Biradical cyclization

An extension of this route has been developed by Diirr et al. It consists in the irradiation of 3//-spiropyrazoles 74 which rearrange to 3//-indazoles 75 prior to nitrogen extrusion and biradical cyclization to cycloproparenes 76. As above, only 1,1 -disubstituted cycloproparenes (mainly with alkoxycarbony 1 groups) are acces-... 

Rather than fragmentation of Norrish Type IT biradicals, cyclization can occur. This is called the Yang Cyclization. ... 

DFT studies on heteroatom diallenes have found, in line with previous experimental data, that biradical cyclization to a five-membered heteroaromatic ring is the preferred reaction pathway, although protonation of the heteroatom has been found to promote a competing cyclization to a six-membered, initially biradical, ring.59... 

The study of Bergman, Myers-Saito and related biradical cyclizations using an unrestricted broken spin symmetry approach refined by single-point energy coupled-cluster calculations has been reviewed, and a simple rule outlined for predicting biradical involvement in such Cope-type rearrangements radicals were found to be probable... 

In contrast, Urry and Trecker had shown in 1962 that a-diketones undergo photocyclization to 2-hydroxycyclobutanones, the intermediate biradical cyclizing but not cleaving (which would form a hydroxyketene) [19]. 

Acylcyclohexanes have received considerable study and, especially as solids, undergo highly stereoselective cyclization, since H-abstraction occurs with the phenyl twisted away from the a-methyl, where it stays until the biradical cyclizes [56]. Addition of a chiral ammonium salt to the benzene ring fosters enantioselective cyclization, presumably by inducing a tilt of the benzoyl group that leads to preferred abstraction from one of the two y carbons [57],... 

Schmittel, M. Keller, M. Kiau, S. Strittmatter, M. A surprising switch from the Myers-Saito cyclization to a novel biradical cyclization in enyne-aUenes formal Diels-Alder and ene reactions with high synthetic potential, Chem. Eur. J. 1997, 3, 807-816. 

Engels, B. Lennartz, C. Hanrath, M. Schmittel, M. Strittmatter, M. Regioselectivity of biradical cyclizations of enyne-allenes influence of substituents on the switch from the Myers-Saito to the novel C —C cyclization, Angew. Chem. Int. Ed. 1998, 37, 1960-1963. 

Similarly, the photosensitized electron transfer mediated reactions of several 1-and 9-(aminoalkyl)phenanthrenes using DCB have been investigated by Lewis and coworkers (Scheme 69) [319], These reactions provide an efficient method of construction of skeletal structures of aporphine, phenanthropiperidine, phenanthroa-zepine and related alkaloids. The mechanism of these reactions involves an initial electron transfer from the ground state amine to the singlet arene followed by N-H proton transfer and biradical cyclization. 

Hydrogen Abstraction.- Xanthone-sensitized irradiation of the dihydroisoquinoline (19) affords a 6X yield of the spiro derivative (20). The reaction is akin to a Norrish Type II process but in this instance hydrogen abstraction by the imine nitrogen is involved. The resultant biradical cyclizes to yield the observed product (20). 

Like (5-alkoxy ketones, (5-amido ketones also undergo photoinduced 8-hydrogen abstraction to give proline derivatives (in <50% chemical yield) (Scheme 6.126).967 The reaction stereoselectivity depends on the biradical cyclization rate, which competes with conformational changes. Whereas singlet biradicals couple without reaching conformational equilibrium, the triplet biradicals allow bond rotation before the ring forms. 

The dienones (128) are photochemically reactive. Irradiation brings about ring opening of the spiro-cyclopropane moiety to yield the biradical intermediate (129). This biradical cyclizes exclusively by a 5-exo-trig path to yield the novel compounds (130). The yields are high, with (130, R = H) obtained in 77%, while (130, R = Me) is obtained in 69%. ° ... 

The diyne cyclizations discussed so far involve the formation of an aromatic system together with the biradical. Surprisingly, though, a new kind of diyne- to -biradical cyclization has been discovered only recently, where the transannular interaction of two triple bonds in a diazacy-... 

The keto ester (42) undergoes photochemical hydrogen abstraction reactions to afford a 1,8-biradical.Again, in this case, the regiochemistry is controlled by the presence of the hetero atom. The resultant biradical cyclizes to yield the azalactone (43), the structure of which was determined by X-ray crystallography. The cyclization process is not stereoselective. 

Dioxepins.—The photolytic decomposition of the epoxide (147) goes via several reaction paths, but in pentane the major product is the dioxepin (148), formed by cleavage of the y5-bond and biradical cyclization of the delocalized allyl radical, " ... 

Several enyne-based biradical cyclizations have been reported. Schmittel et al. have expanded their thermal C —C cyclization of enyne allenes to incorporate heteroatoms. 

For example, a yne-allene 141 is smoothly converted into a benzofulvene 142 via diradical 143 in good yield (Scheme 6.33a) [71]. Similar regioselective biradical cyclizations from aza-analogous enyneallenes 144 to carbazoles 145 have also... 

Schmittel M et al (1998) Two novel thermal biradical cyclizations in theory and experiment new synthetic routes to 6H-indolo 2,3-b quinolines and 2-amino-quinolines frran enyne-carbodiimides. Angew Chem Int Ed 37 2371-2373... 

---