Toward the synthesis of norzoanthamine: complete fragment assembly

The complex marine alkaloid norzoanthamine (2) was envisioned to be assembled from three key building blocks: the C1-C5 fragment A, the C6-C10 fragment B, and the C11-C24 fragment C. The synthesis of fragment A was achieved in 14 steps and 33% overall yield from (R)-gamma-hydroxymethyl-gamma-butyrolactone. Fragment B was made in two steps from PMB-protected 4-pentynol in 76% yield. The C11-C24 fragment C was made from (S)-carvone via (R)-isocarvone in 18 steps (6% overall yield). The convergent stereoselective synthesis of the entire carbon framework (C1-C24) of the target molecule was achieved via the following assemblage. Alkenyl iodide 20 derived from the C11-C24 fragment C was coupled to fragment B (C6-C10) through a high-yielding Stille coupling reaction of these two sterically very demanding coupling partners, affording the key Diels-Alder precursor 24. The intramolecular Diels-Alder reaction proceeded smoothly in excellent yield and diastereoselectivity, generating the tricyclic trans-anti-trans perhydrophenanthrene motif of norzoanthamine (C6-C24). The final fragment coupling between lithiated fragment A (C1-C5) and aldehyde 40 (C6-C24) has also been successfully accomplished affording the entire carbon framework of the natural product.     

Total synthesis of the epidermal growth factor inhibitor (-)-reveromycin B

The total synthesis of the epidermal growth factor inhibitor reveromycin B (2) in 25 linear steps from chiral methylene pyran 13 is described. The key steps involved an inverse electron demand hetero-Diels-Alder reaction between dienophile 13 and diene 12 to construct the 6,6-spiroketal 11 which upon oxidation with dimethyldioxirane and acid catalyzed rearrangement gave the 5,6-spiroketal aldehyde 9. Lithium acetylide addition followed by oxidation/reduction and protective group manipulation provided the reveromycin B spiroketal core 8 which was converted into the reveromycin A (1) derivative 6 in order to confirm the stereochemistry of the spiroketal segment. Introduction of the C1-C10 side chain began with sequential Wittig reactions to form the C8-C9 and C7-C6 bonds, and a tin mediated asymmetric aldol reaction installed the C4 and C5 stereocenters. The final key steps to the target molecule 2 involved a Stille coupling to introduce the C21-C22 bond, succinoylation, selective deprotection, oxidation, and Wittig condensation to form the final C2-C3 bond. Deprotection was effected by TBAF in DMF to afford reveromycin B (2) in 72% yield.     

A practical synthesis of (+)-discodermolide and analogues: fragment union by complex aldol reactions

A practical stereocontrolled synthesis of (+)-discodermolide (1) has been completed in 10.3% overall yield (23 steps longest linear sequence). The absolute stereochemistry of the C(1)-C(6) (7), C(9)-C(16) (8), and C(17)-C(24) (9) subunits was established via substrate-controlled, boron-mediated, aldol reactions of the chiral ethyl ketones 10, 11, and 12. Key fragment coupling reactions were a lithium-mediated, anti-selective, aldol reaction of aryl ester 8 (under Felkin-Anh induction from the aldehyde component 9), followed by in situ reduction to produce the 1,3-diol 40, and a (+)-diisopinocampheylboron chloride-mediated aldol reaction of methyl ketone 7 (overturning the inherent substrate induction from the aldehyde component 52) to give the (7S)-adduct 58. The flexibility of our overall strategy is illustrated by the synthesis of a number of diastereomers and structural analogues of discodermolide, which should serve as valuable probes for structure-activity studies.     

Formal synthesis of amphidinin B

An efficient, convergent, and highly stereoselective formal synthesis of amphidinin B (1) is reported herein. In Amphidinin B both C10–C21 (4) and C1–C9 (5) fragments were derived from geraniol 6 and mono-PMB ether of 1,4-butane diol 7 in 19 and 9 steps, respectively. The key steps involved in this synthesis are Sharpless asymmetric epoxidation, Evans aldol, Julia olefination, oxa-Michael, Keck allylation, Mannich reaction, Evans asymmetric alkylation, and Yamaguchi esterification.     

Formal synthesis of (+)-discodermolide

[structure: see text] Herein we report the formal total synthesis of (+)-discodermolide in 21 steps (longest linear sequence) from commercially available Roche ester. This synthesis features the assembly of C(9-18) and C(19-24) fragments via a metal-chelated aldol coupling reaction.     

Total synthesis of epoxyquinols A, B, and C and epoxytwinol A and the reactivity of a 2H-pyran derivative as the diene component in the Diels-Alder reaction

Full details of two versions of the total synthesis of epoxyquinols A, B, and C and epoxytwinol A (RKB-3564D) are described. In the first-generation synthesis, the HfCl(4)-mediated diastereoselective Diels-Alder reaction of furan with Corey's chiral auxiliary has been developed. In the second-generation synthesis, a chromatography-free preparation of an iodolactone, by using acryloyl chloride as the dienophile in the Diels-Alder reaction of furan, and the lipase-mediated kinetic resolution of a cyclohexenol derivative have been developed. This second-generation synthesis is suitable for large-scale preparation. A biomimetic cascade reaction involving oxidation, 6pi-electrocyclization, and then Diels-Alder dimerization is the key reaction in the formation of the complex heptacyclic structure of epoxyquinols A, B, and C. Epoxytwinol A is synthesized by the cascade reaction composed of oxidation, 6pi-electrocyclization, and formal [4 + 4] cycloaddition reactions. A 2H-pyran, generated by oxidation/6pi-electrocyclization, acts as a good diene, reacting with several dienophiles to afford polycyclic compounds in one step. An azapentacyclic compound is synthesized by a similar cascade reaction composed of the four successive steps: oxidation, imine formation, 6pi-azaelectrocyclization, and Diels-Alder dimerization.     

Total synthesis of calothrixins A and B via oxidative radical reaction of cyclohexenone with aminophenanthridinedione

Bioactive indolo[3,2-j]phenanthridine alkaloids, calothrixin B and its N-oxide derivative calothrixin A have been synthesized via an oxidative free radical reaction. Calothrixin B is generated from the commercially available 2,4,5-trimethoxybenzaldehyde in only seven steps. The key step in this synthesis is the Mn(OAc)₃ mediated oxidative free radical reaction of 9-(benzylamino)phenanthridine-7,10-dione with cyclohexenone to form 12-benzyl-12H-indolo[3,2-j]phenanthridine-7,13-dione.     

Stereoselective synthesis of the C1-C13 fragment of 2,3-dihydrodorrigocin A

[Chemical reaction: see text] The first synthesis of the C1-C13 fragment of 2,3-dihydrodorrigocin A has been achieved from 6-bromohexanoic acid in 14 linear steps and an overall yield of 2%. The configurations of the stereogenic centers C8, C9, and C10 have been determined to be the same as for migrastatin.     

Concise formal synthesis of (+)-pyripyropene A

A concise enantioselective synthesis of A/B bicyclic segment of naturally occurring α-pyrone meroterpenoid pyripyropene A is achieved in 9 steps (LLS) and 7.5% yield starting from R-(?)-carvone. The significant points of the synthesis include: (1) an intramolecular 1, 3-dipolar cycloaddition reaction to construct the A ring and assemble C4 quaternary carbon stereocenter as well; (2) reductive cleavage of the oxazole motif utilized Raney Ni/B(OCH₃)₃.     

Halichondrin B: synthesis of a C1-C14 model via desymmetrization of (+)-conduritol E

[reaction: see text] A model C1-C14 segment (1) of halichondrin B was synthesized from (+)-conduritol E (7) in 18 steps and 2.9% overall yield. Key features of the synthesis include the novel ozonolytic desymmetrization of C(2)-symmetric diol 6, the early-stage construction of the C-ring which accompanies installation of the crucial C12 stereocenter, and the use of an enol ether C14-ketone surrogate as a precursor to the CDE-"caged" ketal.     

Enantioselective total synthesis of (+)-dibromophakellstatin

The first enantioselective total synthesis of (+)-phakellstatin and (+)-dibromophakellstatin was achieved. Key steps in the synthesis were a desymmetrization of the diketopiperazine (S,S)-cyclo (Pro, Pro) via a diastereoselective acylation, an intramolecular Mitsunobu reaction to introduce the C6 aminal, and a tandem Hofmann rearrangement/cyclization to simultaneously introduce the C10 quaternary aminal center and deliver the cyclic urea. The synthesis also demonstrates the unusual stability of pyrrolo aminals. Importantly, this strategy has the potential for producing phakellstatin derivatives, derived from (R,R)-cyclo (Pro, Pro), necessary for biological studies. A similar annulation protocol is also expected to be applicable to the synthesis of palau'amine.     

Total synthesis of an antitumor antibiotic,Fostriecin (CI-920)

The total synthesis of an antitumor antibiotic, fostriecin (CI-920), via a highly convergent route is described. A characteristic feature of the present total synthesis is that the synthesis was achieved via a coupling procedure of three segments A, B, and C. The unsaturated lactone moiety of fostriecin, corresponding to segment A, was constructed from a known Horner-Emmons reagent, and the stereochemistry of the C-5 position was introduced by asymmetric reduction with (R)-BINAl-H. Segment B having a series of stereogenic centers was synthesized from (R)-malic acid and the stereogenic centers at the C-8 and C-9 positions were prepared by a combination of Wittig reaction and Sharpless asymmetric dihydroxylation reaction. The conjugated Z,Z,E-triene moiety of fostriecin, corresponding to segment C, was eventually constructed by Wittig reaction and Stille coupling reaction. The phosphate moiety, which is known to be essentially important for the antitumor activity, was introduced via two routes: (i) direct phosphorylation of the monohydroxyl derivative in which other hydroxyl groups are protected with silyl groups; (ii) cyclic phosphorylation and selective cleavage of the cyclic phosphate derivative. Although the former route is basically the same as those reported by other groups, the latter route is novel and more effective than the former one. The present total synthesis would serve as a versatile synthetic route to not only fostriecin, but also its various analogues including stereoisomers.     

A practical synthesis of (+)-biotin from L-cysteine

Alpha-amino aldehyde 4, which is readily derived from L-cysteine through cyclization and elaboration of the carboxy group, was subjected to the Strecker reaction, which, via sodium bisulfite adduct 16, afforded alpha-amino nitrile 5 with high diastereoselectivity (syn/anti=11:1) and in high yield. Amide 6, derived from 5, was converted to thiolactone 8, a key intermediate in the synthesis of (+)-biotin (1), by a novel S,N-carbonyl migration and cyclization reaction. The Fukuyama coupling reaction of 8 with the zinc reagent 21, which has an ester group, in the presence of a heterogeneous Pd/C catalyst allowed the efficient installation of the 4-carboxybutyl chain to provide 9. Compound 9 was hydrogenated and the protecting groups removed to furnish 1 in 10 steps and in 34 % overall yield from L-cysteine.     

Total synthesis of erythromycin B

We report the details of the first total synthesis of erythromycin B using two different strategies for the end game. The first of these follows a classical approach in which the desosamine and cladinose residues are sequentially appended to a macrocyclic lactone, which was formed by cyclization of a seco acid derivative, to give a bis-glycosylated macrolide intermediate that is converted into erythromycin B. The second strategy features an abiotic approach in which a seco acid bearing a desosamine residue is cyclized to give a monoglycosylated macrocyclic lactone that is then transformed into erythromycin B via a sequence of steps involving refunctionalizations and a glycosylation to introduce the cladinose moiety. Attempts to prepare a bis-glycosylated seco acid by de novo synthesis were unsuccessful. The syntheses of the key seco acid intermediates feature the oxidative transformation of a furan containing C(3)-C(10) to provide a dioxabicyclo[3.3.1]nonenone that served as a template on which to create the stereocenters at C(6) and C(8). A stereoselective aldol reaction was used to establish the C(11)-C(15) segment, and a stereoselective crotylation was implemented to introduce the propionate subunit comprising C(1)-C(2).     

Synthesis of the C(2)-C(13) fragment (the A-B spiroketal unit) of spongistatin 1 (altohyrtin A): use of a common intermediate for the synthesis of both spongistatin spiroketals

[reaction: see text] A convergent synthesis of 14 corresponding to the A-B spiroketal core of spongistatin 1 has been accomplished via an iodo-spiroketalization reaction of glycal 9, which was synthesized in three steps from a late-stage intermediate used in our synthesis of the C-D spiroketal fragment of spongistatin 1. Elaboration of 14 to the A-B spiroketal 15 was accomplished in three steps.     

C60与硅烯环加成反应机理的理论研究

用半经验AM1法研究了C₆₀与单态硅烯环加成反应机理.经Berny梯度法优化得到反应的过渡态,并进行了振动分析确认.计算结果表明:硅烯在C₆₀的66键上的加成反应分两步,第一步反应物生成中间配合物,无势垒;第二步由中间配合物经过渡态变为产物.65键上的加成反应分三步,第一步由反应物生成中间配合物,第二步由中间配合物经过渡态I得到闭环结构的中间体,第三步由中间体经过渡态Ⅱ形成产物.66键加成反应的活化势垒较低,从反应机理和动力学角度解释了66键加成优于65键加成的原因.     

The thiopyran route to polypropionates: enantioselective synthesis of membrenone B from racemic fragments

(6S,7S,8S,9R,10S)-(--)-Membrenone B was synthesized in nine steps (9.4% overall yield) beginning with two-directional aldol coupling of tetrahydro-4H-thiopyran-4-one with racemic 1,4-dioxa-8-thiaspiro[4.5]decane-6-carboxaldehyde. The first aldol reaction occurs with dynamic kinetic resolution to give a single adduct (>98% ee). The second aldol reaction is highly diastereoselective (three of eight possible adducts), and both major products are converted to membrenone B. The route also constitutes a formal synthesis of membrenone A.     

Structure reassignment and synthesis of Jenamidines A1/A2, synthesis of (+)-NP25302, and formal synthesis of SB-311009 analogues

The proposed structures of jenamidines A, B, and C (1-3) were revised to jenamidines A1/A2, B1/B2, and C (8-10). Jenamidines A1/A2 (8) were synthesized from activated proline derivative 43 by conversion to 26 in two steps and 50% overall yield. Acylation of 26 with acid chloride 38d gave 39d, which was deprotected with TFA and then mild base to give 8 in 45% yield from 26. (-)-trans-2,5-Dimethylproline ethyl ester (49) was prepared by the enantioselective Michael reaction of ethyl 2-nitropropionate (51) and methyl vinyl ketone (50) using modified dihydroquinine 60 as the catalyst. Further elaboration converted 49 to natural (+)-NP25302 (12). A Wittig reaction of proline NCA (76) with ylide 79 gave 72 as a 9/1 E/Z mixture in 27% yield, completing a one-step formal synthesis of SB-311009 analogues.     

Metathesis-based synthesis of 3-methoxy α,β-unsaturated lactones: total synthesis of (R)-kavain and of the C1-C6 fragment of jerangolid D

The total synthesis of (R)-kavain and of the C1-C6 fragment of jerangolid D has been achieved in nine and seven steps, respectively, from commercially available dimethyl d-malate. A metathesis reaction of vinyl ethers and a sulfoxide-modified Julia olefination have been employed as the key steps.     

Spirastrellolide studies. Synthesis of the C(1)-C(25) southern hemispheres of spirastrellolides A and B, exploiting anion relay chemistry

Construction of the C(1)-C(25) southern fragments of both spirastrellolide A and B are described. Highlights of the syntheses include effective use of the three component anion relay chemistry (ARC) tactic recently introduced by our laboratory, a stereoselective spirocyclization via concomitant Ferrier reaction to elaborate the BC spiroketal and use of two dithiane unions to install the A ring as well as C(22)-C(25) fragment. The synthesis proceeded with longest linear sequences of 33 and 32 steps, respectively for spirastrellolide A and spirastrellolide B.