Dienes, cyclic

Because of the linear geometry required of cumulated dienes cyclic allenes like cycloalkynes are strained unless the rings are fairly large 1 2 Cyclononadiene is the smallest cyclic allene that is sufficiently stable to be isolated and stored conveniently... 

In the presence of the corresponding copper(II) catalyst, N-acryloyloxazo-lidinone reacted with various conjugated dienes (cyclic and acychc) with good enantioselectivities in all cases, competing with results obtained for classical catalysts in the case of flexible dienes. Thus the cycloadducts obtained from isoprene or 2,3-dimethyl-1,3-butadiene were synthesized in high yields and good enantioselectivities (92% ee in both cases). 

This is slightly more complicated as the diene can be ds, ds, or ds, trans (there are two of these if the diene is unsymmetrical) or trans, trans. We shall look at each case with the same dienophile, an acetylenedicarboxylate, as there is then no stereochemistry in the triple bond Starting with ds, ds-dienes is easy if we make the diene cyclic. 

Any unsubstituted cyclic 1,3-diene cyclic diene gives the same product from 1,2- and 1,4 addition. For example ... 

A report of Backer and Blaas6 is responsible for evolution of the present procedure these workers were first to conduct a Diels-Alder synthesis utilizing a 3-sulfolene in place of the free diene (by heating the cyclic adduct of sulfur dioxide and 2-methyl-3-thiomethyl-1,3-butadiene with maleic anhydride). The generality of the method as a variant of the conventional diene synthesis is limited largely by the availability of the appropriate 3-sulfolene its greatest utility, perhaps, will be presently realized in those diene reactions normally requiring 1,3-butadiene, since 3-sulfolene itself is now the least expensive and most widely available diene cyclic sulfone. 

Fig. 3.1 Routes A, B, C, D to equilibrium E between cycloocta-1,5-diene, cyclic oligomers, and linear high polymer, starting from monomer M, polymer P, oligomers O, or a mixture F of monomer and oligomer, respectively. Starting concentration in all cases equivalent to 2.7 mol C4H6 units litre . Catalyst W[=C(OMe)Ph](CO)4(PPh3)/TiCl4. Solvent O = chloro-benzene = benzene, 30°C (Chauvin 1977, 1978).

Hydroboration of a.,oi-dienes. Cyclic hydroboration of ,cu-dienes can be effected with this reagent. The initial product is often polymeric, but six- to eight-membered cyclic B-chloroboracycloalkanes can be obtained after depolymerization by distillation in good yields. The B-chloroboracycIoaikanes (2) and (3) from 1,4-pentadiene (1) were transformed into the cyclic ketones (6) and (7) by reaction with a,a -dichloromethyl methyl ether (5, 200 203) followed by oxidation. ... 

The cyclization of 1,6-enynes by RhCl(PPh3)3 can generate functionalized 1,3- or 1,4-diene cyclic compounds. For example, treatment of l,6-en)mes containing a haloalkenyl group with Wilkinson s catalyst in dichloromethane at reflux produces cyclization product which incorporates an intramolecular halogen shift a wide spectmm of enynes can be applied in this transformation (eq 75). ... 

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