Protection of the carboxyl group of Boc-L-threonine (I) with [2-(chloromethoxy)ethyl]trimethylsilane (SEM-Cl) (II) by means of Li2CO3 affords protected threonine (III). Separately, Z-L-tyrosine (IV) is treated with dimethyl sulfate and KOH in THF with tetrabutylammonium hydrogen sulfate as a catalyst to furnish N,O-dimethyl-L-tyrosine (V), which is then converted into secondary amine (VI) by coupling with protected threonine (III) either by means of 2,4,6-trichlorobenzoyl chloride in THF and DMAP in benzene or with isopropenyl chloroformate, Et3N and DMAP, followed by elimination of the carbobenzyloxy group by hydrogenation over Pd/C. Coupling of Z-L-leucine (IX) to L-proline methyl ester (X) by means of DCC, HOBt and NMM in CH2Cl2, followed by hydrolysis with LiOH, affords Z-leucylproline (VII), which is then coupled to amine (VI) by means of BOPCl and Et3N in CH2Cl2, and then subjected to deprotection with hydrofluoric acid in acetonitrile affording intermediate (VIII).
Aldol condensation between (S)-2-(benzyloxy)-3-methylbutanal (XI) and thioacetal (XII) by means of LDA, tert-butyldimethylsilyl chloride and tin tetrachloride in THF/HMPA gives syn diastereomer (XIII), which is then reduced with LiAlH4 in ether to provide diol (XIV). Protection of the primary hydroxyl group of (XIV) with tert-butyldimethylsilyl chloride in the presence of DMAP and Et3N, followed by reaction with chloride (XV) and DIEA in CH2Cl2, affords protected derivative (XVI), whose benzyl group is then removed by treatment with Na/NH3. The resulting alcohol (XVII) is acetylated with acetic anhydride in CH2Cl2 in the presence of DMAP to give acetate (XVIII). Treatment of D-alloisoleucine (XX) with N-[[(benzyloxy)carbonyl]oxy]-5-norbornene-2,3-dicarboximide and Et3N in THF/H2O, followed by reaction with 1,1'-carbonyldiimidazole (CDI), provides the corresponding imidazolide (XIX), which is then condensed with derivative (XVIII) to yield beta-keto ester (XXI). Reduction of (XXI) with KBH4 in EtOH, followed by protection with triiospropylsilyl triflate, affords a mixture of two diastereomers from which compound (XXII) is cromatographically isolated. Removal of the carbobenzyloxy group of (XXII) by hydrogenation over Pd/C gives amino derivative (XXIII), which is then coupled to intermediate (VIII) by means of isopropenyl chloroformate and NMM in THF to furnish linear peptide (XXIV).
Removal of the tert-butyldimethylsilyl group of (XXIV) by means of HOAc/THF/H2O yields alcohol (XXV), which is then converted into carboxylic acid (XXVI) by first Swern oxidation with trifluoroacetic anhydride, Et3N and DMSO, followed by reaction with KMnO4 in tert-butyl alcohol with sodium hydrogen phosphate. Cyclization of linear peptide (XXVI) is then performed by first removal of the Z protecting group by hydrogenation over Pd/C in MeOH/AcOEt, followed by treatment of the amine with diphenylphosphoryl azide (DPPA) and NaHCO3 in DMF, yielding protected macrolide (XXVII). Removal of the MOM protecting group of (XXVII) by means of dimethylboron bromide in CH2Cl2 affords compound (XXVIII), which is converted into intermediate (XXIX) by Swern oxidation with trifluoroacetic anhydride, Et3N and DMSO, followed by Boc removal with HCl.
Protection of ethyl lactate (XXX) with tert-butyldimethylsilyl chloride in DMF in the presence of imidazole provides derivative (XXXI), which is then converted into dipeptide (XXXII) by first hydrolysis with LiOH in THF/H2O followed by coupling with L-proline methyl ester (X) by means of DCC, HOBt and NMM. Next, hydrolysis of the methyl ester of (XXXII) with LiOH in THF/H2O yields carboxylic acid (XXXIII). Esterification of Z-D-N-Me-Leu (XXXIV) with 2-(trimethylsilyl)ethanol (XXXV) by means of EDCI, DMAP and Et3N in DMF furnishes leucine derivative (XXXVI), whose Z protecting group is removed by hydrogenation over Pd/C in MeOH/AcOEt to afford secondary amine (XXXVII). Coupling of carboxylic acid (XXXIII) with amine (XXXVII) is then performed by means of BOPCl, Et3N. Treatment with tetrabutylammonium fluoride in DMF allows protecting group removal to provide carboxylic acid derivative (XXXVIII), which is then attached to macrocycle (XXIX) by means of BOP and NMM in CH2Cl2 to furnish the target didemnin.
Activation of Boc-allo-isoleucine acid (I) with CDI in THF followed by reaction with ethyl lithioacetate (II) in THF, affords beta-keto ester (III) (alternatively, (III) can be obtained as follows: formation of the corresponding methyl ester of (I) by reaction with MeI and KHCO3 in DMF, followed by reduction with NaBH4 and LiCl in THF/EtOH and subsequent Swern oxidation with pyr:SO3, Et3N and DMSO provides aldehyde (IV), which is then condensed to ethyl lithioacetate (II) in THF and finally oxidized with PDC in CH2Cl2/HOAc). Stereospecific reduction of beta-keto ester (III) with NaBH4 in EtOH affords beta-hydroxy ester (V) (1-3), which is first saponified with NaOH in EtOH and then esterified with trichloroethanol (VI) by means of DCC and DMAP in CH2Cl2, furnishing ester (VII). Removal of the Boc group of (VII) by means of HCl/dioxane yields amine derivative (VIII), which is then coupled to Boc-Thr(Bzl)-OH (IX) by means of diethyl phosphorocyanidate (DEPC) and DIEA in DMF and treated with HCl for Boc removal, allowing isolation of dipeptide (X). Coupling of (X) with Boc-MeLeu-OH (XI) -- obtained in turn by methylation of Boc-Leu-OH (XII) with MeI and NaH in THF -- with DEPC and Et3N in DMF, followed by O-protection with TBDMSCl and imidazole in DMF, provides tripeptide (XIII). Removal of the benzyl group of (XIII) by hydrogenation over Pd/C in THF affords Boc-Me-Leu-Thr-Ist(TBDMS)-OTce (XIV), which is then coupled with Z-Me-Tyr(Me)-OH (XV) by means of DMAP and DCC in CH2Cl2 to furnish depsipeptide (XVII) (protected amino acid Z-Me-Tyr(Me)-OH (XV) can be obtained in turn by methylation of Z-Tyr-OH (XVI) with MeI and NaH in THF). Deprotection of (XVII) by first treatment with Zn in THF in the presence of NH4OAc yields fragment (XVIII).
Protection of 2-hydroxyisovaleric acid (XIX) with tert-butyldimethylsilyl chloride and imidazole in DMF, followed by condensation with Meldrum's acid (XX) by use of diethyl phosphorocyanidate (DEPC) and Et3N in THF, furnishes derivative (XXI), which is then subjected to reaction with benzyl alcohol (XXII) in refluxing benzene to afford beta-keto ester (XXIII). Next, derivative (XXIII) is methylated with MeI and NaH and its benzyl group is removed by hydrogenation over Pd/C in THF to yield carboxylic acid (XXIV). Coupling between H-Pro-OBzl.HCl (XXV) and Boc-Leu-OH (XXVI) by means of DEPC and Et3N in DMF affords dipeptide (XXVII), which is then converted into fragment (XXVIII) by first condensation with carboxylic acid (XXIV) by means of DCC, HOBt and NMM in THF/DMF and treatment with tetrabutylammonium fluoride for TBDMS protecting group removal. Coupling of fragment (XXVIII) with fragment (XVIII) by means of DCC and DMAP in CH2Cl2 furnishes linear peptide (XXIX), which is hydrogenated over Pd/C in THF and then subjected to cyclization by means of BOPCl and Et3N in CH2Cl2 to give macrolide (XXX). Next, deprotection of (XXX) with TMSOTf affords intermediate (XXXI) (didemnin A).
Treatment of ethyl lactate (XXXII) with benzyl bromide and silver oxide in DMF gives Bzl-Lac-OH (XXXIII), which is then coupled with H-Pro-OMe (XXXIV) by means of DEPC and then saponified with NaOH in MeOH to yield derivative (XXXV). Finally, didemnin B is obtained by coupling of intermediate (XXXI) with (XXXV) by means of BOPCl and Et3N, followed by debenzylation with HCO2H and Pd in MeOH.
Coupling of benzyloxy isovalery propionic acid (Bzl-Hip-OH) (I) with leucine trimethylsilylethyl ester (II) by means of HOBt and DCC followed by hydrogenation over Pd/C in isopropanol for benzyl protecting group removal provides dipeptide (III), which is then converted into compound (V) by first coupling to Boc-Sta(TBDMS)-OH (IV) by means of DMAP and DCC followed by removal of the TMSe group by means of TBAF. Coupling between Z-D-N-MeLeu-OH (VI) and H-Thr-OTMSe (VII) by means of N-hydroxysuccinimide (NHS) and EDC yields protected dipeptide (VIII), which is then condensed with dipeptide (IX) by means of DMAP and DCC and then treated with HCl in EtOAc for Boc removal to afford tetrapeptide (X) (in turn, dipeptide (IX) can be obtained by coupling between Boc-Pro-OH (XI) and Me2Tyr-ONb (XII) by means of EDC followed by p-nitrobenzyl removal by hydrogenation over Pd/C in HOAc). Coupling of tetrapeptide (X) with compound (V) by means of EDC, HOBt leads to formation of linear peptide (XIII), which is partially deprotected by treatment with TBAF and TFA and then cyclized by means of HOBt, EDC and NMM, providing macrolide (XIV).
Removal of the Z protecting group of (XIV) by hydrogenation over Pd/C gives secondary amine (XV), which is then condensed with carboxylic acid derivative (XVI) by means of EDC to furnish benzyl-protected didemnin B (XX) (in turn, synthesis of (XVI) can be performed by coupling of O-benzyl lactyl azide (obtained from ethyl O-benzyl lactate (XVII)) with Pro-OMe (XVIII) to afford methyl ester (XIX), followed by saponification of the methyl ester with NaOH). Finally, didemnin B is obtained by hydrogenation of (XX) over Pd/C for benzyl group removal.