Homoallylic radical

It has been shown" that isomerization of the exocyclic allylic system of the five-membered ring D of kaurenols depends on the orientation of the C(15) hydroxyl group. The total synthesis of methyl atis-16-en-19-oate, a tetracyclic diterpenoid possessing a bicyclo[2.2.2]octane skeleton, has been accomplished" using a homoallyl-homoallyl radical rearrangement process of methyl 12-hydroxykaur-16-en-19-oate monothioimid-azolide (280) as the pivotal step. Two plausible mechanisms have been presented" ... 

Another important rearrangement is that of cyclopropylmethyl radicals to the corresponding homoallyl radicals. This is an exceptionally fast reaction (t1/2 10 8) and has been used as a radical clock to determine the rates of other free-radical reactions.95 Cyclopropylcarbene also undergoes rearrangement, leading to cyclobutene.96... 

The asymmetric rearrangement of peroxy radical (5) has recently been used as the key step in the asymmetric synthesis of Plakorin (Scheme l).17 The thermal isomerization of buta-1,2- to buta-1,3-diene has been studied using ab initio calculations and the mechanism concluded to proceed stepwise via radical intermediates.18 The competition between cyclopropyl formation and the homoallyl-homoallyl radical rearrangement has been studied in the radical (6) and found to give the 3-exo cyclization product (7) and the rearranged product (8) in a 1 5 ratio, respectively, under the conditions shown (Scheme 2).19... 

Formation of trapping product 140 in over 54% yield provides strong support for preferential formation of the first bond at the w-carbon, as could be predicted by the rule of five . Higher trapping yield (65%) was obtained in the irradiation of 137 (R = t-Bu) at 50 °C. Formation of the Z-trapped product 141 and the /Z ratio of the trapped products 140/141 (52 13 respectively) could be attributed to an equilibrium between the cyclopropylcarbinyl radical 138 and its corresponding homoallyl radical. However, further studies are required to verify this assumption (Scheme 30). 

In the example shown in Scheme 6.12, one molecule of organic halide and two molecules of acrolein are coupled under tin hydride-mediated conditions [20]. As the first intermolecular C-C bond-forming process, the homoallyl radical adds to acrolein to form a radical a to a carbonyl group. The subsequent 5-exo cydization produces a nucleophilic alkyl radical, which undergoes addition to the second molecule of acrolein. 

Radical iodine atom transfer [3 + 2]-cycloaddition with alkene (118) using dimethyl 2-(iodomethyl)cyclopropane-l,l-dicarboxylate (117) forms cyclopentane derivative (119), through the formation of an electron-deficient homoallyl radical, followed by the addition to alkene, and cyclization via 5-exo-trig manner as shown in eq. 4.41. 

Also related to pathway b, Hodgson et al. were able to obtain 2-aza-bicyclo[2.2.1]hept-5-enes from 7-azabicyclo[2.2.1]heptadienes by a tandem featuring a thiyl intermolecular addition followed by a homoallylic radical rearrangement [92]. 

The mechanism probably involves radical attack as observed with the cyclopropyl ketones, via formation and opening of an intermediate cyclopropylcarbinyl radical into a homoallylic radical (equation 53) . ... 

However, more rapid events, such as 5-exo cyclization and cyclopropylcarbinyl radical ring opening, can lead to radical formylation (Scheme 4-7). Thus, 1,6-dienes, when exposed to tin hydride/CO conditions, give fair yields of slannylfor-mylation products via cyclization (Scheme 4-7) [20J. On the other hand, treatment of vinylcyclopropanes with CO under similar reaction conditions leads to stannylformylation products via a cyclopropylcarbinyl radical opening to a homoallylic radical (Scheme 4-7) [21]. 

A third strategy was proposed by Saicic [14] and us [15] in order to functionalize more elaborate triquinane frameworks. The key step is a radical [3-1-2] condensation between a homoallylic radical and a Michael acceptor such as acrylonitrile. Using the bromomethyldimethylsilyl (BMDMS) ether of a propargyl alcohol (precursor... 

The synthesis of carbocyclic compounds from acyclic precursors can be accomplished by a sequence of radical reactions, where thiyl radicals act as catalysts. As an example, the generation of homoallyl radicals through the addition of phenyl-thiyl radical to the double bonds of vinyl cyclopropanes results in the multi-step synthesis of cyclopentanoids [62]. The mechanism is shown in Scheme 13. Feldman and coworkers have demonstrated that the success of this strategy is based on the coupling of vinyl cyclopropanes with the electronically complementary functionalized alkenes. A judicious choice of substituents R and X accelerates the rate limiting step (c). 

Methano-5/5-cholestan-3-one (664) suffers photoreduction on irradiation in propan-2-ol, giving the 5jS-methyl-3-ketone (666) in a reaction which parallels that of some simple cyclohexanone analogues. The 4a,5a-methano-compound (667) unexpectedly also gave the 5/S-methyl ketone (666) as major product, with only a little of the 5a-methyl ketone (669). The reactions proceed through carbinyl radicals (665) or (668), and the symmetrical homoallylic radical (670) is postulated as providing a pathway for their interconversion. ... 

We were dehghted by the radical ring-opening reaction described above and envisaged that quenching of in situ-generated homoallyl radical (Fig. 30.1) with allyl tri-n-butyltin should lead to the formation of gem-diallyl derivative. ... 

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