Stereospecific homologation of d-xylo and d-galacto dialdoses by 2-trimethylsilylthiazole

Homologation of the title dialdoses is carried out by diastereogenic addition of 2-trimethylsilylthiazole (1) to the side-chain aldehyde and unmasking the formyl group from thiazole ring; further addition of 1 to the resulting homologated dialdoses exhibited good levels of diastereoselectivity.     

Deoxygenation of carbohydrates by thiol-catalysed radical-chain redox rearrangement of the derived benzylidene acetals

Five- or six-membered cyclic benzylidene acetals, derived from 1,2- or 1,3-diol functionality in carbohydrates, undergo an efficient thiol-catalysed radical-chain redox rearrangement resulting in deoxygenation at one of the diol termini and formation of a benzoate ester function at the other. The role of the thiol is to act as a protic polarity-reversal catalyst to promote the overall abstraction of the acetal hydrogen atom by a nucleophilic alkyl radical. The redox rearrangement is carried out in refluxing octane and/or chlorobenzene as solvent at ca. 130 degrees C and is initiated by thermal decomposition of di-tert-butyl peroxide (DTBP) or 2,2-bis(tert-butylperoxy)butane. The silanethiols (Bu(t)O)3SiSH and Pr(i)3SiSH (TIPST) are particularly efficient catalysts and the use of DTBP in conjunction with TIPST is generally the most effective and convenient combination. The reaction has been applied to the mono-deoxygenation of a variety of monosaccharides by way of 1,2-, 3,4- and 4,6-O-benzylidene pyranoses and a 5,6-O-benzylidene furanose. It has also been applied to bring about the dideoxygenation of mannose and of the disaccharide alpha,alpha-trehalose. The use of p-methoxybenzylidene acetals offers no great advantage and ethylene acetals do not undergo significant redox rearrangement under similar conditions. Functional group compatibility is good and tosylate, epoxide and ketone functions do not interfere; it is not necessary to protect free OH groups. Because of the different mechanisms of the ring-opening step (homolytic versus heterolytic), the regioselectivity of the redox rearrangement can differ usefully from that resulting from the Hanessian-Hullar (H.-H.) and Collins reactions for brominative ring opening of benzylidene acetals. When simple deoxygenation of a carbohydrate is desired, the one-pot redox rearrangement offers an advantage over H.-H./Collins-based procedures in that the reductive debromination step (which often involves the use of toxic tin hydrides) required by the latter methodology is avoided.     

Stereoselective epoxidation of 3-trans-benzylideneisoborneol

2-Benzylidenecyclopentanol ( 2 ) can be epoxidized stereoselectively to furnish 2-trans-benzylidene-1β-hydroxycyclopentane β-oxide ( 4 ) with either t-butyl hydroperoxide in the presence of vanadium catalyst or m-chloroperoxybenzoic acid. Epoxidation of 3-trans-benzylideneisoborneol ( 10 ) with t-butyl hydroperoxidevanadium catalyst furnishes stereoselectively 3-trans-benzylideneisoborneol exo-oxide ( 11 ) whereas epoxidation of alcohol 10 with m-chloroperoxybenzoic acid furnishes stereoselectively 3-trans-benzylideneisoborneol endo-oxide ( 14 ).     

Stereoselective formation of silylketene acetals from esters and trialkylsilyl perchlorates.

Alkanoate esters react with triethylsilyl perchlorate and trialkylamines at low temperature to yield O-silylketene acetals. The $\text{Z}$ isomer is obtained in excellent yield.     

Stereoselective and facile total synthesis of (+)-goniodiol,a styryllactone from carbohydrates

The stereoselective synthesis of (+)-goniodiol, a cytotoxic styryllactone, has been accomplished in 10 steps starting from inexpensive and readily available d-manitol and δ-gluconolactone involving the direct and straightforward reaction conditions of Grignard addition, chain elongation, and hydroboration, thus making the synthesis simple and convenient.     

New epoxidation reagents derived from alumina and silicon

Hydrogen peroxide or tert butyl hydroperoxide in contact with basic alumina will epoxidize olefins. Triphenylsilyl hydroperoxide behaves similarly to peracids toward olefins, ketones and allylic alcohols.     

Molybdenum(VI)-complexes with chiral N,O-ligands derived from carbohydrates: synthesis, structure and catalytic properties in asymmetric olefin epoxidation

Novel chiral 2′-pyridinyl alcohols derived from isopropylidene-protected carbohydrates are reported. They show different characteristics at the hydroxy group, but are all suitable ligands for chiral molybdenum(VI) complexes of the type MoO₂L₂ (L = chiral 2′-pyridinyl alcoholate). MoO₂(acac)₂ served as starting material in the complex syntheses. The structure of one ligand and one dioxo complex were exemplary established by X-ray crystallography. For catalytic runs in the enantioselective epoxidation catalysis trans-methylstyrene was used as model substrate, tert-butylhydroperoxide and cumolhydroperoxide, resp., as the oxidant.     

The chemistry of trifluoromethyl imines and related acetals derived from fluoral

Trifluoromethylated nitrogen-containing molecules have been shown to have important biological effects and their synthesis is in the focus of the pharmaceutical industry. In the last few years, simple nitrogen derivatives of fluoral, i.e. imines, acetals and oxazolidines, have emerged as powerful building blocks to synthesize these target molecules and relevant precursors. This review summarizes the chemistry of these "N-derivatives of fluoral", with special highlight on the syntheses of peptidomimetic units (amino alcohols, amino acids...) and heterocycles (piperidines, beta-lactams...).     

Stereoselective dimerization of benzylic amines derived from indoline

Treatment of benzylic amines derived from 2‐(acyloxymethyl)‐5‐nitroindolines with sodium hexamethyl‐disilazide leads to dimeric products resulting from deprotonation in the benzylic position, oxidation of the resulting carbanion to radical by the nitroarene moiety of another molecule, and stereoselective radical recombination. Only those two of six possible diastereoisomers are formed in which the recombination takes place from the less hindered face in the more stable conformation of the presumably near‐planar radical.     

Regiospecific syntheses of quinones using vinylketene acetals derived from unsaturated esters

A general method of annellating quinones in high yield has been devised using mixed vinylketene acetals obtained directly from the enolate ions of unsaturated esters.     

Stereoselective aldol additions to α-alkoxy aldehydes using thioester silyl ketene acetals,

Relative stereochemistry (chelation) effectively controls internal stereochemistry (syn-anti) in the addition of thiopropionate equivalents to O-benzyl lactic aldehyde.     

Carbon-carbon bond formation using primary tosylates derived from carbohydrates

Primary tosylates derived from carbohydrates proved to be substrates for direct carbon-carbon bond formation. This procedure is compared to a classical three steps sequence: oxidation-Wittig reaction-hydrogenation.     

Stereoselective formation and rearrangement of morpholinium ylides derived from copper carbenoids

Amino diazoacetoacetates 4a–e and 7a,b were prepared from readily available amino alcohols and subjected to copper-catalyzed carbene-transfer reaction. Substrates 4c–e furnished the morpholin-2-ones 5c–e in good yield via the corresponding cyclic ammonium ylides, albeit with poor diastereoselectivity. Substrates 4a,b failed to provide the corresponding morpholinones, perhaps as a result of steric congestion. Cyclic substrates 7a,b underwent conversion to bicyclic morpholinones 8 and 9 in good yield and with moderate to good diastereoselectivity. This result is rationalized via control of the transiently stereogenic ammonium center of the intermediate bicyclic ylides.     

Thiol-catalysed radical-chain redox rearrangement reactions of benzylidene acetals derived from terpenoid diols

The thiol-catalysed radical-chain redox rearrangement of cyclic benzylidene acetals derived from 1,2- and 1,3-diols of terpene origin has been investigated from both synthetic and mechanistic standpoints. The redox rearrangement was carried out either at ca. 70 degrees C (using Bu(t)ON=NOBu(t) as initiator) or at ca. 130 degrees C (using Bu(t)OOBu(t) as initiator) in the presence of triisopropylsilanethiol or methyl thioglycolate as catalyst; the silanethiol was usually more effective. This general reaction affords the benzoate ester of the monodeoxygenated diol, unless rearrangement of intermediate carbon-centred radicals takes place prior to final trapping by the thiol to give the product, in which case structurally rearranged esters are obtained. For the benzylidene acetals of 1,2-diols prepared by vicinal cis-dihydroxylation of 2-carene, alpha-pinene or beta-pinene, intermediate cyclopropylcarbinyl or cyclobutylcarbinyl radicals are involved and ring opening of these leads ultimately to unsaturated monocyclic benzoates. 1,2-Migration of the benzoate group in the intermediate beta-benzoyloxyalkyl radical sometimes also competes with thiol trapping during the redox rearrangement of benzylidene acetals derived from 1,2-diols. Redox rearrangement of the benzylidene acetal from carane-3,4-diol, obtained by cis-dihydroxylation of 3-carene, does not involve intermediate cyclopropylcarbinyl radicals and leads to benzoate ester in which the bicyclic carane skeleton is retained. The inefficient redox rearrangement of the relatively rigid benzylidene acetal from exo,exo-norbornane-2,3-diol is attributed to comparatively slow chain-propagating beta-scission of the intermediate 2-phenyl-1,3-dioxolan-2-yl radical, probably caused by the development of adverse angle strain in the transition state for this cleavage. Similar angle strain effects are thought to influence the regioselectivities of redox rearrangement of bicyclic [4.4.0]benzylidene acetals resulting from selected 1,3-diols, themselves prepared by reduction of aldol adducts derived from reactions of aldehydes with the kinetic lithium enolates obtained from menthone and from isomenthone.     

Stereoselective epoxidation of 2-arylidene-1-indanones and 2-arylidene-1-benzosuberones

Summary Oxidation of the (E) and (Z) isomers of 2-arylidene-1-indanones (1) and 2-arylidene-1-benzosuberones (4) by alkaline hydrogen peroxide (methodi) afforded the spiroepoxidestrans-2a–g andtrans-5a–g from both isomers as sole products in high yields. On the other hand, dimethyldioxirane epoxidation(methodii) of the (E) isomers1a–g and4a–g gave the correspondingtrans spiroepoxides in good yields, whereas the (Z) isomers1a,c,e and4a,c,e led to thecis spiroepoxides in moderate yields. Dimethyldioxirane oxidation (methodii) of (Z)-1c and (Z)-4c,e gave diones3c and6c,e as by-products as well. Epoxidation of (Z)-1a,c,e and (Z)-4a,c,e bym-chloroperoxybenzoic acid (methodiii) resulted inca. 6:1 mixtures ofcis-2a,c,e andtrans-2a,c,e orcis-5a,c,e andtrans-5a,c,e spiroepoxides.Dedicated to Prof.W. Fleischhacker on the occasion of his 65^th birthday     

Asymmetric epoxidation by chiral ketones derived from carbocyclic analogues of fructose.

A number of carbocyclic analogues of the fructose-derived ketone 1 have been prepared and investigated for asymmetric epoxidation. The studies show that the oxygen atom of the pyranose ring of 1 has an impact on the catalyst's activity and selectivity. Conformational, electronic, and steric effects are discussed.     

Amino acid derived heterocycles: lewis acid catalyzed and radical cyclizations from peptide acetals

Bicyclization of peptide acetals via nucleophilic attack of a phenyl group on an endocyclic acyliminium ion 4 was explored as a route to novel amino acid derived heterocycles and peptidomimetic scaffolds. In the presence of protic acid, bridged structures such as 6 are formed readily from phenylalanine derivatives, but the fused-ring analogues 5 could not be obtained in good yield. In contrast, radical cyclization of the bromophenyl dihydropyrazinone 7 provides an effective alternative for the synthesis of 5 (n = 0, 1, 2). Additional versatility in this process was demonstrated by efficient synthesis of a different fused ring system, represented by the antihelmintic praziquantel, 8.     

脯氨酸衍生的胺基吡啶四氮锰配合物催化双氧水参与的烯烃不对称环氧化反应（英文）

自然界中存在许多的金属酶,它们参与促进各种各样的氧化反应,例如羟化反应,环氧化反应等.金属酶催化的反应具有催化效率高、反应条件温和、选择性高等优点.受大自然中的金属酶结构及其性质的启发,人们提出了仿生催化氧化的理念,并开始对金属酶进行模拟,致力于发展清洁氧化的反应方式.在过去的几十年中,科学家们设计合成了一系列仿生金属配合物催化剂.例如,利用非手性的乙二胺骨架设计合成出四齿氮配体MEP(N,N'-dimethylN,N'-bis(2-pyridinylmethyl)ethane-1,2-diamine),将其制备成相应的铁配合物催化剂,该铁催化剂可以很好的实现脂肪族烯烃的环氧化,产率高达90%.2003年,Stack小组首次报道了利用手性N,N-二甲基环己二胺骨架衍生的四齿氮配体金属配合物Mn-MCP-(OTf)2(MCP=N,N-dimethyl-N,N-bis(2-pyridylmethyl)cyclohexane-trans-1,2-diamine)催化的不对称环氧化反应.该反应的对映选择性仅仅为10%.因此,发展新型手性四氮配体金属配合物,用于高产率、高对映选择性的不对称环氧化反应,值得进行深入研究.近年来发展的一些含手性二胺骨架的四齿氮配体,例如PDP(2-[[2-(1-(pyridin-2-ylmethyl)-pyrrolidin-2-yl)pyrrolidin-1-yl]methyl]pyridine),被应用到不对称环氧化反应中,但是其手性二胺骨架为联吡咯,价格昂贵,难以制备.这在很大程度上限制了其在不对称合成中的实际应用.因此,利用一些易于合成的手性二胺骨架,发展结构新颖、催化性能优良的四氮金属配合物,成为实现高效、高选择性不对称环氧化反应的关键.在之前的工作基础上,本文以简单易得、价格低廉的天然氨基酸——L-脯氨酸为起始原料,选取吡啶环和含取代基的吡啶环作为侧基氮供体,制备了三种手性四齿氮配体.随后,我们利用新发展的手性四齿氮配体,合成了相应的锰配合物,并且分别将其运用于烯烃不对称环氧化反应中,仔细评估了这些锰金属配合物的催化性能.建立了以0.2 mol%的锰配合物为催化剂,0.5当量的2,2-二甲基丁酸为添加剂,30%双氧水为氧化剂,反应温度为–30 oC,乙腈为溶剂的催化不对称环氧化反应体系.反应结果显示:该催化剂催化的不对称环氧化反应底物适用性广泛,其中苯乙烯、苯并吡喃、烯酰胺等化合物均可以被成功地转化为相应的环氧化物,得到中等至优异的对映选择性(产率最高可达95%,对映选择性最高可达99%).