Cycloaddition precursor cyclohexene

The acceptor quality of vinyl ketones liberated from methyl 2-alkenyl 2-siloxycyclo-propanecarboxylates can also be used in cycloaddition reactions. Thus y-oxoester 147 adds smoothly to 2-siIoxybutadien 146 affording a cyclohexene derivative which after desilylation gives the tricarbonyl compound 148. This crucial intermediate can be obtained from vinyl cyclopropane 132 as a precursor of 147 in 72 % overall yield 85). Its chemoselective methylation, lactonization, and dehydration make norbisabolid available — a constituent of the root bark of atalantia monophylla. 

Bromo-2-pyrone is not only a valuable precursor for the synthesis of various 3-substituted 2-pyrones,7 but it is also a reactive unsymmetrical diene.8 3-Bromo-2-pyrone undergoes Diels-Alder cycloadditions with a regioselectivity and stereoselectivity that is superior to that of 2-pyrone. Furthermore, 3-bromo-2-pyrone is a chameleon (i.e., ambiphilic) dienophile, undergoing cycloaddition to both electron deficient and electron rich dienophiles. The cycloadducts of bromopyrone with dienophiles are isolable and are useful in the synthesis of diastereomerically pure cyclohexene carboxylates (Scheme 2).8... 

There are few cycloadditions involving either three alkenes, to produce cyclohexanes, or two alkenes and an alkyne, to produce cyclohexenes. ° The appropriate metallacyclic precursors containing the CpCo fragment have in fact been prepared and examined for reactivity. Both cobaltacycloheptanes and cobalt-acycloheptenes undergo -hy(Mde elimination, producing 1-heptenes and 1,6-heptadienes respectively, in preference to reductive elimination of the met and coupling to produce cyclic products. ... 

Nonetheless, there are a small number of systems that do mediate such [2 -i- 2 -t- 2] cycloadditions. With allenes as the alkene , cycloaddition with both acetylene and terminal alkynes proceeds regio-selectively to give 3,5-dimethylenecyclohexenes using Ni catalysts, and mostly 3,6-dimethylenecyclo-hexenes using Ni° catalyst precursors (equation 19). Norbomadiene undergoes so-called homo-Diels-Alder cycloaddition with both alkenes and a ynes in the presence of nickel catalysts. Further elaboration of this chemistry with alkynes but not alkenes has been described using a Co/Al catalyst system (equation 20). Attempts to produce cyclohexenes via all-intramolecular [2 + 2 + 2] cycloaddition of l,13-dien-7-ynes or 1,1 l-dien-6-ynes have been unsuccessful. ... 

Eschenmoser and coworkers achieved 1,4-cycloadditions by using a-chloro nitrones as precursors of 1,3-dipoles which react with unactivated olefins [4]. For example, the cycloadduct 7 is obtained by reaction of cyclohexene (5) and a-chloro nitrone 6... 

The first models for asymmetric induction in [2 + 2] cycloadditions are ketimines derived from precursors such as (25), which react with both cyclic and acyclic alkenes to give adducts with high enantiomeric excesses. Full details have been reported on the regio- and stereo-specificity of the addition of dichloroketene to 1-substituted cyclohexenes. Methyl (phenylthiomethyl)-ketene (26) provides a further example of a ketene which, after cycloaddition to cyclopentadiene, undergoes facile opening of the cyclobutane that is thus formed to provide vicinally substituted cyclopentene derivatives. ... 

Irradiation of cyclohexene in the presence of CuOTf produces the dimers 14 (49%) and 15 (8%), along with the cyclohexylcyclohexene 16 (24%) (Scheme 5)T The stereochemical outcome in Cu(I)-catalyzed dimerization of cyclohexene may be the result of cis-tram isomerization on irradiation of the initially formed Cu(I)-cyclohexene complex to the trans-cyclohexene-CuOTf complex 13, followed by a concerted ground state 2 + 2 cycloaddition of the highly strained tram-cyclohexene to another cyclohexene. Cycloheptene, on the other hand, produces the aU tra s-fused trimer 17 (Scheme 6) as the sole product. A 1 3 CuOTf-trans-cycloheptene complex has been proposed to be the precursor of this product. No dimerization reaction has been observed for cyclooctene and acychc olefins.However, mixed photocycloaddition occurs with cyclooctene if the other olefin is sufficiently reactive. Thus, cyclooctene adds to norbornene to produce the cyclobutane derivative 18 in 40% yield. GDdimerization was also observed when a mixture of cyclohexene and cycloheptene was irradiated in the presence of CuOTf to yield the adduct 19 (Scheme 6). ... 

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