Synthesis of chiral organocatalysts derived from aziridines: application in asymmetric aldol reaction

We report the synthesis of a new series of highly efficient chiral organocatalysts derived via the regio- and stereoselective ring opening of chiral aziridines with azide anions. The catalysts have proved to be very efficient for a direct asymmetric aldol reaction, both with cyclic as well as acyclic ketones in brine with 2 mol % of catalyst loading, and afforded the products in excellent yields (up to 99%) and enantioselectivities (up to >99%). The chiral aldol adduct obtained has further been converted to a chiral azetidine ring via a convenient pathway.     

Studies directed towards the synthesis of botcinolides: synthesis of the nonalactone ring of 2-epibotcinolide

Synthesis of the polyoxygenated nonalactone ring of 2-epibotcinolide was achieved using a highly stereoselective aldol reaction of the titanium enolate from a lactate-derived chiral ketone, a stereoselective dihydroxylation and a Yamaguchi macrolactonization reaction.     

Studies directed towards the total synthesis of botcinic acid, the revised structure of botcinolide: synthesis of the highly substituted tetrahydropyran moiety

Synthesis of the highly substituted polyoxygenated tetrahydropyran ring of botcinic acid, the revised structure of botcinolide is achieved using, as key steps, a highly stereoselective aldol reaction of the titanium enolate from a lactate-derived chiral ketone and a stereoselective dihydroxylation.     

Stereoselective synthesis of anamarine

A stereoselective synthesis of the naturally occurring, α,β-unsaturated lactone anamarine is described. The key step was a highly stereoselective aldol reaction of a protected erythrulose derivative with a chiral aldehyde. Another relevant step was an asymmetric aldehyde allylation with a chiral allylborane. The lactone ring was made by means of a ring-closing metathesis.     

A double ring closing metathesis reaction in the rapid, enantioselective synthesis of NK-1 receptor antagonists

[structure: see text]. The NK-1 receptor antagonist 1 has been prepared in seven steps from phenylglycine methyl ester. The key steps are a double ring closing metathesis reaction of tetraene 7 to prepare spirocycle 6 and a reductive Heck reaction to introduce the aryl moiety. This latter reaction discriminates the olefins of compound 6 and proceeds in a highly regio- and stereoselective manner.     

Total synthesis of khafrefungin using highly stereoselective vinylogous Mukaiyama aldol reaction

[structure: see text] A convergent total synthesis of khafrefungin was accomplished on the basis of (1) the highly stereoselective TiCl4-mediated vinylogous Mukaiyama aldol reaction using vinylketene silyl N,O-acetal and (2) syn-selective aldol reaction of enal 5a and ethyl ketone 6 followed by anti-dehydration under Mitsunobu conditions.     

Studies on the Model Synthesis of the Brassinolide and Dolicholide Side Chains

A stereoselective synthesis of brassinolide and dolicholide, which involves construction of the side chain enantiomers by a highly stereoselective aldol reaction of aldehyde 5 with the anion of a-silyloxy ketone 6 is described.     

Synthesis of polyhydroxylated bicyclic tetrahydrofurans and tetrahydropyrans via a stereoselective domino cyclization/reduction reaction

A novel reaction cascade involving a Lewis acid-induced migration of an isopropylidene protecting group followed by the formation of a pyranose or furanose ring and subsequent reduction of the hemiacetal is described. Depending on the reaction conditions, as well as, the stereochemistry of the substrate, polyhydroxylated tetrahydrofurans or tetrahydropyrans can be obtained in reasonable yields. The synthons used in this transformation were prepared via a highly stereoselective one-pot tandem reaction, consisting of a 1,4-Michael addition of vinylmagnesium bromide to d-glucose-derived cyclohexenone followed by aldol reaction with 2,3-O-isopropylidene-d-glyceraldehyde.     

Synthesis of highly functionalized furanones via aldol reaction of 3-silyloxyfurans

[reaction: see text] The stereoselective aldol reaction of 3-silyloxyfurans with aldehydes in the presence of a Lewis acid is described. N-Bromosuccinimide (NBS)-mediated cyclization of the aldol product leads to the formation of the 2,7-dioxa-bicyclo[2.2.1]heptan-3-one ring system, which represents the formal product of hetero Diels-Alder reaction of the furan with the aldehyde.     

Synthesis of the C1‐C10 Fragment of Muamvatin

This report delineates our efforts towards the synthesis of a stereochemically well‐defined ketone, the C₁?C₁₀ fragment of muamvatin, the first example of a 2, 4, 6‐trioxaadamantane ring skeletal polypropionate marine natural product, using two non‐aldol variants. i) The Shimizu reaction, a Pd(0) mediated stereoselective epoxy‐ring opening of alkenyl oxiranes, was employed for the stereoselective installation of methyl groups in syn‐fashion and ii) Bode's protocol, a NHC‐mediated reaction on β‐epoxy aldehydes, was utilized for stereoselective construction of methyl and hydroxyl groups in anti‐fashion.     

Total synthesis of (+/-)-scopadulin.

The first total synthesis of (+/-)-scopadulin, an aphidicolane diterpene, is described. The core structure (A/B/C/D ring system) was constructed by an initial synthesis of the B/C/D ring system by our reported methods and a subsequent A ring cyclization by intramolecular aldol condensation. A highly stereoselective cyanation of the tetracyclic enone by Et2AlCN gave a trans-fused A/B ring system with a beta-cyanide at C-4. Stereoselective construction of a quaternary carbon at C-4 was achieved by alpha-alkylation of the cyano group and conversion of the sterically hindered cyano group to a methyl group via our novel reaction for conversion of primary aliphatic amines into alcohols. Finally, the total synthesis of (+/-)-scopadulin was accomplished by a highly chemo- and stereoselective methylation at C-16 and modification of the C-4 alpha-functionality. The stereoselectivity observed in the MeTi(O-i-Pr)3-mediated methylation for the generation of a tertiary axial alcohol at C-16 is extremely high.     

Studies on the Model Synthesis of the Brassinolide and Dolicholide‘s Side Chains

A highly stereoselective synthesis of the side Chain enantiomers of brassinolide and dolicholide through an aldol reaction of 2-(4-methoxy benzyloxyl) proplonylaldehyde (5) with the anion of 1- ( tert -butyldimethylsiloxy )-3-methyl-2-butanone ( 6 ) is described.     

Synthesis of enantiopure dihydropyranones: aldol-based ring expansion of dihydrofurans

[reaction: see text] The stereoselective Birch reduction of 3-methyl-2-furoic acids using a readily available chiral auxilairy is described; by coupling this process to an oxidative cleavage/aldol ring closure sequence we were able to produce highly functionalized and enantiopure dihydropyranones in high yield. This sequence has ample flexibility built into it, either by the use of different electrophiles during reductive alkylation or by subsequent derivatization of the dihydropyranone after ring expansion.     

Formation of the BC ring system of upenamide via a Staudinger/aza-Wittig reaction

The BC ring system of upenamide was assembled using a stereoselective Diels-Alder reaction followed by a Staudinger/aza-Wittig/imine hydrolysis reaction. Stereoselective aldol coupling with an aldehyde that incorporates the DE ring system led to an advanced synthetic intermediate en route to the marine alkaloid upenamide.     

Synthesis of enantiomerically pure (R)- and (S)-2-ethoxycarbonylmethyl-2-hydroxy-cyclohexanones

Sulfenylation of 2-p-tolylsulfinyl cyclohexanone can be achieved at −78°C with thiosulfonates. The in situ aldol reaction of these compounds with ethyl acetate enolate is highly stereoselective (1,2-asymmetric induction) and yields diastereomeric mixtures of β-hydroxyesters (the configuration of the major one being dependent on the sulfenylating agent) that can be readily separated and transformed into the enantiomerically pure title ketones.     

A stereoselective aldol reaction via diisopinocampheyl boron-enolate in preparation of chromane carboxylate with quaternary carbon

A highly stereoselective aldol reaction was observed on chromane carboxylate ester 1 via the corresponding diisopinocampheyl boron-enolate using commercially available (−)-DIP-Cl reagent. The aldol product 2c was obtained in 89% yield with 48 dr and 92% ee. Further studies indicate that stereoselective formation of the enolate and proper chiral ligand on boron are responsible for the exceptional diastereo- and enantioselectivity in the aldol reaction.     

Synthetic studies towards pentacyclic quassinoids: total synthesis of unnatural (-)-14-epi-samaderine E and natural (-)-samaderine Y from (S)-(+)-carvone

First total syntheses of unnatural (-)-14-epi-samaderine E (5) and natural (-)-samaderine Y (2) were accomplished from (S)-(+)-carvone (6) in 18 and 21 steps, respectively. The syntheses are short, efficient (with an average yield of 80 % plus for each transformation), enantiospecific, and produce nine new chiral centers. The crucial points of the syntheses included a regioselective allylic oxidation on ring C, regio- and stereoselective reduction of ketone, a stereocontrolled epoxidation, an epoxymethano-bridge formation, a chemoselective Grignard reaction, an intramolecular Diels-Alder reaction, an intramolecular aldol addition, and a newly developed manganese(III)-catalyzed allylic oxidation on ring A.     

Aldol reaction under solvent-free conditions: highly stereoselective synthesis of 1,3-amino alcohols

[formula: see text] A method for the highly stereoselective synthesis of 1,3-amino alcohols based on the indium trichloride-catalyzed Mukaiyama aldol reaction of keto ester (R,S)-1 under solvent-free conditions has been developed. The high selectivity observed can be explained on the basis of the shielding of one face by a remote substituent.     

Stereocontrolled synthesis of (+)-boronolide

Boronolide was synthesized stereoselectively from hydroxyacetylfuran 5 and valeraldehyde 6 using a novel dizinc aldol catalyst. Ring closing metathesis provides the lactone ring. The synthesis requires 12 steps and proceeds in 26% overall yield. [reaction: see text]     

Total synthesis of (-)-bafilomycin A(1)

A highly stereoselective total synthesis of (-)-bafilomycin A(1), the naturally occurring enantiomer of this potent vacuolar ATPase inhibitor, is described. The synthesis features the highly stereoselective aldol reaction of methyl ketone 8b and aldehyde 60c and a Suzuki cross-coupling reaction of the highly functionalized advanced intermediates 12 and 39. Vinyl iodide 12 was synthesized by a 14-step sequence starting from the readily available beta-alkoxy aldehyde 14, while the vinylboronic acid component 39 was synthesized by a nine-step sequence from beta-hydroxy-alpha-methyl butyrate 44 via a sequence involving the alpha-methoxypropargylation of chiral aldehyde 49 with the alpha-methoxypropargylstannane reagent 54. Syntheses of fragments 12 and 39 also feature diastereoselective double asymmetric crotylboration reactions to set several of the critical stereocenters. The Suzuki cross-coupling of 12 and 39 provided seco ester 40, which following conversion to the seco acid underwent smooth macrolactonization to give 41. The success of the macrocyclization required that C(7)-OH be unprotected. The Mukaiyama aldol reaction between aldehyde 60c and the TMS enol ether generated from 8b provided aldol 65 with high diastereoselectivity. Finally, all silicon protecting groups were removed by treatment of the penultimate intermediate 65 with TAS-F (tris(dimethylamino)sulfonium difluorotrimethylsilicate), thereby completing the total synthesis of (-)-bafilomycin A(1).