8-Phenylmenthol-derived esters

Another system which affords rearrangement products of high optical purity uses the diastereofacial selection of an 8-phenylmenthol-derived ester. 

A highly diastereoselective aziridination of 8-phenylmenthol-derived a,ft -unsaturated esters (38) with 3-acetoxyamino-2-ethylquinazolinone (39) has been reported. The (g) yields of the resulting aziridines (40) were greatly improved in the presence of hexamethyldisilazane.51... 

In terms of both regio- and diastereoselectivity metal alkoxycarbenes are superior to their isolobal ester analogs as demonstrated by the cycloaddition of the (-)-8-phenylmenthol-derived chromium carbene 44 with nitrilimine 41 (Scheme 11.17). Whereas complex 44 affords a 55% yield of A -pyrazoline ester 45a/b in a 92 8 mixture of diastereomers, its cinnamate congener 47 produces a 38 62 ratio of regioisomers 45 and 46, although in quantitative yield, as diastereomeric mixtures (d.r. = 32 68 for 45a/b and 71 29for 46a/b). 

Recendy, Darzens reaction was investigated for its synthetic applicability to the condensation of substituted cyclohexanes and optically active a-chloroesters (derived from (-)-phenylmenthol). In this report, it was found that reaction between chloroester 44 and cyclohexanone 43 provided an 84% yield with 78 22 selectivity for the axial glycidic ester 45 over equatorial glycidic ester 46 both having the R configuration at the epoxide stereocenter. 

Mainly sulfoxide groups are introduced as chiral auxiliaries for the modification of a,/J-unsat-urated enones (see Section D.1.5.3.5.). Chiral imine derivatives have also been used (see Section D.1.5.3.1.). Various chiral alcohols, and in particular 8-phenylmenthol, have been successfully used as auxiliaries, mainly in two-fold Michael additions to a,/ -unsaturatcd esters. 

Chiral Auxiliary for Asymmetric Induction. Numerous derivatives of (—)-8-phenylmenthol have been utilized for asymmetric induction studies. These include inter- and intramolecular Diels-Alder reactions, dihydroxylations, and intramolecular ene reactions of a,p-unsaturated 8-phenylmenthol esters. These reactions usually proceed in moderate to good yield with high diastereofacial selectivity. a-Keto esters of 8-phenylmenthol (see 8-Phenylmenthyl Pyruvate) have been used for asymmetric addition to the keto group, as well as for asymmetric [2 -F 2] photoadditions and nucleophilic alkylation. Ene reactions of a-imino esters of 8-phenylmenthol with alkenes provide a direct route to a-amino acids of high optical purity. Vinyl and butadienyl ethers of 8-phenylmenthol have been prepared and the diastereofacial selectivity of nitrone and Diels-Alder cycloadditions, respectively, have been evaluated. a-Anions of 8-phenylmenthol esters also show significant diastereofacial selectivity in aldol condensations and enantiose-lective alkene formation by reaction of achiral ketones with 8-phenylmenthyl phosphonoacetate gives de up to 90%. ... 

C-Silylation of Li-Ester Enolates. Further applications of a-diphenylmethylsilyl carhoxylates have been reported. Gais and co-workers employed a-methyldiphenylsilyl acetate (16), prepared from (—)-8-phenylmenthol acetate (15), by lithiation with LDA and reaction with methyldiphenylchlorosilane, in an asymmetric Peterson reaction with the bicyclo[3.3.0]octan-3-one derivative (17) to yield a 89 11 mixture of the (Z)- and ( )-esters (18) and (19) About 50% of the starting material was recovered in the unoptimized procedure (eq 13). ... 

Another approach to conduct asymmetric conjugate addition reactions has been the use of ester auxiliaries derived from chiral alcohols [81]. Oppolzer [82, 83] reported a notable example using unsaturated esters such as 68 derived from (-)-8-phenylmenthol [84] (Equation 14) [82]. The addition of phenylcopper to 68 was catalyzed by BF3-OEt2 [70] and afforded the acid 70 in > 99 % ee after saponification. Analysis of the stereochemical outcome led to the suggestion that the observed induction results from addition to the exposed olefin face in conformer 69 [82], which is favored because of stabilizing orbital overlap between the enone and aromatic groups [85]. 

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