Paracyclophanes rearrangement

Thermolyses of 3-oxaquadricyclanes with different substituents at C1(C5) and C6(C7) such as carboxy and phenyl groups showed that the reaction generally gives oxepins with the carboxy functions in the 4- and 5-position.24 This is also true when the substituents in the 6- and 7-positions form a bridge of six carbon atoms, e.g. formation of 9.129131 The rearrangement of these 3-oxaquadricyclanes gives access to the [6]paracyclophane system. 

In this case, the benzene dioxide 5 is obtained from a [2,2]paracyclophane diene by photooxygenation and rearrangement of the derived endoperoxide. 

Obviously carboxy derivatives such as 11-19 are simple chiral structures suitable for optical resolutions through diastereomeric salts. For this purpose carboxylic groups have been introduced into [10]- and [8]paracyclophane either by chloro-methylation and oxidation of the carboxaldehydes obtained thereof 39,44) or by lithiation and subsequent carboxylation40). Electrophilic substitution of strained paracyclophanes is not advisable since it may initiate rearrangement to the more stable metacyclophanes. Carboxy[7]paracyclophane (72) was first prepared in 1972 by ring contraction of a diazoketone derived from 4-carboxy[8]paracyclophane (75) 45). 

The known chirality of 23 and of the corresponding methyl derivative 38 has served as the basis for several configurational assignments especially for [m][n]44) and layered [2.2]paracyclophanes 64 as well as for the interpretation of rearrangement reactions to optically active [2.2]metaparacyclophanes 95 and for other stereochemical investigations 57 103) (see 2.6 and 2.8). 

It has long been known that appropriately substituted [2.2]paracyclophanes are chiral, chemically stable and do not racemize under normal reaction conditions. With these seemingly ideal prerequisites for use in chiral synthesis, it is perhaps surprising that only three examples have appeared in the literature, all of them in recent years (Scheme 8). Reich employed [2.2]paracyclo-phane-derived selenides such as 24 to administer chirality transfer in selenoxide [2,3] sigmatropic rearrangements. Using this methodology, he was able to synthesize optically active linalool 25... 

Rearrangement of the vinyl ether of acetylparacyclophane oxime 26, the key intermediate in the Trofimov reaction, gave 2-([2,2]paracyclophan-5-yl)pyrrole (27) in 74% yield (Scheme 4) (04TL5489). The intermediate O-vinyl oxime 26 was synthesized (78%) by vinylation of a Cs-derivative of the oxime of 5-acetyl[2,2]paracyclophane (28) with acetylene (Scheme 4) under pressure in a DMSO-n-pentane two-phase system (04TL5489). 

Scheme 20 psewdo-gemmaZZjy-Substituted bisallenic [2.2]paracyclophane systems via dou ble [2,3]sigmatropic rearrangement of propargylic sulfenates... 

The irradiation of the strained diene (87a) in alcoholic solution affords the rearranged product (88). The authors interpret this as evidence for the isomerization of the starting material (87a) into the 4-paracyclophane (89) which thermally adds solvent to afford the isolated product. A more detailed report of this work has included the photoisomerizations of (87 b.c) which yield mixtures of (90) and (91) on irradiation in ethanol. Spectroscopic evidence confirms the paracyclophanes (89) as the intermediates in the formation of the alcohol addition products (88), (90), and (91). ... 

The dicarbamates 254 were smoothly lithiated by using f-BuLi—TMEDA at —80°C and then allowed to warm to room temperature over 16 hours. Under these conditions a smooth anionic ortfio-Fries rearrangement gave the diamido derivatives 255 in fair yields (25-80%) (equation 117) (see also Reference 220). A similar rearrangement was also described for [2,2]-paracyclophanes. ... 

The fully rearranged diradical 9 from dispiro[2.2.2.2]deca-4,9-diene (7) was also successfully trapped by styrene, and substituted styrenes 13, to produce [2.4]paracyclophanes 14 in an analogous series of thermolyses carried out in the presence of rer/-butyl alcohol. " ... 

Irradiation of paracyclophane diazo ketone 98 affords the ring contraction acid 221 in 25% yield.35 Photolysis of the a-diazobenzocycloheptanone (68) in methanol effected the Wolff rearrangement to give the tetralin ester (222).25 Irradiation of 2-diazoindan-1 -one in THF/H2O produces benzcyclobutane carboxylic acid (223).39... 

Rearrangement of the carbenes generated by photoelimination of nitrogen from cyclic diazo-ketones is frequently accompanied by ring-contraction. This process has found extensive use in synthesis recent examples include the synthesis of the [7]paracyclophane ring system,6 4,8-dihydrodibenzo[cRing-contraction was not observed on irradiation of the unstable... 

Jones developed a new fascinating method for [n]paracyclophane synthesis based on spiro carbene rearrangement. The first synthesis of la was achieved two decades ago by this method through rearrangement of the carbene species generated by thermolysis of the lithium salt of the spiro dienone tosylhydrazone 6 (Scheme 1) [1]. Although this work was epoch-making as the first synthesis of la, the formation of by-products hampered further study of la. 

Paracyclophane (la) suflers from peracid oxidation readily oxidation of la with MCPBA proceeded at 0 °C to yield the dimer 109 quantitatively [5b], The formation of dimer 109 is explained in terms of [4 + 2] dimerization of the cyclohexadienone 108 which was formed by epoxide-carbonyl rearrangement of the initially formed epoxide 107 (Scheme 20). Naphthalenophane 55a and anthracenophane 56a (s. Scheme 13) were more reactive the MCPBA oxidation completed immediately at — 78°C to give unstable dienones 110 and 111 (Structures 21) [50]. 

Reviews of general interest this year have concerned force-field calculations (by Altona), Bredt s rule, adamantane rearrangements," triptycene chemistry, the relation between mass-spectral behaviour and thermolytic and photolytic reactivity, the removal of orbital symmetry restrictions to organic reactions, catalysis by transition metals of symmetry-disallowed reactions, and [2,2]paracyclophanes, many of whose reactions lead to bridged or cage molecules. 

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