US 8,138,330 B2  55  56  Those skilled in the art will appreciate that numerous  changes and modifications may be made to the preferred  embodiments of the present invention and that such changes  and modifications may be made without departing from the  spirit of the invention.  It  is therefore intended that the  appended claims cover all such equivalent variations as fall  within the spirit and scope of the invention.  Notably, throughout this application various citations are  provided. Each citation is specifically incorporated herein by  reference in its entirety.  Those skilled in the art will appreciate that numerous  changes and modifications may be made to the preferred  embodiments of the invention and that such changes and  modifications may be made without departing from the spirit  of the invention.  It  is therefore intended that the appended 15  claims cover all such equivalent variations as fall within the  true spirit and scope of the invention.  What is claimed is:  1.  A method for the solid phase synthesis of oligonucle› otides, wherein each synthesis cycle of the method com- prises:  a) cleaving an acyl front end protective group from a  nucleoside immobilized on a solid support to liberate a  hydroxyl group on the nucleoside, the acyl front end  protective group comprising either an electron with› drawing group or a group capable of neighboring group  participation in close proximity to its carbonyl group  such that the acyl group is cleavable in ten minutes or  less at room temperature by a primary amine, a second› ary amine, or a mixture thereof; and  b) coupling an acyl protected nucleoside phosphoramidite  with the hydroxyl group of the nucleoside immobilized  on the solid support.  2.  The method of claim  1,  wherein the acyl protective group  is selected from the group consisting of an oxalyl monoester  group, an ortho-phenoxycarbonyl benzoyl group, and a  cyclohexyloxydicarbonyl group.  3.  The method of claim  2,  wherein the ortho-phenoxycar› bonyl benzoyl group is selected from the group consisting of  a 2-(4-nitrophenoxy)carbonyl benzoyl group and a 2-(2-cy- anophenoxy)carbonyl benzoyl group.  4.  The method of claim  1,  wherein the acyl protective group  is attached to the 5’-hydroxyl function of the nucleoside.  5.  The method of claim  1,  wherein the acyl protective group  is attached to the 3’-hydroxyl function of the nucleoside.  6.  The method of claim  1,  wherein the cleaving reagent  comprises a primary amine.  7.  The method of claim  6,  wherein the primary amine is  selected from the group consisting ofn-butylamine, n-hexy› lamine, 2-methoxyethylamine, and 2-(N,N-diethylamino)  ethylamine.  8.  The method of claim  1,  wherein the cleavage of the front  end protective group occurs in 1 minute or less at room  temperature.  *  *  *  *  *  23.  The nucleoside phosphoramidite of claim  22,  wherein  the acid-labile 2’-protective group a 4-methoxytetrahydro› pyranyl group.  22.  The nucleoside phosphoramidite of claim  15,  wherein  the nucleoside is a ribonucleoside and the 2’-hydroxyl func› tion of the ribonucleoside is protected with an acid-labile  2’-protective group.  21.  The nucleoside phosphoramidite of claim  15,  wherein  the base moiety of the nucleoside phosphoramidite is pro- tected with an acid-labile nucleobase protective group.  20.  The nucleoside phosphoramidite of claim  15,  wherein  the phosphoramidite group comprises a ~-cyanoethyl phos› phate protective group.  19.  The nucleoside phosphoramidite of claim  15,  wherein  the acyl front end protective group is a cyclohexyloxydicar› bonyl group.  18.  The nucleoside phosphoramidite of claim  17,  wherein  the ortho-phenoxycarbonyl benzoyl group is selected from  the group consisting of a 2-(4-nitrophenoxy)carbonyl ben- zoyl group and a 2-(2-cyanophenoxy)carbonyl benzoyl  group.  17.  The nucleoside phosphoramidite of claim  15,  wherein  the acyl front end protective group is an ortho-phenoxycar› bonyl benzoylgroup.  16.  The nucleoside phosphoramidite of claim  15,  wherein  the acyl front end protective group is an oxalyl monoester  group.  15.  Anucleoside phosphoramidite comprising an acyl front  end protective group and a phosphoramidite group, wherein  the acyl front end protective group comprises either an elec› tron withdrawing group or a group capable of neighboring  group participation in close proximity to its carbonyl group  such that the acyl group is cleavable with a primary amine, a  secondary amine, or a mixture thereof in 10 minutes or less at  room temperature.  14.  The method of claim  1,  wherein the nucleoside phos› phoramidite is a ribonucleoside and the 2’-hydroxyl function  of the ribonucleoside is protected with an acid-labile 2’-pro› tective group.  13.  The method of claim  12,  wherein the aqueous acidic  reagent has a pH in the range of pH 2 to pH  4.  12.  The method of claim  11,  wherein the acidic reagent is  aqueous.  11.  The method of claim  10,  wherein, after synthesis of the  oligonucleotide, the nucleobase protective groups of the oli› gonucleotide are removed with an acidic reagent.  10.  The method of claim  1,  wherein the base moiety of the  nucleoside phosphoramidite is either unprotected or pro› tected with an acid-labile nucleobase protective group.  9.  The method of claim  1,  wherein the first nucleoside of  the oligonucleotide to be synthesized is attached to the solid  support via an acid-labile linker. 