Tether-directed cycloaddition reactions

Figure 1.19. Chiral bis-adducts of Cgo obtained by tether-directed cycloaddition reactions.

They have also reported a direct route to optically pure, fused, or bridged tricyclic (3-lactams (III and IV, Fig. 18) as further advances in the intramolecular nitrone-alkene cycloaddition reactions of monocyclic 2-azetidinone-tethered alkenyl-aldehydes [289]. 

However, yields in the intermolecular cycloaddition reactions of vinylcarbene complexes, formed by intramolecular insertion of an alkynyl tethered metal carbene complex, are higher when molybdenum rather than chromium or tungsten carbene complexes are employed. Mild thermolysis (THF, 65 °C, 1 h) in the presence of ten equivalents of an electronically undemanding alkene directly leads to the 2-alkyl-2-(2-methoxycyclopentenyl)cyclopropanes 31. ... 

Figure 10 Carbohydrate microplate arrays prepared by the noncovalent immobilization of azide-derivatized carbohydrates to microtiter plates via a 1,3-dipolar cycloaddition reaction between alkynes and azides. Carbohydrates displaying terminal azides can be captnred on microtiter plate surfaces through a terminal alkyne attached to a long, ahphatic tether and screened directly on the microtiter plate surface.

Chelation is also thought to play an important part in directing the facial selectivity of cycloadditions with the camphor sultam auxiliary. A variety of dienes can be used and adducts are obtained with very high diastereomeiic excesses. Both inter-and intramolecular cycloaddition reactions are amenable to the use of a chiral auxiliary. An intramolecular example is illustrated in Scheme 3.93, in which the diene and dienophile are tethered and in which cycloaddition leads to predominantly one of the two diastereomeric trans-fusQd bicyclic (endo) products. The dienophile is thought to adopt the s-cis conformation, with the aluminium atom complexed to the carbonyl and one of the two sulfone oxygen atoms. 

Bower reported a directing group enhanced rhodium-catalyzed [(3-i-2)-i-l] cycloaddition reaction of nitrogen-tethered yne-CPs and CO (see (50)) [124]. The authors used the urea as the directing group to induce the oxidative addition at the proximal bond of the amino-cyclopropanes. 

Scheme 6.186) [347]. The condensation of O-allylic and O-propargylic salicylalde-hydes with a-amino esters was carried out either in the absence of a solvent or - if both components were solids - in a minimal volume of xylene. All reactions performed under microwave conditions rapidly proceeded to completion within a few minutes and typically provided higher yields compared to the corresponding thermal protocols. In the case of intramolecular alkene cycloadditions, mixtures of hexa-hydrochromeno[4,3-b]pyrrole diastereoisomers were obtained, whereas transformations involving alkyne tethers provided chromeno[4,3-b]pyrroles directly after in situ oxidation with elemental sulfur (Scheme 6.186). Independent work by Pospisil and Potacek involved very similar transformations under strictly solvent-free conditions [348]. 

The convergence of the nitronate and nitrile oxide cycloadditions has allowed for the direct comparisons of yields and stereoselectivities of the two processes. For intramolecular reactions, the nitronate dipole typically required longer reaction times and/or elevated temperatures (22,98,135), however, the nitronate cycloaddition shows considerably higher diastereoselectivity (Table 2.42). Interestingly, the diastereoselectivity is dependent on the placement of a substituent on the tether. In the case of the silyl nitronate derived from 172, the diastereoselectivity is controlled by the substituent at C(l), while cyclization of the analogous nitrile oxide is governed by the substituent at C(l ) (Scheme 2.10) (124). 

Cycloadditions only proceeding after electron transfer activation via the radical cation of one partner are illustrated by the final examples. According to K. Mizono various bis-enolethers tethered by long chains (polyether or alkyl) can be cyclisized to bicyclic cyclobutanes using electron transfer sensitizer like dicyanonaphthalene or dicyano-anthracene. Note that this type of dimerization starting from enol ethers are not possible under triplet sensitization or by direct irradiation. Only the intramolecular cyclization ci the silane-bridged 2>. s-styrene can be carried out under direct photolysis. E. Steckhan made use of this procedure to perform an intermolecular [4+2] cycloaddition of indole to a chiral 1,3-cyclohexadiene. He has used successfully the sensitizer triphenylpyrylium salt in many examples. Here, the reaction follows a general course which has been developed Bauld and which may be called "hole catalyzed Diels-Alder reaction". 

Another diastereoselective intramolecular Diels-Alder reaction of furan was studied by Keay wherein the methyl group in the tether of (—)—12 directed the facial selectivity of the cycloaddition. Equilibrating conditions using a catalytic amount of Lewis acid gave the tricyclic enone (—)-13, Eq. 10 [23]. 

Similar effects have been observed with azide dipolar cycloadditions. In the system directly analogous to that described in the preceding paragraph, reaction of 4-tert-butylphenylazide with propynamide produces a 2 1 ratio of the 1,4- and 1,5-disubstituted-1,2,3-triazoles, whereas tethering the propynamide to j8-cyclodextrin changes the ratio... 

This type of catalyst was also employed by Christmann et al. to promote the asymmetric intramolecular Diels Alder reaction of tethered a,p-unsaturated dialdehydes in the presence of benzoic acid as a co-catalyst through vinylogous enamine activation. The corresponding cycloadducts were obtained in good yields and excellent enantioselectivities of up to 98% ee, as shown in Scheme 6.6. When one of the aldehyde functions was replaced by an a,p-unsaturated ketone as the acceptor, no formal [4 -I- 2] cycloaddition was observed instead, a direct enantioselective vinylogous Michael addition occurred. 

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