Bicyclic polar compounds

A major difficulty with the Diels-Alder reaction is its sensitivity to sterical hindrance. Tri- and tetrasubstituted olefins or dienes with bulky substituents at the terminal carbons react only very slowly. Therefore bicyclic compounds with polar reactions are more suitable for such target molecules, e.g. steroids. There exist, however, several exceptions, e. g. a reaction of a tetrasubstituted alkene with a 1,1-disubstituted diene to produce a cyclohexene intermediate containing three contiguous quaternary carbon atoms (S. Danishefsky, 1979). This reaction was assisted by large polarity differences between the electron rich diene and the electron deficient ene component. 

Recently, [2+3] cycloaddition reaction of 2-acetyl-[l,2,3]diazaphosphole (6) with 9-diazofluorenes (96) has been reported [105, 106], From the reaction in cyclohexane at rt, bicyclic phosphirane 97 was obtained as a result of the loss of nitrogen from the initial cycloadduct (Scheme 30). The cycloadduct, 3-spiro substituted 3H-[l,2,4]diazaphospholo-fused [l,2,3]diazaphosphole (98) could be isolated in good yield at room temperature in one case (R=/Bu) its stability was assigned to the presence of bulky fert-butyl group at 7-position. Use of polar solvent like dichloromethane led to the cyclic trimeric compound 99 (Scheme 30). 

Nitrosyl halides add to alkenes references are scattered through the litnnture back to 1875 (ref. 194 and references cited therein). The adducts vary enormously in their stability, but when their structures allow they, like nonhalogenated nitroso compounds, isomerize to oximes or dimnize. The orientation of the reaction is consistent with an electrophilic medumism, in which the reagent is polarized as NO Hat. Bicyclic substrates and reaction media of low polarity favor syn addition, suggesting a four-center transition state (Scheme 81). Aziridine synthesis via NOCl/alkene adducts is discussed in Section 3.5.2.1. 

There are no reported cycloadditions to bicyclic quinolizinium salts. The reactions in this section fall into two groups, cycloadditions to 4-quinolizones or related compounds and the very extensively studied cationic polar cycloadditions to benzo[b]quinolizinium salts. 

Homoconjugative interactions between a cyclopropane ring and a double bond were studied for the three bicyclic compounds 7-9, each possessing a locked vinylcyclopropane fragment [84], The polarization patterns in the chloranil-sensitized photoreactions of these substrates were used to probe the spin density distribution in the radical cations. A strong dependence of electronic structure of the radical cations on steric effects was found (see Chart IV). 

To investigate whether the radical cations of the diazadithiafulvalenes would undergo eoupling with carbon radicals at a different rate than TTF+, eomparative experiments were performed with the allyl sulfide 99. This led to the products 100 and 101 as for TTF (Scheme 16). However the yield of the bicyclic produet was higher for the diaza compounds, indicating a slow-down in the kinetics of the recombination reaction. Although this is likely to be a fairly small change, it is clear that the modulation of kinetics in this way may be useful with radical-polar crossover reaetions to permit slower cyclizations to occur before recombination. 

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