Chiral synthesis via organoboranes. 35. Simple procedures for the efficient recycling of the terpenyl chiral auxiliaries and convenient isolation of the homoallylic alcohols in asymmetric allyl- and crotylboration of aldehydes

Total Synthesis of Pulvomycin D

A synthetic route to the pulvomycin class of natural products is presented, which culminated in the first synthesis of a pulvomycin, pulvomycin D. Key elements of the strategy include a pivotal aldol reaction which led to bond formation between the C24‐C40 and the C8‐C23 fragment. The remaining C1‐C7 fragment was attached by a Yamaguchi esterification completing the assembly of the 40 carbon atoms within the main skeleton. Ring closure to the 22‐membered lactone ring was achieved in the final stages of the synthesis by a Heck reaction. The completion of the synthesis required the removal of six silyl protecting groups in combination with olefin formation at C26‐C27 by a Peterson elimination.

Studies towards the Synthesis of (–)-Pulvomycin: Construction of the C12–C40 Segment by a Stereoselective Aldol Reaction

A convergent strategy was developed for the synthesis of the C12–C40 segment of (–)-pulvomycin. Key step was a diastereoselective aldol reaction between a chiral ethyl ketone representing the C24–C40 fragment and a chiral aldehyde representing the C12–C23 fragment. Both compounds were prepared from enantiomerically pure building blocks in a convergent fashion. The longest linear sequence commenced with a known d-fucose-derived glycosyl donor and entailed a total number of 16 steps. The desired anti-aldol product was obtained in a total yield of 5% over these steps and contains 12 out of 13 stereogenic centers present in the natural product.

Bi(cyclopentyl)diol-Derived Boronates in Highly Enantioselective Chiral Phosphoric Acid-Catalyzed Allylation, Propargylation, and Crotylation of Aldehydes

In this study, we disclose the catalytic addition of bi(cyclopentyl)diol-derived boronates to aldehydes promoted by chiral phosphoric acids, allowing for the formation of enantioenriched homoallylic, propargylic, and crotylic alcohols (up to >99% enantiomeric excess (ee), diastereomeric ratio (dr) >20:1). These boronate substrates provided superior enantioselectivities, allowing for the reactions to proceed with low catalyst loading (0.5-5 mol %) and reduced reaction time (15 min at room temperature for aldehyde allylboration). A wide substrate scope was exhibited, and the novel boronates provided high enantiocontrol. Reactions with substituted allylboronates and aldehydes yielded vicinal stereogenic alcohols bearing β-tertiary or quaternary carbon centers. High enantio- and diastereoselectivities were found due to the closed six-membered chair-like transition state, with backbone modifications of the boronate and its interactions with the chiral phosphoric acid being the most likely contributing factor.

Asymmetric synthesis of 2 degrees- and 3 degrees-Carbinols via B-methallyl-10-(TMS and Ph)-9-borabicyclo[3.3.2]decanes

Simple Grignard procedures provide methallylboranes 1a and 1b in enantiomerically pure form from air-stable precursors in 98% and 95% yields, respectively. These reagents add smoothly to aldehydes and methyl ketones, respectively, providing branched 2 degrees- (6, 69-89%, 94-99% ee) and 3 degrees- (10, 71-87%, 74-96% ee) homoallylic alcohols.

A synthetic entry to pladienolide B and FD-895

Presented within are syntheses of the pladienolide B and FD-895 side-chains, as well as models of the essential ring-closing metathesis and Stille coupling that will be used to complete their total syntheses. Several analogs of the pladienolide B side-chain were also prepared in order to evaluate the scope of the methodology and to create a library of structures that could be used for stereochemical and SAR analyses.

Asymmetric Allyl- and Crotylboration with the Robust, Versatile, and Recyclable 10-TMS-9-borabicyclo[3.3.2]decanes

The remarkable versatility and selectivity of the 10-(trimethylsilyl)-9-borabicyclo[3.3.2]decanes (10-TMS-9-BBDs) in the allyl- and crotylboration of representative aldehydes are reported. The new reagents are prepared through air-stable crystalline pseudoephedrine borinic ester complexes of the 10-TMS-9-BBDs (4), which are available in 63% overall yield from B-MeO-9-BBN through a simple two-step procedure. These complexes 4 are directly converted to the corresponding B-allyl-10-TMS-9-BBDs (1) with allylmagnesium bromide, which either can be isolated (98%) or used in situ for the allylations. The remarkable enantioselectivity (96 to > or =99% ee) of these reagents in the rapid (<3 h), asymmetric allylboration process at -78 degrees C is only slightly diminished when it is conducted at 25 degrees C, a phenomenon attributable to its rigid bicyclic structure. In addition to providing the homoallylic alcohols 6 efficiently (68-80%), the procedure also permits the efficient recovery of 4 (68-84%) for the direct regeneration of 1. Alternatively, an oxidative workup procedure can be used for the preparation of 6. The reagent gives predictable stereochemistry and exhibits an extremely high level of reagent control in the allylboration of d-glyceraldehyde acetonide. A simple and efficient procedure has been developed for the preparation of all four geometric and enantiomeric isomers of the B-crotyl-10-TMS-9-BBDs (10) from optically pure enantiomers of B-MeO-10-TMS-9-BBD (3). These reagents 10 also add rapidly (<3 h) and efficiently to representative aldehydes at -78 degrees C, providing ready access to all four of the possible stereoisomers of the beta-methyl homoallylic alcohols 12-15 (69-92%) in high dr (> or =98:2) and ee (94-99%).

Luminescent Organoboron Quinolate Polymers

The synthesis of well-defined luminescent organoboron polymers via a novel three-step procedure starting from silylated polystyrene is reported. Highly selective borylation of poly(4-trimethylsilylstyrene) (PS-Si), followed by replacement of the bromine substituents in poly(4-dibromoborylstyrene) (PS-BBr) with substituted thienyl groups (R = H, 3-hexyl, 5-hexyl), and final introduction of the 8-hydroxyquinolato moiety yields a series of new organoboron quinolate polymers in 67-83% isolated yield. The hexyl-substituted polymers are highly soluble and solution-processable yielding thin films that efficiently emit light at 513-514 nm upon excitation at 395 nm.

Asymmetric total synthesis of (-)-laulimalide: exploiting the asymmetric glycolate alkylation reaction

A concise total synthesis of the potent antitumor macrolide (-)-laulimalide is described. The observation that homoallylic (or latent homoallylic) C-O bonds are present at C5, C9, C15, C19, and C23 led to the strategic decision to rely heavily on the asymmetric glycolate alkylation to construct both the C1-C14 fragment 3 and the C15-C27 subunit 4. A diastereoselective addition of a C1-C14 allylstannane to a C15-C27 alpha,beta-epoxyaldehyde served to join the two advanced fragments. A Mitsunobu macrolactonization of hydroxy acid 2 avoided isomerization of the sensitive 2,3-Z-enoate, which has been observed in base-catalyzed macrolactonizations. Removal of two TBS protecting groups to reveal the C15 and C20 hydroxyls occurred without rearrangement to isolaulimalide.

Facial-selective allylation of methyl ketones for the asymmetric synthesis of alpha-substituted tertiary homoallylic ethers

The asymmetric synthesis of enantiomerically pure a-substituted tertiary homoallylic ethers 4a, 11 and 12a-c by the allylation of ethyl methyl ketone (la) with gamma-substituted allylsilanes 9a-h is described. The allylsilanes were obtained by a nickel-catalysed Grignard cross-coupling reaction of (E)- and (Z)-(3-iodoallyl)trimethylsilane with various Grignard reagents. The reaction of the allylsilanes with la in the presence of the trimethylsilyl ether of N-trifluoroacetylnorpseudoephedrine (3), and catalytic amounts of a mixture of trimethylsilyl triflate and trifluoromethanesulfonic acid led to the homoallylic ethers 4a, 11 and 12a-c with two new stereogenic centres, with a selectivity of 1:9 to >20:1 for the homoallylic and of 1:99 to >60:1 for the allylic centre. The facial selectivity does not depend on the configuration of the allylsilane, and in all reactions the anti product is preferentially formed. Interestingly, a pronounced switch of facial selectivity takes place with increasing length of the alkyl group of the allylsilane.

Synthesis of enantiopure homoallylic ethers by reagent controlled facial selective allylation of chiral ketones

The stereoselective allylation of chiral methyl ketones to give tertiary homoallylic ethers, which can easily be transformed into homoallylic alcohols, is described. Reaction of the enantiopure ketones 8a-d and the racemic ketones 26a-d with the norpseudoephedrine derivative 2 or ent-2 and allylsilane in the presence of a catalytic amount of trifluoromethanesulfonic acid, led to a series of homoallylic ethers with good to excellent diastereoselectivity (85:15 to > 97:3). The allylation is reagent controlled and nearly independent from the stereogenic centers in the substrates. A partial kinetic resolution was observed using the racemic ketones 26a-d. In the reaction of the chiral ketones 8a-d with the achiral reagents ethoxytrimethylsilane and allylsilane only a low diastereoselectivity was observed.

Tandem reduction-chloroallylboration of esters: asymmetric synthesis of lamoxirene, the spermatozoid releasing and attracting pheromone of the laminariales (Phaeophyceae)

The asymmetric synthesis of all four stereoisomers of lamoxirene (cis-2-cyclohepta-2,5-dienyl-3-vinyloxirane), the spermatozoid-releasing and -attracting pheromone of the Laminariales (Phaeophyceae), is reported. Chiral ethyl cyclohepta-2,5-diene carboxylates, prepared by a divinylcyclopropane Cope rearrangement, were effectively alkylated by means of a novel tandem DIBAL-H reduction/asymmetric alpha-chloroallylboration using (Z)-gamma-chloroallyldiisopinocampheylboranes. The ensuing syn-alpha-chlorohydrins were transformed into the corresponding vinyloxiranes with DBU, providing all four isomers of the pheromone in good chemical and excellent optical yield (90-97% ee). Spermatozoid-release assays were conducted with the sympatrically growing species L. digitata, L. hyperborea, and L. saccharina and established (1'S,2R,3S)-1c as the most active isomer in all cases.

Diastereo- and Enantioselective Synthesis of syn-alpha-Vinylchlorohydrins and cis-Vinylepoxides

A new method to generate chiral syn-vinylchlorohydrins and cis-vinyloxiranes is reported. Reaction of (alpha-haloallyl)lithiums with methoxy-9-BBN or Ipc(2)BOMe followed by treatment with BF(3).OEt(2) leads to (Z)-(gamma-haloallyl)boranes which react with aldehydes to yield cis-vinylepoxides (de >/= 90%) upon oxidative workup. Alternatively, addition of ethanolamine to the allylboration product yields syn-alpha-halohydrins (de >/= 90%) that are also easily cyclized to cis-vinylepoxides. Extension of this protocol using [(Z)-gamma-chloroallyl]BIpc(2) leads to chiral syn-alpha-chlorohydrins and cis-vinylepoxides in high de (>/=90%) and ee (90-99%). Enantioselectivity of reactions of chiral (Z)-(gamma-chloroallyl)boranes with aldehydes are more sensitive to reaction conditions than enantioselectivity of reactions of other alpha-or gamma-substituted allylboranes. The effects of proportion of BF(3).OEt(2) and the relative efficacies of LiNR(2) bases on diastereo- and enantioselectivity of the chloroallylation are reported.