Design and concise synthesis of novel vitamin D analogues bearing a functionalized aromatic ring on the side chain

Five novel vitamin D analogues (2a, 2b, 3a, 3b and 4) bearing an aromatic side chain have been designed and synthesized in a convergent manner. The requisite CD-ring synthons (10a–c) were prepared from C22 aldehyde (5) using four- or five-step procedures. Using turbo-Grignard reagents allowed aromatic side chains with a polar functional moiety to be installed in a single step with excellent yields. A preliminary biological evaluation using bovine thymus vitamin D receptor (VDR) suggested that incorporating a carboxylic acid instead of the C25-hydroxy group had a positive effect on the VDR affinity compared with the corresponding esters.     

Synthesis and biological evaluation of all A-ring stereoisomers of 5,6-trans-2-methyl-1,25-dihydroxyvitamin D(3) and their 20-epimers: possible binding modes of potent A-ring analogues to vitamin D receptor.

BACKGROUND: The secosteroid 1 alpha,25-dihydroxyvitamin D(3) (1) has a wide variety of biological activities, which makes it a promising therapeutic agent for the treatment of cancer, psoriasis and osteoporosis. Insight into the structure-activity relationships of the A-ring of 1 is still needed to assist the development of more potent and selective analogues as candidate chemotherapeutic agents, as well as to define the molecular mode of action. RESULTS: All possible A-ring stereoisomers of 5,6-trans-2-methyl-1,25-dihydroxyvitamin D(3) (6a-h) and their 20-epimers (7a-h) were designed and efficiently synthesized. The dependence of the affinities for vitamin D receptor (VDR) and vitamin D binding protein (DBP), as well as the HL-60 cell differentiation-inducing activity, upon the stereochemistry of the A-ring and at C20 in the side chain was evaluated. CONCLUSIONS: The binding affinities and potency of the 5,6-trans and 5,6-cis analogues were enhanced by a 2-methyl substituent in a certain orientation. Molecular docking studies based upon the X-ray crystal structure of VDR suggested that the axial 2-methyl group would be accommodated in a pocket surrounded by hydrophobic amino acid residues in the ligand binding domain, resulting in enhanced interaction.     

Parallel synthesis of a vitamin D(3) library in the solid-phase.

A highly efficient synthesis of the vitamin D(3) system on solid support is described. Two synthetic strategies for the solid-phase synthesis of vitamin D(3) were developed. One is for 11-hydroxy analogues, and the other is for most other synthetic analogues. In the latter strategy, the sulfonate-linked CD-ring 58 was initially immobilized on PS-DES resin to give solid-supported CD-ring 63 (Scheme 10). Similarly, solid-supported CD-ring 63 was prepared by attachment of the CD-ring 10 to the chlorosulfonate resin 64. The vitamin D(3) system was synthesized by Horner-Wadsworth-Emmons reaction of the A-ring phosphine oxide to a solid-supported CD-ring, followed by simultaneous introduction of the side chain and cleavage from resin with a Cu(I)-catalyzed Grignard reagent. Parallel synthesis of the vitamin D(3) analogues was accomplished by a split and pool methodology utilizing radio frequency encoded combinatorial chemistry, and a manual parallel synthesizer for side chain diversification and deprotection. Additionally, we demonstrated the synthesis of various A-rings in a similar protocol for efficient preparation of building blocks.     

Synthesis of the N-((1E)-alkenyl)-(2Z,4Z)-heptadienamide side chain of salicylihalamide A and apicularens A and B

[reaction: see text] The unstable N-((1E)-alkenyl)-(2Z,4Z)-heptadienamide side chain of salicylihalamide A (1) and apicularens A and B (3 and 4) has been prepared in one pot by the addition of (1Z,3Z)-hexadienylcuprate, prepared in situ from EtLi, CuBr.SMe2, and acetylene, to a (1 E)-alkenyl isocyanate.     

Design and synthesis of a 1 alpha,25-dihydroxyvitamin D3 dimer as a potential chemical inducer of vitamin D receptor dimerization

[formula: see text] A dimer comprising two 1 alpha,25-dihydroxyvitamin D3 units linked by an alkyl side chain at C-11 was synthesized with a view to the simultaneous binding of two vitamin D receptor (VDR) molecules and the consequent induction of VDR dimerization. The short, convergent synthesis uses a stereoselective cuprate addition to introduce the linking side chain and a key ruthenlum olefin metathesis as the dimerization step.     

Synthesis of new 18-substituted analogues of calcitriol using a photochemical remote functionalization

A novel convergent synthetic approach to new analogues of calcitriol modified at the C-18 position is reported. The key step in the synthesis is the 20-hydroxyl-directed photochemical iodination of the 18-methyl group in the presence of (diacetoxyiodo)benzene. Using this methodology, two new analogues of calcitriol were prepared: the first contains a hydroxylated alkyl side chain attached at C-18 with the natural side chain replaced by an isopropylidene group; the second is a conformationally locked analogue due to an extra oxacycle between the C-18 and C-20 positions.     

Efficient synthesis of 2-modified 1alpha,25-dihydroxy-19-norvitamin D3 with Julia olefination: high potency in induction of differentiation on HL-60 cells

Six novel 2-substituted analogues of 1alpha,25-dihydroxy-19-norvitamin D(3), 6a,b-8a,b, were efficiently synthesized utilizing (-)-quinic acid as the A-ring precursor. The C2-modified A-rings were prepared as 4-alkylated (3R,5R)-3,5-dihydroxycyclohexanones 12-15 from (-)-quinic acid based on radical allylation at the C4 position of methyl (-)-quinicate. The new type of the CD-ring coupling partner 23 was synthesized from 25-hydroxy Grundmann's ketone 19 to apply to the modified Julia olefination to construct a diene unit between the A-ring and the CD-ring. The coupling yields, including a deprotection step, were 47-62%. After the separation of the diastereomers based on C2 stereochemistry, the structure (2alpha or 2beta) was determined by (1)H NMR experiments and compared to DeLuca's 2-methyl- and 2-ethyl-1alpha,25-dihydroxy-19-norvitamin D(3). Thus, the synthesized 2alpha-(3-hydroxypropyl)-1alpha,25-dihydroxy-19-norvitamin D(3) (8a) showed almost the same potency in binding to the bovine thymus vitamin D receptor (VDR) as the natural hormone 1, while its beta-isomer 8b had only a 3% affinity. Both 2alpha-allyl- and 2alpha-propyl-1alpha,25-dihydroxy-19-norvitamin D(3) (6a and 7a) and their 2beta-analogues (6b and 7b) possessed a weak affinity for the VDR. The strong VDR ligand 8a was ca. 36-fold more potent in induction of HL-60 cell differentiation than 1, and interestingly, even the weaker ligand 8b showed a 6.7-fold higher potency in the cell differentiation activity than that of 1.     

Total synthesis of the calphostins: application of fischer carbene complexes and thermodynamic control of atropisomers

The total syntheses of the potent protein kinase C inhibitors calphostins A, B, C, and D as well as a variety of structural analogues are reported. An aminobenzannulation reaction of an enantiopure chromium Fischer carbene complex is utilized to prepare a pentasubstituted naphthylamine. After optimization of side-chain substituents, conversion of the naphthylamine to an o-naphthoquinone was followed by biomimetic oxidative dimerization using trifluoroacetic acid and air yielding a 1:2 P/M mixture of atropisomeric perylenequinones. Thermal equilibration to a 3:1 P:M atropisomeric ratio and separation of the perylenequinones followed by side chain desymmetrization and functionalization led to the total synthesis of enantio- and diastereomerically pure calphostin C in only twelve steps from commercially available starting materials. In addition, calphostins A, B, D, and several structural analogues were prepared to evaluate biological activities.     

Preparation and properties of chiral 4-pyrrolidinopyridine (PPY) analogues with dual functional side chains

Chiral 4-pyrrolidinopyridine analogues 8-13 with two distinct functional side chains at C(2) and C(4) of the pyrrolidine ring were prepared from 4-hydroxy-l-proline. We examined desymmetrization of meso-1,3-cyclohexanediol through enantioselective acylation using these catalysts and found that introduction of a C(4)-side chain was effective for improving both the chemo- and enantioselectivity of acylation.     

Synthesis of vitamin D(3) and calcitriol dimers as potential chemical inducers of vitamin D receptor dimerization

The design and synthesis of vitamin D(3) dimers 2 and 3 and 1 alpha, 25-dihydroxyvitamin D(3) (calcitriol) dimers 4 and 5 are described. The dimers were designed with a view to doubly binding the vitamin D receptor (VDR) and inducing the receptor homodimerization. In the dimers the units are linked through the C-11 position in ring C by an alkyl side chain of six or 10 carbon atoms, far from the hydroxy groups responsible for the VDR binding. The linker is formed by olefin metathesis of an olefinic side chain at the C-11 position introduced by stereoselective cuprate addition. The synthesis, which is both short and convergent, uses the Wittig-Horner approach to construct the vitamin D triene system and allows the preparation of dimers with a linker of modulated length with the purpose of optimizing the vitamin D(3)-VDR interaction.     

Design and efficient synthesis of 2 alpha-(omega-hydroxyalkoxy)-1 alpha,25-dihydroxyvitamin D3 Analogues, including 2-epi-ED-71 and their 20-epimers with HL-60 cell differentiation activity

A concise and efficient synthetic approach to 2 alpha-(omega-hydroxyalkoxy)-1 alpha,25-dihydroxyvitamin D(3) (4a-c), including 2-epi-ED-71, was developed starting from D-glucose as a chiral template for the construction of the 2 alpha-modified A-ring precursors (11a-c). It was found that the best ligand for the bovine thymus vitamin D receptor (VDR) in this series is 4b, which has 1.8 times greater binding affinity for the bovine thymus VDR than that of the natural hormone 1. Interestingly, potency in the induction of HL-60 cell differentiation for 4a-c was almost the same or weaker than that of 1 despite the strong binding affinity for the VDR. Next, we were interested in the "double modification"of 1 based on 4a-c with C20-epimerization, affording 2 alpha-(omega-hydroxyalkoxy)-20-epi-1 alpha,25-dihydroxyvitamin D(3) (20-epi-4a-c). All three 2 alpha-substituted 20-epi analogues of 1 (20-epi-4a-c) exhibited stronger binding affinities for the VDR, and their conformations in the ligand binding domain of VDR were analyzed by molecular modeling. Double-modified analogues of 20-epi-4a-c showed marked HL-60 cell differentiation activity, and 20-epi-4a possesses an activity 58-fold higher than that of the natural hormone 1.     

Synthesis of 3-epi-6,7-dideoxyxestobergsterol A

3-Epi-6,7-dideoxyxestobergsterol A (2), an analogue of xestobergsterol A, has been synthesized from dehydroepiandrosterone (3) in 15 steps. The key synthetic intermediate, 15beta,16alpha-dioxypregn-17(20)E-ene derivative 8, was prepared from the corresponding 15beta,16beta-epoxide 6 by treating with acetic acid and titanium tetraisopropoxide. The 23-oxo side chain was constructed stereoselectively by orthoester Claisen rearrangement of 8 followed by introduction of an isobutyl group. Basic treatment of the 15,23-diketone 12 followed by deprotection gave the title compound 2.     

A new synthesis of potent antitumor saponin OSW-1 via Wittig rearrangement

OSW-1 and its analogues in which thiophene ring was introduced at the side chain were synthesized employing Wittig rearrangement of 17E(20)-ethylidene-16α-(4′-methyl-2′-thienyl)methyloxy steroid. The synthesis required nine steps from the known 17E(20)-ethylidene-16α-hydroxy steroid in 15.6% overall yield.     

Convergent total synthesis of gymnocin-A and evaluation of synthetic analogues

The first total synthesis of gymnocin-A (1), a cytotoxic polycyclic ether isolated from a notorious red tide dinoflagellate, Karenia mikimotoi, has been accomplished. The synthesis relies heavily on the Suzuki-Miyaura cross-coupling-based methodology to assemble the tetradecacyclic polyether skeleton. Convergent union of the GHI (5) and KLMN (6) rings, both of which were prepared from a common intermediate 7, and the subsequent ring closure of the J ring delivered the GHIJKLMN ring. The crucial coupling between the ABCD and FGHIJKLMN ring fragments (3 and 4, respectively) and stereoselective installation of the C17 hydroxyl group, followed by cyclization of the E ring gave rise to the tetradecacyclic polyether skeleton 2. Finally, incorporation of the 2-methyl-2-butenal side chain completed the total synthesis of gymnocin-A. The convergent nature of the synthesis, which employs three fragments of comparable complexity, is well-suited for preparation of various structural analogues of gymnocin-A to explore the structure-activity relationship. The results of preliminary structure-activity relationship studies of several synthetic analogues are also provided.     

Pretilt angle for BTDA,PMDA and CPDA series polyimides with various side chain lengths

Polyamic acid precursors were prepared by mixing dianhydride of 3,3',4,4'-benzophenone-tetracarboxylic dianhydride (BTDA), 1,2,3,4-benzene-tetracarboxylic dianhydride (pyrromellitic dianhydride PMDA), cis-1,2,3,4-cyclopentane-tetracarboxylic dianhydride (CPDA), the diamine (alkyl 3,5-diaminobenzoate) with side chain, and 4,4'-oxydianiline (ODA) without side chain. Copolyimide films with various side chain lengths were prepared by thermal imidization of polyamic acid precursors. The roughness of rubbed polyimide surface increased with increase in the side chain length. The pretilt angle for the BTDA and PMDA series polyimide (PI) increased exponentially with increase in side chain length. Various pretilt angles were obtained on the synthesized polyimides. In the case of CPDA series PI, the pretilt angle was nearly constant at 0 until the alkyl side chain length reached 12 (C12) and then increased markedly at C18. Models of pretilt angle generation were tested.     

Synthesis of linear tuftsin analogues modified at the ε-amino group of lysine

In this Letter, eight tuftsin analogues, seven of which are novel, are presented. All the linear tuftsin analogues contain an isopeptide bond. Modification of the tuftsin chain was based on the introduction of simple amino acids such as valine, glycine, alanine and β-alanine into the peptide chain at the ε-amino group of lysine. The peptides were synthesized by a solid-phase method using the standard Fmoc procedure. Simultaneous deprotection of the peptide side chain and liberation from the resin was achieved using TFA, and the free novel tuftsin analogues were purified and characterized.     

Thermoresponsive brush copolymers with poly(propylene oxide‐ran‐ethylene oxide) side chains via metal‐free anionic polymerization “grafting from” technique

Thermoresponsive brush copolymers with poly(propylene oxide‐ran‐ethylene oxide) side chains were synthesized via a “grafting from” technique. Poly(p‐hydroxystyrene) was used as the backbone, and the brush copolymers were prepared by random copolymerization of mixtures of oxyalkylene monomers, using metal‐free anionic ring‐opening polymerization, with the phosphazene base (t‐BuP₄) being the polymerization promoter. By controlling the monomer feed ratios in the graft copolymerization, two samples with the same side‐chain length and different compositions were prepared, both of which possessed high molecular weights and low molecular weight distributions. The results from light scattering and fluorescence spectroscopy indicated that the brush copolymers in their dilute aqueous solutions were near completely solvated at low temperature and underwent slight intramolecular chain contraction/association and much more profound intermolecular aggregation at different stages of the step‐by‐step heating process. Above 50 °C, very turbid solutions, followed by macrophase separation, were observed for both of the samples, which implied that it was difficult for the brush copolymers to form stable nanoscopic aggregates at high temperature. All these observations were attributed, at least partly, to the distribution of the oxyalkylene monomers along the side chains and the overall brush‐like molecular architecture. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2320–2328, 2010     

Complex profiles of hydrophobic paralytic shellfish poisoning compounds in Gymnodinium catenatum identified by liquid chromatography with fluorescence detection and mass spectrometry

The presence of hydrophobic analogues of paralytic shellfish poisoning toxins (PSTs) was studied in a Portuguese strain of Gymnodinium catenatum by conventional pre-column oxidation HPLC after a prolonged acetonitrile gradient coupled with fluorescence detection. Prior separation of hydrophobic PSTs analogues from hydrophilic analogues was done by solid-phase extraction (SPE) partitioning on a C18 cartridge. Several unknown oxidation products, with emission spectra similar to known PSTs, appeared after periodate or hydrogen peroxide oxidation. The compounds producing these oxidation products could be grouped into three major sub-groups according to SPE partitioning. The first one eluting with 10 and 20% MeOH, produced the first set of oxidation products observed after the saxitoxin oxidation product. The second one eluting with 30-100% MeOH produced the second set of oxidation products. The third one eluted with acidified 90% MeOH produced the third and last set of oxidation products. Additionally, the oxidation products corresponding to decarbamoyl gonyautoxins and decarbamoyl saxitoxins were also abundant, resulting from ester cleavage of the benzoate side chain of these compounds during the oxidation. Analysis of these fractions by LC-MS demonstrated the second sub-group was constituted by analogues of the 11-hydroxysulfated GC1/GC2, while the third sub-group was constituted by analogues of GC3, which lack the 11-hydroxysulfate. In addition to GC1/GC2 and GC3, novel analogues differing by 16u could be related, respectively, to the N1-hydroxyl analogues of GC1-GC3, designated GC4-GC6. A novel family of GC analogues, differing, by 16u from GC1-GC6, were hypothesized to possess an extra hydroxyl in the benzoate side chain, existing in both N1-hydroxylated and non-N1-hydroxylated variants, and tentatively designated GC1a-GC6a. The first sub-group was hypothesized to constitute an additional novel family of GC analogues with a hydroxysulfate group instead of the hydroxyl group in the benzoate side chain, tentatively designated GC1b-GC6b.     

Total synthesis and biological evaluation of (-)-apicularen A and its analogues

The total synthesis of (-)-apicularen A (1), a highly cytostatic 12-membered macrolide, and its analogues is described. The convergent and distinct approach not only provides 1, but also opens the opportunity to synthesize C10-C11 functional analogues of 1. The key steps of the total synthesis include assembling of iodoalkene 12 and aldehyde 13by Nozaki-Hiyama-Kishi (NHK) coupling, stereospecific construction of 2,6-trans-disubstituted dihydropyran by Pd(II)-catalyzed 1,3-chirality transfer reaction, and Yamaguchi macrolactonization. The (17E,20Z,22Z)-heptadienoylenamine moiety in the side chain is installed by an efficient Cu(I)-mediated coupling to complete the synthesis. Analogues of C11-epi-, C11-deoxy-C10-α-hydroxy-, and C10-C11 dehydrated apicularen A 3-5 were also prepared. Cytostatic activities of (-)-apicularen A and the three analogues for three different cancer cell lines are described.     

Synthesis and anti-tetrabenazine activity of c-3 analogues of dimethyl-2-phenylmorpholines

A series of analogues of 2-phenylmorpholines with alkyl substituents at the C-3 position were synthesized for anti-tetrabenazine (anti-TBZ) testing in mice. The target compounds were prepared by reaction of (2-bromoalkyl) phenyl ketones 21a-h with the appropriate aminoalcohol 20a-b to form morpholinols 22a-h . Hydride reduction of the morpholinols gave aminodiols 23a-h which were cyclized to morpholines 6, 8, 10–12, 14–16, 18 and 19 by acid catalaysis. Compounds 7, 9, 13 and 17 were prepared by reductive formylation. The smaller straight chain substituents of 6, 8, 12 and 15 , and the beta branching of the iso-butyl group of 16 were well tolerated; anti-tetrabenazine ED_50′s were comparable to compounds 2–5 . The α-branched, N-methylated, and side chain aryl derivatives were less active.