An efficient, stereocontrolled synthesis of a potent omuralide-salinosporin hybrid for selective proteasome inhibition

A short and highly stereocontrolled synthesis of the potent proteasome inhibitor 3 from the (S)-threonine-derived oxazoline 4 has been developed. The synthetic sequence is summarized in Scheme 1. Aldol coupling of the zinc enolate of 4 with isobutyraldehyde and subsequent silylation provided the TBS ether 5 diastereoselectively (10:1). Reductive cleavage of the oxazoline ring of 5 followed by Swern oxidation of the resulting amino alcohol afforded amino ketone 6, converted further by N-acylation to the acrylamide 7, whose structure was confirmed by X-ray crystallographic analysis. Acrylamide 7 was cyclized to 8 by a novel application of the Kulinkovich Ti(II)-cyclopentene complex. Silylation of 8 to 9 and radical cyclization at low temperature produced the bicyclic lactam 10 with complete control of all stereocenters. Hydroxy desilylation and N-deprotection of 10 gave the dihydroxy ester 11, which was converted to 3 by a novel three-step sequence: (1) demethylation with [Me2AlTeMe]2, (2) combined beta-lactonization and chlorination, and (3) desilylation to effect cleavage of the TBS ether.     

Studies toward the synthesis of salinosporamide A, a potent proteasome inhibitor

An α-methylenepyrrolidinone bearing all the functionalities and relative configurations of an advanced intermediate in the synthesis of salinosporamide A and analogues has been synthesized from methyl pyroglutamate through regio- and stereoselective N-methylnitrone cycloaddition.     

Nitrone [2+3]-cycloadditions in stereocontrolled synthesis of a potent proteasome inhibitor: (-)-omuralide

A new stereocontrolled synthetic route to omuralide has been developed from methyl pyroglutamate. This route involves regio- and stereoselective N-methylnitrone 1,3-dipolar cycloadditions to appropriate pyrrolinones, beta-eliminations, and highly selective hydrogenations as the main steps.     

Stereoselective formal synthesis of the potent proteasome inhibitor: salinosporamide A

(2R,3S)-α-Methylenelactam 3, the key intermediate in Corey’s syntheses of salinosporamide A, has been synthesized from (S)-methyl 2-hydroxymethylpyroglutamate through chemoselective O-protection, regio- and stereoselective N-methylnitrone cycloaddition and quaternarization-elimination reactions as the main steps.     

Stereocontrolled synthesis of the northern part of potent proteasome inhibitor TMC-95A

[reaction: see text]. A protected version of the northern part of TMC-95A, a potent and selective proteasome inhibitor, was synthesized with full stereochemical control. Highlights of this synthesis include (i) a (Z)-selective Mizoroki-Heck reaction to construct the oxyindole portion, (ii) a diastereoselective epoxidation, (iii) a 6-endo selective epoxide opening by Boc carbonyl group to establish the stereochemistry of C6, and (iv) a 1,3-elimination reaction of the L-allo-threonine derivative under Mitsunobu conditions to afford the (Z)-1-propenylamine.     

A practical synthesis of a gamma-secretase inhibitor

A practical and scaleable synthesis of the gamma-secretase inhibitor 1 is reported. The inhibitor consists of a central trisubstituted cyclohexane core with appended propionic acid, 2,5-difluorophenyl, and 4-chlorophenylsulfonyl moieties. Two alternative synthetic strategies, proceeding by way of a common disubstituted cyclohexanone derivative 5, were studied. In the preferred route, conjugate reduction of acrylonitrile derivative 4 with L-Selectride configures the desired relative stereochemistry of the cyclohexane core with >99.9:0.1 dr. A second strategy, based on catalyst-controlled hydrogenation of racemic cyclohexene derivative 2, is more convergent but less diastereoselective (up to 75:25 dr). The common cyclohexanone intermediate 5 was constructed by a regioselective Diels-Alder condensation of a 1,1-disubstituted vinyl sulfone 6 with 2-trimethylsiloxybutadiene.     

Total synthesis of spiruchostatin A, a potent histone deacetylase inhibitor

The total synthesis of spiruchostatin A was accomplished, unambiguously confirming its structure. Key steps included the use of the Nagao thiazolidinethione auxiliary for a diastereoselective acetate aldol reaction and as an activated acylating agent for amide formation, and macrolactonization by the Yamaguchi protocol. Spiruchostatin A is shown to have biological activity similar to that of FK228, a potent histone deacetylase (HDAC) inhibitor in clinical trials. The spiruchostatin A analogue, epimeric at the beta-hydroxy acid, is inactive, highlighting the importance of stereochemistry at this position for interactions with HDACs.     

A practical synthesis of a COX-2-specific inhibitor

A number of synthetic strategies to the Cox-2 specific inhibitor 1 have been described. These studies have led to the identification of a novel pyridine construction using annulation of ketone 2 using a vinamidinium species 29 and ammonia in 97% assay yield. Three approaches to the synthesis of ketone 2 are described that allow for its preparation in large quantities in >65% overall yield from methyl 6-methylnicotinate.     

A concise,stereocontrolled synthesis of spectinomycin

A simple and concise synthesis of the antibiotic spectinomycin is described. The key step comprises a reaction cascade initiated by the β-selective 5-O-glycosylation of an N,N-blocked myo-1,3-inosadiamine 10 with a suitable actinospectosyl donor—the d-glucose-derived enol benzoate of 4,6-dideoxy-α-d-hex-3-enulosyl chloride 5—which is spontaneously followed by regio- and stereospecific cyclo-hemiketalization and a 3-O→2-O-benzoyl group migration to directly elaborate the cis–cisoid–trans-fused pyran–dioxane–cyclohexane framework of 1. The approach is flexible enough to be applied to other inosadiamines towards the generation of novel spectinomycin analogues.     

Total synthesis of thapsigargin, a potent SERCA pump inhibitor

The enantioselective total synthesis of thapsigargin, a potent, selective inhibitor of the Ca2+ pump SERCA, is described. Starting from ketoalcohol 8, key steps involve regioselective introduction of the internal olefin at C4-C5, judicious protecting group choice to allow chelation-controlled reduction at C3, and chemoselective introduction of the angelate ester function at C3-O. A selective esterification approach completes the total synthesis in a total of 42 steps and 0.61% overall yield (88.6% average yield per step). [reaction: see text].     

A concise, stereocontrolled total synthesis of rippertenol

The first total synthesis of the unique terpene rippertenol, a molecule with dense stereochemical complexity arrayed on a compact framework largely devoid of functional groups, is described. Key elements include orchestrated and unique applications of aldol condensations, Diels-Alder chemistry, and a ring expansion to advance a chiral starting material containing a single chiral center into the final target in a concise and diastereocontrolled manner.     

Asymmetric, stereocontrolled total synthesis of paraherquamide A

The first total synthesis of paraherquamide A, a potent anthelmintic agent isolated from various Penicillium sp. with promising activity against drug-resistant intestinal parasites, is reported. Key steps in this asymmetric, stereocontrolled total synthesis include a new enantioselective synthesis of alpha-alkylated-beta-hydroxyproline derivatives to access the substituted proline nucleus and a highly diastereoselective intramolecular S(N)2' cyclization to generate the core bicyclo[2.2.2]diazaoctane ring system.     

A stereocontrolled synthesis of delta-trans-tocotrienoloic acid

[reaction: see text] A concise stereoselective total synthesis of a naturally occurring polymerase beta inhibitor, delta-trans-tocotrienoloic acid (2), is described. The key step in the synthesis is an acid-catalyzed cyclodehydration reaction. Additionally, this report corrects a previously reported structural assignment, defines the absolute stereochemistry of 2, and defines key structural requirements for polymerase beta inhibition.     

First total synthesis of vialinin A, a novel and extremely potent inhibitor of TNF-alpha production

Vialinin A, a powerful inhibitor (IC50 90 pM) of TNF-alpha production, was synthesized from sesamol in 11 steps with 28% overall yield. The key reactions include a double Suzuki coupling of electron-rich aryl triflate with phenylboronic acid and an oxidative deprotection of bis-MOM ether. In addition, the related synthetic studies also suggest the necessity for structural revision of ganbajunin C, a positional isomer of vialinin A.     

New entry to convertible isocyanides for the Ugi reaction and its application to the stereocontrolled formal total synthesis of the proteasome inhibitor omuralide

The development of a new convertible isocyanide, indole-isocyanide, for ready access to pyroglutamic acids culminated in the formal total synthesis of the proteasome inhibitor omuralide featuring a stereocontrolled Ugi reaction. Indole-isocyanide was named after the facilitation of hydrolysis of the resulting 2-(2,2-dimethoxyethyl)anilide via an N-acylindole intermediate obtained by the Ugi multicomponent condensation reaction followed by the indole formation.     

Formal synthesis of fumonisin B1, a potent sphingolipid biosynthesis inhibitor

The formal total synthesis of the important toxin fumonisin B₁ is achieved from simple raw materials in a convergent manner. The key functionalities are derived from MacMillan α-hydroxylation, sharpless asymmetric dihydroxylation and ring-closing metathesis.     

A simple stereocontrolled synthesis of salinosporamide A

A simple and effective stereocontrolled synthesis of salinosporamide A has been developed. This process, the first synthesis of salinosporamide A, is capable of providing the compound in substantial quantities for further biological studies. Salinosporamide A was of special interest as a synthetic target because of its potent in vitro cytotoxic activity against many tumor cell lines (IC(50) values of 10 nM or less).