Double asymmetric coupling

Shibata and colleagues examined the double asymmetric coupling of diiodoparacyclophanes with alkynes in the presence of a chiral Pd catalyst. After screening a series of... 

SCHEME 13.25. The double asymmetric coupling of diiodopara-cyclophanes with alkynes. 

Note that in aldol reaction IV (from 31 to 42 in Scheme 7-9), the methodology differs from that used in I, II, and III (see Scheme 7 7). Aldol reaction IV is also a double asymmetric reaction involving the coupling of two structurally... 

A study on the combined use of a chiral substrate obtained by alcoholysis of a 4-benzylidene-5(4//)-oxazolone with a chiral alcohol coupled with hydrogenation using a chiral catalyst has also been described. This work shows that the matching effect of double asymmetric induction in hydrogenation can be modulated by a solvent effect. 

Scheme 5.26. Selective coupling of two chiral fragments (double asymmetric induction) in the asymmetric synthesis of 6-deoxyerythronolide B [124,127], For a similar reaction in the synthesis of erythronolide B, see ref. [129],...

Three-component coupling with vinylstannane. norbornene (80). and bro-mobenzene affords the product 91 via oxidative addition, insertion, transme-tallation, and reductive elimination[85]. Asymmetric multipoint control in the formation of 94 and 95 in a ratio of 10 1 was achieved by diastereo-differ-entiative assembly of norbornene (80), the (5 )-(Z)-3-siloxyvinyl iodide 92 and the alkyne 93, showing that the control of four chiralities in 94 is possible by use of the single chirality of the iodide 92. The double bond in 92 should be Z no selectivity was observed with E form[86]. 

Optically active (Z)-l-substituted-2-alkenylsilanes are also available by asymmetric cross coupling, and similarly react with aldehydes in the presence of titanium(IV) chloride by an SE process in which the electrophile attacks the allylsilane double bond unit with respect to the leaving silyl group to form ( )-s)vr-products. However the enantiomeric excesses of these (Z)-allylsilanes tend to be lower than those of their ( )-isomers, and their reactions with aldehydes tend to be less stereoselective with more of the (E)-anti products being obtained74. 

Figure 7.7 (a) Double- and tiiple-axis rocking curves (specimen scans) of a GaN epitaxial layer on (0001) oriented sapphire. 0002 reflection CuK radiation, (b) Coupled specimen-analyser scan in ratio 1 2. 022 (with 17.65° asymmetric cut) Si duMond beam conditioners. 111 symmetric Si analyser... 

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