2- -3-alkenal 1,3-dithiane

The Homer - Emmons reagent (52) is effective in the one carbon homologation of ketones possessing acidic a-hydrogen atoms <96SL875> and electron-deficient alkenes add to 2-phenylseleno-l,3-dithiane in a photo-initiated heteroatom stabilised radical atom transfer process, giving products of considerable synthetic potential <96TL2743>. 

Halogenation of 106 with triphenylphosphine, iodine, and imidazole provided the iodo derivative 109. On treatment with lithium aluminum hydride, 109 was converted into two endocyclic alkenes, 110 and di-O-isopro-pylidenecyclohexanetetrol, in the ratio of 2 1. Oxidation of 110 with dimethyl sulfoxide - oxalyl chloride afforded the enone 111.1,4-Addition of ethyl 2-lithio-l,3-dithiane-2-carboxylate provided compound 112. Reduction of 112 with lithium aluminum hydride, and shortening of the side-chain, gave compound 113, which was converted into 114 by deprotection. ... 

Nenajdenko et al. described the first example of addition of a 1,2-dication to C-C mutiple bonds. The only S-S dication found to participate in this reaction was the highly strained dication 115 derived from 1,4-dithiane. The reaction with alkenes 119 proceeded under mild conditions and led to derivatives of dithioniabicyclo[2.2.2]octane 120 as shown in Equation (33) and Table 21 <1998JOC2168>. This reaction was sensitive to steric factors and proceeded only with mono and 1,2-disubstituted ethylenes. Only alkenes conjugated with aromatic or cyclopropane moieties underwent this reaction. For the 1,2-disubstituted alkenes used in this study, the relative configuration of substitutents at the double bond was preserved and only one diastereomer was formed (see entries 2 and 3). 

In a related smdy, the reactions of a bicyclic dithioether dication (58) (generated from 1,4-dithiane 1-oxide) with alkenes and alkynes has been found to proceed as conjugate addition of two sulfonium groups, giving rise to derivatives of dithioniabicyclo[2.2.2]octane (56) and (57), respectively (Scheme 10). The reaction is sensitive to electronic and steric factors and appears invariably to proceed with retention of the relative arrangement of substiments at the double bond of the original alkene (58). " ... 

A -Protonated 2-azabuta-l,3-diene undergoes 4+- -2-cycloaddition with alkenes activated by electron-donating and electron-withdrawing substituents.The 2+ - - 4-cycloadditions of l,3-dithian-2-ylium ions with buta-1,3-dienes follow second-order kinetics which do not exclude a mechanism involving a concerted pathway. The... 

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... 

Thermolysis of 2-diazo-l,3-dithiane, prepared in situ from the reaction of 2-lithio-2-trimethylsilyl-l,3-dithiane and tosyl azide, occurs already below 0°C. The resulting carbene dimerizes efficiently even in the presence of alkenes and alkynes to give bis(l,3-dithianylidene) in 78% yield (Scheme 41) <1997T9269>. 

The chemistry of the carbene l,3-dithian-2-ylidene, generated from 2-diazo-l,3-dithiane, was briefly discussed in Section 8.11.6.3.1. It reacts poorly with alkenes or alkynes if they are not highly electron deficient. However, it was found that Cso as source of C=C bonds efficiently provides the [2-1-1] cycloaddition product, which can be hydrolyzed to the Cso-cyclopropanone (Scheme 51) <2001HC0223>. 

CEJ1358> and the ruthenium mediated isomerization of double bonds (cf. Scheme 89, Section 8.11.7) <2007TL137> are recent examples of transition metal catalyzed manipulations at the side chain carbon atoms of 1,3-heterocycles. A novel side-chain addition reaction of aldehydes to 6-alkylidene-l,3-dioxin-4-ones was used for the construction of intermediates of lophotoxin <2006CJC1226>. An acid-catalyzed intramolecular cycloaddition of a hydroxy group to an alkene has been effected by the presence of an adjacent 1,3-dithiane moiety <2006TL4549>. 

When thioketones are irradiated alone, a cydodimerizalion may occur to give a 1,3-dithietane <4.98). In the presence of an alkene, different cycloadducts are found, usually ihietanes. If visible radiation is used, electron-rich alkenes are especially effective as addends, and the products can be rationalized on the basis of a two-step mechanism involving the more highly stabilized biradical intermediate 4.99). Sometimes a 1,4-dithiane accompanies the thietane (4.100) as a result of the trapping of the biradical by a further molecule of ground-state thioketones (unlike ketones, thioketones react with radicals quite readily, which is one of the causes of... 

The method ZC3Z + C is used for the preparation of reduced pyrimidines, oxazines and thiazines as well as for dioxanes, dithianes and oxathianes as mentioned above (e.g. 258 — 257, 259 Z, Z = NH, O, S). The Prins reaction yields 1,3-dioxanes (77S661) it involves the acid-catalyzed condensation of alkenes with aldehydes with 1,3-diols as intermediates. 

Routes to benzo-fused derivatives of 1,4-dioxanes, 1,4-oxathianes and 1,4-dithianes make use of anions or dianions of the appropriate 1,2-disubstituted benzene. An alternative approach to the synthesis of 1,4-benzodioxanes involves Diels-Alder addition reactions of alkenes across the quinone function of 1,2-benzoquinones, e.g. (352) — (353). 

Hydrolysis of the dithiane groups with mercury(II) chloride/cadmium carbonate followed by deoxymercuration of the alkene with potassium iodide yielded the diketone... 

The dibromobutene 149 could be monosubstituted using 2-lithio-l,3-dithiane as a carboxylic acid precursor, followed by substitution of the remaining bromide with sodium azide as the amine precursor (Scheme 32). The azide was reduce under Staudinger conditions and acy-lated. After separation of the EIZ isomers, further functional group transformations gave the dimethyl-substituted Gly-Gly alkene isostere 152J123 ... 

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