Addition of Allenylzinc Reagents, Prepared in Situ from Nonracemic Propargylic Mesylates, to Aldehydes. A New Synthesis of Highly Enantioenriched Homopropargylic Alcohols

Pentenolide Analogues of Antifungal Butenolides: Strategies Towards 3,6-Disubstituted Pyranones and Unexpected Loss of Biological Effect

Pentenolide analogues of antifungal 3,5-disubstituted butenolides were prepared by oxidative cyclization of 2-(substituted aryl)hex-5-enoic acids as the key step. Given the limitations of the methodology, another approach to the title compounds based on the Pd-catalyzed carbonylative lactonization of 4-iodo-3-en-1-ols was developed, and the carbonylation conditions were optimized. While the former sequence allows only the introduction of a substituted methyl at C6, pyranones bearing a range of various C-substituents at C6 can be prepared by the latter. Somewhat surprisingly, unlike the corresponding butenolides with the same substitution pattern, the title pentenolides possess no antifungal or cytostatic activity.

(R)- and (S)-4-TIPS-3-butyn-2-ol. Useful Precursors of Chiral Allenylzinc and Indium Reagents

A convenient route to the enantiomers of 4-TIPS-3-butyn-2-ol of >95% enantiomeric purity by reduction of the ynone precursor 4 with the Noyori N-tosyl-1,2-diphenylethylenediamineruthenium cymene catalyst is described. The mesylate derivative of the (S) enantiomer (1c) is converted in situ to an allenylzinc or indium reagent in the presence of a catalyst derived from Pd(OAc)2 and Ph3P and either Et2Zn or InI. A second in situ addition of these reagents to aldehydes leads to anti homopropargylic alcohol adducts. The additions proceed in generally high (60-90%) yield with modest to excellent diastereoselectivity and high enantioselectivity. Only slight mismatching (<5%) is observed with chiral alpha-methyl and alpha-silyloxy aldehydes. Additions to alpha-substituted enals are highly diastereoselective, while beta,beta-disubstituted enals afford ca. 2:1 mixtures of anti and syn adducts.

Highly Stereoselective Palladium-Catalyzed Dithiocarbonylation of Propargylic Mesylates with Thiols and Carbon Monoxide

Highly stereoselective dithiocarbonylation of propargylic mesylates with thiols and carbon monoxide has been developed by the use of tetrakis(triphenylphosphine)palladium(0) as the catalyst at 90 degrees C in THF. The reaction affords the corresponding dithioesters in good to excellent yields. For some secondary and tertiary propargylic alcohols with a terminal or internal triple bond, the reaction stereoselectively produces E-dithioesters as products. The dithiocarbonylation is believed to proceed via allenylpalladium and allenyl ester intermediates, and the high stereoselectivity might be rationalized by a mechanism where nucleophilic attack of a Pd(0)L(n) species on the allenyl sp carbon occurs from the less hindered side of an alkyl substituent.

Formal Total Syntheses of Crocacin A-D

A concise route to the common polyketide fragment5of crocacin A-D (1-4) is presented which has previously been converted into all members of this fungicidal and cytotoxic family of dipeptidic natural products by various means. Our synthesis features asyn-selective titanium aldol reaction controlled by a valinol-derived auxiliary, a zinc-mediated, palladium-catalyzedanti-selective addition of propargyl mesylate10to the chiral aldehyde9, as well as a comparison of palladium-catalyzed Stille and Suzuki cross-coupling reactions for the formation of the diene moiety of the target.

Total Synthesis of Amphidinolide X

A concise total synthesis of the cytotoxic marine natural product amphidinolide X (1) is described. A key step of the highly convergent route to this structurally rather unusual macrodiolide derivative consists of a newly developed, highly syn selective formation of allenol 6 by an iron-catalyzed ring opening reaction of the enantioenriched propargyl epoxide 5 (derived from a Sharpless epoxidation) with a Grignard reagent. Allenol 6 was then cyclized with the aid of Ag(I) to give dihydrofuran 7 containing the (R)-configured quarternary sp3 chiral center at C19 of the target. The anti-configured chiral centers at C10 and C11 were formed by the palladium-catalyzed, Et2Zn-promoted addition of propargyl mesylate 12 to the functionalized aldehyde 11. The key fragment coupling at the C13-C14 bond was achieved by the "9-MeO-9-BBN" variant of the alkyl-Suzuki reaction. Finally, the 16-membered macrodiolide ring was formed by a Yamaguchi esterification/lactonization strategy.

Total Synthesis of Zincophorin and Its Methyl Ester

A total synthesis of the naturally occurring ionophore zincophorin has been realized. The route features an intramolecular oxymercuration of a cyclopropanemethanol and a Carroll-Claisen rearrangement for the respective elaboration of the C1-C12 and C13-C25 subunits, which have been assembled by using a highly diastereoselective titanium-mediated aldol condensation.

Total Synthesis of Zincophorin Methyl Ester

[reaction: see text]. A convergent total synthesis of the methyl ester of zincophorin, an ionophore antibiotic, has been realized relying on a diastereoselective titanium-mediated aldol coupling between the C1-C12 and C13-C25 subunits. The latter fragment was prepared by using a Carroll-Claisen rearrangement.

Indium-Mediated Reaction of Trialkylsilyl Propargyl Bromide with Aldehydes: Highly Regioselective Synthesis of Allenic and Homopropargylic Alcohols

High regioselective preparation of either the allenic alcohols or homopropargylic alcohols using the indium-mediated reaction of trialkylsilyl propargyl bromides with various aldehydes has been accomplished. By just changing the silyl groups and the reaction conditions, both the allenic and homopropargylic alcohols can be obtained in high regioselectivities.

Stereo- and regioselectivity of Pd(0)/InI-mediated allylic additions to aldehydes and ketones. In situ generation of allylindium(III) intermediates from N-acylnitroso Diels-Alder cycloadducts and 1-amino-4-acetoxycyclopentenes

Acylnitroso Diels-Alder cycloadduct (11, 37, and 45)- and cyclopentenyl acetate (8 and 9)-derived allylindium(III) species were generated in situ from palladium(0) catalysts and indium(I) iodide, and the stereo- and regiochemistry of their additions to aldehydes and ketones were investigated. Solvent, catalyst, and ionic effects were examined for the reaction of N-acetyl cycloadduct (11) and benzyloxyacetaldehyde (10). The solvent mixture of THF/H(2)O with Pd(OAc)(2).PPh(3) catalysis was found to be optimal. The addition of N-acetyl cycloadduct to aliphatic aldehydes afforded products in good yields and high regio- and stereoselectivity, with the cis-1,4-isomers constituting 90-95% of the products. The reactions with N-Boc (37a) and N-methylcarbamate (37b) cycloadducts also gave the cis-1,4-products predominantly. The same regio- and stereoselectivity applied to the reactions of 4-acetoxy-1-(N-hydroxyphenyacetamido)cyclopentene (8). 4-Acetoxy-1-phenylacetamidocyclopentene (9), however, afforded trans-1,4-products exclusively. Mechanistic speculations involving chelated transition states are described.

Second-generation synthesis of the polypropionate subunit of callystatin A based on regioselective internal alkyne hydrostannation

[formula: see text] An improved route to the polypropionate segment of callystatin A is described in which the efficient directed hydrostannation of an internal alkyne and subsequent iodinolysis provides a key vinylic iodide intermediate.

Total synthesis of bafilomycin V(1): a methanolysis product of the macrolide bafilomycin C(2)

A synthesis of bafilomycin V(1), a methanolysis product of the macrolide natural product bafilomycin C(2), is described. The route utilizes chiral nonracemic allenylzinc reagents, prepared in situ from propargylic mesylates, to access key segments of this methyl ester. The acetylenic moieties of the derived homopropargylic alcohol adducts play an important role in further elaboration of these subunits. Final assemblage of the 25-carbon chain, containing 12 stereocenters, an alpha-methoxy Z,E 1,3-dienic ester, and an additional E,E 1,3-diene, was achieved through Stille coupling of an acetylene-derived vinyl stannane and vinyl iodide of approximate equal complexity. Attempted cyclization of several C15 hydroxy acid derivatives to the 16-membered lactone bafilomycin A(1), a potent inhibitor of vacuolar ATPases, could not be achieved.

Synthesis of stereopentad analogues of the C14-C22 segment of callystatin A through additions of chiral allenylzinc reagents to stereotriads

The addition of (P)- and (M)-allenylzinc reagents, prepared in situ through Pd-catalyzed metalation of (R)- and (S)-3-butyn-2-ol mesylates, to diastereomeric stereotriad aldehydes 8, 13, 18, and 23 of syn,syn, syn,anti, anti,anti, and anti,syn stereochemistry was examined. Additions to the former two aldehydes afforded the four anti adducts with high diastereoselectivity and negligible mismatching. Significant mismatching was observed with the latter two aldehydes and the (M)-allenylzinc reagent. An evaluation of possible transition states is presented in consideration of steric and dipolar control elements.

Catalytic asymmetric access to alpha,beta unsaturated delta-lactones through a vinylogous aldol reaction: application to the total synthesis of the Prelog-Djerassi lactone

[reaction--see text] A one-step catalytic asymmetric access to alpha,beta unsaturated delta-lactones is described, using a vinylogous Mukaiyama-aldol reaction between a gamma-substituted dienolate and various aldehydes in the presence of Carreira catalyst CuF.(S)-tolBinap. This methodology has been further applied to a straightforward access to the Prelog-Djerassi lactone.

Lipase-mediated resolution of 4-TMS-3-butyn-2-ol and use of the mesylate derivatives as a precursor to a highly stereoselective chiral allenylindium reagent

[reaction: see text]. An improved procedure for the resolution of 4-trimethylsilyl-3-butyn-2-ol has been developed. The mesylate derivatives of the resolved alcohols have been found to undergo highly enantio-, regio-, and diastereoselective additions to aldehydes, leading to homopropargylic alcohol adducts.

Additions of enantioenriched allenylzinc and indium reagents to lactic aldehyde ethers

Allenylzinc and indium reagents were generated in situ through Pd(0)-catalyzed metalation of (R)- and (S)-3-butyn-2-ol methanesulfonate with Et(2)Zn and InI. These reagents add to the benzyl and TBS ethers of (S)-lactic aldehyde to afford diastereomeric stereotriads in moderate to high yield. The (S)/(S) combination afforded the anti,anti adducts with 94:6-100:0 diastereoselectivity. The (R)/(S) combination was mismatched, affording a mixture of anti,syn and syn,anti adducts in diastereomeric ratios of ca. 80:20-85:15. Addition of the racemic allenylmetal reagents to the (S)-lactic aldehyde ethers afforded the products of matched and mismatched pairings in equal amount.

Progress toward the total synthesis of bafilomycin A(1): stereoselective synthesis of the C15-C25 subunit by additions of nonracemic allenylzinc reagents to aldehydes

[reaction: see text] A highly stereoselective synthesis of the C15-C25 subunit (2) of bafilomycin A(1) (1) has been accomplished by a route utilizing additions of chiral nonracemic allenylzinc reagents to aldehydes.

Synthesis of beta-hydroxy ketones and vinylsilanes from homopropargylic alcohols by intramolecular hydrosilation

Catalytic intramolecular hydrosilation of nonracemic homopropargylic alcohols affords the corresponding five-membered cyclic siloxanes regiospecifically. Oxidation or alkylmetal addition converts these intermediates into beta-hydroxy ketones or vinylsilanes under mild conditions.

Preparation of Chiral Allenylmetal Reagents from Enantioenriched Allenyl Iodides and Propargylic Mesylates. A Comparison of Indium, Bismuth, and Tin Derivatives

Chiral allenylmetal halides were prepared starting from enantioenriched ( approximately 95% ee) 1-octyn-3-ol mesylate (1) and 3-butyn-2-ol mesylate (7). Addition of these transient metallo species to aldehydes in situ afforded mainly anti homopropargylic alcohol adducts of varying ee. Three approaches were explored. In the first, the mesylates were converted to allenylstannanes with Bu(3)SnLi.CuBr and the allenylmetal halide was prepared from the resulting enantioenriched allenylstannane by transmetalation with InBr(3), BiBr(3), and SnCl(4) in the presence of cyclohexanecarboxaldehyde. The configurational stability of the transient allenylmetal halide was estimated from the ee of the derived adduct. The second approach involved metalation of the allenyl iodides prepared from mesylates 7 and 1 through S(N)2' displacement with LiCuI(2). From these experiments, it was determined that allenylindium bromides and allenyltin bromides are configurationally stable under the reaction conditions. The adduct obtained from the allenylbismuth bromide was nearly racemic. A superior procedure was developed in which the propargylic mesylates 7 and 1 were converted to allenylindium iodide intermediates through treatment with InI and 5 mol % of a Pd(0) dppf catalyst in THF-HMPA, THF-DMSO, or THF-DMPU. Under these conditions, alpha-branched aldehydes were converted to anti adducts ( approximately 95:5 anti/syn) of >90% ee. Additions to unbranched aldehydes were less diastereoselective, but afforded adducts of high ee. Additions of enantiomeric allenylindium iodide reagents to (R)-beta-ODPS-alpha-methylpropanal (13) afforded diastereomeric adducts (anti,anti and anti,syn) 14, 15 and 18, 19 with excellent diastereoselectivity indicative of a high degree of reagent control. A convenient procedure for the preparation of InI from In and I(2) is also described.