An Enantioselective Ring Expansion Route Leading to Furanose and Pyranose Nucleosides Featuring Spirodiketopiperazines at the Anomeric Position

Glycopyranosylidene-Spiro-Morpholinones: Evaluation of the Synthetic Possibilities Based on Glyculosonamide Derivatives and a New Method for the Construction of the Morpholine Ring

Glycosylidene-spiro-morpholin(on)es are scarcely described skeletons in the literature. In this work, we have systematically explored the synthetic routes towards such morpholinones based on the reactions of O-peracylated hept-2-ulopyranosonamide derivatives of D-gluco and D-galacto configuration. Koenigs-Knorr type glycosylation of 2-chloroethanol, allylic and propargylic alcohols by (glyculosylbromide)onamides furnished the expected glycosides. The 2-chloroethyl glycosides were ring closed to the corresponding spiro-morpholinones by treatment with K₂CO₃. The (allyl glyculosid)onamides gave diastereomeric mixtures of spiro-5-hydroxymorpholinones by ozonolysis and 5-iodomethylmorpholinones under iodonium ion mediated conditions. The ozonolytic method has not yet been known for the construction of morpholine rings, therefore, it was also extended to O-allyl mandelamide. The 5-hydroxymorpholinones were subjected to oxidation and acid catalyzed elimination reactions to give the corresponding morpholine-3,5-dions and 5,6-didehydro-morpholin-3-ones, respectively. Base induced elimination of the 5-iodomethylmorpholinones gave 5-methyl-2H-1,4-oxazin-3(4H)-ones. O-Acyl protecting groups of all of the above compounds were removed under Zemplén conditions. Some of the D-gluco configured unprotected compounds were tested as inhibitors of glycogen phosphorylase, but showed no significant effect.

Deciphering the exceptional selectivity of semipinacol rearrangements in cis-fused β-lactam diols using high-level quantum chemical methods

The semipinacol rearrangement mechanism in cis-fused β-lactam diols has been found to kinetically and thermodynamically favor acyl migration. The semipinacol rearrangement was also investigated for trans-fused β-lactam diol, in which the reversed selectivity that favors the alkyl migration was observed.

Tungsten anchored onto functionalized SBA-15: an efficient catalyst for diastereoselective synthesis of 2-azapyrrolizidine alkaloid scaffolds

We used a novel hybrid catalyst in chemo-, regio-, and diastereoselective multi-component reactions (MCR) for the synthesis of the 2-aza analogue of pyrrolizidine and spirooxindole-2-azapyrrolizidine derivatives. The nanocatalyst, W(iv)/NNBIA–SBA-15 [where NNBIA = N,N′-(ethane-1,2-diyl)bis(2-aminobenzamide)] was synthesized by covalent grafting on chloro-functionalized SBA-15. The synthesis process was followed by the anchoring of WCl₆ to catch the desired catalyst. The quality of the catalyst was assessed using different analytical techniques such as X-ray diffraction spectroscopy (XRD), Fourier-transform infrared spectroscopy (FT-IR), N₂ adsorption analysis, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), ammonia Temperature Programmed Desorption (TPD), X-Ray photoelectron spectroscopy (XPS) and thermogravimetric, differential thermal analysis (TGA-DTA). The catalyst, W(iv)/NNBIA–SBA-15, with high catalytic performance is a good candidate for the diastereoselective synthesis of 2-azapyrrolizidine alkaloid scaffolds. The catalyst could be recovered for reuse without noticeable loss of activity.

We used a novel hybrid catalyst in chemo-, regio-, and diastereoselective multi-component reactions (MCR).

Base mediated green synthesis of enantiopure 2-C-spiro-glycosyl-3-nitrochromenes

A novel green synthetic methodology has been developed to obtain enantiopure (2S)-2-C-spiro-glycosyl-3-nitrochromenes following the oxa-Michael-aldol condensation reaction of sugar derived 3-C-vinyl nitro olefins with substituted salicylaldehydes using Et3N as a base under neat conditions at rt-40 °C. The stereochemistry of the product is confirmed by a single crystal X-ray study. Several advantages are associated with this protocol such as cost effectiveness, easy accessibility, short reaction time, high yields, wide substrate scope and high enantiopurity.

Synthesis of Spironucleosides: Past and Future Perspectives

Spironucleosides are a type of conformationally restricted nucleoside analogs in which the anomeric carbon belongs simultaneously to the sugar moiety and to the base unit. This locks the nucleic base in a specific orientation around the N-glycosidic bond, imposing restrictions on the flexibility of the sugar moiety. Anomeric spiro-functionalized nucleosides have gained considerable importance with the discovery of hydantocidin, a natural spironucleoside isolated from fermentation broths of Streptomyces hygroscopicus which exhibits potent herbicidal activity. The biological activity of hydantocidin has prompted considerable synthetic interest in this nucleoside and also in a variety of analogues, since important pharmaceutical leads can be found among modified nucleoside analogues. We present here an overview of the most important advances in the synthesis of spironucleosides.

"In Water": Organocatalyzed Diastereoselective Multicomponent Reactions toward 2-Azapyrrolizidine Alkaloid Scaffolds

Synthesis of the 2-aza analogues of pyrrolizidine and spirooxindole-2-azapyrrolizidine hybrid, a spiro-tetracyclic scaffold possessing multiple contiguous stereocenters, by an exclusive regio-, chemo-, and diastereoselective multicomponent reaction in water is reported. This logical and didactical tactic has integrated the principles of an ideal organic synthesis, privileged substructure-based diversity-oriented synthesis, and biology-oriented synthesis to access hybrid heterocyclic scaffolds.

The evolution of comprehensive strategies for furanoid glycal synthesis and their applications

Cyclic enol ether frameworks, especially stereochemically pure furanoid and pyranoid glycals are well known highly functionalized chiral building blocks.

Carbamoyl radical-mediated synthesis and semipinacol rearrangement of β-lactam diols

In an approach to the biologically important 6-azabicyclo[3.2.1]octane ring system, the scope of the tandem 4-exo-trig carbamoyl radical cyclization-dithiocarbamate group transfer reaction to ring-fused β-lactams is evaluated. β-Lactams fused to five-, six-, and seven-membered rings are prepared in good to excellent yield, and with moderate to complete control at the newly formed dithiocarbamate stereocentre. No cyclization is observed with an additional methyl substituent on the terminus of the double bond. Elimination of the dithiocarbamate group gives α,β- or β,γ-unsaturated lactams depending on both the methodology employed (base-mediated or thermal) and the nature of the carbocycle fused to the β-lactam. Fused β-lactam diols, obtained from catalytic OsO4-mediated dihydroxylation of α,β-unsaturated β-lactams, undergo semipinacol rearrangement via the corresponding cyclic sulfite or phosphorane to give keto-bridged bicyclic amides by exclusive N-acyl group migration. A monocyclic β-lactam diol undergoes Appel reaction at a primary alcohol in preference to semipinacol rearrangement. Preliminary investigations into the chemo- and stereoselective manipulation of the two carbonyl groups present in a representative 7,8-dioxo-6-azabicyclo[3.2.1]octane rearrangement product are also reported.

Synthesis of conformationally North-locked pyrimidine nucleosides built on an oxabicyclo[3.1.0]hexane scaffold

Beginning with a known 3-oxabicyclo[3.1.0]hexane scaffold (I), the relocation of the fused cyclopropane ring bond and the shifting of the oxygen atom to an alternative location engendered a new 2-oxabicyclo[3.1.0]hexane template (II) that mimics more closely the tetrahydrofuran ring of conventional nucleosides. The synthesis of this new class of locked nucleosides involved a novel approach that required the isocyanate II (B = NCO) with a hydroxyl-protected scaffold as a pivotal intermediate that was obtained in 11 steps from a known dihydrofuran precursor. The completion of the nucleobases was successfully achieved by quenching the isocyanate with the lithium salts of the corresponding acrylic amides that led to the uracil and thymidine precursors in a single step. Ring closure of these intermediates led to the target, locked nucleosides. The anti-HIV activity of 29 (uridine analogue), 31 (thymidine analogue), and 34 (cytidine analogue) was explored in human osteosarcoma (HOS) cells or modified HOS cells (HOS-313) expressing the herpes simplex virus 1 thymidine kinase (HSV-1 TK). Only the cytidine analogue showed moderate activity in HOS-313 cells, which means that the compounds are not good substrates for the cellular kinases.

Studies on the Zn(II)-mediated electrophilic selenocyclization and elimination of 3,4-O-isopropylidene-protected hydroxyalkenyl sulfides: synthesis of a 2-phenylselenenyl glycal

Herein, we describe a mild and efficient Zn(II)-mediated electrophilic selenocyclization reaction of readily available and stable 3,4-O-isopropylidene-protected hydroxyalkenyl sulfides to 2-deoxy-2-phenylselenenyl-1-thio-glycosides. This material was transformed into a 2-phenylselenenyl glycal in a controlled manner using an activation-elimination sequence.

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.

Sulfur Extrusion with Tin Radical: Synthesis of 4‘,5‘-Didehydro-5‘-deoxy-5‘-(tributylstannyl)adenosine, an Intermediate for Potential Inhibitors against S-Adenosyl Homocysteine Hydrolase

A new approach has been developed for the synthesis of potential inhibitors of S-adenosyl-l-homocysteine (AdoHcy) hydrolase. The key intermediate 9-[2,3-bis-O-(tert-butyldimethylsilyl)-5-(Z)-(tributylstannyl)-5-deoxy-beta-d-erythro-pent-4-enofuranosyl]adenine (12) was prepared by sulfur extrusion reaction of 4',5'-didehydro-5'-deoxy-5'-(phenylthio)adenosine (11) with tributyltin radical. It was found that this reaction proceeds stereoselectively, forming 12 irrespective of the geometry of 11. Compound 12 readily underwent iodination, bromination, and chlorination with retention of configuration, whereas fluorination gave both (Z)- and (E)-isomers of vinyl fluoride. Because of the susceptibility of 12 to protodestannylation, the (Z)-vinyl iodide (13), prepared in quantitative yield from 12, was used as a substrate for C-C bond formation. Various types of carbon substituents (phenyl, vinyl, trifluorovinyl, ethynyl, and cyano) were introduced to the 5'-position of the 5-deoxy-beta-d-erythro-pent-4-enofuranosyl structure to open up a new route to potential inhibitors of AdoHcy hydrolase.

Conformational Restriction of Nucleosides by Spirocyclic Annulation at C4‘ Including Synthesis of the Complementary Dideoxy and Didehydrodideoxy Analogues

The concept of spirocyclic restriction, when generically applied to nucleoside mimics, allows for the preparation of diastereomeric pairs carrying either a syn- or anti-oriented hydroxyl at C-5'. Reported herein are convenient synthetic routes to enantiomerically pure 1-oxaspiro[4.4]nonanes featuring fully dihydroxylated end products as well as congeners having dideoxy and didehydrodideoxy substitution patterns. Notable use is made of the capacity for introducing unsaturation in the furanose sector via phenylsulfenylation and the incorporation of uracil and thymine by way of their silylated derivatives under catalysis with stannic chloride.

1-Oxaspiro[4.4]nonan-6-ones. Synthetic access via oxonium ion technology, optical resolution, and conversion into enantiopure spirocyclic alpha,beta-butenolides

A general approach to the synthesis of enantiomerically pure spirocyclic alpha,beta-butenolides is presented where the fundamental framework is rapidly elaborated by acid- or bromonium ion-induced rearrangement of the carbinol derived by addition of 2-lithio-4,5-dihydrofuran to cyclobutanone. Subsequent resolution of the resulting ketones by either sulfoximine or mandelate acetal technology has been applied effectively. The availability of these building blocks makes possible in turn the acquisition of the enantiomers of dihydrofurans typified by 17, 35, and 38 and lactones such as 25 and 31, as well as the targeted title compounds. Complementary reductions of the early intermediates provide the added advantage that the alpha- and beta-stereoisomeric carbinol series can be obtained on demand. These capabilities have been coordinated to allow the crafting of any member of the series in relatively few steps.