Indrajeet Sharma

Epimerization-Free Block Synthesis of Peptides from Thioacids and Amines with the Sanger and Mukaiyama Reagents

Highly activated thioesters formed by the rapid reaction of C-terminal thioacids derived from protected amino acids and peptides with the Sanger reagent and other electron-deficient aryl halides in the presence of a free amine immediately form a peptide bond with the amine. This essentially epimerization-free method was used for the 4+4 block synthesis of a hindered octapeptide (see scheme; Boc, Pbf, and Trt are protecting groups).

Influence of protecting groups on the reactivity and selectivity of glycosylation: chemistry of the 4,6-o-benzylidene protected mannopyranosyl donors and related species.

The genesis and development of the 4,6-O-benzylidene acetal method for the preparation of β-mannopyranosides are reviewed. Particular emphasis is placed on the influence of the various protecting groups on stereoselectivity and these effects are interpreted in the framework of a general mechanistic scheme invoking a series of solvent-separated and contact ion pairs in dynamic equilibrium with a covalent α-glycosyl trifluoromethanesulfonate.

Triblock Peptide and Peptide Thioester Synthesis With Reactivity-Differentiated Sulfonamides and Peptidyl Thioacids

As is the norm for a transformational new paradigm, Kent’s concept[1] of native chemical ligation enabling the block synthesis of peptides has been considerably extended and optimized since its introduction in 1994.[2] A significant number of these improvements have addressed the development of methods for peptidyl thioester synthesis and the limitations posed by the mechanistic requirement of a N-terminal 2-mercaptoethylamine, typically cysteine.[3] Perhaps the most important modification, however, was the introduction by Kent and co-workers[4] of the thiazolidine group as a means of protection for N-terminal cysteine moieties compatible with the native chemical ligation itself, and permitting the ligation of three or more peptides into a single entity. Our laboratory has been engaged in the development of an alternative method of block synthesis for peptides in which a C-terminal peptidyl thioacid reacts with an electron-deficient N-terminal sulfonamide to yield a native amide bond.[5] The mechanism of this reaction, which is not limited to the use of any particular amino acid, involves nucleophilic aromatic substitution by the thiocarboxylate on the electron deficient sulfonamide to give a highly reactive thioester and, after loss of sulfur dioxide, an amine leading ultimately to the amide product (Scheme 1). A variant on the method employs a free amine and an electron deficient aryl halide, such as the Sanger or Mukaiyama reagents, as the condensing agent in place of the sulfonamide.[6] Scheme 1 Amide Forming Reaction To convert this method into one capable of enabling the controlled coupling of three blocks into a single segment with minimal protecting group manipulation we required a set of two sulfonamides with differential reactivity toward thiocarboxylates (Scheme 2). Scheme 2 One Pot Triblock Peptide Synthesis A series of N-sulfonylphenylalanine derivatives were therefore prepared (Supporting Information) and screened for reactivity toward thioacetic acid under a standard set of conditions related to those used in our peptide synthesis (Table 1). Table 1 Reactivity of thioacetic acid towards N-sulfonylphenylalanine derivatives[a] Under the conditions employed a single electron-withdrawing group was found to be insufficient to induce reaction, as was the presence of two trifluoromethyl groups in the 3- and 5-positions. However, 2,4-disubstituted systems in which a single nitro group was complemented by a second, but less potent electron-withdrawing group functioned nicely (Table 1). Nevertheless, all three such systems investigated proved significantly less reactive than the 2,4-dinitrobenzenesulfonamide employed originally, and therefore met our reactivity criteria. A further series of experiments with more elaborate thioesters revealed the reactivity of both the CNS and ENS sulfonamides to be adequate for coupling with primary thioacids but not with electron-deficient or more hindered ones. Thus, while both the CNS and ENS class of sulphonamides were amenable to reaction with primary thioacids (Schemes 3 and ​and4)4) they either failed to react or reacted only very slowly with peptide-based thioacids such as Alloc-Gly-Gly-SH. Scheme 3 Three Component Coupling Scheme 4 Reaction with Glutamic and Aspartic Acid Side Chain Thioacids This overall reactivity pattern was sufficient to enable a first series of triply convergent reactions in which an amino acid or peptide protected at the N-terminus by an ENS or CNS group and carrying a 2,4,6-trimethoxybenzyl[7] (Tmob) thioester at the C-terminus was first treated with triethylsilane and trifluoroacetic acid to release the C-terminal thioacid before exposure to a DNS protected peptide and a mild base (Table 2). This first coupling resulted in the formation of a new peptide bearing the ENS or CNS group at the N-terminus ready for a final coupling with a thioacid, albeit necessarily a primary one (Table 2). This critical series of experiments established the feasibility of generation of a thioacid in the presence of a moderately electron-deficient sulfonamide and the ability of that thioacid to undergo subsequent and selective condensation with the more reactive DNS-class of sulfonamides. The reaction sequence was applied successfully to the synthesis of simple di- and tripeptides (Table 2) and to the synthesis of model octapeptides (Table 2, entry 5 and 6). Finally, the sequence was shown to be ammenable to the use of aqueous buffered media, rather than DMF as solvent, with little loss of yield as is clear from a comparison of entries 4 and 7 of Table 2.[8] Table 2 Triply convergent synthesis: Further investigation revealed the FNS group to be somewhat more reactive than the CNS and ENS groups toward the less reactive α-amino-derived thiocarboxylates as illustrated by the examples in Scheme 5. A critical point in the use of the FNS group in this manner, however, was the switch from cesium carbonate to cesium bicarbonate at the level of the first coupling reaction.[9] Scheme 5 Tricomponent Couplings Employing the FNS Group The block synthesis strategy that we present here is complementary to the methods developed by Kent based on native chemical ligation. However, for maximum flexibility in approaching future targets the ability to incorporate both approaches into a single, harmonious scheme is desirable. For this the compatibility of thioesters with our thiocarboxylate-sulfonamide coupling approach is required. The triblock synthesis set out in Scheme 6, in which a fluorenylmethyl (Fm)[5a] thioester is carried through two coupling steps nicely illustrates that such is the case. Scheme 6 Comatibility of the Thioesters with the Thioacid-Sulfonamide Block Synthesis In addition to the right to left strategy for block peptide synthesis set out above, with its necessary reliance on the use of a series of sulfonamides of decreasing reactivity, we have briefly investigated an alternative left to right strategy. This approach enables the more reactive DNS sulphonamide to be employed in all coupling steps but requires the compatibility of the Tmob thioester function with the sulphonamide coupling reaction. The triblock syntheses set out in Scheme 7 show such a sequence and thereby establish the compatibility of the Tmob thioester. Again with a view to potential interconnection with a native chemical ligation strategy one of the examples is terminated by the incorporation of a further thioester. Scheme 7 Left to Right Strategy Showing Compatibility with Thioesters Overall, we present a combination of powerful new methods for the block synthesis of peptides based on the reaction of thioacids with electron-deficient sulfonamides. The assembly of the various blocks may be conducted in a right to left or left to right manner and may be arranged in such a way as to provide a peptide thioester ready for incorporation in a native chemical ligation sequence.

Influence of the O3 protecting group on stereoselectivity in the preparation of C-mannopyranosides with 4,6-O-benzylidene protected donors.

α-C-Glucopyranosides and mannopyranosides are obtained in 65-85% yields from 4,6-O-benzylidene-protected glucosyl and mannosyl thioglycosides bearing ester functionality at the 3-O-position by a coupling reaction with C-nucleophiles on activation with diphenyl sulfoxide, 2,4,6-tri-tert-butylpyrimidine, and trifluoromethanesulfonic anhydride.

Stable Analogues of OSB‐AMP: Potent Inhibitors of MenE, the o‐Succinylbenzoate‐CoA Synthetase from Bacterial Menaquinone Biosynthesis

MenE, the o‐succinylbenzoate (OSB)‐CoA synthetase from bacterial menaquinone biosynthesis, is a promising new antibacterial target. Sulfonyladenosine analogues of the cognate reaction intermediate, OSB‐AMP, have been developed as inhibitors of the MenE enzymes from Mycobacterium tuberculosis (mtMenE), Staphylococcus aureus (saMenE) and Escherichia coli (ecMenE). Both a free carboxylate and a ketone moiety on the OSB side chain are required for potent inhibitory activity. OSB‐AMS (4) is a competitive inhibitor of mtMenE with respect to ATP (Ki=5.4±0.1 nM) and a noncompetitive inhibitor with respect to OSB (Ki=11.2±0.9 nM). These data are consistent with a Bi Uni Uni Bi Ping‐Pong kinetic mechanism for these enzymes. In addition, OSB‐AMS inhibits saMenE with

Is donor-acceptor hydrogen bonding necessary for 4,6-O-benzylidene-directed β-mannopyranosylation? stereoselective synthesis of β-C- mannopyranosides and α-C-glucopyranosides

{K{{{\rm app}\hfill \atop {\rm i}\hfill}}}

Drug discovery: Diversifying complexity

=22±8 nM and ecMenE with

Direct Fmoc-Chemistry-Based Solid Phase Synthesis of Peptidyl Thioesters

{K{{{\rm OSB}\hfill \atop {\rm i}\hfill}}}

Solvent-dependent divergent functions of Sc(OTf)3 in stereoselective epoxide-opening spiroketalizations

=128±5 nM. Putative active‐site residues, Arg222, which may interact with the OSB aromatic carboxylate, and Ser302, which may bind the OSB ketone oxygen, have been identified through computational docking of OSB‐AMP with the unliganded crystal structure of saMenE. A pH‐dependent interconversion of the free keto acid and lactol forms of the inhibitors is also described, along with implications for inhibitor design.

Three-component coupling approach for the synthesis of diverse heterocycles utilizing reactive nitrilium trapping

2,3-Di-O-benzyl-4,6-O-benzylidene-thiohexopyranosides, on activation with 1-benzenesulfinyl piperidine and triflic anhydride, react with allyl silanes and stannanes, and with silyl enolethers to give C-glycosides. In the mannose series the beta-isomers are formed selectively whereas the glucose series provides the alpha-anomers. This selectivity pattern parallels that of O-glycoside formation and eliminates the need to consider donor-acceptor hydrogen bonding in the formation of the O-glycosides.