Diradicals, cyclization

The [2 + 2] photocycloaddition reaction of enones with allenes was first reported in 1966. A diradical intermediate is formed from a triplet enone via an exciplex. The triplet diradical cyclizes to the product after spin inversion to the singlet state [31,32]. 

Studies with other thiocarbonyl compounds reveal a different reactivity pattern from that of thiones in the presence of olefins. 0-Alkyl thiobenzoates do not form dithiane products. Instead, if an olefin carries no bulky substituents, the intermediate diradical cyclizes directly to the thietane ring (Eq. 4). The excited (n, 71 ) state of thiophosgene reacts with alkenes to... 

Aza-enediynes are a new class of anti-neoplastic agents that produce a cytotoxic effect through an aza-Bergman cyclization (1) of an enediyne core to a 1,4-didehydrobenzene reaction intermediate. Both Bergman and benzothiazolium salt diradical cyclizations are illustrated in Eq. 1 and Eq. 2, respectively. 

In the case of 5 and 6, the intermediate diradical cyclizes at two alternative allylic positions. ... 

Vinylogous urethane (248) was employed in a radical-mediated synthesis of epilupinine (Scheme 48), where its cyclization was induced by treatment with tributyltin hydride-AIBN <89T5269>. In another radical process, a very efficient synthesis of berbine was achieved by photolysis of compound (249), which induced a diradical cyclization process promoted by an excited-state electron-transfer desilylation (Scheme 49) <85TL5867>. 

The deuterium labeling experiments referred to above indicate that the diradical cyclizes to starting material considerably more rapidly than it is converted to products. 

The accumulated spectroscopic evidence clearly points to the participation of both 828 and 82a in the SET-photoinduced degenerate methylenecyclopropane rearrangement of d -lla. When combined with the chemical results, it is possible to propose a cation radical cleavage (CRCL)-diradical cyclization (DRCY) mechanism for this process (Scheme 24). Cation radicals < 2-773 and undergo... 

The investigation of various parameters of the cyclization of enyne-allene 12, using the density frinctional theory (DFT), has been undertaken.This includes the thermodynamics for both C2-C7 (leading to 13) and C2-C6 (leading to 14 and called Schmittel reaction) reaction pathways. Theoretical calculations address the regioselectivity of diradical cyclization in enyne-allene with a different substitution. This study rationalizes the switch between the two radical cyclizations (C2-C7 vs. C2-C6) on the basis of mainly steric (12, = t-Bu) or electronic effects (R =... 

Across C=S bonds Irradiation of thioxanthenethione 23 in the presence of alkyl- and alkoxy substituted butatrienes (R = H = MeO, t-BuO, PhO, or R = R = Me) gives the [2+2] cycloadducts 24 in over 90 % yields. Apparently, the triplet thione attacks the end of the triene system and the generated diradical cyclizes to give the cycloadducts. 

American Chemical Society, 121(37), 8615-8627. (d) Engels, B. and Hanrath, M. (1998) A theoretical comparison of two competing diradical cyclizations in enyne-allenes the Myers-Saito and the Novel C -C cyclization. Journal of the American Chemical Society, 120(25), 6356-6361. 

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