Unsaturated rings

The reaction of the vinylcyclopropanedicarboxylate 301 with amines affords an allylic amine via the 7r-allylpalladium complex 302[50]. Similarly, three-membered ring A -tosyl-2-(l,3-butadienyl)aziridine (303) and the four-mem-bered ring azetidine 304 can be rearranged to the five- and six-membered ring unsaturated cyclic amines[183]. 

Ozonation of Aromatics. Aromatic ring unsaturation is attacked much slower than olefinic double bonds, but behaves as if the double bonds in the classical Kekule stmctures really do exist. Thus, benzene yields three moles of glyoxal, which can be oxidized further to glyoxyUc acid and then to oxahc acid. Substituted aromatics give mixtures of aUphatic acids. Ring substituents such as amino, nitro, and sulfonate are cleaved during ozonation. 

A-ring unsaturated ketones and the sensitive 11 j5-hydroxyl group will survive conditions of the Mattox rearrangement. 

Observed geometrical features or stabilities of small ring molecules containing an endocyclic double bond have been explained in terms of the Ji-a interactions [137, 138]. The n relaxation is predicted to stabilize such small ring unsaturated molecules of heavy atoms recently prepared [139-142],... 

Cobalt complexes of the fused 7-membered ring unsaturated analogs of the dibenzo-[14] tetraazaannulenes, the tropocoronands (H2TC), have also been reported. The crystal structure... 

A diazide was obtained91 as the sole product also by addition of IN3 in acetonitrile to c -bicyclo[6.1.0]nonatriene. In this case, however, at variance with the medium-size ring unsaturated compounds, the reaction did not afford a normal vie-diazide. A mechanism involving a bishomotropylium or cyclopentadienyl cation has been proposed (equation 64). 

The [2,3] Wittig rearrangement has proven useful for ring contraction in the synthesis of a number of medium-ring unsaturated structures, as illustrated by entry 3 in Scheme 6.14. 

This product is then treated with acryloyl chloride. The initial step in this case probably involves the acylation of nitrogen on the enamine conjugate addition then completes the formation of the lactam ring (23-5). Treatment of that product with triethyl silane then reduces the ring unsaturation and cleaves the benzylic nitrogen bond on the auxiliary to yield (23-6) as the optically pure tmns isomer. Displacement of bromine with the mercapto benzthiazole (23-7) completes the synthesis of izonsteride (23-8) [25]. 

HOMOATOMIC RINGS UNSATURATED SYSTEMS 10.2.1 Three-membered Rings... 

Ring unsaturation t with double bond in ring, especially if adjacent to the carbonyl/sulfonyl Cross-linking potential resonance stabilization of the carbonium ion... 

The method is simple, and has the additional virtue of great versatility. The requisite ally lie alcohol can be prepared via the Favorskii reaction of a 20-ketopregnane, or via ethoxyacetylene addition to a 17-ketoandrostane. Additional functional groups may be present prior to these reactions or introduced into the A17(20)-compound later. An aromatic A-ring, A-ring unsaturated ketones and the 11 /1-hydroxyl group are all stable to the oxidative attack on the A17(20)-olefln, and the 21-acetate is not hydrolyzed during the reaction. 

Most steroids are alcohols, and accordingly are named as sterols. Important examples include cholesterol, ergosterol, estradiol, stigmasterol, and other representative sterols given in Table 30-2. As you can see from their structures, most possess the same ring skeleton but vary considerably in their peripheral structural features, stereochemistry, and in the degree of ring unsaturation. 

Fused ring products also may be formed from the transannular cyclization of medium ring unsaturated amine derivatives, as shown in Table 27.218 A transannular cyclization to the loline ring system was reported by Wilson (entry 4).218c Cyclization of a tertiary amine to a pyrrolizidine perchlorate has also been reported.219... 

Based on the data in the Table and on our published results,7 it is clear that five-, six-, and seven-membered cycloalkenones undergo this 4-atom ring enlargement reaction to produce medium ring, unsaturated lactones in overall yields... 

Most recent studies indicate that ARCM can be used to synthesize small and medium ring N-containing unsaturated heterocycles in high yield and with excellent ee through catalytic kinetic resolution and asymmetric synthesis [19]. As the representative data in Scheme 10 illustrate, levels of enantioselectivity can vary depending on the nature of the arylamine (compare 44 to 46). As the synthesis of 48 indicates (cf. Scheme 10), the facility and selectivity with which medium ring unsaturated amines are obtained by the Mo-catalyzed protocol is particularly noteworthy. 

Reactivity of Fully Conjugated Rings (Unsaturated Rings) 194... 

Several saturated parent systems shown in Scheme 1, especially (2), (4) and (7), have been incorporated into spiro compounds or have been fused with a variety of non-aromatic carbocyclic or heterocyclic rings. Unsaturated compounds with an endocyclic carbon-carbon double bond, formally possible in all systems with three adjacent heteroatoms, are only known in very few cases (e.g. cyclic sulfites and sulfates of enediols) or in the form of benzo or other condensed systems (mainly cyclic sulfites and sulfates of aromatic 1,2-dihydroxy compounds). For details concerning the various types of derivatives, see Section 4.33.4.2. 

A. Pyrrole, imidazole and other 5-membered ring unsaturated heterocycles. 

Starting material, we reported the synthesis of four different tricyclic architectures (38-4-1) that were obtained using RCM [29]. For the solid-phase synthesis, compound 42 was obtained from 37 and was successfully loaded onto the alkylsilyl linker-based polystyrene macrobeads, providing 43. In one study, compound 43 was successfully transformed using RCM into the six-membered-ring unsaturated lactam 44. Using this method for library generation remains to be undertaken. 

It has been noted [20] that, during deuteration experiments, more than the expected stoichiometric amount of deuterium was added to the cyclohexyl ring (from density measurements). This isotope exchange event had not been described previously, and requires that some mechanism other than simple transfer of H2 across the sites of aromatic ring unsaturation must be occurring during the hydrogenation reaction. 

When considering sugar ring unsaturation removed from the 1,2 position, many methods are available in addition to methods analogous to those used in glycal formation. As shown in Scheme 6.39, the direct elimination of sulfonates under basic conditions provides a useful strategy [20]. Moreover, in the case of m/i5 -2,3-bis-sulfonates, initial displacement of one sulfonate with lithium iodide can be followed by metallation and tandem elimination, as shown in Scheme 6.40 [69]. 

With regard to their classification, we will distinguish four series of unsaturated derivatives the vinyl series, the allyl series, the acetylenic series, and the ring-unsaturated derivatives that are bioisosteric to aromatic rings. 

Reduction proceeds smoothly with fatty acid esters, but not with aromatic esters whose carboxylic ester group is attached directly to a benzene ring., / -Unsaturated esters give the related saturated alcohols, but double bonds that are not conjugated to the alkoxycarbonyl group are, in general, not reduced by this method. 

Since the early observations of photodeconjugation of conjugated aliphatic carbonyl and carboxyl derivatives [19], the reaction has been generalized to unsaturated esters [20], lactones [21], acids [22], amides [20n], aldehydes [23], aliphatic enones [24], and medium-ring unsaturated enones [25]. 

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