Menthyl group

McKervey and Ye have developed chiral sulfur-containing dirhodium car-boxylates that have been subsequently employed as catalysts for asymmetric intramolecular C-H insertion reactions of y-alkoxy-ot-diazo-p-keto esters. These reactions produced the corresponding ci -2,5-disubstituted-3(2H)-furanones with diastereoselectivities of up to 47% de. Moreover, when a chiral y-alkoxy-a-diazo-p-keto ester containing the menthyl group as a chiral auxiliary was combined with rhodium(II) benzenesulfoneprolinate catalyst, a considerable diastereoselectivity enhancement was achieved with the de value being more than 60% (Scheme 10.74). 

Aratani et al. (21) subsequently found that the use of chiral menthyl diazoacetate esters led to higher trans/cis ratios and improved facial selectivity. A number of bulky diazoesters provided high enantioselectivity in the cyclopropanation reaction, but trans selectivity was highest with /-menthyl esters, Eq. 6. It seems clear from these and subsequent studies that the menthyl group is used because of its bulk and ready availability. The chirality present in the ester has a negligible effect on facial selectivity in the cyclopropanation reaction. Slow addition of diazoester is required (7 h at ambient temperature) for high yields presumably to suppress the formation of fumarate byproducts. 

Methyl isobutyl ketone was reduced with (- )-menthol-LAH in ether to give the (+ )-(S)-carbinol (53) in low optical yield. Methyl neopentyl ketone was similarly reduced to the (-I- )-carbinol, although pinacolone was reduced to only racemic alcohol. Maximum stereoselectivity in the reduction of both ketones and alkenynols was obtained with a 2 1 (-)-menthol-LAH reagent. The observed low stereoselectivity was attributed mainly to insufficient interaction of the remote isopropyl substituent on the menthyl group with the substituents on... 

An alternative route to dibenzothiophenebut3o ic acids has been developed involving reaction of 4-lithiodibenzothiophene with menthyl levulate, yielding, after ester hydrolysis, y-hydroxy-y-methyl-4-dibenzothiophenebutyric acid (139). The bulky menthyl group was employed to direct reaction to the ketone moiety of the levulate. Dehydration followed by catalytic hydrogenation yields y-methyl-4-dibenzothiophenebutyric acid, Friedel-Crafts cyclization of which yielded the ketone (140). ... 

The obtained compounds were purified and characterized by H, C and Sn NMR, using CDCI3 with tetramethylsilane as the standard. The results indicated that the compounds were optically pure, without epimerization in the carbon atom of the menthyl group bonded to the tin atom. 

The enantioselective hydrogenation of acetophenone afforded an excess of the (S)-phenylethanol isomer. An enantiomeric excess (e.e.) of around 20% was obtained with all the chiral organotin compounds tested, which is a good result for acetophenone, a non-activated ketone, and especially because of the high selectivity to PE (over 97%). Previously published results with the classical Pt/cinchoni-dine system showed an e.e. of 17%, corresponding to a yield of 4.7% [133]. The similar e.e.s obtained for the three rather different modifiers employed in this work seems to indicate that chiral induction must be assigned to the presence of at least one menthyl group attached to the surface. 

The synthesis of diastereomeric P-chiral fullerenyl phosphine 27a with a (-i-)-menthyl group was achieved analogously [114, 117]. No sign of epimerization upon heating 27a for 14 h at 80 °C in toluene was observed, indicating the remarkable stability of chirality at the phosphorus of 27a. 

In this context, a logical inconsistency in relation to auxiliaries must be pointed out. Frequently the configuration of the auxiliary group, i.e., after attachment of the auxiliary compound, is given by the optical rotation of the auxiliary (compound), e.g., (—)-menlhyl group instead of (17 )-menthyl group. This practice can lead to serious errors and must be avoided. 

Pernak, J. and Feder-Kubis, J., Chiral pyridinium-based ionic liquids, containing the (lR,2S,5R)-(-)-menthyl group. Tetrahedron Asymmetry, 17,1728,2006. 

An alternative complementary olefination procedure involving alkylidene malonates was also developed that produces (Z) olefins as major stereoisomers when the R substituent is aliphatic (equation 93)130. The stereoselectivity depends on the bimetallic compounds and may be improved by bulky ester substituents (such as menthyl groups)131. 

A remarkable complex (33) with a C2-symmetric semicorrin ligand has been recently developed by Pfaltz and coworkers.64 A copper(II) complex was used as a procatalyst, but (33) was shown to be the active cyclopropanation catalyst. As shown in Table 3, this complex resulted in spectacular enantioselecti-vities in the range of 92-97% ee. Once again, the (15,35,4/ )-menthyl group attenuated the selectivity. Unfortunately, even though respectable yields were obtained with dienes and styrenes, the reaction with 1-heptene was rather inefficient. 

Two classes of phosphetane-based chiral ligands have been developed one consists of monodentate phosphetanes with a chiral substituent, such as a menthyl group, on the four-membered ring. The other consists of ( -symmetric bidentate phosphetanes. 

P-Menthylphosphetane 76, in which the chiral menthyl group was introduced on the phosphorus atom, is a highly hindered, chiral, and electron-rich monodentate ligand. It is expected to provide good activity in asymmetric catalytic applications and has been reported for specific applications in organometallic catalysis <2000CPB315>. 

Desilylation of 135 afforded alcohol 136, which was oxidized to enal 137 by Parikh-Doering oxidation in 62% yield. Removal of the p-nitrobenzoyl group and the menthyl group resulted in completion of the total synthesis of mniopetal F (6). 

Addition of 1 equiv. of Lewis acid to free-radical reduction by the tin hydride bearing chiral menthyl groups leads to a remarkable increase of the enantioselectivity (Equation (18)).63 64... 

In 1998, Christie and co-workers reported the first example where stereoselectivity was induced by a chiral metal complex.42 By making dia-stereomerically pure heterobimetallic Mo-Co alkyne complexes possessing a menthyl group (35 and 36), they were able to obtain diaster-eomeric excesses of >99% (determined by 13C NMR spectroscopy) of the resulting cyclopentenone (37 and 38 - Scheme 13). 

Enantioselective cyclopropanation. The reaction of dimenthyl fumarates (2) with isopropylidene(triphenyl)phosphorane proceeds in 74% de in THF at -78 — 20°. (1S,3S)-3 is obtained when the /-menthyl group is used inductor (lR,3R)-3 is obtained in the same optical yield when the unnatural //-menthyl group is used. The concentration plays an important role enantioselectivity is increased by dilution and decreased by concentration. 

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