Christian Chapuis

Asymmetric diels-alder reactions : facile preparation and structure of sulfonamido-isobornyl acrylates

Abstract The crystalline chiral auxiliaries 2 , 3 and 4 were prepared from camphor-10-sulfonyl chlorides in 2 steps. Their readily accessible acrylates underwent efficient asymmetric diels-alder additions to cyclopentadiene, the topicity of which agrees with X-ray evidence.

Synthesis of Citronellal by RhI-Catalysed Asymmetric Isomerization ofN,N-Diethyl-Substituted Geranyl- and Nerylamines or Geraniol and Nerol in the Presence of Chiral Diphosphino Ligands, under Homogeneous and Supported Conditions

For the asymmetric isomerization of geranyl- or neryldiethylamine ((E)- or (Z)-1, resp.) and allyl alcohols geraniol or nerol ((E)- or (Z)-2, resp.) to citronellal (4) in the presence of a [RhI(ligand)cycloocta-1,5-diene)]+ catalyst, the atropic ligands 5 – 11 are compared under homogeneous and polymer-supported conditions with the non-C2-symmetrical diphosphino ferrocene ligands 12 – 16. The tBu-josiphos ligand 13 or daniphos ligand 19, available in both antipodal series, already catalyse the reaction of (E)-1 at 20° (97% e.e.) and favourably compare with the binap ligand 5 (see Table 1). Silica-gel- or polymer-supported diphosphino ligands usually afford similar selectivity as compared to the corresponding ligands applied under homogeneous conditions, but are generally less reactive. In this context, a polymer-supported ligand of interest is the polymer-anchored binap (R)-6, in terms of reactivity, selectivity, and recoverability, with a turnover of more than 14400.

High asymmetric induction in diels-alder additions of cyclopentadiene to acrylates derived from isoborneol

Starting from (R)-(+)- and from (S)-(−)-camphor the chiral alcohols 1, 2 and 3 have been prepared; their acrylates II underwent TiCl2(OR)2-promoted Diels-Alder additions to cyclopentadiene giving efficiently in a predictable manner either the (2R)- or the (2S)-adducts III with up to virtually quantitative asymmetric induction.

Stereochemical course of the [4+2] cycloaddition of 1-methoxybuta-1,3-diene to N-glyoxyloyl-(2R)-bornane-10,2-sultam. The formal synthesis of compactin and mevinolin

Abstract The chiral heterodienophile N -glyoxyloyl-(2 R )-bornane-10,2-sultam 2 , readily prepared from (2 R )-bornane-10,2-sultam 1 , was used in noncatalyzed atmospheric and high-pressure as well as in [Eu(fod) 3 ]-catalyzed [4+2] cycloadditions with 1-methoxybuta-1,3-diene 3 . All the [4+2] cycloadditions studied led to diastereoisomeric mixtures of 6-substituted derivatives of 2-methoxy-5,6-dihydro-2H-pyran 4–7 . The extent of asymmetric induction in these reactions was established by 1 H NMR analysis and the absolute configuration of the thermodynamically stable products 5 and 7 by X-ray analysis, and independently by chemical correlation. Stereochemical models for both noncatalyzed and [Eu(fod) 3 ]-promoted reactions are proposed. The [4+2] cycloadduct 5 was then effectively transformed into (4 R )-hydroxy-(6S)-hydroxymethyltetrahydropyrone-2 12 , a key synthon for the lactone moiety of compactin 10 and mevinolin 11 .

Asymmetric Diels-Alder reaction of a chiral allenic ester: enantioselective synthesis of (−)-β santalene

The Lewis-acid promoted Diels-Alder reaction of the allenic ester 10 provides the adduct 11 with excellent a-facial selection leading to an efficient enantioselective synthesis of (-)-i3-santalene and to the recovery of the chiral control element 7.

Influence of the Solvent Polarity on the Stereoselectivity of the Uncatalyzed [4+2] Cycloaddition of Cyclopentadiene to N,N′-Fumaroyldi[(2R)-borane-10,2-sultam]

A correlation between the solvent polarity and the logarithm of the diastereoisomer ratio (dr) was found for the uncatalyzed [4+2] cycloaddition of cyclopentadiene to N,N′-fumaroyldi[(2R)-bornane-10,2-sultam]. Using the Abboud-Abraham-Kamlet-Taft parameters, predictive values for this method, allowed an optimum diastereoisomeric excess (de) of 96% (EtOH, −78°). A similar diastereoselectivity was achieved using 5M LiClO4/Et2O or H2O/β-cyclodextrin, and the influence of supercritical CO2 is also reported. Selective cycloadditions of apolar diene in polar solvents are entropically favored by the greater dipole moment of the N-enoylcamphorsultam syn-s-cis conformers and of the C(α)-re transition states. Implications on the stereochemical course of the reaction are discussed.

Efficient synthesis of N-glyoxyloyl-(2R)-bornane-10,2-sultam

Abstract Starting from (2 R )-bornane-10,2-sultam 1 , the useful chiron N -glyoxyloyl-(2 R )-bornane-10,2-sultam 5 is prepared via its crystalline precursor 4 . The hemiacetal 4 whose structure was proved by the X-ray analysis is formed in diastereomerically pure form.

Rhodium catalysed decarbonylation of optically active campholenal analogues

Abstract Campholenal analogues (+)- 1a–d were decarbonylated with a catalytic amount of Rh/Al 2 O 3 with only slight racemization. Experiment with other catalysts showed that racemization increases with the ability of the metal to form a π-allyl complex and to catalyse the hydrogenation of the β-elimination side product. (Ph 3 P) 3 RhCl gave the best results.

Asymmetric Diels-Alder reactions of acrylates derived from 3-alkyl-borneols and -isoborneols

Abstract Starting from (+)-camphor the chiral alcohols 7 and 8 have been prepared; their acrylates 9 and 13 underwent TiCl 2 (OiPr) 2 -mediated Diels-Alder addition to cyclopentadiene with stereoface-differentiations of 66% d.e. and 94% d.e., respectively.

Diastereoselectivity in the hetero [4+2] cycloaddition of cyclopentadiene to N-benzyliminoacetyl derivatives of (2R)-bornane-10,2-sultam and other chiral secondary alcohols

Abstract Various protonated chiral glyoxyloyl-α-imino- N -benzyl hetero-dienophiles have been examined in the diastereoselective exo -cycloaddition to cyclopentadiene at −78°C promoted by CF 3 CO 2 − BF 3 , a dissociated non-nucleophilic counter ion. The best π-facial selectivities were observed with (2 R )-bornane-10,2-sultam (76% d.e.) and (2 R )-10-dicyclohexylsulfonamoyl-isoborneol (80% d.e.) as chiral auxiliaries. These conditions were found to be superior in terms of yields and selectivities as compared to analogous aza-dienophiles treated with simple Lewis acids or under thermal conditions. Absolute configurations were assigned on the basis of an X-ray analysis of the major cycloadduct (3′ S )- 3a as well as by chiroptical comparison with the corresponding new amino alcohol (−)-(3 S )- 4a . Plausible transition states are briefly discussed on the basis of PM3 conformational calculations.