Convergent total synthesis of (+)-mycalamide A

The details of a convergent total synthesis of (+)-mycalamide A are described. Yb(OTf)3-TMSCl-catalyzed cross-aldol reaction conditions are used to synthesize the right segment of mycalamide A. In this reaction, an acid-sensitive aldehyde reacts with methyl trimethylsilyl dimethylketene acetal without epimerization to provide the desired aldol adduct. Additionally, a tetrahydropyran ring, which is the left segment of mycalamide A, is prepared using a novel one-pot delta-lactone formation methodology. Both segments are constructed from a common starting material, d-mannitol. These segments are then coupled in the presence of BuLi, and the functional groups are transformed to complete the synthesis of (+)-mycalamide A.     

A stereoselective synthesis of (+)-herboxidiene/GEX1A

A stereoselective synthesis of (+)-herboxidiene is described. The convergent synthesis utilized a Suzuki cross-coupling reaction to assemble the key segments. The synthesis of the functionalized tetrahydropyran ring utilized an Achmatowicz reaction as the key step. The synthesis of the C10-C19 segment was accomplished using Brown's crotylboration, asymmetric alkylation, and a stereoselective allylic chlorination reactions.     

Selective methodologies for the synthesis of biologically active piperidinic compounds

The synthesis of optically active substituted piperidines has been achieved by using four different methodologies. The first one is an intramolecular nucleophilic displacement of activated alcohol moieties that was used to build up the piperidine ring of (-)-prosophylline and (-)-slaframine, and the second one is a ring-closing metathesis of unsaturated amines which was employed in the synthesis of (+)-sedamine and 4a,5-dihydrostreptazoline. The third methodology is the alpha-functionalization of N-Boc piperidines which was particularly useful in the synthesis of argatroban, and the fourth one is a ring expansion of prolinols to 3-chloropiperidines or 3-hydroxypiperidines which was utilized to synthesize (-)-paroxetine, (-)-pseudoconhydrine, the piperidine ring of (-)-velbanamine and (+)-zamifenacin.     

A formal synthesis of (-)-mycalamide A

Novel strategies are developed for an efficient formal synthesis of (-)-mycalamide A. The left-hand side (-)-7-benzoylpederic acid is synthesized from (2S,3S)-2,3-epoxybutane. The key features include a highly regioselective Ru-catalyzed alkene-alkyne coupling reaction and a novel way to control the challenging C(7) stereocenter. The right-hand side was synthesized from (R)-pantolactone. The complex trioxodecalin core is constructed with two Pd(0)-catalyzed O-pi-allyl cyclizations. The first one is chemoselective, while the second one is highly diastereoselective. Three additional steps would be required to complete a total synthesis of (-)-mycalamide A.     

Total synthesis of the antitumor acetogenin mosin B: desymmetrization approach to the stereodivergent synthesis of threo/trans/erythro-type acetogenins

A total synthesis of the threo/trans/erythro-type acetogenin mosin B and one of its diastereomers has been achieved. The carbon skeleton is assembled in a convergent fashion from two segments (a THF ring segment and a gamma-lactone segment) through the Nozaki-Hiyama-Kishi reaction. The THF ring segment was stereoselectively constructed by a stereodivergent synthesis starting from a common intermediate (4-cyclohexene-1,2-diol) based on a desymmetrization strategy. The gamma-lactone segment was synthesized by coupling a triflate and a chiral alpha-sulfenyl gamma-lactone. By virtue of these synthetic results, we suggest that the absolute configuration of natural mosin B is 1 a. Antiproliferative effects of 1 a and 1 b were also investigated.     

Total Synthesis of (−)‐Himeradine A

(?)‐Himeradine A is a complex lycopodium alkaloid with seven rings and ten stereogenic centers that shows anticancer activity against lymphoma L1210 cells. A total synthesis has been developed that builds off prior work on (+)‐fastigiatine. A 2,4,6‐trisubstitited piperidine ring forms the core of the quinolizidine segment, and was prepared by diastereoselective reduction of a pyridine and classic resolution of an intermediate. The remaining secondary amine was introduced with a catalyst‐controlled Overman rearrangement. The piperidine segment was coupled in a B‐alkyl Suzuki reaction with a bicyclic bromoenone, which was a key intermediate for the synthesis of (+)‐fastigiatine. The final transformation featured a transannular Mannich reaction and cyclization to complete the quinolizidine. Five bonds and four new rings were generated in this one‐pot procedure. (?)‐Himeradine A was prepared in 17 steps in the longest linear sequence.     

利用单片机直接驱动光柱显示器件

介绍一种单片机直接驱动光柱显示器件的方法.该方法采用动态扫描的形式直接驱动两个101线光柱.光柱的十位数叫做段码,个位数叫做线数.与段码对应的段叫做当前段,段码减一的段及其以下的段叫做十线全显段.关显示就是关双光柱.     

The theta-temperature depression caused by topological effect in ring polymers studied by Monte Carlo simulation

We studied equilibrium conformations of linear and ring polymers in dilute solutions over the wide range of segment number N of up to 2048 with Monte Carlo simulation, and evaluated N dependence of the radii of gyration, R(g), of chains. The polymer molecules treated in this study are assumed to be composed of beads and bonds, and they are put in a three-dimensional face-centered cubic (FCC) lattice. The values of Flory's critical exponent, ν, for linear and ring polymers were estimated from the N dependence of R(g), and the temperatures at which ν reach 1/2 were obtained. Here we define those as Θ-temperatures in this report. The simulation result shows that the Θ-temperature for ring polymers is evidently lower than that of the linear polymers, and the origin of the Θ-temperature depression is discussed. Since R(g) of a ring polymer is smaller than that for a linear polymer at the same N and temperature, the segment density for a ring polymer is increased by the topological effect and the repulsive force between segments of a ring polymer at the Θ-temperature for a linear polymer is stronger. Thus, the origin of the Θ-temperature depression for ring polymers is the repulsive force emphasized by the topological effect of rings.     

Topological effect in ring polymers investigated with Monte Carlo simulation

We studied equilibrium conformations of ring polymers in the melt over the wide range of segment number up to 1000 by the Monte Carlo simulations and the bond fluctuation model, and estimated Flory's scaling exponent nu. The radial distribution function of segments for the ring polymers in the melt is obtained. We have found that nu for ring polymers is decreased with increasing segment number N, and nu goes down to 0.365 when N reaches 1000, whose value is apparently smaller than the theoretically predicted one, i.e., 25. Those values are in contrast to the well established nu value of 0.5 for linear polymers in the melt. This is because ring polymer chains in the melt are squeezed both by their own topological effect and the compression effect by the neighboring ring polymer coils which are also squeezed at bulk state. The difference in our result and the theory may be due to the fact that the estimation of topological entropy loss was ignored in the theoretical prediction, while it has been taken into consideration in the present study. If polymer coils repel each other in melt at N --> infinity, we have the limiting nu value of 13, so we conclude that nu is in the range of 13 < or = nu < 0.365 when the molecular weight of a ring polymer is high enough.     

Concise enantioselective total syntheses of (+)-homochelidonine, (+)-chelamidine, (+)-chelidonine, (+)-chelamine and (+)-norchelidonine by a Pd II-catalyzed ring-opening strategy

New enantioselective syntheses of the B/C hexahydrobenzo[c]phenanthridine alkaloids (+)-homochelidonine, (+)-chelamidine, (+)-chelidonine, (+)-chelamine, and (+)-norchelidonine are described. Our rapid and convergent route to this class of natural products involved the development and application of a Pd II-catalyzed asymmetric ring-opening reaction of a meso-azabicyclic alkene with an aryl boronic acid as the key step. By screening a variety of functionalized ortho-substituted aryl boronic acids, chiral ligands and reaction conditions we were able to prepare the requisite cis-1-amino-2-aryldihydronaphthalenes in high yield and in up to 90 % ee. Early attempts to complete the synthesis of (+)-homochelidonine using an N-Boc azabicyclic alkene are described in full. The successful route required a protecting group alteration followed by B ring formation and then stereoselective installation of the C-11 syn-hydroxy group by regioselective epoxide ring-opening using a hydride source. Ring-opening of the same epoxide intermediate with water ultimately led to the synthesis of (+)-chelamidine. The same strategy was then used to synthesize the other structurally similar B/C hexahydrobenzo[c]phenanthridine alkaloids, (+)-chelidonine, (+)-chelamidine, and (+)-norchelidonine.     

Parameters affecting transition temperatures of poly(lactic acid‐co‐polydiols) copolymer‐based polyester urethanes and their shape memory behavior

A series of polyester urethanes (PEUs) comprising poly(lactic acid‐co‐polydiol) copolymers as a soft segment, 4,4′‐diphenylmethane diisocyanate (MDI) and 1,4‐butanediol (BDO) as a hard segment were systematically synthesized. Soft segments, which were block copolymers of L ‐lactide (LA) and polydiols such as poly(ethylene glycol) and poly(trimethylene ether glycol), were prepared via ring opening polymerization. Glass transition temperatures (T_g) of the obtained PEUs were found strongly dependent on properties of copolymer soft segments. By simply changing composition ratio, type and molecular weight of polydiols in the soft segment preparation step, T_g of PEU can be varied in the broad range of 0–57°C. The synthesized PEUs exhibited shape memory behavior at their transition temperatures. PEUs with hard segment ratio higher than 65 mole percent showed good shape recovery. These findings suggested that it is important to manipulate molecular structure of the copolymer soft segment for a desirable transition temperature and design optimal soft to hard segment ratio in PEU for good shape recovery. Copyright © 2011 John Wiley & Sons, Ltd.     

Ring expansions of beta-keto lactones with zinc carbenoids: syntheses of (+)-patulolide A and (+/-)-patulolide B

A one-pot ring expansion/oxidation/elimination method has been developed in which beta-keto lactones are converted efficiently to alpha,beta-unsaturated-gamma-keto lactones. The reaction can be successfully applied to a variety of ring sizes. Alkene stereochemistry is dependent upon ring size and reaction conditions. The method was applied to the synthesis of (+)-patulolide A.     

Total synthesis of (+)-tubelactomicin A. 2. Synthesis of the upper-half segment and completion of the total synthesis

[reaction: see text]. We have completed the total synthesis of natural (+)-tubelactomicin A (1), a 16-membered macrolide antibiotic. This Letter presents a highly efficient synthesis of the upper-half segment (C14-C24) and the completion of the total synthesis featuring a high-yielding Stille coupling for the connection of the upper-half and lower-half segments and Mukaiyama macrolactonization for the construction of the entire structure of 1.     

Synthesis of thionaphtheno [3, 2-b] pyrrole

The interest shown in the preparation of new heterocyclic systems, and functional derivatives of them, led us to synthesize a new condensed ring system, which included the thionaphthene and pyrrole ring systems.     

A general procedure for the synthesis of stereochemically pure conduritol derivatives practical also for solid-phase chemistry

A straightforward synthetic protocol apt to synthesize a library constituted by all conduritol stereoisomers in solution phase is described and successfully applied to some polymer-supported substrates. During the solid-phase sequence, an unprecedented rearrangement of a resin-bound sulfone performed under the Ramberg-B?cklund conditions appears of particular interest. Upon treatment with Me(3)Si-I, thiepanes supported on resin are found to undergo regio- and stereospecific ring contraction to a six-membered ring system with traceless cleavage from the solid support.     

Chain propagation/step propagation polymerization. III. An XPS investigation of poly(oxyethylene)-b-poly(pivalolactone) telechelomer

A combination of chain and step propagations is being used to prepare a series of segmented copolymers containing polyoxyethylene and polypivalolactone segments each with narrow molecular weight distributions. The incompatibility of the two segments coupled with the difference in surface energies of the two segments could result in good phase separation with the polyoxyethylene segment thereby being found on the surface. Poly(oxyethylene)-co-(pivalolactone) telechelomer, which is the precursor to the segmented copolymer, is used as a model to develop an XPS method for surface analysis. An angular resolution study shows that negligible amounts of poly(pivalolactone) segments are present near the surface. Core level spectra of the telechelomer and its model compounds are presented to indicate that poly(oxyethylene) segment is richer near the surface.     

Facile syntheses,morphologies, and optical absorptions of P3HT coil‐rod‐coil triblock copolymers

Here we report syntheses, photophysical properties, and morphologies of a series of coil‐rod‐coil ABA triblock copolymers containing highly regioregular poly(3‐hexylthiophene) (P3HT) as the central rod block. A new methodology, based on the coupling reaction between living polymeric anions [polystyrene, polyisoprene, and poly(methyl methacrylate)] and aldehyde terminated P3HT, was successfully developed to synthesize the triblock copolymers with low polydispersities. This coupling reaction was effective for building blocks with a variety of molecular weights; therefore, a good variation in compositions of the triblock copolymers could be feasibly achieved. The non‐P3HT coil segments and the solvents were found to exhibit noticeable effects on morphologies of the spin‐coated thin films. Attachment of the coil segments to P3HT did not change the optical absorption of the P3HT segment as the block copolymers were dissolved in solution regardless the chemical structure and the molecular weight of the coil segment. Interestingly, different UV–vis absorption behaviors were observed for the spin‐coated thin films of the block copolymers, which closely related to their morphologies. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3311–3322, 2010     

Enantioselective Total Synthesis of (+)‐Ophiobolin A

The enantioselective total synthesis of (+)‐ophiobolin A is described. This total synthesis features the construction of the spiro CD ring of (+)‐ophiobolin A through a stereoselective intramolecular Hosomi–Sakurai cyclization reaction, the joining of the A ring to the CD ring by using a reaction reported by Utimoto, and the construction of the ophiobolin eight‐membered carbocyclic ring through ring‐closing metathesis (RCM), which was performed for the first time in this study. This successful RCM reaction required the use of a substrate that contained either a benzyloxy or a methoxymethoxy group at the C5 position and either an isopropenyl group or its hydroxylated form at the C6 position.     

Flexible strategy for the synthesis of pyrrolizidine alkaloids

A general strategy for the production of pyrrolizidine alkaloids is described, starting from intermediate (+)-9. The key features are diastereoselective dihydroxylation, inversion at the ring junction by hydroboration of an enamine, and ring closure to form the bicyclo ring system. This route is attractive because of its brevity and versatility; four natural products were prepared with differing stereochemistry and substitution patterns. Finally, this work allowed assignment of the absolute stereochemistry of 2,3,7-triepiaustraline and hyacinthacine A 7.     

Total synthesis of (+)-pinnatoxin A

A convergent enantioselective total synthesis of (+)-pinnatoxin A is described. The synthesis capitalizes on the highly diastereoselective Ireland-Claisen rearrangement of an acyclic alpha-branched allylic ester to set the quaternary stereogenic center at the core of the spiroimine ring system along with the adjacent tertiary stereocenter. The all-carbon macrocyclic ring system was formed by ring-closing metathesis.