Keto amide (XXXI) was obtained by acylation of L-proline benzyl ester (XII) with pyruvic acid (XXX). Subsequent hydrogenolysis of the benzyl ester group provided pyruvyl proline (XXXII). The target dehydrodidemnin B was then prepared by coupling of didemnin A (XXIX), from either natural or synthetic sources, with pyruvyl proline (XXXII)
In a related semisynthetic strategy, didemnin A (XXIX) was first acylated by N-Boc-L-proline (XXXIII) yielding amide (XXXIV), which was further deprotected by treatment with HCl in EtOAc. The resultant prolyl didemnin A (XXXV) was then coupled with pyruvic acid (XXX) to furnish the title compound
A modification of the previous synthetic methods was further reported. The protected isostatine (XI) was prepared by a related procedure utilizing benzyl ester protected intermediates instead of the methyl ester analogues. After activation of N-Boc-D-allo-isoleucine (VII) with carbonyl diimidazole, condensation with the lithium enolate of benzyl acetate yielded keto ester (LIV). Reduction of the keto group of (LIV), followed by silylation of the resultant alcohol (LV) and benzyl group hydrogenolysis led to the amino hydroxy acid derivative (XI)
Similarly, (S)-2-hydroxy-3-methylbutyric acid (LV) was protected as the silyl ether (LVI) and then condensed with benzyl acetate, via activation with CDI, to afford keto ester (LVII). Subsequent methylation of (LVII) using iodomethane and NaH provided (XVI) as a diastereomeric mixture. The unstable keto acid (XVII), obtained in situ by catalytic hydrogenolysis of (XVI), was then coupled to the dipeptide ester (XV) to afford, after desilylation, the hydroxy keto amide (XIX). Esterification of alcohol (XIX) with the isostatine derivative (XI) as in Scheme 3, followed by acidic cleavage of the O-silyl and N-Boc protecting groups led to depsipeptide (XXI)
N-Boc-L-Threonine (LVI) was protected as the corresponding phenacyl ester (LVII) by treatment with 2-bromoacetophenone and Et3N. Esterification of (LVII) with the modified tyrosine building block (II) yielded the protected didepsipeptide (LVIII). Removal of the phenacyl ester group of (LVIII) under reductive conditions provided the carboxylic acid (VI). Coupling of acid (VI) with the depsipeptide segment (XXI), followed by deprotection by hydrogenolysis furnished the linear precursor (XXIII). Macrocyclization of (XXIII) using HATU and subsequent acidic Boc group cleavage led to the macrocyclic amine (XXV)
N-Benzyloxycarbonyl-N-methyl-D-leucine (LX) was prepared by methylation of the N-protected D-leucine (LIX) with iodomethane in the presence of NaH. Acylation of macrocyclic amine (XXV) with the N-protected aminoacid (LX) furnished the N-carbobenzoxy didemnin A (LXI). The title compound was then obtained by hydrogenolysis of the N-carbobenzoxy group of (LXI), followed by acylation with pyruvyl proline (XXXII) by means of diisopropyl carbodiimide
The modified tyrosine building block (II) was prepared by alkylation of N-Cbz-L-tyrosine (I) at both the phenolic OH and the NH groups employing dimethyl sulfate under phase-transfer conditions. N-Boc-L-Threonine benzyl ether (III) was converted to the corresponding (triethylsilyl)ethoxymethyl ester, and the O-benzyl group was subsequently removed by catalytic hydrogenolysis to afford (IV). Coupling between the tyrosine (II) and threonine (IV) derivatives using DCC and DMAP gave rise to the protected didepsipeptide (V). The SEM protecting group was then cleaved by means of HF in acetonitrile, yielding intermediate (VI)
Activation of N-Boc-D-allo-isoleucine (VII) with carbonyl diimidazole, followed by condensation with the lithium enolate of methyl acetate provided keto ester (VIII). Keto group reduction in (VIII) by means of KBH4 led to the isostatine derivative (IX), which was further hydrolyzed to acid (X) with NaOH in aqueous dioxane. Silylation of hydroxy acid (X) at both the alcohol and carboxyl groups employing tert-butyldimethylsilyl chloride and imidazole, followed by selective desilylation of the carboxyl group with NaOH afforded the protected amino hydroxy acid (XI)
The protected dipeptide (XIV) was prepared by coupling between L-proline benzyl ester (XII) and N-Boc-L-leucine (XIII) in the presence of DCC and HOBt. Acidic Boc group cleavage in (XIV) then gave amine (XV). The unstable keto acid (XVII) was obtained by hydrogenolysis of the epimeric benzyl esters (XVI), and subsequently coupled to the dipeptide ester (XV) by means of HBTU to afford the keto amide (XVIII) as a diastereomeric mixture. Desilylation of (XVIII) with tetrabutylammonium fluoride furnished the deprotected alcohol (XIX), which was further esterified with the protected isostatine (XI) leading to depsipeptide (XX). Then, removal of the O-silyl and N-Boc protecting groups of (XX) under acidic conditions provided intermediate (XXI)
Coupling between segments (VI) and (XXI) by means of HBTU gave rise to the depsipeptide (XXII). Simultaneous deprotection of the C- and N-terminal functions of (XXII) by hydrogenolysis led to the linear precursor (XXIII), which was then cyclized to (XXIV) in the presence of HATU and HOAt
Removal of the N-Boc protecting group of (XXIV) by means of HCl in dioxane generated the macrocyclic amine (XXV). The protected aminoacid (XXVII) was prepared by treatment of D-leucine (XXVI) with Boc2O, followed by N-methylation under phase-transfer conditions. Coupling of N-Boc-N-methyl-D-leucine (XXVII) with amine (XXV) provided amide (XXVIII). Then, acidic Boc group cleavage in (XXVIII) furnished didemnin A (XXIX)
In an alternative synthesis, the previously reported trichloroethyl ester (XXXVI) was deprotected by treatment with zinc dust in the presence of ammonium acetate yielding acid (XXXVII). This was then coupled to proline trichloroethyl ester (XXXVIII), prepared from N-Boc-L-proline (XXXIII), to furnish amide (XXXIX). Subsequent hydrogenolysis of the O-benzyl group of (XXXIX) provided intermediate (XL)
For the synthesis of the other peptidic moiety (L), removal of the N-Boc group from the isostatine derivative (XLI) by means of trifluoroacetic acid gave amino ester (XLII), which was coupled with N-Boc-L-threonine benzyl ether (III) to afford dipeptide (XLIII). Boc group removal in (XLIII) with HCl provided peptide (XLIV), which was coupled with N-Boc-N-methyl-D-leucine (XXVII) to yield tripeptide (XLV). Saponification of the methyl ester group of (XLV) with NaOH provided acid (XLVI). Sequential protection of the alcohol hydroxyl of (XLVI) as the tert-butyldimethylsilyl ether, and esterification of the carboxyl group with trichloroethanol led to the fully protected tripeptide (XLVII). The benzyl ether group of (XLVII) was then removed by catalytic hydrogenolysis to furnish alcohol (XLVIII)
Esterification of the peptide alcohol (XLVIII) with the modified tyrosine derivative (II) afforded depsipeptide (XLIX). After reductive cleavage of the trichloroethyl ester of (XLIX), using zinc dust and ammonium acetate, the resultant carboxylic acid (L) was coupled to the alcohol segment (XL) in the presence of DIC and DMAP稢F3COOH to furnish the linear precursor depsipeptide (LI)
Sequential deprotection of the trichloroethyl ester and the N-benzyloxycarbonyl groups of (LI) led to (LII) and the linear aminoacid (LIII), which underwent macrocyclization to the N-Boc didemnin A (XXVIII) upon treatment with HATU and HOAt. The title compound was then obtained by acidic Boc group deprotection, followed by coupling with pyruvyl proline as outlined in Schemes 23437701e and 23437701f