Efficient synthesis of 1alpha-fluoro A-ring phosphine oxide, a useful building block for vitamin D analogues, from (S)-carvone via a highly selective palladium-catalyzed isomerization of dieneoxide to dieneol

Synthesis and Characterization of Phosphorus-Containing Sorbent for Basic Dye Removal

A new phosphorus-containing sorbent was prepared by copolymerizing ethylene glycol dimethacrylate (EGDMA) and trimethylvinyl silane (TMVS) with diphenylvinylphoshine oxide (DPVO). It was characterized and applied in the removal of cationic dyes such as C.I. Basic Yellow 2 (BY2), C.I. Basic Blue 3 (BB3) and C.I. Basic Red 46 (BR46) using the batch method. Spectroscopic analysis indicated that the phosphinoyl group was introduced into the sorbent structure. Equilibrium adsorption data were fitted to the Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models. The Freundlich model is the most suitable to describe the adsorption of BB3 (the Freundlich constant kF = 32.3 mg1-1/nL1/n/g) and BY2 on the sorbent (13.8 mg1-1/nL1/n/g), while the Langmuir model is the most adequate to describe the adsorption of BR46 (the monolayer capacity Q0 = 2.7 mg/g). The kinetics of the dye adsorption follows the assumptions of the pseudo-second-order (the rate constants k2 = 0.087 ÷ 0.738 g/mg min) model rather than pseudo-first-order or intraparticle diffusion. The presence of Na2SO4 and cationic surfactant in the aqueous solutions inhibited dye retention by the DPVO-EGDMA-TMVS. Adsorbent regeneration efficiency does not exceed 60% using 1 M NaCl and 1 M HCl solutions in the presence of 50% v/v methanol.

Broadly Applicable Stereoselective Syntheses of Serrulatane, Amphilectane Diterpenes, and Their Diastereoisomeric Congeners Using Asymmetric Hydrovinylation for Absolute Stereochemical Control

A stereogenic center, placed at an exocyclic location next to a chiral carbon in a ring to which it is attached, is a ubiquitous structural motif seen in many bioactive natural products, including di- and triterpenes and steroids. Installation of these centers has been a long-standing problem in organic chemistry. Few classes of compounds illustrate this problem better than serrulatanes and amphilectanes, which carry multiple methyl-bearing exocyclic chiral centers. Nickel-catalyzed asymmetric hydrovinylation (AHV) of vinylarenes and 1,3-dienes such as 1-vinylcycloalkenes provides an exceptionally facile way of introducing these chiral centers. This Article documents our efforts to demonstrate the generality of AHV to access not only the natural products but also their various diastereoisomeric derivatives. Key to success here is the availability of highly tunable phosphoramidite Ni(II) complexes useful for overcoming the inherent selectivity of the chiral intermediates. The yields for hydrovinylation (HV) reactions are excellent, and selectivities are in the range of 92-99% for the desired isomers. Discovery of novel, configurationally fluxional, yet sterically less demanding 2,2'-biphenol-derived phosphoramidite Ni complexes (fully characterized by X-ray) turned out to be critical for success in several HV reactions. We also report a less spectacular yet equally important role of solvents in a metal-ammonia reduction for the installation of a key benzylic chiral center. Starting with simple oxygenated styrene derivatives, we iteratively install the various exocyclic chiral centers present in typical serrulatane [e.g., a (+)- p-benzoquinone natural product, elisabethadione, nor-elisabethadione, helioporin D, a known advanced intermediate for the synthesis of colombiasin and elisapterosin] and amphilectane [e.g., A-F, G-J, and K,L pseudopterosins] derivatives. A concise table showing various synthetic approaches to these molecules is included in the Supporting Information. Our attempts to synthesize a hitherto elusive target, elisabethin A, led to a stereoselective, biomimetic route to pseudopterosin A-F aglycones.

Influence of 18-Crown-6 Ether Coordination on the Catalytic Activity of Potassium and Calcium Diarylphosphinites in Hydrophosphorylation Reactions

The addition of 18-crown-6 ether (1,4,7,10,13,16-hexaoxacyclooctadecane) to tetranuclear [(thf)K(OPAryl₂)]₄ and [(thf)₄Ca(OPAryl₂)₂] yields the corresponding mononuclear complexes [(18C6)K(OPAryl₂)] [Aryl = Ph (1a), Mes (1b)] and [(18C6)Ca(OPAryl₂)₂] [Aryl = Ph (2a), Mes (2b)]. The metathesis reaction of [(thf)K(OPAryl₂)]₄ with CaI₂ yields the calciate [K₂Ca(thf)_x{OPMes₂}₄]. The addition of dimesitylphosphane oxide and crystallization from a hexane solution gives [K₂Ca{OPMes₂}₄{Mes₂P(O)H}] (3). The complexes [(thf)K(OPMes₂)]₄, [(thf)₄Ca(OPMes₂)₂], 1b, 2b, and the calciate 3 are tested as catalysts in the hydrophosphorylation of isopropylisocyanate with dimesitylphosphane oxide, quantitatively yielding N-isopropyl(dimesitylphosphoryl)formamide. The potassium complexes are more efficient catalysts than the calcium congeners, and coordination of 18-crown-6 decelerates the catalytic conversion.

Solvent effects in the nucleophilic substitutions of tetrahydropyran acetals promoted by trimethylsilyl trifluoromethanesulfonate: trichloroethylene as solvent for stereoselective C- and O-glycosylations

The selectivities of nucleophilic substitution reactions of tetrahydropyran acetals promoted by trimethylsilyl trifluoromethanesulfonate depend upon the reaction solvent. Polar solvents favor the formation of S(N)1 products, while nonpolar solvents favor S(N)2 products. Trichloroethylene was identified as the solvent most likely to give S(N)2 products in both C- and O-glycosylation reactions.

Stereoselective synthesis of core structure of cortistatin A

A stereoselective synthesis of the core structure of cortistatin A (1), a novel antiangiogenic steroidal alkaloid from Indonesian marine sponge, is described. An 8-oxabicyclo[3.2.1]octene system, a characteristic B-ring structure of 1, was elaborated by a 7-endo selective intramolecular Heck cyclization and a subsequent acid-mediated oxy-Michael reaction.

Design, synthesis and initial biological evaluation of a novel pladienolide analog scaffold

A novel and simplified synthetic scaffold based on pladienolide was designed using a consensus pharmacophore hypothesis. An initial target was synthesized and evaluated to examine the role of the 3-hydroxy group and the methyl groups present at positions 10, 16, 20, 22 in 1, on biological activity. We report the first totally synthetic analog of this macrolide that shows biological activity. Our novel synthetic strategy enables the rapid synthesis of other new analogs of pladienolide in order to develop selective anticancer lead compounds.

Welwitindolinone C synthetic studies. Construction of the welwitindolinone carbon skeleton via a transannular nitrone cycloaddition

Described is the construction of the N-methylwelwitindolinone C core via an efficient strategy that employs a sequential rhodium carbenoid-mediated O-H insertion, Claisen rearrangement and transannular [3+2] nitrone cycloaddition.

Calcitriol derivatives with two different side chains at C-20. V. Potent inhibitors of mammary carcinogenesis and inducers of leukemia differentiation

Calcitriol is implicated in many cellular functions including cellular growth and differentiation, thus explaining its antitumor effects. It was shown that gemini, the calcitriol derivative containing two side chain at C20, is also active in gene transcription with enhanced antitumor activity. We have now further optimized both the A-ring and the two side chains. The chemical structures of the resulting 18 geminis were correlated with biological activities. Those containing the 1alpha-fluoro A-ring are the least active. Those featuring 23-yne and 23(E) side-chains are generally more active in human breast cancer cell growth inhibition and human leukemia cell differentiation induction than their 23(Z) counterparts. On the basis of these evaluations, we selected as lead compound a 20(R) gemini, related to calcitriol in terms of it is A-ring, where one side chain was modified by introduction of a 23-yne function and replacement of the geminal methyl groups with trifluoromethyl groups, the other created by extension of C21 with a 3-hydroxy-3-trideuteromethyl-4,4,4-trideutero-butyl moiety.

In Pursuit of an Ideal C-C Bond-Forming Reaction: Development and Applications of the Hydrovinylation of Olefins

Attempts to introduce the highly versatile vinyl group into other organic molecules in a chemo-, regio- and stereoselective fashion via catalytic activation of ethylene provided challenging opportunities to explore new ligand and salt effects in homogeneous catalysis. This review provides a personal account of the development of enantioselective reactions involving ethylene.

Ligand tuning in asymmetric hydrovinylation of 1,3-dienes: a stereoselective route to either steroid-C20 (S) or -C20 (R) derivatives

1,3-Dienes derived from steroidal D-ring C 17-ketones undergo Ni(II)-catalyzed hydrovinylation to give 1,2- or 1,4-addition of ethylene. Using finely tuned phosphoramidite ligands, it is possible to synthesize either the C 20 ( R)- or ( S)-derivatives without mutual contamination. The proportion of the 1,4-adduct, which is also formed stereoselectively, can be minimized by optimizing the reaction conditions. Because the two alkenes in the resultant dienes have differing steric demands for many potential reactions, and are ideally juxtaposed for further D-ring functionalization, these intermediates could be useful for the preparation of biologically important compounds such as vitamin D analogs and various antitumor steroidal glycosides.

Total Synthesis and Correct Absolute Configuration of Malyngamide U

The enantioselective synthesis of the previously proposed structure of malyngamide U (1) was accomplished in 18 steps from (S)-(+)-carvone. The key steps involved a hydroxymethylation of (S)-(+)-carvone and an asymmetric Henry reaction of aldehyde (+)-5, as well as condensation with the acid 3. The 1H and 13C NMR data of the synthetic compound 1 were not consistent with the data of the reported malyngamide U. The C-2' epimer of compound 1 was therefore synthesized by a similar reaction sequence. While the NMR data of C-2' epimer 23 were in full agreement with those of the reported product, the discrepancy in the specific rotation data suggested the correct structure of malyngamide U should be structure 2, in which the absolute configuration of the amine part was enantiomeric with that in compound 23. Then the correct absolute configuration of revised malyngamide U (2) was confirmed by the similar synthesis from (R)-(-)-carvone.

Enantioselective Synthesis of a Key “A-Ring” Intermediate for the Preparation of 1α-Fluoro Vitamin D3 Analogues

1Alpha-fluoro A-ring dienol 2, a useful building block for the preparation of fluorinated vitamin D3 analogues, was synthesized in eight steps from 4-{[tert-butyldimethylsilyl]oxy}cyclohexanone. The most distinctive synthetic development to emerge from this new synthesis is an unprecedented substrate-controlled diastereoselective fluorodesilylation of an advanced dienylsilane intermediate. This is the first enantioselective route to compound 2 relying on the use of an electrophilic fluorinating reagent.

Versatile synthesis and biological evaluation of 1,3-diamino-substituted 1α,25-dihydroxyvitamin D3 analogues

A concise route to 1alpha,3beta-diamino-25-hydroxy-3-deoxyvitamin D(3) (5) and 1beta,3alpha-diamino-25-hydroxy-3-deoxyvitamin D(3) (6) has been developed starting from (R)- or (S)-carvone for the construction of the modified A-ring fragments. The conversion of the hydroxyl group to amine function with complete inversion of the configuration was efficiently accomplished by Mitsunobu reaction using phthalimide as nucleophile or activation of the hydroxyl group as mesylate followed by reaction with NaN(3). Diamino 5 and 6 as well as monoamino 3, 4, 30, and 31 vitamin D(3) derivatives have shown poor binding to VDR compared with 1alpha,25-dihydroxyvitamin D(3). The most active compound in the inhibition of MCF-7 cell proliferation and HL 60 cell differentiation was 1alpha-amino analogue 3. Also, very low in vivo calcemic effects of derivatives 3 and 4 were found.

Diastereoselective Reactions at Enantiomerically Pure, Sterically Congested Cyclohexanes as an Entry to Wailupemycins A and B: Total Synthesis of (+)-Wailupemycin B

Wailupemycin A (1) and B (2) are polyketide natural products with a highly substituted cyclohexanone core. Three different routes for the syntheses of these compounds were pursued, which commenced from either (R)-(-)-carvone (ent-5) or (S)-(+)-carvone (5). In the first approach it was attempted to construct the skeleton of wailupemycin A from triol 19 (nine steps from ent-5; 19 % yield) by a sequence of diastereoselective epoxidation, nucleophilic ring opening at C-13 and carbonyl addition at C-5. The synthetic plan failed at the stage of the carbonyl addition to aldehyde 27, which had been obtained in seven steps (18 % yield) from triol 19. The second route included an epoxide ring opening at C-13 and a carbonyl addition at C-7 as key steps. It could have led to either wailupemycin A or B depending on the diastereoselectivity of the addition step. Starting from allylic alcohol 30 (six steps from ent-5; 59 % yield) the cyclohexanone 28 was obtained in five steps (54 % yield). Unfortunately, the carbonyl addition failed also in this instance. In the eventually successful third attempt the skeleton of wailupemycin B was built from cyclohexanone 43 (eight steps from 5; 53 % yield) by highly diastereoselective carbonyl addition reactions at C-7 and C-12. The phenyl group at C-14 was introduced at a late stage of the synthetic sequence. Careful protecting group manipulation finally allowed for the total synthesis of (+)-wailupemycin B. The absolute and relative configuration of the natural product was unambiguously confirmed. The total yield of wailupemycin B amounted to 6 % over 23 steps starting from (S)-(+)-carvone (5).

Cp2TiCl-Promoted Isomerization of Trisubstituted Epoxides to exo-Methylene Allylic Alcohols on Carvone Derivatives

The ring-opening reaction of trisubstituted epoxides promoted by Cp2TiCl led to exo-methylene allylic alcohols as major compounds when 0.5 M solutions of the epoxides were added to 0.1 M solutions of the reagent at room temperature in THF. In most cases, the allylic alcohols were contaminated with saturated alcohols. Normal and reverse addition modes led to the alternate product being favored. The different stereochemical outcome of cis- and trans-epoxy acetates is rationalized in terms of mechanistically biased elimination processes.

Regio- and stereoselective ruthenium-catalyzed hydrovinylation of 1,3-dienes: application to the generation of a 20(S) steroidal side chain

The addition of ethylene to 1,3-dienes and 1-vinylcycloalkenes, catalyzed by two ruthenium complexes, proceeds in a regioselective fashion to afford 3-methyl-1,4-dienes as products. For a steroidal-based 1-vinylcycloalkene, the addition is stereospecific, giving a product with a 20(S) configuration. [reaction: see text]

Efficient synthesis of the A-ring phosphine oxide building block useful for 1 alpha,25-dihydroxy vitamin D(3) and analogues

The 1 alpha-hydroxy A-ring phosphine oxide 1, a useful building block for vitamin D analogues, was synthesized from (S)-carvone in nine synthetic operations and a single chromatographic purification in 25% overall yield. The synthesis features two novel efficient synthetic transformations: the Criegee rearrangement of alpha-methoxy hydroperoxyacetate 10 in methanol to obtain directly the desired secondary 3 beta-alcohol 11 and the highly chemo- and stereoselective isomerization of dieneoxide ester (E)-7 to the 1 alpha-allylic alcohol with an exocyclic double bond (E)-8. Further insight into the selectivity control of the latter rearrangement was obtained from the reactions of (Z)-epimeric substrates. The new synthetic approach leading to the 1 alpha-hydroxy epimers complements our previously reported synthesis of the corresponding 1 beta-epimers, thus producing all stereoisomers of these versatile building blocks efficiently from carvone.

Synthetic studies on the ingenane diterpenes. An improved entry into a trans-intrabridgehead system

[reaction: see text] The efficient construction of an ingenol intermediate exhibiting "insideminus signoutside" intrabridgehead stereochemistry is reported. The sequence features the net conversion of a cis-intrabridgehead compound into a highly strained trans-species via palladium-mediated isomerization of an allylic epoxide followed by a low-temperature alkoxide-accelerated 1,5-hydrogen migration.