Subject synclinic

As demonstrated by the spectra of 1,2-dichloroethane shown in Fig. 4.1-11C, two halogen atoms in 1,2-position also show in-phase as well as out-of-phase vibrations. The antiperiplanar conformation is subject to the exclusion rule op. Raman 750 cm Oa . IR 708 cm. In the case of the synclinal conformation, on the other hand, the in-phase vibration at 645 cm is stronger both in the IR and in the Raman spectrum than the out-of-phase vibration at 677 cm. Freely rotating 1,2-dihalogen compounds therefore show four different C-Cl stretching vibrations (Fig. 4.1-11C). 

When an allylsilane containing a C(3) substituent reacts with an aldehyde, two new stereogenic centers are established. The reaction of ( )- and (Z)-2-butenyI-trialkylsilanes with aldehydes was first reported by Hayashi and Kumada in 1983 [28]. In these studies either ( )- and (Z)-2-butenyltrimethylsilane or ( )- and (Z)-cinnamyltrimethylsilane combined with various aldehydes in the presence of TiCl4 (Scheme 10-9). The ( )-2-butenylsilane and ( )-cinnamylsilane both afford >95 f of the syn diastereomer upon reaction with either propanal, isobutyraldehyde, or pivalaldehyde. When the Z-silanes were subjected to the same reaction conditions much lower selectivities were observed (65-72% syn selectivity). An acyclic transition structure with an antiperiplanar arrangement of double bonds was proposed to account for the diastereoselectivity observed in these reactions. A transition stmcture which includes a synclinal arrangement of double bonds may be necessary to explain the lower selectivities observed with the Z-allylsilanes. 

The western Saharan syncline was subjected to rapid subsidence during the Paleozoic and early Mesozoic. This resulted in the formation of a large basin (7-8 km deep) of Paleozoic sediments. The central and eastern Saharan synclines in the Paleozoic were areas of slower subsidence (in individual periods of the Paleozoic), and marine sediments are comparatively thin (from 2 to 4 km) (Fig. 6.13). During the early Mesozoic, the eastern Saharan syncline, including the Trias, Oued el-Mya, and Ghadames Basins (Figs. 6.12 6.13a,b), subsided rapidly. This resulted in the accumulation of up to 4 km of marine and, in places, continental Triassic, Jurassic, and Cretaceous sediments. 

Soon after the first report of the aldol reaction of silyl enol ethers was disclosed, allylsilanes were reported to show similar reactivity toward aldehydes and ketones when activated by a stoichiometric amount of TiCU (Scheme 3-85). This synthetically important reaction has subsequently become the subject of many synthetic chemists and was improved extensively using various kinds of Lewis acid catalysts. Acyclic transition states are proposed to explain diastereoselectivities of the reaction depending on a Lewis acid and reaction conditions. Particularly, synclinal orientation of reactants is suggested to be more preferable rather than an antiperiplanar one particularly for ( )-allylsilanes based on molecular model studies (Scheme 3-86). High diastereoselectivity observed in the reaction of chiral allylsilanes with aldehydes is understood in terms of this transition state model which is based on the Felkin-type induction (Scheme 3-87). ... 

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