore, activated phosphoramidite intermediates are very sensitive to moisture. More than 50% to 100% of the very expensive phosphoramidites need to be aligned even with anhydrous solvents (<20 ppm moisture content). The presence of trace amounts of moisture results in a significant decrease in yield and an increase in impurities in the deleted sequences. In addition to activating phosphoramidite intermediates, Lewis acids can act as moisture scavengers that minimize phosphoramidite degradation. Thus, the use of Lewis acids for activation of phosphoramidite intermediates improves coupling efficiency, reduces cost, and makes material handling and work convenient.

However, the addition of Lewis acids can lower the pH of the reaction mixture. Increasing acidity of the reaction mixture can cause the removal of N- and O-protecting groups on the phosphoramidite intermediate, leading to undesirable products and lower yields. Thus, the addition of pyridine can be used to increase the pH to a level that phosphoramidite protecting groups can tolerate.

One object of the present invention is to provide Lewis acid activated nucleosides for use in the synthesis, including solid phase synthesis, which do not exhibit any of the drawbacks of the prior art.

Another object of the present invention is to provide a method for the preparation and use of Lewis acid activated nucleosides described below.

SUMMARY OF THE INVENTION In its main embodiment, the present invention discloses a method for activating the follamidite monomer of formula I below.

[0013]

[Chemical 6]

In the above formula, B ¹ is selected from the group consisting of purine bases and pyrimidine bases, R ¹ is a secondary amine (the preferred amine is diisopropylamine), R ² is an alkoxy, alkyl, alkylsulfonylalkoxy, arylsulfonyl alkoxy are selected from cyano alkoxy, and the group consisting of haloalkoxy, R ³ is a hydroxy protecting group (preferred groups are 4,4'-dimethoxytrityl), R ⁴ is hydrogen and -OR ⁷ Selected from the group wherein R ⁷ is a hydroxy protecting group and the phosphoramidite monomer of formula I is treated with an optional amount of pyridine and a Lewis acid, preferred Lewis acids are aluminum chloride, bismuth (III) chloride.
, Boron trifluoride, iron (II) chloride, iron (III) chloride, magnesium bromide, magnesium chloride, magnesium trifluoromethanesulfonate, manganese (II) chloride
, Zinc bromide, zinc chloride and zirconium (IV) chloride.

DETAILED DESCRIPTION OF THE INVENTION All patents, patent applications and references cited in the specification are hereby incorporated by reference in their entirety. In case of conflict, the present disclosure including definitions will control.

Definitions of Terms The meanings of the following terms used in the specification and the appended claims are defined as follows.

The term “alkoxy” as used herein refers to an alkyl group, as defined herein, appended to the parent molecular moiety through an oxy group, as defined herein. Representative examples of alkoxy groups include, but are not limited to, methoxy, ethoxy, propoxy, 2-propoxy, butoxy, t-butoxy, pentyloxy and the like.

The term “alkyl” as used herein refers to a straight or branched chain hydrocarbon containing 1 to 5 carbon atoms. Representative examples of alkyl include, but are not limited to, methyl, ethyl, n-propyl, iso-propyl, n-butyl, s-butyl, isobutyl, t-butyl, n-pentyl and the like.

The term “alkylcarbonyl,” as used herein, refers to an alkyl group, as defined herein, appended to the parent molecular moiety through a carbonyl group, as defined herein.
Representative examples of alkylcarbonyl include, but are not limited to, acetyl, ethylcarbonyl, and the like.

The term “alkylcarbonyloxy” as used herein refers to an alkylcarbonyl group, as defined herein, appended to the parent molecular moiety through an oxy group, as defined herein. Representative examples of alkylcarbonyloxy include, but are not limited to, acetyloxy, ethylcarbonyloxy, t-butylcarbonyloxy, and the like.

The term “alkylsulfonyl” as used herein refers to an alkyl group, as defined herein, appended to the parent molecular moiety through a sulfonyl group, as defined herein.
Representative examples of alkylsulfonyl include, but are not limited to, methylsulfonyl, ethylsulfonyl, and the like.

The term “alkylsulfonylalkoxy” as used herein refers to an alkylsulfonyl group, as defined herein, appended to the parent molecular moiety through an alkoxy group, as defined herein. Representative examples of alkylsulfonylalkoxy include, but are not limited to, 2-methylsulfonylethoxy, 2-ethylsulfonylethoxy, and the like.

The term “amino” as used herein refers to a —NH ₂ group.

The term “amino protecting group” or “N-protecting group” as used herein refers to groups intended to protect the amino group against undesired reactions during synthetic procedures. Commonly used nitrogen protecting groups are described in Green, T .; W. , And Wuts, P .; G. M.
(1991), Protective Groups In Organic.
Synthesis (2nd ed.), New York: John Wil
ey & Sons. Preferred nitrogen protecting groups are formyl, acetyl, benzoyl, pivaloyl, t-butylacetyl, phenylsulfonyl, benzyl, t-butyloxycarbonyl (Boc), benzyloxycarbonyl (
Cbz).

The term “aryl” as used herein refers to an aromatic monocyclic ring system or a bicyclic fused ring system in which one or both of the fused rings are aromatic. Representative examples of aryl include, but are not limited to, azulene, indanyl, indenyl, naphthyl, phenyl, tetrahydronaphthyl, and the like.

The aryl groups of the present invention can be substituted with 1, 2 or 3 substituents independently selected from alkyl, cyano, halogen, haloalkyl and nitro.

The term “arylalkoxy” as used herein refers to an aryl group, as defined herein, appended to the parent molecular moiety through an alkoxy group, as defined herein.
Representative examples of arylalkoxy include, but are not limited to, 2-phenylethoxy,
2-naphthylethoxy, 2- (4-nitrophenyl) ethoxy and the like can be mentioned.

The term “arylsulfonyl”, as used herein, refers to an aryl group, as defined herein, appended to the parent molecular moiety through a sulfonyl group, as defined herein.
Representative examples of arylsulfonyl include, but are not limited to, phenylsulfonyl, naphthylsulfonyl, and the like.

The term “arylsulfonylalkoxy” as used herein refers to an arylsulfonyl group, as defined herein, appended to the parent molecular moiety through an alkoxy group, as defined herein. Representative examples of arylsulfonylalkoxy include, but are not limited to, 2-phenylsulfonylethoxy, 3-phenylsulfonylpropoxy, and the like.

[0030]   The term "carbonyl" as used herein refers to a -C (O)-group.

As used herein, the term “catechol” is defined herein for both oxygen and M
_C 6 _H 4 -1,2- added to _(O-) refers to _two groups.

[0032]   The term "cyano," as used herein, refers to a -CN group.

The term “cyanoalkoxy” as used herein refers to a cyano group, as defined herein, appended to the parent molecular moiety through an alkoxy group, as defined herein. Representative examples of cyanoalkoxy include, but are not limited to, 2-cyanoethoxy, 2-cyanopropoxy, 1-methyl-2-cyanoethoxy, 1,1-dimethyl-2-cyanoethoxy, 1,1-dimethyl-2-. Examples thereof include cyanoethoxy.

The term “halo” or “halogen” as used herein refers to —Cl, —Br,
Refers to -I or -F.

The term “haloalkoxy” as used herein refers to at least one halogen, as defined herein, appended to the parent molecular moiety through an alkoxy group, as defined herein. Representative examples of haloalkoxy include, but are not limited to, 2,2,2-trichloroethoxy, 1,1-dimethyl-2,2,2-trichloroethoxy, trifluoromethoxy and the like.

The term “haloalkyl” as used herein refers to at least one halogen, as defined herein, appended to the parent molecular moiety through an alkyl group, as defined herein. Representative examples of haloalkyl include, but are not limited to, chloromethyl, 2-fluoroethyl, trifluoromethyl, pentafluoroethyl, 2-chloro-3-fluoropentyl, and the like.

The term “hydroxy protecting group” or “O-protecting group” refers to groups intended to protect a hydroxy group against undesired reactions during synthetic procedures. Commonly used hydroxy protecting groups can be found in T.W., incorporated herein by reference. H. Green and P.M. G. M. Wuts, Protective Groups in Org
anic Synthesis, 2nd editon, New York: J
ohn Wiley & Sons, New York (1991). Representative examples of hydroxy protecting groups include, but are not limited to, substituted methyl ethers such as methoxymethyl, benzyloxymethyl, 2-methoxyethoxymethyl, 2- (trimethylsilyl) -ethoxymethyl, benzyl and triphenylmethyl; tetrahydropyranyl ether. Substituted ethyl ethers such as 2
, 2,2-Trichloroethyl and t-butyl; silyl ethers such as trimethylsilyl, t-butyldimethylsilyl and t-butyldiphenylsilyl;
Cyclic acetals and ketals such as methylene acetals, acetonides and benzylidene acetals; cyclic orthoesters such as methoxymethylene; cyclic carbonates; cyclic borate esters; carbonyl derivatives such as acetyl, p-phenyl Azophenyloxycarbonyl, 9-florenylmethoxycarbonyl, 2,4-dinitrophenylethoxycarbonyl, 2- (methylthiomethoxymethyl) benzoyl, 2- (isopropylthiomethoxymethyl) benzoyl, 2- (2,4-dinitrobenzenesul) Phenyloxymethyl) benzoyl, 4- (methylthiomethoxy) butyryl and levulinyl; trityl derivatives such as 4,4'-dimethoxytrityl, 4,4 ', 4 "-tris- (benzyloxy) trityl, 4,4'. 4 "-tris (4,5-dichlorophthalimide) trityl, 4,4 ', 4" -tris- (levulinyloxy) trityl, 3- (imidazolylmethyl) -4,4'-dimethoxytrityl, 4-decycloxy Trityl, 4-hexadecyloxytrityl and 1,1-bis- (4-methoxyphenyl) -1'-pyrenylmethyl; substituted xanthenyl groups such as pixyl (9-phenylxanthen-9-yl), 9- (p- Methoxyphenyl) xanthene-9
-Yl) and 9- (4-octadecyloxyphenyl) xanthen-9-yl.

The term “Lewis acid” as used herein refers to a chemical species other than a proton having one unoccupied orbital or one electron pair. It can be appreciated that Lewis acids can be purchased or prepared as, but not limited to, complexes of etherates, hydrates, thioetherates, and the like. Furthermore, it can be seen that the complexes purchased or prepared for the present invention do not contain acidic protons. Representative examples of Lewis acids include, but are not limited to, aluminum chloride, bismuth (III) chloride, boron trifluoride, iron (II) chloride, iron chloride (I
II), magnesium bromide, magnesium chloride, magnesium trifluoromethanesulfonate, manganese (II) chloride, zinc bromide, zinc chloride, zirconium chloride (
IV) and the like.

The term “methylenedioxy” as used herein refers to a —OC (R ⁸⁰ ) (R ⁸¹ ) O— group in which R ⁸⁰ and R ⁸¹ are independently selected from halogen and alkyl. The oxygen atom of the methylenedioxy group is attached to the parent molecular moiety through two adjacent carbon atoms. Representative examples of methylenedioxy groups include, but are not limited to, 1,
3-dioxolanyl, 2,2-dimethyl-1,3-dioxolanyl, 2-methyl-1,3-dioxolanyl and the like can be mentioned.

[0040]   The term "oxy" as used herein refers to (-O-).

As used herein, the term “purine base” refers to 9H-purine-6-imylamine (
Adenine) and 2-amino-1,9-dihydro-6H-purin-6-one (guanine). The amino group added to adenine can be protected with a nitrogen protecting group.

The term “proton” as used herein refers to H ⁺ .

As used herein, the term “pyrimidine base” refers to 2,4 (1H, 3H) -pyrimidinedione (uracil), 5-methyl-2,4 (1H, 3H) -pyrimidinedione (thymine) and Refers to an organic base selected from 4-amino-2 (1H) -pyrimidinone (cytosine). The amino group added to cytosine may be protected with a nitrogen protecting group.

The term “sulfonyl” as used herein refers to a —SO ₂ — group.

The term “trifluoromethane” as used herein refers to a —CF ₃ group.

The term “trifluoromethanesulfonyl,” as used herein, refers to a trifluoromethane group, as defined herein, appended to the parent molecular moiety through a sulfonyl group, as defined herein.

The term “trifluoromethanesulfonyloxy” as used herein refers to a trifluoromethanesulfonyl group, as defined herein, appended to the parent molecular moiety through an oxy group, as defined herein.

Abbreviations The following abbreviations are used. That is, for dimethylacetamide, DMA, 4,
The 4'-dimethoxytrityl, DMT, the ethyl acetate EtOAc, the equivalent eq, the Material Safety Data Sheet MSDS, the sodium bicarbonate NaHCO _3, the sodium sulphate _Na 2 SO _4, thin layer chromatography TLC.

Synthetic Methods The compounds and methods of this invention will be better understood in connection with the synthetic schemes below which illustrate how the compounds of the invention are prepared.

[0050]

[Chemical 7]

General Procedure Using Lewis Acid as Activator: A solution of phosphoramidite (i) (1.0 eq) and nucleoside (ii) (1.0 eq) in acetonitrile or DMA (0.1M) was added to Lewis solution. Treated with acid (Table 1). The solution was mixed at room temperature and monitored by TLC using EtOAc: triethylamine (95: 5) as the developing solvent. Range from 5 minutes to less than 60 minutes (Table 1
After), the reaction mixture was quenched with NaHCO ₃ and extracted with EtOAc. The organic layer was washed with aqueous NaHCO ₃ , dried (Na ₂ SO ₄ ) and evaporated under reduced pressure to give the dimeric product (iii) in near quantitative yield. ³¹ P NMR (500 MHz, CD ₂ Cl ₂ ) δ 139.5, 139.7; MS (ESI ⁺ ) m / z: 1041.4 (M ⁺ ).

General Procedure Using Lewis Acid as Activator: A solution of phosphoramidite (i) (1.0 eq) and nucleoside (ii) (1.0 eq) in acetonitrile or DMA (0.1 M) was added to pyridine. Treated with (2 eq) then Lewis acid (2 eq) was added. The solution was mixed at room temperature and monitored by TLC using EtOAc: triethylamine (95: 5) as the developing solvent. After less than 5 minutes, the reaction mixture was quenched with NaHCO _3, and extracted with EtOAc. The organic layer was washed with aqueous NaHCO ₃ , dried (Na ₂ SO ₄ ) and evaporated under reduced pressure to give the dimeric product (iii) in near quantitative yield. ³¹ P NMR (500 MHz, CD ₂ Cl ₂ ) δ 139.5, 139.7; MS (ESI ⁺ ) m / z: 1041.4 (M ⁺ ).

[0053]

[Table 1]

It can be seen that the dimer (iii) can be oxidized using standard conditions known to the person skilled in the art to give the phosphate (J. Am. Chem. Soc.
． , (1976), 98, 3655-3661). The 5'-OH of the oxidized dimer can be deprotected and treated with a Lewis acid activated phosphoramidite monomer to form a trimer. This series of steps can be repeated until the desired length of the oligonucleotide has been synthesized so that the method of the invention can be used to prepare the oligonucleotide including solid phase synthesis.

Claims (20)

[Claims] 1. A composition comprising a Lewis acid, an optional amount of pyridine, and a compound of formula I as follows: Where B ¹ is selected from the group consisting of purine bases and pyrimidine bases, R ¹ is —NR ⁵ R ⁶ , R ⁵ and R ⁶ are independently selected from the group consisting of alkyl and arylalkyl, and R ² Is selected from the group consisting of alkoxy, alkyl, alkylsulfonylalkoxy, arylsulfonylalkoxy, cyanoalkoxy, and haloalkoxy, R ³ is a hydroxy protecting group, R ⁴ is selected from the group consisting of hydrogen and —OR ⁷ . The composition wherein R ⁷ is a hydroxy protecting group. 2. Including the Lewis acid of formula II below: In the above formula, M is aluminum, antimony, bismuth, boron, cadmium, cobalt, copper, chromium, gold, hafnium, iridium, iron, lanthanum, magnesium, manganese, mercury, molybdenum, nickel, niobium, osmium, palladium,
Selected from the group consisting of platinum, phosphorus, rhenium, ruthenium, scandium, silver, tantalum, tellurium, tin, tungsten, titanium, vanadium, zinc, zirconium, and yttrium, where R ¹⁰ is alkoxy, alkylcarbonyloxy, trifluoromethanesulfur. Selected from the group consisting of phenyloxy, cyano, and halogen, wherein R ¹¹ is absent or selected from the group consisting of alkoxy, alkylcarbonyloxy, trifluoromethanesulfonyloxy, cyano, and halogen, or one R ¹⁰ and R ¹¹ form a catechol, both oxygen atoms are bound to M and R ¹² is absent or is alkoxy, alkylcarbonyloxy, trifluoromethanesulfonyloxy, cyano, and halogen. Or Is selected from the group consisting of or R ¹³ is absent, or alkoxy, alkylcarbonyloxy, trifluoromethane sulfonyl b alkoxy, is cyano, and the group consisting of halogen, R ¹⁴ is absent or a group consisting of halogen, Selected from
The composition according to. 3. The Lewis acid of formula II, wherein M is aluminum, bismuth, boron, iron, magnesium, manganese,
Selected from the group consisting of titanium, zinc and zirconium, R ¹⁰ selected from the group consisting of alkoxy, cyano, halogen and trifluoromethanesulfonyloxy, R ¹¹ consisting of alkoxy, cyano, halogen and trifluoromethanesulfonyloxy. Selected from the group, or R ¹⁰ and R ^{11 which} are joined together form a catechol, both oxygen atoms are bound to M, R ¹² is absent or selected from the group consisting of alkoxy and halogen. The composition of claim 2, wherein R ¹³ is absent or is selected from the group consisting of alkoxy and halogen and R ¹⁴ is absent. 4. Aluminum chloride, bismuth (III) chloride, boron trifluoride, iron (II) chloride, iron (III) chloride, magnesium bromide, magnesium chloride, magnesium trifluoromethanesulfonate, manganese (II) chloride. The composition of claim 1, comprising the Lewis acid selected from the group consisting of :, zinc bromide, zinc chloride and zirconium (IV) chloride. 5. The composition of claim 1, wherein R ⁵ and R ⁶ are independently selected from the group consisting of alkyl. 6. The composition of claim 1, wherein R ⁵ is isopropyl and R ⁶ is isopropyl. 7. The composition according to claim 1, wherein R ² is selected from the group consisting of cyanoalkoxy. 8. The composition according to claim 1, wherein R ² is 2-cyanoethoxy. 9. The composition according to claim 1, wherein R ¹ is diisopropylamino and R ² is 2-cyanoethoxy. 10. R ³ is 4,4′-dimethoxytrityl, 4,4 ′, 4 ″-
Tris- (benzyloxy) trityl, 4,4 ', 4 "-Tris- (4,5-dichlorophthalimido) trityl, 4,4', 4" -Tris- (revnyloxy) trityl, 3- (imidazolemethyl) -4 , 4'-dimethoxytrityl, pixyl (9-phenylxanthene-9-yl), 9- (p-methoxyphenyl) xanthene-9-yl), 4-decyloxytrityl, 4-hexadecyloxytrityl,
9- (4-octadecyloxyphenyl) xanthene-9-yl, 1,1-bis-
(4-Methoxyphenyl) -1'-pyrenylmethyl, p-phenylazophenyloxycarbonyl, 9-fluorenylmethoxycarbonyl, 2,4-dinitrophenylethoxycarbonyl, 4- (methoxylthioethoxy) butyryl, 2- (methoxylthio) The composition of claim 1 selected from the group consisting of ethoxymethyl) benzoyl, 2- (isopropylmethoxylthiomethoxymethyl) benzoyl, 2- (2,4-dinitrobenzenesulfonyloxymethyl) benzoyl, and levulinyl. . 11. The composition according to claim 1, wherein R ³ is 4,4′-dimethoxytrityl. 12. The composition of claim 1, comprising 1 to 4 molar equivalents of pyridine. 13. Aluminum chloride, bismuth (III) chloride, boron trifluoride, iron (II) chloride, iron (III) chloride, magnesium bromide, magnesium chloride,
Magnesium trifluoromethanesulfonate, manganese (II) chloride, zinc bromide,
Said Lewis acid selected from the group consisting of zinc chloride and zirconium (IV) chloride, 0-4 molar equivalents of pyridine, and R ¹ is diisopropylamino, R ² is 2-cyanoethoxy and R ³ is 4 , 4'-Dimethoxytrityl, comprising the compound of formula I.
The composition according to. 14. A method for the preparation of a compound of formula III as follows: Where B ¹ and B ² are independently selected from the group consisting of purine and pyrimidine bases, and R ² is selected from the group consisting of alkoxy, alkyl, alkylsulfonylalkoxy, arylsulfonylalkoxy, cyanoalkoxy, and haloalkoxy. , R ³ is a hydroxy protecting group, selected from the ^{group R 4} is hydrogen and -OR ^{7, R 8} is -OR ^7, is selected from the ^{group R 9} is consisting of hydrogen and -OR ^7, or R ⁸ and R ⁹ together form a methylenedioxy group, said method comprising a compound of formula IV: A method comprising treating the composition of claim 1 with. 15. The method of claim 14, wherein R ² is alkyl. 16. The method of claim 14, wherein R ² is cyanoalkoxy. 17. The method of claim 14, wherein R ² is 2-cyanoethoxy and the singular R ⁸ and R ⁹ form a methylenedioxy group. 18. R ² is 2-cyanoethoxy, R ³ is 4,4′-dimethoxytrityl, R ⁴ is hydrogen, and R ⁸ and R ^{9 which} are united are methylenedioxy. 15. The method of claim 14, wherein the group is formed. 19. A method of synthesizing an oligonucleotide, wherein a nucleoside phosphoramidite monomer precursor is activated by treatment with a Lewis acid to form an activated intermediate, and the activated intermediate is sequentially added to the oligonucleotide. A process improved by forming a product and using said Lewis acid as a phosphoramidite monomer activator. 20. The Lewis acid is aluminum chloride, bismuth chloride (III
), Boron trifluoride, iron (II) chloride, iron (III) chloride, magnesium bromide, magnesium chloride, magnesium trifluoromethanesulfonate, manganese chloride (II
), Zinc bromide, zinc chloride and zirconium (IV) chloride.