Removal of boronic acid protecting groups by transesterification

A method for the preparation of a compound of formula (II)

• R.sup.1 -X.sub.n --NHCH(R.sup.2)-B(OH).sub.2 (II)
• wherein
• R.sup.1 is
• a) hydrogen,
• b) an N-terminal protecting group,
• c) --SO.sub.2 (CH.sub.2).sub.m -aryl, wherein aryl is phenyl, napthyl or biphenyl substituted with one, two or three substituents selected from the group consisting of halo (F, Cl, Br, I,), --CN, C1-C10-alkyl, C3-C8-cycloalkyl, C2-C10-alkenyl, C2-C10-alkynyl, -OR.sup.7, --NO.sub.2, --CF.sub.3, --S(O).sub.r R.sup.8, --NR.sup.6 R.sup.7, --COR.sup.7, --CO.sub.2 R.sup.7, --CONR.sup.6 R.sup.7 ;
• X is a peptide of 1-20 amino acids;
• R.sup.2 is
• a) C1-C10-alkyl,
• b) C2-C10-alkyl-Y,
• c) --(CH.sub.2).sub.n -aryl, wherein aryl is as defined above;
• Y is
• a) --NHC(NH)NH.sub.2,
• b) --NH.sub.2,
• c) --SC(NH)NH.sub.2,
• d) --OR.sup.9,
• e) --SR.sup.9 ;
• R.sup.5 and R.sup.6 are independently
• a) H,
• b) C1-C8-alkyl,
• c) C1-C8-alkoxy,
• d) C3-C8-cycloalkyl,
• e) --CO.sub.2 R.sup.7,
• f) --(CH.sub.2).sub.m -phenyl;
• R.sup.7 is
• a) H,
• b) phenyl,
• c) benzyl,
• d) C1-C8-alkyl;
• R.sup.8 is
• a) phenyl,
• b) C1-C4-alkyl,
• c) C1-C4-alkoxy,
• d) --CF.sub.3 ;
• R.sup.9 is
• a) H,
• b) C1-C2-alkyl,
• c) phenyl or phenyl optionally substituted with a substituent selected from the group consisting of halo (F, Cl, Br, I), --CN, C1-C10-alkyl, C3-C8-cycloalkyl, C2-C10-alkenyl, C2-C10-alkynyl, --OR.sup.7, --NO.sub.2, --CF.sub.3, --S(O).sub.r R.sup.8, --NR.sup.6 R.sup.7, --COR.sup.7, --COR.sup.7, --CO.sub.2 R.sup.7, --CONR.sup.6 R.sup.7 ;
• n is 0or 1;
• m is 0 to 2;
• r is 0 to 2;
• comprising suspending a compound of the formula
• R.sup.1 -X.sub.n --NHCH (R.sup.2)--BR.sup.3 R.sup.4 (I)
• wherein R.sup.1, R.sup.2, X, Y, R.sup.5, R.sup.6, R.sup.7, R.sup.8, R.sup.9, n, m and r are as defined above;
• and
• R.sup.3 and R.sup.4 are
• a) C1-C8-alkoxy, or
• b) when taken together R.sup.3 and R.sup.4 form a cyclic boronic ester where said chain or ring contains from 2 to 20 carbon atoms and, optionally, a heteroatom selected from the group consisting of N, S, or O,
• in a mixture of water and a water-immiscible organic solvent in which said compound of formula (II) is less soluble than in water, said mixture containing an organic boronic acid acceptor which is more soluble in the water-immiscible organic solvent than in water and is selected from the group consisting of phenyl boronic acid, butyl boronic acid, pentyl boronic acid, hexyl boronic acid or cyclohexyl boronic acid, said organic boronic acid acceptor being present in a amount equal to at least 1 equivalent of said compound of formula (I), wherein the amount of water present in said mixture is sufficient to completely dissolve the compound of formula (II) formed and the amount of water-immiscible organic solvent present in said mixture is sufficient to completely dissolve said organic boronic acid acceptor and the boronic ester reaction product formed;
• stirring the resulting reaction mixture for approximately one hour before allowing the reaction mixture to separate into two distinct phases;
• separating the phases;
• and recovering the compound of formula (II) from the separated aqueous phase.