Syntheses of the type AB -CD

Attention is attracted by the fact that practically all the successful syntheses from the precursors considered in this chapter (with the possible exception of syntheses of the type AB - CD) relate to derivatives of equi-lenin and estrone. This is connected with the nature of the reactions used for the syntheses of the missing rings. These include, in the first place, cyclizations with the participation of acids and ketones, which take place well only with aromatic rings or atoms of carbon activated by the propinquity of aromatic rings. 

The contents of the second section (syntheses of the AB - CD type) include the condensation of vinylbicyclenes with cyclic dienophiles. Among these, the successful total synthesis of estrone (Scheme 19) and the synthesis of nonaromatic D-homosteroids (Scheme 22) deserve to be mentioned. ... 

Bulk CdS possesses wurtzite structure ( 4 type) or zincblende structure ( 3 type). Both phases are formed by stacking of close-packed planes of cadmium and sulfur atoms. Hexagonal wurtzite structure can be represented as the periodic sequence AB AB AB... of closed-packed planes. Cubic zincblende is a sequence ABCABC... of closed-packed planes. For both structures the tetrahedral surrounding of both elements, cadmium and sulfur, takes place. According to the available literature data X-ray diffraction patterns of nanostructured CdS films and powders synthesized by different methods have similar features. Analysis of these features shows that the atomic structure of CdS nanoparticles differs from that of the bulk material. 

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