Annelation special

Polarimetry, circular dichroism (CD) and optical rotatory dispersion (ORD) are the most important tools for the study of properties arising from optical activity. Although many chiral thiophenes have been prepared, there is no secure basis for a systematic discussion of the special effects of thiophene or annelated thiophene rings. For the purpose now at hand it is more expedient to discuss three different areas in which thiophene containing molecules and the related chiroptical techniques are central features. 

Cyclopentenone annelation.7 A new method for this reaction involves Pd(0)-directed C-alkylation of 2-methyl-l,3-cyclopentanedione or 2-methyl-l,3-cyclohexa-nedione with 2-ethoxy-3-acetoxy-l-propene followed by an intramolecular Wittig reaction (equation I). Of special interest, the cyclopentenone 1 can be obtained in optically active form by use of an optically active phosphine. Thus use of (R)-DIOP (4, 273 5, 360 361 6, 309) leads to 1 as a 70 30 mixture of ( + )- and ( —)-enantiomers. Similar results are obtained with (R)-(-)-methylphenylpropyl-phosphine. 

In addition to the above discussed transition metal catalyzed [3+2]-eycloadditions some other methylenecyclopentane syntheses have been developed which are of special interest within the context of annelation procedures. 

Numerous aromatic compounds including several bowl-shaped fullerene fragments have been prepared by this method, e.g. cyclopenta[z/]fluoranthene (22, Scheme8, [54d,56a]), cyclopenta[bc]corannulene (23, [55a]) and diace-naphtho[3,2,l,8-cdejg 3, 2, l, 8 -Zmnop]chrysene (24, see Scheme 9, [55b,c]). Of special importance is this approach for the synthesis of cyclopenta-annelated PAHs, e.g. and the three isomeric dicyclopentapyrenes (25-27, Scheme 9, [54e, 56b]). Using these reference samples, several cyclopenta-annelated PAHs could be identified as byproducts formed in the incomplete oxidation of hydrocarbons in fuel rich flames [57]. 

Operations that construct two or more bonds at once ( 3,4) are of special importance for efficiency. Annelation reactions are of this class. Annelations may be annotated i6) fey the number of ring atoms in each of the two synthons forming... 

Furthermore, the single sequential constructions derived in this protocol will miss some concerted two-construction reactions, most notably cycloadditions like the Diels-Alder reaction. These are simply special cases of annelation. Annela-tions are of central importance to the synthesis of cyclic molecules since they are the family of reactions creating rings from separate, smaller starting materials (see Section 8, item 3, and Ref. )), They deserve separate special treatment as linked pairs of two successive half-reactions on two synthons. This could derive special exhaustive sequence lists for annelations, both those already buried in the present sequence lists and the important cycloadditions, cis /-lists of net structural change. 

Fusion of o-aminoamides with urea to give annelated pyrimidine-2,4-diones was introduced by Abt in 1889.250 Usually, the starting material and an equal weight (sometimes more) of urea are heated at 175°C for 2 hr, but other conditions (from 160° to 200°C and from 20 min to 5 hr) have been advocated in special cases. Yields have varied from good to excellent. At lower temperatures, ureide intermediates of the type 155 have been isolated, indicating the course of the reaction. 

As with cyclopentadienide salts, benzo-annelation leads to decreased stabilisation in the tropylium ring. This is illustrated by the pK + values for the tropylium, benzotropylium, 1,2,4,5-di-benzotropylium and tribenzotropylium cations which are respectively 4-75 [3,70], 1-7 [106], -3-7 [107] and --15 [108]. The latter value suggests that there is little if any special stabilisation in... 

Plenary Lecture The use of kinetically generated unstable enolate ions in the regiospecific formation of carbon-carbon bonds. Special applications to annelation processes. 

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