The SAMP-/RAMP-hydrazone methodology in asymmetric synthesis

Synthetic method for the preparation of 2-aminomethyl-1,3-diene derivatives through indium-mediated 1,3-butadien-2-ylation of imines

An efficient method for the preparation of a variety of 2-aminomethyl-1,3-dienes was developed through the reaction of imines with organoindium reagent generated in situ from indium and 1,3-dibromo-2-butyne. Three-component reactions of aldehydes, amines, and organoindium reagents gave successful results in a one-pot process.

Stereoselective asymmetric synthesis of (+)-sedamine and (+)-allosedamine

A convenient method for the generation of (+)-sedamine and (+)-allosedamine in high optical purity has been elaborated. The key steps are the highly stereoselective 1,2-nucleophilic addition to SAMP hydrazones allowing the installation of the stereogenic center at C2 and ring closing metathesis.

Stereoselective alkylations of chiral nitro imine and nitro hydrazone dianions. Synthesis of enantiomerically enriched 3-substituted 1-nitrocyclohexenes

Dianions of chiral nitro imines (generated by a combination of LDA and s-BuLi) underwent diastereoselective alkylation with methyl, butyl, isopropyl, allyl, and methallyl iodides. In contrast to the behavior of simple metalloenamines, the most selective auxiliary contained no coordinating groups but did possess a large steric difference between the two substituents. The yield and selectivity of the alkylations were improved by the addition of HMPA or DMPU. The use of (S)-1-naphthylethylamine as the auxiliary afforded the R absolute configuration of the alkylation products. This stereochemical outcome could be rationalized by simple steric approach controlled alkylation in a conformationally fixed, internally coordinated dianion. A SAMP nitro hydrazone gave poorer yields and selectivities.

Asymmetric solid-phase alkylation of ketones immobilized via SAMP hydrazone analogue linkers

The preparation and application of new solid supports with chiral linkers, analogues of SAMP hydrazine on solid-phase, are described. The supports were used for immobilization of ketones (diethylketone, cyclohexanone, 4- tert-butylcyclohexanone), and diastereoselective alkylation of formed chiral ketone hydrazones. The enantiomeric purities of cleaved alpha-alkylated chiral ketones ranged from 10 to 73%. The use of chiral lithium amides for metalation of hydrazones of t-butylcyclohexanone increased the enantiomeric excess of the alkylated product by 25-47%.

Competition between planar and central chiral control elements in nucleophilic addition to ferrocenyl imine derivatives

Planar chirality associated with the ferrocene in ferrocenyl oximes and hydrazones bearing chiral auxiliaries effectively competes with or overrides the normally excellent stereocontrol afforded by the auxiliary in determining the diastereoselectivity of addition to the C=N bond.

Asymmetric synthesis of naturally occurring spiroketals

Spiroketals are widely found as substructures of many naturally occurring compounds from diverse sources including plants, animals as well as microbes. Naturally occurring spiroketals are biologically active and most of them are chiral molecules. This article aims at reviewing the asymmetric synthesis of biologically active spiroketals for last 10 years (1998-2007).

Synthesis and structures of crystalline Li, Al and Sn(II) 1-azaallyls and beta-diketiminates derived from [Li{mu,eta3-N(SiMe3)C(Ad)C(H)SiMe3}]2 (Ad = 1-adamantyl)

The crystalline dimeric 1-azaallyllithium complex [Li{mu,eta(3-N(SiMe3)C(Ad)C(H)SiMe3}]2 (1) was prepared from equivalent portions of Li[CH(SiMe3)2] and 1-cyanoadamantane (AdCN). Complex was used as precursor to each of the crystalline complexes 2-8 which were obtained in good yield. By 1-azaallyl ligand transfer, 1 afforded (i) [Al{eta3-N(SiMe3)C(Ad)C(H)SiMe3}{kappa1-N(SiMe3)C(Ad)=C(H)SiMe3-E}Me] (5) with [AlCl2Me](2), (ii) [Sn{eta3-N(SiMe3)C(Ad)C(H)SiMe3}2] (7) with Sn[N(SiMe3)2]2, and (iii) [Li(N{C(Ad)=C(H)SiMe3-E}{Si(NN)SiMe3})(thf)2] (8) with the silylene Si[(NCH(2)Bu(t))2C6H(4)-1,2] [= Si(NN)]. By insertion into the C[triple bond, length as m-dash]N bond of the appropriate cyanoarene RCN, gave the beta-diketiminate [Li{mu-N(SiMe3)C(Ad)C(H)C(R)NSiMe3}]2 [R = Ph (2), C(6)H(4)Me-4 (3)], and yielded [Al{kappa2-N(SiMe3)C(Ad)C(H)C(Ph)NSiMe3}{kappa1-N(SiMe3)C(Ad)=C(H)SiMe3-E}Me] (6). The beta-diketiminate [Al{kappa2-N(SiMe3)C(Ad)C(H)C(Ph)NSiMe3}Me2] (4) was prepared from 2 and [AlClMe2]2. The X-ray structures of 1 and 3-8 are presented. Multinuclear NMR spectra in C6D6 or C6D5CD3 have been recorded for each of 1-8; such data on 8 revealed that in solution two minor isomers were also present.

Asymmetric Total Synthesis of (−)-Pironetin Employing the SAMP/RAMP Hydrazone Methodology

A convergent asymmetric total synthesis of pironetin (1), a polyketide with immunosuppressive, antitumor, and plant-growth regulating activities is described. The synthesis was realized by coupling between the C(8)-C(14) 2 and C(7)-C(2) 15 fragments, respectively, by using a Mukaiyama-aldol reaction. The stereogenic centers of each fragment were generated by employing the SAMP/RAMP hydrazone (SAMP=(S)-1-amino-2-methoxymethylpyrrolidine, RAMP=(R)-1-amino-2-methoxymethylpyrrolidine) methodology as a key step. An asymmetric alpha-alkylation of diethyl ketone permitted the introduction of the C(10) stereogenic center of 2, whereas the stereocenters C(4) and C(5) of 15 were installed by an asymmetric aldol reaction. Finally, the formation of the alpha,beta-unsaturated delta-lactone was achieved by ring-closing metathesis in the presence of catalytic amounts of titanium tetraisopropoxide.

An Enantioselective Chiral Brønsted Acid Catalyzed Imino−Azaenamine Reaction

The enantioselective Brønsted acid catalyzed addition of methyleneaminopyrrolidine to N-Boc imines has been achieved in the presence of chiral phosphoric acids derived from 3,3'-di(phenanthryl)-H8-BINOL. The corresponding aminohydrazones have been isolated in good yields with enantiomeric excesses up to 90%. [reaction: see text]

Enantioselective Synthesis of Natural Polyoxygenated Cyclohexanes and Cyclohexenes from [(p-Tolylsulfinyl)methyl]-p-quinols

Exploitation of the beta-hydroxysulfoxide fragment present in a number of enantiomerically pure (SR)- and (SS)-[(p-tolylsulfinyl)methyl]-p-quinols allowed chemo- and stereocontrolled conjugate additions of different organoaluminium reagents to the cyclohexadienone moiety. The same fragment was also shown to act as an efficient chiral masking carbonyl group, after oxidation to sulfone and retroaddition in basic medium, with elimination of methyl p-tolyl sulfone. Through the use of both transformations as key steps, enantiocontrolled syntheses of different natural products-such as the two enantiomers of dihydroepiepoformin, (-)-gabosine O, (+)-epiepoformin, (-)-theobroxide and (+)-4-epigabosine A (an epimer of the natural product gabosine A)-has been achieved. The presence of the beta-hydroxy sulfone moiety makes the cyclic structures rigid, allowing a number of stereoselective transformations such as carbonyl reductions, enone epoxidations or cis-dihydroxylations, en route to the natural structures. The observed selectivities were dependent on the particular substitution in each substrate, providing evidence of a strong influence of remote groups on the preferred approach of the reactants to the reactive conformations. An advanced precursor of natural (+)-harveynone was also synthesized, but the isolation of the natural product was not possible because of the instability of the corresponding enone, containing a triple bond, under the basic conditions necessary to eliminate the beta-hydroxy sulfone. This demonstrated that the limitations of the use of the beta-hydroxy sulfoxide as a chiral protecting carbonyl group were dependent on the relative stabilities of the final targets in the presence of the required base.