By Brandes H. Smith (Eds.)
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4C A 1 C 1 3 in ether. AICI3 in carbon disulfide. Finely divided A 1 C 1 3 in carbon disulfide—ether. (LXX) to behavior noted already in the cyclophanes. "Ortho" cyclization occurs first. When steric or conformational conditions preclude "ortho" cyclization but the chain length is too short for "meta" [to (LXIX)], a dimer (or in some cases no cyclic product) is the result. Finally substitution occurs at the next possible site. The Friedel-Crafts Cyclization Method 59 Two items, at least, appear t o be wrong in the above scheme.
4]paracyclophane (17). 5]paracyclophanes by the same general procedure (45). The diester precursor, (X), was prepared as follows: the α,ω-diphenylalkane was both Η mono- and diacetylated with the monoacetyl derivative then being propionylated and both diketones converted to diacids by the Willgerodt reaction, to diesters by the usual means, and to the saturated diester by catalytic hydrogenation. 5]paracyclophane (all, remember, are the perhydro compounds). The yield here clearly improved as the strain decreased.
Hansley (24,25) was the first to report a marked improvement in the acyloin yield through use of highly purified xylene or toluene as solvent and the maintenance of a temperature sufficient to keep the sodium metal molten and thus more easily and thoroughly dispersed. It was also suggested that an inert gas atmosphere be used. With these improvements Hansley showed the usefulness of the acyloin cyclization for carbocyclic rings of 12-36 members. Some years later Prelog et al, in their quest for improved synthetic routes to C 9 - C 12 rings, reinvestigated the work of Hansley and suggested additional modifications (26).
Bridged Aromatic Compounds by Brandes H. Smith (Eds.)