Enantioselective dimerization

On the other hand, biosynthetic pathways that do not involve the conversion from coniferyl alcohol to matairesinol have been proposed for lignans composed of two syringyl (3,5-dimethoxy-4-hydroxyphenyl) groups (+)-syringaresinol formation in Liriodendron tulipifera [50] and (H-)-lyoniresinol biosynthesis in Lyonia ovalifolia var. elliptica [51]. Enantioselective coupling of two sinapyl alcohol units was proposed for the selective formation of (H-)-syringaresinol [50], On the other hand, a non-enantioselective dimerization of sinapyl alcohol was proposed for (+)-lyoniresinol biosynthesis and the enantioselectivity in the biosynthesis was ascribed... 

When the aromatic substrates to be coupled do not contain any chiral information, stereodifferentiation may be achieved in those reactions that are mediated by chirally modified metal complexes. Thus, 2-naphthalenol is atropo-enantioselectively dimerized by oxidation with a chiral copper complex20,21. 

The efficient enantioselective dimerization reaction of 97 in the inclusion complex (100) with 12a suggests that two coumarin molecules are arranged in chiral positions in 100 and their [2 + 2] photodimerization occurs by keeping the chirality in 100. The chiral arrangement of 97 was detected by CD spectral measurement of the inclusion complex in Nujol mulls. The 1 1 complex of 97 with 12a and 13a showed CD spectra with a mirror-imaged relation (Fig. 17). After photoirradiation, the CD absorptions of the complex (100) at 225, 275, 300, and 330 nm disappeared, and the new CD absorption due to the inclusion complex (101) of 98 at 240 nm appeared. The photodimerization of 97 was also followed by measurement of infrared (IR) spectra as Nujol mulls. Upon photoirradiation, the nCO absorption of 97 in 100 at 1700 cm 1 decreased gradually and finally disappeared after 4 hr, and new vCO absorption due to 98 in 101 appeared at 1740 cm-1. 

The steric course of the photodimerization reactions of coumarin (37a) and thiocoumarin (37b) succeeded in being controlled almost perfectly by carrying out the reaction in inclusion complexes using various host compounds. Furthermore, enantioselective dimerization reactions of 37a and 37b were found to proceed through a single-crystal to single-crystal process. 

Tab. 38. Enantioselective dimerization of 2-naphthol (and 2-naphthylamine) derivatives promoted by vanadium complexes 219 and 220.

McMillan and co-workers [146] have reported the first example of direct enantioselective aldehyde-aldehyde cross-aldol reaction using small molecules as catalysts. Subsequently, they have described the enantioselective dimerization and cross-coupling of a-oxygenated aldehydes to provide eiythrose architecture. A second L-proline-catalyzed aldol reaction generates hexoses (O Scheme 22) [147]. 

TABLE 1. Enantioselective Dimerization of Bntafliene Followed by Trapping with Formaldehyde... 

TABLE 2. Enantioselective Dimerization of Butadiene Followed by Trapping with Acetaldehyde... 

Scheme 5.6 Enantioselective dimerization of enolizable enones catalyzed by 103b.

In their continuing efforts on developing new NHC-catalyzed reactions, the Scheidt group reported in 2011 a diastereo- and enantioselective dimerization of enals. The desired cyclopentene products were obtained with high levels of diastereo- and enantioselectivity (up to 72% yield, >20 1 dr, 90% ee). The key element for the reactivity and selectivity of this process is the utilization of Ti(0 Pr)4, which is crucial to promote a selective 1,4-conjugate addition to another molecule of enal vs. the typical 1,2-addition which might lead to y-lactones (Scheme 7.56). 

The solid-state chemistry of coumarin has also received considerable attention, mainly due to the biological importance of coumarin derivatives, and has been summarized in several reports. " In a comprehensive study of nearly 30 coumarin derivatives in the solid state, Venkatesan, Ramamurthy, and co-workers discuss the criteria for topochemical reactivity in the solid set forth 20 years earlier by Schmidt. Their results, though not invalidating previous guidelines for solid-state reactivity, indicate that caution, rather than rigor, is recommended in their application. The irradiation of coumarin within chiral inclusion complexes has also been used as a means to achieve enantioselective dimer synthesis. " ... 

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