Cyclophanes 1,2-rearrangement

Stereoselectivity. See Asymmetric induction Axial/equatorial-, Cis/trans-, Enantio-, Endo/exo- or Erythro/threo-Selectivity Inversion Retention definition (e.e.), 107 footnote Steric hindrance, overcoming of in acylations, 145 in aldol type reactions, 55-56 in corrin synthesis, 261-262 in Diels-Alder cyclizations, 86 in Michael type additions, 90 in oiefinations Barton olefination, 34-35 McMurry olefination, 41 Peterson olefination, 33 in syntheses of ce-hydrdoxy ketones, 52 Steric strain, due to bridges (Bredt s rule) effect on enolization, 276, 277, 296, 299 effect on f3-lactam stability, 311-315 —, due to crowding, release of in chlorophyll synthesis, 258-259 in metc-cyclophane rearrangement, 38, 338 in dodecahedrane synthesis, 336-337 in prismane synthesis, 330 in tetrahedrane synthesis, 330 —, due to small angles, release of, 79-80, 330-333, 337... 

Two efficient syntheses of strained cyclophanes indicate the synthetic potential of allyl or benzyl sulfide intermediates, in which the combined nucleophilicity and redox activity of the sulfur atom can be used. The dibenzylic sulfides from xylylene dihalides and -dithiols can be methylated with dimethoxycarbenium tetrafiuoroborate (H. Meerwein, 1960 R.F. Borch, 1968, 1969 from trimethyl orthoformate and BFj, 3 4). The sulfonium salts are deprotonated and rearrange to methyl sulfides (Stevens rearrangement). Repeated methylation and Hofmann elimination yields double bonds (R.H. Mitchell, 1974). 

Whereas normally temperatures above 50 "C are necessary to start the rearrangement, in the case of cyclophane 5 the benzene dioxide opens up to form 6 even at 20 JC.n... 

Racemization and rearrangement mechanisms of chiral [2.2]para cyclophanes iso.isi). 

The photorearrangement of 7V-aryl lactams 34 to the ortho-position offers an interesting possibility for the preparation of cyclic benzo-aza-ketones of type 35, whereas the rearrangement to the para-position should lead to para-cyclophane 144 formation. 7-, 8-, and 13-Membered ring jV-aryl lactams (34, n = 5, 6, and 11) were successfully rearranged in ethanol to 35 in chemical yields of 60, 83, and 80%,13 and in quantum yields of 0.071, 0.11, and 0.082, respectively.14... 

Boekelheide and his collaborators [407] have described a two-step sequence for transforming sulfide linkages to carbon-carbon double bonds — Stevens rearrangement of sulfur ylides and Hofmann elimination — which they found particularly useful for the synthesis of cyclophane derivatives, such as the [2.2]metaparacyclophane-l,9-diene shown. The Ramberg-Backlund rearrangement (see Section 4.3.2) was unsatisfactory for such highly strained molecules. 

Warner and Winstein284 have obtained the stable monocation 106 by protonating l,6-methano[10]annulene in HSO3F. Cram and Cram285 have been successful in protonating [2.2]-pura-cyclophane. The initial protonated species 107 rearranges to the meta-protonated species 108 [Eq. (3.40)]. 

Cyclizations by formation of carbon—selenium bonds represent a modern method with a high synthetic potential in the chemistry of cyclophanes. Selenocyanates such as 16 are accessible usually in excellent yields through the reaction of bromides with KSeCN [27], The reaction with benzylic bromides under reductive conditions using the dilution principle results in good to excellent yields of [3.3]di-selenacyclophanes which can be deselenized photochemically, pyrolytically (without previous oxidation), or by reaction with arynes, Stevens rearrangement and subsequent reaction with Raney nickel. [2.2]Metacyclophane (18), for example, is accessible in 47% total yield by using this sequence of reactions starting with... 

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