Lithium dimethylcuprate synthesis

There have been many applications of conjugate additions in synthesis. Some representative reactions are shown in Scheme 8.2. Entries 1 and 2 are examples of addition of lithium dimethylcuprate to cyclic enones. The stereoselectivity exhibited in Entry 2 is the result of both steric and stereoelectronic effects that favor the approach syn to the methyl substituent. In particular, the axial hydrogen at C(6) hinders the a approach. 

The double bond of butenolides undergoes stereoselective Michael addition of organometallic reagents, affording useful synthetic intermediates. Thus 1,4-addition of lithium dimethylcuprate to 231 gave 236 as a single isomer, which was employed (237) for the synthesis of the bromopentene derivative 237. 

Scheme 2.14 Synthesis of allenes 38 and 40 by reduction of propargyl acetates with lithium dimethylcuprate. THP = tetrahydropyranyl.

The addition of organocuprates to chiral decalin enone systems has been explored in the context of steroid synthesis. For the addition of lithium dimethylcuprate to enones 28, 31, and 34, the major diastereomer obtained can easily be predicted by employment of a qualitative conformational analysis (Scheme 6.6) [11-13]. Thus,... 

An elegant two-step synthesis of lavandulol (22) consists of the 1,4-addition of lithium dimethylcuprate to the allenic ester (103) followed by prenylation and reduction of the intermediate which is formed. ... 

The methyl group was introduced by a two-step procedure. Thus, the hydrazone Michael adducts 52 were converted into the enol pivaloates 53 in excellent yields and diastereomeric excesses de > 96%) by treatment with pivaloyl chloride and triethylamine. After treatment with lithium dimethylcuprate the chiral auxiliary was removed by addition of 6n HCl in order to obtain the 5-substituted 2-methylcyclopentene carboxylate 54 in good yields and with excellent stereoselectivity (de, ee > 96%). Finally, the asymmetric synthesis of dehydroiridodiol (55, R = Me, = H) and its analogues was accomplished by reduction of 54 with lithium aluminum hydride or L-selectride leading to the desired products in excellent yields, diastereo- and enantiomeric excesses (de, ee > 96%). 

Cuprate conjugate additions. One step in a recent synthesis of (-t-)-modhephene (3). a natural sesquiterpene with a (3.3.3)propellane skeleton, involved conjugate addition of lithium dimethylcuprate to 1. The desired reaction proved difficult... 

For such hex-3-enopyranosid-2-uloses as 127 or 129, reaction with lithium dimethylcuprate,261 or with anions of malonic ester-type, methylene components in the presence of bis(2,4-pentanedionate)-Ni(II) catalyst,263 affords, in each case, the 1,4-addition products (128 and 130, respectively), in which the branching group is in the axial orientation. The methyl-branched pyranosides 126 and 128, readily accessible in this way, have been of use as chiral precursors for the synthesis of multistria-tins.264-285... 

Stereocontrolled conjugate addition of lithium dimethylcuprate to the electron deficient 2,3-double bond of allenes 851 leads to 5,6-dihydropyranM-oncs 852 in moderate yield (Equation 343) <2000J(P1)3188>. Similarly, the Ag(l)-catalyzed intramolecular cyclization of the allenic acid 853 is accelerated upon addition of diisopropylethyl-amine to afford the 3,6-dihydropyran-2-one 854, an intermediate during the total synthesis of (—)-malyngolide (Equation 344) <2000JA10470>. 

Chelation control has also been implicated in conjugate addition reactions. For example, during a synthesis of the macrolide antibiotic 6-ep/-erythromycin, Mul-zer and co-workers86 found that the stereochemistry at the anomeric centre (l position) of the tetrahydropyranyl protecting group had a profound effect on the stereochemistry of conjugate addition of lithium dimethylcuprate to the ynone 49.1 [Scheme 1.49]. 

Spiroketalization, The synthesis of talaromycin B (3) with four chiral centers by cyclization of an acyclic precursor presents stereochemical problems. A solution involves cyclization of a protected (3-hydroxy ketone with only one chiral center.1 Because of thermodynamic considerations (i.e., all substituents being equatorial and the anomeric effect), cyclization of 1 with HgCl2 in CH3CN followed by acetonation results in the desired product (2, 65% yield) with a stereoselectivity of —10 1. Final steps involve conversion of the hydroxymethyl group to ethyl by tosylation and displacement with lithium dimethylcuprate (80% yield) and hydrolysis of the acetonide group. 

Nigaki alcohol (18) has been identified by spectroscopic and chemical means as a constituent of Picrasma ailanthoides Planchon. Latia luciferin (19) has been synthesized in a stereoselective manner. A key step in this synthesis involves the addition of lithium dimethylcuprate to an enol phosphate derived from a 8-keto-ester to form an a,/3-unsaturated ester. Dehydro-/8-ionilideneacetic acid (20), an important intermediate in the synthesis of abscisic acid, has been prepared, as have the two nor-abscisic acid derivatives (21). The metabolite (22) of abscisic acid has been identified in the seeds of Robinia pseudacacia... 

As shown below, the attack of epoxide 6 with lithium dimethylcuprate is a key step of Hanessian et al. s erythronolide synthesis [23]. This methodology was also applied to the preparation of other polyketide-derived macrolides. Specific to erythronolide, introduction of the methyl group at C2 was achieved according to Scheme 11.3. 

As illustrated in Scheme 11.10, enone 32, prepared from D-glucal in a multistep sequence, was reacted with lithium dimethylcuprate. Clean axial attack of the reagent at C4 gave the substituted ketone 33. Subsequent Wittig methylation and reduction was used to introduce the second methyl group of 34, a key intermediate in the synthesis of a-multistriatin. The same sequence was used in the preparation of calcimycin (A23187), the 4-C-methyl synthon 33 being used to construct the two required chirons [50]. 

Kim has also studied the corresponding acylation of homocuprates by S-(2-pyridyl) thioates, discussed earlier in the context of total synthesis of monensin and erythronolide A (Sections 1.13.2.2 and 1.13.3.2). Under the standard anaerobic conditions necessary for cuprate formation, good yields of ketones could be derived from acylation of lithium dimethylcuprate (or lithium dibutylcuprate) by S-(2-pyridyl) thiobenzoate and other simple S-(pyridyl) thiol esters (equation 71). Interestingly, if the homocuprate is intentionally placed under an oxygen atmosphere before acylation and then reacted with the S-(2-pyridyl) thioate in oxygen at -78 C, one obtains good yields of the ctnresponding ester (equation 72). 

The efficiency of the above one-pot methylalumination procedure applied to the synthesis of tran.s -farnesol compares favorably with the reported three-step procedure involving preparation of the propargylic alcohol, L1A1H4 reduction of the triple bond, iodination of the vinyl carbon-aluminum bond, and final coupling of the alkenyl iodide with lithium dimethylcuprate. " ... 

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