1,3-dioxole, 1,2-cycloaddition with

Diels-Alder cycloaddition of 1,3-dioxole (7) with 3-tosyl-2-pyranone has been used in natural product synthesis <90T4573>, and l,3-dioxol-2-one (8) undergoes an unusual meta cycloaddition upon photolysis in benzonitrile or anisole to give mixtures of regio- and stereoisomers of the tetracyclic adducts (92) where the CN or OMe group can be at any of the three positions shown... 

Because of possible relevance to mutagenesis, considerable effort has been devoted to study of the photochemical transformations of oxypyrimidines uracil, for example, takes part in a [2 -i- 2] cycloaddition with itself, or with vinylene carbonate (l,3-dioxol-2-one). Uracils undergo radical additions these too are of possible relevance to mutagenesis mechanisms. 

Diels-Alder reaction of pyran-2-ones. Diels-Alder reaction of 2-pyrones, if successful, can provide unusual cyclohexenecarboxylic acids, but thermally promoted cycloadditions with these electron-deficient dienes usually result in decarboxylation and aromatization of the adducts as a result of the required high temperatures (6,291-292). Successful Diels-Alder reactions of 3-bromo-2-pyrone (1) with the electron-rich dioxole 2 can be effected with a catalytic amount of ethyldiisopropylamine at 90° (4 days) to give the major adduct (endo-3) in 63% yield. The adduct is hydrolyzed by p-toluenesulfonic acid in methanol to 4 as the only diastereomer. The trisilyl ether of 4 was transformed to the a,/8-unsaturated ester 5 by radical debromination and DBU isomerization. ... 

Ethenediyl carbonate (l,3-dioxol-2-one vinylene carbonate, 417) is a readily available,270 versatile synthon having pronounced dienophilic properties.270-275 Diels-Alder adducts of 417 with 1,4-di-acetoxy-1,3-butadiene and furan were selectively converted into cy-clitols,256 257-275 and also served as precursors of DL-ribose derivatives258 (see Section IV, 2). Another possibility of applying 417 as an equivalent of a 1,2-dihydroxyethane unit has been demonstrated in a synthesis of racemic apiose. Photochemical cycloaddition of 417 to 1,3-diace-toxy-2-propanone (418) gave the oxetane derivative 419, which, on alkaline hydrolysis, afforded DL-apiose (420) in 23% yield.1... 

In certain cases benzene will undergo photocycloaddition to oxygen and sulfur heterocycles. The two major photoproducts of irradiation of a mixture of furan and benzene are adducts 356 and 357, arising, respectively, by [ 2 + 2] and [ 4 + 4] cycloaddition processes.306 Irradiation of benzene and 2,2-dimethyl-l,3-dioxol (358) similarly affords adducts 359 and 360, together with dimer 361.307 Thiochromone 1,1-dioxide also undergoes photoaddition to benzene.308... 

For photocycloaddition, to benzene the following conclusions were drawn from this empirical correlation [124], Olefins with poor electron-donor or poor electron-acceptor abilities yield mainly meta adducts with benzene (i.e., if AG > 1.4-1.6 eV, all other olefins yield mainly ortho adducts). Even ethene, which had seemed to behave exceptionally, fits into this correlation provided that it acts as the acceptor. The transition area from ortho to meta cycloaddition (i.e., the AG region where ortho meta = 1 1) is relatively large ( 0.2 eV). This is considered not to be surprising because the AG correlation is based on many different types of olefins. When only AG values for derivatives of 1,3-dioxole and for 1,4-dioxene were used, the transition area was narrowed to 0.03 eV. Not only ethene but also vinylene carbonate now fit into the correlation. According to the ionization potential rule, this compound should give only ortho photocycloaddition with benzene. Mattay s empirical rule predicts mainly meta addition, which is indeed found experimentally. 

Continuing work 158) on photoreactions of electron-rich olefins with biacetyl shows that the complexity of product mixtures obtains in these reactions also. Effects of solvent polarity provide further support for the importance of ionic intermediates in these reactions. The reactions of biacetyl with 1,1-diethoxyethylene are proposed to proceed via the triplet state (in contrast to reactions with dioxoles). The reversal of regiospecifity between thermal and photochemical cycloaddition of this olefin with biacetyl is nicely explained by the assumption of excited state electron transfer from olefin to dione to give the corresponding radical ions. 

There have only been a few new reports of reactivity of this type. The Paterno-Buchi reaction of 92 with substituted pyruvates gives the endo- and < ti-products shown (Equation 2) <1996TL1195>. In a later study, the corresponding cycloadditions of l,3-dioxol-2-one with a variety of aldehydes and ketones were also examined <2007ARK(viii)58>. 

Cycloaddition of 3-sulfonyl-2-pyrone 164c with electron-rich dioxoles 357a-c [X = CH2, CMc2, C(CMe3)2] under thermal conditions (80°C) gives bicyclic lactones 358a (endo 78% exo 15%) and 358b (endo 77% exo 23%), while the pressure-promoted reaction (25°C, 11-12.5 kbar) affords 86% of... 

With a,/i-dialkoxynitrones such as 2 the presence of at least one alkyl group on the jS-carbon is essential in order to achieve high diastereoselection in the cycloaddition to both electron-rich62 and electron-poor alkenes63. For instance, the reaction of C-(2,2-dimethyl-4-substituted-1.3-dioxolan-4-yl)-3V-methyl nitrone 2 with l,3-dioxol-2-one (vinylene carbonate) affords the bicyclic products 3 and 4 in 90 10 ratio, while a similar reaction with the 4-unsub-stituted nitrone derivative gives a product with a 50 50 mixture of isomers. 

The opposite type of reaction has also been reported, viz. one in which the heterocyclic molecule reacts via an electron-rich double bond with electron-poor olefins, in particular with tetracyanoethylene. Tanny and Fowler43 found that 2-azabicyclo[3.1.0]hex-3-enes reacted with tetracyanoethylene via a (2 + 2)-cycloaddition of the enamine double bond to give 13. Other electron-deficient reactants, such as JV-phenyl-maleimide, reacted differently, yielding an 8-azabicyclo[3.2.1]oct-2-ene (16). This type of reaction possibly occurs via a concerted [ 2 +ff2 +n2]-cycloaddition.43 At room temperature tetracyanoethylene also readily formed (2 + 2)-cycloadducts with heterocycles that contained a vinyl ether group for instance, 3,4-dihydro-2ff-pyran, 2,3-dihydrofuran, and 2,2-dimethyi-l,3-dioxole afforded the adducts 17-19 in yields of... 

A novel one-pot two-step pathway for the synthesis of 5-vinylbenzo[dihydroxycinnamic acid 4 (13TA285).Anodic oxidation of 6 in the presence of 2,6-lutidine and lithium perchlorate gave a mixture of dioxole 7 and side product 8, which could be converted entirely into 7 upon treatment with p-toluenesulfonic acid (13AGE12865). Boron trifluoride-promoted cycloaddition of P-methoxyvinyl sulfides to... 

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