In this study, we successfully synthesized several kinds of P-modified nucleic acids from boranophosphate DNAs via an acyl phosphite intermediate in solution and on a solid support. In the solution-phase synthesis, phosphorothioate diester, phosphotriester, and phosphoramidate diester were synthesized in a one-pot reaction from boranophosphodiester via the conversion of an acyl phosphite as a key intermediate. In addition, doubly P-modified nucleic acid derivatives which were difficult to synthesize by the phosphoramidite and H-phosphonate methods were also obtained by the conversion reaction. In the solid-phase synthesis, a boranophosphate derivative was synthesized on a solid support using the H-boranophosphonate method. Then, an acyl phosphite intermediate was formed by treatment with pivaloyl chloride in pyridine, followed by appropriate transformations to obtain the P-modified derivatives such as phosphotriester and phosphorothioate diester. Notably, it was suggested that the conversion reaction of a boranophosphate to a phosphorothioate diester proceeded with retention of the stereochemistry of the phosphorous center. In addition, a phosphorothioate/phosphate chimeric dodecamer was successfully synthesized from a boranophosphate/phosphate chimeric dodecamer using the same strategy. Therefore, boranophosphate derivatives are versatile precursors for the synthesis of P-modified DNA, including chimeric derivatives.