Total synthesis of the macrolide antibiotic cytovaricin

Ushikulides A and B, immunosuppressants produced by a strain of Streptomyces sp

Novel immunosuppressants, ushikulides A and B, were isolated from the culture broth of Streptomyces sp. IUK-102. Ushikulides A and B both have the same molecular formula, determined as C40H68O10. The structures of both compounds were elucidated to be novel 22-membered macrolides. Both compounds showed immunosuppressive activity for murine splenocyte proliferation in vitro.

Pectenotoxin-2 Synthetic Studies. 2. Construction and Conjoining of ABC and DE Eastern Hemisphere Subtargets

[reaction: see text] Practical asymmetric synthesis of aldehyde 2 and tetrazolyl sulfone 3 has allowed for their coupling via Julia olefination to generate 32 as a single product. This substance possesses the entire carbon backbone of the A-E substructure of pectenotoxin-2.

Bimetallic isopropoxides M[Al(OC3Hi7)4]3 (M = Pr, Nd)—catalyzed reduction of 2-octanone and benzophenone

Bimetallic alkoxides of praseodymium and neodymium with aluminum, viz., M[Al(OC(3)H(i)(7))(4)](3), where M = Pr(III)/Nd(III), have been used as catalysts in Meerwein-Ponndorf-Verley (MPV) reduction of 2-octanone and benzophenone in isopropanol to give their corresponding secondary alcohols. The reduction of ketones is investigated by following the absorption spectra involving 4f-4f transitions. During the MPV reduction the oscillator strengths (P) and the Judd-Ofelt intensity (T(lambda), lambda = 2,4,6) parameters corresponding to each 4f-4f band (H43-->D21, H43-->P03, H43-->P13, and H43-->P23 in praseodymium, and I9/24-->F3/24, I9/24-->F5/24, I9/24-->F7/24, I9/24-->G5/24, and I9/24-->G7/24 in neodymium alkoxides) at different time intervals have also been calculated and used in the determination of rates. Rates were calculated in terms of complexation of ketone and isopropanol with M[Al(OC(3)H(i)(7))(4)](3) during the MPV reaction, from the changes in both absorbance and oscillator strength with time, and were found to be comparable, verifying the Judd-Ofelt theory. The stronger oxyphilicity of Pr(III) and Nd(III) in M[Al(OC(3)H(i)(7))(4)](3) than of their analogue Al(III) in Al(OC(3)H(i)(7))(3) made the bimetallic alkoxides better catalyst in MPV reduction of ketones.

Asymmetric Transfer Hydrogenation of Aryl Ketones Catalyzed by Salt-Free Two Samarium Centers Supported by a Chiral Multidentate Alkoxy Ligand

[reaction: see text] We synthesized a chiral multidentate ligand, (R,R,R,R)-N,N,N',N'-tetra(2-hydroxy-2-phenylethyl)-1,3-xylylene diamine [(R)-2], which can support two metals at adjacent positions. Asymmetric transfer hydrogenation of acetophenone and its derivatives was conducted by using salt-free bimetallic lanthanoid complexes of (R)-2, and the combination of two samarium atoms and (R)-2 was found to be the best catalyst system for asymmetric transfer hydrogenation of aryl ketones in high enantioselectivity (up to >99% ee).

Total Synthesis of Formamicin

The enantioselective total synthesis of the cytotoxic plecomacrolide natural product formamicin (1) is described. Key aspects of this synthesis include the efficient transacetalation reactions of MOM ethers 28 and 38 to form the seven-membered formyl acetals 29 and 39, a late-stage Suzuki cross-coupling reaction of the highly functionalized vinyl boronic acid 6 and vinyl iodide 7, a highly beta-selective glycosidation reaction of beta-hydroxy ketone 4 with 2,6-dideoxy-2-iodoglucopyranosyl fluoride 3, and the global desilylation of penultimate intermediate 77 mediated by in situ generated Et(3)N.2HF.

Structure assignment, total synthesis, and evaluation of the phosphatase modulating activity of glucolipsin A

The previously unknown stereostructure of glucolipsin A (1), a complex glycolipid endowed with glucokinase-activating properties, was unambiguously elucidated as (2R,2'R,3S,3'S) by comparison of its spectroscopic and analytical data with those of all conceivable C(2)-symmetric stereoisomers. This set of macrodiolides was prepared by a sequence comprising auxiliary guided aldol reactions, glycosidation of the resulting beta-hydroxy acid derivatives with trichloroacetimidate 7, followed by hydrolytic cleavage of the auxiliaries used. The hydroxy acids thus formed were subjected to a macrodilactonization reaction mediated by 2-chloro-1,3-dimethylimidazolinium chloride (22) as the activating agent; this transformation is highly productive only in the presence of admixed potassium cations which likely serve as templates to preorganize two substrate molecules in a favorable head-to-tail arrangement. Glucolipsin and analogues were subjected to enzymatic assays that revealed that glycoconjugates of this type effectively inhibit the activity of the dual specific phosphatase Cdc25A with IC(50) values in the low micromolar range, while being hardly active against the tyrosine phosphatase PTP1B in vitro. This activity profile was compared to that of other glycolipids previously prepared in this laboratory, including cycloviracin B(1) (2), caloporoside (38), woodrosin I (39), sophorolipid lactone (40), and tricolorin G (41).

Enantioselective synthesis of the C1-C11 fragment of bafilomycin A1 using non-Wittig and desymmetrization strategies

The synthesis of the C1-C11 fragment 33 of bafilomycin A(1) was achieved. Intermediate ketone 16 was prepared in six steps from 4-oxopimelate 13. Desymmetrization of this ketone using Koga's chiral base followed by TMSCl quench furnished silyl enol ether 17 with excellent enantioselectivity. Further elaboration led to C5-C11 aldehyde 24, which was coupled with sulfone 3 to give lactone 25 in very good yield. The subsequent reductive elimination created the E-trisubstituted C4-C5 olefin with a 13:1 selectivity. The E C2-C3 double bond was then installed by methanol elimination, and compound 33 was obtained after a few functional group manipulations and a Negishi methyl zirconation.

Total synthesis of the salicylate enamide macrolide oximidine II

The asymmetric synthesis of the salicylate enamide macrolide oximidine II is reported. The synthesis involves a highly regio- and stereoselective ring-closing metathesis of a bis-diene substrate to construct the macrocyclic triene core. Copper(I)-mediated amidation of a (Z)-vinyl iodide was employed to attach the enamide side chain.

2-Deoxy-2-iodo-beta-glucopyranosyl fluorides: mild and highly stereoselective glycosyl donors for the synthesis of 2-deoxy-beta-glycosides from beta-hydroxy ketones

2-Deoxy-2-iodo-beta-glucopyranosyl fluoride 14 is a highly stereoselective glucopyranosyl donor that may be activated under mild conditions. Application of this new glycosyl donor to the glycosidation reactions of a variety of acceptors including beta-hydroxy ketones affords beta-glycosides with high efficiency and stereoselectivity. [reaction--see text]

Enantiospecific synthesis of the phospholipase A2inhibitors (−)-cinatrin C1and (+)-cinatrin C3

The enantiospecific synthesis of (-)cinatrin C1 (3) and (+)-cinatrin C3 (5) from the D-arabinose derivative 9 is described. The stereochemistry at C2 was introduced via a chelation-controlled addition of a carbanion to alpha-hydroxy ketone 8. The best selectivity was achieved by use of the Grignard reagent derived from trimethylsilylacetylene. Transformation of the terminal alkyne into methyl ester 17 followed by acetal hydrolysis and selective lactol oxidation gave cinatrin C1 dimethyl ester (7). Base hydrolysis and acid induced relactonization then gave a 1:1 mixture of cinatrins C1 (3) and C3 (5).

Enantiospecific synthesis of the phospholipase A2 inhibitor (-)-cinatrin B

The first enantiospecific synthesis of phospholipase A2 (PLA2) inhibitor (-)-cinatrin B (2) from the D-arabinose derivative 9 is described. The spirolactone system was formed by an Ireland-Claisen rearrangement of the allyl ester 8 followed by hydrolysis and stereoselective iodolactonization. The stereoselectivity of the rearrangement was controlled by the asymmetry in the allylic alcohol fragment. Ester (S)-8 gave the desired rearrangement product 7 and the epimer 13 in high yield as a 73:27 ratio, respectively. The final stereocenter at C2 was introduced via a chelation-controlled addition of the Grignard reagent derived from trimethylsilylacetylene to alpha-hydroxy ketone 6. Transformation of the terminal alkyne into the methyl ester 21 followed by acetal hydrolysis and selective lactol oxidation afforded cinatrin B methyl ester (22). Base hydrolysis and acid-induced relactonization then gave (-)-cinatrin B (2).

Stereocontrolled total synthesis of (-)-ebelactone A

[structure: see text] The highly stereocontrolled hydroboration of an alkene, a subsequent Suzuki-Miyaura cross-coupling reaction, a silylcupration on a nonterminal acetylene, and an iododesilylation were the key steps in a convergent total synthesis of (-)-ebelactone A.

Synthesis of glucopyranoside-based ligands for D-myo-inositol 1,4,5-trisphosphate receptors

Adenophostins A and B are naturally occurring glyconucleotides that interact potently with receptors for D-myo-inositol 1,4,5-trisphosphate, an important second messenger molecule in most cell types. Here we describe the design and synthesis of glucopyranoside-based analogues of adenophostin A lacking the adenine component. The key synthetic strategy involves glycosylation of selectively protected alcohols, derived from methyl beta-D-ribofuranoside or 1,4-anhydroerythritol, using glycosyl donors synthesised from 2,6-di-O-benzyl-D-glucopyranose derivatives. Further elaboration and deprotection of the coupled products gave two trisphosphate analogues; methyl 3-O-alpha-D-glucopyranosyl-beta-D-ribofuranoside 2,3',4'-trisphosphate ("ribophostin") and (3'S,4'R)-3'-hydroxytetrahydrofuran-4'-yl alpha-D-glucopyranoside 3,4,3'-trisphosphosphate ("furanophostin"). The route to furanophostin was further modified to give (3'S,4'R)-3'-hydroxytetrahydrofuran-4'-yl alpha-D-glucopyranoside 3'-phosphate 3,4-bisphosphorothioate, the first phosphorothioate-containing adenophostin analogue.

Total synthesis of hectochlorin

Hectochlorin (1) is a marine natural product with significant fungicidal activity. A synthesis effort was initiated to develop a flexible route to hectochlorin which would allow access to analogues with potentially improved activity and/or attributes relative to the natural product. A successful total synthesis of hectochlorin is described. [structure: see text]

Anionic cyclization approach toward perhydrobenzofuranone: stereocontrolled synthesis of the hexahydrobenzofuran subunit of avermectin

A facile anionic cyclization approach toward stereocontrolled synthesis of the hexahydrobenzofuran subunit 3 of avermectin is described. As a model study, treatment of iodo compound 7 with n-BuLi at -100 degrees C effected metal-halogen exchange and subsequent anionic cyclization to afford perhydrobenzofuranone 8. For the total synthesis of subunit 3, compound 9 was dihydroxylated to give diol 10. Protection of the hydroxyl groups of diol 10 gave compound 11. Ketone 11 was then converted into the required enone 12 using Saegusa's protocol. On iodination followed by Luche reduction, enone 12 yielded alpha-iodo allylic alcohol 14, which on alkylation afforded ether 15. Conversion of the ester unit of 15 into a Weinreb amide group followed by anionic cyclization gave enone 17. 1,4-Addition of (MeOCH(2))(2)CuCNLi(2) to enone 17 followed by cleavage of the acetal unit afforded ketone 19. Preferential acetylation of the secondary alcoholic function of 19 afforded compound 20. The stereochemistry of 20 is confirmed by single-crystal X-ray analysis. Elimination of HOAc from 20 gave the crucial olefin 21. Hydrolysis of the acetate unit of 21 followed by protection of the resulting alcoholic function yielded tert-butyldimethylsilyl ether 23. Introduction of a hydroxyl group at the ring junction of 23, using Davis's procedure, finally afforded the hexahydrobenzofuran subunit 3.

Intramolecular allenolate acylations in studies toward a synthesis of FR182877

During our efforts to synthesize the cytotoxic natural product FR182877, we discovered intramolecular reductive acylations that offer a stereocontrolled alternative to the classical Knoevenagel condensation for the formation of alpha-alkylidene beta-keto-delta-lactones. Other progress toward a synthesis of FR182877 includes a pi-allyl Stille coupling and a bromo Horner-Wadsworth-Emmons reaction that forms a 12-membered ring. Structural relationships among FR182877, hexacyclinic acid, macquarimicin A, and cochleamycin A are also discussed. [reaction: see text]

Total synthesis of (+)-phorboxazole A exploiting the Petasis-Ferrier rearrangement

A highly convergent, stereocontrolled total synthesis of the potent antiproliferative agent (+)-phorboxazole A (1) has been achieved. Highlights of the synthesis include: modified Petasis-Ferrier rearrangements for assembly of both the C(11-15) and C(22-26) cis-tetrahydropyran rings; extension of the Julia olefination to the synthesis of enol ethers; the design, synthesis, and application of a novel bifunctional oxazole linchpin; and Stille coupling of a C(28) trimethyl stannane with a C(29) oxazole triflate. The longest linear sequence leading to (+)-phorboxazole A (1) was 27 steps, with an overall yield of 3%.

Insights into long-range structural effects on the stereochemistry of aldol condensations: a practical total synthesis of desoxyepothilone F

A processable total synthesis of a potent antitumor agent, desoxyepothilone F (dEpoF, 21-hydroxy-12,13-desoxyepothilone B, 21-hydroxyepothilone D), has been accomplished. The route is highly convergent. The new technology has also been applied to a total synthesis of 12,13-desoxyepothilone (dEpoB). The crucial point of departure from previous syntheses of dEpoB and dEpoF involves presentation of the C1-C11 sector for Suzuki coupling with C3 in reduced form. Hitherto, the required S stereochemistry at C3 had been implemented via reduction of a keto function after Suzuki coupling. Whereas that chemistry worked quite well in a synthesis of dEpoB, it was not transferable to a high-yielding synthesis of dEpoF. The reduction of the keto group at C3 via a Noyori protocol after Suzuki coupling had proved to be very difficult. In our current approach, two consecutive aldol reactions are used to fashion the acyl sector. In the first aldol condensation, C6 becomes attached to C7. Following protection at C7, a two-carbon acetate equivalent is used to join C2 and C3 with very high asymmetric induction at C3. Only after this center has been implemented is the Suzuki reaction conducted. This major advance allowed us to synthesize dEpoF in a straightforward fashion. These findings found ready application in the total synthesis of dEpoB. Another part of the study involved analysis of the factors associated with aldol condensations joining C6 to C7. In the work described herein, the consequences of the status of C3 in promoting the C6-C7 aldol coupling are probed in detail. Dramatic stereochemical long-range effects uncovered during the study are described, and a working model to explain these effects has emerged.

Pseudo-C(3)-symmetric tertiary alcohol building block via group-selective hydroalumination: a synthesis of (-)-malyngolide

The stereocontrolled preparation of tertiary alcohol 4 and its TMS surrogate 5, which share a pseudo-C(3)()-symmetry, is described. Compound 4 was used for the synthesis of (-)-malyngolide.

An aldol approach to the synthesis of the EF fragment of spongistatin 1

A synthesis of the C29-C51 fragment of spongistatin 1, containing the E and F rings, has been completed. The approach relies on four diastereoselective aldol additions and an asymmetric glycolate alkylation to establish eight of the eleven stereogenic centers. The intact chlorodiene side chain was appended by a Lewis acid catalyzed addition of an allylstannane to an epoxy enol ether.

A Direct and Efficient α-Selective Glycosylation Protocol for the Kedarcidin Sugar, L-Mycarose: AgPF6 as a Remarkable Activator of 2-Deoxythioglycosides

The α-selective effect and potency of AgPF₆ on readily prepared 2-deoxythioglycosides allows the direct attachment of a partially protected allo-sugar, such as L-mycarose, to an advanced and hindered aglycon unit of kedarcidin. This method also permits access to other 2-deoxyglycosides in an expedient and mild manner. PMBM=p-methoxybenzyloxymethyl, Alloc=allyloxycarbonyl, TES=triethylsilyl.

Stereoselective synthesis of dihydropyrone-containing marine natural products. Total synthesis and structural elucidation of (-)-membrenone-C

[figure: see text] Three diastereomers of membrenone-C were separately prepared using a common two directional chain extending synthetic strategy. This has established the absolute and relative configuration of the natural product to be as shown in the foregoing graphic. Key steps in the synthesis of all the isomers are a stereoselective aldol coupling and reduction giving the C7-C9 stereocenters, a two direction chain extending double titanium aldol coupling, and the trifluoroacetic acid promoted double cyclization/dehydration giving the two dihydropyrone rings.

Total synthesis and antitumor activity of 12,13-desoxyepothilone F: an unexpected solvolysis problem at C15, mediated by remote substitution at C21

A new epothilone analogue, 12,13-desoxyepothilone F (dEpoF, 21-hydroxy-12,13-desoxyepothilone B, 21-hydroxyepothilone D), was synthesized and evaluated for antitumor potential. A convergent strategy employed for the semipractical synthesis of 12,13-desoxyepothilone B (dEpoB) has been utilized to yield an amount of dEpoF sufficient for relevant biological studies. The results from an in vitro assay reveal that this new analogue is highly active against various tumor cell lines with a potency comparable to that of dEpoB. In particular, the growth of resistant tumor cells is inhibited by dEpoF at concentrations where paclitaxel (Taxol) is basically ineffective. A preliminary assessment of its in vivo activity is also promising. The new analogue, containing an additional hydroxyl group at C21, exhibits advantages over other epothilones in terms of water solubility, and can serve as a readily functionalizable handle to produce other useful compounds for pertinent biological studies.