Enantioselectivity coupling

Entries 4 and 5 are examples of use of the Sakurai reaction to couple major fragments in multistage synthesis. In Entry 4 an unusual catalyst, a chiral acyloxyboronate (see p. 126) was used to effect an enantioselective coupling. (See p. 847 for another application of this catalyst.) Entry 5 was used in the construction of amphidinolide P, a compound with anticancer activity. 

Scheme 8.11 Enantioselective coupling of N-Boc pyrrolidine with aryl bromide 3.

The chiral diol 17 derived from tartaric acid is exploited in the titanium-catalyzed asymmetric pinacol coupling in the presence of Zn and MesSiCl to give the corresponding diol in 11-71 ee % [44], The chiral salen ligands 18-20 are used in the titanium-catalyzed enantioselective coupling reaction, which achieves the higher selectivity [45-47]. The chromium complex with TBOxH (21) efficiently catalyzes the asymmetric coupling reaction of both aromatic and aliphatic aldehydes [48]. 

Scheme 7 Enantioselective coupling, f>-alkoxide elimination sequence...

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... 

A crude enzyme preparation from Vinca rosea catalyzed the enantioselective coupling of coniferyl alcohol to give rise to an optically active C8-C5 neolignan, dehydrodiconiferyl alcohol, with a ratio of (—)-enantiomer to (+)-enantiomer of 2 1 (Fig. 12.10) [66],... 

Scheme 9.28. Ni-catalyzed enantioselective coupling of alkynes, imines, and triethylborane.

The precatalyst 17 produced (S)-benzoin (6, Ar = Ph) in very good yield (83%) and enantioselectivity (90% ee). The condensation of numerous other aromatic aldehydes 4 yielded the corresponding a-hydroxy ketones 6 with excellent ee-values of up to 95%. (For experimental details see Chapter 14.20.2). Electron-rich aromatic aldehydes gave consistently higher asymmetric inductions than electron-deficient (i.e., activated) aromatic aldehydes, with lower reaction temperatures or lower amounts of catalyst leading to slightly higher enantioselectivities coupled with lower yields. 

Highly enantioselective couplings have been established by using chiral Lewis acid catalysts (Equation (14)). 

Shintani and Fu repored enantioselective coupling of terminal alkynes 420 with azomethine imines 419 by copper-catalyzed [3 + 2]-cycloaddition (Scheme 133).195 In the presence of Cul and phosphafer-... 

Scheme 9.13 Enantioselective coupling of 2-naphthols using dinuclear vanadium(v) catalysts bearing a V-O-V linkage, reported by Gong. ...

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