Cyclobutene synthesis, photochemical

Several review articles have dealt with various aspects of cycloaddition reactions. A short review has presented examples to illustrate the use to which tethered alkenes can be put in the synthesis of cyclobutenes. The photochemical (2 + 2)-cycloaddition of enones to ethene has been studied from a theoretical standpoint. A review has highlighted the use of linear templates to control photochemical reactions such as (2 + 2)-photocycloadditions. The stereochemical control of photochemical reactions in clay-intercalated compounds has been discussed. A review has highlighted the interdisciplinary nature of photochemistry. ... 

The construction of the tricyclo[5.2.0.02,6]nonane (26, n = 1) and tricyclo[6.2.0.02,7]decane (26, n = 2) frameworks involved the [2 4- 2] cycloaddition of readily accessible31,32 l,2-bis(trimethyl-siloxy)cyclobutene and cnone 25, n = 1 or 2, respectively.33 The yields (75-80%) were good for adducts 26a, c, e, and g. Lower yields (40-50%) were observed for adducts 26b and 26f, while adduct 26d was only isolated in a trace amount. The most interesting and important reaction, related to the total synthesis of eudesmane sesquiterpenes, was the photochemical reaction of (-)-piperitone (25g) with l,2-bis(trimethylsiloxy)cyclobutene, which gave c/.v,(5wf/u W-2/j,7/i-dimethyl-4/ -isopropyl-l f ,8Jf -bis(triniethylsiloxy)tricyclo[6.2.0.0z 7]dec-3-one (26g) with the relative cis configuration of the methyl (R2) and isopropyl (R3) groups.33,34 Some of the other photochemical [2 + 2] cycloaddition reactions utilizing l,2-bis(trimethyl-siloxy)cyclobutene are shown by the formation of 2735,36 and 28.37... 

Highly strained substrates can be transformed into even more strained isomers by ODPM rearrangement. This has been shown by Murata et al. for the synthesis of a valence isomer of azulene [45]. Albeit the photochemical reaction yielded only 20-25% of the bicyclo[1.1.0]butane derivative 15, the synthesis of the precursor cyclobutene (14) is straight-forward from the bicyclo[3.3.0]octenone 13 (Sch. 18). This substrate has obviously a diverse reactivity pattern when directly excited, however, triplet sensitization reduces these competitive pathways because alkene excitation is excluded. Also benzo-annulated azulene valence isomers were generated by this approach [46]. 

Thermolysis of these yields the diketones (95) which are useful compounds for the synthesis of the perhydroazulenes This specific example (95) was converted into the natural product daucene (96). Others have also described the photocycloaddition of the enone (92) to the cyclobutene (93) to afford the (2+2)-adduct (94) in 24 % yield. The adduct was transformed into the natural product balanitol (97). The triplet state of conjugated enones can be photochemically reduced by sodium borohydride. ... 

Miesch, M., Miesch-Gross, L., Franck-Neumann, M. Total synthesis of ( )-silphinene non photochemical cyclobutenic route to a crucial intermediate. Tetrahedron 1997, 53, 2103-2110. 

Extrusion of sulfur dioxide from the products resulting from the alkylation of tetrahydrothiophene 1,1-dioxides " and the 2-phenylthietane 1,1-dioxides, using n-butyllithium and lithium aluminum hydride - or photochemically leads to the synthesis of the corresponding cyclobutenes - and cyclopropanes, respectively (Scheme 107, entries a and b). 

Photochemical [2 + 2]-cycloaddition of unbridged bis(alkenyl-Cp)zirconocene complexes provides a unique approach for the synthesis of 6 -hydrocarbyl-bridged -precursor leads to the corresponding meso-ansa-zhconoccne. 

The biosynthetic relationship between illudol (379) and fomannosin (380) has inspired Semmelhack and co-workers184-185 to a beautifully constructed synthesis of the two natural products via the common intermediate (378) (Schemes 45 and 46). This tricyclic compound (378) was obtained by a Diels-Alder reaction between (376) and the cyclobutene derivative (377). Another photochemically-based route (Scheme 47) has been used to obtain protoillud-7-ene (381) and several oxygenated derivatives.186... 

A synthesis of 10-epijunenol (417) cf. Vol. 8, p. 102) has been accomplished by photochemical cycloaddition of the cyclobutene ester (413) to piperitenone (414) followed by reductive cleavage of the phosphate ester (416) (Scheme 35). 

Annulation of heterocycles with a cyclobutene ring has been achieved by photochemical (2 + 2)-cycloaddition with acetylenes. Both maleic anhydride and N-substituted maleimides yielded 3-oxa- or 3-aza-bicyclo[3.2.0l-hept-6-ene-2,4-diones (167).,89,9° Vinylene carbonates also entered into a cycloaddition reaction with acetylenes to afford 168, which has been employed as starting material for the synthesis of cyclo-butadiene(tricarbonyl)iron or cyclobutenedione.19,-193 3,4-Dihydro-2/f-pyran and 5-methyl-2,3-dihydrofuran reacted with diphenylacetylene to... 

Armesto D, Horspool WM, Martin N, Ramos A, Seaone C (1989) Synthesis of cyclobutenes by the novel photochemical ring contraction of 4-substituted 2-amino-3,5-dicyano-6-phe-nyl-4 77-pyrans. J Org Chem 54 3069-3072... 

Photochemical closure of the diene (168) to the cyclobutene (169) has been used as a key step in a synthesis of cyclopropa[4,5]benzocyclobutene (170), which is the most strained benzene known. The n.m.r. spectrum of (170) is normal in the sense that it is in agreement with expectations from the spectra of benzocyclopropene and benzocyclobutene, but its u.v. spectrum shows a bathochromic shift relative to benzocyclobutene. 

An olehn metathesis reaction has been effected by photochemical [2 + 2] cycloaddition followed by thermolysis of the cyclobutane (Scheme 17). Thus thermolysis of the tricyclic c ,a ft,c/5-adduct (57) gave the cyclodeca-1,5-diene skeleton (58) present in germacranolides (Scheme 17). A closely related process is the [4C + 2C] annelation which has been utilized in the synthesis of lO-epi-juneol (61) (Scheme 18). The photoadduct (59) from piperitone and methyl cyclobutene-1-carboxylate was converted in four steps into the tricyclic compound (60). The strained tr-bond in (60) was cleaved with lithium naphthalide to give 10-epi-juneoI (61). ... 

Nigel S. Simpkins, now at the University of Birmingham, found (Chem. Commm. 2008, 5390) that the prochiral cyclopropane amide 1 could be deprotonated to give, after alkylation, the substituted cyclopropane 3 with high enanantio- and diastereocontrol. In the course of a synthesis of (-l-)-Lineatin, Ramon Abbes of the Universitat Autonoma de Barcelona optimized (J. Org. Chem 2008, 73, 5944) the photochemical cycloaddition of 4 and 5 to give, after reductive dechlorination, the cyclobutene 6. 

Gleiter et al. suggested an alternative simple approach to the synthesis of cubane derivatives. Osawa et aL reported that the failure to produce the cubane skeleton from sy -tricyclo[4.2.0.0 ]octa-3,7-diene 28 was rationalized by assuming that the sequence of the frontier orbitals was Jt+ above Jt and above n as a result of strong through-bond interaction between the cyclobutene ring and the K orbitals (an unnatural order). As a result of this orbital sequence, photochemical [2 + 2]-cycloaddition of 28 to cubane is a symmetry-forbidden process (Figure 23.2). ... 

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