Oxa-di-7i-methane rearrangement

To examine the viability of CIM a number of photoreactions (electrocyclic reactions, Zimmerman (di-n) reaction, oxa-di-7i-methane rearrangement, Yang cyclization, geometric isomerization of 1,2-diphenyl-cyclopropane derivatives, and Schenk-ene reaction) which yield racemic products even in presence of chiral inductors in solution have been explored (Sch. 40) [187,189-200]. Highly encouraging enantiomeric excesses (ee) on two photoreactions within NaY have been obtained photocyclization of tropolone ethylphenyl ether (Eq. (1), Sch. 40) and Yang cyclization of phenyl benzonorbornyl ketone (Eq. (3), Sch. 40). The ability of zeolites to drive a photoreaction that gives racemic products in solution to ee >60% provides... 

The synthetically most useful applications of the oxa-di-7i-methane rearrangement involve 2,5-bridged cyclohex-3-enones and cyclohept-3-enones or molecules eontaining such structural moieties. 

The formation of two of the four possible oxa-di-7c-methane products on sensitized irradiation of (+)-3-ethyl-3-methyl-5-phenylpent-4-en-2-one has been interpreted in terms of a stepwise, rather than a concerted, mechanism.Evidence for a stepwise pathway in the oxa-di-7i-methane rearrangement is also available from investigations on ( —)-(378). Triplet-sensitized reaction of (—)-(378) gave ( —)-(lS,9S)-(379) and (—)-(lS,9R)-(380) enantiomeric purities were 42% for recovered (378) and 48% each for (379) and (380). The essentially full retention of enantiomeric purity during the rearrangement and the formation of endo- and exo-products with identical C-1 configuration eliminate both a-cleavage and concerted mechanisms. 

Hixon SS, Mariano PS, Zimmerman HE (1973) The di-Jt-methane and oxa di-7i-methane rearrangements, Chem Rev 73 531... 

Maiti, B.C., Singh, R., and Lahiri, S., Photoreactivity in 5-keto-P,y-enones oxa-di-7i-methane rearrangements from states other than Tl(jt,Jt ), /. Photochem. Photobiol. A Chem., 91, 27, 1995. 

Singh, V. and Raju, B.N.S., cycloaddition synthesis and oxa-di-7i-methane rearrangement... 

Singh, V.K., Deota, RT., and Bedekar, A.V., Studies in the synthesis of polycyclopentanoids synthesis, oxa-di-7i-methane rearrangement of annulated bicyclo[2.2.2]octenones and cyclopropane ring cleavage of tetracyclo[6.3.0.0. > 0 > ]undecenones,/. Chem. Soc., Perkin Trans. 1, 903,1992. 

Ring contraction of cyclopentenone and cyclohexenone derivatives to cyclobutanes was observed during direct irradiation of various j5,y-unsaturated cyclohexenones. By way of a Nor-rish type I cleavage in n,7i excited singlet (5t) and n,7t excited triplet (T2) states, an acyl/allyl diradical is formed which recombines in the allyl position to form the product of a 1,3-acyl migration,102 while sensitized irradiation leads mainly to oxa-di-rr-methane rearrangement (see Section 4.2.3.). 

The next three chapters by Fleming, Armesto, and Rao deal with different aspects of alkene photochemistry alkene [2+2]-photocycloaddi-tions to other alkenes, di-7i-methane (DPM) rearrangements of 1,4-dienes and oxa-di-7i-methane (ODPM) rearrangements of 3,y-unsaturated carbonyl compounds. Photocycloaddition of an ether-tethered 1,6-diene by Cu(I)-catalysis leads to the exo-selective formation of the bicyclic tetrahydrofuran derivative 4 [4]. By direct electronic excitation of a... 

A plano-convex lens was used to collimate and focus, an IR mirror was employed to eliminate non-productive IR irradiation and minimize associated heat, and a variety of UV filters could be employed to control wavelength. After evaluation of 728 reactions, they identified five which were further explored. Examples included a wavelength dependent [2-I-2]-cycloaddition of indole 27 to afford a mixture of indole 28 cyclobutane 29 (Scheme 5A). They observed that the photo-Fries rearrangement was predominant at shorter wavelengths while the [2 -I- 2]-cycloaddition was favoured at longer wavelengths. A second reaction they identified was an oxa-di-7i-methane reaction with bicycle 39 to afford the caged cyclopropane 35 in 70% yield after 2 min (Scheme 5B). In the screen they found both direct excitation and triplet sensitization conditions for the reaction. They also identified a 1,3-acyl shift with ketol 32 to afford cyclohutanone 33 in modest yield (Scheme 5C). This platform demonstrated that flow photochemistry can enable automated reaction discovery. 

In a number of compounds there exists the possible choice between an oxa-di-7t-methane and a di-7r-methane rearrangement. Both modes of reaction are found. With 15c [Eq. (7)] only the oxa-di-n-methane product 17 c is obtained and not the corresponding di-Ti-methane product. 20) On sensitized irradiation of 42 with R=H, 44a is formed, which is the 1,2-acyl shift product and not 44b and/or 44c which are the benzo-vinyl-methane rearrangement products. 44) On the other hand, 45 gives only 46, a di-ir-methane reaction in which one of the participating double bonds is part of an a,P-enone, and none of the oxa-di-re-methane or benzo-vinyl methane products 47 and 48, respectively, are formed.46). In the same way 49 only gives 50, and not the oxa-di-Tc-methane or the divinylmethane product. 45) in the reaction of 51 to 52 it has not yet been established whether an oxa-di-Tc- or a di-7i-methane pathway is taken. 46)... 

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