Diastereoselective synthesis of all eight l-hexoses from L-ascorbic acid

[reaction: see text] A novel versatile method for the synthesis of all eight diastereomerically pure L-hexoses was developed. L-Ascorbic acid was converted to two diastereomers A. These alpha-hydroxy esters were transformed into four gamma-alkoxy-alpha,beta-unsaturated esters C via the intermediates B and subsequent Wittig olefination reactions. Each one of compounds C was subjected to dihydroxylation to provide a set of two diols D. Anti/syn-differentiation in diol formation was manipulated by using (DHQD)2PHAL and (DHQ)2PHAL as chiral ligands. Further two-step reaction sequence affords all eight diastereopure L-hexoses.     

Diastereoselective total synthesis of 8-epigrosheimin

The first diastereoselective total synthesis of 8-epigrosheimin was accomplished relying entirely on substrate-controlled methods. 8-Epigrosheimin, isolated as an amoebicidal and antibiotic compound from Crepis virens, is a multi-chiral-centered guaianolide with a cis-hydroazulene and a trans-annulated γ-butyrolactone ring. Our approach featured that the γ-butyrolactone unit was formed firstly before the construction of the cycloheptane ring system. The key steps of the synthesis involved (1) a stereoselective Mukaiyama aldol addition; (2) an oxidative γ-lactonization; and (3) an intramolecular aldehyde-ene cyclization.     

Diastereoselective total synthesis of (+)-morusimic acid B, an amino acid from Morus alba

An efficient, flexible and diastereoselective synthesis of the naturally occurring pyrrolidine amino acid, (+)-morusimic acid B, has been accomplished. Starting from chiral, optically active (+)-(3S)-hydroxy butyric acid methyl ester the key steps of our synthesis are diastereoselective α-alkylation of its dianion to introduce the main part of the side chain, Curtius rearrangement of the hydrazide derivative to a 2-oxazolidinone followed by N→π-cyclization with mercury(II) acetate to generate the cis-2,5-disubstituted pyrrolidine ring. The remote C-3 stereocentre is established after chain elongation with the dianion of methyl acetoacetate and asymmetric hydrogenation of the resulting β-oxoester with Noyori's Ru(II)-(R)-BINAP catalyst.     

An efficient synthesis of isocarbacyclin starting from furfural

Two synthetic routes to isocarbacyclin starting from furfural are described. One route involves the regioselective hydrogenation of the disubstituted olefin in the presence of two other olefinic double bonds, and the other includes the highly regioselective organocopper coupling with the allylic etectrophile as a key step.     

Diastereoselective total synthesis of both enantiomers of epolactaene

A stereocontrolled total synthesis of both the (+)- and (-)-epolactaene ((+)- and (-)-1) enantiomers from tetrahydropyran-2-ol is described. The following reactions in this synthesis are particularly noteworthy: (1) the stereoselective construction of the conjugated (E,E,E)-triene by a combination of kinetic deprotonation and thermodynamic equilibration, (2) the E-selective Knoevenagel condensation of beta-ketonitrile 33 with a chiral 2-alkoxyaldehyde, (3) a diastereoselective epoxidation achieved using a bulky nucleophile (TrOOLi) and an appropriate protecting group, (4) the mild hydrolysis of an alpha-epoxy nitrile by silica gel on TLC facilitated by hydroxyl-mediated, intramolecular assistance.     

Efficient synthesis of suitably protected beta-difluoroalanine and gamma-difluorothreonine from L-ascorbic acid

[reaction: see text] Fluorinated amino acids are useful building blocks for the preparation of biologically active peptides and peptidomimetics with increased metabolic stability. We report here the synthesis of two fluorinated amino acids, beta-difluoroalanine and gamma-difluorothreonine, as analogues of Ser and Thr, respectively. These compounds were suitably protected for Fmoc-based solid-phase peptide synthesis. Once incorporated into peptides, they may serve as alternative substrates or inhibitors of lantibiotic synthetases that posttranslationally dehydrate Ser and Thr residues to dehydroalanine and dehydrobutyrine, respectively.     

Diastereoselective synthesis of (+)-nephrosterinic acid and (+)-protolichesterinic acid

A diastereoselective synthesis of (+)-nephrosterinic acid and (+)-protolichesterinic acid, common members of the paraconic acids is described. The synthesis is based on a diastereoselective orthoester Johnson–Claisen rearrangement of a (Z)-allyl alcohol with a vicinal dioxolane moiety as key steps. The synthesis is completed in 10 steps and with overall yields of 15.9% for (+)-nephrosterinic acid and 16.4% for (+)-protolichesterinic acid.     

Diastereoselective nitrenium ion-mediated cyclofunctionalization: total synthesis of (+)-castanospermine

The asymmetric total synthesis of the α-glucosidase inhibitor (+)-castanospermine is reported. The central theme in our approach to this polyhydroxylated alkaloid is the simultaneous generation of the piperidine ring and the C-1/8a erythro stereodiad through the diastereoselective, oxamidation of an unsaturated O-alkyl hydroxamate. This process is believed to proceed sequentially via singlet acylnitrenium and aziridinium ion intermediates.     

正交试验法优化合成环己酮Vc缩酮

缩醛或缩酮是一类具有广泛用途的高档新型香料[1].近年来,国内不少学者对环己酮缩酮的合成作过一些报道,主要工作是探讨催化剂对环己酮缩酮合成反应的影响,所用的醇大多为乙二醇[2-6].     

Diastereoselective ring-closing metathesis: synthesis of P-stereogenic phosphinates from prochiral phosphinic acid derivatives

[reaction: see text] The preparation of phosphorus-containing trienes featuring two diastereotopic vinyl moieties followed by a diastereoselective ring-closing metathesis is described. This methodology allowed for the synthesis of novel highly functionalized P-stereogenic heterocycles featuring both an exo- and an endocyclic double bond. An investigation into the factors influencing the diastereochemical outcome of the ring-closing metathesis is also presented, revealing that the geometry of the double bonds conjugated to phosphorus is important and that 1,3-stereoinduction is superior to 1,4-stereoinduction for these reactions.     

Diastereoselective synthesis of beta-Aryl-C-nucleosides from 1,2-anhydrosugars

The cis opening of glycal epoxides with arylaluminum reagents provides strict stereocontrol in C-glycosylation. beta-Aryl-C-2-deoxynucleosides are obtained from known glycals by an epoxidation-glycosylation-deoxygenation sequence.     

Diastereoselective titanium enolate aldol reaction for the total synthesis of epothilones

[formula: see text] The development of a highly diastereoselective addition of the titanium enolate derived from ketone 1 to aldehyde 2 offers an efficient entry to the total synthesis of the epothilone family. The new aldol process is robust and tolerates a wide range of functional groups.     

Alkaloid synthesis using 2nd generation palladium-catalyzed cycloalkenylation. Diastereoselective total synthesis of α-skytanthine

An efficient approach to the synthesis of 3-azabicyclo[3.3.0]octanes and 3-azabicyclo[4.3.0]nonanes by 2nd generation palladium-catalyzed cycloalkenylation is described. Additionally, this reaction is used for the diastereoselective total synthesis of (±)-α-skytanthine.     

Diastereoselective synthesis of allosecurinine and viroallosecurinine from menisdaurilide

A new and versatile synthetic route to Securinega alkaloids is reported. The first synthesis of allosecurinine has been accomplished in seven steps and 40% yield, starting from (+)-menisdaurilide, using a vinylogous Mannich reaction as the key transformation. Similarly, viroallosecurinine has been synthesized from (-)-menisdaurilide.     

Diastereoselective synthesis of strained spiro-cyclopropanooxindoles from cyclic diazoamides

Reaction of cyclic diazoamides and cyclic olefins or heteroaromatic systems using copper(I) triflate as a catalyst furnished a variety of strained spiro-cyclopropanooxindoles in a diastereoselective manner under mild reaction conditions. The effect of copper(I) triflate and rhodium(II) acetate catalysts on the cyclopropanation was also studied.     

Quantification of L-ascorbic acid and total ascorbic acid in fruits and spinach by capillary zone electrophoresis

A standard curve for the quantification of L-ascorbic acid (L-AA) by capillary zone electrophoresis (CZE) was established, and the quantification of ascorbic acid and total ascorbic acid in fruits (lemon, Sunkist, and pineapple) and spinach were performed using D-isoascorbic acid (D-IAA) as an internal standard. The minimum detection limits (MDLs) for L-AA and D-IAA were determined to be 1 and 2 microg/mL, respectively, at 265 nm. Dehydroascorbic acid (DHAA) in fruits and spinach was quantified in the presence of DL-homocysteine. The recoveries for L-AA in these juices were between 95 and 105%.     

Diastereoselective synthesis of cyclosaligenyl-nucleosyl-phosphotriesters

A diastereoselective synthesis of cycloSal-phosphotriesters (cycloSal=cycloSaligenyl) based on chiral auxiliaries has been developed that allows the synthesis of single diastereomers of the cycloSal-pronucleotides. In previously described synthesis routes, the cycloSal-compounds were always obtained as 1:1 diastereomeric mixtures that could be separated in only rare cases. However, it was shown that the diastereomers have different antiviral activity, toxicity, and hydrolysis stabilities. Here, first a chiral thiazoline derivative was used to prepare nonsubstituted and 5-methyl-cycloSal-phosphotriesters in 48 and ≥95% de (de=diastereomeric excess). However, this approach failed to give the important group of 3-substituted cycloSal-nucleotides. Therefore, two other chiral groups were discovered that allowed the synthesis of (R(P))- and (S(P))-3-methyl-cycloSal-phosphotriesters as well. The antiviral activity was found to be five- to 20-fold different between the two individual diastereomers, which proved the importance of this approach.     

Improved synthesis of isocarbacyclin using regioselective alkylation of allylic alcohols

Two efficient syntheses of isocarbacyclin (1) have been realized using highly regioselective direct alkylation of both - and -allylic alcohols (2 and 3)