Enol acetates single-crystal

Reaction of tetrazolyl enole 45 with copper(II) acetate yields the 3D-coordination polymer [ CulL19)21 (46), the structure of which is unequivocally established by single-crystal X-ray diffraction. The formation of 46 is understandable if the enolate of 45 is considered as tridentate chelate ligand and if the intermediate formation of the coordinatively unsaturated self-complimentary copper(II) building block 47 is assumed. The monomers 47 are bidentate coordinating by the two CN donors, which leads to linking of monomers and to coordinative saturation at the copper(II) center of 47 with formation of three-dimensional 0 [Cu(L19)2] (46) (Scheme 17, Fig. 18) [163, 164],... 

S) -( — )-2-Methyl-2-hydroxy-y-butyrolactone (176a) is a useful synthon for the asymmetric construction of acyclic tertiary a-hydroxy acids found in natural products such as the pheromone frontalin and mevalonolactone, the biosynthetic precursor of terpenoids and steroids. This compound is readily prepared from lactone (175) using the asymmetric enolate oxidation protocol and dimethoxy oxaziridine ( + )-(158) <95JOC6l48>. The a-hydroxy lactone (176b), isolated as the benzoate, was obtained in 84% ee and 70% yield. A single crystallization from ethyl acetate improved the ee to >94% (Equation (41)). 

The pinwheel shape of a f-butyl propionate derived silylketene acetal (see Section 2.4.2.1) was revealed by a single-crystal X-ray diffraction analysis. Several different catalysts were reported to promote the aldol-type condensation of alkyl enol ethersand silyl enol ethers with aldehydes, acetals and various other electrophiles. In some cases the reaction proceeded with high simple stereoselection. The mechanism of the Lewis acid mediated additions to acetals (see Section 2.4.2.3) was investigated in detail, as well as the uncatalyzed aldol reaction of silyl enol ethers with aldehydes promoted by the hydrophobic effect (see Section 2.4.2.1). 

A milder preparation of these 1,3-dioxolanones is illustrated in Scheme 10, this time utilizing 2. Rhodium triflate, [Rh(CH3CN)3(triphos)] (CF3S03 )3, catalyzed acetalization of pivaldehyde with 2 followed by a single crystallization from ether/pentane furnishes pure cis 50 in high yield (80—90%). Diastereoselective alkylation of the lithium enolate of 50 with... 

Oppolzer s auxiliary opened, in addition, an access to a/iti-configured aldol adducts 272 (Scheme 4.62). For this purpose, silyl ketene N,0-acetal 271 was prepared from propionic sultam 92, obtained as a single diastereomer, according to the NMR spectra of the crude product, and isolated as a crystalline compound it was characterized as a cis-silicon enolate by a crystal structure analysis. For the subsequent Mukaiyama aldol addition, titanium tetrachloride was found to be the optimum Lewis acid to yield the awti-diastereomers 272 in excellent diastereoselectivity. Their formation under attack of the enolate to the Re-face of the aldehyde is consistent with an open transition state 275, wherein the Lewis acid-coordinated aldehyde is located on the face opposite to the sulfonyl group (Scheme 4.62) [136b]. An alternative approach to the a fi-aldol adducts was also elaborated, based upon cA-boron enolates 267 when they are reacted with aldehydes in the presence of titanium tetrachloride, an ti-selective aldol addition occurs leading to the products 272 rather than to sy -aldols 268 that result in the absence of the Lewis acid [136c]. 

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