Asymmetric α‐Hydroxylation of a Lactone with Vinylogous Pyridone by Using a Guanidine–Urea Bifunctional Organocatalyst: Catalytic Enantioselective Synthesis of a Key Intermediate for (20S)‐Camptothecin Analogues

We have developed a catalytic asymmetric synthesis of (S)‐4‐ethyl‐6,6‐(ethylenedioxy)‐7,8‐dihydro‐4‐hydroxy‐1H‐pyrano[3,4‐f]indolizine‐3,10(4H)dione ( 5 a ), a synthetic intermediate for (20S)‐camptothecin analogues. A key step in this synthesis is an asymmetric α‐hydroxylation of a lactone with a vinylogous pyridone structure ( 8 a ) by using a guanidine–urea bifunctional organocatalyst. The present oxidation was successfully applied to the synthesis of C20‐modified derivatives of (+)‐C20‐desethylbenzylcamptothecin ( 13 ).     

Total Synthesis of Tubulysin U and Its C‐4 Epimer

The Tup fragments of tubulysins were synthesized with a tandem reaction as the key step, and unexpected diastereoselectivity was observed in the first Grignard addition stage. The coupling of the enolate of a thiazolyl ketone with chiral sulfinimines furnished the backbone of the Tuv fragment with over 100:1 d.r. and high yield. Thus, tubulysin U and C‐4 epi‐tubulysin U were prepared in a highly selective and efficient manner. The results of the MTT assay furthermore indicated that C‐4 epi‐tubulysin U maintained significant growth inhibition activities against several cancer cell lines.     

Total Synthesis of (−)‐Apratoxin A, 34‐Epimer,and Its Oxazoline Analogue

A concise and convergent total synthesis of the highly cytotoxic marine natural product apratoxin A is accomplished by an 18‐step linear sequence. The high sensitivity of the thiazoline, bearing an adjacent β‐hydroxyl group at the C35‐position, results in the assembly process requiring the inclusion of appropriate protecting groups and the careful optimization of all individual transformations. In the synthesis of 3,7‐dihydroxy‐2,5,8,8‐tetramethylnonanoic acid (Dtena), the three reagent‐controlled asymmetric reactions enables us to introduce four chiral carbon centers in a dihydroxylated fatty acid moiety. Formation of the hindered ester and sterically‐unfavorable N‐methylamide bonds were successfully demonstrated. The thiazoline in apratoxin A was constructed by Tf₂O and Ph₃PO‐mediated dehydrative cyclization, and final macrocyclization was achieved between N‐methylisoleucine and proline residues. Moreover, an oxazoline analogue and a C34 epimer of apratoxin A have also been elaborated in a similar approach. This synthetic route would enable assembly of other analogues differing in stereocenters of Dtena and their amino acids.     

Gold(I) as an Artificial Cyclase: Short Stereodivergent Syntheses of (−)‐Epiglobulol and (−)‐4β,7α‐ and (−)‐4α,7α‐Aromadendranediols

Three natural aromadendrane sesquiterpenes, (?)‐epiglobulol, (?)‐4β,7α‐aromadendranediol, and (?)‐4α,7α‐aromadendranediol, have been synthesized in only seven steps in 12, 15, and 17 % overall yields, respectively, from (E,E)‐farnesol by a stereodivergent gold(I)‐catalyzed cascade reaction which forms the tricyclic aromadendrane core in a single step. These are the shortest total syntheses of these natural compounds.     

(-)-（3S, 6R）-3, 6-羟基-10-甲基十一酸及其三聚体的全合成

以廉价易得的异戊基溴为起始原料,以烯丙基二异松莰烷基硼烷参与的不对称烯丙基化反应和Yamaguchi酯化反应为关键步骤,实现了对(-)-(3S,6R)-3,6-二羟基-10-甲基十一酸(总收率27.5％)及其三聚体(总收率24.5％)的不对称全合成。     

Practical synthesis of (S)‐7‐(2‐isopropylamino)ethylcamptothecin hydrochloride,potent topoisomerase I inhibitor

A practical semi‐synthetic method of (S)‐7‐(2‐isopropylamino)ethylcamptothecin hydrochloride has been developed. The Mannich reaction of (S)‐7‐methylcamptothecin with isopropylamine hydrochloride in dimethyl sulfoxide as a formaldehyde source gave the desired product in moderate yield.     

20(S)-O-取代苯甲酸-7-乙基喜树碱酯类化合物的合成及其抗肿瘤活性

在AcOH/98％ H2S04体系中,喜树碱和正丙醛经烷基化反应制得7-乙基喜树碱(2);以1-乙基-3-(3-二甲基氨基丙基)碳二亚胺盐酸盐为脱水剂,4-二甲氨基吡啶为催化剂,2与取代苯甲酸直接酯化合成了一系列新型的20(S)-O-取代苯甲酸-7-乙基喜树碱酯类化合物(4a -4k),其结构经1H NMR,IR和MS表征.采用MTT法初步考察了4a～4k对人肺癌细胞(LLC-E9FP),人肺腺癌细胞(95D),人胃癌细胞(BGC-803),人胃癌细胞( HGC-27)和人肝癌细胞(7721)的抑制活性.结果表明,4b和4e对LLC-E9FP具有明显强于喜树碱的抑制活性;4h～4k对HGC-27,95D,7721也有明显的抑制活性.     

Synthesis of Murisolin, (15R, 16R, 19R, 20S)‐Murisolin A,and (15R, 16R, 19S, 20S)‐16,19‐cis‐Murisolin and Their Inhibitory Action with Bovine Heart Mitochondrial Complex I

The asymmetric total synthesis of murisolin, (15R, 16R, 19R, 20S)‐murisolin A, and (15R, 16R, 19S, 20S)‐16,19‐cis‐murisolin was performed by using an epoxy alcohol as a versatile chiral building block for synthesizing the stereoisomers of mono‐THF annonaceous acetogenins. The inhibitory activity of these murisolin compounds was examined with bovine heart mitochondrial complex I, and they showed almost the same activity.     

Total Synthesis of (6R,10R,13R,14R,16R,17R,19S,20R,21R,24S, 25S,28S,30S,32R,33R,34R,36S,37S,39R)‐Azaspiracid‐3 Reveals Non‐Identity with the Natural Product

A convergent and stereoselective total synthesis of the previously assigned structure of azaspiracid‐3 has been achieved by a late‐stage Nozaki–Hiyama–Kishi coupling to form the C21?C22 bond with the C20 configuration unambiguously established from l ‐(+)‐tartaric acid. Postcoupling steps involved oxidation to an ynone, modified Stryker reduction of the alkyne, global deprotection, and oxidation of the resulting C1 primary alcohol to the carboxylic acid. The synthetic product matched naturally occurring azaspiracid‐3 by mass spectrometry, but differed both chromatographically and spectroscopically.     

TiCl3‐Mediated Synthesis of 2,3,3‐Trisubstituted Indolenines: Total Synthesis of (+)‐1,2‐Dehydroaspidospermidine, (+)‐Condyfoline,and (−)‐Tubifoline

2,3,3‐Trisubstituted indolenine constitutes an integral part of many biologically important monoterpene indole alkaloids. We report herein an unprecedented access to this skeleton by a TiCl₃‐mediated reductive cyclization of tetrasubstituted alkenes bearing a 2‐nitrophenyl substituent. The proof of concept is demonstrated firstly by accomplishing a concise total synthesis of (+)‐1,2‐dehydroaspidospermidine featuring a late‐stage application of this key transformation. A sequence of reduction of nitroarene to nitrosoarene followed by 6π‐electron‐5‐atom electrocyclization and a 1,2‐alkyl shift of the resulting nitrone intermediate was proposed to account for the reaction outcome. A subsequent total synthesis of (+)‐condyfoline not only illustrates the generality of the reaction, but also provides a mechanistic insight into the nature of the 1,2‐alkyl shift. The exclusive formation of (+)‐condyfoline indicates that the 1,2‐alkyl migration follows a concerted Wagner–Meerwein pathway, rather than a stepwise retro‐Mannich/Mannich reaction sequence. Conditions for almost quantitative conversion of (+)‐condyfoline to (?)‐tubifoline by way of a retro‐Mannich/1,3‐prototropy/transannular cyclization cascade are also documented.     

Palladium‐Catalyzed Decarbonylative Dehydration for the Synthesis of α‐Vinyl Carbonyl Compounds and Total Synthesis of (−)‐Aspewentins A,B, and C

The direct α‐vinylation of carbonyl compounds to form a quaternary stereocenter is a challenging transformation. It was discovered that δ‐oxocarboxylic acids can serve as masked vinyl compounds and be unveiled by palladium‐catalyzed decarbonylative dehydration. The carboxylic acids are readily available through enantioselective acrylate addition or asymmetric allylic alkylation. A variety of α‐vinyl quaternary carbonyl compounds are obtained in good yields, and an application in the first enantioselective total synthesis of (?)‐aspewentins A, B, and C is demonstrated.     

Stereochemical Definition of the Natural Product (6R,10R,13R, 14R,16R,17R,19S,20S,21R,24S,25S,28S,30S,32R,33R,34R,36S,37S,39R)‐Azaspiracid‐3 by Total Synthesis and Comparative Analyses

The previously accepted structure of the marine toxin azaspiracid‐3 is revised based upon an original convergent and stereoselective total synthesis of the natural product. The development of a structural revision hypothesis, its testing, and corroboration are reported. Synthetic (6R,10R,13R,14R,16R,17R,19S,20S,21R,24S,25S,28S,30S,32R, 33R,34R,36S,37S,39R)‐azaspiracid‐3 chromatographically and spectroscopically matched naturally occurring azaspiracid‐3, whereas the previously assigned 20R epimer did not.     

Total Synthesis of (+/−)‐Frondosin B and (+/−)‐5‐epi‐Liphagal by Using a Concise (4+3) Cycloaddition Approach

A recently developed (4+3) cycloaddition between dienes and furfuryl alcohols, as precursors of oxyallyl‐type cations, has been used as a key step in the racemic syntheses of two natural products: frondosin B and liphagal. This work demonstrates the synthetic potential of this cycloaddition reaction, and offers a short synthetic route to an interesting family of natural products. A full account of these synthetic studies is presented, further illustrating the mechanism, scope, and limitations of this straightforward synthetic method for seven‐membered rings.     

20(S)-O-喜树碱肉桂酸酯衍生物的合成及其抗肿瘤活性研究

为了提高喜树碱内酯环的稳定性降低其毒性, 增加其抗肿瘤效能, 以喜树碱为先导化合物, 通过酯化反应直接合成了17个20(S)-喜树碱肉桂酸酯衍生物, 并采用MTT法测定了对于人胃癌细胞SGC-7901的体外抗肿瘤活性, 活性测试结果表明有些化合物的抑制活性明显高于母体化合物喜树碱.     

Asymmetric Total Synthesis of (−)‐Stenine and 9a‐epi‐Stenine

A route for the asymmetric synthesis of (?)‐stenine, a member of the Stemona alkaloid family used as folk medicine in Asian countries, is described. The key features of the sequence employed include stereoselective transformations on a cyclohexane ring controlled by a chiral auxiliary unit and an intramolecular Mitsunobu reaction to construct the perhydroindole ring system. By using an intermediate in the route to (?)‐stenine, an asymmetric synthesis of 9a‐epi‐stenine was also executed. The C(9a) stereocenter in 9a‐epi‐stenine was installed by using a Staudinger/aza‐Wittig reaction of a keto–azide precursor followed by reduction of the resulting imine. The results of this effort demonstrate the applicability of the chiral auxiliary based strategy to the preparation of naturally occurring alkaloids that contain highly functionalized cyclohexane cores.